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  1. 1. 20 Animal Development: From Genes to Organism The whale blows its nose from the top of its head—as in “thar she blows,” the whalers’ exclamation. The spout from the blowhole is the whale’s exhalation coming out of its nasal pas- sages. It is convenient for a marine mammal to breathe out of the top of its head because not much of its body has to come out of the water, and it can continue moving through the water as it breathes. But in most terrestrial mammals, the nose is on the front of the head. How did the whale’s nose get to the top of its head? This is an evolutionary question, but the answer is to be found in development—the processes whereby a fertilized egg becomes an adult organism. The vertebrate body varies enormously among species in form and function, yet its basic structural design does not. For example, the whale flipper, the bat wing, and the human arm all have the same bones. However, during development, these bones assume different shapes and dimensions to adapt the forelimbs to various functions: swimming, flying, and tool use. Thar She Blows! The nasal passages of Similarly, all vertebrates have the same bones in their heads, but through devel- the whale Orcinus orca are on top of its opment, these bones grow differentially, and therefore the skull takes on different head because of the extreme growth of its jaw bones during development. shapes in different species. In both whales and humans, the nasal passages are in the nasal bone, which is just above the bones of the upper jaw. In the human, that places the nasal bone just above the jaw on the front of the face. Things are different in the whale. During development of the whale skull, the bones of the upper jaw grow enormously relative to the other bones of the skull, and project far forward to form the cavernous mouth. As a result of this differential for- ward growth of the jaw bone, the nasal bone ends up on the top of the skull, rather than on the front. Thus, the answer to why the whale’s nose is on the top of its head and how its forelimbs become flippers is found in the processes of development. These processes form and shape the components of the basic vertebrate body plan. In the previous chapter, we learned that the processes of development include deter- mination, differentiation, growth, and mor-
  2. 2. ANIMAL DEVELOPMENT 409 phogenesis. In this chapter we will see how these processes are carried out in the early stages of development. Development begins with the joining of sperm and egg. The fertilized egg goes through an initial rapid series of cell divisions without growth that subdivides the egg cytoplasm into a mass of smaller undifferentiated cells. Although this mass of cells shows no hints of the eventual body plan, the uneven distribution of molecules in the cytoplasm of the fer- tilized egg provides positional information that will result in the determination of cells and set up the body plan. The body plan then unfolds through orderly movements of cells that create multiple cell layers and set up new cell-to-cell contacts that trigger signal transduction cascades and further steps of determination. These inductive interactions influence the temporal and spatial expression of the genes that control the growth and differentiation of cells, leading to the emergence of the organs of the new individual. To appreciate both the diversity and the similarity in the 20.1 Sperm and Egg Differ Greatly in Size This artificially colored development of different animals, we will discuss these early micrograph of human fertilization illustrates the size difference developmental steps in a few model organisms that have between the two types of gametes in mammals. The large egg (blue) been studied extensively by developmental biologists: sea contributes more cytoplasm to the zygote than the much smaller sperm (yellow). urchins (invertebrates), and frogs, chickens, and humans (all vertebrates). comes the centrosome of the zygote, which produces the mi- totic spindles for subsequent cell divisions. Development Begins with Fertilization It had long been assumed that the one thing that sperm Fertilization is the union of a haploid sperm and a haploid and egg contributed equally to the zygote was their haploid egg to produce a diploid zygote. Fertilization does more, nuclei. However, we now know that even though they are however, than just restore a full complement of maternal equivalent in terms of genetic material, mammalian sperm and paternal genes. The entry of a sperm into an egg acti- and eggs are not equivalent in terms of their roles in devel- vates the egg metabolically and initiates the rapid series of opment. In the laboratory, it is possible to construct zygotes cell divisions that produce a multicellular embryo. Also, in in which both haploid nuclei come from the mother or both many species, the point of entry of the sperm creates an come from the father. In neither case does development asymmetry in the radially symmetrical egg. This asymme- progress normally. Apparently, in mammals at least, certain try is the initiating event that enables a bilateral body plan genes involved in development are active only if they come to emerge from the radial symmetry of the egg. We will de- from a sperm, and others are active only if they come from scribe the mechanisms of fertilization in Chapter 43. Here an egg. This phenomenon, called genomic imprinting, was de- we take a closer look at the cellular and molecular interac- scribed in Chapter 17. tions of sperm and egg that result in the first steps of devel- opment. Fertilization causes rearrangements of egg cytoplasm The entry of the sperm into the egg stimulates changes in and The sperm and the egg make different contributions rearrangements of the egg cytoplasm that establish the po- to the zygote larity of the embryo. The nutrients and molecules in the cy- Nearly all of the cytoplasm of the zygote comes from the egg toplasm of the zygote are not homogeneously distributed, (Figure 20.1). Egg cytoplasm is well stocked with nutrients, and therefore, they are not divided equally among all daugh- ribosomes, and a variety of molecules, including mRNAs. Be- ter cells when cell divisions begin. This unequal distribution cause the sperm’s mitochondria degenerate, all of the mito- of cytoplasmic factors sets the stage for the signal transduc- chondria (and therefore all of the mitochondrial DNA) in the tion cascades that orchestrate the sequential steps of devel- zygote come from the mother. In addition to its haploid nu- opment: determination, differentiation, and morphogenesis. cleus, the sperm makes one other important contribution to Let’s examine these earliest developmental events in the frog, the zygote in some species: a centriole. This centriole be- an organism in which they have been well studied.
  3. 3. 410 CHAPTER T WENT Y The rearrangements of egg cytoplasm in some frog species sperm centriole rearranges the microtubules in the vegetal are easily observed because of pigments in the egg cyto- hemisphere cytoplasm into a parallel array that presumably plasm. The nutrient molecules in an unfertilized frog egg are guides the movement of the cortical cytoplasm. Organelles dense, and they are therefore concentrated by gravity in the and certain proteins from the vegetal hemisphere move to lower half of the egg, which is called the vegetal hemisphere. the gray crescent region even faster than the cortical cyto- The haploid nucleus of the egg is located at the opposite end plasm rotates. of the egg, in the animal hemisphere. The outermost (cortical) As a result of these movements of cytoplasm, proteins, and cytoplasm of the animal hemisphere is heavily pigmented, organelles, changes in the distribution of critical developmen- and the underlying cytoplasm has more diffuse pigmenta- tal signals occur. A key transcription factor in early develop- tion. The vegetal hemisphere is not pigmented. ment is β-catenin, which is produced from maternal mRNA The surface of the frog egg has specific sperm-binding and is found throughout the cytoplasm of the egg. Also pres- sites located only in the animal hemisphere, so sperm always ent throughout the egg cytoplasm is a protein kinase called enter the egg in that hemisphere. When a sperm enters, the GSK-3, which phosphorylates and thereby targets β-catenin cortical cytoplasm rotates toward the site of sperm entry. This for degradation. However, an inhibitor of GSK-3 is segregated rotation reveals a band of diffusely pigmented cytoplasm on in the vegetal cortex of the egg. After sperm entry, this inhibitor the side of the egg opposite the site of sperm entry. This band, is moved along microtubules to the gray crescent, where it pre- called the gray crescent, will be the site of important devel- vents the degradation of β-catenin. As a result, the concentra- opmental events (Figure 20.2). tion of β-catenin is higher on the dorsal side than on the ven- The cytoplasmic rearrangements that create the gray cres- tral side of the developing embryo (Figure 20.3). cent bring different regions of cytoplasm into contact on op- Evidence supports the hypothesis that β-catenin is a key posite sides of the egg. Therefore, bilateral symmetry is im- player in the cell–cell signaling cascade that begins the posed on what was a radially symmetrical egg. In addition process of cell determination and the formation of the em- to the up–down difference of the animal and vegetal hemi- bryo in the region of the gray crescent. But before there can spheres, the movement of the cytoplasm sets the stage for be cell–cell signaling, there must be multiple cells, so let’s the creation of the anterior–posterior and left–right axes. In turn first to the early series of cell divisions that transforms the frog, the site of sperm entry will become the ventral the zygote into a multicellular embryo. (belly) region of the embryo, and the gray crescent will be- come the dorsal (back) region. Since the gray crescent also marks the posterior end of the embryo, these relationships Cleavage: Repackaging the Cytoplasm specify the anterior–posterior and left–right axes as well. The transformation of the diploid zygote into a mass of cells occurs through a rapid series of cell divisions, called cleav- age. Because the cytoplasm of the zygote is not homoge- Rearrangements of egg cytoplasm set the stage neous, these first cell divisions result in the differential dis- for determination tribution of nutrients and cytoplasmic determinants among The molecular mechanisms underlying the first steps in frog the cells of the early embryo. In most animals, cleavage pro- embryo formation are beginning to be understood. The ceeds with rapid DNA replication and mitosis, but no cell growth and little gene expression. The em- bryo becomes a solid ball of smaller and The cortical cytoplasm smaller cells, called a morula (from the Latin Animal cortical rotates relative to the word for “mulberry”). Eventually, this ball Animal inner cytoplasm. cytoplasm forms a central fluid-filled cavity called a (pigmented) pole A blastocoel, at which point the embryo is called a blastula. Its individual cells are Inner called blastomeres. cytoplasm The pattern of cleavage, and therefore Sperm entry the form of the blastula, is influenced by point Vegetal two major factors. First, the amount of nu- cortical V trient material, or yolk, stored in the egg Vegetal cytoplasm The gray crescent is pole differs among species. Yolk influences the (unpigmented) created by the rotation. pattern of cell divisions by impeding the 20.2 The Gray Crescent Rearrangements of the cytoplasm of frog eggs after fertilization pinching in of the plasma membrane to create the gray crescent. form a cleavage furrow between the daugh-
  4. 4. ANIMAL DEVELOPMENT 411 ter cells. Second, cytoplasmic determinants stored in the egg (a) Fertilization by the mother guide the formation of mitotic spindles and Egg Animal pole the timing of cell divisions. β-Catenin (orange) is distributed throughout cytoplasm. Sperm The amount of yolk influences cleavage GSK-3 (blue), which targets β-catenin for degradation, In embryos with little or no yolk, there is little interference is also found throughout with cleavage furrow formation, and all the daughter cells are cytoplasm. of similar size; the sea urchin egg provides an example (Fig- Vegetal pole ure 20.4a). More yolk means more resistance to cleavage fur- A protein that inhibits row formation; therefore, cell divisions progress more rapidly GSK-3 is contained in vegetal pole vesicles. in the animal hemisphere than in the vegetal hemisphere, (b) Cortical rotation where the yolk is concentrated. As a result, the cells derived from the vegetal hemisphere are fewer and larger; the frog egg provides an example of this pattern (Figure 20.4b). Ventral Dorsal In spite of this difference between sea urchin and frog (V) (D) eggs, the cleavage furrows completely divide the egg mass Vesicles in vegetal pole move on microtubule in both cases; thus these animals are said to have complete tracks to side opposite cleavage. In contrast, in eggs that contain a lot of yolk, such as sperm entry. the chicken egg, the cleavage furrows do not penetrate the yolk. As a result, cleavage is incomplete, and the embryo (c) Dorsal enrichment forms as a disc of cells, called a blastodisc, on top of the yolk inhibitor mass (Figure 20.4c). This type of incomplete cleavage, called The vesicles release GSK-inhibiting protein… discoidal cleavage, is common in fishes, reptiles, and birds. Another type of incomplete cleavage, called superficial V D cleavage, occurs in insects such as the fruit fly (Drosophila). In the insect egg, the mass of yolk is centrally located (Figure 20.4d). Early in development, cycles of mitosis occur without cytokinesis. Eventually the resulting nuclei migrate to the pe- riphery of the egg, and after several more mitotic cycles, the plasma membrane of the egg grows inward, partitioning the (d) Dorsal inhibition nuclei into individual cells. of GSK-3 …so GSK-3 cannot degrade β-catenin on the dorsal side… The orientation of mitotic spindles influences V D …but does degrade it the pattern of cleavage on the ventral side. The positions of the mitotic spindles during cleavage are not random; rather, they are defined by cytoplasmic determi- nants that were produced from the maternal genome and stored in the egg. The orientation of the mitotic spindles de- (e) Dorsal enrichment termines the planes of cleavage and, therefore, the arrange- of b-catenin Thus there is a higher ment of the daughter cells. β-catenin concentration If the mitotic spindles of successive cell divisions form in the dorsal cells of the parallel or perpendicular to the animal–vegetal axis of the V D early embryo. zygote, the cleavage pattern is radial, as in the sea urchin and the frog. In these organisms, the first two cell divisions are parallel to the animal–vegetal axis and the third is perpen- dicular to it (Figure 20.4a,b). Another cleavage pattern, spi- 20.3 Cytoplasmic Factors Set Up Signaling Cascades ral cleavage, results when the mitotic spindles are at oblique Cytoplasmic movement changes the distributions of critical develop- angles to the animal–vegetal axis. Mollusks have spiral mental signals. In the frog zygote, the interaction of the protein kinase GSK-3, its inhibitor, and the protein β-catenin are crucial in cleavage, and a visible expression of this is the coiling of specifying the dorsal–ventral (back–belly) axis of the embryo. snail shells.
  5. 5. 412 CHAPTER T WENT Y FERTILIZED 2-CELL 4-CELL 8-CELL EGG STAGE STAGE STAGE (a) Sea urchin Animal Blastomeres (lateral view) pole Yolk platelets are Early cleavage results evenly distributed. in blastomeres of similar size. Complete cleavage 0.15 mm Vegetal pole (b) Frog Animal pole Cleavage (lateral view) furrow Blastomeres at the animal pole are smaller, and those at the vegetal pole are larger. Gray Yolk is concentrated crescent at the vegetal pole. Vegetal pole 0.5–1 mm (c) Chick Blastomeres The embryo develops (view from top) on top of the yolk as a disc of cells, called a Incomplete blastodisc. cleavage Cleavage is incomplete. ~25 mm Single (d) Drosophila cell layer Yolk core (lateral section) Superficial cleavage Nucleus Yolk Multiple The nuclei migrate to the periphery, and 0.5 mm nuclei plasma membranes form between them. 20.4 Patterns of Cleavage in Four Model Organisms Differences in patterns of early embryonic development reflect differences in the way the egg cytoplasm is organized. Cleavage in mammals is unique in cleavage. In species such as sea urchins and frogs, gene ex- Several features of mammalian cleavage are very different pression does not occur in the blastomeres, and cleavage is from those seen in other animal groups. First, the pattern of directed exclusively by molecules that were present in the cleavage in mammals is rotational: the first cell division is par- egg prior to fertilization. allel to the animal–vegetal axis, yielding two blastomeres. As in other animals that have complete cleavage, the early The second cell division occurs at right angles: one blas- cell divisions in a mammalian zygote produce a loosely as- tomere divides parallel to the animal–vegetal axis, while the sociated ball of cells. However, at about the 8-cell stage, the other divides perpendicular to it (Figure 20.5a). behavior of the mammalian blastomeres changes. They Cleavage in mammals is very slow; cell divisions are 12–24 change shape to maximize their surface contact with one an- hours apart, compared with tens of minutes to a few hours in other, form tight junctions, and become a very compact mass non-mammalian species. Also, the cell divisions of mam- of cells (Figure 20.5b). malian blastomeres are not in synchrony with each other. Be- At the transition from the 16-cell to the 32-cell stage, the cause the blastomeres do not undergo mitosis at the same cells separate into two groups. The inner cell mass will be- time, the number of cells in the embryo does not progress in come the embryo, while the surrounding cells become an the regular (2, 4, 8, 16, 32, etc.) progression typical of other encompassing sac called the trophoblast, which will be- species. come part of the placenta. Trophoblast cells secrete fluid, Another unique feature of the slow mammalian cleavage creating a cavity (blastocoel) with the inner cell mass at one is that the products of genes expressed at this time play roles end (see Figure 20.5b). At this stage, the mammalian embryo
  6. 6. (a) ANIMAL DEVELOPMENT 413 Parallel Plane of first plane cell division A 20.5 The Mammalian Zygote Becomes a Blastocyst (a) Mammals have rotational cleavage, in which the plane of Perpendicular the first cleavage is parallel to the animal–vegetal (A, V) axis, plane but the planes of the second cell division (shown in beige) are at right angles to each other. (b) Starting late in the 8-cell stage, the mammalian embryo undergoes compaction of its cells, resulting in a blastocyst—a dense inner cell mass on top of a hollow blastocoel, completely surrounded by trophoblast cells. V (b) Later 8-cell stage Blastocyst Early 8-cell stage (compaction) 16-cell stage (32-cell stage) Blastocoel Tight junctions have The inner cell mass will Zona pellucida Trophoblast formed between the cells. form the embryo. is called a blastocyst to distinguish it from the blastulas of The blastocoel prevents cells from different regions of the other animals. blastula from interacting, but that will soon change. During Fertilization in mammals occurs in the upper reaches of the the next stage of development, the cells of the blastula will mother’s oviduct, and cleavage occurs as the zygote travels move around and come into new associations with one an- down the oviduct to the uterus. When the blastocyst arrives in other, communicate instructions to one another, and begin to the uterus, the trophoblast adheres to the endometrium (the differentiate. In many animals, these movements of the blas- uterine wall). This event begins the process of implantation that tomeres are so regular and well orchestrated that it is possible embeds the embryo in the wall of the uterus (see Figure 20.14). to label a specific blastomere with a dye and identify the tis- In humans, implantation begins on about the sixth day after sues and organs that form from its progeny. Such labeling ex- fertilization. As the blastocyst moves down the oviduct to the periments produce fate maps of the blastula (Figure 20.6). uterus, it must not embed itself in the oviduct wall, or the re- sult will be an ectopic or tubal pregnancy—a very dangerous condition. Early implantation is normally prevented by an ex- ternal proteinaceous layer called the zona pellucida, which sur- Animal pole rounds the egg and remains around the cleaving ball of cells. Ectoderm will form epidermal At about the time the blastocyst reaches the uterus, it hatches layer of skin. The neural ectoderm will from the zona pellucida, and implantation can occur. form the nervous system. The gray crescent is Specific blastomeres generate specific the site where major cell movement will tissues and organs begin. In all animal species, cleavage results in a repackaging of the egg cytoplasm into a large number of small cells surround- ing a central cavity. Little cell differentiation occurs during Endoderm will form Vegetal pole Mesoderm will form muscle, cleavage, and in most nonmammalian species, none of the the lining of the gut, bone, kidneys, blood, gonads, the liver, and the lungs. and connective tissues. genome of the embryo is expressed. Nevertheless, cells in dif- ferent regions of the resulting blastula possess different com- 20.6 Fate Map of a Frog Blastula The colors indicate the portions plements of the nutrients and cytoplasmic determinants that of the blastula that will form the three germ layers, and subsequently were present in the egg. the frog’s tissues and organs.
  7. 7. 414 CHAPTER T WENT Y 20.7 Twinning in Humans Division of blastomeres during …produces monozygotic Because humans have regulative early blastula formation… twins with separate placentas. Two chorions development, remaining cells can compensate when cells are lost in Inner cell mass Uterus early cleavages. Monozygotic (identi- cal) twins can result when cells in the early blastula become physically sep- arated and each group of cells goes on to produce a separate embryo. Embryos 2-cell embryo Two amnions Trophoblasts Blastomeres become determined—committed to specific the digestive tract, respiratory tract, and circulatory sys- fates—at different times in different species. In some species, tem and make up other internal tissues such as the pan- such as roundworms and clams, blastomeres are determined creas and liver. by the 8-cell stage. If one of these blastomeres is experimen- The cells remaining on the outside of the embryo become tally removed, a particular portion of the embryo will not the outer germ layer, the ectoderm. The ectoderm will form. This type of development has been called mosaic de- give rise to the nervous system, the skin, hair, and nails, velopment because each blastomere seems to contribute a sweat glands, oil glands, and milk secretory ducts. specific set of “tiles” to the final “mosaic” that is the adult an- Other cells migrate between the endoderm and the ecto- imal. In contrast, other species, such as sea urchins and ver- derm to become the middle germ layer, or mesoderm. tebrates, have regulative development: The loss of some cells The mesoderm will contribute tissues to many organs, during cleavage does not affect the developing embryo be- including blood vessels, muscle, bones, liver, and heart. cause the remaining cells compensate for the loss. If some blastomeres can change their fate to compensate Some of the most challenging and interesting questions in for the loss of other cells during cleavage and blastula for- animal development have concerned what directs the cell mation, are those cells capable of forming an entire embryo? movements of gastrulation and what is responsible for the To a certain extent, they are. During cleavage or early blas- resulting patterns of cell differentiation and organ formation. tula formation in mammals, for example, if the blastomeres In the past 25 years, scientists have answered many of these are physically separated into two groups, both groups can questions at the molecular level. In the discussion that fol- produce complete embryos (Figure 20.7). Since the two em- lows, we’ll consider the similarities and differences among bryos come from the same zygote, they will be monozygotic gastrulation in sea urchins, frogs, reptiles, birds, and mam- twins—genetically identical. Non-identical twins occur when mals. We’ll also review some of the exciting discoveries about two separate eggs are fertilized by two separate sperm. Thus, the mechanisms underlying these phenomena. while identical twins are always of the same sex, non-identi- cal twins have a 50 percent chance of being the same sex. Invagination of the vegetal pole characterizes gastrulation in the sea urchin Gastrulation: Producing the Body Plan The sea urchin blastula is a simple, hollow ball of cells that is The blastula is typically a fluid-filled ball of cells. How does this only one cell thick. The end of the blastula stage is marked by simple ball of cells become an embryo, made up of multiple tis- a dramatic slowing of the rate of mitosis, and the beginning of sue layers, with head and tail ends and dorsal and ventral gastrulation is marked by a flattening of the vegetal hemisphere sides? Gastrulation is the process whereby the blastula is trans- (Figure 20.8). Some cells at the vegetal pole bulge into the blas- formed by massive movements of cells into an embryo with tocoel, break free, and migrate into the cavity. These cells be- multiple tissue layers and visible body axes. The resulting spa- come primary mesenchyme cells—cells of the middle germ layer, tial relationships between tissues make possible the inductive the mesoderm. (Mesenchyme cells are unconnected to one an- interactions that trigger differentiation and organ formation. other and act as independent units, in contrast to epithelial cells, During gastrulation, the animal body forms three germ which are tightly packed into sheets or tubes.) layers (also called cell layers or tissue layers): The flattening at the vegetal pole results from changes in the shape of the individual blastomeres. These cells shift Some blastomeres move together as a sheet to the inside from being rather cuboidal to become wedge-shaped, with of the embryo, creating an inner germ layer called the constricted outer edges and expanded inner edges. As a re- endoderm. The endoderm will give rise to the lining of sult of these shape changes, the vegetal pole bulges inward,
  8. 8. ANIMAL DEVELOPMENT 415 1 The vegetal 2 Some cells change 3 Other cells break 4 More cells break free, 5 The archenteron 6 The mouth will form pole of the shape and move free, becoming forming secondary elongates by where the archenteron blastula flattens. inward to form primary mesenchyme. Thin rearrangement meets ectoderm. the archenteron. mesenchyme. extensions of these of cells. cells attach to the Animal overlying ectoderm. Secondary hemisphere mesenchyme Ectoderm Endoderm Archenteron Vegetal Primary hemisphere Blastopore mesenchyme 7 The blastopore will 20.8 Gastrulation in Sea Urchins During gastrulation, cells move to new positions form the anus of and form the three germ layers from which differentiated tissues develop. the mature animal. or invaginates, as if someone were poking a finger into a hol- velopment of a complete larva. It has been proposed that the low ball. The cells that invaginate become the endoderm and reason for these differences is an uneven distribution of var- form the primitive gut, the archenteron. At the tip of the ious transcriptional regulatory proteins in the egg cytoplasm. archenteron more cells break free, entering the blastocoel to As cleavage progresses, these proteins end up in different form more mesoderm, the secondary mesenchyme. combinations in different groups of cells. Therefore, specific The early invagination of the archenteron is due to the sets of genes are activated in different cells, determining their changes in cell shapes, but eventually it is pulled by the sec- different developmental capacities. Let’s turn now to gastru- ondary mesenchyme cells. These cells, attached to the tip of lation in the frog, in which a number of key signaling mole- the archenteron, send out extensions that adhere to the over- cules have been identified. lying ectoderm and contract. Where the archenteron eventu- ally makes contact with the ectoderm, the mouth of the ani- mal will form. The opening created by the invagination of the Gastrulation in the frog begins at the gray crescent vegetal pole is called the blastopore; it will become the anus Amphibian blastulas have considerable yolk and are more of the animal. than one cell thick; therefore, gastrulation is more complex What mechanisms control the various cell movements of in amphibians than in sea urchins. Furthermore, there is con- sea urchin gastrulation? The immediate answer is that spe- siderable variation among different species of amphibians. cific properties of particular blastomeres change. For exam- In this brief account, we will mix results from studies done ple, some vegetal cells migrate into the blastocoel to form the on different species to produce a generalized picture of am- primary mesenchyme because they lose their attachments to phibian development. neighboring cells. Once they bulge into the blastocoel, they Amphibian gastrulation begins when certain cells in the move by extending long processes called filopodia along an gray crescent change their shape and their cell adhesion extracellular matrix of proteins that is laid down by the ec- properties. The main bodies of these cells bulge inward to- todermal cells lining the blastocoel. ward the blastocoel while they remain attached to the outer A deeper understanding of gastrulation requires that we surface of the blastula by slender necks. Because of their discover the molecular mechanisms whereby certain blas- shape, these cells are called bottle cells. tomeres develop properties different from those of others. The bottle cells mark the spot where the dorsal lip of the Cleavage divides up the cytoplasm of the egg in a very sys- blastopore will form (Figure 20.9). As the bottle cells move tematic way. The sea urchin blastula at the 64-cell stage is ra- inward, they create this lip, over which successive sheets of dially symmetrical, but it has polarity. It consists of tiers of cells will move into the blastocoel in a process called involu- cells. As in the frog blastula, the top is the animal pole and tion. The first involuting cells are those of the prospective en- the bottom the vegetal pole. doderm, and they form the primitive gut, or archenteron. If different tiers of blastula cells are separated, they show Closely following are the cells that will form the mesoderm. different developmental potentials (see Figure 19.7). Only As gastrulation proceeds, cells from the animal hemisphere cells from the vegetal pole are capable of initiating the de- move toward the site of involution in a process called epiboly.
  9. 9. 416 CHAPTER T WENT Y Animal pole Blastocoel 20.9 Gastrulation in the Frog Embryo The colors in this diagram are matched to those in the frog fate map (Figure 20.6). 1 Gastrulation begins when cells just below the center of the gray crescent move Bottle cells inward to form the dorsal lip of the future blastopore. The blastopore lip widens and eventually forms a Dorsal lip of blastopore complete circle surrounding a “plug” of yolk-rich cells. As cells continue to move inward through the Vegetal pole blastopore, the archenteron grows, gradually dis- placing the blastocoel. As gastrulation comes to an end, the amphibian Blastocoel embryo consists of three germ layers: ectoderm on the outside, endoderm on the inside, and meso- derm in the middle. The embryo also has a dor- sal–ventral and anterior–posterior organization. Most importantly, however, the fates of specific re- Dorsal lip of gions of the endoderm, mesoderm, and ectoderm blastopore have been determined. The discovery of the events whereby determination takes place in the amphib- ian embryo is one of the most exciting stories in an- Blastocoel Archenteron imal development. displaced 2 Cells of the animal pole Mesoderm spread out, pushing surface cells below them toward and The dorsal lip of the blastopore organizes across the dorsal lip. These embryo formation cells involute into the interior Dorsal lip of of the embryo, where they blastopore In the 1920s, the German biologist Hans Spemann form the endoderm and was studying the development of salamander mesoderm. eggs. He was interested in finding out whether the Endoderm nuclei of blastomeres remain totipotent—capable of directing the development of a complete embryo. Archenteron Ectoderm With great patience and dexterity, he formed loops from a single human baby hair to constrict fertil- Mesoderm ized eggs, effectively dividing them in half. 3 Involution creates the (notochord) archenteron and destroys the When Spemann’s loops bisected the gray cres- blastocoel. The blastopore lip forms a circle, with cells Dorsal lip of cent, both halves of the zygote gastrulated and de- moving to the interior all blastopore veloped into complete embryos (Experiment 1 in around the blastopore; the Figure 20.10). But when the gray crescent was on yolk plug is visible through Yolk plug the blastopore. only one side of the constriction, only that half of the Ventral lip of zygote developed into a complete embryo. The half blastopore lacking gray crescent material became a clump of undifferentiated cells that Spemann called the “belly piece” (Experiment 2 in Figure 20.10). Spemann thus Neural plate of brain Neurula Notochord hypothesized that cytoplasmic determinants in the region of the gray crescent are necessary for gas- Endoderm trulation and thus for the development of a normal Neural plate Mesoderm organism. Ectoderm To test his hypothesis, Spemann and his student Hilde Mangold conducted a series of delicate tissue 4 Gastrulation is followed by transplantation experiments. They transplanted neurulation, which is marked by the development of the nervous pieces of early gastrulas to various locations on system from ectoderm. Blastopore other gastrulas. Guided by fate maps (see Figure 20.6), they were able to take a piece of ectoderm
  10. 10. ANIMAL DEVELOPMENT 417 EXPERIMENT they knew would develop into skin and transplant it to a re- Question: Are cytoplasmic factors necessary for development gion that normally becomes part of the nervous system, and segregated within the fertilized egg? vice versa. Experiment 1 Experiment 2 When they performed these transplants in early gastrulas, the transplanted pieces always developed into tissues that were appropriate for the location where they were placed. Using a baby’s hair, the Donor presumptive epidermis (that is, cells destined to be- zygote is constricted along the plane of first come skin in their original location) developed into host neu- cleavage. ral ectoderm (nervous system tissue), and donor presumptive neural ectoderm developed into host skin. Thus, the fates of the transplanted cells had not been determined before the One constriction transplantation. bisects the gray crescent; the other In late gastrulas, however, the same experiment yielded restricts it to one opposite results. Donor presumptive epidermis produced half of the zygote. patches of skin cells in the host nervous system, and donor Gray crescent presumptive neural ectoderm produced nervous system tis- sue in the host skin. Something had occurred during gastru- lation to determine the fates of the embryonic cells. In other words, as Spemann and Mangold had hypothesized, the path of differentiation a cell would follow was determined Only those halves during gastrulation. with gray crescent develop normally. Spemann and Mangold next did an experiment that pro- “Belly duced momentous results: They transplanted the dorsal lip piece” of the blastopore (Figure 20.11). When this small piece of tis- Normal Normal Normal sue was transplanted into the presumptive belly area of an- Conclusion: Cytoplasmic factors in the gray crescent are crucial for other gastrula, it stimulated a second site of gastrulation, and normal development. second whole embryo formed belly-to-belly with the origi- 20.10 Spemann’s Experiment Spemann’s research revealed that nal embryo. Because the dorsal lip of the blastopore was ap- gastrulation and subsequent normal development in salamanders parently capable of inducing the formation of an entire em- depended on cytoplasmic determinants localized in the gray crescent. bryo, Spemann and Mangold dubbed it the primary embryonic organizer, or simply the organizer. MOLECULAR MECHANISMS OF THE ORGANIZER. In recent years, 20.11 The Dorsal Lip Induces Embryonic Organization In a famous experiment, Spemann and Mangold transplanted researchers have studied the primary embryonic organizer the dorsal lip of the blastopore. The transplanted tissue intensively to discover the molecular mechanisms involved induced a second site of gastrulation and the formation of a second embryo. EXPERIMENT Question: Can some cells induce other cells to follow a particular developmental path? Presumptive Blastocoel Neural Notochord notochord tube Somite Dorsal Presumptive Primary Endoderm blastopore endoderm involution lip Transplanting the early gastrula …initiates a secondary …secondarily induced …and a complete secondary dorsal blastopore lip… involution… structures… embryo attached to the first. Conclusion: The dorsal lip of the blastopore can induce other cells to participate in embryogenesis.
  11. 11. 418 CHAPTER T WENT Y in its action. The distribution of the transcription factor β-catenin in the late blastula corresponds to the location of the organizer in the early gastrula, so β-catenin is a candi- Gray crescent date for the initiator of organizer activity. To prove that a protein is an inductive signal, it has to be shown that it is 1 Repression of siamois both necessary and sufficient for the proposed effect. In other prevents expression 2 β-Catenin in vegetal of organizer-specific cells below the gray words, the effect should not occur if the candidate protein genes. crescent blocks Tcf-3 is not present (necessity), and the candidate protein should repression of siamois gene expression. be capable of inducing the effect where it would otherwise No β-catenin not occur (sufficiency). Tcf-3 proteins siamois gene β-Catenin proteins siamois gene The criteria of necessity and sufficiency have indeed been repressed activated satisfied for the transcription factor β-catenin. If β-catenin DNA mRNA transcripts are depleted by injections of antisense RNA into the egg (see Chapter 16), gastrulation does not oc- Transcription cur. If β-catenin is experimentally overexpressed in another region of the blastula, it can induce a second axis of embryo formation, as the transplanted dorsal lip did in the Spe- 3 TGF-β-related signal proteins act synergistically with Siamois protein mann–Mangold experiments. Thus, β-catenin appears to be Siamois to activate goosecoid. both necessary and sufficient for the formation of the primary goosecoid gene embryonic organizer—but it is only one component of a com- activated plex signaling process. What follows is a summary of some of the critical early steps in this signaling cascade. This description may contain Transcription a confusing amount of detail. However, it is not the arcane names of the genes and gene products involved that are im- 4 Goosecoid protein activates numerous portant to remember. Rather, we hope to provide a basic un- genes in the organizer. derstanding of how these signaling molecules—their interac- tions and their gradients—can create and convey positional 20.12 Molecular Mechanisms of the Primary Embryonic Organizer and temporal information. The organizing potential of the gray crescent depends on the activity Studies of early gastrulas revealed that primary embry- of the goosecoid gene, which in turn is activated by signaling path- ways set up in the vegetal cells below the gray crescent. onic organizer activity is induced by signals emanating from vegetal cells just below the gray crescent. The protein β-catenin appears to play critical roles in generating these signals. One signal critical to stimulating the expression of become the primary organizer. Cells that receive other com- organizer genes is the transcription factor Goosecoid. Ex- binations of signaling molecules are induced to become dif- pression of the goosecoid gene appears to depend on two sig- ferent types of mesoderm. naling pathways, both of which involve β-catenin. The first of these pathways involves a goosecoid-promot- MOLECULAR MECHANISMS OF LEFT–RIGHT AXIS FORMATION. We ing transcription factor called Siamois. The siamois gene is have seen how the distribution of cytoplasmic determi- normally repressed by a ubiquitous transcription factor nants in the egg can set up a dorsal–ventral axis, and how called Tcf-3, but in cells where β-catenin is present, an inter- the site of sperm entry can set up an anterior–posterior axis. action between Tcf-3 and β-catenin induces siamois expres- What about the left–right body axis? After all, not every- sion (Figure 20.12). But Siamois protein alone is not sufficient thing in the animal is bilaterally symmetrical. The internal for goosecoid expression. organs of a vertebrate have many left–right asymmetries: In Vegetal cells receive mRNA transcripts from the original humans, the heart is tilted to the right side of the body, the egg cytoplasm for proteins in the TGF-β (transforming aorta comes off of the left side of the heart and the pul- growth factor β) superfamily of cell signaling molecules. One monary artery comes off of the right side of the heart; the or more of these proteins (candidates include Vg1 and spleen is on the left side of the body; and the large intestine Nodal) interact with Siamois protein by cooperatively bind- goes from right to left, to name only a few. ing to the promoter of the goosecoid gene and thereby con- We now know that there are a number of genes that are trolling its transcription (see Figure 20.12). Thus it is a par- necessary for normal left–right organization of the body. If ticular combination of factors that determine which cells one of these genes is knocked out, it can randomize the
  12. 12. ANIMAL DEVELOPMENT 419 left–right organization of the internal organs, with serious, cells on top of the yolk (see Figure 20.4c). We will use the even lethal, consequences. What triggers the asymmetrical chicken egg to show how gastrulation proceeds in a flat disc expression of these genes? of cells rather than in a ball of cells. We do not know the complete answer to this question, Cleavage in the chick results in a flat, circular layer of cells but it appears that the mechanism involves a left–right dif- called a blastodisc. Between the blastodisc and the yolk mass ferential distribution of some of the transcription factors that is a fluid-filled space. Some cells from the blastodisc break act very early during gastrulation. For example, in frogs, one free and move into this space. Other cells grow into this space of the TGF-β proteins involved in organizer determination from the posterior margin of the blastodisc. These cells come is also responsible for determining the left–right axis. In together to form a continuous layer called the hypoblast, mammals, there are cilia that cause a differential flow of which will later give rise to extraembryonic membranes that fluid in the yolk sac cavity. If these cilia are inactivated, the will support and nourish the developing embryo. The over- normal left–right asymmetries of the internal organs become lying cells make up the epiblast, which will form the embryo random. proper. Thus, the avian blastula is a flattened structure con- sisting of an upper epiblast and a lower hypoblast, which are joined at the margins of the blastodisc. The blastocoel is the Reptilian and avian gastrulation is an adaptation fluid-filled space between the epiblast and hypoblast. to yolky eggs Gastrulation begins with a thickening in the posterior re- The eggs of reptiles and birds contain a mass of yolk, and gion of the epiblast caused by the movement of cells toward therefore the blastulas of these species develop as a disc of the midline and then forward along the midline (Figure 20.13). The result is a midline ridge called the primitive streak. A depression called the primitive groove forms along the Chick embryo viewed from above length of the primitive streak. The primitive groove functions as the blastopore, and cells migrate through it into the blas- Yolk tocoel to become endoderm and mesoderm. 1 Cells at the posterior 2 Cells move toward the 3 The primitive 4 …forming the primitive 5 Cells passing over Hensen’s node epiblast move inward. primitive streak, down streak narrows groove—the chick’s migrate anteriorly and form head through it, and forward. and lengthens… blastopore. structures and notochord. Anterior Embryo Yolk Posterior Primitive streak Primitive streak Cells moving over sides of primitive streak form mesoderm and endoderm somites. Epiblast Endoderm Blastocoel The hypoblast is 20.13 Gastrulation in Birds Because of displaced by spreading the large yolk mass in bird and reptile Yolk Hypoblast endoderm. eggs, these embryos display a pattern of gastrulation very different from that of sea urchins and amphibians. Cross section through chick embryo
  13. 13. 420 CHAPTER T WENT Y In the chick embryo, no archenteron forms, but the endo- Uterus derm and mesoderm migrate forward to form the gut and other structures. At the anterior end of the primitive groove is a thickening called Hensen’s node, which is the equivalent of the dorsal lip of the amphibian blastopore. In fact, many signaling molecules that have been identified in the frog or- ganizer are also expressed in Hensen’s node. Cells that pass over Hensen’s node become determined by the time they Human embryo at 9 days (blastocyst) reach their final destination, where they differentiate into cer- Wall of uterus tain tissues and structures of the head and dorsal midline Developing (but not the nervous system). placenta Hypoblast Mammals have no yolk, but retain the avian–reptilian Inner cell mass Epiblast gastrulation pattern Mammals and birds both evolved from reptilian ancestors, Trophoblast so it is not surprising that they share patterns of early devel- Blastocoel opment, even though the eggs of mammals have no yolk. Earlier we described the development of the mammalian tro- phoblast and the inner cell mass, which is the equivalent of Endometrium the avian epiblast. As in avian development, the inner cell mass splits into an Amnion Chorionic Blood upper layer called the epiblast and a lower layer called the villi vessel hypoblast, with a fluid-filled cavity between them. The em- 20.14 A Human Blastocyst at Implantation Adehesion molecules bryo will form from the epiblast, and the hypoblast will con- and proteolytic enzymes secreted by trophoblast cells allow the blas- tribute to the extraembryonic membranes (Figure 20.14). The tocyst to burrow into the endometrium. Once implanted within the epiblast also contributes to the extraembryonic membranes; wall of the uterus, the trophoblast cells send out numerous projec- specifically, it splits off an upper layer of cells that will form tions—the chorionic villi—which increase the embyro’s area of con- tact with the mother’s bloodstream. the amnion. The amnion will grow to surround the develop- ing embryo as a membranous sac filled with amniotic fluid. Gastrulation occurs in the mammalian epiblast just as it does in the avian epiblast. A primitive groove forms, and epiblast gestive tract. Following these first cells over the dorsal lip are cells migrate through the groove to become layers of endo- those that will become mesoderm (see Figure 20.9). The dor- derm and mesoderm. sal mesoderm closest to the midline (the chordomesoderm) will become a rod of connective tissue called the notochord. The notochord gives structural support to the developing em- Neurulation: Initiating the Nervous System bryo; it is eventually replaced by the vertebral column. After Gastrulation produces an embryo with three germ layers that gastrulation, the chordomesoderm induces the overlying ec- are positioned to influence one another through inductive in- toderm to begin forming the nervous system. teractions. During the next phase of development, called Neurulation involves the formation of an internal neural organogenesis, many organs and organ systems develop si- tube from an external sheet of cells. The first signs of neuru- multaneously and in coordination with one another. An early lation are flattening and thickening of the ectoderm overly- process of organogenesis that is directly related to gastrula- ing the notochord; this thickened area forms the neural plate tion is neurulation, the initiation of the nervous system in (Figure 20.15). The edges of the neural plate that run in an an- vertebrates . We will examine this event in the amphibian terior–posterior direction continue to thicken to form ridges embryo, but it occurs in a similar fashion in reptiles, birds, or folds. Between these neural folds, a groove forms and and mammals. deepens as the folds roll over it to converge on the midline. The folds fuse, forming both a cylinder, the neural tube, and a continuous overlying layer of epidermal ectoderm. The stage is set by the dorsal lip of the blastopore The neural tube develops bulges at the anterior end, which The first cells to pass over the dorsal lip of the blastopore become the major divisions of the brain; the rest of the tube move anteriorly and become the endodermal lining of the di- becomes the spinal cord.
  14. 14. ANIMAL DEVELOPMENT 421 At the start of neurulation: In the middle of neurulation: Late in neurulation: The neural plate, which forms from ectoderm As the edges of the neural plate move upward When the edges of the neural plate grow together above the notochord, is well defined. and grow toward one another, the center of and fuse, a hollow cylinder forms and detaches the plate sinks, forming the neural groove. from the ectoderm to become the neural tube. (a) Dorsal Midsagittal plane Neural groove Fused surface view neural folds Neural fold Transverse plane Blastopore Neural plate Blastopore Notochord Neural plate Neural tube Notochord Neural plate Neural fold Neural Blastopore fold Blastopore (b) Midsagittal section Ectoderm Cavity Archenteron of gut Neural groove Notochord Neural tube Notochord Neural plate Notochord Neural plate Cavity Neural fold Cavity of gut of gut (c) Transverse section Endoderm Mesoderm Archenteron Mesoderm Ectoderm 20.15 Neurulation in the Frog Embryo Continuing the sequence from Figures 20.6 and 20.9, these drawings outline the development of the frog’s neural tube. ments are most evident as the repeating patterns of vertebrae, ribs, nerves, and muscles along the anterior–posterior axis. As the neural tube forms, mesodermal tissues gather along the sides of the notochord to form separate blocks of In humans, failure of the neural tube to develop normally cells called somites (Figure 20.16). The somites produce cells can result in serious birth defects. If the neural folds fail to that will become the vertebrae, ribs, and muscles of the trunk fuse in a posterior region, the result is a condition known as and limbs. spina bifida. If they fail to fuse at the anterior end, an infant The nerves that connect the brain and spinal cord with tis- can develop without a forebrain—a condition called anen- sues and organs throughout the body are also arranged seg- cephaly. Whereas several genetic factors that can cause neu- mentally. The somites help guide the organization of these pe- ral tube defects have been identified, there are also environ- ripheral nerves, but the nerves are not of mesodermal origin. mental factors, including dietary ones. The incidence of When the neural tube fuses, cells adjacent to the line of closure neural tube defects used to be about 1 in 300 live births, but break loose and migrate inward between the epidermis and the we now know that this incidence can be cut in half if pregnant somites and under the somites. These cells, called neural crest women have an adequate amount of folic acid (a B vitamin) cells, give rise to a number of structures, including the periph- in their diets. eral nerves, which grow out to the body tissues and back into the spinal cord. As development progresses, the segments of the body be- Body segmentation develops during neurulation come different. Regions of the spinal cord differ, regions of Like the fruit flies whose development we traced in Chapter the vertebral column differ in that some vertebrae grow ribs 19, vertebrates have a body plan consisting of repeating seg- of various sizes and others do not, forelegs arise in the ante- ments that are modified during development. These seg- rior part of the embryo, and hind legs arise in the posterior
  15. 15. 422 CHAPTER T WENT Y 2-Day chick embryo terior of the embryo. The Hox genes closer to the 5′ end of the Neural tube 1 Repeating blocks of gene complex are expressed later and in a more posterior part tissue–somites–form on of the embryo. As a result, different segments of the embryo Epidermis either side of the neural tube. receive different combinations of Hox gene products, which Somites serve as transcription factors (Figure 20.17). What causes the Notochord linear, sequential expression of Hox genes is unclear. Whereas Hox genes give cells information about their po- 2 Each somite divides sition on the anterior–posterior body axis, other genes give into three layers of 4-Day chick embryo cells. The upper will cells information about their dorsal–ventral position. Tissues Neural crest contribute to skin… in each segment of the body differentiate according to their cells dorsal–ventral location. In the spinal cord, for example, sen- 3 …the middle to muscles… sory nerve connections develop in the dorsal region and mo- tor nerve connections develop in the ventral region. In the 4 …and the lower somites, dorsal cells develop into skin and muscle and ven- will form cartilage tral cells develop into cartilage and bone (see Figure 20.16). of the vertebrae and ribs. 7-Day chick embryo 5 Neural crest cells Hox genes are clustered migrate between in four gene complexes. these layers and will produce nerves and a1 a2 a3 a4 a5 a6 a7 a9 a10 a11 a13 other tissue. Hoxa genes b1 b2 b3 b4 b5 b6 b7 b8 b9 20.16 The Development of Body Segmentation Repeating Hoxb blocks of tissue called somites form on either side of the neural tube. genes c4 c5 c6 c8 c9 c10 c11 c12 c13 Skin, muscle, and bone form from the somites. Hoxc genes d1 d3 d4 d8 d9 d10 d11 d12 d13 Hoxd genes region. How is a somite in the anterior part of a mouse em- 3′ 5′ Hindbrain Trunk bryo programmed to produce forelegs rather than hind legs? The genes closest to the 3′ …and those closest to end are expressed in the the 5′ end are Hox genes control development along the anteriormost positions… expressed more anterior–posterior axis b1 b2 b3 b4 b5 b6 b7 b8 b9 Homeobox genes are central to the process of anterior– Hoxb posterior determination and differentiation. In Chapter 19, we saw how homeotic genes control body segmentation Expression gradients from in Drosophila. In the mouse, four families of homeobox anterior to genes, called Hox genes, control differentiation along the posterior of anterior–posterior body axis. embryo Each mammalian Hox gene family resides on a different chromosome and consists of about 10 genes. What is remark- For example, Hoxb1 …and Hoxb9 in able is that the temporal and spatial expression of these genes is expressed in the the spinal cord. hindbrain… follows the same pattern as their linear order on their chro- mosome. That is, the Hox genes closest to the 3′ end of each Hindbrain Spinal co gene complex are expressed first and are expressed in the an- rd Midbrain Cervical Tho Forebrain raci c r ba m 20.17 Hox Genes Control Body Segmentation Hox genes are expressed along the Lu anterior–posterior axis of the embryo in the same order as their arrangement between the 3′ and 5′ ends of the gene complex. 12-Day mouse embryo
  16. 16. ANIMAL DEVELOPMENT 423 An example of a gene that provides dorsal–ventral infor- along the inside of the eggshell, both over the embryo and mation in vertebrates is sonic hedgehog, which is expressed in below the yolk sac. Where they meet, they fuse, forming two the mammalian notochord and induces cells in the overlying membranes, the inner amnion and the outer chorion. The neural tube to have fates characteristic of ventral spinal cord amnion surrounds the embryo, forming the amniotic cavity. cells. (As with the Hox genes, sonic hedgehog is homologous The amnion secretes fluid into the cavity, providing a pro- to a Drosophila gene, which is known simply as hedgehog.) tective environment for the embryo. The outer membrane, One family of homeobox genes, the Pax genes, plays the chorion, forms a continuous membrane just under the many roles in nervous system and somite development. One eggshell (Figure 20.18). It limits water loss from the egg and of these genes, Pax3, is expressed in those neural tube cells also works with the enlarged allantoic membrane to ex- that will develop into dorsal spinal cord structures. Sonic change respiratory gases between the embryo and the out- hedgehog represses the expression of Pax3, and their interac- side world. tion is one source of dorsal–ventral information for the dif- ferentiation of the spinal cord. After the development of body segmentation, the forma- Extraembryonic membranes in mammals tion of organs and organ systems progresses rapidly. The de- form the placenta velopment of an organ involves extensive inductive interac- In mammals, the first extraembryonic membrane to form is tions of the kind we saw in Chapter 19 in the example of the the trophoblast, which is already apparent by the fifth cell vertebrate eye. These inductive interactions are a current fo- cus of study for developmental biologists. 5-Day chick embryo Embryo Allantoic Amnion membrane Extraembryonic Membranes Gut There is more to a developing reptile, bird, or mammal than Amnionic the embryo itself. The embryos of these vertebrates are sur- cavity rounded by several extraembryonic membranes, which orig- Chorion inate from the embryo but are not part of it. The extraem- Yolk bryonic membranes function in nutrition, gas exchange, and waste removal. 1 The first extraembryonic mem- 2 The mesoderm and ectoderm brane is the yolk sac, which is extend beyond the embryo to form Extraembryonic membranes form with contributions forming in the 5-day embryo. the chorion and the amnion. from all germ layers We will use the chicken to demonstrate how the extraem- 9-Day chick embryo bryonic membranes form from the germ layers created dur- Embryo ing gastrulation. The yolk sac is the first extraembryonic Gut membrane to form, and it does so by extension of the endo- Amnion dermal tissue of the hypoblast layer along with some adja- Amnionic cent mesoderm. The yolk sac grows to encloses the entire cavity body of yolk in the egg (Figure 20.18). It constricts at the top Chorion Allantois to create a tube that is continuous with the gut of the embryo. Yolk sac Yolk Allantoic However, yolk does not pass through this tube. Yolk is di- membrane gested by the endodermal cells of the yolk sac, and the nu- trients are then transported to the embryo through blood ves- 3 The mesodermal and ectodermal 4 Mesodermal and endodermal sels that form from the mesoderm and line the outer surface layers fuse below the yolk so that tissues form the allantois, a the chorion lines the shell. sac for metabolic wastes. of the yolk sac. The allantoic membrane is also an outgrowth of the extraembryonic endoderm plus adjacent mesoderm. It 20.18 The Extraembryonic Membranes In birds, reptiles, forms the allantois, a sac for storage of metabolic wastes. and mammals, the embryo constructs four extraembryonic Just as the endoderm and mesoderm of the hypoblast membranes.The yolk sac encloses the yolk, and the amnion grow out from the embryo to form the yolk sac and the al- and chorion enclose the embryo. Fluids secreted by the amnion fill the amniotic cavity, providing an aqueous environment for lantoic membrane, ectoderm and mesoderm combine and the embryo.The chorion, along with the allantois, mediates gas extend beyond the limits of the embryo to form the other ex- exchange between the embryo and its environment.The allantois traembryonic membranes. Two layers of cells extend all stores the embryo’s waste products.