Reproduction

1. Gamete generation and selection
Fertilization
Preimplantation development
Implantation and formation of the bilaminar embryonic disc
Extraembryonic structures and blood formation
Gastrulation and the formation of three germ layers
Formation of the major body axes
Neurulation

2. Gamete generation and selection
The production of gametes for reproduction requires, in the case
of the male, sperm that are mobile and functional, and in the
female, the ovulation of an egg that is effectively the best of all
those developing in the ovary.
In the selection of the ‘best’ gamete(s) there is enormous wastage
of both male and female gametes which occurs at different stages
in their generation.
In males, selection occurs primarily after ejaculation.

3. Millions of spermatozoa are produced by each male on a daily
basis, calculated at 1000 per second in the human, however the
number of sperm that actually reach the site of fertilization is
understood to be remarkably low, with only one spermatozoa
actually required for fertilization.
Therefore, the vast majority of male gametes are unsuccessful in
the pursuit of reproduction.
Whereas in women, selection occurs in the ovary by a variety of
mechanisms with several follicles growing but ultimately only one
egg is ovulated in the vast majority of cases.

4. It is not yet known how the egg that is selected for ovulation in a
normal cycle differs to those that undergo atresia and die.
Ovarian stimulation in women allows a whole cohort of follicles to
develop and multiple eggs to be ovulated, and yet we have little
knowledge about which eggs should be used first as the ‘best’
eggs for assisted reproduction.
Therefore, furthering our understanding of ovulation rate and the
mechanisms that regulate it are critical to developing more
natural ways of obtaining eggs and to enhancing our selection of
the best eggs.

5. Spermatozoa are produced in the testes which are external to
the body cavity in most mammals.
Temperature regulation is critical to the production of functional
sperm in humans.
The question arises as to why sperm production in mammals
requires a lower temperature in some species.

6. It is possible that it is an evolutionary advantage for sperm to die
at body temperature and therefore with each new fertilization,
new sperm are required, ensuring that for each conception, the
sperm that fertilizes is from the current fittest sire.
Alternately, females who are unwell with an elevated
temperature will enhance sperm death, thus aiding the
prevention of pregnancy in women who are unwell.

7. Figure 1.1 Male gametes. (a) human sperm; (b) mouse
sperm; (c) wood mouse sperm trains; (d) angler fish
Spermatozoa are produced in the testes
from puberty till death, ensuring there is
a continuous supply throughout the
reproductive life of mammals.
Therefore men can continue to
reproduce late into their dotage.
The oldest father on record is an
Australian who fathered his last child at
92 years of age.
This is in direct contrast to the limited
number of eggs that exist in females.
However, although sperm production is
continuous, sperm production and
quality are known to decline as men age.

8. Fertilization
One question is why sperm binding is species-specific if it occurs
within the reproductive tract of sexually reproducing species?
The answer is that it is most likely a remnant from our early
ancestry when fertilization occurred externally and has not been
lost.
However the exact mechanisms that regulate sperm binding to the
egg zona pellucida in mammals have yet to be elucidated.
There is considerable controversy in the field, with numerous
hypotheses based on clear and convincing data, albeit conflicting.

9. Embryo development and gestation
Preimplantation embryos generated during assisted reproduction that
are surplus can be stored for further reproductive cycles.
Currently this requires cryopreservation; however this does result in a
degree of embryo damage and loss.
Therefore, since these embryos are extremely precious, developing
new methods to improve viability of preserved embryos would be
advantageous.
For instance, a number of marsupials, including the tammar wallaby,
generate a blastocyst which remains quiescent in the female’s
reproductive tract for almost a year until the environment is once
again optimal for reproduction.

10. This blastocyst is generated to enable the tammar wallaby to
rapidly resume pregnancy if the existing offspring dies.
Understanding the mechanisms that can maintain a viable
blastocyst at this stage for this long period of time would of
course be of great use clinically in the preservation of blastocysts,
as this would prevent loss during the cryopreservation procedure.
One of the most interesting and unexpected discoveries in recent
years is that mothers often retain a small number of cells from
the fetus they have carried.

11. Therefore mothers are effectively chimaeras.
A high proportion of fetal cells in mothers have been linked to an
increased incidence of autoimmune disease.
Understanding the mechanisms of not only how these cells cross
the placenta but also how they contribute to the onset of
autoimmune disease is a field of active scientific research.

12. Fertilization and egg activation
pp.98

13. Introduction
Sexual reproduction is a survival strategy employed by species for
procreation, and is characterized by the transfer of haploid genetic
material from each parent to produce diploid offspring, ensuring
continued genetic diversity.
In mammals, males produce vast numbers of gametes called sperm in
the testes (53–55µm in length), which are deposited in the female
reproductive tract in a liquid medium (semen) via the process of
ejaculation.

14. Females produce much larger gametes, eggs (100µm in diameter),
which are surrounded by a thin glycoprotein layer, the zona pellucida
(ZP), and generated in follicles contained in the ovaries which,
following maturation, are released and transported through the
fallopian tube to the uterus.
Sexual reproduction involves the concerted release of these two
components, which subsequently fuse to re-form the diploid
chromosome number, resulting in a new, genetically unique
individual, a process termed fertilization.

15. Fertilization involves a number of key, sequential steps, these are:
(1) the acquisition of sperm motility and chemotaxis;
(2) capacitation and the acrosome reaction;
(3) sperm-egg fusion; and
(4) egg activation and the initiation of embryo development.

16. Chemotaxis is a phenomenon by which single cells or multicellular
organisms direct their movements according to certain chemicals in
their environment.
How human sperm are attracted towards the egg is still a matter of
debate, with multiple mechanisms proposed.
In some non-mammalian species, peptides or proteins are known to
act as chemoattractants (agents of chemotaxis), such as unsaturated
fatty acids.
In mammalian species, sperm chemotaxis towards follicular fluid has
been observed, and progesterone secreted from cumulus cells is
thought to be the main chemoattractant.

17. Textbook of Clinical Embryology
Kevin Coward
Dagan Wells
Cambridge University Press 2013
Early embryogenesis
pp. 109-121

18. Overview and general concepts
The process of embryogenesis is the basis for establishing the final
form of the adult.
Embryonic development does not consist simply of the growth in size
of a ‘preformed’ miniature fetus, but is a tremendously dynamic
process, characterized by a great deal of cell movement and tissue
rearrangement.
An embryo starts out as a single cell, the fertilized egg, which divides
to form a mass of cells.
Over the course of embryogenesis, a pattern emerges from this
initially inchoate collection of cells.

19. This pattern is manifest as changes in cells, resulting in the
generation of different types of tissues, and in the stereotypic
arrangement of these cells and tissues to generate recognizable fetal
form.
Coordinated with the morphogenetic events that shape the embryo
is the progressive differentiation of cells, to achieve functional
specialization.
Differentiation is generally accompanied by a reduction in the
potential of those cells (in terms of cell types they can subsequently
differentiate into).
Embryogenesis starts with the zygote, said to be totipotent because
it can give rise to all fetal and placental tissues.

20. The zygote undergoes repeated rounds of division to yield cells
that differentiate into pluripotent cells, which can give rise to many
cell types (including all those found in the fetus), but not certain
cell types that make up the placenta.
In the course of embryogenesis, these pluripotent cells
differentiate further, giving multi, oligo and unipotent cell types.
Some of these cell types can be described as stem cells, capable of
indefinite self-renewal (given the appropriate niche), while also
retaining the potential to differentiate into specific cell types.

21. Differentiation is mediated by changes to the gene expression
profile of cells.
These changes are brought about by many different mechanisms,
such as the differential inheritance of asymmetrically localized
cytoplasmic determinants during cell division or in response to
inductive signals from neighbouring cells.
Such mechanisms set off a cascade of events that ultimately lead
to specific genes being activated or repressed, defining the gene
expression profile of the cell.

22. Networks of genes that guide development are frequently reused
in different contexts.
It is therefore not uncommon for a specific gene to play an
important role in the development of completely different
structures – for example, the gene Sonic Hedgehog (SHH) is key to
the proper patterning of the neural tube as well as the limb.
This chapter gives a broad overview of the major developmental
events that take place during early embryogenesis, from
fertilization up to the formation of the first organs.

23. Networks of genes that guide development are frequently reused
in different contexts.
It is therefore not uncommon for a specific gene to play an
important role in the development of completely different
structures – for example, the gene Sonic Hedgehog (SHH) is key to
the proper patterning of the neural tube as well as the limb.
This chapter gives a broad overview of the major developmental
events that take place during early embryogenesis, from
fertilization up to the formation of the first organs.

24. Preimplantation development is considered first, during which the
embryo undergoes the first differentiation events essential in
delineating placenta and the fetal lineages.
The next major event is gastrulation, during which the fundamental
tissues types used to build the embryo are generated.
The chapter ends with a description of embryonic turning, the
process by which the initially flat embryo is converted to a tubular
form.
These areas are per force described in brief, but the reader is
referred to several excellent texts for greater detail on specific
topics.

25. Preimplantation development
Placental mammals are unique in developing inside a womb, which
first requires the formation of a placenta as an interface between
the mother and fetus.
The early days of embryogenesis therefore are devoted to the
formation of cell types that can be used to contribute to the
placenta.
An oocyte is full of maternally encoded mRNA and proteins, stored
during oogenesis.
This stored material enables the zygote to jump-start embryonic
development as soon as fertilization occurs.

26. The loss or depletion of maternal factors causes embryos to stop
development.
For example, the lack of heat shock factor (HSF1) results in one-cell
arrest.
The embryo depends on maternal transcripts until around the two-
cell stage, when its genome is activated in a process called maternal-
to-zygotic transition (MZT).
This occurs in two phases – first, a subset of maternal transcripts is
eliminated, then in the second phase, zygotic transcription is
initiated.

27. Figure 12.1 Preimplantation embryonic development. The zygote undergoes cleavage divisions as it
travels down the oviduct towards the uterus. At roughly day 6, the blastocyst hatches from the zona
pellucida in preparation for implantation
During MZT, both maternal and zygotic transcripts
are active.
The majority of maternal transcripts are degraded by
the end of the two-cell stage, when the switch to
zygotic transcripts is finished.
After fertilization, the zygote undergoes a series of
cleavage divisions – cell division without growth –
resulting in a progressive reduction in cell volume.
As cleavage proceeds, the individual cells are now
called blastomeres, and form a morula (from the
Latin for mulberry, because of the distinctive
appearance of the clump of cells).
Cleavage division is relatively slow at first, so that on
day 1 after fertilization the embryo consists of two
blastomeres, on day 2 of four blastomeres and on
day 3, the embryo consists of eight blastomeres (Fig.
12.1).
Separation of blastomeres at this early stage can
result in mono-zygotic twins, with the two fetuses
having completely separate placenta.

28. Generally at the eight-cell stage, the morula undergoes a process
called compaction, whereby the blastomeres start forming
intercellular junctional complexes (gap, adherens and tight
junctions), resulting in them becoming more closely apposed,
making the surface of the morula smooth in appearance.
This is accompanied by the blastomeres becoming polarized, with
exposed apical membrane and embedded basolateral membranes
acquiring different properties (Fig. 12.1).
As compaction proceeds, blastomeres lose their totipotency.
External cells differentiate to form abundant gap and tight
junctions, reorganize their cytoskeleton to flatten their shapes and
increase adhesion with one another through E-cadherin.

29. Inner cells remain apolar and round in shape. The process of
compaction is completed by about the 32-cell stage, by which
time the embryo starts to become a blastocyst, expanding due to
the accumulation of fluid-filled spaces within it.
These fluid-filled spaces coalesce to form an asymmetrically
positioned blastocyst cavity.
Inside cells remain round in shape, positioned on one side and
become the pluripotent inner cell mass (ICM), while outside cells
become the trophoblast (Fig. 12.1).

30. The ICM (inner cell mass) gives rise ultimately to the fetus (as well
as contributing to the placenta) while the trophoblast contributes
exclusively to the placenta.
The region of the trophoblast abutting the ICM is called polar
trophoblast, while that lining the blastocyst cavity is called mural
trophoblast.
Monozygotic twins with a shared placenta (one chorion and two
amnions or one chorion and one amnion) result from the ICM
splitting after the blastocyst stage.

31. Time-lapse movies of human embryo cleavage and blastocyst
formation are available through the work of Mio and Maeda.
It is from the ICM that embryonic stem cells (ES cells) are derived.
The ICM and trophoblast lineages are characterized by the
expression of different genes.
For example, ICM cells notably express pluripotent markers such
as NANOG and zygotic OCT4, while trophoblast cells express CDX2.

32. Implantation and formation of the bilaminar embryonic disc
Fertilization takes place in the fallopian tube, after which the
cleaving embryo travels towards the uterus.
Approximately five days after fertilization, the embryo reaches the
uterus.
By that time it is a blastocyst, but is still enclosed by the zona
pellucida.
The zona pellucida is important in preventing the embryo from
implanting prematurely in the oviduct, resulting in ectopic
pregnancy.

33. Once it reaches the uterus, the blastocyst ‘hatches’ out of the zona
pellucida (Fig. 12.1).
This is achieved by localized enzymatic digestion of the zona
pellucida by the embryo, creating a hole through which the
embryo can slip out.
The embryo is now ready to undergo implantation, a process that
begins on roughly day 6.
Around this time, the ICM differentiates into two cell layers, the
epiblast (adjacent to the polar trophectoderm) and hypoblast
(adjacent to the blastocyst cavity).

34. As the embryo implants, rapidly dividing cells of the polar trophoblast
lose their cell membrane and coalesce with one another to form the
syncytiotrophoblast.
In contrast, cells of the mural trophoblast remain as individual cells,
forming the cytotrophoblast (Fig. 12.2).
The syncytiotrophoblast facilitates implantation by secreting
metalloproteases and collagenases to digest the extracellular matrix
of the uterine endometrium.
After implantation, the syncytiotrophoblast ultimately comes to
envelop the entire embryo, which now consists of an outer layer of
cytotrophoblast enclosing the epiblast, hypoblast and blastocyst
cavity (Fig. 12.2).

35. In female embryos, X chromosome inactivation (XCI) occurs in cells
of the epiblast.
This is a process of dosage compensation that ensures both males
and females produce the same amount of proteins encoded by the
X chromosome, despite females having two (XX) and males only
one X chromosome (XY) in each cell.
The inactivated X chromosome condenses into heterochromatin
and is called the Barr body.
XCI is triggered by the expression of XIST transcript from the X
chromosome to be silenced.

36. Either the maternal or paternal X chromosome is randomly
inactivated.
As a result, the female fetus (and adult) is a genetic mosaic,
composed of a mixture of cells expressing genes from the paternal
or maternal X chromosome.
Around day 8, the amniotic cavity forms between the epiblast and
overlying trophoblast.
Cells from the epiblast spread out to line this cavity, ultimately
forming the amnion.

37. Cells from the hypoblast spread out along the cytotrophoblast,
forming Heuser’s membrane.
This encloses the blastocyst cavity, which now comes to be called
the primary yolk sac (Fig. 12.2).
The epiblast and hypoblast separating the amniotic cavity from
the primary yolk sac is now called the bilaminar embryonic disc.

39. Figure 12.2 Formation of the bilaminar embryonic disc. By this stage, the embryo has implanted in the uterine
endometrium (not shown). Heuser’s membrane grows to enclose the primary yolk sac. At about day 12, the chorionic
cavity forms as a space within the extraembryonic mesoderm and spreads to enclose the epiblast (arrows).

40. Extraembryonic mesoderm, thought to be derived from the
hypoblast or primary yolk sac, forms between Heuser’s membrane
and the overlying cytotrophoblast, partially occluding the cavity of
the primary yolk sac.
It also grows over the amnion, resulting in the embryonic disc
(along with the amniotic and yolk sac cavities) being embedded
within a mass of extraembryonic mesoderm.

41. As the embryo grows, a new space emerges within the
extraembryonic mesoderm – the chorionic cavity.
This space gradually grows around the embryonic disc, until
around the end of week 2; the embryonic disc is suspended
within this cavity by a connecting stalk derived from
extraembryonic mesoderm.
While the chorionic cavity is expanding, portions of the primary
yolk sac get pinched off, leaving behind a smaller definitive or
secondary yolk sac (Fig. 12.2).

42. Extraembryonic structures and blood formation
Primitive hematopoiesis and vasculogenesis first occur in the yolk
sac, leading to the production of blood and vessels respectively.
Blood islands, clusters of primitive erythrocytes surrounded by
endothelial cells, arise from the extraembryonic mesoderm in the
yolk sac.
These islands grow and connect with each other to form the
vasculature (and blood) of the fetal part of the uteroplacental
circulation.

43. Beginning day 9, vacuoles form within the syncytiotrophoblast
and fuse with one another to form trophoblastic lacunae(Fig.
12.2). Maternal uterine capillaries in the near vicinity anastamose
with these lacunae between the 11th and 13th days.
As this process is ongoing, the cytotrophoblast grows into the
syncytiotrophoblast to form elongated villi.
Syncytiotrophoblast-covered villi protrude into the trophoblastic
lacunae, forming primary chorionic stem villi.
Around day 16, extraembryonic mesoderm invades their cores,
transforming them into secondary chorionic stem villi.

44. These in turn become tertiary chorionic stem villi upon formation
of vasculature from the extraembryonic mesoderm.
Thus maternal and fetal gas and nutrient exchange occurs between
these villi and the trophoblastic lacunae.

45. Gastrulation and the formation of three germ layers
Gastrulation is a crucial process in embryogenesis. It results in:
the generation of the three primary germ layers from which all
fetal tissues are derived; the conversion of the bilaminar
embryonic disc into a trilaminar disc; the establishment of the
primary (rostrocaudal) body axis; and the organization of the
basic body plan.
Around day 14 a midline thickening, the primitive streak, appears
in the epiblast in the caudal region of the bilaminar embryonic
disc.

46. Epithelial epiblast cells within the primitive streak undergo an
epithelial-tomesenchymal transition, delaminate and ingress into
the interior of the embryo to displace the underlying hypoblast
(Fig. 12.3).
These epiblast-derived cells re-epithelialize, forming the
definitive endoderm that forms the inner lining of the gut tube as
well as associated organs.
Cells that ingress through the streak subsequent to the formation
of the definitive endoderm (around day 16) form mesoderm (Fig.
12.3) that gives rise to most internal organs.

47. Cells of the epiblast that do not ingress through the streak
differentiate into ectoderm that gives rise to the central nervous
system and epidermis of the skin.
As a result of this process the bilaminar embryonic disc,
consisting of epiblast and hypoblast, is converted into a
trilaminar embryonic disc consisting of ectoderm, endoderm and
an intervening layer of mesoderm.
At the rostral end of the primitive streak is a specialized
expanded structure called the primitive node.

48. The node acts as an embryonic organizer, in that it is responsible
for organizing surrounding tissues without itself contributing to
them.
The properties of embryonic organizers was demonstrated by
Hilde Mangold and Hans Spemann in seminal experiments where
they transplanted the structure analogous to the node from one
amphibian embryo to another and showed that this resulted in
the formation of a second body axis in the recipient embryo.
Towards the end of the third week, two depressions form in the
ectoderm, at the rostral and caudal ends of the embryo disc.

49. The ectoderm in the depressions fuses tightly with the endoderm
beneath, excluding the mesoderm and forming bilaminar
membranes in these regions.
The rostral membrane is the oropharyngeal membrane which
later degenerates to form the opening of the oral cavity.
The caudal membrane is the cloacal membrane, which
degenerates later to form the opening of the digestive, urinary
and genital tracts.

50. The ingression of cells through the primitive streak continues for
several days.
This is accompanied by the regression of the streak towards the
caudal end, so that as the embryo develops, the streak grows
shorter.
Thus, a rostrocaudal ‘temporal gradient’ of development exists,
with rostral structures being more developmentally advanced
than caudal structures.

51. Figure 12.3 Gastrulation. Panel (a) shows the epiblast surface in a dorsal view of the embryonic disc. The arrows depict
streams of mesodermal cells migrating beneath the epiblast. Panel (b) shows a section view along the dotted line in
panel (a). Cells of the epiblast undergo an epithelial to mesenchymal transition and delaminate to form the mesoderm

52. Formation of the major body axes
The orientation of the primitive streak defines the rostrocaudal
axis of the embryos.
On the basis of experiments in chick and mouse embryos, it is
understood that the hypoblast (or a subset of cells within it) is
responsible for determining the position at which the primitive
streak forms and consequently, the orientation of the
rostrocaudal axis.
In one sense, the dorsoventral axis is determined already at the
bilaminar disc stage, from the way the epiblast and hypoblast are
arranged relative to one another.

53. However, it is only after gastrulation and the formation of the
notochord that there is overt patterning along the dorsoventral axis,
manifest for example in the patterning of the neural tube in
response to signals from the notochord and surface ectoderm.
The left-right axis is the last to be established.
Though externally humans are bilaterally symmetrical, internally
there is asymmetry in the position or structure of organs – the
heart is on the left, the liver is mostly on the right, the two lungs
have different numbers of lobes, the two kidneys are at different
levels, etc.

54. This normal asymmetry is referred to as situs solitus and is
generated by the ventral cells of the node.
These cells have characteristic cilia that rotate.
As the cilia are tilted, their rotation creates a directional leftward
nodal flow.

55. Neurulation
Around day 18, the node induces the ectoderm immediately rostral
to it to change shape.
These cells elongate to become pseudostratified and the area
thickens to become the neural plate, composed of neuroectoderm.
Thus as the node regresses towards the posterior, neural induction
follows in its wake.
As the streak regresses, it lays down a rod-like mesodermal
structure in the midline, the notochord, formed from cells that
ingress through the node.

56. The notochord induces the overlying neural plate to fold in and
eventually pinch off to form the neural tube.
The neural tube gives rise to the central nervous system.
The notochord and surface ectoderm play central roles in the
dorso-ventral patterning of the neural tube.
The neural plate folds into a tube by elevating its edges toward the
midline by the movement of surface ectoderm.

57. In a process called neural tube closure, the edges form medial
hinge points, meet medially and fuse to pinch off the neural tube,
which sinks below the surface ectoderm.
Closure defects can result in severe congenital abnormalities such
as craniorachischisis or spina bifida.
Neural crest cells are migratory mesenchymal cells originating
from the crest of the neural fold as it closes.
They are internalized by the ectoderm when the neural tube is
pinched off.

58. Neural crest cells contribute to a very wide range of tissues and
organs.
For example, they give rise to neurons and glia of the peripheral
nervous system, the cartilage and bones of the face, melanocytes
and contribute to the septation of the developing heart.
Endodermal and mesodermal derivatives The endoderm forms
the embryonic gut tube, which gives rise to the inner lining of the
digestive tract.
It also contributes to organs that arise from evaginations of the
embryonic gut tube, such as the lung, liver and pancreas.

59. The thyroid, thymus and parathyroid glands are also derived from
the endodermal cells of the pharyngeal pouches.
The cells to emerge from the rostral third of the primitive streak
form an arc of cardiogenic mesoderm rostral to the primitive
streak called the cardiac crescent.
This mesoderm gives rise to the primitive heart tube and much of
the adult heart, particularly the left ventricle and the two atria.
The remaining mesoderm can be subdivided into axial, paraxial,
intermediate and lateral-plate mesoderm on the basis of position
from the centre of the embryo.

60. Axial mesoderm lies in the midline and forms the notochord.
Paraxial mesoderm lies to either side of the neural tube. In the
head region it forms the head mesoderm which gives rise to
striated muscles of the head, jaw and neck regions.
In the trunk, the paraxial mesoderm condenses into paired
segmental blocks called somites.
Somites are a manifestation of the segmental architecture of the
basic mammalian body plan.
Each somite gives rise to a sclerotome, myotome and dermatome,
which form bone, muscle and the dermis of skin respectively.

61. Somites form and develop in rostrocaudal progression, so in the
more rostral region somites differentiate into bone and muscle
while at the same time in more caudal regions, they are just
being formed from the paraxial mesoderm. Intermediate
mesoderm lies just lateral to paraxial mesoderm.
It gives rise to the pronephros, mesonephros and metanephros in
rostrocaudal succession.
The metanephros form the definitive kidneys. Intermediate
mesoderm also gives rise to the gonads and associated ducts.

62. Lateral plate mesoderm is the most peripheral mesoderm and
gives rise to vasculature, lymphatic system, blood cells and the
covering of visceral organs.
Germ cells Early in gastrulation, a group of cells called primordial
germ cells (PGC) are set aside to ultimately produce the gametes
that give rise to the next generation.
They are derived from the epiblast, but migrate to reside in the
yolk sac.
Later, between the fourth and sixth weeks, they migrate from the
yolk sac through the wall of the gut tube, to the dorsal body wall.

63. Here they populate a region of intermediate mesoderm that they
induce to form paired genital ridges from which the gonads form.
Embryo folding The final human body plan is one of a tube within a
tube with the inner gut tube enclosed by an outer tube of
ectoderm.
However, as described, the embryo starts out as a flat disc, initially
bilaminar, and then after gastrulation, trilaminar. Embryonic folding
refers to the process by which this flat disc is converted to the final
tubular form.
This is achieved primarily through differential rates of growth in
different regions of the embryo.

64. During the fourth week, the embryo and overlying amnion
undergo rapid growth which, in combination with convergent
extension movements of cells along the embryonic midline,
result in the elongation of the body.
The yolk sac in contrast does not grow, resulting in the margins of
the embryonic disc being pulled down towards it (Fig. 12.4).
Axial structures like the notochord and neural tube keep the
dorsal midline of the embryo relatively rigid, causing the lateral
margins to account for most of the folding movement.
The cranial and caudal ends of the embryonic disc also fold in.

65. Cranial, caudal and lateral folding movements result in the
endoderm being enveloped, pinching it off internally to form the
gut tube.
The cranial and caudal ends of the gut tube are initially blind
ending.
The mouth and anus are formed later, by apoptosis of cells of the
oropharyngeal and cloacal membranes respectively.
In addition to enclosing the endoderm to form the gut tube,
embryonic folding movement also encloses mesoderm, which
comes to line the gut tube and the body wall.

66. In between these two layers of mesoderm is a space that forms
the intraembryonic coelom.
A wedge of mesoderm called the septum transversum grows
across the coelom, contributing to the formation of the
diaphragm and separating the coelom into abdominal and
thoracic cavities. Cranial folding also brings the primitive heart
tube, which is initially rostral to the developing brain, to its more
caudal thoracic position.
The folds meet ventrally and fuse, forming the ventral body wall.

67. This fusion occurs first in the cranial and caudal ends, with the yolk
sac cavity in between, communicating directly with the gut tube.
The regions of ventral fusion gradually move towards each other,
constricting the yolk sac into a narrow stalk, still connected to the
midgut.
Early in the fourth week, a small diverticulum called the allantois
(proximal regions of which give rise to the urinary bladder)
emerges from the forming hindgut and grows into the connecting
stalk.

68. The yolk sac also comes to be pressed against the connecting
stalk, which ultimately comes to be enclosed within the growing
amniotic sac, to give the umbilical cord (Fig. 12.4).
These changes transform a sheet-like embryo into a tube, with
the basic tissues roughly in place to undergo organogenesis.

69. Figure 12.4 Embryonic folding. Transverse (panels a, b and c) and cross-sectional views (panel d) of different stages of
embryonic folding. The diagrams are not to scale

70. Copyright © 2019 by Bruce Alberts, Dennis Bray, Karen Hopkin,
Alexander Johnson, the Estate of Julian Lewis, David Morgan,
Martin Raff, Nicole Marie Odile Roberts, and Peter Walter
pp.491-730
Textbook of Clinical Embryology
Kevin Coward
Dagan Wells
Cambridge University Press 2013
pp.1-8
pp. 98-121

71. Thank you for attention