Development biology of 7 th semester zoology

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Developmental biology

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The Diploblasts, Protostomes, and Deuterostomes

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Diploblasts
Diploblastic animals are those that have ectoderm and endoderm, but no true mesoderm.
The diploblasts have traditionally included the cnidarians (jellyfish and hydras) and the ctenophores
(comb jellies).
It has long been thought that these two phyla had radial symmetry and no mesoderm.
Triploblastic
Triploblast phyla (all other animals) had bilateral symmetry and a third, mesodermal, germ layer.
Most metazoans have bilateral symmetry and three germ layers and are thus considered to be
triploblastic animals.
The evolution of the mesoderm enabled greater mobility and larger bodies because it became the
animal's musculature and circulatory system.
The flatworms have a fully developed mesoderm and are often considered among the most primitive
of the bilateral phyla.

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Interestingly the flatworms and the cnidarians do not have a strict separation of germ line and
somatic line.
Among flatworms, for instance, are many species that have retained a population of totipotent
stem cells.
In other words, even as sexually mature adults, they have a population of cells that can become
any cell type in the body-including gametes.
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Animals of the Bilataria are further classified as either protostornes or deuterostomes.

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Protostomes
Protostomes (Greek, "mouth first"), which include the mollusc, arthropod, and worm phyla, are so
called because the mouth is formed first, at or near the opening to the gut, which is produced during
gastrulation.
The anus forms later at another location.
The coelom, or body cavity, of these animals forms from the hollowing out of a previously solid cord of
mesodermal cells.
There are two major branches of the protostomes.
1. The Ecdysozoa includes the animals that molt their exterior skeletons.
Examles: Arthropoda (insects, arachnids, mites, crustaceans, and millipedes).
1. The second major group of protostomes is the Lophotrochozoa. These animals are characterized
by a common type of cleavage (spiral), a common larval fonn (the trochophore), and a distinctive
feeding apparatus (the lophophore) found in some species.
Examples: flatworms, bryozoans, annelids, and molluscs.

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Deuterostomes
First, in deuterostomes ("mouth second"), the oral opening is formed after the anal opening. Also,
whereas
1. protostomes generally form their body cavities by hollowing out a solid block of mesoderm
(schizocoelous formation of the body cavity),
2. most deuterostomes form their body cavities from mesodermal pouches extending from the gut
(enterocoelous formation of the body cavity).
Example: chordates and echinodems. (humans, fish, and frogs with starfish and sea urchin).

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Exceptions to these generalizations
The evolution of organisms depends on inherited changes in their development.
1. One of the greatest evolutionary advances is the amniote egg-occurred among the
deuterostomes.
Example: chicken egg (Figure 2.19), is thought to have originated in the amphibian ancestors of
reptiles about 255 million years ago. The amniotic egg allowed vertebrates to roam on land, far from
existing ponds. Whereas most amphibians must return to water to lay their eggs, the amniotic egg
carries its own water and food supplies. It is fertilized internally and contains yolk to nourish the
developing embryo.
Moreover, the amniotic egg contains four sacs:
1. the yolk sac, which stores nutritive proteins;
2. the amnion, which contains the fluid bathing the embryo;
3. the allantois, in which waste materials from embryonic metabolism collect; and
4. the chorion, which interacts with the outside environment, selectively allowing materials to reach
the embryo.
The entire structure is encased in a shell that allows the diffusion of oxygen but is hard enough to
protect the embryo from environmental assaults and dehydration.
2. A similar development of egg casings enabled arthropods to be the first terrestrial invertebrates.
Thus, the final crossing of the boundary between water and land occurred with the modification of the
earliest stage in development: the egg.

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Development in Flowering Plants

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 Like animals " plants are multicellular organisms.
 But the divergence of the plant and animal lineages occurred so early in evolutionary history
that the most recent ancestor shared by both groups was almost certainly unicellular.
 The evolutionary divergence of plants and animals resulted in dramatically different life cycles,
morphologies, biochemistry, and genetics; equally distinctive are the strategies by which
members of the two kingdoms reproduce and develop.
 The different groups within the plant kingdom display diverse life cycles.
 Among the land plants (also known as embryophytes), most of these life cycles include both
diploid and haploid multicellular stages, a phenomenon referred to as alternation of generations.
Alternation of generations results in two different multicellular body plans over the individual's
life cycle.

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 Some angiosperms contain both male and female sexual organs in the same plant. These are the
monoecious angiosperms, such as corn and cucumbers. Many monoecious plants (such as
apples plums, and tomatoes) even have functional male and female parts in the same flower.
Here, haploid sperm are enclosed in the pollen that is made by meiosis in the anther, while
haploid eggs are produced by meiosis in the ovule.
 In dioecious plants, the male and female organs are on different individuals, such that some
individuals are males (pollen producers) and others are females (fruit producers).
 Maples, ash, date palms, and figs are examples of dioecious plants.

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Fertilization occurs when the pollen grain lands on the stigma containing the ovary.
Pollen is actually multicellular, containing a tube cell and two sperm cells.
The tube cell interacts with the stigma to elongate a long tube through which the sperm cells
travel.
When the pollen tube enters the ovule containing the egg, the two sperm cells are released and a
double fertilization occurs (Figure 2.14).
One sperm cell fuses with the egg to produce the zygote that will form the embryo.
The second sperm cell fuses with a multinucleated somatic cell to produce the triploid endosperm
that will nourish the developing embryo.

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• The major differences between animal and plant embryos can be traced to the fundamental
difference between animal and plant cells: the plant cell wall.
• This cellulose-based structure supports the plant cell and constrains cell expansion and mobility.
1. It prevents cell migration in plants (plants don't undergo gastrulation, for instance) and
2. prevents neighboring cells in a plant from having the types of intercellular interactions that are
common in animal cells and that are so crucial in the processes of animal development.
• Plant embryos grow by mitosis, and in the developing plant, cell fate is determined primarily by
its position.
• And, even though there is a general fate map, plant cell fates are not as rigidly determined as in
animal embryos.
• Indeed, unlike most animal cells, plant cells are totipotent, and individual cells can produce an
entire plant.

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Vegetative Propagation
This ability of the plant to retain totipotent cells makes possible the cyclic "death" and renewal of
plants in temperate climates.
The above-ground portion of many flowering plants shrivels away each fall, only to grow and bloom
again in the springtime. As the plant grows, these totipotent cells form growing regions called
meristems.
The meristematic cells proliferate at the opposite tips of the plant (the root and shoot meristems) as
well as along the stem (the ground meristem). This ability of a plant to retain totipotent cells
throughout its life also allows one to grow an entire plant from cuttings.
Thus, if you have a particularly nice dorm plant, you can often share it with a friend by cutting off a
single branch and planting it in soil; an entire plant will grow from that small section. This is called
vegetative propagation, and some plants (such as strawberries) routinely use this as a way of
reproducing.

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Developmental Plasticity
Plants also have a great deal of developmental plasticity. That is to say, the development of
plant organs can be controlled to a large degree by the environment.
Plants growing in low moisture or nutrients may decrease their root diameter compared to
plants grown in high moisture and nutrient conditions. This narrowing increases the length and
the surface area of the roots, allowing them to absorb more nutrients.
Similarly, plants grown in the shade often change their leaf structures and the amount of
branching to better harvest the small amount of sunlight (Figure 2.15).
This remarkable degree of plasticity may help compensate for a plant's inability to move to a
different location.

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Do practice it