GASTRULATION AND ITS TYPES

Submitted by:

Dr. Sakshi Verma

Assistant Professor

Zoology Department

Gastrulation

• Gastrula is the stage of the embryo after blastula. The gastrula stage is an important stage in embryonic development. During this stage, the blastula is reorganized into gastrula.
• Gastrulation is a fundamental phase of animal embryogenesis during which germ layers are specified, rearranged, and shaped into a body plan with organ rudiments. The term gastrulation, derived from the Greek word gaster, denoting stomach or gut, is a fundamental process of animal embryogenesis that employs cellular rearrangements and movements to reposition and shape the germ layers, thus creating the internal organization as well as the external form of developing animals

• GASTRULATION is a complex series of cell movements that: a. rearranges cells, giving them new neighbors. These rearrangements put cells in a new environment, with the potential to receive new signals. b. results in the formation of the 3 GERM LAYERS that will form most of the subsequent embryo: ECTODERM, ENDODERM and MESODERM.

Gastrula Layers�

• In triploblastic organisms, the gastrula is trilaminar (three-layered). These three germ layers are the ectoderm (outer layer), mesoderm (middle layer), and endoderm (inner layer). In diploblastic organisms, such as Cnidaria and Ctenophora, the gastrula has only ectoderm and endoderm. The two layers are also sometimes referred to as the hypoblast and epiblast.  Sponges do not go through the gastrula stage.

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• Gastrulation takes place after cleavage and the formation of the blastula, or blastocyst. Gastrulation is followed by organogenesis, when individual organs develop within the newly formed germ layers

Morphogenetic movements or Movements of gastrulation

• The following general types of morphogenetic movements have been recognized:

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• a. Movements causing expansion of epithelium:

i. EPIBOLY

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ii. INTERCALATION:

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iii. CONVERGENT EXTENSION

EPIBOLY

• It is the spread of an outside cell layer to envelop a yolk mass or deeper layer.
• In other words, it is the spreading and thinning of the ectoderm while the endoderm and mesoderm layers move to the inside of the embryo.
• When undergoing epiboly, a monolayer of cells must undergo a physical change in shape in order to spread. Alternatively, multiple layers of cells can also undergo epiboly as the position of cells is changed or the cell layers undergo intercalation.

Intercalation

• During intercalation, two or more rows of cells move between one another, creating an array of cells that is longer (in one or more dimensions) but thinner. The overall change in shape of the tissue results from cell rearrangement. Intercalation can be a powerful means of expanding a tissue sheet.

Convergent extension

• Convergent extension (CE), sometimes called convergence and extension (C&E), is the process by which the tissue of an embryo is restructured to converge (narrow) along one axis and extend (elongate) along a perpendicular axis by cellular movement.
• During convergent extension, two or more rows of cells intercalate, but the intercalation is highly directional. Cells converge by intercalating perpendicular to the axis of extension, resulting in the overall extension of the tissue in a preferred direction.

• Movements causing Displacement of cells:

i. INVAGINATION -local inward buckling of an epithelium

ii. INVOLUTION -inward movement of a cell layer around a point or edge

iii. INGRESSION -movement of individual cells or small groups from an epithelium into a cavity.

iv. DELAMINATION -splitting 1 cell sheet into 2 or more parallel sheets.

INVAGINATION:

• The infolding of a region of cells, much like the indenting of a soft rubber ball when it is poked.
• A small depression is formed in the region occupied by the grey crescent area. This depression grows inwards and forms the archenteron or gastrocoel or secondary body cavity. The outer opening of the archenteron is called the blastopore. As the archenteron increases in size the blastocoel gets reduced. Ultimately only a slit like semicircular cavity indicates the remnants of the blastocoel. The blastopore meanwhile becomes expanded and becomes ring shaped.

Invagination

INVOLUTION:

• The inturning or inward movement of an expanding outer layer so that it spreads over the internal surface of the remaining external cells
• During this process the cells which have grown backward during epiboly now roll inside at the margin of the blastopore. The endoderm is the first to roll inside. The cells of the notochord and mesoderm which were formed outside now migrate over the lip of blastopore and become internal and arrange themselves on the roof, sides and the floor of the archenteron. The notochord cells are found on the roof along the midline. While the endoderm forms the anterior, lateral and ventral walls, the mesoderm forms wing like extensions in the archenteron.

Involution

INGRESSION:

• The migration of individual cells from the surface layer into the interior of the embryo. The cells become mesenchymal (i.e., they separate from one another) and migrate independently.

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• Unipolar ingression: in which individual cells migrate inwards at one end of a blastoderm.eg. Porifera and Coelenterata
• Multipolar ingression: in which individual cells migrate inwards from all points of the blastocoels. Eg. Echidna

Ingression

DELAMINATION:

• The word delamination means mass separation of groups of cells from other cell groups. In other words, the splitting of one cellular sheet into two or more parallel sheets.
• The separation of endodermal, mesodermal and notochordal cells from each other in teleost fishes is a good example for delamination. According to a widely accepted view the endoderm formation in birds takes place by delamination

Process of Delamination in chick embryo

Fate of Three Germ Layers:

• Each germ layer forms specific tissues, organs and organ-systems.
• The three germ layers produce tissues, organs and organ-system in following manner:
• Ectoderm:
• It forms:
• (i) Epidermis of skin, epidermal derivatives like epidermal glands, hair, nail etc.
• (ii) Nervous system,
• (iii) Medulla of adrenal gland, posterior and intermediate lobes of pituitary gland, pineal gland,
• (iv) Eye (conjunctiva, cornea, lens, retina, iris and ciliary muscles),
• (v) Internal ear,
• (vi) Nasal and olfactory epithelia,

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Fate of Three Germ Layers:

• Ectoderm:
• (vii) Enamel of teeth,
• (viii) Epithelium of fore gut and hind gut
• (ix) Some glands—sweat glands, oil glands, mammary glands, salivary glands and lacrimal glands.

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Fate of Three Germ Layers:

• Mesoderm:
• It forms:
• (i) Dermis of skin,
• (ii) Muscles except iris and ciliary muscles,
• (iii) Connective tissues,
• (iv) Kidneys,
• v) Gonads,
• (vi) Notochord,
• (vii) Heart, blood and lymph vessels,
• (viii) Urinary and reproductive ducts,

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Fate of Three Germ Layers:

• Mesoderm:
• (ix) Most of skeleton,
• (x) Coelomic epithelium,
• (xi) Pericardium and pleura,
• (xii) Dentine of teeth,
• (xiii) Cortex of adrenal gland,
• (xiv) Mesenteries,
• (xv) Sclera and choroid of eyes,
• (xvi) Wall of the gut except its lining.

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Fate of Three Germ Layers:

• Endoderm:

It forms:

• (i) Lining of gut except for gut and hind gut,
• (ii) Some glands—pancreas, liver, gastric glands, intestinal glands, thyroid, parathyroid, thymus and larger part of prostate,
• (iii) Inner layer of tympanic membrane,
• (iv) Lining of middle ear,
• (v) Trachea, bronchi and lungs,
• (vi) Urinary bladder,
• (vii) Urethra.

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GASTRULATION IN SEA URCHIN

• The process is easiest to understand in radially cleaving eggs with little yolk, such as the much-studied eggs of sea urchins
• One wall of the blastula (the vegetal pole) bulges inward, or invaginates, as if someone were poking a finger into a hollow ball. The new space that forms is the archenteron, or “first gut.” The opening into the archenteron is the blastopore. Recall that in deuterostomes, the blastopore becomes the anus and the mouth (the second opening) forms opposite it. The outer layer of cells is now ectoderm, and the wall of the archenteron is endoderm and future mesoderm. The archenteron elongates, assisted by contractions of wandering cells called mesenchyme cells. The archenteron will eventually squeeze the blastocoel out of existence.

• Gastrulation in a Sea Urchin Embryo During gastrulation, cells move to new positions. Invagination to form endoderm in this non-yolky egg is illustrated. The archenteron, or “first gut,” remains connected to the exterior via the blastopore. Mesenchyme cells assist with gastrulation and will also form the temporary larval skeleton.

GASTRULATION IN AMPHIBIAN EGG

• In eggs such as those of amphibians, which contain a moderate amount of yolk, cleavage continues to produce many ever-smaller cells in the animal hemisphere and larger cells in the vegetal hemisphere. To initiate gastrulation, some cells in the gray crescent invaginate to form a slitlike blastopore. Small blastomeres of the animal pole then begin to roll like a sheet over the dorsal lip of the blastopore and push into the blastocoel. At the same time, cell divisions continue to increase the number of cells. �

• Because the small cells of the animal hemisphere continue to divide more rapidly than the larger cells of the vegetal hemisphere, these smaller cells grow over and surround the larger cells of the vegetal hemisphere. As these smaller cells reach the blastopore, they too move inward so that the inward migration occurs all around the blastopore. These cells will form much of the endoderm and mesoderm of the developing embryo. The blastopore becomes the anus.

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Gastrulation in a Frog Embryo