GASTRULATION

 Is a process by which the hollow ball of cells, blastula, becomes transformed into a
layered embryo; the gastrula ,by mechanisms of cell movements and selective adhesion
 Gastrulation is important process that facilitates these key cellular rearrangements

Fate Maps

 These are projections of how specific areas of early embryos will develop at later stages.
It helps understand the movements that occur during gastrulation. In the fate maps and
also in the gastrula stage embryos that will be described, the term prospective or
presumptive is used to denote what the specific region will become in the more advanced
embryo.
 N/B: Fate maps describe the fate of specific regions of the early embryo

Fig. 4-1

(a)AMPHIIOXUS
(Projected upon an uncleaved zygote lateral view)

Fig…

(b)AMPHIBIAN
(Projected upon late blastula lateral view)
 Fig…

(c)BIRD
(Projected upon epiblast (top) layer of early gastrula (late blastula). The hypoblast (inner
layer) is mostly prospective endoderm.
*Prospective non-notochordal mesoderm

Amphioxus and Amphibian Fate Maps

 Fate maps of (see figures) amphioxus and amphibian are similar
 The animal region of each embryo gives rise to epidermis and neural plate (which
becomes, the nervous system and associated structures)
 The prospective epidermis and prospective neural plate will make up the outer layer of
the gastrula, the ectoderm
 Middle or marginal zone of each embryo gives rise to the middle region of the gastrula,
the mesoderm, which is composed of prospective notochord and prospective non-
notochordal mesoderm
 Notochord is a rod that develops below the neural tube, that may help support
developing embryo. It disappears in adult
 Non-notochordal mesoderm develops into many structures including limbs, heart,
muscles, kidneys and gonads
 The vegetal region of amphioxus and amphibian embryos gives rise to the endoderm, the
inner embryonic layer that forms gut and derivatives of gut tube

BIRD FATE MAP

 Bird fate map is different from the other two

 This is because the bird blastula is not a hollow ball but instead is a layered cap
sitting atop the yolk
 Fate map of the top (epiblast) layer gives rise to the extraembryonic ectoderm
 The membranes that are outside the embryo itself; the epidermis; neural plate;
notochord and non-notochordal mesoderm; the yolk sac; and the gut endoderm
 The lower (hypoblast) region gives rise to endoderm and some of the notochord
 Thus, epiblast and hypoblast are not equivalent to ectoderm and endoderm
Gastrulation in Sea Urchin

Fig

Sequence of Events (studied by time lap cinematography)

 Sea urchin is isolecithal with even distribution of relatively small amount of yolk
 No excessive quantity of yolk to interfere with the buckling in or invagination of the
blastula
 Gastrulation in sea urchin begins with cellular shape changes that occur in the vegetal
plate
 Some of the cells lose adhesion with their neighbours and are forced into blastocoel;
these are primary mesenchyme cells
 As partial result of this loss of vegetal plate cells, the vegetal area begins to buckle in or
indent
 This indentation forms a cavity or tube in the gastrula, the archenteron or primitive gut
 The opening of this tube to the exterior at the vegetal end is the blastopore
 The tip of the archenteron advances toward the animal pole
 At the tip of this advancing tube, cells extend projections called filopodia
 The cells that extend these filopodia are secondary mesenchyme cells, that become
mesoderm
 The tips of the filopodia appear to ‘feel’ the inner surface of gastrula and finally stick to
the animal end. They then contract, pulling the archenteron tube toward the animal end

Mechanism of Gastrulation in Sea Urchin

 Cell movement or cell motility is one important mechanism as seen from mesenchyme
cell activity
 Selective adhesiveness is another mechanism i.e. the primary mesenchyme cells lose
adhesiveness with their neighbors in the vegetal plate while secondary mesenchyme cells
appeared to preferentially stick. (via filopodia) to the animal end
 Contractility of the filopodia also plays a role in pulling the gut tube towards the animal
pole
 Gastrulation transforms sea urchin blastula a hollow ball of cells into a gastrula with gut
tube which begins to resemble body of adult sea urchin
  During Gastrulation, mechanism of cell motility, selective adhesiveness and contractility
control the cellular rearrangement needed for embryo to begin to take form of adult

Gastrulation in Amphioxus

 Gastrulation in amphioxus also occurs by invagination but not with aid of secondary
mesenchyme cells
 Instead, the vegetal area flattens and bends inward or invaginates
 The embryo begins to resemble a punched-in ball
 The outer portion of the embryo consist of prospective epidermis and prospective neural
system- which makes up the ectoderm
 The inner part of the cup, where your fist would touch as ball is pushed in consists of
endoderm, prospective gut and gut derivatives
 The mesoderm (prospective notochord and non-notochordal mesoderm) moves into the
cup, from the rim into the pocket.

Figure

 Amphioxus gastrula now possesses a primitive gut or archenteron

 The opening of the archenteron to the outside is a blastopore

Gastrulation in Amphibian

 Amphibian egg and embryo is moderately telolecithal- yolk is unevenly distributed and
vegetal region contains much more yolk than animal region
 Cleavage in amphibian embryo is also unequal; the yolk retards cleavage in vegetal
region
 Gastrulation is also influenced by yolk
 In amphibian unlike amphioxus and sea urchin, invagination or “punching in” of the
embryo does not occur. Instead, cells move from the exterior to interior of the embryo by
active migration of the cells through a groove that begins to form at surface of the
embryo
  The vegetal region of amphibian is too thick and too yolk laden to allow the type of
invagination found in sea urchin and amphioxus

Sequence of Events- Surface View

 A slit forms just below the equator; the blastopore

 Cells from surface of embryo move inside the embryo through the blastopore
 Migration first occurs in a small region below equator between animal and vegetal
hemisphere
 The area just dorsal to this beginning cleft is termed the dorsal lip of blastopore
 The dorsal lip forms at the site of the grey crescent
 Cells migrate over this lip through blastopore and into the embryo
 The blastopore lengthens and becomes crescent shaped, then semicircular and finally it
forms a full circle
 This results from cells on surface of embryo moving inward
 The first cells move in from the dorsal area of the embryo; as the blastopore becomes
crescent shaped, cells from lateral regions of the embryo move in
 Finally ventral cells move in, completing the circular blastopore
 The lateral lips and ventral lip of the blastopore are regions to the lateral and ventral over
which cells migrate into the embryo through blastopore
 When blastopore is complete, forming a full circle, the center of circle is filled with yolky
endoderm cells; yolk plug

Mechanism of Gastrulation

 Cellular shape changes first occur at the dorsal lip region and cellular movements begins
 Cells expand and contract like the changes in secondary mesenchyme cells of sea urchin
embryo
 Cellular expansion and contraction appear to play important roles in inward movement of
the active cells as well as those cells attached to the active cells
 The native of forces involved in amphibian gastrulation is not well understood
Sequence of Events- Cross Section

 Examining what occurs inside amphibian embryo during gastrulation

 Notochord forms from cells that move from the dorsal surface of the embryo over dorsal
lip region, through blastopore to the interior of the embryo below prospective neural tube
 Notochord forms the roof of the archenteron
 The prospective epidermis and prospective nervous system (together comprising the
ectoderm) expand over the entire surface of the embryo
 The blastopore becomes circular, the notochordal mesoderm enters dorsally and the non-
notochordal mesoderm enters laterally and ventrally
 The yolk plug endoderm disappears from surface of the embryo as the rim of the
blastopore contracts, pulling the yolk plug inside.

Cross-section gastrulation in amphibian

GASTRULATION IN BIRD EMBRYO

 In bird, gastrulation begins when the blastoderm separates into two layers, the top
epiblast layer and the bottom hypoblast layer
 The space between these layers is the cleft space
 2nd step in bird gastrulation is the formation of a thickening at one end of the blastodisc
(cytoplasmic cap) by movement of lateral cells towards the center
 This thickening is called primitive streak

 An indentation called primitive groove forms down the midline of the primitive streak
 Primitive groove serves the same function as blastopore of the other embryos
 The primitive streak elongates and cells move from epiblast surface through the primitive
groove into cleft space
 Some of epiblast cells that move in, form endoderm
 These cells enter hypoblast
 The other epiblast cells that move in, form the middle mesoderm layer
 In bird gastrulation occurs only in the non-yolky cap of cells
 The yolk remains uncleaved and uninvolved in gastrulation process