Download Cleavage and Gastrulation in Metazoa: Formation of Germ Layers and Morphogenetic Movements and more Lecture notes Developmental biology in PDF only on Docsity!
UNIT 14 CLEAVAGE AND
GASTRULATION
Structure
14.1 Introduction Objectives 14.2 Cleavage Yolk and Cleavage Planes of Cleavage Pattern of Cleavage Roducts of Cleavage (Morula and Blastula) Mechanism of Cleavage 14.3 Gastrulation Fate Maps Morphogenetic Movements GastrulationinSome Animals 14.4* Summary 14.5 Terminal Questions 14.6 Answers
14.1 INTRODUCTION
In Unit 13 ~f this Block you have studied that spermatozoa reach the ovum by either chance or chemical attraction. Eventually, one spermatozoon fuses wi the ovum to restore the diploid genomic condition and activates all the potentials i r the fertilised egg cell or zygote to develop into a new individual of the next generation. But the zygote is one cell and the adult body in the Metazoa is constituted of many cells - from a few hundred to many billions of cells. It implies that the unicellular zygote must enter the phase of rapid divisions in quick succession to convert itself into a
multicellular body. Such a series of divisions of the zygote is known as cleavage or
segmentation. In this multicellular structure formed as a result of cleavage the various cells or cell groups later become tearranged as layers and sublayers during a process called gastrulation.
Cleavage and gasuulation are significant phases in the ontogenetic development because
cleavage transforms the unicellular zygote into a mullicellular body and gastrulation lays
the foundation of primary organ rudiments so as to initiate the formation of organs
according to the body plan of the particular metazoan group of animals to which the particular individual belongs.
0 bjectives
After you have read this unit you should be able to:
explain the various planes of cleavage furrows.
list different cleavage patterns explain the purpose of gastrulation e discuss the process and mechanism of gastrulation in some animals describe the influence and the role of yok in determining the pattern and course of cleavage and gastrulation.
142 CLEAVAGE
iii) Mesolecihal eggs wih moderabe mount of yolk, e.g. tunicares and amphibians (Fig. 14.2 C).
Clesvage and Castrulaliun
Cleavage or segmentation is a series of cell divisions of the fertilised .egg through whic'h it is converted into .a multiccllular suucture, called blastula. The main characteristics of cleavage include:
i) The unicellular^ fertilised^ egg is transformed by consecutive^ miroric^ divisions into a multicellular body. ii) hacticdly no growlh takes place during cleavage.
The cell divisibns in the somatic cells is mitotic. The daughter cells or blastomeres or
cleavage cells are also derived as a result of miototic divisions of the zygote. We may
ask whether there are any dificrences between mitotic divisions of somatic cells and, of
the zygote and the blastomeres derived from it during cleavage. From the following you will learn th3t the mitosis in the phase of cleavage has spme striking pcculiruities:
a) Synchronisation of cell divisions of blastomeres: The early blastomeres divide sirnultaneuusly (synchronously) producing two blastomeres from zygote followed by 4,8,16.32 and so on, in most cases. Howevei, such synchronisation is lost, during later cleavage divisions.
b) No interphase between two successive divisions, i.e. here is no growh in he amount of cytoplasm in the derived blastomeres with the result that the size of daughter blastomeres continues to decrease during successive cleavages.
c) The size of the nucleus remains practically unchanged. Therefore, h e nucleus:
cytoplasm ratio, which is very small in h e fertilized egg cell or zygote, continues to increase in the blastomeres derived from successive cleavage divisions.
d) Rate of cell divisions is very rapid and very large number of cells are produced during cleavage (Fig. 14.1). This is possible due to absence of interphase. The rate. slows down later on
Hours at 150C Flg. 14.1: Increase In the number of cells during early development of frog, Note the dlflerence In the rate of celi.dlvlsions during cleavage and gastrulation.
14.2.1 Yolk and Cleavage
Apart from the importance of yolk as nutritive material for the deleloping embryo, yolk
or deutoplasm determines he type and structure of the egg, and it also influences the
rate and pattern of its cleavage. In other words, cleavage depends, to a large extent, upon the amount, distribution and orientation of yolk in the egg.
Types of Eggs
Depending on the amount of yolk, the eggs in various animal groups are of the following types (Fig. 14.2):
i) Alecithal^ or yolkless-eggs^ as^ in the^ eutherian~mainmals.(Fig.^ 14.2^ A). ii) Microlecithal or oligolecithal eggs have liule yolk in the form of granules, e.g., echinoderms, Amphioms, rnohscs (except cephalopods), annelids, flativorrns (Fig. 14.2 B).
Influence of Yolk on Cleavage Cleavage^ and^ G.strul.Ua
Though biological significance of yolk is to provide nourishment to the developing embryo, it is not part of the active cytoplasm. Yolk is dead and inert component not participating in the cellular activities, but, it influences cleavage in ,the following ways:
i) With gradual increase in the amount of stored yolk, the total amount of^ the^ active cytoplasm tends to decrease. ii) Cell division is the activity of only the nucleus and cytoplasm. With increase in the yolk amount the formation of spindles, cell membranes and cleavage furrows takes
place in the active cytoplasm which is restricted to relatively smaller areas of the
zygote and its daughter blastomeres. iii) The speed of cleavage is inversely proportional to the amount of yolk present. In the telolecithal eggs, blastomeres nearer to the animal pole divide at rr faster rate
L than the blastomeres^ located^ towards the vegetal pole because the passive^ behaviour of the inert yolk in the yolky parts of the zygote and its daughter blastomeres obsnucts the formation of cleavage furrows.
Therefore, the nature of various metabolic activities of the egg and the blastomeres derived from it depends upon the amount and placement of yolk mass.
Principles Governing Cleavage
a) The nucleus and mitotic achromatic figure tend to occupy the centre of active cytoplasmic density of the dividing cells, e.g., in isolecithal eggs or microlecithal eggs, the spindle is formed centrally in the cell while in the telolecithal eggs, it is formed nearer the animal pole.
b) Each new cleavage furrow tends to intersect the plane of the preceeding cleavage furrow at right angle.
c) The cells or blastomeres tend to divide into two equal sized daughter cells unless yolk is unevenly distributed. d) Free sides of the blastomeres tend to become rounded.
SAQ 1
Fill in the blanks with appropriate words:
i) During cleavage zygote and^ tlastomeres^ divide by^ .............................
ii) There is no ........................... or ...........................between two consecutive divisions of blastomeres during cleavage.
iii) The egg of frog isdescribed as ...........................and ...........................:
iv) Large size of hen's egg is due to ...........................amount of yolk.
v) Achromatic figure or ........................... tends to be formed in the centre ,of cytoplasmic ...........................
14.2.2 Planes.of Cleavage
The'ava of most of the animal groups (except some specific cases like insects) are spherical or nearly spherical having their own actual centre comparable to earth shape. Similar to north and south poles on earth, the egg has animal and vegetal poles. The yolk platelets have more density than active cytoplasm and are concentrated more towards vegetal hemisphere. Therefore, when the egg lies in any fluid medium (the fundamental feature of most of the eggs even in the a p p ~ n t l yterrestrial eggs like those of birds etc.), the vegetal pole tends to face the centre of gravity and animal pole away from it.
With this picture in mind, we can now define the planes of cleavage 'of zygote or b!astomeres, keeping in mind the imaginary lines (latitudes and longitudes) drawn on the earth surface (Fig. 14.3).
Anlmd Development I
Fig. 143: A-Meridians (longitudes), B-Latitudes (imaginary lines on the earth surf'ace which are comparable to the cleavage planes d a spherical egg).
The basic planes along which the egg and its daughter blastomeres are divided during early cleavage are:
i) Meridional^ Plane^ -^ the cleavage furrow passes from the animal pole to the vegetal
pole through the centre of the spherical egg or the blastomere so as to divide the
egg into two equal halves, e.g., first cleavage furrow in the chick and first as well as second cleavage furrows in the frog's egg (Fig. 14.5 A, B).
ii) Vertical Plane - the cleavage furrow forms parallel to animal-vegetal axis but a
little away from the centre of the blastomers, e.g., thud cleavage plane of chick blastoderm (Fig. 31 C). iii) Equatorial Plane - the cleavage furrow bisects the egg at right angle to the median axis exactly half way between the animal and vegetal poles The cleavage furrow appears along the equator of the spherical egg., e.g.. the third cleavage plane of sea urchin (Fig. 14.5 C). iv) Latitudinal or transverse or horizontal plane - it is like equatorial but the cleavage furrow passes through the egg cytoplasm on either side of equator along the latitudes of the egg sphere, e.g., third cleavage plane of amphibian eggs (Fig. 14.6 C).
14.2.3 Patters of Cleavage
In most of the animal groups with spherical or almost spherical egg and little or moderate amount of yolk (micro-or mesolecithal eggs), the first and second divisions result in four blastomeres of almost equal size (Fig. 14.5, A, B). Because of greater concentration of yolk platelets in the vegetal hemisphere, the third cleavage divides the 4 blastomeres in latitudinal plane giving rise to 8 cells arranged in two tiers of 4 blastomeres each including one tier of 4 small blastomeres (micromeres) in the animal hemisphere and the second tier of 4 large blastomeres (macromeres) in the vegetal hemisphere (Fig. 14.6 C). The arrangement of blastomeres in these two tiers is very distinct and on this basis, the cleavage may be of:
a) Radial type: If each of the blastomeres of upper tier lies exactly over the corresponding blastomere of the lower tier the paltern of cleavage is radially symmetiical i.e. blastomeres are arranged along the radii of the sphere, e.g. echinoderms, Amphioxus, amphibians (Fig. 14.5, D-F).
b) Spiral type: The upper tier of blastomeres of 8-cell stage embryo may be shifted
with respect to the lower tier so that radially symmetrical arrangement of
blastomeres is distorted in various degrees. In such a case the blastomeres of the upper tier do not lie exactly over the corresponding blastomeres of the lower tier. Instead, all the blastomeres of upper tier are shifted in the same direction with respect to all the blastomeres of the lower tier. This position results from the oblique position of mitotic spindle so that, from the start, two daughter blastomeres
Animal Development I Animal^ oole^ Meridional Equatorial cleavage
Vegetal pole B c
Animal half Animal pole
vegetzd half E D Fig. 145: Holoblastic and radial cleavage In the microlecithal egg of Synapta digita (Echinoderm) leading to the hollow blastula (C). A-B indicate the merldlonal planes of 1st and 2nd cleavage; C-equatorial plane (3rd cleavage); D-G radlal arrangement of blasborneres.
........^ ...-^ Micromeres
.. ....
'i
..
Macromeres
- A Flg. 14.6: Unequal Holoblastic Cleavage In frog's egg (A-F) cleavage furrows a r e designated, by Roman numerals Indicating the order of appearance; r-mohibian blastula in section Micromeres
M acrom-ere Flg. 14.7: Cleavage In the-ganoid h h Amla.
b) Meroblastic or partial cleavage: The egg does not divide completely because divisions are restricted to only a part of the egg while the rest of the egg remains entirely uncleaved. It is of two types:
i) Discoidal^ merdblastic^ cleavage: It takes place in the heavily^ yoked macrolecithal and highly telolccithal eggs, as for example in cephalopod molluscs, reptiles, birds (Fig. 14.8) and monotremes (egg laying mammals). The cleavage is restricted to the cytoplasmic germinal disc situated at the animal pole. Even the germinal disc divides incompletely while the entire yolk mass remains undivided. ii) Superficial meroblastic cleavage: This occurs in the cenuolecithal eggs of insects. Cell divisions are restricted to the peripheral cytoplasmic layer while the centrally located yolky is left undivided (Fig. 14.9).
Sub germinal cavity
I
Rp. 14.8: Dlagrams^ of^ sectlons^ of the^ fertllind^ chlck^ egg.^ DLsc01d.I^ memblastic^ cleavage^ In Its biastodlx lying on top of yolk.
Flg. 14.9: Diagramatlc representation of superflclal cleavage In Insect embryo. (A) Undivlded zygote nucleus In the yolk. (B) - (E) After-lst, Znd, 3rd and more divisions OF the zygote nucleus. (F) Daughter nuclei have mlgrated from Interior of the egg to peripheral cytoplasm which Is still undivided. (C) Perlpheral cytoplasm dlvided Into separate cells to form cellular blastoderm around undivided yolk.
cleavage and Gastrulation
The structure of blastula becomes modified in various animal groups. The modifications (^) Cleavage and Gastrulation are related to the amount of yolk deposited in the egg, as you will see from the accounts of blastula structure in some deuterostome animals.
The blastula of the echinoderm sea cucumbers consists of a fluid fillcd blastocoel surrounded by a single layer of cuboidal blastomeres, which constitute the simple epithelial blastoderm. In the blastulae of sea urchin and Auphioxus, the blastoderm surrounding the blastocoel is an epithelium consisting of a single layer of columnar cells (blastomeres). However, the vegetal blastomeres are larger than animal blastomeres so that the epithelium is thicker at the vegetal pole and thinner at the animal pole. Thus the polarity of the egg persists in the blastula (Fig. 14.10, a, b).
yolk
subgerminal cavlty
Fig. 14.10: Diagrammatic comparison of blastulae of echlnderm. (A) Amphloxus (B) Amphibian (C) bony flsh (D) blrd (E) blastocyst of mammals (0 and Insect (G).
The blastulae of the echinoderms and Amphioxus have been described as coe!oblastula.
Animals with larger amount of yolk (e.g. amphibians) show considerable differences in
size among the blastomeres of blastular blastoderm, and the blastocoel is distinctly
eccentric nearer to the animal pole instead of being in the centre. The blastoderm is not a simple epithelium of a single layer of cells but is two or more cells thick (Fig. 14.10,
c). The cells of the inner side of blastoderm are loosely connected to one another but those at the external surface adhere with each other very f m l y because of the presence of tight junctions between them. The blastoderm at the animal pole and most of the animal hemisphere is made up of micromeres forming the dome-shaped roof of the blastocoel while the blastomeres of the vegetal hemisptiere form the floor of the blastocoel (Macromeres). The amphibian blastula is also a coeloblastula but it is
modified as described above due to larger quantity of yolk mostly located in the vegetal
hemisphere of the egg (Fig. 14.10, c).
The embryonic stage comparable to blastula occurs in a still more modified form in the
sharks, bony fishes, reptiles, birds and egg laying mammals, all with mamlecithal and highly telolecithal eggs. In the egg laying amni6tes reptiles, birds, monotremes the active cytoplasm is restricted to a small disc (cytoplasmic germinal disc) on top of the yolk
near the animal pole. Cleavage is meroblastic occuring only in the germinal disc and
gives rise to a disc-shaped blastoderm made of several layers of cells lying on top of the uncleaved yolk. Such a blastula is called discoblastula. Between the blastodenn and yolk there is a narrow space called sub-germinal space (or segmentation cavity), which is not comparable to blastocoel. In the birds a true blastocoel appears later between the upper layer of blastodem (epiblast) and the lower layer (hypoblast) farmed by cells (^) 1 migrating from the blastoderm (Fig. 14.10 e). In the insects having centrolecithal eggs, the blastula stage does not have any cavity. It is characterized by bne cell thick epithelial blastoderm enclosing the yolk filled intaia. Such a blastula is called superficial blastula (Fig. 14.10 f). Cleavage of the yolkless eggs of eutherian mammals gives rise to a solid ball of cells (morula). Fluid is secreted into the space between the cells of monila which grows in size to become the blastocyst. This is the blastula stage of the embryos of eutherian mammals. Structurally it consists of a single layer of cells (trophectodenn) enclosing a large fluid filled blastocoel. At one end of the blastocoel pressed up against the inner
surface of uophectoderm there is a group of cells referred to as the inner cell mass
(ICM). The entire body of the embryo is formed from cells of ICM (Fig. 14.10, g).
SAQ 3
i) In spite of little or no yolk in the eggs of echinoderms and eutherian mammals, the cleavage follows entirely different courses in two p u p s , why?
ii) There is a list of various animal given below. Mention the type of cleavage and the resultant blastulae:
Animals ~y pes'of Cleavage Type of blastula
a) Ciona (Tunicate) ........................ ........................
b) Rat ......................... ........................
c) Labeo rohita (rohu) ......................... ........................
e) Rana tigrina ........................ ........................
0 Calotes versicolar ......................... ......................... (garden lizzard)
g) Pigeon ........................ ........................
h) Sea (^) ,Urchin (^) ........................ ..........................
i). ~ e r &(Annelids) ........................ ..........................
Animal Dcvdopmcnt I (^) so far. The available evidence suggests that mitotic spindle dictates the location of
.cleavage furrows. The furrow always forms perpendicular to the long axis of the spindle. Formation of the cleavage furrow depends upon the presence of a pair of asters, one at either end of the spindle. Disruption of astral rays inhibits the formation of the furrow and hence cytokinesis. Although karyokinesis and cytokinesis are coordinated they are independent processes. Nuclear divisions can take place without being followed by cytoplasmic division. As you have learnt cytokinesis can be inhibited by treatment with cytochalasin B but the nuclear division proceeds to completion resulting in binucleate or multinucleate cells. It occurs in nature also, e.g., in the insect egg the zygotic nucleus and its daughter nuclei divide mitotically many times to produce hundreds of nuclei but the cytoplasmic divisions take place only later when all these nuclei have migrated to the peripheral cytoplasm (Refer to Fig. 14.9). Similarly cleavage of cytoplasm can take place even if karyokinesis is blocked, e.g. if the zygotic nucleus of a fertilized egg is removed the enucleate egg cytoplasm undergoes cleavage divisions upto about blastula stage.
The Formation of New Membranes
Divisions of the egg or a blastomere increase the total surface area of the two daughter
cells to be covered by membrane:at each cleavage. The existing membrane of the parent cell is insufficient. From the evidgnce available so far, it is indicated that this insufficiency of membranes for daughter blastomeres during cleavage is made up from two sources:
i) A portion of the membranes covering the daughter cells is provided by stretching and extension of the original plasma membrane of the zygote or the blastomeres. ii) A portion of the cell membrane is newly synthesized by the daughter cells. Thus, the furrow membranes are a mosaic of different parts.
SAQ 4
i) Define:
. a) Karyokinesis .....................................................................................
b) Cytokinesis ..................................................................................... ...................................................................................................... ii) What are the basic structural units of: a) Spindle fibres: b) Contractile ring of microfilaments: c) Astral rays.
iii) How is cleavage division affected by treaunent of the egg with: a) Colchicine b) Cytochalasin B.
14.3 GASTRULATION
The end of cleavage of the unicellular zygote results in the formation of multicellular blalstula, which may be a solid structure without a cavity (stereoblastula), or its cells
k I may be arranged in th form of a one cell or several cells thick epithelium around a cavity (coeloblastula) or around or on top of the yolk. (Superficial blastula; Discoblastula). In either case the blastula has no resemblances to the shape or organization of the body. Therefore, through the subsequent developmental stage the simple blastula should transform itself into a more complex embryonic structure (gastrula) upon which the adult like body may be built up. Such' a process of transformation is known as gastrulation. It is a very significant phase of ontogenetic development, which mark the beginning of the development of form and organization of adult body. In the metazoans (except in sponges and coelenterates), the various tissues and organs of the body develop from cells which become arranged in the form of three layers, the outer ectoderm, the inner endoderm and the mesoderm between these two layers. The three layers are called the germinal layers. With the exception of some parasitic flatworms a new cavity called the archenteron (future alimentary canal) is f h e d surrounded by endoderm. In the blastula all the cells are located on the surface forming the blastoderm. During gastrulation there occurs displacement of the parts of blastoderm so that the presumptive endodermal and mesodermal cells are removed from the surface of blastula and brought
I into the interior of embryo where the respective organs are formed in the^ course^ of further development. The cells of the presumptive ectoderm remain on he surface. Thus, the single layer of cells, the blastoderm, gives rise to three germinal layers viz. ectoderm, mesoderm and endoderm. Therefore, gastrulation is a dynamic process involving large scale movement of blastula cells resulting in their arrangement in a way
which establishes the basic body plan according to which the embryo has to develop
further. Since these movements lay the foundation of the form and organization of h e body they are called morphogenetic movements. They involve movements of epithelial layers of cells as a whole as well as independent movements of cells which break loose from epithelium and become mosencphymal. The extent of morphogenetic movements during gastrulation depends, to a certain degree, on the number of cells in the blastoderm of the completed blastula. For example, relatively simple and less movements occur during gastrulation in the ascidian styela (tunicate) in which there are only about 100 cells at the end of blastula stage. In animals such as frog in which the blastoderm consists of many thousands of cells already at blastula stage very large scale and complicated movements are required during gastrulation for their reatrangement into the three germinal layers. The important features of the gastrulation are:
a) rvangement of cells of the embryo by means of morphogenetic movements b) rhythm of cell divisions slows down (Fig. 14.1) c) growth, if any, is insignificant. d) there is intensification of oxidation. b e)^ the nuclei become more active in controlling the activities of^ he^ embryonic cells. The influence of paternal genes becomes evident during gastrulation. I f ) proteins of many new types that were not present in the egg or blastula^ begin^ to^ be synthesized. I
14.3.1 Fate Maps
The details of the process of gastrulation are not easy to understand without the knowledge of positions of the cells of the future germinal layers in the blastula. A chart or diagram showing the prospective fzte of each part of blastula or embryo at any stage of development is called a "fate m~p".
There are various ways of constructing the fate maps of blastulae or any other developmental stage of different animals.
By Natural Markings
For some animals it is possible to make use of the peculiarities of cytoplasm in certain parts of egg, such as pigment granules in the cytoplasm. The descendent blastomeres of such cytoplasmic areas tend to carry the pigment granules where ever they happen to be present in the later stage. Pigment granules of various colours .are present in different cytoplasmic areas of the egg and in different blastomeres subsequent to cleavage in
Ckrvage and CI8buIatlon
Fig. 14.13: Fate map of chkk blastoderm immediately prior to gastrulation. The prlmitive streak Is not yet formed but it will extend eventually to the reglon of the notochord.
In three to eight hours the tritiated thymidine is incorporated into the chromosomal DNA of dividing blastodermal cells. The embryo labelled in such a way by uitiated thymidine serves as a donor. Another embryo at the same stage of development as attained by the labelled donor in the meantime is then selected to serve as the host A small area of the host embryo is excised and replaced by a corresponding piece from the donor of which the fate is to be determined. (Fig. 14.14). Healing usually occurs quickly and the development is not impaired if the operation has been done carefully. The thymidine does not pass out of the nuclei of the labelled cells but remains in the chromosomes of their descendents. Although the radioactive thymidine present in the DNA is gradually diluted with each subsequent chromosomal replication radioactivity remains for a " considerable time. Such composite embryo (partly from donor and partly from the host) is tested at a later stage of development for radioactivity by special ~ h n i q u e ssuch as autoradiography etc. Only the part(s) or structure(s) developed from the grafted piece show the presence of radioactivity thus establishing the fate of particular area taken from the donor.
donor host Fig. 14.14: Dlagram explalnlng the method of testlng the fate of particular part of a chlcv blastoderm by Implanting a corresponding part born a trltiated thymMlne labelled donor into unlabelled reclplenthost.
14.3.2 Morphogenetic Movements
Gastrulation is a dynamic process involving a variety of coordinated movements of cells of different areas of the blastula.
The movements of cells in the embryo from one place to another to establish a particular form or structural arrangement are referred to as morphogenetic movement. (Morphos = shape; genesis = formation). Such movements occur during embryonic development (from the beginning of gastrulation onwards) as well as in the adult body. In the adult body, these are reversible but the movements occurring during gastrulation ace irreversible.
and
Gastrulation begins and proceeds as a result of the onset of various types of morphogenetic movements which are inherrent to the particular category of. cells. For the sake of convenience, these are described separately but it should be understood that two or more of them may occur simultaneously. Broadly, there are two groups of morphogenetic movements in embryonic development i.e., Epiboly and Emboly.
i) Epiboly or Epibolic Morphogenetic Movements
Epiboly means to throw on or to extend upon (Fig. 14.15). It occurs only in the
presumptive ectodermal blastomeres (epidermal and neural areas). The cells of this area
have an inherrent property of flattening, expansion and stretching. The cells of the presumptive ectodermal areas expand and extend but they remain on the surface eventually forming the outer layer covering the entire embryo and enveloping the inwardy migrating presumptive mesodermal and endodermq blastomeres.
Flg. 14.15: Dhgrm showing the analogy with epiboly. Vlscous llqukl poured over a sphere slowly spreads covering I t s surface.
ii) Emboly or Embolic Morphogenetic ~ o v e r n e k
Emboly 'means to throw in or to thrust in. Such movements bring about the m i e o n of presumptive mesodermal and endodermal cells from the external surface of the embryo into its interior. Emboly includes several different m s of movements:
Invagination
It specifically includes the process of insinking & infolding of presumptive endodermal
- areas (Fig. 14.16) and is the most widely observed embolic movement during gastrulation in most animals, e.g. echinoderms, Amphioxus and amphibians etc. * Invagination may be: - A animal pole B animal pole
lip of blastopore vegetal pole vpgetal pole Fig. 14.16: Diagrammatic representation of gastruletlon by Invagination. Sectlons of blastula (A) and gastrula (B).
movements
Flg. 14.18: Convergent and dlveqent morpbgeaetk movements of cells mlgratlng Into the blastopore and then under the surface h~ gastrulathg rmphlblan embryo.
Fig. 14.19: Morpbogeactk movement d u r h g gPstrulatlon In chlck embryo, cbatLnuous Uncs
(on left sWc) l o w movement In the epiblast and broken lines (on rrght sMe)
hdlcate the cellular movements which have lmmlgrated h t o the interior through tbc primltlve streak.
Polyinvagination or Ingression
Ia this process, individual cells or small groups of cells in different parts of the
blastoderm or bl&stodisc invaginate (or ingress) and migrate into the cavity ar spaces
developed within the entbryo (Fig. 14.20). Since such invagination may occur at
diffe~entpoints at the same time it is also @led pdyinvagination. Ingression and
polyinvagination have similar meanings. Primary mesodermal cells of sea urchin embryo
become internal by this process.
Hg. 142& Dlagrammatlc representatton ol the lngnsslon of primary mesenchymal cells
@lack) in sea urchin embryo. (A) Early blnstuIa with clllated mkromeres. (B) Late blastula with cllla bclng withdrawn from mlcmmeres and the cell rounding up. (C) Mesenchyme +blastula,wlth micromeres detached Itom the h y d h layer and entered the blastocoel aa prlmary mesenchyme cells. @)
Early p t n l o 4 t h mobile cells (El Mldgastrula, wlth formatlm of syncytium
prlor to deposltlon of skeletal matrlx.
SAQ 5 Cleavage and Gastrulation
i) Fill^ m^ the blanks with appropriate words: Gastrulation is ........................process caused by the ........................of blastomeres from the snrface of ......................... The cytoplasmic areas of blastula praclically show the same ........................ as that of ........................... As a result of .........................single layered blastula is ........................ into a two layered or ........................layered ..................
(ii) List the various morphogenetic movements through which the gastrulation may take place.
(iii) List the various methods of constructing the fate maps of blastoderm of embryos in difiercnt animal groups.
14.3.3 Gastrulation in some Animals
As mentioned earlier gastrulation marks the beginning of morphogenesis i.e. development of body form and organisation of cells in the embryo. By the end of this process the groups of cells destined to form different tissues and organs are arranged in their respective proper positions within the three germinal layers (ectoderm, mesoderm, endoderm) and the primitive basic body phn of the animal is established. However, in different groups of animds gaqtrulation takes place in different ways determined mainly by the type of egg and subsequent pattern of cleavage and the structure of blastula. In this section you will study gasmlatim as it occurs in echinoderms. amphibians, birds
and eutherian mammals. The description will help you w understand the difference in
the method of gastrulation due to the influence of the amount of yolk and pattern of its distribution in the egg, subsequent pattern of cleavage and t!e ultimate structure of the blastula.
i) Gastrulation in Sea Urchin
The small, isolecithd eggs of sea urchin have very little yolk and undergo holoblastic cieavage. The resultant free swimming ciiiated blastula is a sphere consisting of a single Iayer of cells surrounding a large blastocwl (Fig. 14.21).
Formation of Primary Mesencychyme
t Gastrulation is iniuated by fiattening of the cells of vegetal region forming a vegetal plate (Fig. 14.21H). Small cells in the centre of this plate lose the cilia of their external surface, show pulsating movements at the inner end which becomes rounded and
i
i attachment with adjacent cells is lost.^ Erentually,^ these cells separate from the vegetal piate and migrate as individual cells into the blastocoel by ingression. They move around in the blastocoel for some time before setting down near the vegetal plate (Fig. 14.21 I). These cells constitute the primary mesetlchyme which gives rise to skeleton of the larva (Fig. 14.21 J, M).
First Stage of Invagination
The large endodermal cells remaining in the vegetal phte move laterally towards the centre of the plate. and fill the gap in the vegetal plate caused by ingression of h e primary mesenchyme cell. As a result, the vegetal plate becomes even more flattened. Soon thereafter the plate bends inwards (invagination) in the centre initiating the formation of a new cavity called the archenteron (primitive gut). Its opening at the vegetal pole is the blastopore. Invagination proceeds until the archenteron extends into the blasmcwl a b u t one third the distance betwcen the vegetal and animal poles and then stops (Fig. 41.2i J). The endodermal cells invaginate even if the vegetal plate is isolated and cultured in vibro indicating that invagination occurs dne rl, Inninsic 5 1
