Gastrulation is a nicely integrated, dynamic process one, which is controlled largely by intrinsic forces bound up in the specific physicho-chemical conditions of the various presumptive organ forming areas of the late blastula and early gastrula. These internal forces in turn are correlated with external conditions. In most vertebrates, it includes different kinds of morphogenetic movements.
The movement of the blastomeres from one place of the embryo to another to establish a particular form or structure is a common embryological procedure. This type of cell movement is described as formative or morphogenetic movement, because, it results in the generation of a particular form or structural arrangement (Vogt, 1925, Trinkaus, 1969 and Berrill, 1971). Such morphogenetic movements occur in the embryo during blastulation, gastrulation, tubulation and organogenesis. They do not occur only during the embryonic stages, but also in a adult animal. But, there is a fundamental difference in the two, that the morphogenetic movements of an adult animal are of reversible nature, while, the morphogenetic movements of gastrula, etc., are of irreversible nature, i.e., each part remains in the position into which it has been brought by the preceding movements.
During gastrulation, each type of blastomere performs a peculiar and specific kind of morphognetic movement to meet its prospective density. All kinds of morphogenetic movements which culminate into gastrulation may be classified as follows:
(i) Epibolic morphogenetic movements or epiboly
(ii) Embolic morphogenetic movements or emboly
(i) Epibolic morphogenetic movements (Epiboly)- The epibolic morphogenetic movements occur only in the prospective ectodermal blastomeres, which have a inherent property of flattening and forming a cohering epithelial layer. Thus, epiboly may be referred to the motility displayed by amoebocytes. Further, besides flattening, due to multiplication and rearrangement of ectodermal cells, the ectoderm expands and extends in the antero-posterior direction and eventually, envelops the inwardly migrating prospective mesodermal and endodermal blastomeres. Such type of morphogenetic movement of ectodermal cells has been observed in holoblastic eggs (e.g., Porifera, cephalochordata and Amphibia) as well as, meroblastic eggs, during the gastrulation. In the rounded blastula of Amphioxus, frog, etc., the tendency of prospective ectodermal cells to extend antero-posteriorly produces an enveloping movement in the antero-posterior direction. As a result, the prospective ectodermal blastomeres, actually engulf and surround the inwardly moving presumptive notochordal, mesodermal blastomeres. In flattened blastulae of teleost fishes, reptiles, and birds, epibolic morphogenetic movements are concerned largely with antero-posterior extension, associated with peripheral migration and expansion of the ectodermal blastomerres.
Epiauxesis- In Ctenophora, in various spirally cleaving eggs (e.g., Annelids and Mollusca) and in some vertebrata (Pisces), a modified type of epibolic morphogenetic gastrular movement called epiausesis occurs (Lovtrup, 1974). Four conditions must be fulfilled for epiauxesis to occur, namely, absence of a hyaline membrane, presence of a surface to which the blastomeres adhere and on which they may spread, a certain intercellular adhesion and that they are mitotically active. In Annelida and Mollusca epiauxesis involves overgrowth of ectodermal (micromeres) over yolk-laden vegetal pole cells (macromeres). Overgrowth of micromeres not only depends upon the subdivision of the existing micromeres, but also upon the addition of new ones budding off from macromeres.
In certain animals where gastrulation occurs through invagination, epiauxesis may be involved in neurulation.
(ii) Embolic morphogenetic movements (Emboly)- The embolic morphogenetic movements are concerned with inward migration of prospective chorda-mesodermal and endodermal blastomeres from the external surface of the blastula and their extension along antero-posterior axis of the developing embryo. The tendency of inward migration of mesodermal and endodermal cells is inherent to these cells and there is a convincing reason for such a peculiar behaviour of these cells. Balinsky (1970) has explained the reason, that why do these blastomeres move inward, not to the outward direction in following manner: “…………….. in the stage preceding gastrulation (the blastula stage) the cells are in the form of an epithelium (the blastoderm) which is more or less distinctly polarized. As in typical epithelium, it has a distal surface, in contact with the external environment, which is different from the proximal surface facing the inner miliue of the embryo. Now it has been noticed (Gundernatsch, 1913), epithelium as ameboid or mesenchyme cells, the direction of movement is nearly always in the proximal end of the cell that is more likely to start forming pseudopodia and embarking an ameboid locomotion, and that is why in an intact embryo the movement of both mesoderm ane endoderm is inward and not outward.”
The inward migration of prospective chorda-mesodermal and endodermal blastomeres in different choardates may occur by following embolic methods:
A. Invagination- The process of insinking or infolding of a layer of cells (endoderm or mesoderm) to form a cavity called archenteron is called invagination. Invagination is a best mean of embolic morphogenetic movement in most of the animals, such as Amphioxus, amphibians, and birds, and however fundamentally, it differs in different animal groups in its achievement. Many attempts have been made throughout the present century to explain how and why the invaginative movement is accomlished. Certain important views regarding the cause of invagination are following:
(1) It has been assumed generally that continued increase in the number of cells by division and growth in the wall of blastula (viz., blastoderm) results in a lateral pressure among the cells, which causes an infolding of the weakest part of the blastula sphere. However, by this hypothesis, it is difficult to explain why the thicker vegetal cells should first invaginate. Moreover, it has also been noted that invagination could take place without the increase in cell number.
2. Rhumbler (1902) was probably the first to suggest that infolding (i.e., invagination) resulted from a change in shape of the vegetal cells.
3. Assheton (1966) suggested that the invagination might be brought about, if there were an attraction between cell of the blastula with the greatest force being exerted along a line passing through the periperally located nuclei.
(b) Certain modern views about the physiology of invagination- Recently many new interpreatations have been forwarded regarding the invagination by modern embryologists. The most important theories of invagination are following:
(i) Tension theory of Invagination- Moore and Burt (1939) and Moore (1941) attempted to analyze by operative means the question whether the entire blastula takes part in the gastrulation process, or whether the forces of gastrulation are localized in a special region. When the upper half or two thirds of the early gastrula is cut away leaving an intact vegetal or endodermal plate, gastrulation is not damaged. The experiment disposes of the idea that differences between the blastocoel fluid and the external medium play a part during invagination. In the cases where the endodermal plate was isolated by cutting away the remainder of embryo, the plate continued to roll up its rim, eventually closing to form a small spherical invaginated gastrula. These experiments led Moore and Burt to the conclusion that the forces of gastrulation do not act in a median plane through the poles of the embryo, but in the plane of the endodermal plate at right angles to the animal-vegetal axis. Thus, tension theory suggests that the forces having to do with invagination exist in the endodermal plate and in intensity and direction are disposed from the centre outward in the form of a radial gradient. It also suggests that the differential cohesion of cells may be an important factor in producing pocketing.
(ii) Inward cell migration theory- Holtfreter (1943, 1944) correlated the movements of the several properties of amphibian cell groups, and to the responses of these cell groups to stimuli emanating from their environment. The primary causes of gastrulation in the amphibians, according to him, is a higher relative alkalinity of the blastocoel fluid, which brings about local surface tension changes in the membranes of certain flask-shaped cells. These cells (e.g., endodermal cells) are attached at their-outer boundaries to the “surface coat” and exert contractile tension, when they elongate in the direction of blastocoel in response to the surface tension changes at their blastocoel.
1. The mechanism of the inward migration of mesenchyme cells during the gastrulation of sea urchin, has been studied in minutest details by microcinenatography (Gustafson and Kinnander, 1956). It has been found that the prospective mesenchymal micromeres exhibit a pulsatory activity, loosen contact with one another and with the hyaline layer, and move into the blastocoel before the vegetal plate as a whole shows signs of invagination. Once inside they throw out numerous thin, long, temporary, pseudopodia and move about, eventually taking up a characteristic pattern relating to the form of the skeleton finally produced.
Following the migration of the primary mesenchyme cells, i.e., the micromere descendants, the cells remaining in the vegetal plate do not lose contact with the hyaline layer and are typically pear-shaped; yet they exhibit a pulsatory activity and tend to round up, indicating a decreased contact among themselves. The periphery of this region consists of cells with full contact. Gustafson has suggested that for geometrical and mechanical reasons this combination of a fixed ring of cells surrounding an area of cells with decreased mutual contact brings about an invagination. This is a debatable point for it is uncertain to what extent the changes in cell contact primarily depend on (1) decreases in adhesion, (2) the effect of increase of tension at the surface produced by rounding up of cells, and (3) active pulsatory movements of cells.
2. In amphibians, the invagination of endodermal and mesodermal cells includes following processes- (i) The endodermal and mesodermal cells migrate inwardly by ameboid movement and during their migration the shape of cell changes temporarily, so that each cell becomes elongated, resembles with a flask or bottle. The change in the shape of these cells is not due to a lateral compression from the neighbouring cells, but is performed by the cells themselves. The microfilaments and microtubules of cytoplasm have significant role in such changes in the cell shape. (ii) The shape of the individual cells reflects the direction of their movement. (iii) The boundaries of the inner blastomeres of blastula are not continuous like the wax in a honeycomb, but, each cell is a discrete unit, interconnected with its neighbour only by slender processes. (iv) All peripheral cells or blastomeres of blastula remain firmly held together by a common surface film of plastic elasticity.
3. In chick, not all the cells in the primitive streak undergo the change in shape at the same time, while some cells become bottle-shaped, others retain the original broad columnar form. Eventually, the external ends of the bottle-shaped cells lose their contact with the surface and the cells slip downward, emerging on the inner surface of the epiblast. Presumably, the migrating cells are not held as firmly together at their external ends as the cells of the amphibian blastopore, so, instead of pulling the surface inward, they detach themselves from the surface without causing the formation of a pocket (the archenteron).
B. Involution and convergence- The word, involution, as used in gastrulation, denotes a ‘turning in” or inward rotation of cells which have migrated to the blastoporal margin due to the process of convergence. In doing so a migration of cells from outside surface of blastula to the external margins of the blastoporal lips takes place by process called convergence. Now, the cells which are located along the external margin of a blastoporal lip, move over the lip to the inside edge of the lip. This is called involution. The inturned or involuted cells, thus, are deposited on the inside of the embryo along the inner margin of the blastopore. In frog, for example, most of the chorda-mesodermal blastomeres first converge to the dorsal blastoporal lip and then rotate inwardly to occupy a position just beneath the ectodermal blastomeres. Likewise, in chick, the same essential movements are present, namely, a convergence of prospective mesodermal cells to the primitive streak and then an inward rotation of cells through the substance of the streak to the inside.
Further, the prospective mesodermal blastomeres of these animals are found to involute by performing a gliding movement. They are found to have also an inherent tendency to stretch in the antero-posterior direction, so, when prospective mesodermal cells rotate inwardly, they spread along the highly adhesive lower surface of the ectoderm in both direction.
C. Concrescence- The movement of masses of cells toward each other and their fusion into one cell mass is called concrescence. It occurs during the development of the feathers of birds.
D. Cell proliferation- The gastrulation of Amphioxus and frog, somehow depends on increase in cell number due to mitosis.
E. Polyinvagination or Ingression- The process in which individual or small groups of cells in different parts of the external layer of the blastula or blastodisc invaginate of ingress into the segmentation (blastocoel) cavity. That is there are several different and separate inward migrations of one or more cells. Polyinvagination occurs in reptiles, birds and eutherian mammals, during the formation of endoderm and mesoderm. It also occurs in prototherian mammals.
F. Cell transformation- In gastrular germ layer segregation, the transformation of the cell type is also involved. Both the sc-transformation [i.e., transformation of solocytes into colligocytes] and the lf-transformation[i.e., a transformation of lobocytes into filocytes] ar found to involved in germ-layer segregation. The sc-transformation occur in the gastrulation of stereoblastulae, while, lf transformation occur in the gastrulation of stereoblastulae, while, lf transformation involved in the mesoderm segregation in those cases (e.g., Echinoidea) where all the blastomeres remain attached to a hyaline membrane.
G. Divergence- The phenomenon of divergence is the opposite of convergence. For, example, after cells have involuted over the blastoporal lips during gastrulation, they migrate and diverge to their future positions within the developing embryo. This movement particularly is true of the lateral plate and ventral mesoderm in the frog, or of lateral plate and extra- embryonic mesoderm in the reptiles, birds or mammals.
H. Extension- During extension entire gastrula begins to elongate in the antero-posterior axis. The elongation of the presumptive neural and epidermal areas externally and of the notochordal, mesodermal and endodermal materials after they have moved inward beneath the neural plate ane epidermal material are the examples of extension. The extension of cellular masses is a pr0minent factor in gastrulation in all Chordata from Tunicata to Mammalia.