In this article we will discuss about the morphogenetic movements of embryo during gastrulation in animals.
During cleavage and up to the blastula stage the embryo retains roughly the shape of the egg from which it started developing. It is spherical in most cases, sometimes oval if the egg is oval, or elongated, as in the case of many insects and Cephalopods among the molluscs, which have elongated eggs. The internal structure of the blastula is also simple, consisting, as it does, of one layer of cells around a cavity. (Special cases are present in some animals with large yolky eggs, such as birds).
With the onset of gastrulation the embryos structure changes. Through the development of the archenteron and then the separation of the mesoderm, the embryo acquires a more complicated internal structure, departing from the geometric simplicity of the blastula of animals with oligolecithal eggs. The structure becomes further complicated with the formation of the primary organ rudiments, which closely follows gastrulation.
Gradually the external shape of the embryo starts to change. As primary organ rudiments give rise to secondary and tertiary organ rudiments, the internal structure and the external shape of the embryo approach those of the adult animal. (The embryo or larva may leave the egg long before the final goal is achieved.)
Thus, the new organism not only acquires a diversity of parts that did not exist in the egg but also acquires the form typical of an animal of its species. By form we mean not only the external shape of the animal but also the structural organization, that is, the body’s being composed of a number of parts, placed in a typical disposition inside and on the surface of the animal’s body.
The new elements of the embryo’s organization appearing in gastrulation (the archenteron, etc.) are produced by movements and changes in the shape of cells and groups of cells of the embryo. These movements and changes in shape are also involved in subsequent processes of form creation.
The movements which we are referring to are very different from the movements of parts of an adult animal. Whereas the movement of parts of an adult is usually of a reversible nature, the gastrulation movements are irreversible; each part remains in the position into which it was brought by the preceding movement.
As a result of the movements, the structure of the embryo is changed, or in other words, new structural elements are created, such as the archenteron, the neural tube, and the notochord, in place of the simple layer of cells found in the blastula stage. The movements have created new shapes, new forms. They have therefore been designated as the morphogenetic movements.
The morphogenetic movements appear to be movements of large parts of the whole embryo, which stretch, fold, contract, or expand. The question arises, how are these movements achieved? They cannot be ascribed to contractility in the narrow sense of muscle contractility.
Neither can they be interpreted as an ameboid movement of the embryo as a whole, because each moving part consists of numerous cells, and the movement of the whole, we should expect, would be an integrated result of the movements of the individual cells.
That gastrulation cannot be a function of the embryo as a whole has been proved by investigating isolated parts of the young gastrula. We know that the presumptive ectoderm contributes to gastrulation by expanding its surface. The expansion is an active process depending on the presumptive ectoderm itself.
If large pieces of the animal region of an amphibian blastula or early gastrula are cut out and cultivated in a suitable medium, the presumptive ectoderm rounds itself up into a vesicle, and later the epithelium of this vesicle increases its surface greatly and is thrown into a series of irregular folds as it does so.
Also, if presumptive ectoderm is combined with cells of presumptive endoderm and mesoderm in such a proportion that the ectoderm is far in excess of the endodermal and mesodermal parts which it has to cover, the ectoderm tends to form folds. These experiments show that the expansion of the ectoderm is active and proceeds independently of the other movements involved in gastrulation.
Similarly, the movements of invagination can be performed by parts of the blastoderm independently of their surroundings. The most suitable method of testing this is to transplant small pieces of the dorsal lip of the blastopore into some other part of the embryo, the animal region or the ventral part of the marginal zone.
The transplanted piece will invaginate and form an archenteric cavity which may be completely independent of the archenteric cavity of the host embryo. The mechanism by which the invagination is achieved is the change in shape and the movements of the cells of the marginal zone.
The change in the shape of the cells is not due to forces exercised by the embryo as a whole but is performed by the cells themselves. This property is best seen if the cells are isolated by tearing the embryo to pieces; the cells preserve their shape, and moreover, the in-folding of the surface layer may even be facilitated by releasing a piece of it from its connections with the surrounding parts.