In this article we will discuss about:- 1. Introduction to Epithelial Layers 2. Thinning Out of Epithelial Layers 3. Local Thickenings of the Epithelial Layer 4. Separation of Epithelial Layers 5. Folding of the Epithelial Layer 6. Thickenings Followed by their Excavation to form Tubes or Vesicles 7. Breaking Up of Epithelial Layers to Produce Mesenchyme.
- Introduction to Epithelial Layers
- Thinning Out of Epithelial Layers
- Local Thickenings of the Epithelial Layer
- Separation of Epithelial Layers
- Folding of the Epithelial Layer
- Thickenings Followed by their Excavation to form Tubes or Vesicles
- Breaking Up of Epithelial Layers to Produce Mesenchyme
1. Introduction to Epithelial Layers:
During the early stages of cleavage the blastomeres are rather loosely connected to one another, but before gastrulation and the morphogenetic movements set in, the cleavage cells in most animals join to form more closely knit structures in the form of layers of cells—the epithelia.
Thus, the first morphogenetic movements involve epithelia, and also at later stages of development many organ rudiments are formed by epithelial layers. It will therefore be convenient at this stage to deal in greater detail with the processes of morphogenesis in the epithelial layers of the embryo.
2. Thinning Out of Epithelial Layers:
This process is a transformation which has an important role in gastrulation in the form of epiboly, the “overgrowth” of the surface of the embryo by presumptive ectoderm. In fact, growth (increase in living mass) is not involved, and the process is one of stretching of the epithelial layer, so that increase in surface proceeds at the expense of a thinning out of the epithelium.
Further expansion of the ectoderm occurs at the stage of neurulation, when the neural plate ectoderm disappears from the surface and the entire exterior of the embryo has to be covered by epidermis. The presumptive ectoderm of an amphibian blastula consists of several layers of cells; the number of cell layers becomes reduced during gastrulation and still further reduced during neurulation.
In the process, some of the cells originally lying in the interior become intercalated between the superficially lying cells. The whole change is thus brought about by a re-arrangement of the cells of the epithelium. After neurulation the epidermis in amphibians, as well as in many other vertebrates, consists of two layers of cells.
In invertebrates the epidermis is usually a single layer of columnar cells. Once the epithelium is reduced to two or even one layer of cells, a further thinning out may be achieved by the flattening of the cells constituting the layer.
3. Local Thickenings of the Epithelial Layer:
Local thickenings may be multiple, leading to the formation of a number of similar organs or structures within an epithelial layer; there may be a pair or several pairs of similar thickenings, as in the development of paired organs; or there may be only one organ rudiment starting its development as a thickening of the epithelium. The formation of a neural plate takes place in this way.
The thickening is brought about by the epithelial layer flowing, as it were, toward the region in which the organ rudiment is to appear, in this case the mid-dorsal region of the embryo. As the thickening becomes discernible, the epithelial cells may become elongated in a direction perpendicular to the surface of the epithelial layer.
This elongation of cells is very distinct in the neural plate. The cells become columnar, and in the case of the urodele amphibians, at least, single cells stretch from the external to the internal surface of the neural plate, in spite of its increased thickness.
This means that the formation of the neural plate involves a re-arrangement of the cells of the epithelial layer, because the ectoderm of the late gastrula in amphibians consists of several layers of cells, whereas in the neural plate a single layer of columnar cells is found. It has been suggested that the elongation of individual cells of the epithelium is the mechanism by which the thickening of the layer is achieved.
In the case of multiple organs, more than one thickening may develop simultaneously in the same sheet of epithelium; a clear-cut example of this may be seen in the development of the hair follicles of mammals. Before the appearance of the rudiments of the hair follicles, the epidermis is a two-layered sheet of epithelium, with cells more or less flattened and of uniform thickness. Then, rather suddenly, numerous concentrations become visible, scattered at more or less regular intervals throughout the epithelial layer.
In each concentration the cells lie closer together, and the individual cells, especially in the lower layer of epithelium, are less flat than before and approach the cuboidal or even the columnar shape. The concentration of cells is brought about by the cells coming closer together and not by a local increase in the proliferation of cells.
This assertion has been proved by two different methods. Counting mitoses in the early hair follicles and the epidermis in between did not show greater numbers of mitoses in the follicles. Providing the epidermis with radioactive precursor (tritiated thymidine) showed that nuclei taking up thymidine (a reliable sign of the cells preparing for mitosis) were evenly distributed at the time of hair follicle formation.
After the epidermal thickenings have appeared, however, growth and cell proliferation set in. As a result, the number of layers of cells in the epidermal thickenings increases. What was originally a thickened area becomes a rounded peg intruding into the dermis.
4. Separation of Epithelial Layers:
Epithelial layers or masses of cells may be subdivided into parts by the appearance of crevices between groups of cells, the cells on the opposite sides of a crevice losing connection with one another. The crevices may appear at any spot, but perhaps the most common occurrence is for the crevices to be either parallel to the surface of the epithelium or perpendicular to it.
In the first case, the epithelial layer is split into two layers lying one on top of the other. The original crevice may be increased by the secretion of fluid into it and may become a more or less spacious cavity. This is the case with the formation of the parietal and visceral layers of the lateral plate mesoderm and the coelomic cavity between them.
An instance of crevices appearing perpendicular to the surface of the epithelium is found in the development of the mesodermal somites. The upper edge of the mesodermal mantle becomes subdivided into segments by crevices running between cells perpendicular to the surface of the epithelium and at the same time perpendicular to the main axis of the body.
The masses of cells thus formed (the somites) are of approximately the same size. This suggests that each somite may possibly be formed around a center of attraction whose force is limited, so that only a certain number of cells can be kept together, the cells losing connection with one another where the attraction of the center is too weak. A rhythmic pattern of differentiation could thus be produced.
Splitting of epithelial masses often follows the formation of local thickenings. Thus, the neural tube eventually loses its connection with the epidermis.
5. Folding of the Epithelial Layer:
The folding of epithelial layers is perhaps the most important form of morphogenetic movement. The invagination of the archenteron as it occurs in animals such as the sea urchin or Amphioxus and also in the early stages of gastrulation in the amphibians is obviously a case of in-folding of an epithelium. The in-folding is generally accepted to be the result of a change in the shape of the cells.
If cells of a columnar epithelium contract at their external ends and expand at their internal ends, so that instead of being prismatic they become pyramidal, the whole epithelial layer must inevitably change its shape and become bent in toward the interior.
This is what actually takes place in the blastopore region; the external parts of the cells here become attenuated, and the opposite ends of the cells expand. The cells acquire a bottle shape, with the neck of the bottle keeping the cells in touch with the surface, while the bodies of the cells are drawn away from the surface.
The area occupied by a given group of cells on the surface of the embryo contracts to a small fraction of what it had been originally. The contraction at the external surface during the formation of the blastopore in the frog embryo is so violent that the plasmalemma of the cells is thrown into deep folds, and the deeper sections of the folds are nipped off as vesicles which sink into the cytoplasm.
To understand the final result one has to consider, however, one further factor – namely, that the cells are held together at their external (superficial) ends. The cells at the surface of the blastoderm are more firmly held together than the cells which do not reach the surface.
It has been claimed that the adhesion of cells in the surface layer is due to the presence of an extracellular continuous cuticular membrane, the “surface coat,” to which all the cells reaching the surface are firmly connected. With the aid of the electron microscope it can be shown that no such membrane exists, but that the cells reaching the surface adhere to one another very closely just at their distal ends and are probably joined by some cementing substance, which is not easily broken.
As a result, when numerous cells in the blastopore region start moving inward, they cause the surface of the embryo to be pulled inward with the formation of a pocket—the rudiment of the archenteron. Similarly, when the movement of the cells is a general expansion or concentration, the cells, being joined together at their external ends, move in concord, as if borne by a common force.
The folding of epithelial layers can occur either in the form of linear folds, resulting in the formation of grooves, or as approximately round depressions, forming pockets. The direction of folding is, as a rule, toward the originally proximal surface of the epithelial layer.
In the ectoderm, and later in the epidermis, the folding is therefore directed inward. In the invaginated parts of the embryo, as in the whole of the endoderm, the folding is directed outward, that is, away from the lumen of the alimentary canal.
If a linear fold becomes closed in and separated from the original epithelial layer, the resulting structure is a hollow tube. If a pocket-like depression becomes closed in and separated from the original epithelial layer, the result is a hollow vesicle.
An example of the first formation is the neural tube and of the second formation is the rudiment of the inner ear—the ear vesicle. Most of the glands are developed as pocket-like invaginations of the epithelium, an indication of how common this type of morphogenetic process is.
6. Thickenings Followed by their Excavation to form Tubes or Vesicles:
By no means are tubes or vesicles always produced by in-folding of the epithelial layer. Quite often the initial stage is a solid thickening of the epithelium. Such a solid thickening may acquire an internal cavity secondarily by the separation of cells in the middle.
This cavity may or may not be connected with the external surface of the epithelium. It is remarkable that one and the same organ in different animals may develop in different ways – as an in-folding in some and as a solid thickening becoming excavated later in others.
This is the case with the development of the neural tube in various vertebrates. It is formed by in-folding in the elasmobranchs, in the urodele amphibians, and in the amniotes; it develops as a longitudinal thickening that later separates itself from the epidermis and acquires a central cavity in the Myxinoidea and in the bony fishes.
In the Anura a sort of intermediate state is found, the groove being very shallow and the neural tube being, in part, solid at the beginning. Other organs such as the eye, the ear, and the lens may develop either as hollow pocket-like invaginations or as solid masses of cells. The underlying mechanism of both formations must therefore be similar in nature.
If the thickening of an epithelial layer is directed outward, it takes the form of an “outgrowth.” The formation of an outgrowth is probably never caused by a local increase in growth rate, that is, increased multiplication or increase in size of the cells. Much as in all other cases of organ formation, the outgrowths are caused by the concentration of cells and their rearrangement.
Such outgrowths are formed on the surface of the chorion in mammals (the chorionic villi) or on the gills in amphibians and fishes (the gill filaments). The outgrowths may be simple and finger-like or may develop secondary outgrowths when the whole becomes dendritic or plumose.
The tips of the epithelial outgrowths may be solid, but once they have appeared, they become hollowed out, so that connective tissue and blood vessels may penetrate into them. Blood capillaries develop in a similar way – first as solid strands of endothelial cells, pushing forward from existing blood vessels and being hollowed out later.
The cavity in this case is the lumen of the capillary. The speed with which the solid outgrowths are hollowed out differs from case to case, and no strict boundary can be drawn between these and outwardly directed folds or evaginations which are hollow from the start.
7. Breaking Up of Epithelial Layers to Produce Mesenchyme:
The breaking up of epithelial layers is a very important morphogenetic process, even if it does not lead directly to the formation of organ rudiments, because it furnishes the material from which other organ rudiments may develop. Transformation of epithelial cells into mesenchyme – type cells may start very early in development and may be involved in the formation of the germinal layers during gastrulation.
An epithelial layer or a portion of it may sometimes break up into mesenchyme altogether, so that the epithelium as such disappears or a gap is left in the previously continuous layer. An example of this process is found in the development of the neural crest.
A part of the ectodermal epithelium forming the edge of the neural folds splits up into disconnected cells, leaving a gap between the neural tube and the epidermis. (Sometimes, however, the neural crest cells are given off from the dorsal wall of the neural tube after its separation from the epidermis.) The other possibility is for individual cells to slip out from an epithelial layer, so that the latter remains intact though depleted of part of its cells.
Invagination of the endoderm and mesoderm into the interior of the embryo, as we have seen, cannot always be interpreted as the in-folding of an epithelial layer. It often takes the form of immigration of individual cells into the blastocoele, as in the case of the primitive streak of amniotes. A study of the cells in the primitive streak of birds with the electron microscope shows that the behavior of migrating cells in this case is very similar to the behavior of cells in the amphibian blastopore.
The cells of the epiblast of the chick are roughly cuboidal (or rather, low columnar). The cells participating in the downward migration start by changing their shape. They elongate in a vertical direction, their inner ends expand, and their outer ends become narrowed like necks of bottles reaching to the surface of the epiblast.
Contrary to what happens in the amphibian blastopore, however, 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 contact with the surface, and the cells slip downward, emerging on the under (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 that instead of pulling the surface inward, they detach themselves from the surface without causing the formation of a pocket (the archenteron). It follows that the presence or absence of an archenteric cavity may be accounted for by a different degree of mutual adhesion of cells involved in the inward movement at the time of gastrulation.
The mechanism of the migration of mesenchyme cells has also been studied in the early gastrulae of sea urchin embryos. Cinemicrography was used in these studies. Gastrulae of sea urchins were photographed at 1- to 18-second intervals.
When the film, taken in this way, is projected at normal speed, the movements of the cells of the gastrula become accelerated and thus are made more readily visible. Before gastrulation, the blastoderm of a sea urchin embryo is a regular columnar epithelium, somewhat thickened (the cells being larger and taller) at the vegetal pole.
The cells of the primary mesenchyme start “bubbling” or “pulsating” on their inner surface (the surface facing the blastocoele); that is, the cytoplasm produces rounded protrusions or broad pseudopods which may collapse and then be formed again. Of course, the bulging of the cytoplasm on the inner surface cannot proceed without the cytoplasm being withdrawn from those parts of the cells which are nearer to the exterior.
It appears that the adhesion of these cells to other cells is lost or reduced. Eventually, the mesenchymes cells are released into the blastocoele, where they become spherical in shape—an indication of complete or almost complete loss of adhesion between them.