Essay on the Growth and Differentiation in Animal Cells

Although some of the lower multicellular animals reproduce by the development of new individuals from single, uniparental cells or aggregates of cells, by far the most common method is that in which a male reproductive cell unites with a female reproductive cell to form a zygote.

This new cell, which is made up of cellular materials from both parents, undergoes successive divisions to form an embryo. Regional differentiation of cells occurs in such a way that sheets of cells, the primary germ layers, are soon formed.

In all except the least complex metazoa, three such germ layers are developed: the ectoderm, the endoderm, and the mesoderm. It is as though the embryo covering of organs which attach to it, while that part which lies next to the ectoderm gives rise to structures such as bones and muscles, and to the inner layer to the skin. Tissue of the higher animal body After development it well advanced and the many cells of the higher animal body have become in large part highly specialized, they tend to be associated together as tissues according to function.

Four types of animal tissues are generally recognized by histologist. These are termed epithelial, nervous, muscular, and connective.

Nervous tissue is derived entirely from ectoderm, muscle and connective tissue generally arise from medoderm, and epithelium may come from ectoderm, mesoderm, or endoderm, depending upon its location in the body.

Epithelial tissue is essentially protective in its function, and it serves to cover or line surfaces. One is inclined to think only of the external body surface in this connection. The outermost layer or layers of cell are epithelial in multicellular animals, but great many- internal surfaces exist, and epithelium is also found as a protective tissue for these. The gastro-intestinal tract is lined in this fashion on its internal surface.

The external; surfaces of organs which lie within or adjacent to the body cavity of a given coelomate animal are covered by peritoneum, an epithelium of single-cell thickness. Blood vessels and tubules of various sorts exhibit this characteristic also.

Epithelium frequent’ assumes other roles in addition to that of protection. One of these is that of secretion; in which certain epithelial cells produce some particular substance or substances: an aggregation of such specialized cells which perform a common secretary function is called a gland. Another secondary role played by epithelial cells is that of absorption, as is the case in cells which surround the lumen of the small intestine.

We have already seen that in the movement of digested foods from the small intestine of man to the blood and lymph streams, they must be absorbed into the epithelium, a process which is, in a sense, somewhat the reverse of secretion. It is the specialized function of nervous tissue to transmit impulses throughout the animal body.

In their organization, nerve cells form a coordinated system which allows for the reception of external or internal stimuli, the transmission of impulses arising from such stimuli, and an orderly distribution of these impulses to organs of action. The typical cell is well adapted to this function, consisting as it does of a cell body whose cytoplasm may possibly extend for considerable distances in the form of nerve fibres.

These fibres maintain connections with fibres of other nerve cells, and there is thus a systematic mechanism for reception, transmission, and action. It is significant those nervous tissues are found in all multicellular animals except sponges, and that the morphology of nerve cells is remarkably uniform among the animals that possess them.

Like nervous tissue, muscle is found in all multicellular animals except sponges. It is specialized for contraction, and the animal possessing it thus is able to exhibit a considerable degree of motility.

Muscle cells are somewhat elongate, and contraction occurs when a complex series of chemical reactions within a given cell cause it to become shortened and thickened. In all but the least complex animals, many muscle cells may be bound together to form a muscle, in which case their contraction is very highly coordinated in the performance of work. In the vertebrates three types of muscle cells are recognized.

There are those which axe striated, so called because small bands or striation axe seen when the cell is highly magnified, those which are smooth, lacking such striations, and a third type are known as cardiac cells, which are found only in the heart.

Striated cells, or fibres, are usually fairly long and are multinucleate. They are associated with the endoskeleton in vertebrates, to which they attach in groups as muscles. Smooth muscle cells are found in the internal organs. They are uninucleate, and contract much more slowly, as a rule, than do striated fibres.

They may be grouped together in sheets or bands, or they may exist as somewhat isolated units. Cardiac muscle presents a rather complex, branching appearance, with individual cells lying alongside and across one another in close contact. Muscle cells or fibres usually contract through nerve-transmitted stimuli.

As the name implies, connective tissues serve chiefly to bind the other tissues together in the organism, although some are specialized for other functions, as is described below. One characteristic which all connective tissues share is that nonliving fibres are closely associated with cells, both of which are surrounded by a nonliving matrix.

Both the fibres and the matrix are produced by the cells. Three general types of connective tissues are recognized by histologists, namely, binding, supporting, and fluid tissues. Binding tissue serves to connect the outer epithelium to underlying tissues such as muscle; it ties nerve fibres into bundles, and so on Ligaments and tendons which connect bones to each other and to muscles, respectively, represent a type of binding connective tissue in which fibres are sufficiently numerous that there replace much of the matrix ordinarily present. Supporting tissue is represented in the higher vertebrates by bone in which the matrix becomes impregnated with calcium salts and is thus solid, and by cartilage, in which the matrix is fewer firms.

It should be bone in mind that supporting tissues, although unusual compact, contain living cells which are continuously active in maintaining the fibres and matrix. In fluid connective tissues, of which the blood of vertebrates is most typical, the matrix exists as a liquid and fibres are only potentially present in the form of a blood protein called fibrinogen. Whenever blood is induced to clot through the initiation of certain complex reactions, fibrinogen is precipitated out of solution and fibres are formed.

A blood clot consists of these fibres plus any blood cells that may become enmeshed in the fibrous network. The various types of blood cells represent the living portion of this fluid tissue. The four types of tissue are bound together within the animal body in the formation of organs. These layers, along with blood vessels and smooth muscle fibres.

These layers, along with blood vessels and nerves, are held together by connective tissue. Even in an organ such as a muscle, where one type of tissue predominates, other tissues are present. Binding connective tissue ties the muscle cells together in this case, and nerve fibres supply them with impulses.