Biochemistry - Chemical Reactions in Living Cells, Vol. 3 - D. Metzler 1980

Cell Growth, Differentiation, and Chemical Communication
Tissue Differentiation and Developmental Biology
Animal Embryonic Development

When considering The process of embryonic Cell Differentiation, it should first be noted that the nearly spherical egg (ovum) exhibits a strong polarity. The Nucleus is located closer to one end, traditionally called the animal pole. The opposite end is termed the vegetal pole; in many eggs, it contains a high concentration of yolk granules. In amphibians, the animal pole of the egg is heavily pigmented, whereas the vegetal pole is less pigmented, and on one side, just below the equatorial line, There is a grey crescent. In some animals, spermatozoa penetrate the egg in a region opposite to this crescent (Fig. 16-13). The grey crescent corresponds to the future DORSAL SIDE OF the Organism, while the opposite region of the egg represents the future ventral side. There is compelling evidence that many substances in the mature egg are unevenly distributed in a bilaterally symmetrical pattern. The critical role of this distribution is demonstrated by the following fact: centrifugation of the egg prior to Fertilization frequently leads to The formation of abnormal embryos due to the displacement of modified Ribosomes and other cellular components. From a biochemical standpoint, the process of egg fertilization is highly complex. The final Stages of the last mitotic division are often delayed until the sperm penetrates the egg. This latter event somehow triggers the "activation" of the egg. In lower organisms, a similar activation can often be induced by chemical or physical Treatment of the unfertilized egg, thereby stimulating The Development of parthenogenetic offspring.

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FIG. 16-13. Cleavage of the egg and gastrula formation. Left: fertilization and cleavage of an amphibian egg; middle right: spiral cleavage typical of Mollusks; bottom right: gastrula stage in Echinoderms and certain primitive invertebrates.

The fertilized (activated) egg, or ovum, undergoes several mitotic divisions not accompanied by an overall increase in volume. This process is known as cleavage. While the number of Cells increases and The amount of DNA doubles with each division, the total volume of the resulting cell cluster remains equal to the original volume of the uncleaved egg (Fig. 16-13). Soon, the process reaches a stage characterized by the formation of an internal cavity surrounded by a single layer of cells (referred to at this stage as blastomeres); this is the blastula. In sea urchins, the blastula consists of a single cell layer, whereas in other organisms, such as frogs, the cells are arranged in two or more layers. In mammals, the process begins with the formation of a compact mass of cells (the morula), which subsequently develops into a blastocyst, i.e., a spherical Structure with an internal cavity.

At the next stage of embryonic development, characteristic of many invertebrates and amphibians, an invagination appears at the vegetal pole of the blastula and gradually deepens, thereby forming the gastrula. At this developmental stage, the ectoderm and endoderm cell layers become clearly distinguishable in the embryo. The cavity formed during Gastrulation that opens to the exterior is called the gastrocoel (archenteron); it will eventually give rise to the gastrointestinal tract, or enteron. In frogs, gastrulation proceeds in a more complex manner, while in humans, the process is not only more intricate but also follows a somewhat different pathway.

In all animals except the most primitive ones, a Third Layer of mesosomal cells forms between the ectoderm and endoderm. Subsequently, these three germ layers differentiate as follows: the ectoderm gives rise to the Skin and Nervous system; the mesoderm forms the Skeleton, Muscles, Connective Tissue, and Circulatory system; and the endoderm develops into the digestive tract, Lungs, other Internal Organs, and Germ Cells.

The experiments of Gurdon [148, 149] provided a striking demonstration of the totipotency of differentiated amphibian embryonic cells. Using a nuclear transplantation technique, the author replaced the nucleus of an egg cell with a nucleus derived from intestinal epithelial cells or other Tissues.

In certain instances, normal toads developed from such eggs. This demonstrated that differentiated cells contain all the Genetic information required for the development of an adult organism. However, similar results could not be achieved using neuronal nuclei. This implies that the possibility of irreversible differentiation in certain cell types cannot be entirely ruled out.