Useful notes on the Origin of Primordial Germ Cells !
Gametogenesis is arbitrarily designated as the first stage of embryonic development.
In embryology, the gametes are usually discussed first, as they provide both the blue print and the raw material from which the embryo is formed.
Gametogenesis i.e. formation of gametes in the sexes is tailored to their future roles in reproduction. The female gamete is usually non-mobile, larger and nutrient filled cell, the ovum or egg.
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The female gamete must be competent to be fertilized, which means that it must develop a number of specialized properties to enable it to interact with the sperm. The male gamete is usually small and mobile sex cell, the spermatozoon or sperm. The formation of female gamete, ova or egg is known as oogenesis, whereas formation of male gamete, sperm is termed as spermatogenesis.
In some animals, the spermatozoa and ova are produced by a single individual which is called hermaphrodite or monoecious or bisexual, whereas in others the two types of gametes are produced by different individuals, and such individuals are called Dioecious or unisexual. This type of seperation of individuals is called gonochorism.
Both classes of gametes, spermatozoon and ova make an equal contribution to the nucleus of the zygote. It is said that egg and sperm possess the ‘information’ that is needed to build a new organism in the encoded form. During the development of egg, the encoded information is decoded. The decoding or reading of the information is equivalent to the process of ontogenetic development (i.e. transformation of zygote into new adult individual).
Another aspect of gametogenesis requires that chromosome number be reduced from the diploid number to the haploid condition. Sexual reproduction involves union of gametes from two different individuals, so either gamete possesses one half the number of chromosomes of the parents.
The reduction of chromosome number is accomplished by meiotic (Z-44/D.B.) division. In both the sexes of initial cells (germinal cells) giving rise to the gametes are very similar, and the steps in the production of gametes include (i) proliferation of cells by mitosis (ii) growth and (iii) maturation.
Origin of Primordial Germ Cells:
The initial cells of the early embryo which migrate to a new site to produce sperm or ova are called primordial germ cells. The primordial germ cells of both sexes are indistinguishable from one another. The primordial germ cells may arise at some distance from the presumptive gonads. After the gonads are formed, the germ cells migrate into them, become established and increase in number by mitosis. Regarding the place of origin of primordial germ cells, a great costroversy exists among the embryologists.
(1) In Lepidosteus and Amia the germ cells arise from the endoderm, whereas in Petromyzon the germ cells derive from the coelomic epithelium.
(2) In urodeles, the germ cells originate in the postero-ventral part of the lateral mesoderm (Nienwkoop, 1946). In anurans, the germplasm of primordial germ cells originates before cleavage, within the subtropical area of the vegetal pole of the egg. In amphibians, the primordial germ cells migrate to the gonads by amoeboid movements.
(3) In reptiles and birds, the germ cells have been assigned an extragonadal origin. In chick, the primordial germ cells are said to originate from the endodermal layer of the extra-embryonic part of the blastoderm just anterior to the head region of the embryo.
The primordial germ cells have been observed to leave the endoderm epithelium to move slowly into the space between endoderm and mesoderm, and then to penetrate into the blood vessels of the area vasculosa, via the blood vessel, they migrate into the developing gonad.
Origin of primordial germ cells in mammals:
In mammals the primordial germ cells have extragonadal origin. Earlier it was believed that in mouse the germ cells originated from the germinal epithelium of the gonad, but recent studies suggest that in mouse like the vertebrates, germ cells originate into the endoderm before migrating into the gonad. By utilizing the marking capacity of alkaline phosphatase enzyme of the germ cells, it was possible to determine the location of these cells within the caudal endoderm and their subsequent migration into the developing gonad.
Similar histochemical observation on the primordial germ cells of the human embryo have revealed that the primordial germ cells originate in the endodermal epithelium of the yolk sac in the vicinity of the allantoic stalk and from there these germ cells migrate into the adjoining mesenchyme and eventually take up their position in the germinal ridges (Witschi, 1948).
Thus, it becomes evident that the primordial germ cells have an extragonadal origin in most invertebrates and vertebrates. Then they migrate by their own or float passively through the blood vessels to reach their final abode in the gonads.
According to some workers, these primordial germ cells later give rise to the functional gametes of adult animals. But some other workers have found that these primordial germ cells degenerate before they are converted into functional gametes, and certain other cells of the gonads later on give origin to functional germ cells.