In recent years sex determination has been differentiated from sex differentiation, and sex determination mechanism is explained more on the basis of the specific genes located on sex chromosomes and autosomes. Sex determination is recognized as a process in which signals are initiated for male or female developmental patterns.
During sex differentiation, events occur in definite pathways leading to the development of male and female phenotypes and secondary sexual characters. Significant progress has been made in understanding the mechanism of sex in human beings and other mammals and new genes have been identified.
The primary germ cells in mammals are located on the floor of the yolk sac. From this place, they migrate to the genital ridges that develop dorsal to the hindgut. In females the germ cells become completely surrounded by follicle cells.
These germ cells in later stages of development grow enormously to become oocytes. In males the germ cells associate with cells of the genital ridge in a topological configuration similar to that of seminiferous tubules (Fig. 14). In this form the germ cells differentiate into spermatogonia.
Germ cells can differentiate into oocytes or spermatogonia irrespective of the fact that whether they carry XX (female) chromosomal component or XY (male) configuration of chromosomes. Their fate is determined by their contact with the genital ridge.
This indicates cellular interaction determining the pathway of differentiation. Male embryos form genital ridge cells with the ability to direct differentiation to spermatogonia whereas female embryos form genital ridge cells with the ability to direct oocyte differentiation. Germ cells of either XX or XY embryos respond to these signals.
A specific protein or family of proteins referred to as H-Y antigen plays a role in the association of germ cells with the tissue of the genital ridge. H-Y antigen was first recognized by specific antibodies that react far more strongly with male embryonic tissue than with female embryonic tissue. H- Y antigen is the product of a gene on the X-chromosome that is found in both males and females.
However, a gene on the Y-chromosome found only in the males is required to activate the H-Y gene and thus it is expressed only in male embryo. Antibodies specific to H-Y antigen lend evidence that the H-Y antigen controls in males. When dissociated cells of newborn male rat testes are coated with the antibody, they re-associate in a form similar to the ovaries. The germ cells become completely surrounded by follicular cells.
If the antibody is absent, the testes cells associate in such a way as to form tubules from which they come. These results show that the H-Y antigen on the surface of the genital ridge cells of male embryo leads to the male tubular configuration and directs all subsequent differentiation of germ cells in the pathway to form spermatogonia.
The genital ridges of XX individuals direct germ cells to differentiate into oocytes whereas that of XY individuals directs germ cells to differentiate into spermatogonia irrespective of their sex chromosomes complement.
The flexibility of differentiation of germ cells can also be observed in some fish. In sheep-shead fish, all young fish start life as small pink females. As they grow older, the color changes to black and the sex changes to males. In the beginning the germ cells differentiate into eggs whereas later in life they differentiate into sperm. In the sea bass, females remain as females in the company of males.
The visual stimulus generated by the presence of the male ensures the hormonal balance directs the differentiation of the germ cells into eggs. Removal of the male results in females changing their sex to become males.
The first transformed male then represses all the others. In that individual, the germ cells now differentiate into sperm. Although the final form of sperm and eggs differs about as much as any two differentiated cells, germ cells clearly are poised to follow either pathway. The final decision comes from the hormonal balance in the fish.
Hormonal Control of Sex Differentiation:
The hormonal system that regulates the internal or physiological environment of the organism does not directly influence the fundamental process of sex determination. It is important, however, to the development of secondary sexual characters. Sex hormones are synthesized by the ovaries, testes and adrenal glands with stimulation from the pituitary hormones.
The adrenals produce steroids that are chemically related to those of the gonads and that also influence the secondary sex characters. Ovaries and testes each have a dual function. They produce gametes and sex hormones. Sex hormones influence the development of secondary sex characters.
Sperms and eggs are produced within the body and require ducts to find their way out. In mammals, the genital ducts differentiate from primitive excretory ducts of the embryo. The oviducts are derived from the Mullerian ducts while the vas deferens develop from the Wolffian duct. In early embryos both these ducts are present but under the influence of hormones, in the later stages of development, one of these two ducts degenerates (Fig. 15).
In male mammals, the gonads produce the steroid hormone testosterone, which circulating through blood transforms the Wolffian duct into the vas deferens and prevents it from degenerating. Female embryos produce less testosterone, and in them the Woffian duct degenerates.
In males the gonads also produce a peptide hormone ‘X’ that directs the degeneration of the Mullerian duct so that the female organs are not formed. In females, hormone X is not produced and therefore, the Mullerian duct differentiates into the oviduct.