Sexual Differentiation
The sexual differentiation takes place in consecutive steps, viz. chromosomal, gonadal, somatic and psychic sex differentiation.
Though, the two sexes can be recognized by their morphological features, the complete sexual differentiation includes the total morphological, physiological and psychological differences that distinguish male and female.
In all living organisms, reproduction is essential to continuity of the species. In mammals, reproduction depends upon the union of anisogametes produced by dimorphic individuals designated as male and female.
With the union of sperm and the ovum there is formation of zygote which undergoes several mitoic divisions. The period of embryo consists of the time from blastocyst development until there is differentiation of the organ systems in the embryo.
Chromosomal sexual differentiation
Chromosomal sex is determined by a single pair of sex chromosomes. The sex having similar sex chromosomes is the homozygous sex, whereas the one with dissimilar sex chromosomes is heterozygous sex. Among mammals the female is the homogametic sex (XX) and the male is heterogametic sex (XY). In poultry, hens are heterozygous with ZW and males are homozygous with ZZ chromosomes.
The chromosomal sex of an individual is determined at the time of fertilization itself, though at this stage there is no morphological or physiological differentiation.
Chromosomal sex can be ascertained by examining interphase nuclei of somatic cells. The nuclei of somatic cells containing two X chromosomes contain a characteristic chromatin mass, the ‘Barr body’. Thus, the females are chromatin positive (nuclei containing Barr bodies), whereas the males are chromatin negative.
Gonadal Sexual differentiation
Although the genetic sex of the conceptus is unalterably determined by its sex chromosome complement, each embryo is potentially capable of developing the genitalia of either sex, since the primitive gonad has all the cellular elements to give origin to a testicle or an ovary. The differentiation of sexual characteristics depends upon the quantitative relationship between male and female determining genes and their integration with the internal environment.
Genes associated with female characteristics are believed to be located on the X-chromosome. Thus XX chromosomes are necessary for the differentiation of the normal ovary. Genes for male development are located on autosomes and on the Y chromosome. The presence of a Y chromosome is responsible for the differentiation of the testes.
The mammalian embryo has an inherent tendency to develop as a female. Therefore, castration of male embryo at early stage of embryogenesis results in their development as females.
Gonadogenesis begins with the formation of the genital ridges (in close association with the mesonephros). Primordial germ cells migrate from the yolk sac endoderm to the genital ridges. The gonads at this stage are still sexually bipotent and consist of an inner medulla and an outer cortex. Primordial germ cells invade the medulla to form primary sex cords. At this time, gonadal differentiation takes place, and the gonad loses its bipotentiality.
When genes for masculanity of an XY individual prevail, the medulla persists, cortical development is inhibited, and a testis develops.
When genes for femaleness predominate in the XX embryo, the medulla is inhibited, and the cortex develops. The persisting pri-modial germ cells in the epithelium invade the cortex as the secondary sex cords, and an ovary develops.
During the indifferent stage the gonad is potentially capable of either testis or ovary formation. The ovaries/testes being the primary sex organs but once there is differentiation of gonads is complete, the individual/organism can be distinguished as MALE or FEMALE.
Somatic sexual differentiation
Following gonadal differentiation, the accessory genitalia develop under the influence of the gonad. In an indifferent stage, there are 2 duct systems, the Mullerian duct system and the Wolffian duct system.
When the testis has formed, the female (Mullerian) duct system regresses to vestigial rudiments and the male (Wolffian) elements develop under the influence of secretions from the newly formed testis.
When an ovary has formed, the absence of the testicular morphogenic agents permit the Mullerian system to develop, and the Wolffian duct and tubules regress to vestigial rudiments.
From these two duct systems eventually there is development of the complete male/female reproductive organs. Thus at the time of birth of individual has morphologically differentiable identification either male or of reproductive organs.
Psychic sex differentiation
At the time of birth despite of having either male or female reproductive organs an individual can not behave, feel or act like a male or the female. The organism does not have a sex drive or libido. The psychic sex establishment is the final step in the differentiation of the 2 sexes. The psychic sex the sexual behavior is basically under the control of gonadotropic and gonadal hormones secreted in proper amount and specific ratio. In addition some external factors like season/ also play an important role.
Prepubertal gonadectomy usually prevents mating behavior. Ovariectomy of sexually mature females immediately abolishes mating behavior. Adequate doses of estrogen administered to ovariectomized adult female restores manifestation of estrus and mating behavior. Similarly in males prepubertal orchiectomy prevents normal mating patterns. Adult castration is followed by a gonadal diminution of copulatory responses and sex drive.