Sexual Differentiation - Anatomy & Physiology

Genetic Sex

• Established at the time of fertilisation.
• The foetus receives its genetic endowment in two equal portions from the male gamete (Spermatozoon) and female gamete (Oocyte).
• The X sex chromosome is larger than the Y sex chromosome.
• Females have XX chromosomes. All oocytes contain one X chromosome, they are the homogametic sex.
• Males have XY chromosomes. Spermatozoa form two distinct populations, one containing X chromosomes and one containing Y chromosomes. They are the heterogametic sex.

• The Y chromosome is required to initiate male development.
• In absence of the Y chromosome, female gonads (Ovaries) will develop.
• The prime male determining gene is SRY (sex determining region of the Y chromosome.

• SRY is a very small piece of DNA that acts as a molecular switch to regulate structural genes and autosomal chromosomes, determining if and when they are to be activated.
• The presence of SRY alone is sufficient to ensure development of male gonads (Testes).
• Female is the default sex, in absence of SRY female gonads (Ovaries) will develop.

• Subsequent gonadal development is dependent on a normal population of germ cells.
• Normal Oocyte growth requires the activity of both X chromosomes.
• Normal Spermatogenesis requires the activity of no more than one X chromosome.

Internal Genetalia

• Early development of gonads proceeds similarly in both sexes. The gonads are derived from two distinct tissues:
  • Somatic mesenchymal tissue forms the matrix of the gonad
  • Primordial germ cells form the gametes

• Genital ridge forms from somatic mesenchyme.
• Primordial germ cells migrate to the genital ridge.
• At this stage it is not possible to discriminate between male and female gonads.
• Y chromosomal determination of gonadal sex is only visible when primordial germ cell colonisation is complete.

MALE

• Sex cord cells proliferate to form the testes cord. During this period SRY is expressed for the first time within Sertoli cells (SRY is NOT expressed in primordial germ cells).
• Sex cords with a basement membrane become Semniferous Cords which give rise to Semniferous Tubules.
• Within the semniferous cords are two cell populations:
  • Primordial germ cells - Prospermatogonia which give rise to Spermatozoa
  • Mesodermal cord cells which give rise to Sertoli cells

External Genetalia

Turner's Syndrome

Kleinfelter's Syndrome

Testicular Feminisation

Adrenogenital Syndrome

Hermaphroditism

Bovine Freemartinism