Endocrine Signalling & Reproductive Biology 3

Sex Determination

  • Genetic Basis:
    • Female mammals are XX, producing X germ cells.
    • Male mammals are XY, producing X or Y germ cells.
    • Thus, sex determination is fundamentally based on whether the sperm contributes an X or Y chromosome.

Sex Differentiation

  • Process Overview:
    • Begins post-sex determination, involving the establishment of gonads (ovaries for females, testes for males) and phenotypic characteristics (external and internal sexual traits).
    • Gonadal sex influences phenotypic sex.

Key Biological Concepts

  • Gonadal Development:
    • Males: Requires presence of the Y chromosome for testes development.
    • Testosterone promotes male genitalia development (penis and scrotum).
    • Dihydrotestosterone (DHT) conversion is critical in target organs for external male anatomy.
    • Females: Absence of high androgen levels allow for the development of clitoris, labia, and vagina.

Hormonal Influences on Development

  • Androgens and Differentiation:
    • Testosterone impacts primary (gonads) and secondary sexual characteristics (like body hair distribution).
    • Differences in liver function and hormonal balance exist between sexes.

Genetic Conditions and Insights

  • Guevedoces:
    • A condition in the Dominican Republic, individuals appear female at birth due to a mutation affecting the 5-alpha reductase enzyme.
    • At puberty, testosterone surges lead to development of male characteristics (penis, scrotum).
    • These individuals demonstrate typical heterosexual preferences, suggesting biological factors have more influence than rearing.

Reproductive Development Timeline

  • Critical Phases:
    • Around 6 weeks gestation, germ cells migrate to gonad precursors.
    • Primary sexual differentiation occurs as tissues develop into gonads.
    • Secondary sexual differentiation includes developing external features and secondary sexual characteristics.

Development Mechanisms

  • Role of Testosterone:
    • Supports development of Wolffian ducts and male reproductive system.
    • Lack of testosterone in females allows the paramesonephric ducts to develop into female reproductive organs.
  • Mesonephric Ducts:
    • Presence or absence of anti-Müllerian hormone (AMH) from testes influences reproductive tract development.

Testicular Descent

  • Cryptorchidism:
    • Condition where testes fail to descend, leading to infertility if untreated.
  • Process Overview:
    • Testes descend from near kidneys into the scrotum regulated by hormonal actions (testosterone and its metabolites).

Germ Cell Development in Females

  • Primordial Follicle Formation:
    • Completed by 28 weeks gestation.
    • Peak germ cell number (~7 million) occurs before birth, decreasing significantly by time of birth to about 1 million.
    • Primordial follicles undergo meiosis arrests, essential for fertility.
  • Ovary Function:
    • Females typically release one ovulatory follicle per menstrual cycle with surplus follicles undergoing atresia.

Modern Factors Influencing Fertility

  • Contributing Factors:
    • Delayed childbearing age and obesity can reduce fertility or increase the risk of infertility.

Summary

  • The process of sex determination and differentiation in humans is complex, intricately linked by hormonal influences and genetic factors that shape reproductive capabilities. Understanding these processes provides insight into both normal and atypical sexual development and reproductive health.