Week 9: Sex determination

Sex organs

Gonads: Organs (testicles & ovaries) which produce gametes (Sperm & ovum)

Internal genitalia: Accessory glands & ducts within the ECF

External genitalia: All external reproductive structures

Diploid cells

Diploid (2n) cells → All somatic cells (body cells sans gametes)

• 23 homologous chromosome pairs, 46 chromosomes total

  • 22 pairs are autosomal, 1 pair of sex chromosomes

  • Autosomal = genes of growth & development

  • Sex chromosomes (X or Y) = genes of sex organ development

Haploid cells

Haploid (n) cells → Only gametes (sperm & ova)

• 23 chromosomes total → NO PAIRS

  • 22 chromosomes are autosomal, 1 copy of sex chromosomes

Process of conception:

1 haploid (ovum) + 1 haploid (sperm) → Diploid cell (zygote)

Sex determination in chromosomes — extra scenarios

XXY = zygote will develop into a male

YO = zygote will die

XO = zygote will develop into a female (Turner syndrome)

• When ovaries develop, one X chromosome randomly clumps into nuclear chromatin called a “Barr body”

Bipotential Gonads

Differentiation of gonads in an embryo happens at week 7.

Prior to week 7, the undifferentiated gonad is termed a “bipotential gonad”

Bipotential gonads have: (think of adrenal glands)

• Outer cortex — develops into ovaries (ABSENCE of signal)

• Inner medulla — develops into testes (PRESENCE of signal)

This signal is provided by the SRY gene

SRY gene: Male embryonic development

Directs male embryonic development.

Testicular development does NOT require male sex hormones such as testosterone.

Once the bipotential gonad → testicles

3 hormones secreted:

1. Testicular Sertoli cells secrete anti-Müllerian hormone (AMH; a glycoprotein).

2. Testicular interstitial (Leydig) cells secrete androgens: testosterone and DHT

AMH causes embryonic Müllerian ducts to regress.

Testosterone converts the Wolffian ducts into male accessory structures.

Later, testosterone controls migration of the testes from the abdomen into the scrotum. Remaining male sex characteristics, are controlled primarily by DHT.

Female embryonic development

Multiple genes required for ovarian development.

In the absence of testicular AMH, the Müllerian ducts develop into the upper portion of the vagina, the uterus, & fallopian tubes

Without testosterone, the Wolffian ducts degenerate.

Without DHT, the external genitalia take on female characteristics

Sperm vs Ovum

Females are born with all the ova they will have.

Males continuously produce sperm when they reach maturity.

Gametogenesis (a.k.a meiosis)

Hormonal control of gamete genesis

Just look at bottom right corner of flow chart.

Female hormones: Progesterone & estrogens

Male hormones: Androgens

Sex hormone control pathway

GnRH is released in pulses.

If released in a constant, steady stream, does not effectively stimulate the anterior pituitary.