Reproductive System: Gonads and Germ Cells

Reproductive System

• Gonad: an organ where gametes are produced (e.g., testis and ovaries) and controls sexual characteristics via hormones.
• Germ cells: cells that become gametes, often indicated by cells before meiosis (e.g., spermatogonia and oogonia).

Origin of Germ Cells

• Primordial germ cells (PGCs) arise from the posterior yolk sac and migrate to the gonads (future testis or ovaries) during embryogenesis.

Spermatogenesis

• PGCs form 3-6 weeks into embryogenesis and migrate to the gonads 2-3 months into embryogenesis.
• At puberty, spermatogonia occurs, and seminiferous tubules in the testis mature, allowing spermatogonia to further proliferate and enter meiosis.
• Diploid spermatogonia proliferate by mitotic cell division inside the testis.
• Spermatogonia enter meiosis, becoming secondary spermatocytes and then spermatids.
• Spermatids finally differentiate into spermatozoa (sperm) through a process involving:
  • Nucleus condensation
  • Formation of acrosome (head) and flagellum
  • Mitochondria gathering at the midpiece
  • Removal of excess cytoplasm

Sertoli Cells

• Span from the basement membrane to the lumen, surrounding differentiating spermatogonia.
• Produce enzymes and growth factors required for spermatogenesis.
• Function as phagocytes to clean up residual cytoplasm of sperm.
• Spermatogenesis takes place in close relation to Sertoli cells.

Leydig Cells (Interstitial Cells)

• Reside outside seminiferous tubules.
• Secrete testosterone in response to luteinizing hormone (LH).
• Sperms are released to the lumen and transferred to the epididymis.

Epididymis

• Reservoir of sperm with a passage taking 1-21 days.
• Concentrates fluid by 100x.
• Long single duct with peristaltic contraction.
• Sperm undergoes further maturation, acquiring mobility, and increasing dependence on fructose for energy production.

Oogenesis

• Primordial germ cells (PGCs) form 3-6 weeks into embryogenesis and migrate to the gonads (future ovaries) 6-12 weeks into embryogenesis.
• Oogonia proliferate via mitotic cell division inside ovaries.
• After 12 weeks of embryogenesis, oogonium enters meiosis-1's prophase and becomes a primary oocyte.
• Follicular cells in the ovary surround the oocyte, forming the primordial follicle.
• At puberty, a few primordial follicles re-commence meiosis every day, with only one ovulated per month.

Maturation of Follicles

• Primordial follicle, with flat follicular cells.
• Primary follicle, with multi-layer follicular cells.
• Secondary follicle with antrum.
• Secondary follicle with antrum and "Graafian follicle".

Menstrual Cycle

• Pituitary hormones: LH reaches peak during ovulation, and FSH levels remain steady throughout, with slight increases during maturing follicle stage.
• Ovarian hormones: progestogen increases gradually during the secretory phase, and oestrogen fluctuates, increasing at the end of the proliferation phase and decreasing till day 14.
• Ovulation occurs on day 14.

Uterus

• Consists of two uterine tubes that lead to the body with a back wall called the fundus.
• The inner wall of the uterus is called the endometrium, with the cervix being the entrance to the body.

Ovulation

• Ovum is collected by the trumpet-shaped distal end of the uterine tube (infundibulum) with fimbriae (finger-like projections).

Fertilisation

• Can occur at the ampulla (the curve at the uterine tube).
• Out of 300 million sperm, only 200 reach oocytes.
• Sperm must migrate through follicular cells (corona radiata), zona pellucida, using enzyme kin acrosome, and finally fuse with the membrane of the oocyte.
• The fertilised egg travels to the uterus.

Implantation

• The fertilised egg travels to the uterus, and by day 4, it is 8 cells and at the end of the uterine tube.
• By day 5, an early blastocyst forms, and by day 7, hatching occurs where the blastocyst comes out of zona pellucida.
• Implantation occurs on day 8-9.

Reproductive System

• Gonad: an organ where gametes are produced (e.g., testis and ovaries) and controls sexual characteristics via hormones.
• Germ cells: cells that become gametes, often indicated by cells before meiosis (e.g., spermatogonia and oogonia).

Origin of Germ Cells

• Primordial germ cells (PGCs) arise from the posterior yolk sac and migrate to the gonads (future testis or ovaries) during embryogenesis.

Spermatogenesis

• PGCs form 3-6 weeks into embryogenesis and migrate to the gonads 2-3 months into embryogenesis.
• At puberty, spermatogonia occurs, and seminiferous tubules in the testis mature, allowing spermatogonia to further proliferate and enter meiosis.
• Diploid spermatogonia proliferate by mitotic cell division inside the testis.
• Spermatogonia enter meiosis, becoming secondary spermatocytes and then spermatids.
• Spermatids finally differentiate into spermatozoa (sperm) through a process involving:
  • Nucleus condensation
  • Formation of acrosome (head) and flagellum
  • Mitochondria gathering at the midpiece
  • Removal of excess cytoplasm

Sertoli Cells

• Span from the basement membrane to the lumen, surrounding differentiating spermatogonia.
• Produce enzymes and growth factors required for spermatogenesis.
• Function as phagocytes to clean up residual cytoplasm of sperm.
• Spermatogenesis takes place in close relation to Sertoli cells.

Leydig Cells (Interstitial Cells)

• Reside outside seminiferous tubules.
• Secrete testosterone in response to luteinizing hormone (LH).
• Sperms are released to the lumen and transferred to the epididymis.

Epididymis

• Reservoir of sperm with a passage taking 1-21 days.
• Concentrates fluid by 100x.
• Long single duct with peristaltic contraction.
• Sperm undergoes further maturation, acquiring mobility, and increasing dependence on fructose for energy production.

Oogenesis

• Primordial germ cells (PGCs) form 3-6 weeks into embryogenesis and migrate to the gonads (future ovaries) 6-12 weeks into embryogenesis.
• Oogonia proliferate via mitotic cell division inside ovaries.
• After 12 weeks of embryogenesis, oogonium enters meiosis-1's prophase and becomes a primary oocyte.
• Follicular cells in the ovary surround the oocyte, forming the primordial follicle.
• At puberty, a few primordial follicles re-commence meiosis every day, with only one ovulated per month.

Maturation of Follicles

• Primordial follicle, with flat follicular cells.
• Primary follicle, with multi-layer follicular cells.
• Secondary follicle with antrum.
• Secondary follicle with antrum and "Graafian follicle".

Menstrual Cycle

• Pituitary hormones: LH reaches peak during ovulation, and FSH levels remain steady throughout, with slight increases during maturing follicle stage.
• Ovarian hormones: progestogen increases gradually during the secretory phase, and oestrogen fluctuates, increasing at the end of the proliferation phase and decreasing till day 14.
• Ovulation occurs on day 14.

Uterus

• Consists of two uterine tubes that lead to the body with a back wall called the fundus.
• The inner wall of the uterus is called the endometrium, with the cervix being the entrance to the body.

Ovulation

• Ovum is collected by the trumpet-shaped distal end of the uterine tube (infundibulum) with fimbriae (finger-like projections).

Fertilisation

• Can occur at the ampulla (the curve at the uterine tube).
• Out of 300 million sperm, only 200 reach oocytes.
• Sperm must migrate through follicular cells (corona radiata), zona pellucida, using enzyme kin acrosome, and finally fuse with the membrane of the oocyte.
• The fertilised egg travels to the uterus.

Implantation

• The fertilised egg travels to the uterus, and by day 4, it is 8 cells and at the end of the uterine tube.
• By day 5, an early blastocyst forms, and by day 7, hatching occurs where the blastocyst comes out of zona pellucida.
• Implantation occurs on day 8-9.