02. Testicular Function

• the nucleus of a sperm cell is too compact for transcription to occur
• there is no endoplasmic reticulum or cytoplasm in the sperm cell

  • translation is not needed as transcription does not occur
• sperm cells move through the mucus in the cervix and enters the uterus to find an oviduct

  • moves to the very end of the oviduct into the ampulla section
• spermatogenesis = production of mature sperm cells
• spermatogenesis occurs in the seminiferous tubules

  • once the sperm are made they are released and pushed into the epididymis - allows for further maturation
  • stored in vas deferens
  • testis are stored in the scrotum - provides cooler environment compared to the body
• spermatogenesis starts in puberty, initiated by the pulsatile release of gonadotropin releasing hormone from the hypothalamus
• three phases of spermatogenesis:

  • proliferation - mitosis occurs as spermatogonia A form spermatogonia B
  • division - meiosis occurs as spermatogonia B form spermatocyte I (diploid) which form spermatocyte II (haploid)
  • differentiation - spermiogenesis, divide to form spermatids

  continually pushed towards the lumen of the tube as this occurs
• within the seminiferous tubules Sertoli cells nurture germ cells in testis

  • between tubules are interstitial cells and Leydig cells which produce androgens
• Sertoli cells are critical for all aspects of spermatogenesis and maintain spermatogonial stem cell niche

  • tight junctions with other Sertoli cells form a blood-testis barrier which separates the germ cells from the immune system
  • form syncytium like epithelial monolayer in which germ cells are embedded with gap junctions for exchanging molecules and communication
  • secrete fluid into the lumen of seminiferous tubules which flush sperm into the epididymis
• at puberty, prospermatogonia are reactivated and undergo mitosis in the basal compartment of the seminiferous tubule - act as reservoir of self-renewal stem cells
• round spermatids elongate to form spematozoa
• acrosome = formed from Golgi apparatus which migrates to one end of the nucleus

  • contains hydrolytic enzymes which are released upon binding to the zona pellucida of the egg and aid penetration
• flagellum = centrioles migrate to opposite end of the nucleus to the acrosome and for axoneme

  • important for movement
  • sperm provide centriole to the egg allowing division of the embryo
• mitochondria of the sperm is helically arranged around the first part of the flagellum providing energy for motility
• sperm DNA is highly condensed with histones replaced by protamines
• spermiation = releasing the sperm cells from the syncytium

  • released into lumen of the testis
• sperm cells form a syncytium during mitosis and meiosis as cytokinesis is incomplete - creating intracellular bridges

  • occurs as the protein for the tail of the sperm is within the X chromosome
  • therefore need to share cytoplasm so Y carrying sperm can develop tail
• GnRH produced by the hypothalamus acts on the anterior pituitary to release FSH and LH

  • LH acts of Leydig cells to induce testosterone production
  • FSH acts on Sertoli cells to maintain spermatogenesis
• testosterone is important for development of testis in the embryo

  • carried by androgen binding protein
  • converted into DHT by enzyme produced by Sertoli cells
• inhibin is produced by Sertoli cells and acts on the pituitary gland, inhibiting FSH production
• androgen binding protein binds to testosterone preventing it leaving the testis

  • maintains high levels of testosterone in the blood
• sperm released from the testis are immotile until they traverse the epididymis

  • fluid secretions from Sertoli cells and periodic contraction push sperm out of the testis
  • need to undergo maturation in the epididymis
  • also mature (capacitation) in the female tract
• the epididymis has 3 main regions

  1. caput - provides motility to sperm
  2. corpus - provide fertilising ability to sperm
  3. vas deferens - storage
• semen = sperm (5%) and seminal plasma

  • seminal plasma is secreted by the accessory sex glands
  • provides fluid for transport, nutrition and buffering