Gametogenesis (HL) (HL IB Biology)

• Sexual reproduction involves fertilisation; the fusion of sex cells, or gametes, from two parents
  • Gametes are specialised, haploid cells 
  • The fusion of gametes forms a zygote
• In animals, male gametes are sperm cells and female gametes are ova (singular ovum), also known as oocytes
• Gametes are produced in pairs of glands known as gonads
  • Sperm are produced in the male testes (singular testis)
  • Ova are produced in the female ovaries (singular ovary)
• The process of gamete formation is known as gametogenesis, and it involves both mitosis and meiosis
  • Specifically the production of sperm cells is spermatogenesis and production of ova is oogenesis
• Gametogenesis in both males and females involves the following stages:
  • Cells dividing by mitosis to produce many new cells, all of which have the potential to develop into gametes
  • Cells growing and differentiating
  • Cells dividing by meiosis to produce haploid gametes

Spermatogenesis

• The production of sperm takes place in the testes in males from puberty onwards
• The testes contain many small tubes, or tubules, known as seminiferous tubules
  • The gaps, or interstices, between the seminiferous tubules are filled with cells called interstitial cells, sometimes known as Leydig cells
  • The interstitial cells produce the male sex hormone testosterone
• Spermatogenesis begins in the germinal epithelium, a layer of cells that makes up the outer layer of the seminiferous tubules
• Cells in the germinal epithelium divide by mitosis, producing diploid cells called spermatogonia (singular spermatogonium)
  • Of the two daughter spermatogonia cells, one will go on to eventually become a sperm cell, while the other remains in the germinal epithelium where it can continue development
• Spermatogonia begin to migrate from the germinal epithelium towards the lumen of the seminiferous tubules; they do this by moving through the gaps between Sertoli cells
  • Sertoli cells form the inner lining of the seminiferous tubules
• Spermatogonia differentiate into immature sperm cells called primary spermatocytes
• Primary spermatocytes mature and divide by meiosis
  • Meiosis I forms secondary spermatocytes
  • Meiosis II forms spermatids 
• Spermatids formed during meiosis remain associated with the Sertoli cells as they mature into sperm cells, also known as spermatozoa
• Once fully matured, the sperm cells detach from the Sertoli cells and move along the seminiferous tubule lumen towards the sperm duct
  • On their way towards the sperm duct, the mature sperm pass through a coiled tube called the epididymis

Spermatogenesis diagram

Spermatogenesis begins in the germinal epithelium of the seminiferous tubules and the new cells migrate towards the tubule lumen as they differentiate.  Both mitosis and meiosis are involved.

Oogenesis

• The production of ova begins in the ovaries of the female foetus before birth
• The ovaries are surrounded by an outer layer of cells called the germinal epithelium; the cells in this layer divide by mitosis throughout the first 7 months of foetal development to form diploid cells called oogonia (singular oogonium)
  • The oogonia migrate throughout the tissues of the ovaries
  • This process of oogonia formation stops after 7 months, by which time several million oogonia have been produced
  • These are all the oogonia that the ovaries will produce in the female’s lifetime
• During the few months leading up to birth, the oogonia in the foetus’ ovaries grow in size and enter meiosis I, and a layer of cells called follicle cells develop around them
  • The partially divided oogonia together with their layer of follicle cells are known as primary follicles
• Once the oogonia have developed into primary follicles the oogenesis process pauses until the start of puberty
  • Although the ovaries contain several million primary follicles at birth, many of them degrade throughout a woman’s life and never reach maturity
• When puberty begins, the hormone FSH stimulates the continued development of several primary follicles in the ovary
  • Only one of the stimulated follicles will reach maturity
• Meiosis I continues and the primary follicle divides to form two new cells
  • The division of cytoplasm is not equal and the result of meiosis I is a secondary oocyte along with a very small cell called a polar body
  • The polar body cell has very little cytoplasm and does not mature further
• The secondary oocyte formed at the end of meiosis I enters meiosis II; at this point, it leaves the ovary, together with its layer of follicle cells, in the process of ovulation
  • The remains of the follicle that are left behind in the ovary develop later into the corpus luteum
• The secondary oocyte doesn’t finish meiosis II until after a sperm cell enters it, at which point meiosis II finishes just before the nuclei fuse
  • The secondary oocyte becomes an ovum very briefly between the end of meiosis II and the fusion of the two nuclei
  • A second polar body is produced at the completion of meiosis II

Oogenesis diagram

Oogenesis also begins in the germinal epithelium before new cells migrate through the tissues of the ovary. Both mitosis and meiosis are involved.