Unit 12 - Spermatogenesis & Oogenesis

Spermatogenesis

• Spermatogenesis

  • sperm formation in males

  • takes place in the testes

    • contained within the sac like scrotum

  • inside each testis are diploid cells called spermatogonia

    • derived from embryonic germ cells

    • are stem cells and divide by mitosis to produce more spermatogonia

    • in this way a continuous supply of spermatogonia is always available

  • during embryonic development hormones stimulate some of the spermatogonia to form primary spermatocytes

  • at puberty increased production of the hormone testosterone stimulates the spermatocytes to begin meiosis

• Meiosis 1 and Meiosis 2

  • in the first meiotic division (meiosis 1) a primary spermatocyte generates two secondary spermatocytes

  • each haploid secondary spermatocyte then undergoes the second meiotic division (meiosis 2)

  • in meiosis 2 both secondary spermatocytes generate two haploid spermatids

  • the result is the production of four haploid spermatids

    • two from each secondary spermatocyte

  • these spermatids eventually mature into sperm cells

  • so during spermatogenesis one diploid spermatocyte gives rise to four haploid sperm cells

  • the sperm cells produced in the testis are stored in the epididymus

  • they complete their maturation to functional sperm there

Oogenesis

• egg formation in females

• takes place within the ovaries

  • each ovary contains diploid cells called oogonia that are derived from embryonic germ cells

• Mitosis and Meiosis

  • during fetal development in a female the oogonia divide by mitosis

    • the result is more oogonia some of which develop into primary oocytes

      • primary oocytes are immature egg cells contained within masses of cells called follicles

        • the primary oocytes then enter meiosis 1

        • the process stops at this stage and no more primary oocytes form after this point

        • so at birth each female has a finite number of primary oocytes available for reproduction

• Meiosis at Puberty

  • at puberty a complex series of hormonal events stimulates changes in the follicle cells that induce some primary oocytes to complete their first meiotic division

  • the division of cytoplasm and cell organelles is unequal

    • as a result each primary oocyte forms one large secondary oocyte (haploid) and one small polar body (haploid)

    • in this way most of the cellular cytoplasm remains in the developing oocyte

  • the polar body often degenerates

  • the follicle containing the secondary oocyte continues to mature until a surge of the luteinizing hormone (LH) initiates ovulation

    • the mature follicle ruptures releasing the secondary oocyte into the oviduct also called the Fallopian tube

  • at ovulation the secondary oocyte begins meiosis 2

    • this second meiotic division will not be completed unless sperm penetration occurs

    • the secondary oocyte enters the oviduct where a sperm cell can penetrate it

    • the entry of a sperm cell into the cytoplasm of the secondary oocyte triggers the completion of meiosis 2

  • the cytoplasm divides unequally generating a mature ovum and a second polar body

  • the fusion of the haploid sperm cell nucleus and the haploid egg cell nucleus (process of fertilization) produces a diploid zygote