Ovulation and Fertilization

Oocyte: female gamete (inside of follicle)

Spermatogenesis: physiological process of growing an maturing spermatozoa

What is Oogenesis and Why is it Important?

• male: billion of copies per ejaculate

• female: few million eggs in her entire lifetime

• Embryo transfer: the process of super-ovulating and retrieving embryos from a donor animal and transferring those embryos into recipient

  • ~10-15 embryos when attempting

Folliculogenesis

• follicles are located on the ovary and make functional eggs

• ovary → follicle → oocyte hormone drive

• of the 75k to 2 million primordial follicles only 0.05% will reach the point of developing into a Graafian follicle and ovulation

• Steps:

  • primary follicle (primary oocyte inside)

  • secondary and tertiary stages

  • Graafian follicle

    • highly structured relationship between the outer layers of theca and the inner layers of granulosa separated by the basement membrane

    • basement membrane creates a physical barrier that allows fluids to pool and the final structures of the follicle to form, it also creates a blood barrier

    • As the follicle develops, the egg develops its own barrier, the Zona pelludica.

      • Zona Pelludica: protein structures surrounding the oocyte that serves a barrier to fertilization

Oogenesis—the Creation and the Maturation of Egg Cells

• Embryonically, Oogonium: the first form of egg cells which contain two sets of chromosomes identical to other cells of the body

• Oogonium undergo mitotic divisions to create the 75k to 2 million primary (1°) oocytes

• These mitotic divisions cease before birth, and the number of potential oocytes becomes FIXED and will begin decreasing even before birth

• Any oocytes attempting to continue development before puberty will become atretic

• Any oocytes attempting to continue development before puberty will become atretic

• Large difference in male and female reproductive systems: order in which they go through meiosis and metamorphosis

  • the male goes through spermatocytogenesis and then through spermiogenesis

• Male reduces gametes’ chromosome number first and then transforms its shape to look like a sperm cell. = Makes more copies

• Female changes the shape of her gamete first (zona pellucida around the structure) and then goes through meiosis. 

• Seems like a small difference but leads to males remaining fertile their entire life and females having limited gametes.

• In the female:

  • Metamorphic change: follicle is developing from the primary to the secondary stage of development, the egg will begin to enlarge; increase cytoplasm and organelles

  • 2x or 3x: develop the zonal pelludica, granulosa cells known as the corona radiata

  • this arrangement = very large cell

    • only nutrients the embryo will have until the outer wall, the zona pelludica attach to uterine wall

• Meiosis in the gg is a two-step process (spermatozoa)

  • Two-Part Meiosis: “cell waking up”

    • Meiosis I:

      • cell begins growing without duplication of the chromosomes

      • the arms of the paired chromosomes will exchange genetic material in a process called cross over

        • Cross-over: exchange of genetic material between paired chromosomes

          • leads to genetic “survival of the fittest”

      • ends with the production of two nuclei each with one set of chromosomes (haploid). These nuclei are genetically different

      • One of the nuclei will be “thrown out.” Polar body. Remaining Nuclei referred to as: secondary oocyte

• Many hightech procedures require healthy, mature oocytes. Successful vs. unsuccessful

• second division, still needed to become mature