Fertilization & Lactation

Fertilization

  • Approx. 150-600 million sperm deposited per ejaculation

  • Approx. 50-100 sperm reach the end of the fallopian tube (approx. 5 mins after copulation)

  • Cilia & fallopian tube propel oocyte (egg) toward the uterus

  • Fimbria sweep the oocyte into the fallopian tube where fertilization of the oocyte occurs

  • Viability of gamete:

    • Egg: 12-24 hours

    • Sperm: 2 days

  • Window of opportunity:

    • 3 days: 2 days before ovulation & 1 day after ovulation

Capacitation

  • Definition: removal or modification of the protective protein coat from the sperm cell membrane of the acrosome; within the female reproductive tract

  • Result: sperm’s tail begins to beat more vigorously

Penetration

  • Zona Pellucida: the outer membrane surrounding the egg

    • contains sperm-ZP3 glycoprotein

    • Sperm-ZP3 binding initiates acrosomal reaction (↑ Ca+ )

  • Sperm releases hydrolyzing enzymes to dissolve the zona pellucida

  • Sperm’s head penetrates the zona pellucida

Fusion of Sperm and Oocyte

  • After sperm penetrates the oocyte’s membrane:

    • Sperm and oocyte membranes fuse

    • ↑ Ca in sperm → exocytotic release of acrosomal contents into oocyte

    • Oocyte undergoes a cortical reaction after sperm - oocyte penetration

    • Oocyte increases Ca and triggers oocyte’s 2nd meiotic division

Cortical Reaction

  • Definition: causes the release of granules by the cortical vesicle within the oocyte leading to:

    • Completion of the second meiotic division of the oocyte

    • Formation of the female pronucleus from the nucleus of the mature oocyte

    • Granules contain enzymes that cause the zona pellucida to harden, thus preventing polyspermy

      • Polyspermy: egg fertilization by more than one sperm → resulting in embryo death d/t abnormal chromosome numbers

Completion of Fertilization

  • The nucleus of the sperm transforms into the male pronucleus

  • Male and female pronuclei fuse → forms zygote with 46 chromosomes (23 from each parent)

Lactation

  • Lactation: the secretion and ejection of milk

    • Controlled by both hormonal and physical stimulation

  • Milk: an emulsion of fats in an aqueous solution containing lactose, proteins, and electrolytes

  • Differs from colostrum:

    • Colostrum: thin, yellowish, milk-like substance produced during the first 4 days after birth

    • Contains very few fats and less lactose compared than milk

Hormonal Control of Lactation

  • Prolactin: responsible for milk production and secretion

    • Produced and secreted by the anterior pituitary gland

    • Initiation of milk production depends on a decrease in estrogen and progesterone levels occurring after birth (aka parturition)

    • Dopamine inhibits the release of prolactin

Mechanism of Prolactin Release

  • Suckling is the most powerful stimulus for prolactin release

    • Nerve impulses generated by nipple stimulation travel from the breast → spinal cord → hypothalamus

    • Causes inhibition of dopaminergic neurons in hypothalamus to decrease dopamine release

    • Decreased dopamine → removal of inhibition in prolactin release

    • Dopamine is aka prolactin-inhibitory hormone

Role of Oxytocin in Lactation

  • Suckling stimulation also causes hypothalamus to produce and release oxytocin

    • Oxytocin then Moves to the posterior pituitary and into systemic circulation

  • Oxytocin enhances milk ejection

    • causes myoepithelial cell contraction → initiates milk ejection

  • Breast-feeding could suppress the production of gonadotropin-releasing hormone (GnRH) and decrease FSH and LH production