Fertilization & Lactation

Overview of Fertilization and Lactation
Objectives

  • Understand the process of fertilization.

  • Understand the mechanism of lactation.

Process of Fertilization

  • Sperm Count and Journey: 150-600 million sperm are typically deposited per ejaculation, with approximately 50-100 making it to the ampullary portion of the fallopian tube within 5 minutes post-copulation.

    • Fimbriae sweep the oocyte into the fallopian tube.

    • Cilia along the fallopian tube help propel the oocyte toward the uterus.

  • Viability:

    • Sperm: Up to 2 days in the female reproductive tract.

    • Egg: 12-24 hours post-ovulation.

  • Window of Opportunity: 3 days (2 days before ovulation and 1 day after).

Capacitation

  • Definition: A physiological process that removes or modifies the protective protein coat from the sperm’s acrosome within the female reproductive tract.

    • Results in increased motility as the sperm's tail beats more vigorously.

Acrosomal Reaction

  • Penetration Mechanism:

    • Sperm binds to the zona pellucida (ZP) via ZP3 glycoproteins, initiating the acrosomal reaction which is mediated by an increase in intracellular calcium concentration [Ca++]i[Ca^{++}]_i.

    • Sperm releases hydrolytic enzymes to dissolve the zona pellucida, allowing head penetration.

Fertilization (Continued)

  • Fusion of Membranes: The sperm and oocyte cell membranes fuse upon penetration.

    • The oocyte undergoes a cortical reaction, leading to an increase in [Ca++]i[Ca^{++}]_i, which triggers its second meiotic division.

    • Sperm's increase in [Ca++]i[Ca^{++}]_i causes the exocytotic release of acrosomal contents into the oocyte.

Cortical Reaction

  • Granule Release: The cortical reaction leads to the release of granules from cortical vesicles that contain enzymes.

  • This process hardens the zona pellucida to prevent polyspermy (fertilization by multiple sperm).

Completion of Fertilization

  • The nucleus of the mature oocyte becomes the female pronucleus, while the sperm's nucleus forms the male pronucleus.

  • When male and female pronuclei fuse, the zygote is formed containing 46 chromosomes.

Mechanism of Lactation

  • Definition: Lactation is the secretion and ejection of milk, controlled by hormonal and physical stimulation.

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

    • Colostrum: A thin, yellowish substance produced during the first few days post-parturition, containing less fat and lactose compared to mature milk.

Hormonal Regulation of Lactation

  • Prolactin: Produced and secreted by the anterior pituitary gland; it is the principal hormone for milk production and secretion.

    • Initiaition requires decreased levels of estrogens and progesterones post-parturition and inhibition of its release by dopamine.

Stimulus for Milk Production

  • Nerve Impulses: Generated by nipple stimulation during suckling.

    • These impulses travel to the hypothalamus, leading to a decrease in dopamine release which normally inhibits prolactin release.

Oxytocin and Milk Ejection

  • Role of Oxytocin: Suckling stimulates oxytocin release from the hypothalamus and its transport to the posterior pituitary.

    • Oxytocin induces contraction of myoepithelial cells around the alveoli and ducts, facilitating milk ejection (let-down).

    • Breastfeeding may suppress gonadotropin-releasing hormone (GnRH) production, subsequently decreasing FSH and LH levels and affecting the ovarian cycle.