Endocrinology and Reproductive Endocrinology Basics

Housekeeping

  • Caitlin and Jeremy will present a video introducing the unit.

  • Labs commence Wednesday of week one.

  • Pre-lab work required on LMS before attending the lab session.

Basics of Endocrinology

  • Endocrinology is the study of hormone actions.

  • Hormones are involved in almost all reproductive processes across the lifespan.

  • Multicellular organisms require communication between cells and tissues, achieved by:

    • Nervous system: Neurons send electrical impulses.

    • Endocrine system: Hormones (chemical messengers) travel in blood to target organs/cells.

Endocrine System

  • The endocrine system is essential for communication and control of reproductive processes.

  • Brain as an endocrine organ: The hypothalamus synthesizes and secretes hormones affecting the anterior pituitary.

  • The pituitary gland synthesizes and secretes hormones that travel to the gonads.

  • In males, the testes produce gametes and hormones that affect the penis, seminal vesicles, body hair, and muscle development.

  • Feedback mechanisms: Hormones can influence the hypothalamus and pituitary.

  • In females, similar processes occur with the hypothalamus and pituitary affecting the ovaries, which produce gametes and hormones influencing the vagina, uterus, and breasts.

  • Feedback mechanisms also present in females.

Hormone Action

  • Hormones travel in the blood to target cells or organs.

  • Effects are relatively slow (seconds to minutes) compared to the nervous system but are long-lasting and widespread.

  • Specificity is achieved through hormone-receptor binding, like a lock and key.

  • Two types of hormone receptors:

    • Nuclear receptors: Paired with steroid hormones.

    • Cell surface receptors: Associated with peptide or protein hormones.

  • Hormone-receptor binding alters cellular activity.

  • Hormones can act in extremely low concentrations but have very large effects (e.g., puberty, menstrual cycle, pregnancy).

Mechanisms of Hormone Action

  • Hormones can act at variable distances from source to target:

    • Endocrine: Hormone produced by an endocrine cell travels through the bloodstream to a distant target cell with a specific receptor.

      • Example: Anterior pituitary LH acts on Leydig cells in the testis to produce testosterone.

    • Paracrine: Hormone released from a paracrine cell travels through interstitial fluid to an adjacent target cell with a receptor.

      • Example: Testosterone diffuses from Leydig cells to seminiferous tubules to affect Sertoli cells.

    • Autocrine: Hormone produced by an autocrine cell acts on the same cell, which has the receptor for that hormone.

      • Example: Granulosa cells in the female follicle produce estrogen, which affects the granulosa cells themselves.

    • Exocrine: Substance is produced from a gland and secreted into a duct. Not a typical hormone action, but arguably…

      • Example: Pancreas secretes digestive enzymes into the gut.
        *Exocrine Manner: Estrogen secreted from ovary affects fallopian tube for oocyte mobility

Major Classes of Hormones

  • Three main categories: steroid hormones, iacasanoid hormones, and protein hormones.

Steroid Hormones

  • Produced as required, not stored.

  • Derived from cholesterol.

  • Enzymatic reactions convert cholesterol into different classes of steroid hormones.

  • Examples: progesterone, testosterone, estradiol, cortisol, aldosterone.

  • Aromatase: Converts testosterone into estradiol.

    • Estradiol production requires testosterone production.

    • Testosterone production requires progesterone.

  • Steroids are hydrophobic and require carrier proteins for transport in blood.

  • Carrier proteins - Vast majority (>95%) is bound, a small proportion (2-5%) is unbound.

  • Only free (unbound) hormone can pass through membranes to activate a target cell.

  • Hormone receptors are found on the nuclear membrane or within the nucleus (nuclear receptors).

  • Steroid hormone action:

    • Hormone becomes unbound and crosses biological membranes.

    • Finds its steroid receptor.

    • Hormone-receptor complex acts as a transcription factor, altering gene transcription.

    • This leads to the production of mature RNA, which then exits the nucleus, finds itself a ribosome, and then that ribosome leads to the production of a new peptide or a new protein.

    • Alters transcription of specific genes.

    • Example: Testosterone affects Sertoli cells to increase peptide and protein secretion for gamete production.