Endocrine Signalling and Reproductive Biology

Introduction to Reproductive Biology

  • Overview of lecture goals:

    • Revise basic human reproductive biology and endocrinology.

    • Discuss fundamentals of human reproductive development, steroids, and gonadotropins.

    • Analyze human and animal models for endocrine signaling in reproduction.

Part One: Basic Concepts

  • Target: Understanding human reproductive hormones and feedback systems.

Endocrine Signaling Types

  1. Endocrine Signaling

    • One cell secretes a hormone that travels through the bloodstream to other distant target cells.

    • Example: Luteinizing Hormone (LH) from the pituitary gland to gonads.

  2. Paracrine Signaling

    • One cell communicates with neighboring cells of a different type.

    • Example: Oestradiol in the ovary affecting surrounding cells.

  3. Corticotropin-Releasing Hormone (CRH)

    • Acts on the same cell type (hypothalamic cells) for feedback regulation.

  4. Neurohormones

    • Hormones released by neurons into the bloodstream.

    • Example: Gonadotropin-Releasing Hormone (GnRH) travels to the anterior pituitary.

  5. Neurotransmitters

    • Direct signaling between neurons.

    • Example: Acetylcholine.

  6. Pheromones

    • Signaling molecules released into the environment.

    • Example: Boar taint in male pigs signaling reproductive receptiveness to females.

    • Humans: Suggested pheromones include androstenone and estratetraenol, but studies are limited and often inconclusive.

Human Pheromones

  • Evidence for existences, such as maternal nipple secretion stimulating suckling in infants.

  • Pheromones play an ancient role in communication and reproduction.

Conventional Reproductive Hormones

  1. Produced by Gonads:

    • Ovaries: Oestrogens and Progesterone.

    • Testes: Androgens.

    • Non-steroidal hormones: Inhibin A, Inhibin B, Anti-Müllerian Hormone (AMH), and Relaxin-like factor.

  2. Produced by Hypothalamus:

    • Gonadotropin-Releasing Hormone (GnRH).

  3. Produced by Anterior Pituitary:

    • Luteinizing Hormone (LH), Follicle Stimulating Hormone (FSH), Prolactin (PRL).

  4. Pineal Gland:

    • Melatonin, involved in reproductive rhythms.

  5. Placenta:

    • Produces Chorionic Gonadotropin (hCG) and steroid hormones, influencing pregnancy and reproduction.

Hormonal Regulation

  • Hormonal levels and their metabolic fate (e.g., sulfate modification) affect activity.

  • Hormones can bind to transport proteins (e.g., Sex Hormone Binding Globulin) impacting bioavailability and activity.

  • Hormone excretion influenced by binding state (free vs. bound) and requires metabolic conversion (bio-transformation).

Hypothalamic-Pituitary-Gonadal Axis

  • Main control system for reproduction:

    • GnRH acts in a pulsatile manner stimulating LH and FSH release.

    • GnRH triggers LH release, LH then stimulates testosterone production in males.

    • Feedback loops: Negative feedback from gonadal hormones on GnRH and LH to maintain homeostasis.

Feedback Mechanisms

  1. Negative Feedback:

    • Stabilizes and regulates hormone levels (e.g., LH and testosterone).

  2. Positive Feedback:

    • Reinforces signals in certain situations (e.g., childbirth) leading to increased hormone output.

    • Oxytocin release during labor creates a positive feedback loop until childbirth occurs.

  3. Short and Long Feedback Loops:

    • Ultra-short: e.g., CRH reduces its own secretion.

    • Short: Hormones affecting their own release from the pituitary.

    • Long: Comprehensive feedback from multiple organs in the reproductive axis.

Summary

  • Understanding the complex interplay of hormonal signaling is crucial in reproductive biology, providing insights into the regulation of sexual development, reproductive cycles, and overall hormonal health.

  • Continuous research is necessary to further unravel the roles and mechanics of these hormones in both humans and models to attempt to ascertain the intricacies of reproduction.

  1. Endocrine Signaling

    • One cell secretes a hormone that travels through the bloodstream to distant target cells.

    • Example: Luteinizing Hormone (LH) from the pituitary gland to gonads.

  2. Paracrine Signaling

    • One cell communicates with nearby cells of a different type.

    • Example: Oestradiol in the ovary affecting surrounding cells.

  3. Autocrine Signaling

    • A cell releases a hormone that binds to receptors on its own surface, influencing its own activity.

    • Example: Certain immune cells producing signals that affect their own behavior.

  4. Neurohormones

    • Hormones released by neurons into the bloodstream.

    • Example: Gonadotropin-Releasing Hormone (GnRH) travels to the anterior pituitary.

  5. Neurotransmitters

    • Direct signaling between neurons.

    • Example: Acetylcholine.

  6. Pheromones

    • Signaling molecules released into the environment.

    • Example: Boar taint in male pigs signaling reproductive receptiveness to females.

    • Humans: Suggested pheromones include androstenone and estratetraenol, but studies are limited and often inconclusive.