Introductory Endocrinology & Female Physiology – Lecture Review

Nervous System vs. Endocrine System

• Both are communication networks, yet differ fundamentally:

  • Nervous system

  • Uses neurotransmitters.

  • Chemical is released into a synaptic cleft (microscopic gap).

  • Acts on a downstream neuron or effector cell (e.g.
    heart, lung, skeletal muscle) across an extremely small distance.

  • Response is rapid (milliseconds–seconds) and highly localized.

  • Endocrine system

  • Uses signaling molecules called hormones.

  • Hormones are secreted directly into the bloodstream and therefore circulate system-wide.

  • Only tissues with the appropriate receptors respond; cells lacking the receptor are unaffected.

  • Response is generally slower (seconds–days) and longer-lasting.

General Features of Hormones

• Distribution

  • Blood (primarily water) carries hormones to every vascularized tissue.

  • Specificity is determined by receptor expression, not by the hormone’s route.

• Physiologic roles (non-exhaustive)

  • Growth & development (embryonic, post-natal, repair).

  • Sexual maturation/reproduction (ovaries → estrogen, testes → testosterone).

  • Behavior modulation.

  • Metabolism

  • Anabolism: building molecules.

  • Catabolism: breaking molecules down.

  • Fluid & electrolyte balance

  • Regulation of H2O reabsorption/excretion.

  • Ion homeostasis (e.g.
    extracellular [Na^+] must remain high relative to intracellular space).

• Concentration range

  • Effective at picogram levels: 1\text{ pg mL}^{-1}=10^{-12}\;\text{g mL}^{-1} ("one millionth of a millionth of a gram").

• Feedback mechanisms

  • Negative feedback: output suppresses further hormone release when the physiologic goal is met (dominant control).

  • Positive feedback: output amplifies stimulus; examples

  • Oxytocin → uterine contractions → more oxytocin release.

  • Estrogen surge pre-ovulation (see below).

• Release patterns

  • Pulsatile, diurnal, seasonal, or developmental.

  • Onset of action varies widely:

  • Catecholamines (epinephrine/norepinephrine): minutes.

  • Thyroid hormone, growth hormone: hours to days.

• Solubility & transport

  • Steroid hormones are lipophilic → require plasma-binding proteins to travel through aqueous blood.

Chemical Classes of Hormones

• Protein / polypeptide hormones (majority).

• Steroid hormones

  • Produced by adrenal cortex, gonads, placenta.

• Tyrosine-derived hormones

  • Catecholamines: epinephrine (primary) & norepinephrine.

What Constitutes the Endocrine System?

• An organ is "endocrine" if it releases a signaling molecule into blood.

• Classic endocrine glands

  • Hypothalamus & pituitary (brain).

  • Thyroid, parathyroids, adrenals, pancreas, gonads, placenta.

• Non-traditional endocrine tissues (blue items in lecture)

  • Adipose tissue → leptin (appetite control).

  • Stomach → gastrin (digestion).

  • Heart → atrial natriuretic peptide (ANP) (reduces BP).

  • Liver → angiotensinogen (RAS pathway → raises BP).

  • Skin → precursor to vitamin D3 (sunlight dependent; final activation in liver & kidney → intestinal Ca^{2+} absorption).

  • Kidney → erythropoietin (EPO, RBC synthesis), renin (initiates RAS).

Hypothalamus–Pituitary Axis (HPA) Overview

• Hypothalamus (part of diencephalon) produces tropic (a.k.a.
  trophic) hormones that act on anterior pituitary.

• Pituitary

  • Anterior (adenohypophysis): synthesizes & secretes hormones such as FSH, LH, GH, ACTH, TSH, prolactin.

  • Posterior (neurohypophysis): stores & releases hypothalamic hormones (oxytocin, vasopressin/ADH).

• Tropic hormones often trigger secondary endocrine glands to secrete their own hormones (multi-tier control).

Female Reproductive Anatomy Refresher

• Uterus → cervix → vaginal canal (midline structures).

• Fallopian (uterine) tubes extend laterally with fimbriae sweeping ova.

• Ovaries (paired, lateral) house follicles containing immature ova (female gametes).

• Lifecycle facts

  • Female is born with all primary oocytes she will ever need.

  • Reproductive years ≈ ages 12–50 (++ inter-individual variability).

  • Menstrual/ovarian cycles average 28 days (range ≈ 20–45; can even be absent).

Key Hormones of Female Reproductive Endocrinology

• GnRH (gonadotropin-releasing hormone)

  • Origin: hypothalamus.

  • Released in pulses (≈ every 2 h, 10–15 min per pulse).

  • Stimulates anterior pituitary to release:

  • FSH (follicle-stimulating hormone).

  • LH (luteinizing hormone).

• Estrogens (primarily \beta-estradiol; others: estrone, estriol)

  • Synthesized in ovarian follicles.

  • Steroid hormones → produced on demand (cannot be vesicle-stored due to lipophilicity).

  • Dominant "female" sex hormone, but present in males as well.

Follicular Development & Ovulation (Endocrine Control)

• Thousands of primordial follicles reside in each ovary from birth.

• At puberty, cyclical endocrine cascade begins:

  1. Pulsatile GnRH → anterior pituitary.

  2. Anterior pituitary secretes FSH > LH early in cycle.

  3. Granulosa cells (follicular epithelium) express FSH receptors → FSH binding →

  • Secretion of estrogen-rich fluid into follicle.

  • Positive feedback: rising estrogen ↑ number of FSH receptors, speeding follicle growth.

  1. Estrogen + FSH promote insertion of LH receptors on granulosa cells.

  2. Mid-cycle LH surge triggers ovulation (release of a single ovum into abdominal cavity → swept into fallopian tube).

Physiologic Roles of Estrogen

• Skeletal system

  • Pubertal estrogen spike → closure of epiphyseal plates (halts longitudinal bone growth).

  • In adult life, estrogen inhibits osteoclast-mediated bone resorption → preserves bone density.

  • Post-menopausal estrogen decline → unchecked osteoclastic activity → osteoporosis; hormone-replacement therapy (HRT) may mitigate.

• Reproductive tissues, secondary sex characteristics, cardiovascular effects, CNS behavior modulation (detailed coverage in later lectures).

GnRH: Clinical & Experimental Insights

• Essential for sexual maturation (both sexes).

• Pulsatile exogenous GnRH (mimicking physiologic pattern) → promotes pubertal development in GnRH-deficient children.

• Continuous exogenous GnRH

  • Initially ↑ FSH/LH ("flare" effect), but rapidly causes down-regulation of GnRH receptors on pituitary → decreased FSH/LH secretion → decreased estrogen (or testosterone in males).

  • Clinical use:

  • Treatment of estrogen-positive breast cancer (lower estrogen deprives tumor of growth stimulus).

  • Analogous approaches in prostate cancer (reduce testosterone).

Biochemical Relationship Between Androgens & Estrogens

• Both originate from a common androgen precursor.

• Enzyme aromatase converts androgens → estrogens.

• Highlights biochemical continuity: labeling testosterone "male" and estrogen "female" is oversimplified; both sexes produce both hormones, differing mainly in relative concentrations.

Quantitative & Miscellaneous Data Mentioned

• 1\text{ pg}\;=10^{-12}\,\text{g} (hormone working concentration as low as 1\text{ pg mL}^{-1}).

• GnRH pulses: q ~2 h, release lasts 10–15 min.

• Menstrual cycle length: average 28 d (range ~20–45 d).

Ethical, Practical & Clinical Connections

• Endocrine manipulation in sports: synthetic EPO abuse in endurance cycling ↑ RBC mass (kidney-derived hormone).

• Sunlight/vitamin D3 deficiency → impaired Ca^{2+} absorption, risk of rickets/osteomalacia.

• Positive-feedback loops (oxytocin, estrogen) illustrate scenarios where amplification—not homeostasis—serves physiologic goals.

• Terminology precision: "intuitive" systems (cardiovascular) vs. more abstract signaling networks (endocrine) highlight teaching challenges.

These notes capture all major & minor details, illustrative examples, quantitative data, physiologic principles, and clinical implications discussed throughout the transcript. They are organized to serve as a complete stand-alone study guide, replacing the need to re-watch the original lecture.