Endocrine physiology II Flashcards
Week 6
Explain the regulation of release and functions of thyroid hormones produced by the Thyroid gland.
Thyroid gland - - > Thyroid hormone
* Two hormones - thyroxine (T4) & triiodothyronine (T3)
* Structurally similar but different number of iodine atoms bound:
- T4 is the major hormone secreted by thyroid gland (90%)
- T3 is more biologically active & also formed at target tissues by conversion from T4
* Amino acid-based but lipid-soluble –> enters target cells
Inside a target cell, binds to receptors in 3 locations:
* Cytoplasm: held in storage and released upon ↓ TH levels inside cell
* Mitochondria: ↑ in ATP production
* Nucleus: activates genes that control enzyme synthesis for energy transformation and use, e.g., Na+/K+ ATPase production, glycolysis, etc.
“Metabolic Hormone”
- metabolic rate & body temperature
- Tissue growth and development
- Maintain blood pressure
Thyroid hormone synthesis, storage & secretion
* Large quantities of TH stored in extracellular colloid-filled follicle
- enough for 2-3 months
- TSH from anterior pituitary binds to receptors on follicular cells:
1. secrete stored TH
2. synthesise more TH-containing colloid to refill cavity
Regulation of thyroid hormone secretion (negative feedback loop)
1. Stimulus: Homeostasis disturbed by decreasing T3 & T4 concentrations in blood and low body temp.
2. Receptor: Hypothalymus releases thyroid releasing hormone (TRH) to anterior lobe
3. Anterior lobe releases thyroid stimulating hormone (TSH)
4. Effector: Thyroid gland follicles release T3 and T4
5. Restored: T3 and T4 concentrations increase and body temp rises… Homeostasis Restored.
Explain the regulation of release and functions of calcitonin hormones produced by the Thyroid gland.
- Stimulated by ↑ blood Ca2+ levels
- Targets kidneys to increase Ca2+ excretion
- In childhood – bone growth & skeletal muscle development
- In starvation & late pregnancy – bone ‘sparing’
- In non-pregnant healthy adult – role on bones is unclear
Explain the regulation of release and functions of parathyroid hormones produced by the Parathyroid glands.
Most important for Ca2+ balance : Ca2+ controls nerve impulses,
muscle contraction, blood clotting, intracellular signalling
Explain the regulation of release and functions of hormones produced by the Adrenal glands.
Adrenal cortex: corticosteroids
* Mineralocorticoids regulate salt & water balance (mainly aldosterone)
* Glucocorticoids regulate metabolism & stress responses, anti-inflammatory effects (mainly cortisol)
* Gonadocorticoids minor role compared with gonadal hormones (mainly androgens)
- Synthesis: multistep involving cholesterol-varying intermediates
- Release: not stored in cells, so rate of release depends on rate of synthesis
- Action: lipid-soluble so bind intracellular receptors & modify gene transcription
Regulation of corticosteroid release – HPA axis (Negative feedback loop)
1. Stimulus: stress
2. Hypothalymus release corticotropin-releasing hormone (CRH) to anterior lobe
3. Anterior lobe releases ACTH
4. Effector: Adrenal cortex releases Glucocorticoids (mainly cortisol), Gonadocorticoids (androgens), and Mineralocorticoids (mainly aldosterone) to target cells
5. Glucorticoids prompt ↑ blood amino acids, ↑ blood fatty acids, ↑ blood glucose & anti-inflammatory effects. Aldosterone prompts ↑ sodium & water retention for ↑ blood volume & BP.
Adrenal medulla: catecholamines
Reinforces & prolongs fight-or-flight response
- Produces & secretes catecholamines: adrenaline (Ad) & noradrenaline (NAd)
- Stored & released in 80:20 ratio (Ad:Nad)
- Stimulated by: acute stress
- Action: water-soluble so bind plasma membrane receptors
Explain the regulation of release and functions of hormones produced by the Pineal gland.
Pineal gland: melatonin
Controls biological clock & regulates physiological rhythms
- Targets:
- Hypothalamus “biological clock” –> body temperature, sleep, & appetite
- CNS neurons –> antioxidant effects
- Stimulated by: visual pathways, i.e., intensity & duration of daylight –> diurnal cycle, peaks during night & lowest around noon
- Action: water-soluble so bind plasma membrane receptors
Explain the regulation of release and functions of hormones produced by
the major organs of the endocrine system – endocrine pancreas
- Mixed gland: endocrine & exocrine cells
- Endocrine islets have 4 cell types:
- α-cells produce glucagon: ↑ blood glucose
- β-cells produce insulin: ↓ blood glucose
- δ-cells produce GHIH: suppresses glucagon & insulin, slows food absorption
- PP cells produce pancreatic polypeptide: effects on metabolism & appetite
Endocrine pancreas: insulin
Decreases blood glucose levels
- Peptide-based hormone
- Targets & actions:
- Glucose uptake into cells & ATP production (mainly skeletal muscle & adipose)
- Glycogenesis: glucose molecules join to synthesise glycogen (mainly liver)
- Triglyceride & protein synthesis (adipose & muscle)
- Stimulated by: ↑ blood glucose, ↑ blood amino acids & fatty acids, parasympathetic NS (acetylcholine), incretins
- Inhibited by: GHIH, sympathetic nervous system
Define spermatogenesis, spermiogenesis, and oogenesis
Spermatogenesis, spermiogenesis, and oogenesis are all processes involved in the production of gametes, or sex cells, in humans:
Spermatogenesis
- Sperm formation in males begins at puberty until later life (testes)
- Spermatogenesis involves:
1. mitosis of stem cells & differentiation: spermatogonia –> primary spermatocytes
2. meiosis I & II: primary spermatocytes –> secondary spermatocytes àspermatids
3. spermiogenesis: spermatids –> mature sperm
Spermiogenesis
The process of remodelling and differentiation of spermatids into mature sperm. Spermatids undergo spermiogenesis as they travel from the seminiferous tubules to the epididymis.
Oogenesis
- Oocyte formation in females begins before birth, speeds up at puberty & ceases at menopause (ovaries)
- Oogenesis involves:
1. mitosis of stem cells before birth (oogonia –> primary oocytes)
2. meiosis I begins before birth but stalls & re-commences from puberty (primary oocytes –> secondary oocytes)
3. meiosis II begins before ovulation but completes only if fertilised (secondary oocyte –> mature ovum) - if not fertilised, meiosis II does not complete & secondary oocyte disintegrates
The process of gametogenesis, which includes spermatogenesis and oogenesis, is crucial for the continuation of generations. When a sperm and egg fuse, they form a zygote that develops into an embryo.
Explain hormonal control of male reproductive functions
- Male reproductive functions include spermatogenesis, spermiogenesis, bone & muscle growth, secondary sex characteristics, & male libido
- Regulated by hormone interactions involving hypothalamus, pituitary, & gonads (testes)
Testosterone:
* Released from interstitial endocrine/Leydig cells within testes
* Steroid hormone > travels in blood bound to transport proteins > binds intracellular receptors & DNA
* Tightly regulated - under negative feedback control
Inhibin:
* Released from Nurse/Sertoli cells within testes
* Peptide hormone > travels free in blood > binds to plasma membrane receptors
* Provides negative feedback control of spermatogenesis
Explain the ovarian and uterine cycles, and understand their interrelationship
- Female reproductive functions include oogenesis, ovulation (ovarian cycle), uterine preparation for pregnancy (uterine cycle), mammary preparation for breastfeeding, secondary sex characteristics, & female libido
- Regulated by hormone interactions involving hypothalamus, pituitary, & gonads (ovaries)
Oestrogens (mainly oestradiol):
* Produced & released by developing follicles (oocyte surrounded by layers of follicular cells) within ovary
* Steroid hormone
Progesterone:
* Produced & released by corpus luteum within ovary
* Steroid hormone
The cycles are coordinated & last ~28 days on average:
Ovarian cycle:
* Monthly cycle involving maturation & ovulation (release) of an oocyte
* Phases: follicular (before ovulation) & luteal (after ovulation)
Uterine/menstrual cycle:
* Monthly cycle that prepares the uterus for implantation of a fertilised ovum
* Phases: menstrual & proliferative (before ovulation) & secretory (after ovulation)
Outline the actions of major hormones of pregnancy
HORMONES OF PREGNANCY
* Human chorionic gonadotropin (hCG)
o increased levels in blood or urine confirms pregnancy
o maintains corpus luteum function & associated progesterone secretion (prevents menstruation)
* Human placental lactogen (hPL)
o alters maternal metabolism (enhances nutrient delivery to fetus)
o prepares mammary tissue for breastmilk production
* Relaxin
o relaxes public symphysis (and other joints) so pelvis can widen
o cervical dilation in labour
o inhibits oxytocin release to prevent early contractions
* Progesterone
o produced by corpus luteum in early pregnancy, then placenta
o maintains endometrial lining of the uterus
o prepares mammary tissue for breastmilk production
* Oestrogens
o near end of pregnancy, placenta produces ↑↑ oestrogens, which contribute to initiation of labour
o prepares mammary tissue for breastmilk production
Placenta: a temporary organ in uterine wall that produces hormones & exchanges materials between mother & fetus
Outline the actions of major hormones of lactation
HORMONES OF LACTATION
* Prolactin
o manufactured & released by anterior pituitary (and placenta)
o stimulates growth of mammary tissue & milk production
* Oxytocin
o manufactured by hypothalamus & released by posterior pituitary
o stimulates milk ejection (& uterine contractions in labour)
