Anatomy & Physiology II BIOL 215 Dr. Bharathi Aravamudan Endocrinology- Part I Outline • Introduction to Endocrinology and Hormones • Pituitary, Hypothalamus and other sites of hormone production • Hormone function (mechanisms) • Hormone dysfunction/ pathology (clinical relevance) Endocrinology: the Study of Hormones • • • Endocrine System: Glands, tissues and cells that secrete hormones Endocrinology: the study of this system, and the diagnosis and the treatment of its disorders What is a Hormone? A chemical messenger transported by the bloodstream and stimulate physiological responses in cells of another tissue or organ that is often far away (also keep paracrines or ‘local hormones’ in mind!) v Hormones help regulate: – Extracellular fluid amount & components – Metabolism rate & actions – Reproductive cycles – Contraction of cardiac & smooth muscle – Glandular secretion – Immune function – Growth & development v Biochemical nature of hormones are varied v Specificity of hormone function rendered by: i) receptors on target cells/ tissues, or, ii) the presence in the target cells/ tissues of a processing mechanism (such as enzymes) that convert the circulating, non-functional form of the hormone into the active form Major Organs of the Endocrine System ★ Endocrine vs. Exocrine Glands ★ o A gland is a cell or an organ that secretes substances for use elsewhere in the body (secretion) or for elimination as waste (excretion) o Glands can be classified as exocrine or endocrine • Endocrine glands – Ductless release of hormones; into the bloodstream – Examples: thyroid, pituitary, adrenal etc. • Exocrine glands – Release of secretions via ducts – Examples: mammary, tear ducts etc. • The distinction is not so clear in some cases (e.g. liver) Exocrine Gland Epithelial cells proliferate into the connective tissue Endocrine Gland Epithelial cells connecting to the surface degenerate Due to apoptosis, the core is hollowed out and a duct is created Secretory tissue becomes infiltrated with blood capillaries Gland remains connected to the surface via the duct and releases its secretions to the epithelial surface Secretory cells secrete their hormones into the blood Hormone Nomenclature ★ ★ Anatomy of the Brain- Recap (Learn: locations of the hypothalamus, the pituitary and the pineal glands Anatomy of the Brain- Recap (medial aspect) Hypothalamus and Pituitary Gland ★ Both are “master” endocrine glands since their hormones control other endocrine glands (these are tropic hormones) The Hypothalamus v v v v v Major control center of the endocrine and autonomic nervous systems Shaped like a flattened funnel Connects to the pituitary gland by infundibulum (= funnel) Controls the pituitary with 7 different releasing & inhibiting hormones Its nuclei have centers that play a multitude of visceral functions: ² Hormone secretion ² Autonomic effects ² Thermoregulation ² Food and water intake ² Sleep and circadian rhythms ² Memory ² Emotional behavior and sexual response The Hypothalamus and the Pituitary ★ (See Table 17.3 for a list of releasing and inhibiting hormones that affect the anterior pituitary) Hypothalamo-Pituitary-Target Organ (Axis) Relationships ★ Essential Recap (Thalamic and hypothalamic nuclei and their functions) ★ ★ Hormones of the Hypothalamus ★ The Pituitary Gland (Hypophysis) v Located in a depression of the sphenoid bone (sella turcica) v ~1.3 cm wide, roughly the same size and shape as a kidney bean v Grows ~50% larger during pregnancy v Is composed of two functionally independent structures: ² Anterior lobe (Adenohypophysis) • Hormone release is controlled by the hypothalamus. But no nervous connection to the hypothalamus; link is via blood vessels of the hypophyseal portal system • It produces 6 hormones: o PRL (Prolactin) o GH (Growth hormone/ Somatotropin) o FSH (Follicle Stimulating Hormone) o LH (Luteinizing Hormone) o TSH (Thyroid Stimulating Hormone) o ACTH (Adrenocorticotropic Hormone) ★ ² Posterior lobe (Neurohypophysis) • In actuality, this is not a gland but a nervous tissue (nerve fibers and neuroglia) • Stores and releases OT and ADH (hormones of the neurohypophysis made by the hypothalamic neuroendocrine cells and transported down to the posterior lobe by hypothalamo-hypophyseal tract) Helpful Hints: o Learn material on Table 17.4 o Learn full names as well as abbreviations of hormones and which tissue releases which hormone Subdivisions of the Pituitary Histology of the Pituitary Gland Anterior Pituitary Hormones (6) ★ 1 & 2 are Gonadotropins, i.e., they target ovaries and the testes (the gonads) 1. FSH (Follicle Stimulating Hormone) • Secreted by the gonadotrope cells in the pituitary • In the ovaries, it stimulates the secretion of ovarian sex hormone, and the development of bubble-like follicles that contain the eggs. In the testes, it stimulates sperm production 2. LH (Leutenizing Hormone) • Also secreted by the gonadotrope cells in the pituitary • In females, it stimulates ovulation, the release of the egg • So named because after ovulation, the follicle becomes corpus luteum (meaning ‘yellow body’) • Also stimulates corpus luteum to secrete progesterone, a hormone important in pregnancy • In males, LH stimulates the testes to produce testosterone ★ 3. TSH (Thyrotropin/ Thyroid Stimulating Hormone) • Secreted by the thyrotrope cells in the pituitary • Stimulates growth of thyroid gland and the secretion of thyroid hormone, with widespread effects on metabolic rate, body temperature etc. 4. PRL (Prolactin) • Secreted by the lactotrope (also called mammotrope) cells in the pituitary • Plays a role lactation (hence the name prolactin) • During pregnancy, lactotropes increase in number and size, concurrent with increase secretion of PRL; however, PRL exerts its effect on the mammary glands only after the woman gives birth., by stimulating them to secrete milk ★ 5. ACTH (Adrenocorticotropic Hormone) or Corticotropin • Secreted by corticotrope cells • Target organ is adrenal cortex (and hence the name) • Stimulates adrenal cortex to secrete hormones called glucocorticoids (especially cortisol), which regulate glucose, protein and fat metabolism, and play an important role during body’s response to stress 6. Growth Hormone (Somatotropin) • Secreted by somatotrope cells, the most numerous in the pituitary, which produces at least a 1000 times more GH than any other hormone • Increased mitosis, cellular differentiation and thus promote growth – Promotes protein production – Promotes lipid metabolism – Glucose sparing (maintains high plasma glucose levels, by making energy available from non-glucose sources) • Childhood – Bone, cartilage and muscle growth • Adulthood – Maintenance of muscle and bone mass Posterior Pituitary Hormones ★ ADH (Antidiuretic Hormone) • Increase water retention by the kidneys, reduces urine volume and helps prevent dehydration • Also functions as a neurotransmitter, under the name of arginine vasopressin, as it can cause vasoconstriction at very high concentrations (seen only during pathological conditions) OT (Oxytocin) • Has several reproductive functions (from intercourse to breastfeeding) • Increases in amounts (both in males and females) during sexual arousal and orgasm (helps sperm ejaculation and transport) • Also helps develop sexual satisfaction, and emotional bonding between partners • Stimulates labor contractions during childbirth, and helps flow of milk from the mammary gland to the nipple in lactating mothers • Is important for mother-child bonding as well, as data suggest that female mammals neglect their young ones in the absence of OT Hormones of the Pituitary ★ Hypothalamic Control of Anterior Pituitary Secretion of Posterior Pituitary Hormones Animation: Hypothalamic and Pituitary Hormone Secretion https://anatomy.mheducation.com/html/apr.html?animal=human& Control of Oxytocin Secretion
