Endocrinology: Hormones and Glands

Introduction to Endocrinology

• Hormones are the chemical messengers used by the endocrine system
• The pituitary gland is a classical endocrine gland
• Paracrines are hormones that act on neighboring cells
• Hormones have the characteristics of binding to specific receptors, acting on distant target cells and acting as chemical messengers
• A fast onset of action is NOT a characteristic of hormones
• Steroid hormones are derived from cholesterol

Hypothalamic-Pituitary Axis

• The infundibulum connects the hypothalamus to the pituitary gland
• The anterior pituitary is also known as the adenohypophysis
• ADH (antidiuretic hormone) is released by the posterior pituitary
• The hypothalamic-hypophyseal portal system carries hormones from the hypothalamus to the anterior pituitary
• CRH (corticotropin-releasing hormone) is a hypothalamic hormone

Thyroid Gland

• The thyroid gland is located in the neck
• The follicle is the functional unit of the thyroid gland
• Thyroid hormones are stored in the thyroid gland as part of the colloid
• C-cells (parafollicular cells) in the thyroid gland secrete calcitonin
• Thyroid hormones are derived from the amino acid tyrosine
• Metabolic rate is primarily affected by thyroid hormones

Parathyroid Glands

• Most species have four parathyroid glands
• The primary function of the parathyroid glands is to regulate blood calcium
• PTH (parathyroid hormone) is produced by parathyroid glands
• PTH increases blood calcium levels by increasing calcium reabsorption in the kidneys
• Chief cells in the parathyroid gland produce PTH

Adrenal Glands

• The adrenal glands are located near the kidneys
• The cortex is the outer layer of the adrenal gland
• The adrenal cortex produces cortisol
• Aldosterone is the primary mineralocorticoid produced by the adrenal gland
• The adrenal medulla produces catecholamines
• Cortisol is the main glucocorticoid in most animals
• Angiotensin II is the primary stimulus for aldosterone secretion
• Cortisol is released in response to hypoglycemia
• The main effect of aldosterone on the kidneys is to increase sodium reabsorption
• Cortisol increases blood glucose levels by stimulating gluconeogenesis
• Epinephrine and norepinephrine are responsible for the "fight or flight" response

Pancreas

• The pancreas has both endocrine and exocrine functions
• The endocrine portion of the pancreas is made up of Islets of Langerhans
• Beta cells in the pancreas secrete insulin
• Insulin's primary effect is to decrease blood glucose levels
• Insulin promotes the storage of glucose in the form of glycogen
• Alpha cells in the pancreas secrete glucagon
• Glucagon's primary effect is to increase blood glucose levels
• Glycogenolysis is the process called when glucagon stimulates the breakdown of glycogen in the liver
• Delta cells secrete somatostatin
• Diabetes mellitus is caused by insulin deficiency or resistance

Reproductive System

• Sexual differentiation begins at the zygote stage
• Anti-Müllerian Hormone (AMH) is produced by Sertoli cells in males
• The absence of the Testis determining factor (TDF) leads to the development of ovaries
• The male hypothalamus lacks the surge center
• In oogenesis, meiosis I results in a secondary oocyte and a polar body
• The chromosome number of a spermatid is n (haploid)
• Crossing over occurs during Meiosis I
• Estradiol is the hormone responsible for the LH surge
• Progesterone is the primary hormone produced by the corpus luteum
• Fertilization occurs in the ampulla of the uterine tube
• Capacitation of sperm occurs in the female reproductive tract
• Sperm capacitation achieves the block to polyspermy
• In ruminants, maternal recognition of pregnancy involves the secretion of Interferon tau (IFNτ)

General Endocrinology

• Hormones are chemical messengers
• Direct enzymatic activity is NOT a characteristic of hormone action
• Oxytocin is an example of a neuroendocrine hormone
• The hypothalamus controls the anterior pituitary by releasing and inhibiting hormones
• Down-regulation of receptors results in decreased hormone sensitivity