Endocrine System: histology and disorders (lecture 12)

Endocrine Histology

• The pituitary gland divides into the anterior and posterior lobes.
• Epithelial cells in the anterior lobe produce and secrete hormones.
• Neuroendocrine cells in the posterior lobe release hormones into the bloodstream.
• Signals from the hypothalamus reach the posterior pituitary via nerve cell axons/terminals.
• These signals trigger the release of hormones like oxytocin and vasopressin.

Pineal Gland

• Signals from the optic nerve help sense light and dark.
• This information regulates melatonin production and sleep patterns.
• Some nerve fibers travel from the optic nerve to the superior colliculi, spinal cord, sympathetic chain ganglion, and then the pineal gland.
• Melatonin production at night is regulated by the circadian rhythm, with serotonin as a precursor.
• The circadian rhythm is the internal clock that dictates when we feel awake and sleepy.
• Melatonin production is inhibited during the day when there is light, keeping humans alert.
• Melatonin production increases at night in the absence of light, signaling the body that it is time to sleep.
• Pinealocytes convert serotonin to melatonin.
• Damage to the pineal gland can cause sleep disturbances, insomnia, or irregular sleep patterns due to decreased melatonin levels, and mood disorders such as depression and anxiety.

Thymus

• Located behind the manubrium, above the heart.
• T cell maturation occurs.
• Killer T cells attack and destroy targeted cells, such as infected and cancerous cells.
• Helper T cells determine how the immune system reacts to threats.
• Regulatory T cells maintain immune tolerance, preventing autoimmune issues by stopping the immune system from attacking the body's own cells.
• Secretions include thymopoietin, thymosin, and thymulin, which ensure effective defense against infections and diseases, regulate T cell activity, and develop lymphatic tissue and T cells.
• The blood-thymus barrier in the cortex protects developing T cells.
• T cells decrease if the thymus is damaged, impairing infection and disease defense, weakening the immune system, and increasing cancer risk.
• A lobe is composed of a lobule, cortex, and medulla, separated by connective tissue trabeculae.

Thyroid Gland

• Located in the neck, it regulates metabolism and growth through hormone production.
• Follicles are composed of follicular cells (principal cells) that release TH (90% T4, thyroxine) and colloid.
• Parafollicular cells (C-cells) secrete calcitonin, which regulates calcium levels in the blood.
• Each follicle is surrounded by capillaries.
• TH increases nutrient breakdown, oxygen consumption, heat production, and promotes growth of bones, skin, hair, nails, and teeth and fetal nervous system development.
• Follicles are surrounded by capillaries that transport hormones into the bloodstream.
• Damage can cause hypothyroidism, hyperthyroidism, goiter, and metabolic issues.

Parathyroid Gland

• Monitors blood Ca2+ levels.
• Secretes parathyroid hormone (PTH) which regulates calcium levels in the blood.
• Chief cells release PTH when calcium levels are low, acting on bones, kidneys, and intestines to increase calcium.
• Chief cells have calcium-sensing receptors and a G protein system.
• The function of oxyphil cells is unknown, but they are larger than chief cells.
• More active cells producing and secreting PTH have darker cytoplasm.
• Damage can cause high BP, diabetes, depression, and other problems.

Adrenal Gland

• Regulates BP, stress response, and metabolism.
• The outer layer, cortex, is responsible for corticoid production.
• The zona glomerulosa produces mineralocorticoids like aldosterone, which regulate BP.
• The zona fasciculata and zona reticularis secrete glucocorticoids like cortisol, regulating metabolism and stress responses.
• The inner layer (medulla) has chromaffin cells (like postganglionic cells) that release catecholamines to regulate fight or flight.

Pancreas

• Islets of Langerhans are small clusters of cells.
• Alpha cells secrete glucagon between meals when blood glucose is below 100 mg/dL, acting on the liver to breakdown glycogen and conduct gluconeogenesis.
• Beta cells secrete insulin and amylin during and immediately following meals; amylin slows stomach emptying, promoting satiety.
• Delta cells secrete somatostatin, a regulator of glucagon and insulin secretion that lowers stomach acid and inhibits the release of the growth hormone GH.
• Acinar cells produce digestive enzymes that help break down food in the small intestine and absorb nutrients.
• Glucagon and insulin regulate blood sugar.
• Breakdown of glycogen into glucose occurs when the body needs a quick source of energy and blood glucose levels drop; this breakdown is called glycogenolysis, and it releases glucose into the bloodstream, further elevating blood sugar levels.
• Gluconeogenesis, glucose synthesis from fats and proteins, is increased during the production of glucose and raises blood sugar levels.
• In times of plentifully bound nutrients, insulin stimulates cells to absorb glucose, fatty acids, and amino acids for storage or metabolism, lowering blood glucose levels.
• Beta cells secrete insulin and amylin simultaneously.
• Amylin helps reduce spikes in blood glucose by slowing the emptying of the stomach, contributing to the feeling of fullness.

Gonads

• Ovaries and testes serve as both endocrine and exocrine glands.
• Sperm and eggs are part of the exocrine function, released into the reproductive tract.
• Estradiol, progesterone, and testosterone are key hormones involved in many processes.
• Estradiol, a form of estrogen, is produced by developing follicles in the ovaries, influencing menstruation and developing secondary sexual characteristics.
• Progesterone is produced by the corpus luteum of the ovaries; after ovulation, progesterone prepares the uterine lining for potential pregnancy by making it suitable for implantation of a fertilized egg.
• Interstitial cells in testes produce testosterone, which develops secondary male characteristics, such as promoting muscle mass and a deep voice, and also plays a role in sperm production.

Endocrine Disorders

• Diabetes affects blood sugar regulation.
• Type I diabetes is caused by no insulin production; it is an autoimmune disease where the body's immune system attacks and destroys cells in the pancreas that produce insulin.
• Type II diabetes is caused by insulin resistance, where the body produces insulin but does not respond to it well.
• Diabetes Insipidus is caused by ADH hyposecretion, where the body does not produce enough ADH, leading to a lot of urine production, which can lead to dehydration.
• "Insipidus" refers to the lack of sweetness of the urine in contrast to the sugary urine of diabetes mellitus.
• GH disorders are pituitary disorders that affect growth.
• Gigantism results from hypersecretion of growth hormone (GH) in childhood, leading to individuals who are much taller than average.
• Pituitary dwarfism is caused by GH hyposecretion during childhood, leading to individuals who are shorter than average.
• Acromegaly is caused by overproduction of GH during adulthood, causing thickening of bones and soft tissues, with noticeable effects on the hands, feet, and face.
• TH disorders affect the thyroid.
• Hypothyroidism is caused by the thyroid gland not producing enough TH, causing lowered metabolism, weight gain, and lethargy.
• Hyperthyroidism is caused by a thyroid gland producing too much thyroid hormone, causing an accelerated metabolism, weight loss, and an increased heart rate.
• Goiter is caused by a lack of iodine in the diet, causing the gland to be unable to synthesize TH. Without TH, the pituitary gland receives no feedback and acts as if the thyroid were understimulated, causing it to secrete extra TSH, which makes the thyroid produce more colloids. Accumulated colloid cannot be iodinated to make TH and produces visible swelling in the neck.
• Hypoparathyroidism, is when not enough PTH produced, which causes low blood Ca2+, increased bone density, and issues with muscle contraction especially the heart.
• A fatal tetany can result, which are suffocating spasms of muscles of the larynx which can happen in two to three days after a complication of thyroid surgery where the parathyroid was accidentally removed or degenerated from a cut-off blood supply.
• Hyperparathyroidism results from increased secretion of PTH, increasing blood calcium and can be caused by adenomas, which are benign tumors. Bones become soft, fragile, and deformed, also promoting the formation of renal calculi (kidney stones).
• Central precocious puberty is when the body matures earlier than normal, generally at the age of 8 for females and 9 years for males, and causes include early GnRH release, childhood obesity, brain/pituitary tumors, meningitis, cancer treatments, and genetics.
• Androgen Insensitivity Syndrome occurs when the body does not respond to androgens. People with AIS are "male" because they have XY chromosomes, but they don't have a scrotum or penis. The testes are undescended in the abdomen and produce normal male levels of testosterone, however, the target cells lack receptors for it. This causes them to not be able to develop facial hair, deep voices, or muscle mass. People with AIS may have a female appearance as if no testosterone were present. They do not have a uterus or menstraution, but do have a vagina and labia.
• Polycystic Ovary Syndrome (PCOS) is characterized by the presence of multiple cysts on the ovaries, as well as irregular periods and excess androgen levels.
• Multiple Endocrine Neoplasia (MEN) is a group of disorders that affect the endocrine glands.
• MEN is characterized by tumors in 2+ endocrine glands, which can possibly be benign or malignant.
• Type 1 MEN is characterized by tumors in the parathyroid, pancreas, and pituitary; hyperparathyroidism can cause kidney stones, bone thinning, and high HP.
• Type II MEN is characterized by tumors in the thyroid and parathyroid glands; 50% of times there are also tumors on the adrenal gland called pheochromocytoma. This tumor causes the adrenal gland to produce too much adrenaline and noradrenaline, which can cause panic attack symptoms such as high BP and rapid HR. Mucosal neuromas are noncancerous tumors that can develop in the mucous membranes of the mouth, lips, and digestive tract.