Endocrine System: histology and disorders (lecture 12)

Endocrine Histology

• The pituitary gland is divided into two main lobes, the anterior and posterior.
• The anterior lobe has epithelial cells that produce and secrete hormones.
• The posterior lobe has neuroendocrine cells, which are specialized nerve cells that release hormones into the bloodstream.
• Axons/terminals of the nerve cells carry signals from the hypothalamus to the posterior pituitary.
• These signals trigger the release of hormones like oxytocin and vasopressin.

Pineal Gland

• Receives signals from the optic nerve, which helps to sense light and dark.
• This information is used to regulate melatonin production and sleep patterns.
• It integrates light signals and information about hours of light and dark.
• Some nerve fibers come from the optic nerve, then go to the superior colliculi, spinal cord, sympathetic chain ganglion, and finally the pineal gland.
• Circadian rhythm regulates melatonin production at night and serotonin as precursor for production
• Pinealocytes convert serotonin to melatonin and are the main cell type in the pineal gland.
• Circadian rhythm is the internal clock that dictates when we feel awake and when we feel sleepy.
• During the day, when there is light, melatonin production is inhibited, keeping humans alert.
• At night, in absence of light melatonin production increases, signaling the body its time to sleep.
• Damage to the pineal gland causes sleep disturbances, insomnia, or irregular sleep patterns, due to decreased melatonin levels, and mood disorders such as depression and anxiety.

Thymus

• It is located behind the manubrium, above the heart.
• WBC (T cell) maturation occurs
• Killer T cells attacks and destroys targeted cells such as infected cells and cancerous cells.
• Helper T cells determine how the immune system reacts to threats.
• Regulatory T cells maintain immune tolerance, avoiding autoimmune issues. It prevents the immune system from attacking the body's own cells (autoimmune diseases)
• Secretions include thymopoietin, thymosin, and thymulin, which ensure that the body can effectively defend itself against infections and diseases, regulate T cell activity, and develop lymphatic tissue & T cells
• The blood-thymus barrier in the cortex protects developing T cells.
• If it is damaged, then T cells decrease, impairing the body's ability to fight off infection and disease, weakening the immune system, and increasing the risk of cancer.
• Lobe is composed of lobule, cortex, and medulla and is separated by connective tissue trabeculae.

Thyroid Gland

• The thyroid gland is located in the neck and plays a crucial role in regulating metabolism and growth through the production of hormones.
• Follicles are made up of follicular cells (principal cells) that release TH (90% T4 is thyroxine (T4)) and colloid.
• Parafollicular cells (C-cells) secrete calcitonin, a hormone that helps regulate calcium levels in the blood.
• Capillaries surround each follicle
• TH increases nutrient breakdown, oxygen consumption, produces heat, and promotes growth of bones, skin, hair, nails, teeth, and development of the fetal nervous system.
• The follicles are surrounded by capillaries, which transport hormones into the bloodstream.
• Damage causes hypothyroidism, hyperthyroidism, goiter, and metabolic issues.

Parathyroid Gland

• Monitors blood Ca2+
• Secretes PTH, which regulates calcium levels in the blood.
• Chief cells release PTH when calcium levels are low and act 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.
• Calcium-sensing receptors play a vital role in monitoring calcium levels through a protein signaling pathway.
• Oxyphil cells are larger than chief cells.
• The more active the cells are in producing and secreting PTH, the darker their cytoplasm appears.
• If damaged, then it causes high BP, diabetes, depression, and other problems.

Adrenal Gland

• It regulates BP, stress response & metabolism
• The cortex (outer layer of adrenal gland) is responsible for corticoid production.
• Zona glomerulosa produces mineralocorticoids such as aldosterone, which regulates BP.
• Zona fasciculata and zona reticularis secrete glucocorticoids like cortisol, regulating metabolism and stress responses.
• The adrenal medulla secretes catecholamines such as epinephrine and norepinephrine, which regulate fight or flight.
• The medulla (inner layer of the adrenal gland) has chromaffin cells (like postganglionic cells) that release catecholamines.

Pancreas

• Islets of Langerhans are small clusters of cells found in the pancreas.
• Alpha cells secrete glucagon between meals when blood glycose is below 100 mg/dL, which has two actions on the liver: breakdown of glycogen and gluconeogenisis.
• 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. Blood glucose promotes glycogenolysis, which 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 to store or metabolize them; therefore, it lowers the level of blood glucose
• Beta cells secrete insulin and amylin simultaneously.
• Amylin helps reduce spikes in blood glucose by slowing the emptying of the stomach, and it contributes to the feeling of fullness (satiety).

Gonads

• Ovaries and testes function as both endocrine and exocrine glands.
• Sperm and eggs are part of the exocrine function that is released into the reproductive tract.
• Estradiol, progesterone, and testosterone are key hormones with many processes and production.
• Estradiol, a form of estrogen, is produced by the developing follicles in the ovaries, which influences menstruation and develops secondary sexual characteristics.
• Progesterone is produced by the corpus luteum of ovaries, which after ovulation secretes progesterone to prepare the uterine lining for potential pregnancy, making it suitable for implantation of a fertilized egg.
• Testosterone is produced by interstitial cells in testes to develop secondary male characteristics such as promoting muscle mass, deep voice, etc. It plays a role in sperm production.

Endocrine Disorders

• Diabetes is a disease that affects how the body regulates blood sugar.
• 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. It is a condition where the body produces insulin but does not respond to it as well as it should.
• Insipidus is caused by ADH hyposecretion. It is a condition where the body does not produce enough ADH. ADH helps the body regulate the amount of water in the blood. Without enough ADH, the body produces a lot of urine, 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 is caused by hypersecretion of GH in childhood, leading to individuals who are much taller than average.
• Pituitary dwarfism is caused by hyposecretion of GH during childhood, leading to individuals who are much shorter than average.
• Acromegaly is caused by overproduction of GH during adulthood, causing thickening of the bones and soft tissues, with especially noticeable effects on the hands, feet, and face.
• TH disorders affect the thyroid gland.
• 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. This causes the pituitary to secrete extra TSH, which makes the thyroid produce more colloids. However, the colloid can't be iodinated to make TH, so it accumulates in the gland, producing visible swelling in the neck.
• Hypoparathyroidism, a condition where not enough PTH is 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. This can happen in as little as 2 or 3 days after a complication of thyroid surgery where the parathyroid was accientally removed or dengenerated from a cut off blood supply.
• Hyperparathyroidism is a result of increased secretion of PTH which increases blood calcium and can be caused by adenoma, which are benign tumors. It can cause bones to become soft, fragile, and deformed. It also promotes the formation of renal calculi, which are kidney stones.
• Central precocious puberty is when the body matures earlier than normal and occurs generally at the age of 8 for females, and 9 years for males, Causes of this 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" becuase 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 devleop in the mucous membranes of the mouth, lips, and digestive tract.

The pituitary gland has anterior epithelial cells, and posterior neuroendocrine cells. Nerve signals from the optic nerve regulate melatonin production and circadian rhythm in the pineal gland. These signals help regulate sleep patterns.