Lecture 12: Endocrine Gland Histology

• The main topics covered are the histology of endocrine glands, identification of different cells and tissues of those glands, endocrine disorders, and causes and treatments of those disorders.

Pituitary Gland

• Has an anterior portion composed of epithelial cells
• Has a posterior portion composed of neuroendocrine cells and axons/terminals

Pineal Gland

• Functions to integrate light signals based on hours of light and dark
• Some fibers come from the optic nerve, then travel to the superior colliculi, spinal cord, sympathetic chain ganglion, and finally to the Pineal gland
• Regulates the circadian rhythm
• Melatonin is produced at night
• Serotonin serves as a precursor to melatonin
• Contains pinealocytes

Thymus

• Located behind manubrium, above the heart
• Responsible for WBC (T cell) maturation
• Killer T cells destroy targeted cells
• Helper T cells determine how the immune system reacts
• Regulatory T cells promote immune tolerance to avoid autoimmune issues
• Secretes thymopoietin, thymosin, and thymulin
• Development of lymphatic tissue and T cells
• Regulates T cell activity
• Blood-thymus barrier exists in the cortex
• Lobes of the thymus are separated by trabeculae
• Lobes contain a lobule, cortex, and medulla

Thyroid Gland

• Follicles composed of follicular (principal) cells release TH (90% T4)
• Follicles contain a colloid
• Parafollicular cells (C-cells) produces calcitonin
• Follicles are surrounded by capillaries
• TH increases nutrient breakdown and oxygen consumption, and heat production, and promotes GH secretion
• Follicles are composed of a ring of cuboidal epithelial cells, also called principal cells, that are bounded by a basement membrane, and surround a lumen filled with pink-staining colloid consisting primarily of thyroglobulin
• C-cells, or parafollicular cells, are scattered in the spaces between follicles
• C-cells secrete calcitonin to control calcium homeostasis

Parathyroid Gland

• Monitors blood Ca2+ levels and secretes PTH
• Chief cells contain calcium-sensing receptors and a G protein system
• Oxyphil cells' function is currently unknown
• Contains a large amount of adipose tissue, which expands in puberty
• The PTH-producing chief cells are numerous, with central nuclei and pale cytoplasm
• The more active the cells are in producing and secreting PTH, the darker their cytoplasm appears
• Clusters of Oxyphil cells are visible, whose function is unknown

Adrenal Gland

• The cortex produces corticoids
• The zona glomerulosa produces mineralocorticoids
• The zona fasciculata secretes glucocorticoids
• The zona reticularis secretes glucocorticoids
• The medulla contains chromaffin cells (like postganglionic cells)
• Produces catecholamines

Pancreas

• Islets contain alpha, beta, and delta cells
• Alpha cells produce glucagon when glucose levels are below 100 mg/dL to stimulate the breakdown of glycogen and gluconeogenesis
• Beta cells produce insulin and amylin, which slows stomach emptying and creates satiety
• Delta cells produce Somatostatin, to lower stomach acid, and inhibit GH
• Acinar cells produce digestive enzymes
• The islets of Langerhans appear as distinct islands in a sea of pancreatic acinar cells and constitute a small percentage (~2%) of pancreatic tissue
• The beta cells (green), which produce insulin, are typically located in the center of the islets
• The alpha cells (red), which produce glucagon, are located on the periphery

Gonads

• Ovaries and testes
• Engaged in exocrine (sperm and eggs) and endocrine functions
• Estradiol is produced by developing follicles
• Progesterone is produced by the corpus luteum
• Testes produce testosterone in interstitial cells

Endocrine Disorders

• Diabetes Type I is characterized by a lack of insulin production
• Diabetes Type II is characterized by insulin resistance
• Diabetes Insipidus is characterized by ADH hyposecretion

GH Disorders

• Gigantism is hypersecretion of GH in childhood
• Pituitary dwarfism is hyposecretion of GH in childhood
• Acromegaly is hypersecretion of GH in adulthood

TH Disorders

• Hypothyroidism causes lowered metabolism and lethargy
• Hyperthyroidism causes accelerated metabolism and weight loss and increases heart rate
• Goiter is caused by a lack of iodine, which then causes no TH to be produced, and therefore high TSH levels, which leads to excess thyroglobulin production.

Parathyroid Disorders

• Hypoparathyroidism causes low blood Ca2+ levels, resulting in issues with muscle contraction (especially the heart) and increased bone density, potentially leading to fatal tetany (spasms in larynx, hypocalcemic tetany within days), or can come as a complication of thyroid surgery, or rarely, resistance to PTH.
• Hyperparathyroidism causes increased PTH production/secretion, often caused by adenomas (benign tumors)
• Hyperparathyroidism causes bones to become soft, fragile, and deformed, which promotes renal calculi formation

Central Precocious Puberty

• The body matures earlier than normal, with puberty happening before the age of 8 in females and 9 in males
• Males are more likely to have a specific trigger
• Potential causes include early GnRH release, childhood obesity, brain/pituitary tumors, meningitis, cancer treatments and genetics.

Androgen Insensitivity Syndrome

• Individuals are "male" with respect to genes and gonads but not with respect to genitalia (no scrotum or penis)
• Karyotype of XY chromosomes
• Abdominal testes
• Produce normal male levels of testosterone, but target cells lack receptors for it
• External genitalia develop female anatomy as if no testosterone were present
• No uterus or menstruation

Polycystic Ovary Syndrome (PCOS)

• Characterized by small cysts in ovaries as indicated by fluid-filled sacs. Fluid-filled sacs are not diagnostic.
• Hypersecretion of androgens induces a set of symptoms
• No ovulation leads to production of cysts and androgens produced
• Menstrual cycle abnormalities may occur
• May be related to insulin resistant individuals, where insulin buildup increases androgen levels

Multiple Endocrine Neoplasia (MEN)

• Presence of tumors in 2+ glands that may be benign or malignant
• Type I affects the parathyroid, pancreas, and pituitary, leading to hyperparathyroidism, kidney stones, bone thinning, and high BP
• Type II affects the thyroid and parathyroid, sometimes the adrenal gland (50%)
• Type II causes Pheochromocytoma, high BP, rapid HR (panic attack symptoms), and mucosal neuromas