L27. Histology - Microanatomy of Endocrine Glands. Introduction and The Hypophysis

The pituitary gland, also known as the hypophysis, is a small but vital endocrine gland located at the base of the brain. It plays a crucial role in regulating various bodily functions.

The pituitary gland is composed of two main lobes: the anterior pituitary (adenohypophysis) and the posterior pituitary (neurohypophysis).

The anterior pituitary (adenohypophysis) is responsible for producing and releasing hormones that regulate growth, metabolism, and reproduction.

The posterior pituitary (neurohypophysis) stores and releases hormones produced by the hypothalamus, specifically oxytocin and antidiuretic hormone (ADH).

The adenohypophysis is derived from an outpouching of the oral ectoderm during embryonic development.

The neurohypophysis develops from an extension of the diencephalon, a part of the brain.

The pineal gland, also known as the epiphysis, is a small endocrine gland located in the brain. It primarily produces melatonin, a hormone that regulates sleep-wake cycles and circadian rhythms.

The pars intermedia is a part of the pituitary gland that is poorly developed in humans and contains colloid-filled cysts.

The pars intermedia's function remains largely unknown in humans.

Pituicytes are the primary cell type in the neurohypophysis, resembling astrocytes.

The neurohypophysis is a neurosecretory site where neurons from the hypothalamus store and release hormones.

Herring bodies are visible dilations containing neurosecretory material found in the neurohypophysis.

Somatotropes are cells in the pars distalis of the pituitary gland that synthesize and secrete somatotropin (growth hormone).

Lactotrope cells in the pars distalis produce prolactin, a hormone responsible for stimulating mammary gland development and milk production.

Basophils are cells in the pars distalis that stain blue with trichrome staining and have a basophilic appearance.

Thyrotropes are basophil cells in the pars distalis that produce thyrotropin (thyroid stimulating hormone, TSH), a hormone that stimulates thyroid hormone production.

Gonadotropes are basophil cells in the pars distalis that produce follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which regulate the gonads (testes and ovaries).

The pars distalis of the pituitary gland is a vital component of the anterior pituitary that is composed of various hormone-secreting cells.

The anterior pituitary, also known as the adenohypophysis, is responsible for producing and releasing hormones that regulate growth, metabolism, and reproduction.

The posterior pituitary, also known as the neurohypophysis, stores and releases hormones produced by the hypothalamus, specifically oxytocin and antidiuretic hormone (ADH).

The adenohypophysis is the anterior lobe of the pituitary gland. It is responsible for producing and releasing hormones that regulate various bodily functions, including growth, metabolism, and reproduction.

The pars distalis is a region of the adenohypophysis, the anterior pituitary lobe. It synthesizes and releases various hormones, including growth hormone (GH), prolactin (PRL), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH).

The pars tuberalis is a small, funnel-shaped portion of the adenohypophysis, located around the infundibulum, the stalk connecting the pituitary to the hypothalamus.

The pars intermedia is a small region of the adenohypophysis, located between the pars distalis and the pars nervosa, the posterior pituitary lobe.

The neurohypophysis is the posterior lobe of the pituitary gland. It stores and releases hormones produced by the hypothalamus, specifically oxytocin and antidiuretic hormone (ADH), into the bloodstream.

The pars nervosa is the main part of the neurohypophysis, the posterior pituitary lobe. It stores and releases hormones, particularly oxytocin and antidiuretic hormone (ADH). These hormones are synthesized in the hypothalamus and transported down to the pars nervosa.

The infundibulum is a stalk or funnel-shaped structure that connects the hypothalamus to the pituitary gland. Along with the median eminence, it forms the neurohypophyseal system, which mediates the release of hormones from the posterior pituitary.

Rathke's pouch is an invagination of the oral ectoderm during embryonic development. This pouch eventually forms the adenohypophysis, the anterior lobe of the pituitary gland.

The superior hypophyseal arteries, branches of the internal carotid arteries and the circle of Willis, are the main blood supply for the anterior lobe of the pituitary gland. They form a portal system, a specialized circulatory network, to deliver hormones from the hypothalamus to the anterior pituitary.

The inferior hypophyseal arteries, branches of the internal carotid arteries, are the main blood supply for the posterior lobe (pars nervosa) of the pituitary gland. They give rise to fenestrated capillaries which drain into hypophyseal veins, ensuring hormonal release into the circulatory system.

The parathyroid glands are small, oval-shaped structures typically located on the posterior surface of the thyroid gland. They consist of two cell types: chief cells, responsible for producing parathyroid hormone (PTH), and oxyphil cells, whose function is still debated. Histologically, the parathyroid gland is composed of tightly packed cells organized into cords and clusters, surrounded by a network of capillaries.

Chief cells are the primary cell type in the parathyroid gland. They synthesize and secrete parathyroid hormone (PTH), which plays a vital role in regulating calcium levels in the blood. Oxyphil cells are less abundant and their exact function remains unclear, but they are believed to be involved in parathyroid gland development and possibly calcium homeostasis.

As we age, parathyroid glands may show some changes. The number of chief cells can decrease, contributing to reduced PTH production. However, these age-related changes are typically subtle and do not significantly impact parathyroid function.

PTH secretion from parathyroid chief cells is primarily controlled by blood calcium levels. When calcium levels are low, PTH secretion increases, leading to bone resorption, increased calcium absorption from the gut, and renal reabsorption of calcium. This negative feedback loop ensures calcium levels are maintained within a narrow range.

The adrenal glands, also known as suprarenal glands, are located atop the kidneys. They have an outer cortex and an inner medulla, each producing distinct hormones. The cortex, composed of three zones (zona glomerulosa, zona fasciculata, and zona reticularis), secretes steroid hormones like cortisol, aldosterone, and sex hormones. The medulla, derived from neural crest cells, secretes catecholamines, primarily epinephrine and norepinephrine.

The adrenal cortex exhibits three distinct zones, each producing different hormones. The zona glomerulosa, the outermost layer, secretes aldosterone, a mineralocorticoid that regulates sodium and potassium balance. The zona fasciculata, the middle layer, produces cortisol, a glucocorticoid involved in stress response and metabolism. The zona reticularis, the innermost layer, produces small amounts of sex hormones like androgens.

The adrenal medulla, derived from neural crest cells, is the inner region of the adrenal gland. Histologically, it consists of chromaffin cells, arranged in cords and clusters around blood vessels. These cells produce and secrete catecholamines, primarily epinephrine (adrenaline) and norepinephrine (noradrenaline).

Blood supply to the adrenal gland is crucial for delivery of precursors and nutrients. The adrenal cortex receives blood from the superior and inferior suprarenal arteries, creating a rich capillary network. The adrenal medulla receives blood from the cortex and a direct branch from the aorta, ensuring rapid delivery of hormones into the bloodstream.

Secretion of epinephrine and norepinephrine from the adrenal medulla is primarily controlled by the sympathetic nervous system. Pre-ganglionic sympathetic neurons release acetylcholine, activating nicotinic receptors on chromaffin cells, triggering the release of these catecholamines. This rapid response prepares the body for 'fight or flight' situations.