Anatomy of Adrenal Glands

Adrenal Glands

• Each adrenal gland is approximately 3-5cm in height, 2-3cm in width, and 1cm thick, weighing about 3.5 to 5gm.
• During embryonic development, the adrenal gland differentiates into two structurally and functionally distinct parts: the outer cortex (80 to 90% of the gland) and the inner medulla.

The Outer Zone

• The outer zone is the zona glomerulosa, which secretes mineralocorticoids that affect mineral (electrolyte) homeostasis.
• The middle zone is the zona fasciculata, which is the widest of the three zones and secretes mainly glucocorticoids.
• The inner zone is the zona reticularis, arranged in branching cords that synthesize a small amount of weak androgen with masculinizing effects.

Adrenal Medulla

• The adrenal medulla is a modified sympathetic ganglion of the autonomic nervous system.
• It develops from the same embryonic tissue as all other sympathetic ganglia, but its cells don't have axons and form clusters around small veins.
• The adrenal medulla is composed of neurosecretory tissue, consisting of neurons specialized to secrete their hormones directly into the blood rather than across a synapse.
• The two major hormones secreted by the adrenal medulla are epinephrine (adrenaline), accounting for about 80% of the medullary secretion, and norepinephrine (noradrenaline), accounting for the other 20% of the medulla's secretion.

Pancreatic Islets

• The pancreas consists of two major types of secretory tissues, referred to as dual function.
• The pancreas is an exocrine gland that secretes digestive juice and an endocrine gland that releases hormones.
• The hormone-secreting cells of the pancreas are grouped in clusters, or islets, in close proximity to blood vessels.
• There are three main types of cells in the pancreatic islets (islets of Langerhans): alpha or A cells that secrete glucagon, beta or B cells that secrete insulin, and delta or D cells that secrete somatostatin (GHIH).

Pituitary Gland

• The anterior pituitary is supplied with arterial blood indirectly, first passing through a capillary bed in the hypothalamus.
• The blood then passes through the pituitary portal system, which transports blood to the anterior pituitary.
• Release of anterior pituitary hormones is stimulated by releasing hormones and suppressed by inhibiting hormones from the hypothalamus.
• The neurohypophysis (posterior pituitary) does not synthesize hormones, but serves as a storage and release site for two hormones: antidiuretic hormone (ADH) and oxytocin (OT).

Endocrine System

• The endocrine system is composed of endocrine glands located throughout the body.
• The endocrine glands are ductless glands, as the hormones they release pass directly into the bloodstream.
• The major endocrine glands include the hypothalamus, pituitary gland, pineal gland, parathyroid gland, thyroid gland, thymus, kidney, adrenal gland, pancreas, and ovaries and testes.

Pineal Gland

• The pineal gland is a tiny, cone-shaped structure, reddish-brown in color, and surrounded by a capsule.
• It is located deep between the cerebral hemispheres, attached to the upper portion of the thalamus near the roof of the ventricle.
• The pineal gland secretes the hormone melatonin in response to light conditions outside the body.

Thyroid Gland

• The thyroid gland is a butterfly-shaped gland located in the neck, on the anterior and lateral surfaces of the trachea, just below the larynx.
• The weight of the gland in an adult is variable, but is around 30 gm, and it is a highly vascular gland surrounded by a fibrous capsule.

Parathyroid Glands

• There are small parathyroid glands, two embedded in the posterior surface of each lobe of the thyroid gland.
• Each parathyroid gland is surrounded by a thin capsule of connective tissue and consists of many tightly packed secretory cells, spherical in shape, closely associated with capillary networks.
• The parathyroid glands contain two types of epithelial cells: principal cells that produce parathyroid hormone (parathormone), and oxyphil cells, whose function is not known.