Glandular Epithelium - Endocrine Type
Glandular epithelium can also form endocrine glands. These glands are characterised by the absence of ducts and release their secretory products, called hormones, directly into the bloodstream. Hormones act on target cells throughout the body, regulating various physiological processes. Endocrine glands are typically composed of epithelial cells, or cells of epithelial origin, arranged in cords or clusters surrounded by a rich network of capillaries. This arrangement facilitates efficient uptake of hormones into the circulation.
Examples and Functions of Endocrine Glands
The following are some examples of endocrine glands and their functions:
Pituitary Gland (Hypophysis): Located beneath the brain, this gland is composed of two main parts: the anterior pituitary (adenohypophysis) and the posterior pituitary (neurohypophysis). The anterior pituitary secretes a variety of hormones, including growth hormone, prolactin, and hormones that regulate other endocrine glands. The posterior pituitary releases hormones produced in the hypothalamus, such as oxytocin and vasopressin.
Adrenal Glands: These glands sit atop the kidneys and comprise an outer cortex and an inner medulla. The adrenal cortex produces steroid hormones, including mineralocorticoids (e.g., aldosterone), glucocorticoids (e.g., cortisol), and weak androgens (e.g., DHEA). The adrenal medulla releases catecholamines like epinephrine and norepinephrine, which are involved in the stress response.
Pancreatic Islets: Embedded within the pancreas, these clusters of cells primarily secrete insulin and glucagon, crucial for regulating blood sugar levels. They also produce somatostatin, which inhibits the secretion of other hormones.
Thyroid Gland: Situated in the neck, this gland produces thyroid hormones (thyroxine and triiodothyronine), which regulate metabolism, and calcitonin, which plays a role in calcium homeostasis.
Parathyroid Glands: Found on the posterior surface of the thyroid gland, these glands secrete parathyroid hormone, essential for maintaining calcium balance.
Pineal Gland: Located in the brain, this gland secretes melatonin, a hormone involved in regulating circadian rhythms.