How does the secretion from an endocrine gland get into the blood?
Substances secreted by cells can have several actions:
Autocrine: the substance acts upon the cell that secreted it–an autofeedback mechanism
Paracrine: the substance acts upon other cells in the immediate vicinity–a localized effect
Endocrine: the substance acts upon other cells at a distance
Thus, the hormones secreted by the endocrine glands enter the circulation and are carried throughout the body to act upon target cells located far away from the secreting glands. Endocrine glands secrete directly into the bloodstream, not through a duct system. Thus, endocrine glands are highly vascularized with many small capillaries among the nests of endocrine cells.
The pituitary develops embryologically from two sources. The anterior pituitary (adenohypophysis, or pars distalis) is derived from an upward evagination of pharyngeal epithelium known as Rathke’s pouch. Occasionally, remnants of Rathke’s pouch persist between anterior and posterior pituitary that are nests of cells have a squamous appearance. The posterior pituitary (neurohypophysis, or pars nervosa) is derived from a downgrowth of neural tissue from the hypothalamus. Between these two parts is the pars intermedia that consists of small cystic structures lined by a cuboidal epithelium and containing pink proteinaceous material–and rarely Rathke’s pouch remnants. The pars intermedia doesn’t do anything. The median eminence of the hypothalamus above continues down as an infundibular stalk that connects to the pituitary below in the sella turcica. The pituitary is surrounded by meninges.
The adenohypophysis has three major cell types based upon their appearance with H&E staining:
Acidophils: these cells stain bright pink. They produce either growth hormone (GH) or prolactin (Prl).
Basophils: these cells stain purple and produce either thyroid stimulating hormone (TSH), adrenocorticotrophic hormone (ACTH), or the gonadotrophic hormones follicle-stimulating hormone (FSH) and luteinizing hormone (LH) known in the male as interstitial cell stimulating hormone (ICSH).
Chromophobes: these are pale-staining cells that are not associated with specific hormone secretion.
The blood supply of the pituitary consists of hypophyseal arteries that form a superficial network of capillaries that become a portal plexus of veins that supply the substance of the pituitary with blood. This portal system drains blood from the median eminence, where hypothalamic releasing factors are formed, to the pituitary, where the releasing factors signal cells of the adenohypophysis to release their hormones.
The neurohypophysis is essentially just the termination point of axons that derive from the supraoptic and paraventricular nuclei in the hypothalamus. The hormones vasopressin (antidiuretic hormone, or ADH) and oxytocin are synthesized in the hypothalamus and transported down the infundibular stalk to the pars nervosa, from which they are released.
The adrenals lie atop the kidneys in the retroperitoneum. The adrenals are covered with a thin connective tissue capsule. Arterial blood is supplied to the adrenal from three sources: phrenic artery branch, renal artery branch, aortic branch. The adrenal vein drains directly to the inferior vena cava. The adrenal has two major components: cortex and medulla. Each adrenal weighs about 4 to 6 grams.
In fetal life, the adrenals first develop during the fifth week of gestation, when coelomic epithelial cells proliferate in response to induction by the ureteric bud, giving rise to the adrenal cortical primordium. These mesodermal cells arise from a wide plate of coelomic epithelium in the most internal region of the mesonephric blastema between the mesenteric root and the gonadal primordium and penetrate the underlying mesenchyme, forming the primitive adrenal cortex, also known as the fetal adrenal cortex, which eventually becomes the zona reticularis. The developing fetal adrenal cortex is then invaded by sympathogonia of neural crest origin that appear as neuroblasts and form the adrenal medulla. At the end of the first trimester, another proliferation of coelomic epithelium occurs, penetrating the mesenchyme and surrounding the fetal cortex to eventually become the adult adrenal cortex that forms the zona glomerulosa and zona fasciculata.
Zona glomerulosa: the outermost layer of cortex produces mineralocorticoids, principally aldosterone.
Zona fasciculata: the middle and largest layer of adrenal cortex produces glucocorticoids, principally cortisol.
Zona reticularis: the innermost layer of cortex produces sex steroids, principally dehydroepiandrosterone (DHEA).
The medulla is composed of large, irregular cells that produce the catecholamines norepinephrine and epinephrine. Within the medulla may be seen the adrenal vein with its eccentrically placed smooth muscle and sympathetic ganglion cells.
The thyroid is located anterior to the thyroid cartilage just below the larynx in the neck. The thyroid is derived embryologically from a downward migration of epithelium from the foramen cecum of the tongue along the thyroglossal duct, forming the fetal thyroid. Remnants of this migrational path are known as thyroglossal duct cysts. The thyroid consists of a right and left lobe connected by an isthmus. The weight is between 10 and 30 grams.
The thyroid parenchyma consists of many follicles with a rich vascularized stroma containing scattered “C” cells (parafollicular cells) and minimal collagenous septae. The follicles are lined by a cuboidal epithelium and filled with a pink-staining colloid material which is the stored gel-like thyroglobulin. With thyroid stimulating hormone (TSH) secretion by the pituitary, thyroglobulin is processed by the follicular epithelial cells to thyroxine (T4), and also a small amount of triiodothyronine (T3), which are then released into the bloodstream. TSH stimulation causes the follicular epithelium to become more columnar. Trace amounts of iodine in the diet are needed to form thyroid hormones.
The “C” cells in the interstitium between the follicles produces calcitonin, which in humans has minimal functionality.
The parathyroid glands are embryologically derived from the 3rd and 4th pharyngeal pouches. Since the thymus is also derived from the 4th pharyngeal pouches, thymic tissue may be seen adjacent to parathyroid, and parathyroid may be found in the anterior mediastinum, which is of no practical significance unless you are a surgeon trying to find a parathyroid adenoma. There are normally four parathyroids, but there can rarely be two or six of them. A normal parathyroid gland weighs about 80 to 100 milligrams and is an ovoid structure 2 x 3 x 5 mm in size.
The parathyroid glands have a thin connective tissue capsule. There is a rich vascular supply to the stroma. The parenchyma consists of chief cells that secrete parathyroid hormone (parathormone) under the influence of decreasing serum calcium. There are also variable numbers of oxyphil cells in small nodules. The pink oxyphil cells have cytoplasm packed with mitochondria, so they must be doing something, perhaps transitioning to chief cells. A variable number of steatocytes are scattered in the parathyroid parenchyma.
This obscure gland resides at the base of the brain, attached near the habenula. It is a diencephalic structure, but has an endocrine function. This 5 x 8 mm gland has rounded pinealocytes with surrounding interstitial cells that function similar to neuroglial cells. Beginning in childhood and increasing throughout life, calcified concretions (“brain sand”) appear in the pineal.
The pinealocytes secrete the hormone melatonin, which may play a role in regulation of circadian rhythms and reproductive cycles. Peak melatonin production occurs at night. The pineal is innervated by a circuitous pathway from postganglionic sympathetic nerves that arise in the superior cervical ganglia in the neck. These nerve endings release norepinephrine to control melatonin production.