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Endocrine system

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Main organs of the endocrine system.

The endocrine system is made up of endocrine glands located throughout the body. These glands produce different endocrine hormones and release them into the bloodstream via which they travel to specific target organs. This way the endocrine system acts as a signaling pathway regulating a number of important body functions such as growth, metabolism, temperature, reproductive development, and many other bodily functions and processes.

Here, we will discuss the microscopic appearance of three components of the endocrine system: the parathyroid, thyroid and suprarenal glands.

Parathyroid gland

The parathyroid glands are four small circular glandular structures embedded in the posterior surface of the thyroid gland. They are an endocrine gland, producing parathyroid hormone (parathormone, PTH) which acts to control the level of calcium and phosphorus in the blood.

Structurally, each parathyroid gland is encased by a thin connective tissue capsule that separates it from the thyroid. Septa extend from the capsule into the gland to divide it into irregular lobules and to separate the densely packed cords of cells. Principal cells and oxyphil cells form the epithelium of the parathyroid gland.

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Thyroid gland

The thyroid gland is a bilobular endocrine gland that is found in the neck, anterior and inferior to the larynx. The main purpose of this organ is to produce, store and secrete the iodine-based hormones triiodothyronine (T3) and thyroxine (T4).

The thyroid gland is encased by a thin connective tissue capsule that enters the substance of the lobes to further subdivide the gland into irregular lobular units. Each lobule contains a cluster of follicles, which are the structural and functional units of the thyroid gland.

Each follicle is surrounded by thin connective tissue stroma rich in fenestrated capillaries. These follicles are lined with follicular epithelium which is simple epithelium consisting of low columnar, cuboidal or squamous cells depending on the level of activity of the follicle. When the follicles are active, they appear cuboidal to low columnar, but when they are inactive the cells are squamous.

Another cell type that can be found here is parafollicular cells, also known as C (clear) cells. They can be found within the basal lamina of the thyroid follicles without extending into the follicular lumen or between thyroid follicles in the interfollicular space, either singly or in the form of groups.

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Suprarenal (or adrenal) glands

The suprarenal (or adrenal) glands are paired retroperitoneal endocrine glands situated over the medial aspects of the upper poles of each kidney. They play a vital role in the body's fight or flight response by generating stress hormones that activate physiological adaptations to counteract changes in the external environment.

The suprarenal glands are covered with a thick connective tissue capsule from which trabeculae extend into the parenchyma.

Deep to the capsule, is the suprarenal cortex, which is the steroid-secreting portion of the gland. The cortex can be subdivided into three zones:

  • Zona glomerulosa - responsible for secreting mineralocorticoids such as aldosterone
  • Zona fasciculata - secretes glucocorticoids to increase the overall blood glucose level
  • Zona reticularis - production of suprarenal androgens which serve as precursors for testosterone.

A thin, grey medulla can be found at the center of the suprarenal gland. Chromaffin cells, ganglion cells and dilated capillaries reside here. Chromaffin cells are responsible for the production of catecholamines, namely epinephrine (adrenaline), norepinephrine (noradrenaline) and dopamine. Epinephrine is released directly into the medullary capillaries and carried to their site of action via systemic circulation. Ganglion cells modulate the secretory activity of the suprarenal cortex and their axons extend to the splanchnic nerves to innervate abdominal organs.

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