Endocrine gland

The major endocrine glands: 
1 Pineal gland
2 Pituitary gland
3 Thyroid gland
4 Thymus
5 Adrenal gland
6 Pancreas
7 Ovary
8 Testicle
Endocrine glands in the human head and neck and their hormones

Endocrine glands are ductless glands of the endocrine system that secrete their products, hormones, directly into the blood.

- Endocrine gland

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Pineal gland

Pineal gland or epiphysis (in red)
Pineal gland parenchyma with calcifications
Micrograph of a normal pineal gland – very high magnification
Micrograph of a normal pineal gland – intermediate magnification
Diagram of the operation of the pineal gland for Descartes in the Treatise of Man (figure published in the edition of 1664)
Mesal aspect of a brain sectioned in the median sagittal plane
Dissection showing the ventricles of the brain
Hind- and mid-brains; antero-lateral view
Median sagittal section of brain
Pineal gland
Brainstem; posterior view

The pineal gland, conarium, or epiphysis cerebri, is a small endocrine gland in the brain of most vertebrates.

Hormone

Any member of a class of signaling molecules in multicellular organisms, that are transported by intricate biological processes to distant organs to regulate physiology and behavior.

Left: A hormone feedback loop in a female adult. (1) Follicle-Stimulating Hormone, (2) Luteinizing Hormone, (3) Progesterone, (4) Estradiol. Right: Auxin transport from leaves to roots in Arabidopsis thaliana
The left diagram shows a steroid (lipid) hormone (1) entering a cell and (2) binding to a receptor protein in the nucleus, causing (3) mRNA synthesis which is the first step of protein synthesis. The right side shows protein hormones (1) binding with receptors which (2) begins a transduction pathway. The transduction pathway ends (3) with transcription factors being activated in the nucleus, and protein synthesis beginning. In both diagrams, a is the hormone, b is the cell membrane, c is the cytoplasm, and d is the nucleus.
Blood glucose levels are maintained at a constant level in the body by a negative feedback mechanism. When the blood glucose level is too high, the pancreas secretes insulin and when the level is too low, the pancreas then secretes glucagon. The flat line shown represents the homeostatic set point. The sinusoidal line represents the blood glucose level.

In vertebrates, endocrine glands are specialized organs that secrete hormones into the endocrine signaling system.

Pituitary gland

Located at the base of the brain, the pituitary gland is protected by a bony structure called the sella turcica of the sphenoid bone.
Location of the human hypothalamus.
The Hypothalamus-Pituitary Complex.
The Limbic Lobe.
Histology of pituitary gland
A normal-sized hand (left) and the enlarged hand of someone with acromegaly (right)
Location of the pituitary gland in the human brain
Pituitary and pineal glands
The arteries of the base of the brain.
Mesal aspect of a brain sectioned in the median sagittal plane.
Pituitary
Pituitary gland
Cerebrum.Inferior view.Deep dissection.

In vertebrate anatomy, the pituitary gland, or hypophysis, is an endocrine gland, about the size of a pea and weighing, on average, 0.5 g in humans.

Gland

Group of cells in an animal's body that synthesizes substances for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland).

Human submandibular gland. At the right is a group of mucous acini, at the left a group of serous acini.
This image shows some of the various possible glandular arrangements. These are the simple tubular, simple branched tubular, simple coiled tubular, simple acinar, and simple branched acinar glands.
This image shows some of the various possible glandular arrangements. These are the compound tubular, compound acinar, and compound tubulo-acinar glands.
Here is a diagram that shows the differences between endocrine and exocrine glands. The major difference is that exocrine glands secrete substances out of the body and endocrine glands secrete substances into capillaries and blood vessels.
Histopathology of sclerosing adenosis of the breast.

Endocrine glands secrete substances that circulate through the blood stream.

Endocrine system

Messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs.

Main glands of the endocrine system
Female endocrine system
Male endocrine system

In humans, the major endocrine glands are the thyroid gland and the adrenal glands.

Thyroid

The human thyroid (tan), as viewed from the front; and arteries (red) supplying the gland.
The thyroid gland surrounds the cricoid and tracheal cartilages and consists of two lobes. This image shows a variant thyroid with a pyramidal lobe emerging from the middle of the thyroid.
Clear pyramidal lobe (center) as viewed from the front
Section of a thyroid gland under the microscope. 1 colloid, 2 follicular cells, 3 endothelial cells
Floor of pharynx of embryo between 35 and 37 days after fertilization.
The thyroid hormones T3 and T4 have a number of metabolic, cardiovascular and developmental effects on the body. The production is stimulated by release of thyroid stimulating hormone (TSH), which in turn depends on release of thyrotropin releasing hormone (TRH). Every downstream hormone has negative feedback and decreases the level of the hormone that stimulates its release.
Synthesis of the thyroid hormones, as seen on an individual thyroid follicular cell: 
 - Thyroglobulin is synthesized in the rough endoplasmic reticulum and follows the secretory pathway to enter the colloid in the lumen of the thyroid follicle by exocytosis.
 - Meanwhile, a sodium-iodide (Na/I) symporter pumps iodide (I−) actively into the cell, which previously has crossed the endothelium by largely unknown mechanisms. 
 - This iodide enters the follicular lumen from the cytoplasm by the transporter pendrin, in a purportedly passive manner.
 - In the colloid, iodide (I−) is oxidized to iodine (I0) by an enzyme called thyroid peroxidase.
 - Iodine (I0) is very reactive and iodinates the thyroglobulin at tyrosyl residues in its protein chain (in total containing approximately 120 tyrosyl residues).
 - In conjugation, adjacent tyrosyl residues are paired together.
 - The entire complex re-enters the follicular cell by endocytosis.
 - Proteolysis by various proteases liberates thyroxine and triiodothyronine molecules, which enters the blood by largely unknown mechanisms.
Child affected by Congenital iodine deficiency syndrome, associated with a lack of iodine.
The thyroid was named by Thomas Wharton after the ancient Greek shield of a similar pronunciation. Shown is an example of such a shield, as engraved on a coin dating from 431 to 424 BCE.
Goat affected by a goitre

The thyroid, or thyroid gland, is an endocrine gland in vertebrates.

Anterior pituitary

Glandular, anterior lobe that together with the posterior lobe (posterior pituitary, or the neurohypophysis) makes up the pituitary gland (hypophysis).

Median sagittal through the hypophysis of an adult monkey. Semidiagrammatic.
The anterior pituitary complex
Microanatomy of the pars distalis showing chromophobes, basophils, and acidophils
Eosinophilic follicles are a common normal finding in the anterior pituitary.
The anterior pituitary is the anterior, glandular lobe of the pituitary gland.

Tropic hormones are named for their ability to act directly on target tissues or other endocrine glands to release hormones, causing numerous cascading physiological responses.

Organ (biology)

Organ is a collection of tissues joined in a structural unit to serve a common function.

Many of the internal organs of the human body
The liver and gallbladder of a sheep
Relationship of major animal lineages with indication of how long ago these animals shared a common ancestor. On the left, important organs are shown, which allows us to determine how long ago these may have evolved.
The flower is the angiosperm's reproductive organ. This Hibiscus flower is hermaphroditic, and it contains stamen and pistils.
Strobilus of Equisetum telmateia
Human viscera

Endocrine system: communication within the body using hormones made by endocrine glands such as the hypothalamus, pituitary gland, pineal body or pineal gland, thyroid, parathyroids and adrenals, i.e., adrenal glands.

Adrenal gland

The adrenal glands lie above the kidneys.
Adrenal glands, anterior (left) and posterior (right) surface.
Section of human adrenal gland under the microscope, showing its different layers. From the surface to the center: zona glomerulosa, zona fasciculata, zona reticularis, medulla. In the medulla, the central adrenomedullary vein is visible.
Different hormones are produced in different zones of the cortex and medulla of the gland. Light microscopy at magnification × 204.
Steroidogenesis in the adrenal glands – different steps occur in different layers of the gland
Negative feedback in the HPA axis
Characteristic skin hyperpigmentation in Addison's disease
Incidences and prognoses of adrenal tumors.

The adrenal glands (also known as suprarenal glands) are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol.

Parathyroid gland

Diagram showing structures in the human neck. The four green shaded areas represent the most common position of the parathyroid glands, which are generally four in number and situated behind the lateral lobes of the thyroid gland (shaded orange).
3D Medical Animation still shot showing Hyperparathyroidism
Intermediate magnification micrograph. H&E stain. The white round structures are fat cells. Adipose tissue comprises 25–40% of normal parathyroid gland tissue.
High magnification micrograph. H&E stain. The small, dark cells are chief cells, which are responsible for secreting parathyroid hormone. The cells with orange/pink staining cytoplasm are oxyphil cells
Gross pathology of a parathyroid gland (white arrow), next to the thyroid gland
Scheme showing development of branchial epithelial bodies. I, II, III, IV. Branchial pouches.

Parathyroid glands are small endocrine glands in the neck of humans and other tetrapods.