ENDOCRINE GLANDS

endocrine glands 
Endocrinology is becoming more difficult to define in a meaningful way. The concept has been accepted for many years that an endocrine gland produces a chemical substance (hormone) that is carried by the circulation to a target organ at some considerable distance from the original endocrine gland.

 As an example of some difficulty of classification under this definition, the adrenal medulla produces epinephrine, which is carried by the blood to distant target organs. 

Sympathetic nerves, however, produce a similar substance, norepinephrine, that acts in the immediate vicinity of the nerve ending. It is difficult to classify one substance as a hormone and not the other, because they have so many similarities,

THE PITUITARY GLAND

The pituitary gland (hypophysis cerebri) is located at the base of the brain in the sella turcica (Turkish saddle), a depression in the sphenoid bone. on the floor of the cranial cavity. The pituitary gland consists of an anterior lobe, an intermediate lobe, and a Posterior lobe

The anterior lobe and intermediate lobe are formed from Rathke' s pocket or pouch, a structure derived from the mucous membrane of the embryonic pharynx. This accounts for the epithelial structure of these lobes.

The posterior lobe (pars nervosa or neurohypophysis) originates from the embryonic brain, and in the adult is still connected to the brain by means of the pituitary stalk. The infundibulum of the stalk attaches to the tuber cinereum, a cone-shaped projection from the brain located at the base of the brain between the optic chiasm (crossing of the optic nerves) and the mammillary body.

The anterior lobe of the pituitary has a projection called the pars tuberalis that extends a variable distance along the front of the pituitary stalk toward the brain.

ADRENAL GLAND
The adrenal glands (also called suprarenal glands from their position in man) are located close to the kidneys. Shape, size, and exact location vary from one species to another.

 Each adrenal gland consists of an outer (peripheral) zone (the cortex) and an inner zone (the medulla), with the entire gland surrounded by a connective tissue capsule.

The parenchymal cells of both the cortex and medulla are arranged in clumps that are related to blood vessels.
thyroid gland
The thyroid gland consists of two lobes located near the thyroid cartilage of the larynx. One lobe is found on each side of the larynx., and an isthmus may or may not connect the two lobes, depending on the species.

A connective tissue capsule covers the gland and sends into the substance of the thyroid, septa which give support and conduct vessels to the epithelial cells.
PARATHYROID GLAND
The parathyroid glands are small nodules located within or near the thyroid gland. Commonly there are two parathyroid glands on each side, but the exact number and location vary with the species.

 Accessory parathyroid glands may be found at a considerable distance from the usual glands. These accessory glands may cause inconstant results in experiments involving removal of the parathyroid glands.

 Parenchyma of the parathyroid glands consists of clumps and cords of epithelial cells interspersed with capillaries. Two types of cells are described, chief cells and oxyphil cells. Chief cells are small cells with dark-staining nuclei and either granular or clear cytoplasm. The closeness of the nuclei to each other gives the tissue a dark.

THE PANCREAS
The islets of _Langerhans are clumps of pale-staining cells scattered among the alveoli and ducts of the pancreas. Cells of the islets, the endocrine portion of the pancreas, are arranged in irregular cords separated by capillaries.

 Special stains are used to demonstrate the four types of epithelial cells found in the islets of Langerhans. These are the A or alpha cells, B or beta cells, C cells and D cells.

The B cells are the most common and produce the hormone insulin, which is necessary to prevent diabetes mellitus.

 A cells produce the hormone, glucagon, that is antagonistic to insulin. The C and D cells look much the same, and their significance is unknown.

THE PINEAL BODY
The pineal body (epiphysis cerebri) is sometimes included as an endocrine gland. It contains a high content of serotonin and an enzyme which converts this to melatonin.


The enzyme, hydroxyindole-O-methyl transferase, has not been found in any tissue other than the pineal body. The fact that it functions with a circadian rhythm and hence does not release its products constantly, points to consideration of photo-periods having an influence in animals via this body, The pineal body is located above the thalamus and is attached to the roof of the third ventricle.

 Parenchymal cells, neuroglial cells, and nerve fibers are found in the substance of the pineal body. There is some evidence that the pineal body tends to inhibit development of gonads in the male.
THE THYMUS GLAND
The thymus gland is the true sweetbread, although the pancreas is sometimes classed as a sweetbread. The thymus is a lymphoid organ found on both sides of the trachea within the cranial mediastinal space and along the neck for a variable distance, depending on the age and species of animal.

 During early development of the thymus it resembles an endocrine gland, because of the arrangement of parenchymal cells in cords. However, production of a hormone has not been proven.

 As development progresses, the parenchymal cells are crowded by cells that appear to be identical to small lymphocytes, but are called thymocytes.

 Odd groups of degenerating cells called thymic corpuscles (Hassal's corpuscles) are scattered throughout the substance of the thymus.

 Current evidence suggests that the thymus is the source of a blood borne factor which induces the differentiation of lymphoid precursor or stem cells, making them capable of taking part in immune reactions.