Endocrine Organs
Communication between cells is necessary to maintain homeostasis and
coordinate growth and development. The primary function of two major
organ system, the nervous system and the endocrine system , is intercellular
communication .
The endocrine system communicates through the release of hormones ,
secretory products of endocrine cells and organs , that pass into the
circulatory system for transport to target cells that posses receptors for the
hormone .
The endocrine system produces a slower and more prolonged response than
the nervous system . The release of hormones also induces a more
generalized response as the hormones reach target cells in widely separated
organs or tissues .
Functionally , these two system are closely interrelated and may overlaped
in function . Both systems may act simultaneously on the same target cells
and tissues , and some nerve cells secrete hormones .
1-The hypothalamus
A part of the brain , as one of the major controlling centers of the
autonomic nervous system , also it releases many of the releasing and
inhibiting hormones .
2-Hypophysis ( PITUITARY GLAND )
The hypophysis is a pea sized, compound endocrine gland . it is located in
the base of the skull in a depression of the sphenoid bone , called the sella
turcica . A short stalk , the infundibulum , attaches the pituitary to the
hypothalamus.It consists of two major subdivisions :
1- Adenohypophysis
Is composed of three parts : Pars tuberalis , pars distalis and pars
intermedia.
 Pars tuberalis,
Is composed of numerous cuboidal cells whose
function is not known .
 Pars distalis , contains numerous parenchymal cells arranged in thick
cords . The parenchymal cells are classified into two main categories :
those whose granules readily take up stain, chromophils , and those cells
that do not possess a strong affinity for stains , chromophobes .
Chromophils are of two types , acidophils and basophils . Hormones that
modulate the secretory functions of the pituitary – dependent endocrine
glands are somatotropin , thyrotropin (TSH) , follicle – stimulating hormone
(FSH), luteinizing hormone (LH) , prolactin , adrenolating hormone (ACTH)
and melanocyte – stimulating hormone (MSH) .
 Pars intermedia. Contains colloid – filled cysts lined by different cells
with basophils predominantly .
2- Neurohypophysis
 Infundibular Stalk
 Pars Nervosa
It is composed of pituicytes , cells believed to be neuroglial in nature that
my fulfill a supporting function for the numerous unmyelinated axons of the
pars nervosa. These axons whose cell bodies are located in the supraoptic
and paraventricular nuclei of the hypothalamus , enter the pars nervosa via
the hypothalmo-hypophyseal tract . They contain oxytosin and antidiuretic
hormone (ADH, vasopressin) , two neurosecretory hormones that are stored
in the pars nervosa but are manufactured in the cell bodies in hypothalamus.
3-Thyroid Gland
The thyroid gland consists of right and left lobes that are interconnected
by a narrow isthmus across the thyroid cartilage and upper trachea. The
parenchymal cells of the gland are arranged in numerous follicles, composed
of a simple cuboidal epithelium lining a central colloid filled lumen . The
colloid, secreted and resorbed by the follicular cells, is composed of the
thyroid hormone that is bound to a large protein and the complex is known
as thyroglobulin . An additional secretory cell type, parafollicular cells , is
present in the thyroid, they manufacture the hormone calcitonin .Throid
hormone is essential for regulation basal metabolism , calcitonin helps
control calcium concentrations in the blood.
4- Parathyroid Glands
They are usually four in number, are embedded in the fascial sheath of the
posterior aspect of the thyroid gland. Two types of cells are present :
a) numerous small chief cells which produced parathyroid hormone
(PTH) , the most important regulator of calcium in the body .
b) a smaller number of large acidophilic cells, the oxyphils.
F: follicle, FC: follicular cells, PF: Para follicular cells ,CT: connective t. , Cl : colloid
5- Adernal gland
The adrenal glands are invested by a connective tissue capsule.
 Cortex
The cortex is subdivided into three concentric regions or zones . The
outermost region , just beneath the capsule , is the zona glomerulosa , where
the cells are arranged in arches and spherical clusters with numerous
capillaries surrounding them . The second region, the zona fasciculata , is
the most extensive , its parenchymal cells , usually known as spongiocytes ,
are arranged in long cords, with numerous capillaries between the cords .
The innermost region of the cortex , the zona reticularis, is arranged in
anastomosing cords of cells with a rich intervening capillary network . Three
types of hormones are produced by the suprarenal cortex , namely ,
mineralocorticoids (zona glomerulosa ), glucocorticoids (zona fasciculata
and , to some extent , zona reticularis ) , and some androgens (zona
fasciculata and zona reticularis ) .
 Medulla
The cells of the medulla, disposed in irregularly arranged short cords
surrounded by capillary networks, contain numerous granules that stain
intensely when the freshly cut tissue is exposed to chromium salts. This is
referred to as the chromaffin reaction, and the cells are called chromaffin
cells. There are two types of chromaffin cells: one produces epinephrine,
while the other manufactures norepinephrine, the two hormones of the
suprarenal medulla. Since these cells are innervated by preganglionic
sympathetic nerve fibers, chromaffin cells are considered to be related to
postganglionic sympathetic nerurons.
6-Pineal Body
The pineal body (epiphysis) is a projection of the roof of the
diencephalon. The main cellular elements of the pineal body are
pinealocytes and neuroglial cells. The pinealocytes
manufacture
melatonin and serotonin, while neuroglial cells lend support to
pinealocytes. Interestingly, serotonin is only produced during daylight, while
melatonin is manufactured only at night. The intercellular spaces of the
pineal body contain calcified granular material known as brain sand
(corpora arenacea) whose significance, if any, is not known.