Dr.Abdulla … ……….………….Histology………………………Endocrine system
The endocrine system:
It consisting of ductless glands, distinct clusters of cells within certain organs of the body, and diffuse neuroendocrine cells, regulates metabolic activities in certain organs and tissues of the body, thereby helping to bring about homeostasis brought about by chemical substances called hormones, which are released into the bloodstream to influence target cells at remote sites. pituitary gland (Hypophysis)
The hypophysis (Gr. hypo, under, + physis, growth), or pituitary gland. It lies in a cavity of the sphenoid bone—the sella turcica.the hypophysis develops partly from oral ectoderm and partly from nerve tissue. Because of its dual origin, the hypophysis consists of two glands:the neurohypophysis and the adenohypophysis. the neurohypophysis, the part of the hypophysis that develops from nerve tissue, consists of a large portion, the pars nervosa, and the smaller infundibulum. Which is composed of the stem and median eminence. The part of the hypophysis that arises from oral ectoderm is known as the adenohypophysis and is subdivided into three portions: a large pars distalis, or anterior lobe; a cranial part, the pars tuberalis, which surrounds infundibulum; and the pars intermedia (Figure -1).
Figure -1 pituitary gland
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Dr.Abdulla … ……….………….Histology………………………Endocrine system
Blood Supply of the Pituitary Gland
The pituitary gland is connected to the brain by neural pathways; it also has a rich vascular supply from vessels that supply the brain, secretion of nearly all of the hormones produced by the pituitary gland is controlled by either hormonal or nerve signals from the hypothalamus.
The arterial supply for the pituitary gland is provided from two pairs of vessels that arise from the internal carotid artery. The superior hypophyseal arteries supply the pars tuberalis and the infundibulum. They also form an extensive capillary network, the primary capillary plexus, in the median eminence. Inferior hypophyseal arteries primarily supply the posterior lobe (Figure -1).
Hypophyseal portal veins drain the primary capillary plexus of the median eminence, which delivers its blood into the secondary capillary plexus, located in the pars distalis. Hypothalamic neurosecretory hormones, manufactured in the hypothalamus and stored in the median eminence, enter the primary capillary plexus and are drained by the hypophyseal portal veins, which connect to the secondary capillary plexus in the anterior lobe. the neurosecretory hormones leave the blood to stimulate or inhibit the parenchymal cells.
Axons of neurons that originate in various portions of the hypothalamus terminate around these capillary plexuses.
A-The adenohypophysis
1-Pars Distalis
The main components of the pars distalis are cords of epithelial cells interspersed with capillaries. The hormones produced by these cells are stored as secretory granules.
The pars distalis possesses three types of cells, acidophils and basophils
(together known as chromophils ) and chromophobes .
Chromophobes generally have less cytoplasm than chromophils do, and they may represent either nonspecific stem cells or partially degranulated chromophils.
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Dr.Abdulla … ……….………….Histology………………………Endocrine system
Immunocytochemical methods and electron microscopy distinguish five types from acidophils and basophils.
There are two types of acidophils: somatotrophs and mammotrophs .
Somatotrophs secrete somatotropin (growth hormone); they are are stimulated by SRH and inhibited by somatostatin . Somatotropin has a generalized effect of increasing cellular metabolic rates. This hormone also induces liver cells to produce insulin-like growth factors I and II which stimulate the mitotic rates of epiphyseal plate chondrocytes and thus promote elongation of long bones.
Mammotrophs release the hormone prolactin, which promotes mammary gland development during pregnancy as well as lactation after birth.
Basophils house granules that stain blue with basic dyes. There are three types of basophils, corticotrophs, thyrotrophs, and gonadotrophs .
Corticotrophs, are round to ovoid cells, each with an eccentric nucleus and relatively few organelles. They secrete adrenocorticotropic hormone (ACTH) and lipotropic hormone (LPH). Secretion is stimulated by CRH. ACTH stimulates cells of the suprarenal cortex to release their secretory products.
Thyrotrophs can be distinguished by their small secretory granules which contain TSH, also known as thyrotropin. Secretion is stimulated by TRH and inhibited by the presence of thyroxine (T4) and triiodothyronine (T3) (thyroid hormones) in the blood.
Gonadotrophs are round cells whose secretory granules vary in diameter from
200 to 400 nm. They secrete FSH and LH; sometimes LH is called interstitial cell– stimulating hormone (ICSH), because it stimulates steroid hormone production in interstitial cells of the testes. Secretion is stimulated by GnRH.
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Dr.Abdulla … ……….………….Histology………………………Endocrine system
2-Pars Intermedia made up of cords and follicles of weakly basophilic cells that contain small secretory granules. α-Melanocyte-stimulating hormone (α-MSH).
3-Pars Tuberalis
The pars tuberalis is a funnel-shaped region surrounding the infundibulum of the neurohypophysis (Figure 20–2). Most of the cells of the pars tuberalis secrete gonadotropins (follicle-stimulating hormone and luteinizing hormone).
B-The neurohypophysis:
consists of the pars nervosa and the neural stalk
The distal terminals of the axons of the hypothalamohypophyseal tract end in the pars nervosa and store the neurosecretions that are produced by their cell bodies, which are located in the hypothalamus. These axons are supported by glia-like cells known as pituicytes. One population of axons contains membrane-bound granules of vasopressin and that another population contains oxytocin. These hormones are housed in dilatations of the axons called Herring bodies. The contents of these granules are released into the perivascular space near the fenestrated capillaries of the capillary plexus. Vasopressin acts on the nephron of the kidney and oxytocin facilitates contraction of the uterine myometrium.
Adrenal (Suprarenal) Glands
The adrenal glands are paired organs that lie near the superior poles of the kidneys, embedded in adipose tissue (Figure -2). They are flattened structures is divided into two histologically and functionally different regions: an outer yellowish portion, called the suprarenal cortex, and a small, dark, inner portion called the suprarenal medulla (figure-2) . The adrenal cortex and the adrenal medulla can be considered two organs with distinct origins, functions, and morphological characteristics that became united during embryonic development. The cortex arises
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Dr.Abdulla … ……….………….Histology………………………Endocrine system from the coelomic epithelium, whereas the cells of the medulla derive from the neural crest, from which sympathetic ganglion cells also originate.
The suprarenal cortex:
It is divided into three concentric regions, the outermost zona glomerulosa, the middle (and largest) zona fasciculata, and the innermost zona reticularis.
1-The zona glomerulosa : the layer beneath the connective tissue capsule is the zona glomerulosa, in which columnar or pyramidal cells are arranged in closely packed surrounded by capillaries when stimulated by ACTH, synthesize and release the hormones aldosterone and deoxycorticosterone.
2-The zona fasciculata: layer of cells is known as the zona fasciculata because of the arrangement of the cells in one- or two-cell thick straight cords that run at right angles to the surface of the organ and have capillaries between them when stimulated by ACTH, synthesize and release the hormones cortisol and corticosterone.
3-The zona reticularis ,: it contains cells disposed in irregular cords that form an anastomosing network. when stimulated by ACTH, synthesize and release dehydroepiandrosterone, androstenedione, and some glucocorticoids.
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Dr.Abdulla … ……….………….Histology………………………Endocrine system
Figure-2 :adrenal gland
Adrenal Medulla :
The adrenal medulla is composed of polyhedral cells arranged in cords and supported by a reticular fiber network. capillary supply intervenes between adjacent cords, and there are a few parasympathetic ganglion cells. Thus, the cells of the adrenal medulla can be considered modified sympathetic postganglionic neurons that have lost their axons and dendrites during embryonic development and have become secretory cells.
Medullary cells have dense secretory granules, These granules contain one or the other of the catecholamines, epinephrine or norepinephrine.
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Dr.Abdulla … ……….………….Histology………………………Endocrine system
3-The pancreas:
It produces exocrine and endocrine secretions. The endocrine components of the pancreas, islets of Langerhans, are scattered among the exocrine secretory acini
( figure-3). The Islets have five cell types, alpha cells (produce glucagon), beta cells (produce insulin), PP cells (produce pancreatic polypeptide), G cells
(manufacture gastrin), and delta cells (manufacture somatostatin)
Table -1
.
Table -1. Cell Types in Islets of Langerhans.
Cell
Type
A
B
D
Hormone
Produced
Glucagon
Hormonal Function
Insulin
Acts on several tissues to make energy stored in glycogen and fat available through glycogenolysis and lipolysis; increases blood glucose content
Acts on several tissues to cause entry of glucose into cells and promotes decrease of blood glucose content
Somatostatin Inhibits release of other islet cell hormones
.somatostatin also lower gastric motility pp Pancreatic polypeptide
G cells gastrin
Control of gastric secretion and Control of secretion of the exocrine pancreas? manufacture gastrin
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Dr.Abdulla … ……….………….Histology………………………Endocrine system
Figure -3The pancreas with secretory acini, their cell types, and the endocrine islets of Langerhans.
Thyroid gland :
The thyroid gland, located in the cervical region anterior to the larynx, consists of two lobes united by an isthmus (Figure -4). Its function is to synthesize the hormones thyroxine (T
4
) and triiodothyronine (T
3
), which are important for growth, for cell differentiation, and for the control of oxygen consumption and the basal metabolic rate in the body. Thyroid hormones affect the metabolism of proteins, lipids, and carbohydrates.
Thyroid tissue is composed of 20–30 million microscopic spheres called thyroid follicles.
The follicles are lined by a simple epithelium and their central cavity contains a gelatinous substance called colloid which is composed of a glycoprotein called thyroglobulin.
(Figures -4). The thyroid epithelium rests on a basal lamina. Its cells exhibit the characteristics of a cell that simultaneously synthesizes, secretes, absorbs, and digests proteins .
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Dr.Abdulla … ……….………….Histology………………………Endocrine system
Another type of cell present in the thyroid, the parafollicular, or C, cell, is found as part of the follicular epithelium or as isolated clusters between thyroid follicles (Figures -4). These cells are responsible for the synthesis and secretion of calcitonin, a hormone whose main effect to lower blood calcium levels by inhibiting bone resorption. Secretion of calcitonin is triggered by an elevation in blood calcium concentration. The gland is covered by a loose connective tissue capsule that sends septa into the parenchyma.
Figure -4 thyroid and parathyroid glands
Control of the Thyroid:
The major regulator of the anatomic and functional state of the thyroid is thyroid-stimulating hormone (TSH; thyrotropin), secreted by the anterior pituitary
(Figure 20–8). TSH stimulates all stages of production and release of thyroid hormones. Thyroid hormones inhibit the synthesis of TSH maintaining an adequate quantity of T4 and T3 in the organism. TSH increases the height of the follicular epithelium and decreases the quantity of the colloid and the size of the follicles. The
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Dr.Abdulla … ……….………….Histology………………………Endocrine system cell membrane of the basal portion of follicular cells is rich in receptors for thyrotropin.
The parathyroid glands:
The parathyroid glands, usually four in number, are located on the posterior surface of the thyroid gland; each gland is enveloped in its own thin, collagenous connective tissue capsule.
The major functional parenchymal cells of the parathyroid glands are the slightly eosinophilic-staining chief cells which house PTH containing secretory granules (figure -4). PTH acts on bone, kidneys, and the intestines in maintaining the optimal calcium concentrations in blood and interstitial tissue fluid.
The second cell type located in the parathyroid glands is the oxyphil cell. Its function is unknown.
Action of Parathyroid Hormone & Its Interrelation with Calcitonin
Parathyroid hormone binds to receptors in osteoblasts. This is a signal for these cells to produce an osteoclast-stimulating factor, which increases the number and activity of osteoclasts and thus promotes the absorption of the calcified bone matrix and the release of Ca2+ into the blood. The resulting increase in the concentration of
Ca2+ in the blood suppresses the production of parathyroid hormone. Calcitonin from the thyroid gland also influences osteoclasts by inhibiting both their resorptive action on bone and the liberation of Ca2+. Calcitonin thus lowers blood Ca2+ concentration and increases osteogenesis; its effect is opposite to that of parathyroid hormone.
These hormones constitute a dual mechanism to regulate blood levels of Ca2+, an important factor in homeostasis.
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Dr.Abdulla … ……….………….Histology………………………Endocrine system
Pineal Gland:
The pineal gland ( pineal body ) is an endocrine gland whose secretions are influenced by the light and dark periods of the day. It is a cone-shaped, midline projection from the roof of the diencephalon, with a recess of the third ventricle extending into the stalk that is attached to it. The gland is covered by pia mater, forming a capsule from which septa extend and divides the pineal gland into incomplete lobules. Blood vessels enter the gland via the connective tissue septa. The parenchymal cells of the gland are composed primarily of pinealocytes and interstitial cells .
The pineal gland also contains concretions of calcium phosphates and carbonates, which are deposited in concentric rings around an organic matrix. These structures, called corpora arenacea (“brain sand”) . Although it is unclear how they are formed or function, they increase during short photoperiods and are reduced as the pineal gland is actively secreting.
The pinealocytes produces melatonin and serotonin that may influence reproduction. Melatonin is secreted at night, whereas serotonin is produced during the day. Interstitial cells are believed to be astrocyte-like neuroglia(
Figure- 5 )
.
Figure- 5 Pineal gland staining structures are brain sand (BS) scattered among the pinealocytes (Pi).
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