Endocrine System Part 1 PDF

ENDOCRINE SYSTEM part 1 DR. SARAH KHUDHAIR OBAYE S M.B.CH.B F ICMS.PATH  Endocrine glands Hormones: They are liberated by specialized cells that are called endocrine cells. Endocrine cells usually aggregate as endocrine glands, there are many isolated cells in...

ENDOCRINE SYSTEM part 1 DR. SARAH KHUDHAIR OBAYE S M.B.CH.B F ICMS.PATH  Endocrine glands Hormones: They are liberated by specialized cells that are called endocrine cells. Endocrine cells usually aggregate as endocrine glands, there are many isolated cells in the body such as endocrine cells of digestive tract, the cells of placenta, the cells of the heart that produce atrial natriouretic factor and the JG cells of the kidney. Most hormones act at a distance from the site of their secretion. Therefore the endocrine cells are always very close to blood capillaries. Many cells produce molecules that act at a short distance by diffusion through the extracellular matrix, this is called paracrine secretion e.g. insuline secretion from the islet of langerhan's. However autocrine control, cells secrete molecules that act on themselves..  The tissues on which the hormones act are called target tissues, they have receptors that specifically recognize and respond to the hormones. The target cells respond to the respective hormones even when they are present in very small concentrations in the blood. Endocrine glands are also target tissues to control hormone secretion through a feedback mechanism. The endocrine organs interact with the nervous system and immune system Hypophysis: Hypophysis or pituitary gland, weighs about 0.5gm, and its normal dimensions are about 10×13×6 mm. It lies in sella turcica. Because of its dual origin, the hypophysis consists of two glands –the neurohypophysis and the adenohypophysis. The neurohypophysis, the part of hypophysis that develop from nerve tissue consist of two parts: the pars nervosa and the infundibulum or neural stalk. The part of the hypophysis that develops from oral ectoderm is known as the adenohypophysis which is subdivided into three portions: pars distalis or anterior lobe, the parts tuberalis and the pars intermedia. Blood supply: From above, the superior hypophyseal arteries supply the neural stalk, from below the inferior hypophyseal arteries supply the neurohypophysis. The superior hypophyseal arteriea divide into a plexus of fenestrated capillaries that irrigate the neural stalk. These capillaries then rejoin to form veins that develop into a secondary capillary plexus in the adenohypophysis. This hypophyseal portal system carries neurohormones from the neural stalk to the adenohypophysis. Adenohypophysis: Pars distalis: The main components of pars distalis are cords of epithelial cells interspersed with capillaries. The hormones produced by these cells are stored as secretory granules. The pars distalis account for 75 % of the mass of hypophysis. Common stains allow the recognition of three cell types in the pars distalis: chromophobes and two types of chromophils called basophils and acidophils. The subtypes of basophils and acidophils are named for the hormones they produce.  With the exception of the gonadotropic cells, which produce two hormones, the other cells produce only a single hormone. Control of the pars distalis: The cells of the pars distalis are controlled by many mechanisms through peptide hormones produced in the hypothalamic aggregates of neurosecretory cells and stored in the neural stalk they are transported to the pars distalis through the capillary plexus. Most of these hormones are hypothalamic releasing and inhibiting hormones.  A second control mechanism is the direct effect of hormones secreted by endocrine cells on the release of peptides from the neural stalk and the pars distalis. A third mechanism depends on the action of nerve impulses or of molecules that are produced neither in the hypothalamic nuclei nor in the target tissue such as: 1 :Inhibin and activin produced in the gonads which control the secretion of FSH 2 :Dopamine produced in the central nervous system 3 :Oxytocin liberated at the neurohypophysis stimulates the secretion of prolactin. Pars tuberalis: Is a funnel shaped region surrounding the infundibulum of the neurohypophysis. Most of the cells of the pars tuberalis secrete gonadotropins (FSH and LH). Pars intermedia : The pars intermedia develop from the dorsal portions of Rathke's pouch. It is a rudimentary region made up of cords and follicles probably produce MSH (Melanocyte Stimulating Hormone). Neurohypophysis: The neurohypophysis consists of the pars nervosa and the neural stalk. It is composed of unmyelinated axons of secretory neurons situated in the supraoptic and paraventricular nuclei. The secretory neurons have all the features of typical neuron, but have well developed Nissl bodies related to the production of the neurosecretory material. The neurosecretions are transported along the axons and accumulate at their nerve endings to form what's called the Hering's bodies, which contain the neurosecretory granules which are released and enter the fenestrated capillaries. The neurosecretory materials consist of two hormones Anti- diuretic hormone (vasopressin) ADH and oxytocin. Vasopressin and oxytocin are released into the blood because impulse in the nerve fiber from the hypothalamus. Cells of the neurohypophysis: The neurohypophysis consists mainly of axons from hypothalamic neurons, about 25 % of its volume consists of a specific type of highly branched glial cells called a pituicyte. Actions of the hormones of the neurohypophysis: ADH is released in response to increased tonicity of the blood, which is recognized by osmoreceptor cells present in the hypothalamus. The main effect of ADH is to increase the permeability of collecting tubules of the kidney to water, so more water is reabsorbed instead of being eliminated in the urine. Thus vasopressin helps to regulate the osmotic balance of the internal media.  Oxytocin stimulates contraction of the myoepithelial cells that surround the alveoli and ducts of the mammary glands during nursing and of the smooth muscle of the uterine wall during copulation and childbirth. The secretion of oxytocin is stimulated by nursing or by distension of the vagina or the uterine cervix. This occurs via nerve tracts that act on the hypothalamus. The neurohormonal reflex triggered by nursing is called the milk-ejection reflex. Figure 1. The effects of various hypophyseal hormones on target organs and the feedback mechanisms that control their secretion. Adrenal (suprarenal) gland: The adrenal glands are paired organs that lie near the superior poles of the kidneys. They are flattened structures with a half moon shaped, they are about 4-6cm long, 1-2cm wide and 4- 6mm thick. Together they weigh about 8gm. It is formed by two concentric layers: a yellow peripheral layer, the adrenal cortex; and a reddish brown central layer, the adrenal medulla. The adrenal cortex and the adrenal medulla can be considered two organs with distinct origins, functions, and morphological characteristic. The cortex arises from coelomic epithelium, whereas the cells of the medulla derive from the neural crest. Blood supply: The adrenal glands are supplied by several arteries that are divided into three groups: arteries that irrigate the capsule; cortical arteries and medullary arteries which pass through the cortex and form an extensive capillary network in the medulla. The cells in the medulla are thus bathed with both arterial blood from the medullary arteries and venous blood originating from the capillaries of the cortex. Capillaries of the medulla, together with capillaries that supply the cortex, form the medullary veins, which join to constitute the adrenal or suprarenal vein. Figure 2. General architecture and blood circulation of the adrenal gland. Adrenal cortex: The cells of the adrenal cortex have the typical ultrastructure of steroid secreting cells. They synthesize and secrete steroid hormones upon demand. Steroids are lipid soluble molecules diffuse through plasma membrane and do not require the specialized process of exocytosis for their release. Because of the difference in appearance of its cells, the adrenal cortex can be subdivided into three concentric layers: zona glomeruloza, the zona fasciculate, and zona reticularis. The layer immediately beneath the connective tissue capsule is the zona glomeruloza, in which columnar cells are arranged in closely packed rounded or arched cords surrounded by capillaries. The next layer of cells is known as the zona fasciculate because of the arrangement of the cells in one or two thick straight cords that run at a right angle to the surface of the organ and have capillaries between them. The cells are polyhedral with the presence of great number of lipid droplets in their cytoplasm, so they appear vacuolated in common histological preparation. Because of their vacuolization, the cells of the fasciculate are called spongiocytes. The zona reticularis lies between the zone fasciculate and the medulla, it contain cells disposed in irregular cords that form an anastomosing network. Lipfuscin pigment granules in the cells are large and quite numerous. Figure 3. Photomicrographs of several regions of the adrenal cortex. A: A low-power general view showing the gland’s layers. Low magnification. B: The capsule, the zona glomerulosa, and the beginning of the zona fasciculata. One of the arcuate cords of this zona is delineated. Medium magnification. Cortical hormones and their action: Adrenal steroids originate from cholesterol. Cholesterol is converted to the final hormones partly in the mitochondria and partly in the SER. The steroids are secreted by the cortex can be divided into three groups: mineralocorticoids (aldesterone), glucocorticoids (cortisol) and androgens (dehydroepiandrosterone DHEA). The mineralocorticoids act mainly on the distal tubules as well as on the gastric mucosa, colon, and salivary and sweat gland, stimulating the reabsorption of sodium by epithelial cells. The glucocorticoids affect the metabolism of carbohydrates by increase the production of glucose which may lead to hyperglycemia, promote protein and lipid degradation. Glucocorticoids also suppress the immune response by inhibiting the mitotic activity in lymphocyte forming organs and controlling secretion of cytokines. DHEA is weak androgen that exerts its action after being converted into testosterone in several tissues. Adrenal medulla: The adrenal medulla is composed of polyhedral cells arranged in cords or clumps and supported by reticular fiber network. A profuse capillary supply intervenes between adjacent cords, and there are few parasympatheitic cells. The medullary cells arise from the neural crest as do the postganglionic neurons of sympathetic and parasympathetic ganglia. 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 become secretory cells. Medullary cells have abundant secretory granules, which contain one or the other type of the catecholamine, epinephrine or norepinephrene. The secretory granules also contain ATP, protein called chromogranin (binding protein for catecholamines), dopamine and opiate like peptide (enkephaline). Unlike the cortex, which does not store steroids, cells of the medulla accumulate and store their hormones in granules. The adrenal medullary cells are innervated by cholinergic endings of preganglionic sympathetic neurons.