Histology of the Endocrine System PDF

HISTOLOGY OF THE ENDOCRINE SYSTEM BY DR. OLUNFUNTO ADELEYE  CONTENTS The Hypophysis Cerebri The Pineal Gland The Thyroid Gland The Parathyroid Glands The Suprarenal Glands Some other Organs Having Endocrine Functions Endocrine tissue is mad...

HISTOLOGY OF THE ENDOCRINE SYSTEM BY DR. OLUNFUNTO ADELEYE  CONTENTS The Hypophysis Cerebri The Pineal Gland The Thyroid Gland The Parathyroid Glands The Suprarenal Glands Some other Organs Having Endocrine Functions Endocrine tissue is made up essentially of cells that produce secretions which are poured directly into blood. The secretions of endocrine cells are called hormones. Hormones travel through blood to target cells whose functioning they may influence profoundly. A hormone acts on cells that bear specific receptors for it. Some hormones act only on one organ or on one type of cell, while other hormones may have widespread effects. Along with the autonomic nervous system, the endocrine organs co-ordinate and control the metabolic activities and the internal environment of the body. Endocrine cells are distributed in three different ways. Some organs are entirely endocrine in function. They are referred to as endocrine glands (or ductless glands). They are: the hypophysis cerebri (or pituitary), the pineal gland, the thyroid gland, the parathyroid glands, and the suprarenal (or adrenal) glands.  Groups of endocrine cells may be present in organs that have other functions. Examples are: the islets of the pancreas, the interstitial cells of the testes, and the follicles and corpora lutea of the ovaries, some cells in the kidneys, the thymus, and the placenta. Isolated endocrine cells may be distributed in the epithelial lining of an organ. Examples of these are found in the epithelium of the gut the epithelium of the respiratory passages. The Hypophysis Cerebri The hypophysis cerebri is also called the pituitary gland. It is suspended from the floor of the third ventricle (of the brain) by a narrow funnel shaped stalk called the infundibulum, and lies in a depression on the upper surface of the sphenoid bone. The hypophysis cerebri is one of the most important endocrine glands. It produces several hormones some of which profoundly influence the activities of other endocrine tissues. Its own activity is influenced by the hypothalamus, and by the pineal body. It is divided into an anterior part (the pars anterior); an intermediate part (the pars intermedia); and a posterior part the pars posterior (or pars nervosa).  The pars posterior contains numerous nerve fibres. It is directly continuous with the central core of the infundibular stalk which is made up of nervous tissue. These two parts (pars posterior and infundibular stalk) are together referred to as the Neurohypophysis.  The area in the floor of the third ventricle (tuber cinereum) immediately adjoining the attachment to it of the infundibulum is called the median eminence.  The pars anterior (which is also called the pars distalis), and the pars intermedia, are both made up of cells having a direct secretory function. They are collectively referred to as the Adenohypophysis.  An extension of the pars anterior surrounds the central nervous core of the infundibulum. Because of its tubular shape this  ADENOHYPOPHYSIS  PARS ANTERIOR The pars anterior consists of cords of cells separated by fenestrated sinusoids. Several types of cells, responsible for the production of different hormones, are present. Using routine staining procedures the cells of the pars anterior can be divided into; 1. chromophil cells(alpha cells): cells that have brightly staining granules in their cytoplasm. they are further classified into a. acidophil when their granules stain with acid dyes (like eosin) b. basophil when the granules stain with basic dyes (like haematoxylin). 2. Chromophobe cells(beta cells): cells in which granules are not prominent., and the basophils are called beta cells.  Types of Acidophil Cells (1) Somatotrophs produce the somatropic hormone (also called somatotropin [STH], or growth hormone [GH]). This hormone controls body growth, specially before puberty. (2) Mammotrophs (or lactotrophs) produce the mammotropic hormone (also called mammotropin, prolactin (PRL), lactogenic hormone, or LTH) which stimulates the growth and activity of the female mammary gland during pregnancy and lactation. Types of Basophil Cells (1) The corticotrophs (or corticotropes) produce the corticotropic hormone (also called adreno-corticotropin or ACTH). This hormone stimulates the secretion of some hormones of the adrenal cortex. The granules in the cells contain a complex molecule of pro-opio-melano-corticotropin. This is broken down into ACTH and other substances. (2) Thyrotrophs (or thyrotropes) produce the thyrotropic hormone (thyrotropin or TSH) which stimulates the activity of the thyroid gland. (3) Gonadotrophs (gonadotropes, or delta basophils) produce two types of hormones each type having a different action in the male and female. (a) In the female, the first hormone is the follicle stimulating hormone (FSH) which stimulates the growth of ovarian follicles. It also stimulates the secretion of oestrogens by the ovaries. In the male the same hormone stimulates spermatogenesis. (b) In the female, the second hormone stimulates the maturation of the corpus luteum, and the secretion by it of progesterone. It is called the luteinizing hormone (LH). In the male the same hormone stimulates the production of androgens by the interstitial cells of the testes, and is called the  PARS TUBERALIS The pars tuberalis consists mainly of undifferentiated cells. Some acidophil and basophil cells are also present.  PARS INTERMEDIA This is poorly developed in the human hypophysis. In ordinary preparations the most conspicuous feature is the presence of colloid filled vesicles. These vesicles are remnants of the pouch of Rathke. Beta cells, other secretory cells, and chromophobe cells are present. Some cells of the pars intermedia produce the melanocyte stimulating hormone (MSH) which causes increased pigmentation of the skin.  NEUROHYPOPHYSIS  Pars Posterior The pars posterior consists of numerous unmyelinated nerve fibres which are the axons of neurons located in the hypothalamus. Most of the nerve fibres arise in the supraoptic and paraventricular nuclei. Situated between these axons there are supporting cells of a special type called pituicytes. The pars posterior of the hypophysis is associated with the release into the blood of two hormones (1) Vasopressin (also called the antidiuretic hormone or ADH): This hormone controls reabsorption of water by kidney tubules. It is produced mainly in the supraoptic nucleus (2) Oxytocin: It controls the contraction of smooth muscle of the uterus and also of the mammary gland. It is produced mainly in the paraventricular nucleus.  THE PINEAL GLAND (EPIPHYSIS CEREBRI) The pineal gland (or pineal body) is a small piriform structure present in relation to the posterior wall of the third ventricle of the brain. The pineal has for long been regarded as a vestigial organ of no functional importance. (Hence the name pineal body). However, it is now known to be an endocrine gland of great importance. Sections of the pineal gland stained with haematoxylin and eosin reveal very little detail. The organ appears to be a mass of cells amongst which there are blood capillaries and nerve fibres. A distinctive feature of the pineal in such sections is the presence of irregular masses made up mainly of calcium salts. These masses constitute the corpora arenacea or brain sand. The organ is covered by connective tissue. The best known hormone of the pineal gland is the amino acid melatonin. The synthesis and discharge of melatonin is remarkably influenced by exposure of the animal to light, the pineal gland being most active in darkness. The neurological pathways concerned involve the hypothalamus and the sympathetic nerves. Because of this light mediated response, the pineal gland may act as a kind of biological clock which may produce circadian rhythms (variations following a 24 hour cycle) in various parameters.  THE THYROID GLAND The thyroid gland is covered by a fibrous capsule. Septa extending into the gland from the capsule divide it into lobules. On microscopic examination each lobule is seen to be made up of an aggregation of follicles. Each follicle is lined by follicular cells, that rest on a basement membrane. The follicle has a cavity which is filled by a homogeneous material called colloid (which appears pink in haematoxylin and eosin stained sections). The spaces between the follicles are filled by a stroma made up of delicate connective tissue in which there are numerous capillaries and lymphatics. The capillaries lie in close contact with the walls of follicles. The thyroid gland also contains C-cells (or parafollicular cells) which intervene (here and there) between the follicular cells and the basement membrane. They may also lie in the intervals between the follicles. Connective tissue stroma surrounding the follicles The Follicular Cells: 1. Have varying shape depending on the level of their activity. Normally (at an average level of activity) the cells are cuboidal, and the colloid in the follicles is moderate in amount. When inactive (or resting) the cells are flat (squamous) and the follicles are distended with abundant colloid. Lastly, when the cells are highly active they become columnar and colloid is scanty. Different follicles 2. Secretes two hormones that influence the rate of metabolism. Iodine is an essential constituent of these hormones. a. One hormone containing three atoms of iodine in each molecule is called triiodothyronine or T3. b. The second hormone containing four atoms of iodine in each molecule is called tetraiodothyronine, T4, or thyroxine. T3 is much more active than T4. 3. With the EM a follicular cell shows the presence of apical microvilli, abundant granular endoplasmic reticulum, and a prominent supranuclear Golgi complex. Lysosomes, microtubules and microfilaments are also present. The apical part of the cell contains many secretory The C-Cells (Parafollicular Cells) They are also called clear cells, or light cells. The cells are polyhedral, with oval eccentric nuclei. Typically, they lie between the follicular cells and their basement membrane. They may, however, lie between adjoining follicular cells; but they do not reach the lumen. With the EM the cells show well developed granular endoplasmic reticulum, Golgi complexes, numerous mitochondria, and membrane bound secretory granules. C-cells secrete the hormone thyro-calcitonin. This hormone has an action opposite to that of the parathyroid hormone on calcium metabolism. This hormone comes into play when serum calcium level is high. It tends to lower the calcium level by suppressing release of calcium ions from bone. This is achieved by suppressing bone resorption by osteoclasts.  THE PARATHYROID GLANDS The parathyroid glands are so called because they lie in close relationship to the thyroid gland. Normally, there are two parathyroid glands, one superior and one inferior, on either side; there being four glands in all. Sometimes there may be as many as eight parathyroids. Each gland has a connective tissue capsule from which some septa extend into the gland substance. Within the gland a network of reticular fibres supports the cells. Many fat cells (adipocytes) are present in the stroma. The parenchyma of the gland is made up of cells that are arranged in cords. Numerous sinusoids lie in close relationship to the cells. The cells of the parathyroid glands are of two main types: 1. Chief cells (or principal cells) 2. Oxyphil cells (or eosinophil cells) With the light microscope the chief cells are seen to be small round cells with vesicular nuclei. Their cytoplasm is clear and either mildly eosinophil or basophil. In contrast the oxyphil cells are much larger and contain granules that stain strongly with acid dyes. Their nuclei are smaller and stain more intensely than those of chief cells. The chief cells produce the parathyroid hormone (or parathormone). This hormone tends to increasethe level of serum calcium by: (a) increasing bone resorption through stimulation of osteoclastic activity; (b) increasing calcium resorption from renal tubules (and inhibiting phosphate resorption);  THE SUPRARENAL GLANDS As implied by their name the right and left suprarenal glands lie in the abdomen, close to the upper poles of the corresponding kidneys. In many animals they do not occupy a ‘supra’ renal position, but lie near the kidneys. They are, therefore, commonly called the adrenal glands. Each suprarenal gland is covered by a connective tissue capsule from which septa extend into the gland substance. The gland is made up of two functionally distinct parts: (1) A superficial part called the cortex (2) A deeper part called the medulla. The volume of the cortex is about ten times that of the medulla. Layers of the Cortex The suprarenal cortex is made up of cells arranged in cords. Sinusoids intervene between the cords. On the basis of the arrangement of its cells the cortex can be divided into three layers as follows: (a) The outermost layer is called the zona glomerulosa. Here the cells are arranged as inverted U-shaped formations, or acinus-like groups. The zona glomerulosa constitutes the outer one-fifth of the cortex. The cells are seen to be small, polyhedral or columnar, with basophilic cytoplasm and deeply staining nuclei. It produces the mineralocorticoid hormones aldosterone and deoxycorticosterone. (b) The next zone is called the zona fasciculata. Here the cells are arranged in straight columns, two cell thick. Sinusoids intervene between the columns. This layer forms the middle three fifths of the cortex. The cells of the are large, polyhedral, with basophilic cytoplasm and vesicular nuclei. They are very rich in lipids also contain considerable amounts of vitamin C. it produce the glucocorticoids hormones cortisone, cortisol (dihydro-cortisone), small amounts of dehydroepiandrosterone (DHA) which is an androgen. (c) The innermost layer of the cortex (inner one-fifth) is called the zona reticularis. It is so called because it is made up of cords that branch and anastomose with each other to form a kind of reticulum. The cells of the zona reticularis are similar to those of the zona fasciculata, but the lipid content is less. Their cytoplasm is often eosinophilic. The cells often contain brown pigment. It produce some glucocorticoids; and sex hormones, both oestrogens and androgens. The suprarenal cortex is essential for life. Removal or destruction leads to death unless the hormones produced by it are supplied artificially. Increase in secretion The Suprarenal Medulla Both functionally and embryologically the medulla of the suprarenal gland is distinct from the cortex. When a suprarenal gland is fixed in a solution containing a salt of chromium (e.g., potassium dichromate) the cells of the medulla show yellow granules in their cytoplasm. This is called the chromaffin reaction, and the cells that give a positive reaction are called chromaffin cells. The cells of the suprarenal cortex do not give this reaction. The medulla is made up of groups or columns of cells. The cell groups or columns are separated by wide sinusoids. The cells are columnar or polyhedral and have a basophilic cytoplasm. Functionally, the cells of the suprarenal medulla are considered to be modified postganglionic sympathetic neurons. Like typical postganglionic sympathetic neurons they secrete noradrenalin (norepinephrine) and adrenalin (epinephrine) into the blood. This secretion takes place mainly at times of stress (fear, anger) and results in widespread effects similar to those of stimulation of the sympathetic nervous system (e.g., increase in heart rate and blood pressure). Thank you guys weldone