Pituitary Gland PDF

LECTURE TWO DEPARTMENT OF PHYSIOLOGY DR. HANAN AL- TAEE OBJECTIVES After studying lecture 2&3, you should be able to:  Describe the structure of the pituitary gland and how it relates to its function....

LECTURE TWO DEPARTMENT OF PHYSIOLOGY DR. HANAN AL- TAEE OBJECTIVES After studying lecture 2&3, you should be able to:  Describe the structure of the pituitary gland and how it relates to its function.  Define the cell types present in the anterior pituitary and understand how their numbers are controlled in response to physiologic demands.  Define the effects of the growth hormone in growth and metabolic function, and how insulin-like growth factor I (IGF-I) may mediate some of its actions in the periphery.  List the stimuli that regulate growth hormone secretion and define their underlying mechanisms.  Understand the basis of conditions where pituitary function and growth hormone secretion and function are abnormal, and how they can be treated. Pituitary gland Introduction Pituitary gland or hypophysis is a small endocrine gland with a diameter of 1 cm and weight of 0.5 to 1 g. It is situated in a depression called ‘sella turcica’, present in the sphenoid bone at the base of skull. It is connected with the hypothalamus by the pituitary stalk or hypophyseal stalk. Pituitary gland is divided into two divisions:  Anterior pituitary or adenohypophysis  Posterior pituitary or neurohypophysis. Both the divisions are situated close to each other.Still both are entirely different in their development, structure and function. Between the two divisions, there is a small and relatively avascular structure called pars intermedia. 1  Figure (1): demonstrates parts of pituitary gland Development of pituitary gland Both divisions of pituitary glands develop embryologically from different sources. Anterior pituitary is ectodermal in origin and arises from the pharyngeal epithelium as an upward growth known as Rathke,s pouch. Posterior pituitary is neuroectodermal in origin and arises from the hypothalamus as a downward diverticulum. Rathke pouch and the downward diverticulum from hypothalamus grow towards each other and meet in the midway between the roof of the buccal cavity and base of brain. There, the two structures lie close together. Regulation of secretion The relationship between hypothalamus and pituitary gland is called hypothalamo-hypophyseal relationship. Hormones secreted by hypothalamus are transported to anterior pituitary and posterior pituitary. But the mode of transport of these hormones is different. 1 Anterior pituitary or adenohypophysis: Anterior pituitary is also known as the master gland because it regulates many other endocrine glands through its hormones. Anterior pituitary consists of three parts (Figure 1) 1. Pars distalis 2. Pars tuberalis 3. Pars intermedia. Histology Anterior pituitary has two types of cells, which have different staining properties: 1. Chromophobe cells 2. Chromophil cells. Chromophobe Cells These do not possess granules and stain poorly. These cells form 50% of total cells in anterior pituitary.Chromophobe cells are not secretory in nature,but are the precursors of chromophil cells. Chromophil Cells Contain large number of granules and are darkly stained. Are classified by two methods.  Classification on the basis of staining property:  Acidophilic cells or alpha cells, which form 35%  Basophilic cells or beta cells, which form 15%.  Classification on the basis of secretory nature:  Somatotropes, which secrete growth hormone (GH).  Corticotropes, which secrete adrenocorticotropicHormone (ACTH).  Thyrotropes, which secrete thyroid-stimulating hormone (TSH).  Gonadotropes, which secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH).  Lactotropes, which secrete prolactin. 1 Somatotropes and lactotropes are acidophilic cells, whereas others are basophilic cells.Somatotropes form about 30% to 40% of the chromophil cells. Pituitary tumors that secrete large quantities of human growth hormone are called acidophilic tumors. We can classify hormones of the anterior pituitary also into 1. Glycoprotien: TSH, LH, FSH, and HCG (human chorionic gonadotropin), which is secreted during pregnancy from the placenta. Composed of two peptide chains alpha and beta. Alpha chains are similar in all but beta is different, so their actions depend on beta chains. 2. Somatomammotrphins: (single peptide) prolactin, GH and HPL (human placental lactogen). 3. ACTH related peptide: these are close related peptides are derived from parent compounds known as (BIG ACTH) which has 120-130 amino acid(aa) residues this undergoes enzymatic cleavage leading to large fragment (beta lipoprotein), this gives rise to smaller peptides, the most important of which is ACTH. ACTH is composed of 39 aa the activity of ACTH resides in the first 24 aa, the last 13 aa resembles that of beta melanocytes stimulating hormone therefore if we have increment of ACTH above the physiological level we have increase in the pigmentation of the skin. Other important fragment of beta lipotrophs is beta endorphin which is present in the brain and GIT and acts as neurotransmitter (have analgesic property) Regulation of anterior pituitary secretion Hypothalamus controls anterior pituitary by secreting the releasing and inhibitory hormones (factors), which are called neurohormones. These hormones from hypothalamus are transported to the anterior pituitary through hypothalamo-hypophyseal portal vessels. 1 Some special nerve cells present in various parts hypothalamus send their nerve fibers (axons) to median eminence and tuber cinereum. These nerve cells synthesize the hormones and release them into median eminence and tuber cinereum. From here, the hormones are transported by blood via hypothalamo-hypophyseal portal vessels to anterior pituitary. Releasing and Inhibitory Hormones Secreted by Hypothalamus 1. Growth hormone-releasing hormone (GHRH): Stimulates the release of growth hormone. 2. Growth hormone-releasing polypeptide (GHRP): Stimulates the release of GHRH and growth hormone. 3. Growth hormone-inhibitory hormone (GHIH) or somatostatin: Inhibits the growth hormone release hormone. 4. Thyrotropic-releasing hormone (TRH): Stimulates the release of thyroid stimulating hormone. 5. Corticotropin-releasing hormone (CRH): Stimulates the release of adrenocorticotropin. 6. Gonadotropin-releasing hormone (GnRH): Stimulates the release of gonadotropins, FSH and LH. 7. Prolactin-inhibitory hormone (PIH): Inhibits prolactin secretion. It is believed that PIH is dopamine. Hormones secreted by anterior pituitary 1. Growth hormone (h GH) or somatotropic hormone (STH) 2. Thyroid-stimulating hormone (TSH) or thyrotropic hormone 1  3. Adrenocorticotropic hormone (ACTH) 4. Follicle-stimulating hormone (FSH) 5. Luteinizing hormone (LH) in females, or interstitialcell- stimulating hormone (ICSH) in males. 6. Prolactin (figure 2). Tropic Hormones First five hormones of anterior pituitary stimulate the other endocrine glands. Growth hormone also stimulates the secretory activity of liver and other tissues.Therefore, these five hormones are called tropic hormones. Prolactin is concerned with milk secretion. Figure 2: demonstrates releasing factor of hypothalamus and stimulating hormone of pituitary gland. 1 Gonadotropic Hormones Follicle-stimulating hormone and the luteinizing hormone are together called gonadotropic hormones or gonadotropins because of their action on gonads. LH is a glycoprotein made up of one α-subunit and one β-subunit. The α- subunit has 92 amino acids andβ-subunit has 141 amino acids. The half- life of LH is about 60 minutes. Follicle-stimulating hormone is a glycoprotein made up of one α-subunit and a β-subunit. The α-subunit has 92 amino acids and β-subunit has 118 amino acids. The half-life of FSH is about 3 to 4 hours. Prolactin Prolactin is a single chain polypeptide with 199 amino acids secreted by lactotroph cells.Its half-life is about 20 minutes. Prolactin is necessary for the final preparation of mammary glands for the production and secretion of milk.Prolactin acts directly on the epithelial cells of mammary glands and causes localized alveolar hyperplasia. In female it causes milk secretion after estrogen and progesterone priming effect. „ 1