Endocrine System Dr. Rajaa Ali
Structure and Function of the Pituitary Gland Anterior Lobe of the Pituitary Gland (Adenohypophysis) The anterior lobe of the pituitary gland regulates other endocrine glands. Most of the anterior lobe of the pituitary gland has the typical organization of endocrine tissue. The cells are organized in clumps and cords separated by fenestrated sinusoidal capillaries of relatively large diameter. These cells respond to signals from the hypothalamus and synthesize and secrete a number of pituitary hormones.
Four hormones of the anterior lobe adrenocorticotropic hormone (ACTH), thyroidstimulating (thyrotropic) hormone (TSH, thyrotropin), follicle-stimulating hormone (FSH), and luteinizing hormone (LH) are called tropic hormones because they regulate the activity of cells in other endocrine glands throughout the body. The two remaining hormones of the anterior lobe, growth hormone (GH) and prolactin (PRL), are not considered tropic because they act directly on target organs that are not endocrine.
The cells within the pars distalis vary in size, shape, and staining properties. The cells are arranged in cords and nests with interweaving capillaries. Histologists identified three types of cells according to their staining reaction, namely, basophils (10%), acidophils (40%), and chromophobes (50%)
Pars Intermedia. The pars intermedia surrounds a series of small cystic cavities that represent the residual lumen of Rathke s pouch fig.(2). The parenchymal cells of the pars intermedia surround colloid-filled follicles. The cells lining these follicles appear to be derived either from folliculo-stellate cells or various hormone-secreting cells, pars intermedia have vesicles larger than those found in the pars distalis.. The pars intermedia contains basophils and chromophobes (Fig. 2). Frequently, the basophils and cystic cavities extend into the pars nervosa.
Pars Tuberalis. The pars tuberalis is an extension of the anterior lobe along the stalklike infundibulum fig.(2). It is a highly vascular region containing veins of the hypothalamohypophyseal system. The parenchymal cells are arranged in small clusters or cords in association with the blood vessels. Nests of squamous cells and small follicles lined with cuboidal cells are scattered in this region. These cells often show immunoreactivity for ACTH, FSH, and LH.
Posterior Lobe of the Pituitary Gland (Neurohypophysis) The posterior lobe of the pituitary gland is an extension of the central nervous system (CNS) that stores and releases secretory products from the hypothalamus, consists of the pars nervosa, the infundibulum& median eminance that connects it to the hypothalamus fig.(2). The pars nervosa, contains the unmyelinated axons and their nerve endings of approximately 100,000 neurosecretory neurons whose cell bodies lie in the supraoptic nuclei and paraventricular nuclei of the hypothalamus.
The axons form the hypothalamohypophyseal tract and are unique in two respects: First, they do not terminate on other neurons or target cells but end in close proximity to the fenestrated capillary network of the pars nervosa. Second, they contain secretory vesicles in all parts of the cells, i.e., the cell body, axon, and axon terminal.
The posterior lobe of the pituitary gland is not an endocrine gland. Rather, it is a storage site for neurosecretions of the neurons of the supraoptic and paraventricular nuclei of the hypothalamus.
The nonmyelinated axons convey neurosecretory products to the pars nervosa. Other neurons from the hypothalamic nuclei also release their secretory products into the fenestrated capillary network of the infundibulum, the first capillary bed of the hypothalamohypophyseal portal system. There are neurosecretory vesicles in the nerve endings of the pars nervosa, aggregate to form Herring bodies that contain either oxytocin or antidiuretic hormone (ADH; also called vasopressin).
Oxytocin promotes contraction of smooth muscle of the uterus and myoepithelial cells of the breast.
The pituicyte is the only cell specific to the posterior lobe of the pituitary gland, Because of their many processes and relationships to the blood, the pituicyte serves a supporting role similar to that of astrocytes in the rest of the CNS.
Blood Supply & the Hypothalamo-Hypophyseal Portal System: The blood supply derives from two groups of vessels coming off the internal carotid artery. The pituitary blood supply is derived from two sets of vessels: Superior hypophyseal arteries supply the pars tuberalis, median eminence, and infundibulum. Inferior hypophyseal arteries primarily supply the pars nervosa. These vessels arise solely from the internal carotid arteries. An important functional observation is that most of the anterior lobe of the pituitary gland has no direct arterial supply.
The hypothalamohypophyseal portal system provides the crucial link between the hypothalamus and the pituitary gland. The arteries that supply the pars tuberalis, median eminence, and infundibulum give rise to fenestrated capillaries (the primary capillary plexus). These capillaries drain into portal veins, called the hypophyseal portal veins, which run along the pars tuberalis and give rise to a second fenestrated sinusoidal capillary network (the secondary capillary plexus).
Nerve Supply The nerves that enter the infundibulum and pars nervosa from the hypothalamic nuclei are components of the posterior lobe of the pituitary gland. The nerves that enter the anterior lobe of the pituitary gland are postsynaptic fibers of the autonomic nervous system and have vasomotor function.
Hypothalamus: The hypothalamus regulates pituitary gland activity. The hypothalamus is located in the middle of the base of the brain, and it encapsulates the ventral portion of the third ventricle. It coordinates most endocrine functions of the body and serves as one of the major controlling centers of the autonomic nervous system.
Some of the functions that it regulates include blood pressure, body temperature, fluid and electrolyte balance, body weight, and appetite. The hypothalamus produces numerous neurosecretory products. In addition to oxytocin and ADH, hypothalamic neurons secrete polypeptides that promote and inhibit the secretion and release of hormones from the anterior lobe of the pituitary gland.
Pineal Gland: The pineal gland (pineal body, epiphysis cerebri) is an endocrine or neuroendocrine gland that regulates daily body rhythm. It develops from neuroectoderm of the posterior portion of the roof of the diencephalon and remains attached to the brain by a short stalk, it is located at the posterior wall of the third ventricle near the center of the brain. The pineal gland is a flattened, pine cone shaped structure, hence its name. The pineal gland contains two types of parenchymal cells: Pinealocytes and interstitial (glial) cells.
Pinealocytes are the chief cells of the pineal gland. They are arranged in clumps or cords within lobules formed by connective tissue septa that extend into the gland from the pia mater that covers its surface. The interstitial (glial) cells constitute about 5% of the cells in the gland. They have staining and ultrastructural features that closely resemble those of astrocytes. In addition to the two cell types, the human pineal gland is characterized by the presence of calcified concretions, called corpora arenacea or brain sand (Fig. 3). The human pineal gland relates light intensity and duration to endocrine activity.the pineal gland is a photosensitive organ and an important timekeeper and regulator of the day/night cycle (circadian rhythm).
Fig.(1) Photomicrograph of human pineal gland. This higher-magnification photo micrograph characteristic concretions called brainsand or corpora arenacea. shows the
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