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Hypothalamus & pituitary gland

The hypothalamus is a region of the brain that controls an immense number of bodily functions. It is located in the middle of the base of the brain, and encapsulates the ventral portion of the third ventricle The pituitary gland, also known as the hypophysis, is a roundish organ that lies immediately beneath the hypothalamus, resting in a depression of the base of the skull called the sella turcica ("Turkish saddle").

Careful examination of the pituitary gland reveals that it composed of two distinctive parts: The anterior pituitary or adenohypophysis is a classical gland composed predominantly of cells that secrete protein hormones. The posterior pituitary or neurohypophysis is not really an organ, but an extension of the hypothalamus. It is composed largely of the axons of hypothalamic neurons which extend downward as a large bundle behind the anterior pituitary. It also forms the so-called pituitary stalk, which appears to suspend the anterior gland from the hypothalamus

Development and Anatomy of the Pituitary Gland

Adenohypohysis: Pars distalis - the largest section Pars tuberalis - a collar of tissue that usually surrounds the infundibular stalk Pars intermedia - a narrow band that is usually separated from the pars distalis by a hypophyseal cleft

Neurohypohysis: Pars nervosa - the bulk of the posterior pituitary Median eminence - the upper section of the neurohypophysis above the pars tuberalis Infundibular stalk - the "stem" that connects the pars nervosa to the base of the brain

Parvicellular neurons Magnocellular neurons Superoptic N. Paraventricular N. Parvicellular neurons

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Overview of Hypothalamic and Pituitary Hormones The pituitary gland is often portrayed as the "master gland" of the body. Such praise is justified in the sense that the anterior and posterior pituitary secrete a battery of hormones that collectively influence all cells and affect virtually all physiologic processes. The pituitary gland may be king, but the power behind the throne is clearly the hypothalamus. As alluded to in the last section, some of the neurons within the hypothalamus - neurosecretory neurons - secrete hormones that strictly control secretion of hormones from the anterior pituitary. The hypothalamic hormones are referred to as releasing hormones and inhibiting hormones, reflecting their influence on anterior pituitary hormones. Hypothalamic releasing and inhibiting hormones are carried directly to the anterior pituitary gland via hypothalamic-hypophyseal portal veins. Specific hypothalamic hormones bind to receptors on specific anterior pituitary cells, modulating the release of the hormone they produce. As an example, thyroid-releasing hormone from the hypothalamus binds to receptors on anterior pituitary cells called thyrotrophs, stimulating them to secrete thyroid-stimulating hormone or TSH. The anterior pituitary hormones enter the systemic circulation and bind to their receptors on other target organs. In the case of TSH, the target organ is the thyroid gland.

Hormones of the hypothalamus Adrenocorticotrophic Hormone-releasing hormone (ACT-RH) Follicle Stimulating Hormone-releasing hormone (FSH-RH) Luteinizing Hormone-releasing hormone (LH-RH) Growth hormone- releasing hormone (GH-RH) Thyroide Stimulating Hormone-releasing hormone (THS-RH) Prolactin- Inhibiting Hormone (PRl-IH)

Pituitary Hormones Hormone& Major target organ(s) & Major Physiologic Effects Anterior Pituitary Growth hormone Liver, adipose tissue, Promotes growth (indirectly), control of protein, lipid and carbohydrate metabolism Thyroid-stimulating hormone Thyroid gland Stimulates secretion of thyroid hormones Adrenocorticotropic hormone Adrenal gland (cortex), Stimulates secretion of glucocorticoids Prolactin Mammary gland ,Milk production Luteinizing hormone Ovary and testis ,Control of reproductive function Follicle-stimulating hormone Ovary and testis Control of reproductive function Posterior Pituitary Antidiuretic hormone Kidney ,Conservation of body water Oxytocin Ovary and testis ,Stimulates milk ejection and uterine contractions

Histology of the pituitary gland

Histology of the Adenohypophysis The bulk of the adenohypophysis is pars distalis. That tissue is composed of winding cords of epithelial cells flanked by vascular sinusoids. In sections stained with dyes such as hematoxylin and eosin, three distinct cell types are seen among epithelial cells: Acidophils have cytoplasm that stains red or orange Basophils have cytoplasm that stains a bluish color Chromophobes have cytoplasm that stains very poorly This figure shows pars distalis from a cat at two magnifications (H&E stain).

Histology of the Adenohypophysis

The pars intermedia is closely associated with pars nervosa and separated from the pars distalis by the hypophyseal cleft. This lobe of the pituitary shows considerable variation in size among species. It is small in man, but much larger in species such as amphibians. The pars intermedia contains large pale cells that often surround follicles filled with ill-defined "colloid". Melanocyte-stimulating hormone is the predominant hormone secreted by the pars intermedia. The images below show pars intermedia from a cat at low and higher magnification. The hypophyseal cleft is seen in the middle of the left image. In the right image, the three round, clear areas are follicles characteristic of this tissue.

Histology of the Neurohypophysis The neurohypophysis is known also as the pars nervosa. Anatomists distinguish between three areas of this organ, starting closest to the hypothalamus: the median eminence infundibular stalk infundibular process The infundibular process froms the bulk of the neurohypophysis is what is usually referred to as the posterior pituitary. The bulk of the neurohypophysis is composed on largely unmyelinated axons from hypothalamic neurosecretory neurons. These axons have their cell bodies in the paraventricular and supraoptic nuclei of the hypothalamus. These neurons secrete oxytocin or antidiuretic hormone. Roughly 100,000 axons participate in this process to form the posterior pituitary. In addition to axons, the neurohypophysis contains glial cells and other poorly-defined cells called pituicytes.

Each lobule of this gland has a darker staining cortex (left A) and a lighter staining medulla (left B). One outstanding feature of this organ is the presence of thymic roses/thymic corpuscles (C). This organ, large until adolescence, is important for the development of the immune system. The hormone produced by this gland is thymosin.