Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Endocrine System
Classification, structure, and functions of endocrine glands
Pituitary gland, pituitary hormones

The Pituitary Gland is a small organ measuring 10–15 mm and weighing 0.5–0.7 g. It is located in the hypophyseal fossa of the Sphenoid bone's sella turcica. The infundibulum (pituitary stalk) connects the pituitary gland to the Hypothalamus. The pituitary coordinates the Functions of many other endocrine Organs. Furthermore, it is both anatomically and functionally closely linked to the hypothalamus, which regulates numerous vital life functions.

The pituitary gland comprises three lobes—the anterior, middle (intermediate), and posterior lobes—which differ in their embryological origin and Structure. The anterior and intermediate lobes are collectively referred to as the adenohypophysis.

The anterior lobe is formed by epithelial glandular Cells (adenocytes) arranged in cords (trabeculae). Wide Blood capillaries lie between these cords. The Cells of the glandular cords vary in Structure and function: some cells (somatotropes) produce Growth Hormone, while others (mammotropes) secrete prolactin.

Gonadotropes produce follitropin. Thyrotropes secrete thyrotropin. Corticotropic endocrinocytes of the adenohypophysis (corticotropes) release adrenocorticotropic hormone (ACTH), also known as corticotropin.

The intermediate part of the pituitary gland forms a narrow strip situated between the main (distal) part of the anterior lobe and the posterior (neural) lobe. Endocrinocytes of the intermediate lobe synthesize melanocyte-stimulating hormone (melanotropin), which influences melanin pigment METABOLISM, as well as lipotropin, a hormone that stimulates Lipid Metabolism.

The hormone-producing function of the entire pituitary gland, including its anterior lobe, is controlled by the hypothalamus of the Diencephalon. Neurosecretory products from hypothalamic Nerve Cells reach the adenohypophysis via the hypophyseal portal Veins, where they regulate its functions.

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Fig. 77. Location of Endocrine glands in The Human Body:

1 — Pituitary and Pineal Glands; 2 — parathyroid gland; 3 — Thyroid Gland; 4 — Adrenal Glands; 5 — pancreatic islets; 6 — Ovary; 7 — Testis

The posterior lobe of the pituitary gland does not synthesize Hormones. Instead, it releases into the bloodstream BIOLOGICALLY ACTIVE SUBSTANCES (hormones) that are produced in the neurosecretory nuclei of the hypothalamus and transported to the pituitary via the nerve fibers of the hypothalamo-hypophyseal tract. The hormones oxytocin and vasopressin (antidiuretic hormone) are produced by the neurosecretory cells of the hypothalamic nuclei. Vasopressin, or antidiuretic hormone (ADH), promotes the reabsorption of Water from primary urine within the renal nephron tubules. A deficiency of vasopressin—caused by impaired neurosecretory function of the hypothalamic nuclei or reduced function of the posterior pituitary—results in The excretion of a very large volume of sugar-free urine and causes severe thirst. This condition is known as diabetes insipidus. Oxytocin (OXT) stimulates the contraction of the myometrium (smooth Muscle of the Uterus) during childbirth.

The pituitary gland not only synthesizes and releases numerous hormones into the blood but also regulates the internal secretory activity of other endocrine glands through these hormones, influencing various metabolic processes in the body (Fig. 78). Because of this regulatory role, Pituitary Hormones are called tropic hormones (from the Greek word tróphe, meaning nourishment). For instance, growth hormone, or somatotropin (STH), stimulates growth and affects protein, carbohydrate, and lipid metabolism. Overproduction of this hormone enhances body growth, leading to conditions such as gigantism, where a person's height can reach 2.5 m. Conversely, a deficiency of growth hormone in early childhood stunts growth, resulting in dwarfism. A body length of less than 125 cm is classified as Pituitary dwarfism. In such cases, normal body proportions and psychological development are preserved.

Hypopituitarism in adults leads to profound Disorders of protein, carbohydrate, and lipid metabolism, resulting either in generalized (pituitary) obesity or severe emaciation (pituitary cachexia). Hyperpituitarism in adults, occurring after growth has ceased, leads to the enlargement of specific body parts. The hands, feet, and lower jaw increase markedly in size, and facial features change. This condition is known as acromegaly.

Pituitary gonadotropic hormones (follicle-stimulating hormone / follitropin, luteinizing hormone / lutropin, and lactogenic hormone / prolactin) stimulate the Functions of the Gonads. Follitropin (FSH) influences the development and maturation of ovarian follicles in females, and sperm production and prostate development in males. Lutropin (LH) stimulates the function of endocrinocytes (internal secretory cells) in the Ovaries and Testes, promoting their secretion of Sex Hormones (estrogens and androgens). Prolactin stimulates progesterone production in the corpus luteum of the ovary and Lactation (milk production). Adrenocorticotropic hormone (ACTH, or corticotropin) stimulates the function of the adrenal cortex cells and the release of corticosteroids. The secretion and stimulatory action of ACTH increase during various heightened emotional states (stress).

Fig. 78. Interrelationships of organs within the hypothalamo-hypophyseal system:

1 — adenohypophysis (anterior pituitary lobe) with a secondary capillary network; 2 — hypophyseal portal vein; 3 — pars tuberalis of the adenohypophysis; 4 — median eminence with a primary capillary network; 5 — neurosecretory nuclei of the hypothalamus; 6 — hypothalamo-hypophyseal tract; 7 — infundibulum (pituitary stalk); 8 — posterior pituitary lobe (neurohypophysis); 9 — intermediate (middle) pituitary lobe.

Pituitary hormones and their directions of action:

ADH — antidiuretic hormone, OXT — oxytocin, PRL — prolactin, TSH — thyrotropin, FSH — follitropin, LH — lutropin, ACTH — adrenocorticotropic hormone, STH — somatotropin

Thyroid-stimulating hormone (thyrotropin — TSH) stimulates thyroid function and the secretion of its hormones. The action of TSH increases the secretory activity of The Thyroid Gland. The secretion of TSH depends on the blood levels of THYROID HORMONES. When blood levels of thyroid hormones rise, The production of TSH by the pituitary decreases. Conversely, when blood levels of thyroxine and other thyroid hormones drop, thyrotropin secretion increases. Thus, the interaction between the pituitary gland and the thyroid gland operates on a feedback principle.