Pituitary gland
Dr. Madhu Gupta Assistant Professor Department of Zoology University of Lucknow Lucknow
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Pituitary gland
a. Location: lies in the hypophyseal fossa of the sella turcica of the sphenoid bone. It attaches to the hypothalamus by a stalk, the infundibulum. b. Structure: a pea-shaped structure that measures 1–1.5 cm in diameter. The pituitary gland is actually two fused glands. The anterior pituitary is a true endocrine gland of epithelial origin, derived from embryonic tissue that formed the roof of the mouth. It is also called the adenohypophysis {adeno-, gland + hypo-, beneath + phyein, to grow}, and its hormones are adenohypophyseal secretions. The anterior pituitary consists of two parts in an adult: The pars distalis is the larger portion, and the pars tuberalis. The posterior pituitary, or neurohypophysis, is an extension of the neural tissue of the brain. It also consists of two parts: the pars nervosa, the larger bulbar portion, and the infundibulum. It consists of axons and axon terminals of more than 10,000 hypothalamic neurosecretory cells. The cell bodies of the neurosecretory cells are in the paraventricular and supraoptic nuclei of the hypothalamus; their axons form the hypothalamohypophyseal tract. This tract begins in the hypothalamus and ends near blood capillaries in the posterior pituitary. The neuronal cell bodies in the paraventricular nucleus (PVN) synthesize the hormone oxytocin and the neuronal cell bodies in the supraoptic nucleus (SON) produce antidiuretic hormone (ADH), also called vasopressin. Both are neurohormones.
c. Portal system: Usually, blood passes from the heart through an artery to a capillary to a vein and back to the heart. In a portal system, blood flows from one capillary network into a portal vein, and then into a second capillary network before returning to the heart. The name of the portal system indicates the location of the second capillary network. In the hypophyseal portal system, blood flows from capillaries in the hypothalamus into portal veins that carry blood to capillaries of the anterior pituitary. Hypothalamic hormones that release or inhibit anterior pituitary hormones reach the anterior pituitary through a portal system. d. Hormones secreted: Hormones secreted by anterior pituitary gland Those anterior pituitary hormones that act on other endocrine glands are called tropic hormones or tropins. Five types of anterior pituitary cells—somatotrophs, thyrotrophs, gonadotrophs, lactotrophs, and corticotrophs—secrete seven hormones.
Cell type Hormone secreted Somatotrophs Human growth hormone (hGH), also known as somatotropin Thyrotrophs Thyroid-stimulating hormone (TSH), also known as thyrotropin Gonadotrophs Follicle-stimulating hormone (FSH) Gonadotrophs Luteinizing hormone (LH) Lactotrophs Prolactin (PRL) Corticotrophs Adrenocorticotropic hormone (ACTH), also known as corticotropin Corticotrophs Melanocyte-stimulating hormone (MSH)
Hormones released by posterior pituitary gland The posterior pituitary is the storage and release site for two neurohormones: oxytocin and vasopressin. e. Physiological actions of anterior pituitary hormone: Hormone Physiological actions hGH • stimulates liver, muscle, cartilage, bone, and other tissues to synthesize and secrete insulin like growth factors (IGFs); • IGFs promote growth of body cells, • protein synthesis, • tissue repair, • lipolysis, and • elevation of blood glucose concentration. TSH • stimulates synthesis and secretion of thyroid hormones by thyroid gland. FSH • in females, initiates development of oocytes and • induces ovarian secretion of estrogens. • in males, stimulates testes to produce sperm. LH • In males, stimulates testes to produce testosterone. • In females, stimulates secretion of estrogens and progesterone, • ovulation, and • formation of corpus luteum. PRL • Together with other hormones, promotes milk production by mammary glands. ACTH • Stimulates secretion of glucocorticoids (mainly cortisol) by adrenal cortex. MSH • Exact role in humans is unknown but may influence brain activity; when present in excess, • can cause darkening of skin.
f. Physiological actions of posterior pituitary hormone: Oxytocin (OT) • During delivery of a baby, oxytocin enhances contraction of smooth muscle cells in the wall of the uterus. • Promotes expulsion of the placenta. • After delivery, it stimulates milk ejection (“letdown”) from the mammary glands in response to the mechanical stimulus provided by a suckling infant. Antidiuretic Hormone (ADH) or Vasopressin • Decreases urine production and causes the kidneys to return more water to the blood, thus decreasing urine volume. • Decreases the water lost through sweating and causes constriction of arterioles, which increases blood pressure. g. Termination of actions: • Release of anterior pituitary hormones is stimulated by releasing hormones and suppressed by inhibiting hormones from the hypothalamus. • Negative feedback regulation
• Neurosecretory cells of hypothalamus secrete OT in response to uterine distension and stimulation of nipples.
• Neurosecretory cells of hypothalamus secrete ADH in response to elevated blood osmotic pressure, dehydration, loss of blood volume, pain, or stress; inhibitors of ADH secretion include low blood osmotic pressure, high blood volume, and alcohol.
References: • Guyton, Arthur C. Textbook of medical physiology, 11th Edition. Elsevier, Inc. • Tortora, G J and Derrickson, B. Principles of Anatomy & Physiology, 13th Edition. John Wiley & Sons, Inc. • Silverthorn, D U. Human physiology: an integrated approach, 6th Edition. Pearson Education, Inc.