Endocrine System - Part 1

ENDOCRINE SYSTEM

PART 1

ENDOCRINE GLANDS AND HORMONES
•    Endocrine glands lack ducts and are hence, called ductless glands. Their secretions are called hormones.
•    Hormone is a chemical produced by endocrine glands and released into the blood and transported to a distantly located target organ.
•    Hormones are non-nutrient chemicals which act as intercellular messengers and are produced in trace amounts.
•    Invertebrates possess very simple endocrine systems with few hormones whereas a large number of chemicals act as hormones and provide coordination in the vertebrates. The human endocrine system is described here.
•    The endocrine glands and hormone producing diffused tissues/cells located in different parts of our body constitute the endocrine system. Pituitary, pineal, thyroid, adrenal, pancreas, parathyroid, thymus and gonads (testis in males and ovary in females) are the organized endocrine bodies in our body.
•    In addition to these, some other organs, e.g., gastrointestinal tract, liver, kidney, heart also produce hormones.
•    A brief account of the structure and functions of all major endocrine glands and hypothalamus of the human body is given in the following sections. 

HYPOTHALAMUS
•    Hypothalamus is the part of the forebrain and it regulates a wide spectrum of body functions.
•    It contains several groups of neurosecretory cells called nuclei which produce hormones.
•    These hormones regulate the synthesis and secretion of pituitary hormones.
•    However, the hormones produced by hypothalamus are of two types, the releasing hormones (which stimulate secretion of pituitary hormones) and the inhibiting hormones (which inhibit secretions of pituitary hormones).
•    For example a hypothalamic hormone called Gonadotrophin releasing hormone (GnRH) stimulates the pituitary synthesis and release of gonadotrophins.
•    On the other hand, somatostatin from the hypothalamus inhibits the release of growth hormone from the pituitary.
•    These hormones originating in the hypothalamic neurons, pass through axons and are released from their nerve endings. These hormones reach the pituitary gland through a portal circulatory system and regulate the functions of the anterior pituitary. The posterior pituitary is under the direct neural regulation of the hypothalamus.

PITUITARY GLAND
•   The pituitary gland is located in a bony cavity called sella tursica and is attached to hypothalamus by a stalk.
•   It is divided anatomically into an adenohypophysis and a neurohypophysis.
•   Adenohypophysis consists of two portions, pars distalis and pars intermedia.
•  The pars distalis region of pituitary, commonly called anterior pituitary, produces
              •    Growth Hormone (GH),
              •    Prolactin (PRL),
              •    Thyroid Stimulating Hormone (TSH),
              •    Adrenocorticotrophic Hormone (ACTH),
              •    Luteinizing Hormone (LH) and
              •    Follicle Stimulating Hormone (FSH).

•    Pars intermedia secretes only one hormone called Melanocyte Stimulating Hormone (MSH).
•    However, in humans, the pars intermedia is almost merged with pars distalis.
•    Neurohypophysis (pars nervosa) also known as posterior pituitary, stores and releases two hormones called oxytocin and vasopressin, which are actually synthesised by the hypothalamus and are transported axonally to neurohypophysis.
•    Over-secretion of GH stimulates abnormal growth of the body leading to gigantism and low secretion of GH results in stunted growth resulting in pituitary dwarfism.
•    Prolactin regulates the growth of the mammary glands and formation of milk in them.
•    TSH stimulates the synthesis and secretion of thyroid hormones from the thyroid gland.
•    ACTH stimulates the synthesis and secretion of steroid hormones called glucocorticoids from the adrenal cortex.
•    LH and FSH stimulate gonadal activity and hence are called gonadotrophins.
•    In males, LH stimulates the synthesis and secretion of hormones called androgens from testis. In males, FSH and androgens regulate spermatogenesis.
•    In females, LH induces ovulation of fully mature follicles (graafian follicles) and maintains the corpus luteum, formed from the remnants of the graafian follicles after ovulation. FSH stimulates growth and development of the ovarian follicles in females.
•    MSH acts on the melanocytes (melanin containing cells) and regulates pigmentation of the skin.
•    Oxytocin acts on the smooth muscles of our body and stimulates their contraction. In females, it stimulates a vigorous contraction of uterus at the time of child birth, and milk ejection from the mammary gland.
•    Vasopressin acts mainly at the kidney and stimulates resorption of water and electrolytes by the distal tubules and thereby reduces loss of water through urine (diuresis). Hence, it is also called as Anti-Diuretic Hormone (ADH).


To be Continued in Part 2