ENDOCRINOLOGY: PINEAL GLAND & THYMUS GLAND

Pineal body

· The pineal gland or epiphysis cerebri is a small, reddish-grey organ that lies dorsal to the superior colliculi (brain).
· In the adult, it weighs about 150 mg, is about 8 mm long and 4 mm wide.

· In infants, the pineal gland is large and begins to involute before puberty.

· So it shows retrogressive changes.

· The gland is surrounded by a capsule and is made up of several lobules separated by connective tissue septa.
· The lobules contain cords and follicles of parenchymal cells called pinealocytes.

· Several types of pinealocytes are present and are rich in cytosolic organelles concerned with secretory function.

· In addition, other cell types, like neuroglia, macrophages are also present.

· The adult gland shows calcification and is known as 'brain sands'.

· They are calcium-containing hydroxyapatite crystals in the extracellular matrix.
· The pineal gland has a very rich blood supply and the capillaries are fenestrated.
· The gland is innervated by the nervi conarii which relay sympathetic fibres from the superior cervical ganglion.

· These sympathetic fibres carry information about tight -dark cycle from the environment by a complex pathway.

· This pathway arises from the retina, and via the suprachiasmatic nucleus of the hypothalamus, is conveyed through the superior cervical ganglion.

· The pineal gland is active throughout the life of an individual.

· Functions: The pineal gland secretes the hormone, Melatonin.

· SYNTHESIS AND SECRETION OF MELATONIN:
· L-Tryptophan is the precursor for the synthesis of melatonin.

· It is first converted to serotonin which is present in high concentrations in the pineal gland.

· Serotonin is converted to melatonin by the action of several enzymes and cofactors.

· The synthesis and secretion of melatonin follow a cyclic pattern.

· Melatonin release is increased during the dark period of the day and decreases during the daylight period.

· This diurnal variation is controlled by the sympathetic nerves.

BIOLOGICAL ACTIONS
1. Neuroendocrine Function:
· The pineal gland influences the activity of several endocrine glands.

· It is said to modify the activity of the pituitary gland, endocrine pancreas, para­thyroids, adrenal glands and the gonads.
· Melatonin acts at the level of the hypothalamus on the pituitary gland.

· It is required for the normal functioning of the parathyroids.

· It has an inhibitory effect on the functioning of the para­thyroids.
· It has an inhibitory effect on the thyroids, adrenal cortex and the endocrine pancreas.

2. Reproductive Function:
· It inhibits the development of sex organs before puberty.

· Destructive tumours of the pineal cause precocious puberty in boys.

· Pineal tumours that secrete melatonin result in suppression of gonadal function.

3. Effect on sleep:
· The secretion of melatonin increases during sleep. Melatonin can induce deep sleep and modify sleep patterns.

· Melatonin secretion is disturbed in the elderly, who normally suffer from disturbed sleep.

· Serotonin is one of the neurotransmitters involved in the initiation and maintenance of sleep.

· Melatonin by its control over serotonin secretion is said to affect sleep.

· DISORDERS:
· Jet Lag: International travellers by air have to go through different time zones. This disturbs their normal biological rhythmic functions, especially sleep. Melatonin has been found to be beneficial in correcting such disturbances.

THYMUS GLAND

· The thymus is a primary lymphoid organ.

· It is situated in the mediastinum around the trachea.
·  The size of the gland varies with age.

· It is large at birth weighing 10-15 g, grows to 20-30 g at puberty, and remains at that level.

· In old age, it atrophies and weighs 3-6 g.
· The thymus is endodermal in origin, arising from the third branchial pouch.
· It consists of an outer cortex densely packed with lymphocytes, and a central medulla with fewer lymphocytes.

· A loose network of epithelial cells forms the matrix of cortex and medulla.
· The lymphocytes are arranged in packets surrounded by epithelial cells, which form a barrier between the capillaries and lymphocytes.

· The medulla contains ‘Hassall's corpuscles' – (spherical structures composed of keratin and concentric layers of spindle-shaped cells that may be macrophages.)

· Non­-lymphoid secretory cells are also present in the medulla.

· The thymus is innervated by sympathetic fibres from the stellate ganglion.

· The vagus supplies parasympathetic fibres.

THYMIC HORMONES

· The epithelial cells of the thymus, called epitheliocytes, secrete hormones and thymic factors.
· The well-characterized hormones are thymulin, thymosins, thymopoietin and thymic humoral factor.
· Thymulin is a Zn- dependent hormone.

· The thymosins are a family of hormones.

· Thymopentin and Thymin humoral factors have been sequenced, but no homology has been found between any of the thymic hormones.

BIOLOGICAL ACTIONS
1. Immunological Function
· The thymus promotes the development of immunologically competent T-cells for cell-mediated immunity.
· The thymus gland provides the environment in which the precursors of mature T-cells undergo development, differentiation and clonal expansion.

· In the thymus, the T-cells acquire specificity, and develop immune tolerance to the body's own tissues - 'self-recognition'.
· The immature T-cells interact with the epitheliocytes and other cells and their secretions aid in this process of development and differentiation.

· Hence, removal of the thymus in the newborn severely affects the immune mechanisms, because of atrophy of all lymphoid tissues and a failure to produce antibodies.

· These effects can be prevented by a thymus graft.

2. Endocrine effects :
· The thymic hormones affect the function of other endocrine organs and are in turn influenced by the hormones released from other endocrine glands.

· The thymic epitheliocytes contain receptors for many hormones.

· The thymic epitheliocytes influence the functioning of the pituitary gland.
· This action is exerted directly or indirectly through the hypothalamus.

· The endocrine system, by mutual interaction with the thymus gland, and its secretions, exerts direct and indirect effects on the immune system of the body.

DISORDERS
· Autoimmune Diseases
· Tumours of the thymus gland can result in autoimmune diseases, like myasthenia gravis, haemolytic anaemia and Hashimoto's thyroiditis.

Also, Read our Other Notes related to Endocrinology of Mammals:

• Endocrinology of Mammals: Introduction
• Hypothalamus & Pituitary Gland
• Thyroid & Parathyroid Gland
• Adrenal Glands: Cortex & Medulla
• Pancreas & Gonads
• Pineal Gland & Thymus Gland
• Heart, Tissues & Kidney as Endocrine Glands