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Medical Importance And Clinical Significance Of The Pineal And Thyroid Glands
The Pineal Gland
The Pineal Gland
The Pineal and thyroid glands are of huge significance to the human body. They secrete hormones which take part in major metabolic processes and hence are active in sustaining life.
Anatomy: The pineal gland which weighs 120 mg lies beneath the posterior border of the corpus callosum and between the superior colliculi. Embryologically, it develops from the ependyma lining the roof of the third ventricle. The parenchymal cells- the pinealocytes- are of neuroepithelial origin. Pineal gland synthesis melatonin (which is a methoxyindole) from serotonin. In addition to melatonin, three hypothalamic hormones (TRH, LH-RH and somatostatin) and an octapeptide, arginine vasotocin (Vasopressin), have also been detected in the pineal gland.
Function: Though the exact functional significance of this gland in humans is not clear, it is likely that its secretion exerts inhibitory effects on gonadal and thyroid functions and also modulates behaviour.
Calcification of the pineal: This occurs in a matrix of ground substance secreted by pinealocytes. Calcification begins in early childhood and is completed by about puberty. Calcification is not associated with any functional disturbance.
Tumours: The pineal may be the seat of tumours. These may be pinealomas, teratomas, glial tumours or vascular tunours. Tumours lead to mechanical effects due to their presence and endocrine disturbances. Pressure effects include rise in intracranial tension, internal hydrocephalus due to pressure on the aqueduct of sylvius and paralysis of upward gaze due to occulomotor involvement (Parinaud’s syndrome).
Endocrine disturbance manifests as abnormalities of sexual maturation such as sexual precocity, delayed puberty and hypogonad states. Evidence of hypothalamic involvement like diabetes insipidus, polyphagia, somnolence, obesity or behavioural disturbances are seen in 70% of patients with pinealoma and precocious puberty.
Treatment: Surgical treatment is beneficial in trained hands, though the procedure is difficult. Most of the pineal tumours are at least partially readiosensitive and irradiation, is effective. Chemotherapy with adriamycin, vincristin and bleomycin has also been found to be successful.
The Thyroid Gland
Anatomy: The thyroid gland weighs 15 to 25g, and it is made up of an isthmus and two lateral lobes. The isthmus lies just below the cricoids cartilage. The parathyroid glands which are four in number are embedded in the substance or sheath of the thyroid, behind the lateral lobes. The thyroid develops as early as the third or fourth week of gestation. The thyroid is formed as an outgrowth on the ventral wall of the pharynx in association with the parathyroids which are derived from the third and fourth brachial arches and the ultimobranchial body derived from the sixth branchial arch. The ‘C’ cells which produce calcitonin are formed from the cells of the ultimobranchial body. In the fetus, thyroid hormones are synthesized under the influence of TSH at about the eleventh to twelfth week of gestation. Persistence of the remnants of the thyroid stalk may give rise to the pyramidal lobe, which lies just to the left of the midline extending upwards from the isthmus or to thyroglossal cyst. Failure of the thyroid stalk to descend results in the formation of lingual thyroid.
Iodine metabolism and hormone Synthesis: The average Indian diet contains variable quantities of iodine ranging from very low values to more than 200 ug/day. Ingested iodine is reduced to idodide in the gastrointestinal tract and absorbed throughout the gut, the maximum being in the small intestine. Thyroid traps this iodine. Plasma inorganic iodide level is usually less than 1.0 ug/dl. About 90% of the total iodine is in the thyroid gland. Thyroxine (T4) and tri-iodothyronine (T3) are formed by sequential reactions occurring in the thyroglobulin molecule under the control of TSH. The follicular cells synthesise thyroglobulin and store it within the follicles as colloid. This colloid acts as a reservoir of thyroid hormones.
Release of thyroid hormones from thyroglobulin: The follicular cells take up thyroglobulin colloid droplets and release T3 and T4 by its degradation. The thyroglobulin molecule is split by proteases and peptidases into T3 and T4 which are released into circulation and this is controlled by TSH. The total T4 secretion is about 60-120 ug and T3 is 40-60 ug per day.
The Pineal and thyroid glands are of huge significance in maintaining and sustaining the human body.
© 2014 Funom Theophilus Makama