THE EPIPHYSIS
The epiphysis or pineal gland, named from the fancied resemblance of the human structure to a pine cone, is a small, darkly pigmented outgrowth from the dorsal aspect of the brain at the caudal end of the roof of the third ventricle and directly before the rostral colliculi (see Figure 8-22/11).
In certain species it is related to a large outpouching (epiphysial recess) of the pia- ependyma that roofs the ventricle. It is concealed between the cerebral hemispheres and cerebellum in the intact brain.The epiphysis is solid but is not always homogeneous as foci of calcification (“brain sand”) often develop with advancing age. Its functions were long obscure. It produces melatonin, an indolamine derived from serotonin, which possesses an antigonadotropic circadian effect. The existence of this hormone was first postulated from the observation that tumors that destroy the secretory tissue are frequently associated with precocious puberty.
The driving endogenous circadian clock is located in the hypothalamic suprachiasmatic nucleus (SCN), and its rhythm controls the rhythm of melatonin secretion by the pineal gland by a polysynaptic pathway. The autonomic innervation of the pineal gland runs via the superior cervical ganglion. Melatonin is secreted as a sleeping hormone during the night and acts on many brain areas, including the SCN and the pituitary. The brain knows that it is day by the enhanced activity of the SCN and knows that it is night by the secretion of melatonin. The action of melatonin on the pars tuberalis is important for seasonal hormonal fluctuations. Fine-tuning of the biological clock in the SCN can be achieved by gradual changes in daylight, which regulate both long-term (seasonal) and short-term (diurnal) variation in gonadal activity.