Discussion
Molecular clocks located throughout the body in peripheral tissues are organized into a coherent, hierarchical system by a "master" clock is located :
Answer: D
The master clock is in the suprachiasmatic nucleus (SCN) of the hypothalamus.
The SCN, a pair of small nuclei just above the optic chiasm, receives light information directly from the retina. It keeps its own near-24-hour rhythm and synchronises the peripheral clocks of the liver, heart and other organs through nerve signals, hormones and body temperature cycles. Destroying the SCN abolishes the coordinated circadian rhythms of the whole animal.
The pineal gland secretes melatonin under the SCN's control; it is an output of the clock, not the master. The hypophysis (pituitary) and its anterior lobe, the adenohypophysis, are endocrine glands, not the circadian pacemaker.
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