Publisher Summary The overall architecture of the pineal gland is extremely variable. Some glands have a well defined, lobular structure, strands of connective tissue separating the groups of parenchymal cells. In others the connective tissue is more extensive and the bands are wider. In some of these glands only small islands of parenchymal cells surrounded by connective tissue are seen. On the other hand, the connective tissue may be extremely sparse so that in these glands there are sheets of parenchymal cells with only the occasional fine connective tissue septa containing blood vessels. It is important to note that there may be a good deal of variation in the amount of the connective tissue in different parts of the gland. The amount of connective tissue in the pineal gland that should be regarded as normal is a matter for debate, but although some workers have found that there is some increase in the amount of fibrous tissue in the gland with age this is minimal and it should be noted that even in old age the majority of the glands still contain large numbers of parenchymal cells. That it is possible to measure melatonin in the blood as well as many of the hypothalamic and pituitary hormones, it is likely that the mechanisms whereby pineal tumors produce their endocrine effects may be eventually resolved. In fact there is no reason to doubt that both, effects from the secretion of endocrinologically active substances by pineal tumors and interference with the hypothalamic centers and tracts, may well be significant in individual cases.