The principal aspect of the pineal gland anatomy is the pineal parenchymal cell, or pinocyte. The pineal gland develops during the second month of gestation as a diverticulum in the diencephalic roof of the third ventricle. It is flanked by the posterior and habenular commissures in the rostral portion of the midbrain directly below the splenium of the corpus callosum. The velum interpositum is found rostral and dorsal to the pineal gland and contains the internal cerebral veins, which join to form the great cerebral vein (of Galen). 
The anatomy of the pineal gland, along with the pituitary gland, is displayed in the image below.
In their 1954 pineal tumor study, Ringertz and colleagues defined the pineal region as being bound by the splenium of the corpus callosum and tela choroidea dorsally, the quadrigeminal plate and midbrain tectum ventrally, the posterior aspect of the third ventricle rostrally, and the cerebellar vermis caudally.  In treating lesions in this area, important anatomic considerations include the presence of deep venous structures.
The principal cell of the pineal gland is the pineal parenchymal cell or pinocyte. This cell is a specialized neuron related to retinal rods and cones. The pinocyte is surrounded by a stroma of fibrillary astrocytes, which interact with adjoining blood vessels to form part of the blood-pial barrier. Pineal region tumors are derived from cells located in and around the pineal gland.
Pineal gland innervation
The pineal gland is richly innervated with sympathetic noradrenergic input from a pathway that originates in the retina and courses through the suprachiasmatic nucleus of the hypothalamus and the superior cervical ganglion. Upon stimulation, the pineal gland converts the sympathetic input into hormonal output by producing melatonin, which has regulatory effects upon hormones such as luteinizing hormone and follicle-stimulating hormone.
The pineal gland is a neuroendocrine transducer that synchronizes hormonal release with phases of the light-dark cycle by means of its sympathetic input. However, the exact relationship between the pineal gland and human circadian rhythm remains unclear and is an active area of investigation.
Pineal gland tumors
Tumors of the pineal region have a varied histology that generally can be divided into germ cell and non–germ cell derivatives. Most tumors are a result of displaced embryonic tissue, malignant transformation of pineal parenchymal cells, or transformation of surrounding astroglia. No specific genetic mutations have been associated with sporadic pineal region tumors. [3, 4]
A pineal tumor is displayed in the image below.