The pineal gland is host to a spectrum of neoplasms. Those considered to be derived from or differentiating toward pineal parenchymal cells are rare. Traditionally, pineal parenchymal tumors (PPTs) have been divided into 3 types: pineocytomas, pineoblastomas, and mixed or transitional tumors. Their characterization has been far from adequate and no firm diagnostic criteria, light microscopic or ultrastructural, have been established. In an attempt to provide more precise prognostic diagnostic criteria, we undertook a detailed ultrastructural analysis of 17 PPTs and found them to exhibit light microscopic and ultrastructural features strikingly similar to those of pineal parenchymal cells in varying stages of development, ranging from undifferentiated primitive neuroepithelial cells to mature pineal parenchymal cells. We endorse classification of PPTs based on a combination of their light microscopic and ultrastructural features. Accordingly, PPTs can be divided into three categories: 1) pinealoblastoma, 2) PPTs of intermediate or mixed differentiation, and 3) pineocytoma, a tumor of mature-appearing pineocytes. In keeping with this classification, our 3 pinealoblastomas behaved as highly malignant tumors. A correlation of morphology and prognosis was less evident between intermediate tumors and pineocytomas, perhaps the result of considerable variation in surgical and other therapies. Evidence of neurosensory differentiation, a feature noted to a varying extent in all but the pineoblastomas, included club-shaped ''nerve endings'' in 7 tumors, small numbers of dense core granules in 8, clear vesicles in 7, and structures suggestive of synapses in 4. With the exception of 3 undifferentiated PPTs or pinealoblastomas lacking nerve endings, all pineocytomas exhibited some combination of these markers of neuronal specialization. In that the ultrastructural features of these PPTs were more indicative of their aggressiveness than was their degree of light microscopic differentiation or grade, we consider electron microscopy a useful adjunct, not only in diagnosis but also in therapeutic decision-making and prognostication.
