Electron Microscopic Observations on the Mouse Pineal, with Particular Emphasis on Its Secretory Nature

The pineal body of the mouse was studied by electron microscopy especially as to whether this organ is secretory in nature. The cytoplasmic processes of the pinealocytes were frequently observed together with the axons around the blood capillaries. The pinealocyte processes contain variable amounts of vesicles of various sizes and appearances. Large vesicles are of two kinds: granular (cored) and agranular (clear). The granular vesicles measure a mean diameter of 1160A. Small vesicles contain no electron dense core, resembling those in axon terminals in size and shape. It is most likely that the large granular vesicles are formed in the Golgi apparatus in the cell bodies and transported to the cell processes. By a morphological analogy to the secretory vesicles of the neurosecretory cells the large granular vesicles would be considered as the secretory material of the pinealocytes. The nerve axons in the perivascular spaces are loaded with numerous small, granular and agranular vesicles which measure approximately 600A in diameter. They are considered sympathetic and adrenergic in nature.The endothelial cells of the capillaries which are surrounded by the tissue spaces are attenuated and possess numerous fenestrations with an average diameter of 650A. The occurrence of the fenestrations may also suggest the possible secretory nature of the pineal body.