The subcommissural organ of the adult rabbit

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Abstract
The subcommissural organ of the adult rabbit was examined after the animals had been exposed to mild, nonspecific stress. The apical portion of the subcommissural cells contained few microvilli and cilia. Occasional small secretory granules were present. In the perinuclear zone there were numerous Golgi lamellae and vesicles and secretory granules. The electron-dense secretory material was delimited by both smooth and rough-surfaced endoplasmic reticulum. The largest secretory granules were approximately 2 [mu] long and 300 m[mu] wide and were enveloped by a smooth single membrane. Basal cytoplasmic processes containing secretory material extended into the posterior commissure. Capillaries surrounded by an extracellular space were encountered deep within the subcommissural organ. Subcommissural cell processes abutting on basement membranes and included within the extracellular space were filled with rodlike particles containing electron-dense secretory material. These particles measured approximately 80 to 140 m[mu] in length. The relationship between the large particles in the perinuclear zone and the rodlike particles in the extracellular space is not clear. It is possible that this morphologic difference is either a reflection of a decrease in size before molecular dispersion into the bloodstream or an indication that the subcommissural organ is capable of producing more than one type of secretory product. Morphologic evidence was obtained for basal secretion into the extracellular space, thus supporting an endocrine function for the subcommissural organ of the rabbit.