Light- and electron-microscopic structure of cells protruding into the mesencephalic ventricle of Scyllium stellare (Elasmobranchii, Selachii).

In the elasmobranch fish, Scyllium stellare, a complex group of cells protrudes into the cavity of the mesencephalic ventricle of the optic tectum. It consists of six to seven large spherical perikarya which resemble neurons of the mesencephalic nucleus of the Vth cranial nerve. The bundled processes of these cells form a stalk connecting the protrusion with the brain tissue. The protrusion is located in the region where the mesencephalic ventricle joins the cerebral aqueduct. This complex was not found in all specimens examined in the present study. The functional role of this peculiar group of cells, which contain dense core granules and are bathed in the cerebrospinal fluid, is open to discussion.