A light-microscopic study on pineal organ structure and innervation in the catfish, Heteropneustes fossilis

In Heteropneustes fossilis, the pineal organ consists of a long hollow stalk (PS) and an elongate spindle-shaped end vesicle (EV) which lies beneath a highly specialized window. The EV is formed of a dorsal thin and ventral thick parenchymal layer, separated by a lumen and contains photoreceptor, supportive and ganglion cells. The numerous photoreceptor cells are characterized by apical processes which are AF- and PAS-positive. In Holmes' silver preparation, the apical processes showed parallel silver-positive lamellae embedded in luxol fast blue coagulum. The supportive cells are ependymal or glial-like. The ganglion cells or neurons are bipolar or multipolar with prominent dendrites and long, beaded and occasionally branched axonal processes. The neurons are strongly acetylcholinesterase (AChE)-positive. The EV also showed strong toluidine blue-positive mast cells and AF-positive granules and fibres. The PS is composed of the same cellular structures. The neurons are small and AChE-positive. Pineal tract fibres were arranged into dorsal and ventral bundles in the middle and proximal regions of the PS. Silver-positive neurons were found among or close to the pineal tracts. The pineal tract fibres innervate the habenular commissure, habenular nucleus, subcommissural organ, posterior commissure and pretectum, and are AChE-positive. At the origin of the stalk, the pineal tract fibres were seen to project into the ventricle and innervate the subcommissural organ both supraependymally and transependymally.