An electron microscope observation of the right and the two left portions of the habenular nuclei of the frog.

The right dorsal habenula (RDH), the left dorsal habenula, lateral portion (LDH-LP) and the left dorsal habenula, medial portion (LDH-MP) of the adult frog Rana esculenta have been studied by the electron microscope with the use of three types of fixatives: osmium tetroxide, aldehydes and potassium permangarate. The study has demonstrated in the habenulae of both sides a variety of syraptic vesicles, special synaptic patterns (en passant, serial, axo-somatic) containing small spherical transparent vesicles, never mixed with other types of vesicles, and large pale processes containing big granules. However, a particular type of synaptic vesicles characterized by small clear spherical vesicles with a tiny dark granule revealed by potassium permanganate fixition has been demonstrated only in the neuropil of the LDH and not in the RDH. Further, a peculiar type of intracytoplasmic inclusion appears in the form of myelin-like and crystal-like formations only in the LDH-MP. On the basis of these consistent ultrastructural differences, the dorsal habenular nuclei of the frog can be considered as composed of three individual nuclei, each with its own morphological characterization, dislocated, for some unknown reason, one on the right and two on the left side of the epithalamus. The results suggest that the LDH-MP is a nucleus metabolically different from the others and that the projections to the dorsal habenular nuclei are different on the right and left sides of the brain.