Ultrastructure of pars nervosa and pars intermedia of the Lamprey, Lampetra tridentata.

The pars intermedia of the adult lamprey is separated by perivascular spaces and a capillary plexus from the pars nervosa. No penetration of nerve fibers into the pars intermedia was found. The pars nervosa, which constitutes the posterior wall of the infundibulum, consists of an ependymal layer and a fuchsinophilic fiber layer; the latter contains at least four different types of axonal endings. The pars intermedia is avascular and is composed of a small proportion of non-secretory cells and a large proportion of secretory cells. The secretory granules in the cells of the pars intermedia seem to be discharged toward the capillaries that separate the pars intermedia from the pars nervosa. Although no direct nervous or vascular connections were found between the pars nervosa and pars intermedia, a mechanism of control of secretory activity in the pars intermedia cells by the central nervous system appears likely.