Larval electroreceptors in the epidermis of mormyrid fish: I. Tuberous organs of type A and B

Abstract

Two types of larval electroreceptors, type A and B, are described in the epidermis of the head of larvae of three mormyrid species, Campylomormyrus cassaicus, Mormyrus rume proboscirostris and Pollimyrus isidori, bred in captivity. In each of these electroreceptor organs, a single sensory cell is found inside an intraepidermal cavity, sitting on a platform of accessory cells. The cavity is filled with microvilli originating both from the sensory cell and from the epidermal covering cells lining the intraepidermal cavity. These two types of tuberous larval electroreceptors differ in their distribution in the epidermis of the head, in the composition of their accessory cells, and by their innervation. The innervation found in type B organs is similar to that already described for electroreceptors of adult mormyrids. The sensorineural junction is composed of primary afferent terminal boutons, which contact the base of the sensory cell. Opposite each terminal bouton, a ribbon-like synaptic bar surrounded by vesicles is found in the cytoplasm of the sensory cell. In contrast, the base of the sensory cell in type A larval electroreceptors is not contacted by nervous terminal boutons, but instead forms closed appositions with specialized prolongations of accessory cells of the platform. The base of the sensory cell presents membrane evaginations, with hemispheric synaptic structures and few synaptic vesicles. These two types of electroreceptor organs degenerate at the time of the degeneration of the larval electric organ and the functional differentiation of the adult electric organ. The functional role of two tuberous electroreceptor types is examined.

Comparative Anatomy of the Electrosensory Lateral Line Lobe of Mormyrids: The Mystery of the Missing Map in the Genus <i>Stomatorhinus</i> (Family: Mormyridae)

“…The finding that B-cells are maintained despite the lack of trophic support from an afferent fiber implies that B-cells could serve some other function, such as assuring proper development or structural maintenance of the mormyromast. The uninnervated B-cells might be unique for Stomatorhinus adults among mormyrids, but uninnervated tuberous electroreceptors have already been described for larval mormyrids [Bensouilah et al, 2002]. Finally, the unusual position of the B-cell in Stomatorhinus, outside the inner cavity of the receptor organ, suggests a possible role of the afferent nerve in promoting cellular migration of B-cells in other mormyrid species.…”

Larval electroreceptors in the epidermis of mormyrid fish: II. The promormyromast

“…Adult fish were purchased from registered wholesale dealers in Germany and had been imported from their wild habitat in Africa. Development of the gonads and reproduction were based on progressive decrease of water conductivity using the method of Schugardt and Kirschbaum (1998; see also Kirschbaum and Schugardt, 2002). Free embryos generally hatched about 3 days after spawning.…”

“…However, the two types of larval tuberous organs differ in the composition of their accessory cells, in their distribution in the epidermis, and by their innervation. For example, no synaptic terminals were found at the base of the LTOA sensory cell (Bensouilah et al, 2002) and this cell in fact appears to lack a primary afferent innervation. The LTOA and LTOB both degenerate at the same time that the larval electric organ disappears.…”

“…In the larval stage, in addition to the ampullary organs and knollenorgan similar to those of the adult, the epidermis contains three other sensory organs: two different types of larval tuberous organs, designated types A and B (LTOA and LTOB) (Denizot et al, 1998;Bensouilah et al, 2002) and also the promormyromast. Each of the larval tuberous organs comprises a single sensory cell, which presents the typical characteristics of an electroreceptor sensory cell.…”

“…In addition, this study has made it possible to compare larval mormyromast organs-promormyromasts-with the two other types of electroreceptors specific to the larval stage (Bensouilah et al, 2002).…”