Functional anatomy of the peripheral olfactory system of the african lungfish Protopterus annectens owen: Macroscopic, microscopic, and morphometric aspects

Abstract

An anatomical functional study of the olfactory system of Protopterus annectens along the course of the primary olfactory neuron was made by means of macroscopic and microscopic methods on samples of specimens weighing from 3 to 1.250 gm. The olfactory organ, opening by anterior and posterior nostrils into the oral cavity, may be considered a macrosmatic fish type according to the repartition of receptor areas along parallel grooves and on the basis of its active irrigation mechanism. A morphometric study of Protopterus, using a set of seven specimens weighing from 4 to 350 gm, is compared with two morphometric studies on Silurid fishes and reveals some original features concerning the increase of the surface area of the receptor epithelium. The olfactory organ is meant to function in water; its isolation from the aerial environment is carried out by closing the apertures with edema linked to the hypothyroid and cholinergic crisis during the starvation stage; it represents an adaptation to the life in a special environment. This study does not support the assimilation of the posterior intrabuccal opening to a choana, nor the presence of a functional vomeronasal organ. The olfactory nerve shows a definite degree of organization, but it was not possible to recognize any vomeronasal nerve linked with the former. The histological organization of the bulbar relay is of a primitive type; it was impossible to locate an accessory olfactory bulb corresponding to a functional vomeronasal organ, the presence and the innervation of which by the nervus terminalis is discussed.

Olfactory Projections in the Lepidosirenid Lungfishes

“…In contrast, most studies [Rudebeck, 1945;Derivot, 1984;Reiner and Northcutt, 1987;Nieuwenhuys, 1998a;González and Northcutt, 2009;Northcutt, 2009Northcutt, , 2011a have placed the caudal border of the main olfactory bulb more rostrally, as in the present study. In the first experimental study of the primary olfactory projections in Protopterus , von placed horseradish peroxidase (HRP) in the olfactory organ and demonstrated that the main olfactory bulb of Protopterus ends more rostrally than claimed by Clairambault and Capanna [1973].…”

“…In contrast, Clairambault and Capanna [1973] recognized a lateroventral segment of the olfactory bulb as an accessory olfactory bulb, which appears to be approximately the same area as recognized by Gerlach [1933]. Reiner and Northcutt [1987] and Northcutt [2009] noted this segment of the lateroventral olfactory bulb but interpreted it as a matrix zone, as did Derivot [1984], which they assumed would give rise to additional neurons with continued development of the olfactory bulb. Neither Derivot [1984] nor von Bartheld et al [1988] recognized an accessory bulb in Protopterus .…”

“…Reiner and Northcutt [1987] and Northcutt [2009] noted this segment of the lateroventral olfactory bulb but interpreted it as a matrix zone, as did Derivot [1984], which they assumed would give rise to additional neurons with continued development of the olfactory bulb. Neither Derivot [1984] nor von Bartheld et al [1988] recognized an accessory bulb in Protopterus . Nieuwenhuys and Meek [1990] and Nieuwenhuys [1998a] did not recognize a vomeronasal system in lungfishes, nor did they note the earlier controversy over its possible existence.…”

Terminal Nerve Complex

“…Wirsig-Wiechmann [911 also observed GnRH immunoreactivity in the motor nerves that supply the smooth mus 90 Demski Terminal Nerve Complex cle of the naris constrictor which, as ihe name implies, con trols closure of the external nares. This unexpected finding is the first to document TN innervation of muscle tissue.…”

“…89| and lungfish [90] have revealed nonolfactory axons in the ON and OE that appear to be of several types including small (less than I pm) unmyelinated fibers that contain DCVs of several size ranges that are suggestive of some of the elasmobranch TN axons (see above). In amphibians, the DCV-containing fibers end in relation to blood vessels, melanocytes and cells of Bowman's glands [89], At least some of the nonol factory axons probably belong to the TN.…”