Original Paper
Department of Neuroanatomy. University of Gottingen Medical School, Gottingen. Germany, and Departments of Otolaryngology and Physiology & Biophysics, University of Washington. Seattle
Oculomotor and Sensory Mesencephalic Trigeminal Neurons in Lungfishes: Phylogenetic Implications
Key Words
Abstract
Fluorescent tracer Eye muscle Mesencephalic trigeminal nucleus Trochlear nerve Proiopterus Neoceratodns Acetylcholinesterase Oculomotor Lungfish
The location and number of neurons in the brainstem with projections to the eve muscles were investigated by means of fluorescent tracers in the African lungfish Proiopterus dolloi. The oculomotor nucleus (M III) projects bilaterally with a ratio of 3: L(70 ipsilateral. 20 contralateral neurons). Three subdivisions of this nucleus can be differentiated: one projects exclusively ipsilatcrally. another pro jects exclusively contralatcrally, and a third component projects bilaterally. The trochlear nucleus (M IV) is located caudally. distinct from M III. and projects predominantly to contralateral eye muscles with a ratio of 6:1 (18:3 neurons). The abducens nucleus (M VI) contains about 30 neurons with ipsilateral projec tions only. There is no evidence for an accessory abducens nucleus in Proiopte rus. Intraocular injections of tracers do not reveal any retinopetal projections in Proiopterus. The mesencephalic trigeminal nucleus (Mes V) of Proiopterus and Neoceratodns contains about 540-590 neurons on each side. In juvenile Proio pterus, up to 75 Mes V neurons arc located caudally in a ventral projection of the tectum above the velum medulläre anterius. Fifty-five Mes V neurons (10% of the total number) have processes that exit the brain with the trochlear nerve. The relatively large number of Mes V neurons in lungfishes correlates with the well developed jaw musculature. The present study provides the First conclusive evidence for the location of oculomotor subdivisions in the brain of a lepidosirenid lungfish. The organiza tion of the oculomotor nucleus is consistent with the observation that lungfishes possess the pattern of eye-muscle innervation seen in elasmobranchs and sup ports the unconventional view that lungfishes may be the sistergroup of elasmo branchs.
Christopher S. von Bartheld
Introduction Lepidosirenid lungfishes share many features with amphibians, particularly urodeles [Northcutt, 1977, 1986; Rosen et a!., 1981], Some investigators have postulated a sistergroup relationship between lungfishes and tetrapods.
or have suggested a classification of lungfishes with amphibians [reviewed by Rosen et al., 1981). Alterna tively, similarities between the two groups may have arisen by retention of ancestral juvenile characters [paedomorphosis: Bemis. 1984; Northcutt, 1986] and independent adaptation to similar ecological niches. Convergent evolu-
Dr. Christopher S. von linrihcli! Department of Physiology an
Department of Neuroanatomy. University of Gottingen Medical School, Gottingen. Germany, and Departments of Otolaryngology and Physiology & Biophysics, University of Washington. Seattle
Oculomotor and Sensory Mesencephalic Trigeminal Neurons in Lungfishes: Phylogenetic Implications
Key Words
Abstract
Fluorescent tracer Eye muscle Mesencephalic trigeminal nucleus Trochlear nerve Proiopterus Neoceratodns Acetylcholinesterase Oculomotor Lungfish
The location and number of neurons in the brainstem with projections to the eve muscles were investigated by means of fluorescent tracers in the African lungfish Proiopterus dolloi. The oculomotor nucleus (M III) projects bilaterally with a ratio of 3: L(70 ipsilateral. 20 contralateral neurons). Three subdivisions of this nucleus can be differentiated: one projects exclusively ipsilatcrally. another pro jects exclusively contralatcrally, and a third component projects bilaterally. The trochlear nucleus (M IV) is located caudally. distinct from M III. and projects predominantly to contralateral eye muscles with a ratio of 6:1 (18:3 neurons). The abducens nucleus (M VI) contains about 30 neurons with ipsilateral projec tions only. There is no evidence for an accessory abducens nucleus in Proiopte rus. Intraocular injections of tracers do not reveal any retinopetal projections in Proiopterus. The mesencephalic trigeminal nucleus (Mes V) of Proiopterus and Neoceratodns contains about 540-590 neurons on each side. In juvenile Proio pterus, up to 75 Mes V neurons arc located caudally in a ventral projection of the tectum above the velum medulläre anterius. Fifty-five Mes V neurons (10% of the total number) have processes that exit the brain with the trochlear nerve. The relatively large number of Mes V neurons in lungfishes correlates with the well developed jaw musculature. The present study provides the First conclusive evidence for the location of oculomotor subdivisions in the brain of a lepidosirenid lungfish. The organiza tion of the oculomotor nucleus is consistent with the observation that lungfishes possess the pattern of eye-muscle innervation seen in elasmobranchs and sup ports the unconventional view that lungfishes may be the sistergroup of elasmo branchs.
Christopher S. von Bartheld
Introduction Lepidosirenid lungfishes share many features with amphibians, particularly urodeles [Northcutt, 1977, 1986; Rosen et a!., 1981], Some investigators have postulated a sistergroup relationship between lungfishes and tetrapods.
or have suggested a classification of lungfishes with amphibians [reviewed by Rosen et al., 1981). Alterna tively, similarities between the two groups may have arisen by retention of ancestral juvenile characters [paedomorphosis: Bemis. 1984; Northcutt, 1986] and independent adaptation to similar ecological niches. Convergent evolu-
Dr. Christopher S. von linrihcli! Department of Physiology an
