The mormyrid mesencephalon. II. The medio-dorsal nucleus of the torus semicircularis: afferent and efferent connections studied with the HRP method.

The connections of the medio-dorsal nucleus (nMD) were established with the HRP tracing method in two mormyrid species, Brienomyrus niger and Gnathonemus petersii. The medio-dorsal nucleus (nMD) is the second largest nucleus of the mormyrid torus semicircularis which comprises 7 nuclei. According to our histological observations, the nMD is composed of three regions: anterior (a), medialis (m) and posterior (p), each has distinct connections. The nMD receives medullary, rhombencephalic inputs, and mesencephalic inputs. The rhombencephalic afferents arising from the acoustico-lateral area constitute the most extensive projection: they arise from nucleus octavius, nuclear anterior and a 'crest cell layer'. The largest part of these rhombo-mesencephalic connections is bilateral. The bifurcated axons are gathered in the pars medialis of the lemnisci laterales and end, respectively, in the three subdivisions--pars posterior, pars medialis, pars anterior--of the nMD. The contralateral projections are larger than the ipsilateral ones except for the octavo-mesencephalic projection, where the ipsilateral is predominant. Collaterals of these axons end bilaterally in the mesencephalic paralemniscal nuclei (nPL). The mesencephalic afferents originate in the contralateral nPL, while medullary afferents arise in the contralateral nucleus subfunicularis (nSF). None of these projections show topological specificity within the nMD. Concerning the efferent connections, the nMD projects to the ipsilateral tectum opticum and valvula cerebelli.

Development of the gigantocerebellum of the weakly electric fish Pollimyrus.

The morphogenesis of the "hypertrophied" mormyrid cerebellum was investigated in Pollimyrus (Pisces). Two adults and 36 larvae and young fish raised in captivity were used. Two Gnathonemus petersii adults were taken for comparison. The ontogenetic development of the various cerebellar structures was analysed in inverse chronological order with the aid of serial sagittal and frontal brain sections. Special attention was given to the trilobed corpus cerebelli (C1, C2, C3), the lobi transitorii et caudales, the valvula, the crista cerebelli, the eminentia granularis and the lobus lineae lateralis. 1. The cerebellar structures are of bilateral origin; they develop from the cerebellar and acoustico-lateral "anlage" of the rhombencephalon behind the rhombomesencephalic fissure, either through budding or individualisation and appear between the 4th and 11th day after spawning. The midline fusion of the symmetrical structures is accomplished somewhat later, between the 8th and 23rd days. 2. The cerebellar structures acquire their definitive spatial organisation within 38 days, except for the valvula whose development takes much longer. Recognisable from the 11th day, the valvula upon which ridges are visible from the beginning continues to grow after the 38th day beyond the mesencephalic ventricle, finally overlying the telencephalon frontally and the different rhombencephalic structures caudally. This development, which includes a large antero-lateral folding of the valvula, takes 240 days. 3. Cytological differentiation is just as complex as the general development of the cerebellar structures. Cortical stratification first begins on the 8th to the 11th day in the corpus cerebelli and in the valvula from day 21 to 23 onwards. This differentiation is characterised throughout almost the entire cerebellum by a downward migration of the superficial undifferentiated cells which then constitute the granular layer. In the valvula, the majority of the undifferentiated cells leave the ridges to form a continuous granular layer at the base of the ridges. 4. A differentiation gradient was observed on the antero-posterior axis. 5. In spite of its complexity, the mormyrid cerebellum develops much more rapidly than the cerebellum of the trout.