R. Glenn Northcutt

Organization of the Amphibian Telencephalon

Living amphibians comprise three different groups or orders: Anura (frogs and toads, approximately 2600 species), Urodela (salamanders, approximately 300 species), and Apoda (caecilians, approximately 150 species). Each group is clearly separated from the others by extensive structural variation, and each is characterized by a distinct life style. These three orders are easily distinguished from their earliest appearance in the fossil record, but they also share a number of unique characters which suggest common ancestry. These characters include teeth with a weak, uncalcified segment between the base and crown (pedicellate teeth) and similarities in the middle ear bones and vertebral-skull articulation. Parsons and Williams (1963) concluded from these characters that modern amphibians represent a monophyletic group, the Lissamphibia. Most workers employ this tenable model of amphibian affinities; however, additional data are needed to accept this hypothesis (Thomson, 1968; Estes and Reig, 1973), particularly for the apodans.

New Observations on the Organization and Evolution of the Telencephalon of Actinopterygian Fishes

The Osteichthyes comprise at least three groups of bony fishes which are generally believed to share a common ancestor: the Crossopterygii, the Dipnoi, and the Actinopterygii. The intergroup relationships have long been disputed by paleontologists, and there is no consensus at the present time. It is usually argued either that the Crossopterygii and the Dipnoi are more closely related to one another than either is to the Actinopterygii (Romer, 1966; Gardiner, 1973) or that the three have been distinct groups since their earliest appearance (Schaeffer, 1969; Miles in Moy-Thomas, 1971). Jarvik (1968), however, has favored a closer affinity between the Crossopterygii and the Actinopterygii than between either of these groups and the Dipnoi.

The monoamine-containing neurons in the brain of the garfish, Lepisosteus osseus

The morphological organization of monoamine (MA)-containing neurons in the brain of the longnose gar (Lepisosteus osseus) was studied by means of fluorescence histochemical methods. In this species, one of the few living representatives of the holostean group of actinopterygian fishes, by far the largest number of MA cells is found within the preoptico-hypothalamic complex. Multitudinous small-sized MA cells, with a short club-like process protruding into the third ventricle, are present along the ependymal wall of the lateral and posterior hypothalamic recesses. This population of CSF-contacting MA cells, comprising both catecholamine (CA) and serotonin (5-HT) type cells, gives rise to numerous efferent fibers, some of which proceed rostrally toward the telencephalon while others course dorsally to reach the midbrain tegmentum and optic tectum. Many fibers, however, arborize directly within the hypothalamus, particularly around blood vessels where they form patches of highly fluorescent material. Small CA cells are also scattered along the preoptic recess wall. They do not directly contact the CSF but also appear to contribute to the CA innervation of telencephalon. At brain stem levels, a few CA cells are scattered at the base of the rostral midbrain, in the isthmal tegmentum, and in the central and dorsal (vagal lobe) portions of the medulla. Some CSF-contacting CA cells are also present around the central canal at upper spinal cord levels. One of the most striking features of the MA systems in Lepisosteus is the remarkable development of the 5-HT neuronal network. A prominent 5-HT cell column extends rostrocaudally in the raphe region from the caudal midbrain to upper spinal cord levels. In the caudal midbrain and isthmus, the 5-HT, cells also invade the lateral tegmentum and profusely innervate various brain stem structures as well as large portions of telencephalon, particularly the dorsal nucleus of area ventralis (Vd). The CA innervation of telencephalon is relatively weak, except in the olfactory bulb where numerous CA varicosities were found. These findings in Lepisosteus suggest that the pattern of MA system organization in the holostean brain is far more similar to that seen in primitive vertebrates, such as cyclostomes--where the 5-HT systems are highly elaborated relative to the CA systems--than it is to the pattern in more advanced fishes such as teleosts.

Central Auditory Pathways in Anamniotic Vertebrates

The late nineteenth and early twentieth centuries witnessed rapid growth in descriptive neuroanatomy. This period of intensive study of nervous systems in a wide variety of vertebrates resulted in several hypotheses concerning the origin and subsequent evolution of the otic and lateralis systems. These hypotheses possess two common features: they are based on descriptive anatomical material and were not tested experimentally as the appropriate techniques did not yet exist; and they reflect certain supposed anatomical relationships among an amniotic vertebrates that were believed to form a linear series of increasingly complex groups.

Segregation of electro- and mechanoreceptive inputs to the elasmobranch medulla

The anterior lateral line nerve of the thornback ray consists of fibers that innervate head electroreceptive ampullary organs and mechanoreceptive neuromasts. As the anterior lateral line nerve enters the medulla it divides into dorsal and ventral roots. Single unit responses of dorsal root fibers to electric field and mechanical stimuli indicate that the dorsal root consists only of ampullary fibers, whereas the ventral root consists only of mechanoreceptove fibers. The dorsal and ventral roots of the anterior lateral line nerve terminate in the dorsal and medial octavolateralis nuclei respectively, indicating that the dorsal nucleus is the primary electroreceptive nucleus of the elasmobranch medulla and the medial nucleus is the mechanoreceptive nucleus. Averaged evoked potential responses to electric field stimuli could be recorded from the dorsal but not the medial nucleus, further evidence that the dorsal nucleus is the electroreceptive nucleus. A second evoked response to electric field stimuli was elicited from the lateral reticular nucleus, suggesting that the reticular formation may be a secondary target of efferents of the dorsal octavolateralis nucleus. A dorsal octavolateralis nucleus exists not only in elasmobranchs, but also in agnathan, chondrostean, dipnoan, and crossopterygian fishes, suggesting that all of these taxa are also electroreceptive.

Retinofugal pathways in fetal and adult spiny dogfish, Squalus acanthias

Retinofugal pathways in fetal and adult spiny dogfish were determined by intraocular injection of [3H]proline for autoradiography. Distribution and termination of the primary retinal efferents were identical in pups and adults. The retinal fibers decussate completely, except for a sparse ipsilateral projection to the caudal preoptic area. The decussating optic fibers terminate ventrally in the preoptic area and in two rostral thalamic areas, a lateral neuropil area of the dorsal thalamus and more ventrally in the lateral half of the ventral thalamus. At this same rostral thalamic level, a second optic pathway, the medial optic tract, splits from the lateral marginal optic tract and courses dorsomedially to terminate in the rostral tectum and the central and periventricular pretectal nuclei. The marginal optic tract continues caudally to terminate in a superficial pretectal nucleus and also innervates the superficial zone of the optic tectum. A basal optic tract arises from the ventral edge of the marginal optic tract and courses medially into the central pretectal nucleus, as well as continuing more caudally to terminate in a dorsal neuropil adjacent to nucleus interstitialis and in in a more ventrally and medially located basal optic nucleus. Comparison of the retinofugal projections of Squalus with those of other sharks reveals two grades of neural organization with respect to primary visual projections. Squalomorph sharks possess a rostral dorsal thalamic nucleus whose visual input is primarily, if not soley, axodendritic, and an optic tectum in which the majority of the cell bodies are located deep to the visual terminal zone. In contrast, galeomorph sharks are characterized by an enlarged and migrated rostrodorsal thalamic visual nucleus, and an optic tectum in which the majority of the cell bodies are located within the visual terminal zone. These data suggest that evolution of primary visual pathways in sharks occurs by migration and an increase in neuronal number, rather than by the occurrence of new visual pathways.

Projections of the optic tectum in the longnose gar,lepisosteus osseus

Efferent projections of the optic tectum were studied with the anterograde degeneration method in the longnose gar. Ascending projections were found bilaterally to 3 pretectal nuclei -- the superficial pretectal nucleus, nucleus pretectalis centralis and nucleus pretectalis profundus -- and to a number of targets which lie further rostrally -- the central posterior nucleus, dorsal posterior nucleus, accessory optic nucleus, nucleus ventralis lateralis, nucleus of the ventral optic tract, rostral part of the preglomerular complex, suprachiasmatic nucleus, anterior thalamic nucleus, nucleus ventralis medialis, nucleus intermedius, nucleus prethalamicus and rostral entopeduncular nucleus. Projections of the tectum reach the contralateral side via the supraoptic decussation and are less dense contralaterally than ipsilaterally. Descending projections resulting from tectal lesions include: (1) a tectal commissural pathway to the core of the torus longitudinalis bilaterally and the contralateral tectum and torus semicircularis; and (2) a pathway leaving the tectum laterally from which fibers terminate in the ipsilateral torus semicircularis, an area lateral to the nucleus of the medial longitudinal fasciculus, lateral tegmental nucleus, nucleus latealis valvulae, nucleus isthmi and the reticular formation. A component of this bundle decussates at the level of the lateral tegmental nucleus to project to the contralateral reticular formation. On the basis of comparisons of these findings with the pattern of retinal projections in gars and other data, it is argued that the nuclei previously called the lateral geniculate and rotundus in fish are not the homologues of the nuclei of those names in land vertebrates but are rather pretectal cell groups. The overall organization of both retinal and tectal projections in gars is strikingly similar to that in land vertebrates; at present, the best candidate for a rotundal homologue is the dorsal posterior nucleus.

New thalamic visual nuclei in lizards

As many as 5 distinct, dorsal thalamic cell populations receive retinal projections in birds 25,32. At least 4 and perhaps all 5 of these cell populations project to the visual Wulst in the telencephalonlg,~a, 25,26,31. However, most retinal projection studies in reptiles, utilizing the anterograde degeneration technique, have identified only a single target in the dorsal thalamus8,16. These studies would suggest that the avian visual thalamus can be characterized by an increase in the number of visual nuclei. However, as more recent studies have indicated that more than one dorsal thalamic retinal target may also be present in reptiles2,4Y-29, a re-examination of the retinal projections in two lizards, utilizing anterograde axonal transport of.tritiated proline, was undertaken. A preliminary report of these data has been published elsewhere ~0. Two juvenile specimens of Iguana iguana (12 and 15 cm snout-vent length) and three adult specimens of Gekko gecko (12-14 cm snout-vent length) received intra-ocular injections of 40-60/~Ci of L-[4,5-~H]proline, 9.4/~Ci/#l, under sodium pento-barbital anesthesia (20 mg/kg, i.p.). Injections were carried out with a 10 #1 Hamilton syringe and 26-gauge needle fitted with a polyethylene sleeve which limited penetration into the eye to 3 ram. Aqueous radioactive solutions were evaporated to dryness with nitrogen to remove 3H20 and redissolved in 0.86 ~ saline immediately before use. Following postoperative survival times of 1 and 6 days for the iguanas and 1-3 days for the geckos at 28 °C, the animals were perfused with AFA (90 ml of 80 ~ ethanol, 5 ml formalin, and 5 ml glacial acetic acid). The brains were removed and stored in AFA for at least one week prior to dehydration and embedding in paraffin. The brains were then cut at 15 #m in the transverse plane. Autoradiographic procedures followed those described by Kopriwa and Le-blond 24. Kodak NTB3 nuclear track emulsion was diluted 1:1 with distilled water at 40 °C, and the slides were dipped into the emulsion and dried for approximately 1 h. Following an exposure time of 20 days, the slides were developed in Kodak Dektol and stained with cresyl violet. With the autoradiographic method, the pattern of retinal projections to the

Experimental determination of the primary trigeminal projections in lampreys

The trigeminal complex of lampreys, like that of anamniotic gnathostomes, consists of profundus and mandibulo mandibulomaxillary branchesa, s. The profundus branch possesses a distinct ganglion (supraorbital ganglion) separate from the ganglion (suboptical ganglion) of the mandibulomaxillary brancheslL However, both ganglia send their processes into the medulla as a single trigeminal sensory root (Figs. 1A, 2A). The lamprey trigeminal complex differs from that of other anamniotes in that the anterior lateral line and facial ganglia are not closely associated with the trigeminal ganglia but are located beneath and within the optic capsule a,12. This separation clearly simplifies experimental analysis of the trigeminal projections, as the trigeminal ganglia can be lesioned or the sensory roots transected without damaging facial or lateralis inputs. Earlier studies claim that scattered lateral line receptors on the dorsal surface of the head are innervated by the trigeminal nerve~, ~. This condition has not been reported for any other vertebrate and has been interpreted as supporting the hypothesis that a lateralis component existed ancestrally in each branchiomeric cranial nerve. It is also claimed that the trigeminal nerve in lampreys projects directly to the cerebellum, the octavolateralis area and the funicular nucleus (presumed precursor of the dorsal column nuclei4,6). This trigeminal projection pattern has been interpreted as supporting the claim that the dorsal column nuclei, octavolaterlis area and cerebellum arose from a single somatic sensory column, and that lampreys essentially retain that primitive condition 6. Finally, a distinct mesencephalic trigeminal nucleus has not been identified in lampreys and is assumed to have arisen with the evolution of jawed vertebrates. These claims are of considerable importance regarding the evolution of the trigeminal system and were the impetus for the present study. The trigeminal ganglia in 5 transformed specimens (11-16 cm in length) of Ichthyomyzon unicuspis were unilaterally destroyed by transection or heat cauterization of the ganglia under MS222 anesthesia. The animals survived 3, 5, 7 and 12 days at 23 °C before they were reanesthetized, the brains dissected from the neurocranium and emersed in 107o formalin. The brains were embedded in 25~ gelatin and processed by the Wiitanen procedure 14 for demonstrating degenerating axons and their terminals. All the survival times were adequate to reveal degenerating trigeminal

Central projections of the eight cranial nerve in lampreys

The octavolateralis area in anamniotic vertebrates constitutes the bulk of the hindbrain alar plate and is the primary target of three cranial nerves : anterior lateral line, octavius, and posterior lateral line 2,3,6-s. There is no agreement in the literature regarding the exact rostrocaudal extent of the octavolateralis area or the nomenclature applied to this area, but most workers have recognized 3 longitudinally arranged cell groups in lampreys, cartilaginous fishes, and chondrostean fishes 1,3,5,7,9-11,13. The nomenclature adopted in this study is that of Pearson 11. The octavolateralis area of lampreys consists of dorsal, medial and ventral nuclei (Figs. l B, 2C). Rostrally, the dorsal nucleus begins immediately caudal to the cere-bellar plate and continues caudally in the medulla where it ends slightly beyond the entry of the anterior lateral line nerve (Fig. 1D-E). Rostrally, the medial nucleus is continuous with the cerebeIlar plate, and caudally it can be traced into the medulla where it ends just rostral to the obex (Fig. 1E). The dorsal and medial nuclei possess similar patterns of organization: both are capped laterally by the cerebellar crest-a layer of fine fibers; both consist of centrally situated neuropils supplied by the lateral line nerves (Fig. 2B); and both possess prominent periventricular cell plates (Fig. 2B, C). Rostrally, the ventral nucleus forms the lateral edge of the cerebellar plate, and caudally it continues to obex levels (Fig. 1). The ventral nucleus is bordered laterally by the octavius nerve (Figs. 1D, 2C, D), and the cells of the ventral nucleus are more scattered than those of the dorsal and medial nuclei. The ventral nucleus contains three distinct aggregations of large neurons, termed the anterior, intermediate and posterior octavomotor nuclei (Figs. 1, 2A, C). Thus the ventral nucleus is a long column of scattered small and medium-sized cells, within which three collections of much larger neurons (the octavomotor nuclei) can be identified. Octavius fibers have been claimed to project to only the ventral nucleus 1, to both the medial and ventral nuclei 3, and primarily to the medial nucleus ~3. In addition, most reports claim that primary octavius fibers project to the ipsilateral cerebellum, as well as to the contralateral cerebellum by decussating in the cerebellar commissure immediately dorsal to the trochlear nucleusl,3, ~ (Figs. IA, 2A). In an attempt to resolve these discrepancies, the central projections of the octavius nerve were determined experimentally. The membranous labyrinth in six