Organization of the hypobranchial motor column of the clearnose skate, Raja eglanteria, with comparisons to tetrapods.

Motoneurons that supply the clearnose skate's hypobranchial musculature, via the occipital nerve and first seven ventral spinal nerve roots, are located within a column that extends from a level just caudal to the obex through the corresponding rostral spinal cord segments. Individual muscle motoneuron pools within the column are considerably intermingled and overlap. Comparisons with tetrapods, particularly mammals, where the hypobranchial musculature is greatly modified, reveal general conserved features. The motor column's multisegmental organization is retained although, in mammals, the column begins rostrally at medullary levels, where hypobranchial muscle motoneurons are intimately associated with motoneurons to lingual muscles, and it is restricted caudally to fewer spinal cord segments. In addition, despite an intermingling of motoneurons that supply individual hypobranchial muscles there is a shared rostrocaudal sequence of the motor pools. Rostral most hypobranchial motoneurons supply the most ventral and anterior muscles (i.e., m. coracomandibularis, and likely m. coracohyoideus, of skate and the suprahyoid musculature, m. geniohyoideus, of tetrapods). Caudal hypobranchial motoneurons supply the skate's mm. coracohyomandibularis, coracoarcualis communis and coracobranchialis and the tetrapod's entire infrahyoid muscle complex. The intermingling of multisegmental motoneuron populations innervating different hypobranchial muscles might be attributed to intermixing of premuscle mesoderm derived from several postotic somites but the musculotopic organization along the rostrocaudal axis indicates that pre- and posthyoid muscle mesoderm may partially keep its identity during its migration to the floor of the pharynx and oral cavity.

Motor components of the trigeminal nerve and organization of the mandibular arch muscles in vertebrates. Phylogenetically conservative patterns and their ontogenetic basis.

The peripheral branching pattern of the mandibular ramus of the Vth cranial nerve and the organization of the trigeminal motor column are highly conserved in craniate phylogeny regardless of the vast modifications in attachments and structure of the mandibular arch musculature. Proximal, intermediate and distal series of mandibular nerve branches supply three major muscle groups and are in register with three neuronal populations of the trigeminal motor column. The adult branching pattern is established in response to the differentiation of mandibular muscles and is important in determining the organization of the motor nucleus of V. The innervation of the muscles of the mandibular segment of the head, and the location of motoneurons reflect their segmental origins and are reliable criteria for homologizing mandibular muscles among the craniates.

Organization of the vagus in elasmobranchs: its bearing on a primitive gnathostome condition.

The vagus nerve of the clearnose skate, Raja eglanteria, on the basis of its central and peripheral patterns and in light of the embryonic origin of its innervation fields, is viewed as a collector of separate elements. The peripheral elements include a series of branchial nerves to a segmented pharynx, an intestinal nerve to an unsegmented gut, a nerve or nerves to the heart, and an accessory nerve to a cucullaris muscle. The central elements include a sensory column, a dorsal motor column, and a ventral motor column. The dorsal motor column and sensory column are segmented in register with the branchial and intestinal nerves. Motoneurons that supply the branchial muscles of somitic origin are only located in the rostral segmented portion of the dorsal motor column. Preganglionic parasympathetics to the enteric plexus, presumably derived from circumpharyngeal crest, form the caudal portion of the dorsal motor column and are probably also present in the rostral segmented portion. Cardiac preganglionic parasympathetics to a visceral field of cardiac crest origin occur in the rostral portion of the ventral motor column as well as in the dorsal motor column. Accessory motoneurons that supply the cucullaris, likely a part of the general body musculature, are unrelated to other vagal motoneurons and form a separate nucleus (caudal ventral motor nucleus) located at spinal levels. The central and peripheral vagal nerve patterns of elasmobranchs suggest a highly conserved, ancestral gnathostome condition.

Central location of the motoneurons that supply the cucullaris (trapezius) of the clearnose skate, Raja eglanteria.

A complex of three muscles (one lateral, one intermediate and one medial in position) in the clearnose skate, Raja eglanteria, is believed to be wholly, or in part, homologous to the cucullaris (trapezius). The retrograde transport of horseradish peroxidase was used to discover the central location of the motoneurons that supply each of these muscles. Motoneurons that project to the lateral muscle occupy the caudal part of the ventral nucleus of X. This nucleus is situated ventrolateral to the dorsal vagal motor column at caudal medullary levels, and lateral to the main ventral motor column of the rostral spinal cord. The axons of these motoneurons exit the medulla within the caudal vagal rootlets and course peripherally within the intestinal (visceral) ramus of the vagus nerve. Motoneurons that innervate the intermediate and medial muscles are located along the ventral border of the ventral column of gray at spinal cord segments 10-15. Their axons course peripherally within the ventral roots of spinal nerves. The caudal ventral nucleus of X, the nerve that supplies the lateral muscle, and the lateral muscle are likely homologues of the accessory nucleus, accessory nerve, and cucullaris (trapezius), respectively, among other fishes and tetrapods. Intermediate and medial muscles, based on the central location of motoneurons that supply them, are part of the longitudinal epaxial musculature and are not part of a trapezius complex.

Topography and nerve supply of the cucullaris (trapezius) of skates.

Dissections of Sudan black B stained specimens reveal that, of a complex of medial, intermediate, and lateral muscles of skates, presumed homologous to the cucullaris of sharks, only the lateral muscle is innervated by a branch or branches of the vagus and is inserted, in part, to the fused pharyngobranchials of the caudal visceral arches. The medial and intermediate muscles are supplied by separate branches of rostral spinal nerves and do not attach to the branchial skeleton. The lateral muscle therefore is the most likely homologue of the cucullaris (trapezius) of sharks and perhaps other fishes and tetrapods. The medial and intermediate muscles appear to be part of the axial musculature.

Medullary and mesencephalic pathways and connections of lateral line neurons of the spiny dogfish Squalus acanthias.

The neuronal connections of the electrosensory dorsal and the mechanosensory medial octavolateralis nuclei of the spiny dogfish Squalus acanthias were studied by horseradish peroxidase, autoradiographic and axonal degeneration methods. Efferents from each nucleus, in addition to extensive commissural components, give rise to ipsilateral and contralateral lemnisci that ascend to midbrain levels and terminate among the cells of the lateral mesencephalic nucleus (LMN). Within LMN, electrosensory and mechanosensory neurons distribute dorsolateral and ventromedial in position, respectively. Ascending fibers of both modalities also terminate within the central zone of the optic tectum. The LMN of spiny dogfish sharks that possess a primitive pattern of midbrain organization is homologous to parts of the lateral mesencephalic nuclear complex of batoids that possess a more derived pattern of midbrain organization. Other fiber connections of the dorsal and medial octavolateralis nuclei appear to differ from each other, indicating that electrosensory and mechanosensory lateral line information is carried over separate pathways at least to midbrain levels of the brain stem. For example, nucleus B, a feedback center, occupies a position in the descending lateral line pathways of sharks and skates similar to nucleus praeeminentialis of many electrosensory teleosts. The dorsal octavolateralis nucleus of sharks and skates receives afferents from nucleus B but there is no evidence that nucleus B directly feeds back to the medial octavolateralis nucleus of the spiny dogfish. Moreover, unlike the dorsal nucleus, the medial nucleus of Squalus is reciprocally linked with the octaval system.

Connections of the lobus inferior hypothalami of the clearnose skate Raja eglanteria (Chondrichthyes).

The afferent and efferent connections of the lobus inferior hypothalami of the clearnose skate were demonstrated by the anterograde and retrograde transport of horseradish peroxidase. The main source of afferents is from the midbrain tegmentum and telencephalon. The major midbrain input is from cells of the ipsilateral nucleus tegmentalis lateralis and the caudal tegmental area. Another prominent, mostly ipsilateral, projection arises from nucleus F of the isthmic region. A few labeled cells also occur in the nucleus interpeduncularis, nucleus raphes superior, and lateral reticular formation. Afferents from the telencephalon arise from cells of the area preoptica, area superficialis basalis, striatum, nucleus septalis lateralis, and area subpallialis 1. Of the pallial structures, the pallium mediale and anterior as well as posterior subdivisions of the pallium dorsale pars centralis appear to have strong projections to the inferior lobe. Efferent connections of the inferior lobe consist of ascending and descending pathways. Fibers of the main ascending efferent pathway course within the basal forebrain bundle and distribute to subpallial areas. The descending efferent pathways course within the tractus lobobulbaris and tractus lobocerebellaris. Of these, the former is traceable to the level of the facial motor nucleus, issuing fibers enroute to the midbrain tegmentum and to the lateral reticular formation. The lobocerebellar tract courses dorsolateral to the lobobulbar tract, and its fibers terminate within the ipsilateral granular ridge of the rostral pole of the cerebellar corpus. There appears to be a topological organization of the inferior lobe connections. In general, pallial areas project mainly to the lateral subdivision of the inferior lobe nucleus at midlobic levels, whereas connections with the brainstem arise from or terminate within the dorsal and intermediate subdivisions at midlobic as well as caudal levels. The widespread ascending and descending connections indicate that the hypothalamic inferior lobe of the clearnose skate is a major relay center between the telencephalon and brainstem.

The spiracular organ of sharks and skates: anatomical evidence indicating a mechanoreceptive role.

The elasmobranch spiracular organ is a specialized receptor associated with the first visceral pouch. The structure of the sensory epithelium of the spiracular organ and the pattern of central termination of the afferent neurons that innervate it show that the spiracular organ is a mechanoreceptor closely related to the lateral line system of sense organs. Its position and orientation within the spiracular cleft suggest that it plays a role in proprioception or equilibrium-audition.

Ascending lateral line pathways to the midbrain of the clearnose skate, Raja eglanteria.

Efferent projections of the electroreceptive dorsal and the mechanoreceptive intermediate octavolateralis nuclei were revealed, in the brain of the clearnose skate Raja eglanteria, by means of silver degeneration and autoradiographic methods. Efferents from each nucleus give rise to ipsilateral and contralateral lemnisci and commissural components. Commissural fibers terminate within their respective nuclei of the opposite side. The lemnisci from each nucleus parallel each other as they course from medullary to mesencephalic levels; those from the dorsal nucleus assume a lateral position and terminate within the lateral part of the nucleus of the lateral line lemniscus and the lateral nucleus of the midbrain; those within the medial part of the lateral line lemniscal nucleus and the dorsomedial mesencephalic nucleus. A substantial number of fibers of each class course through the mesencephalic nuclei and terminate bilaterally within the central tectal zone. The segregation of electroreceptive and mechanoreceptive information is maintained from medullary to mesencephalic levels although there probably is convergence within the central tectal zone and the magnocellular nucleus, the only octaval center to receive terminals from secondary lateral line fibers.

Medullary and cerebellar projections of the statoacoustic nerve of the dogfish, Scyliorhinus canicula.

The statoacoustic nerve of the dogfish, Scyliorhinus canicula, was transected medial to the ganglion for the purpose of elucidating its central pathways and terminal fields. Following two to six weeks postoperative survival times, transverse, horizontal, and sagittal sections of the brain stem were stained by the Fink-Heimer silver-impregnation method to reveal degenerating axons and terminals. Fragmented axons enter the medulla and give rise to medial, descending, and ascending pathway. Fibers of the medial pathway terminate about the soma and lateral dendrites of the large cells that comprise nucleus magnocellularis; descending and ascending fibers terminate on the dendrites of the cells of ventral and superior nuclei respectively. In addition, fibers emanate from fascicles of the descending pathway to form a large field of degenerating axons and terminals within the ventromedial part of the medulla, and a substantial proportion of the fibers of the ascending pathway continues beyond the superior nucleus to terminate among the granule cells of the medial part of the vestibulolateral lobe of the cerebellum. No fragmented axons are traceable to the lateral part (auricles) of the vestibulolateral lobe, cerebellar nucleus or corpus, or those nuclei associated with the lateral-line lobes. It appears therefore that octavus terminal fields are separate from those of the lateral line at both cerebellar and medullary levels, at least at the level of the first-order neuron.

The topography of the superificial roots and ganglia of the anterior lateral line nerve of the smooth dogfish, Mustelus canis.

The topographic relationships of the superficial roots and ganglia of the anterior lateral line nerve (NLLa) to each other and to cranial nerves V, VII and VIII were studied, in the smooth dogfish Mustelus canis, by dissection of Sudan Black B stained specimens. NLLa consists of four branches: namely, superficial ophthalmic, buccal, otic and external mandibular. Each branch carriers lateral line neurons exclusively and forms a dorsal root and a ventral root which enter the anterior lateral line lobe and posterior lateral line lobe of the medulla respectively. It is estimated that slightly more than 50% of the fibers of the superficial ophthalmic, approximately 60% of the fibers of the buccal, at least 35% of the fibers of the otic and about 20% of the fibers of the external mandibular constitute the dorsal root. The ventral root is comprised of less than 50% of the fibers of the superficial ophthalmic, 40% of the fibers of the buccal, at least 50% of the fibers of the otic, and 80% of the fibers of the external mandibular. These results are correlated with the peripheral distribution and central termination of NLLa and it is concluded that the dorsal root carries axons from ampullary receptors and the ventral root carries fibers that innervate ordinary lateral line sense organs.

Cytological differentiation in the uropygial gland.

Ultrastructural changes were studied in the cells undergoing secretory differentiation in zone I of the tubules of the uropygial gland of White Plymouth Rock chickens. A layer of basal cells and four secretory stages are recognized as the cells migrate from the periphery to the lumen of tubules and progressively elaborate a secretion product. Basal cells, containing rough endoplasmic reticulum and free ribosomes, rest on the basement membrane and are the source from which secretory cells arise. Dilated perinuclear cisternae and the proliferation of smooth endoplasmic reticulum in the form of vesicles, invaginated sacs and cusp-shaped cisternae indicate the onset of lipgenesis in stage I cells. The perinuclear cisternae are more dilated and the endoplasmic reticulum is composed on saccules and cisternae in stage II cells. Stage III cells are characterized by concentric lamellae of endoplasmic reticulum surrounding secretory droplets. Dilated cisternae of endoplasmic reticulum and secretory droplets both contain a reticular substance. The perinuclear cisternae of stage III cells have returned to normal dimensions. Large mature lucent secretory droplets, lined with electron-dense material, fill the cytoplasm ostage IV cells which degenerate and release their secretory product into the tubule lumen. Spherical membrane-bound compartments containing a mottled substance of moderate electron density occur in basal cells and all subsequent secretory stages. These mottled bodies are surrounded by saccules of endoplasmic reticulum in stage II cells and are intimately associated with secretory droplets in stage III cells, but there is no evidence that they give rise to secretory droplets and their role in secretory differentiation is unknown.