Brain and Sense Organ Anatomy and Histology of the ... - Lannoo Lab

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BRAIN AND SENSE ORGANS OF ELEGINOPS MACLOVINUS 91 Figure 4. (Continued.) optic nucleus are abundant in the dorsal preoptic area and the endopeduncular nucleus is large (Fig. 4G). In the central diencephalic region, the preoptic region contains both parvocellular and magnocellular cells (Fig. 4G,H). The habenula is medium-sized and thalamic nuclei, including the ventromedial nucleus, are present and exhibit a normal appearance. Cells comprising the lateral tuberal nucleus are large and form a prominent cluster (Fig. 4H). The pituitary is also large and appears well organized (not shown). At the level of the posterior commissure, narrow subependymal expansions are present (Fig. 4I,J), thalamic nuclei, including the dorsoposterior and centroposterior nuclei, and hypothalamic nuclei, including the dorsal and ventral nuclei, appear in normal positions Journal of Morphology DOI 10.1002/jmor
92 J.T. EASTMAN AND M.J. LANNOO and proportions. Nucleus preglomerulosus is normally proportioned. The inferior lobes are small to medium sized. Farther caudally, the nucleus glomerulosus is large and circular shaped, the posterior tuberal nucleus crosses the midline (Fig. 4K,L). Immediately ventral to the posterior tuberal nuclei the mammillary bodies meet along the third ventricle. At this level the inferior lobes are medium sized, cells of the nucleus diffusus are small and sparse, and the lateral recess is positioned within the lobe. The saccus vasculosus is small. Farther caudally, at the level of the oculomotor nerve (Fig. 4L,M), cells within the nucleus diffusus are well organized. Near the caudal-most inferior lobes, nucleus diffusus cells are less well organized and the lateral recesses become subpial (Fig. 4N). Mesencephalon. The tectum is small and unusually proportioned. Rostrally (Fig. 4I) the superficial white matter, consisting of the superficial white and gray zone, the central zone, and the deep white zone, is proportional along the dorsal and lateral tectal surfaces, but thickens to form a deep neuropil ventrally. At about the mid-tectal level, the tectum remains small and thin. The thick appearance of the white matter is no longer apparent, but the ventrolateral portion of the tectum makes an acute angle (Fig. 4J). The rostral portions of the torus longitudinales are present. The rostral torus semicircularis is flattened along its dorsal surface—it protrudes very little into the tectal ventricle (Fig. 4K). Caudally, the torus semicirculares form narrow lobes that project deeply into the tectal ventricle (Fig. 4L–N). Motor neurons of the oculomotor complex are large but sparse at about the midtectal level (Fig. 4L). At the level of oculomotor nerve exit (Fig. 4M) oculomotor nuclei are numerous. Farther caudally (Fig. 4N) they are again less numerous but clustered, and positioned immediately dorsal to the medial longitudinal fasciculus. Cerebellum. At its rostral-most level the valvula cerebellum is single lobed, consisting of a central molecular layer and lateral granule cell regions (Fig. 4L). Just caudally, at the level of oculomotor nerve exit (Fig. 4M), the valvula consists of two large, stacked lobes, which caudally become broader, with lobules separated dorsoventrally—a morphology that does not appear to be artifact and that we have not seen before (Fig. 4N). The rostral corpus cerebellum is tall, comprising more than 50% of the dorsal extent of the brain at the level of the rostral rhombencephalon (Fig. 4O,P). Farther caudally, the corpus becomes broad (Fig. 4P,Q); the eminentia granulares appear as large lateral granule cell masses (Fig. 4Q). At the point where it becomes a lobe, the corpus and the vestibulolateral lobe are large and well differentiated; the crista cerebellaris is large (Fig. 4R). As it forms a lobe, the corpus is large, with about the Journal of Morphology DOI 10.1002/jmor same cross-sectional area as the brainstem proper (Fig. 4S). Rhombencephalon. The brainstem of Eleginops is typically proportioned. Rostrally, neurons of the trigeminal motor nuclei are prominent (Fig. 4O–Q). The rostral portions of the crista cerebellares are large (Fig. 4R) and at the level of the mid-fourth ventricle form a decussation (Fig. 4S). At the level of the caudal rhombencephalon, the fourth ventricle is again open dorsally, the crista cerebellares become small, and the sensory nuclei of the vagal nerve are present (Fig. 4T). At this level, subependymal expansions (arrows) are prominent and contain cell bodies and axons. At the level of the brainstem junction with the spinal cord, the spinal sensory nuclei are moderately sized (Fig. 4U). Subependymal expansions are prominent and form a septum below the central canal. The rostral spinal cord is narrower dorsally than it is ventrally; motor neurons are large (Fig. 4V). Sensory Systems Visual system Eye size. At 11%–13% of head length (HL) or 3% of SL, the eye diameter of Eleginops maclovinus (Figs. 2 and 7A) is about twofold smaller than in many other notothenioids. For comparison, eye diameter of the artedidraconid Dolloidraco longedorsalis is 31–37% of HL, bathydraconids are 16–31%, and channichthyids are 19–22% (Eastman and Lannoo, 2003a,b, 2004). Ocular vasculature. The generalized teleostean eye (Walls, 1942) as well as the phyletically basal notothenioid eye, exemplified by Bovichtus diacanthus (Eastman, 2006; Eastman and Lannoo, 2007), possess three ocular vascular structures supplying the retina and ocular adnexa: the choroid rete mirabile, the lentiform body (also a rete), and the falciform process. In Eleginops maclovinus the embryonic choroid fissure is closed and therefore the falciform process is not present (Fig. 5A). There is a well-developed choroid rete (Fig. 5B) but this was not filled by our ventral aortic perfusions of Microfil because the perfusate is unable to pass distal to the capillary bed of the pseudobranch. The ophthalmic artery, a vessel originating in the pseudobranch, supplies the choroid rete. The eye of Eleginops also receives blood from a second vessel, the optic artery (60 lm diameter). This vessel, a branch of the carotid complex, is the afferent supply to the lentiform body and was filled by Microfil (Fig. 5C). The lentiform body, located ventral to the optic nerve (Fig. 5B,C), is a small complex (1.2 mm by 0.5 mm) of two to three orders of short, small diameter (15 lm) arterial capillaries. Since the venous capillaries are extrinsic and part of the general choriocapillaris network, they were not filled by Microfil. The falciform artery (also
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