Structural characteristics of cell and fiber populations in the optic tectum of the frog (Rana catesbeiana)

Abstract

In the optic tectum Cajal's method I1 for neurofibrils reveals two major fiber systems in which the fibers are oriented perpendicular to one another. The outer fiber system, composed of seven laminae, lies in the superficial zone with its fibers oriented in the anteroventral to posterodorsal direction; Golgi preparations show that optic tract fibers enter the outer system laminae at the tectal margin, run within single laminae, and end in arborizations that are elongated i n the dominant fiber direction. The intermediate fiber system is composed of bundles of fibers in lamina 7 of Ramon and the superficial zone; its fibers are oriented in the anterodorsal to posteroventral direction. A third deep fiber system, forms fibrous laminae in the region between lamina 7 and the ependyma.In Golgi impregnations the deep cells are of two major classes: pyramidal cells with relatively narrow dendritic arbors in the superficial zone and candelabra cells with broad arbors. Many pyainidal cell dendrites have lamellar branching patterns such that terminal bushes occur within one or another of the outer system laminae. The multipolar cells within lamina 6 and lamina 7 of Ram6n send large, unbranched axons directly into the deep medullary lamina and appear to be the major efferent neurons. Near the pia the perikarya of superficial granule cells give rise to single processes that branch repeatedly within the superficial zone.The majority of the studies describing the normal structure of the anuran optic tectum have been based on Golgi preparations. Only the brief descriptions of cytoarchitecture by Ariens Kappers and Hammer ('18) and Huber and Crosby ('33) were primarily dependent upon other techniques. Since the original extensive description of Golgi impregnations of the frog optic tectum was given by Ram6n (1894), several reports based on similar preparations have appeared (Rubaschkin, '03; Rambn y Cajal, '04; Larsell, '31; Kiro, '48; Leghissa, '62; Lazar and Szekely, '67). Although the reports agree in recognizing the presence of the major cellular laminae and the radially oriented dendritic trunks of most tectal cells, the various studies do not present uniform descriptions of the forms and dispositions of the dendritic arborizations and of the axons of tectal cells. None of the studies deal specifically with the orderly distribution and orientation of fibers within the tectum. These properties are of current interest as the result of recent experimental studies of the J. COMP. NEUR., 136: 203-232. retinotectal projection system. Gaze ('58) demonstrated electrophysiologically that the retinal projection to the tectum is topically organized in a "point-to-point" fashion. The capacity of the retinal fibers to form this retinotopic projection during their regeneration following optic nerve section (Gaze and Jacobson, '63a) as well as during their development (Jacobson, '68) has been studied. In addition to the problems related to the organized distribution of retinotectal fibers across the tectum, the internal organizat...

Relationships between segregated afferents and postsynaptic neurones in the optic tectum of three-eyed frogs

“…In tangential views, the dendritic arbors were roughly circular and typically 3-6 stripe widths in diameter. These cells resemble the ganglionic (Szekely and Lazar, 1976;Antal et al, 1986;Matsumoto et al, 1986) and multipolar (Potter, 1969) cells described previously. Using cobalt-lysine and HRP backfilling techniques, Lazar et al (1983) have shown that these are among the efferent neurons of the tectum with axons in both rostra1 and caudal tectal projection zones.…”

“…The use of flattened, unsectioned tecta greatly facilitated this examination since it allowed visualization of the entire dendritic tree of even the largest tectal neurons without the truncations necessarily produced in sectioned material. In contrast, most previous studies have emphasized radial organization of tectal dendrites in sectioned material and classified cells accordingly (e.g., Potter, 1969;Lazar, 1973;Szekely and Lazar, 1976;Antal et al, 1986;Matsumoto et al, 1986). As a result of this difference in approach, our assignments are not directly comparable to those used for tectal neurons in previous studies.…”

“…16). These probably correspond to multipolar cells of Potter (1969) and to some of the ganglionic neurons of Szekely and Lazar (1976). In striped tecta the primary branches frequently extended over 6-8 stripe widths, making them the cells with the largest arbors we encountered.…”

Cell size predicts morphological complexity in the brains of frogs and salamanders.

“…In general, frogs have more complex brain morphology than do salamanders, having morphologically distinct nuclei that often lie in migrated positions in the diencephalon, the pretectum, and the mesencephalic tegmentum (2). In addition, multiple lamination (an alternation of cellular and fibrous layers) is found in the tectum opticum (3,4), the torus semicircularis (5), and a number of diencephalic nuclei (2). The brain of salamanders long has been known to be morphologically much simpler than that of frogs and other vertebrates (6,7).…”

Retinal Projections in the Cane Toad, <i>Bufo marinus</i>

“…In the superficial part of the contralateral OT (layers 9, 8, 7 and 6), coarse fibers formed four well defined laminae, corresponding to B, D, F of layer 9 and lamina G between layers 7 and 8 of Potter, 1969 ( fig.8B). In addition, a few coarse fibers also extended to layer 6, and fine fibers were present in laminae A, C and E. On the ipsilateral side optic fibers entered the OT via the mMOT as well as the IMOT.…”