Studies on the retina. The structure of the retina of Alligator mississippiensis and its photomechanical changes

Abstract

Estudios sobre la retina.La estructura de la retina de Slligator mississipiensis y sus cambios fotomechnicos.El ojo de Alligator posee un tapetum retinal bien desarrollado, formado por la inclusi6n de guanina en las c6lulas epiteliales de las porciones dorsal y posterior, a una distancia de 1.5 mni. de la entrada del nervio 6ptico. El pecten consiste en una especie de copa pigmentada ligeramente elevada, que cubre la entrada del nervio 6ptico. E n toda la retina se encuentran conos y bastones, pero la proporcih de anibos es diferente en distintas regiones. Los bastones son todos del mismo tipo, 10s conos de dos tipos: grueso y delgado. Los primeros son mhs numerosos, presenthdose especialmente en las regiones posterior y ventral de la retina. Los conos del segundo tip0 se encuentran solamente en la porcibn ventral y no son numerosos. T a m b i h existen conos dobles. Ninguno de 10s conos y bastones contiene gotas de grasa.Los nlicleos de 10s bastones son de forma oval y la mayor parte de ellos se proyectan a travks de la membrana limitante externa en una extensi6n variable. Los nGcleos de 10s conos son piriformes y ocupan un nivel m8s profundo que el de 10s nficleos de 10s bastones, formando una segunda fila. Los bastones presentan un cambio de longitud media de unas 4 micras, y son m8s largos a la luz. Los conos sencillos presentan un cambio medio de 2.1 micras y son m8s cortos a la luz. Los mienibros mas pequefios de 10s conos dobles presentan un cambio medio de longitud de 3.5 micras, y 10s mayores de 2.7 micras. La emigracih del pigmento es ligera, y su media es 1.6 micras, per0 cuando se combina con el cainbio de longitud de las c6lulas visuales produce una emigraci6n efecti va igual a su sunia. El trabajo termina con consideraciones te6ricas sobre 10s cambios fotomec&nicos y la teoria de la duplicidad bajo un punto de vista comparado.

Spatial resolving power and spectral sensitivity of the saltwater crocodile, Crocodylus porosus, and the freshwater crocodile, Crocodylus johnstoni

“…The gross morphology of the eye of Australian crocodiles confirms previous descriptions of the crocodilian eye in other species (Laurens and Detwiler, 1921). Both Australian species possess a bright yellow iris, a slit pupil and a relatively large lens.…”

“…The crocodilian eye possesses a relatively large lens, a rod photoreceptor-dominated retina and a guanine-based tapetum lucidum, all features commonly associated with optimised sensitivity in dim light environments (Abelsdorff, 1898;Laurens and Detwiler, 1921). The eye is also equipped with a mobile slit pupil, which provides a high degree of control over the amount of light that reaches the retina during the day (Walls, 1942).…”

“…The eye is also equipped with a mobile slit pupil, which provides a high degree of control over the amount of light that reaches the retina during the day (Walls, 1942). Previous studies have described the retinal fine structure of the spectacled caiman, Caiman crocodilus, and the American alligator, Alligator mississippiensis (Tafani, 1883;Chievitz, 1889;Laurens and Detwiler, 1921), and measured the absorbance spectra of the visual pigments in C. crocodilus, A. mississippiensis and the Nile crocodile, Crocodylus niloticus (Dartnall and Lythgoe, 1965;Govardovskii et al, 1988;Sillman et al, 1991).…”

Vertebrate Photoreceptors

“…In this study it appeared that there was an inverse relation between the number of droplets present and the amount of deeply staining granular material in the rod outer segment, and a definite relation of these droplets to the pigment epithelium as Kolmer assumed was not supported. The eye of the field-mouse, with its preeminently rod retina, if not entirely so, showed the same condition, whereas in the retina of the diurnal lizard (Eremias) with cones only there was no evidence of these granular structures (figures 29,30,31).…”

The Visual Cells of Lampreys

“…Direction of migration in light and darkness.--This criterion was made use of by Laurens and Detwiler (1921) on the alligator, and depends upon the rule that if the visual cells of a given retina migrate at all, they move in opposite directions. If Cell "A" elongates in light and contracts in darkness while Cell " B " elongates in darkness and contracts in light, then Cell "A" is the rod and Cell " B" the cone.…”