Foveal Development in the Marmoset Monkey

Abstract

Abstract: : Purpose: The common marmoset (Callithrix jacchus) is an established new world primate model of eye growth and refractive development. We have used this model species for an investigation of the mechanisms of foveal development. In this study we present the first report of the developmental features for the marmoset fovea from birth to adulthood. Methods: Six marmoset eyes were examined at 12-36 hrs after birth, 1 and 8 mo of age, and adulthood (>2 yrs of age). One eye from each animal was fixed in Carnoys, embedded in paraffin and serially sectioned. The other eye was fixed in paraformaldehyde, prepared as a retinal whole mount, and labeled for photoreceptor antigens using immunocytochemistry. Peak foveal cone density was determined from the paraffin sections, and from 7 additional retinal wholemounts 5mo to 10yrs of age. Foveal pit dimension was measured in sections and the temporal relationship was determined between changes in the geometry of the pit and cone density. Results: Mature marmosets have well developed foveas with maximum cone packing density on average of 198,000/mm² and a pit ~100um deep. At birth a very shallow foveal pit is present, mainly involving the ganglion cell layer (GCL). Cones form a regular monolayer and have elongated and slightly tilted axons. By 1mo the GCL, inner nuclear and inner plexiform layers are fused into a layer 2-3 cells deep at the bottom of the pit which is narrow and 1.5x as deep as adult depth. Cone nuclei now are multilayered, highly elongated, and their axons are markedly tilted away from the pit center. Based on counts from sections, foveal cone density increases from 17,000/mm² at birth to 83,000/mm² at 1mo. This suggests that cone density increases are associated with, and may begin following pit formation. Cone density continues to increase while pit depth decreases and the fovea widens up to 6-8mo. Conclusions: The common marmoset, a new world primate, exhibits a developmental sequence in foveal pit formation and cone packing that is remarkably similar to that reported for old world macaque and human fovea. Because of the relative immaturity of the marmoset fovea at birth and its rapid postnatal development, the marmoset provides a new and more efficient model to study the mechanisms of foveal development in primates.

Keywords: retinal development • anatomy • macula/fovea