The decussation of the optic nerves at the chiasm is essential for the development of binocular vision and stereopsis. With decussation, visual information from corresponding retinal areas in each eye runs through the lateral geniculate body (lateral geniculate nucleus) and optic tracts to the visual cortex, where the information from both eyes is commingled and modified by the integration of various inputs. See BCSC Section 5, Neuro-Ophthalmology, Chapter 1, for further discussion.
In the human retina, most of the ganglion cells are generated between 8 and 15 weeks’ gestation, reaching a plateau of 2.2–2.5 million by week 18. After week 30, the retinal ganglion cell (RGC) population decreases dramatically during a period of rapid cell death (a process termed apoptosis) that lasts 6–8 weeks. Thereafter, RGC death continues at a low rate into the first few postnatal months. The RGC population is reduced to a final count of approximately 1.0–1.5 million. The loss of about 1 million RGC axons may serve to refine the topography and specificity of the retinogeniculate projection by eliminating inappropriate connections.
The continued development of visual function after birth is accompanied by major anatomical changes, which occur at all levels of the central visual pathways. The fovea is still covered by multiple cell layers and is sparsely packed with cones, which, in addition to neural immaturity, contributes to the estimated visual acuity of 20/400 in the newborn. During the first years of life, the photoreceptors redistribute within the retina, and foveal cone density increases fivefold to achieve the configuration found in the mature retina. In newborns, the white matter of the visual pathways is not fully myelinated. Myelin sheaths enlarge rapidly in the first 2 years after birth and then more slowly through the first decade of life. At birth, the neurons of the lateral geniculate body are only 60% of their average adult size. Their volume gradually increases until age 2 years. Refinement of synaptic connections in the striate cortex continues for many years after birth. The density of synapses declines by 40% over several years, attaining final adult levels at approximately age 10 years.
See BCSC Section 2, Fundamentals and Principles of Ophthalmology, for further discussion of ocular development.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.