Foveal & Macular Anatomy

The Macula: Boundaries and Dimensions

The macula is bounded by the superior and inferior temporal vascular arcades, a roughly circular zone about 5.5 mm in diameter with the fovea at its center. That boundary is vascular, not just descriptive: clinicians find the macula by the vessels ringing it. Histologically the macula is distinct from peripheral retina in one key respect, it carries more than one layer of ganglion cell nuclei. That multilayering reflects the much higher concentration of ganglion cells needed to serve central, high-acuity vision.

The Parafovea

Immediately surrounding the fovea, the parafovea is a roughly 0.5-mm-wide ring where the retina reaches its greatest thickness anywhere: the ganglion cell layer (GCL), inner nuclear layer (INL), and outer plexiform layer (OPL) are all at their maximum here. The fovea itself has none of these inner layers; they are cleared away entirely so light can reach the photoreceptors with minimal scatter. The parafovea is where those displaced layers pile back up, just outside the foveal pit.

The Foveal Avascular Zone

At the center of the macula lies the foveal avascular zone (FAZ), a capillary-free area typically 250 to 600 micrometers across. It has no retinal capillary network of its own. Oxygen and nutrients instead diffuse up from the underlying choriocapillaris, through the RPE and photoreceptor layer, rather than arriving by way of retinal vessels. The central fovea depends on choroidal rather than retinal blood flow, and that single fact explains the cherry red spot of central retinal artery occlusion: the surrounding retina infarcts once retinal arterial supply is lost, while the fovea, still fed by the choroid, is spared.

Because the FAZ's size and shape track the health of the surrounding perifoveal capillary network, it is checked routinely on fluorescein angiography and OCT angiography. An enlarged or irregular FAZ is a sensitive marker of capillary dropout in diseases such as diabetic retinopathy.

Macular Pigment: Lutein and Zeaxanthin

The macula's yellow color comes from macular pigment: the carotenoids lutein and zeaxanthin, concentrated in the inner retinal layers of the macula, particularly the fibers of Henle and the plexiform layers. The two carotenoids are not evenly distributed. Zeaxanthin concentrates in the cone-dense fovea itself, while lutein predominates in the rod-dense retina surrounding it. At the very center of the macula, the lutein-to-zeaxanthin ratio runs about 1:2.4 (sometimes simplified to roughly 1:2), shifting toward lutein dominance farther out toward the periphery.

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