Therefore, in the case of rods it seems that they are never generated in the fovea center. While there has been progress investigating the inductive influences on progenitor cells to stimulate rod differentiation [89, 90], the signals that control the exclusive differentiation of cones remain to be established. Therefore, the first two steps of foveal development are intricately linked, although the exact mechanism utilized to establish foveal location and exclude rods remains elusive (Fig. 3E).

Cones, Ganglion Cells, and Initial Pit Formation

The lack of rods in the fovea appears to be an early developmental patterning event; however, other characteristics of the adult fovea such as the accumulation of a high cone photoreceptor density and the formation of the pit appear to be later developmental events. When the foveal region becomes identifiable at fetal week 11, foveal neurons are postmitotic, synapses have been formed, all the retinal layers are present, and foveal cones can be identified as short, fat cuboidal cells lying in a single layer at a density of 11,200 cones/mm2 (Fig. 4A) [2, 26, 83, 85, 91, 92]. Which forces act on the foveal cones to influence them to change from a cuboidal shape to the thin, elongated cone in the mature fovea? The first observable change in the foveal cone is the elongation of the axonal process (Figs. 4 and 5). This alteration of cell shape is necessary for the cones to maintain their synaptic connections as they pack tightly into the fovea to reach the average adult cone density of approximately 200,000 cones/mm2 [3]. The exact mechanism by which the cones begin to change their morphology such that they can pack more tightly in the center of the fovea is unknown; however, it has been proposed that the mechanism for this elongation is the differential expression of the fibroblast growth factor (FGF) family members and their receptors on foveal cones [93, 94]. FGF signaling may mediate the morphological cell shape changes of foveal cones that are associated with increased density in the foveal cone mosaic in the early phase of photoreceptor accumulation (Fig. 5) [92, 95].

In humans at Fetal week 14, the maximum spatial density of foveal ganglion cells is approximately 22,500 cells/mm2, and this density increases to about 31,500 cells/mm2 at Fetal weeks 16–17 [85]. Therefore, at the same time that the cones are changing shape, the ganglion cells begin to accumulate in the incipient foveal region (Fig. 4B, arrow) [28]. The timing of this peak in maximum ganglion cell density corresponds with the appearance of a distinctive dome in the GCL (Fig. 4B,C, arrow) [27, 28, 91]. Once the GCL reaches a critical thickness (approximately 7–9 cells thick), the initial formation of the pit is initiated between fetal weeks 24 and 28 (Fig. 4C,D, arrow) [28]. The beginning of the foveal pit begins as a shallow depression and is observed as a progressive decrease in the depth of the inner retinal layers [2]. By fetal week 28, the fovea contains a clearly defined pit with ganglion cells and inner retinal neurons beginning to move away from the foveal center (Fig. 4D, arrow). The foveal cones are further elongating, their pedicles are being pulled away from the foveal center, and the density of foveal cones increases to 22,268 cells/mm2 [26, 92]. The cells of the inner and outer nuclear layers move away from the foveal center (centrifugal migration), while at the same time foveal cones translocate in the opposite direction (centripetal), becoming more densely packed (18,500 cones/mm2) (Fig. 4E) [26, 92]. It is at this point that the third stage of foveal development is completed. In the next stage, there are additional influences on the

Fig. 5. Drawing of the central foveal cones throughout development. At fetal week 11, the cones are cuboidal and then begin to slowly elongate from fetal weeks 16, 20, and 24. These ages correspond with the increase in cone packing that may be mediated by fibroblast growth factor (FGF) signaling. By fetal week 32, the cones begin to elongate further, such that after birth, they are significantly thinner with longer axons compared with fetal week 11. These cones are changing shape due to proposed mechanical influences that form the deep foveal pit. (Cones were traced from images in Fig. 4 and from images from [91].)

forming foveal region that mediate the further increase in cone cell density and complete pit formation.

Retinal blood vessels are first detected emerging from the optic disk at approximately fetal week 14 and rapidly spread into the superior and inferior quadrants of the retina; however, the vascularization of the macular region is delayed. By fetal week 22, inner retinal vessels have grown out to cover greater than 50% of the retina, but the ring of vessels that will form the avascular zone in the adult has not yet begun to encircle the central retina. This ring of vasculature starts forming between fetal weeks 25 and 28, but the precise timing of the formation of the foveal avascular zone in humans has not been established. Interestingly, the foveal region is never vascularized during normal development, suggesting that there is a repulsive cue acting to repel the vessels [96–98]. There does seem to be a correlation between the formation of the avascular zone and the formation of the fovea; however, this has yet to be tested. A role for the vasculature in the formation of mature foveal pit is discussed in the next section.

Deep Foveal Pit Formation

The next stage of foveal development is the refinement of the pit to exclude all nonphotoreceptor cells and increase cone photoreceptor numbers. The initial increase in cone density follows a shallow slope; however, between fetal week 32 and birth, the slope flattens (Figs. 4E and 6). After birth, there is a marked change in the slope such that the accumulation of cones is accelerated (Fig. 4F). As cells of the inner and outer nuclear layers are displaced, they maintain their initial synaptic relationships, which are critical for the maintenance of acuity in the one–one circuitry of the midget system (Figs. 1C, 4F, and 5).