By establishing an avascular, highly elastic, region within the fetal
area of high acuity (AHA), the developing primate eye has created a unique
substrate on which the mechanical forces of intraocular pressure (IOP) and
growth-induced retinal stretch (stretch) can act. We proposed (Springer & Hendrickson, 2004b) that these
forces generate both the pit and high cone density found in the adult AHA.
In this paper, we use quantitative measures to determine the temporal
relationships between nasal and temporal retinal elongation, changes in
pit depth, cone packing, and cone morphology over M. nemestrina
retinal development. Retinal length increased rapidly to about 105 days
postconception (dpc; Phase 1) and then elongation virtually ceased (Phase
2) until just after birth (180 dpc). Retinal elongation due to stretch
resumed during Phase 3 until approximately 315 dpc (4–5 months),
after which time the retina appeared mature (Phase 4). The pit appeared
during the quiescent Phase 2, suggesting that IOP acts, in conjunction
with molecular changes in the inner retina, on the highly elastic,
avascular, AHA to generate a deep, narrow pit and causes inner retinal
cellular displacements. Subsequently (Phase 3), the pit widened, became
50% shallower and central inner retinal lamina thinned slightly due to a
small amount of retinal stretch occurring in the AHA. Centripetal movement
of cones was minimal until just after birth when the pit reached 88% of
its maximal depth. Accelerated cone packing during Phase 3 was temporally
correlated with increased stretch. A slight stretching of the central
inner retina generates “lift” forces that cause the pit to
become shallower and wider. In turn, these “lift” forces draw
cones toward the center of the AHA (Springer,
1999). Localized changes in cone morphology associated with
packing, included smaller cell body size, a change from a monolayer to a
multilayered mound of cell bodies, elongation of inner segments and
tilting of the apical portion toward the AHA. These changes began in cones
overlying the edges of the pit, not its center. Henle cone axons formed
initially in association with centrifugal displacement of the inner retina
during pit formation, with an additional subsequent elongation due to
cones moving centripetally. An integrated, two-factor model of AHA
formation is presented. Initially, during the second half of gestation
(Phase 2), IOP acts on the hyperelastic avascular zone of the AHA to
generate a deep pit in the inner retina. In the first 4 months after birth
(Phase 3), central retinal stretch generates tensile “lift”
forces that remodel the pit and pack cones by drawing them toward the AHA
center.