Development of prestriate visual projections in the monkey and human fetal cerebrum revealed by transient cholinesterase staining.

Cholinesterase (ChE) staining was used to reveal the timing and pattern of development of afferents to the prestriate visual cortex (areas 18, 19, 20, and 21 of Brodmann) in a series of developing human and monkey fetal brains. This investigation was possible because the nucleus pulvinaris of the thalamus, the main source of subcortical projections to the prestriate cortex, displays positive reactivity after thiocholine incubation during the last three quarters of gestation, while neighboring thalamic nuclei that project to the adjacent neocortical areas are unstained. Staining of the pulvinar and its prestriate projections passes through six broad stages. Stage I begins in both species at the end of the first third of gestation. Positively stained fibers originate from the pulvinar and enter but do not extend beyond the hemispheric stalk. During stage II, pulvinar axons gradually invade the intermediate zone of the occipital lobe, and in stage III they reach the level of the subplate zone. In stage IV, which occurs around mid-gestation in both species, cholinesterase-positive fibers accumulate within the subplate zone subjacent to the developing prestriate cortex. During stage V, ChE-positive fibers penetrate the prospective prestriate cortex but do not yet form the alternating columnar pattern characteristic of pulvinar input to this area in the adults. Rather, ChE activity is concentrated in two continuous bands situated within prospective layers III-IV and VI; also a narrow band is visible in upper layer I. In stage V a clear histochemical border forms between prestriate and striate areas with ChE activity in prospective area 17 limited mostly to the superficial strata of layers I and II. This histochemical differentiation precedes the emergence of cytoarchitectonic landmarks. During stage VI, which begins in the last fifth of gestation in both species, the pulvinar become progressively less stainable and its projections can no longer be traced by ChE histochemistry.