Cytoarchitectonic Cortical Fields in the Primate: Effects of Bilateral Enucleation in the Fetal Monkey on the Boundaries, Dimensions, and Gyrification of Striate and Extrastriate Cortex

Bilateral enucleation was performed at different fetal ages during corticogenesis, and the brains were prepared for histological examination. Early-enucleated fetuses (operated prior to embryonic day 77) showed morphological changes at the level of the thalamus and the cortex. In the thalamus, there was a loss of lamination and a decrease in size of the lateral geniculate nucleus. There was a decrease in the size of the inferior pulvinar, but there was no change in the lateral pulvinar. The border of striate cortex was as sharp in the enucleates as it was in the normal monkeys. In three of the four early enucleates, we observed an interdigitation of striate and extrastriate cortex. In three of the early enucleates, we observed a small island of nonst,riate cortex near the striate border that was surrounded entirely by striate cortex. Enucleation led to an age-related reduction of striate cortex. This reduction was greater in the operculum than in the calcarine fissure. The reduction of striate cortex was accompanied by an increase in the dimensions of extrastriate visual cortex, so that the overall dimensions of the neocortex remained invariant. The extrastriate cortex in the enucleated animals presented a uniform cytoarchitecture and was indistinguishable from area 18 in the normal animal. There were changes in the gyral pattern that were restricted mainly to the cortex on the operculum. A deepening of minor dimples as well as the induction of a variable number of supplementary sulci led to an increase in the convolution of the occipital lobe. These results are discussed with respect to the specification of cortical areas. They demonstrate that the reduction in striate cortex was not accompanied by an equivalent reduction in the neocortex; rather, there was a border shift, and a large volume of cortex that was destined to become striate cortex appears to be cytoarchitectonically normal extrastriate cortex. ,i 19% Wiley-Liss. Inc. Indexing terms: development, area 17, monkey, plasticity, vision Surgical manipulation of the sensory periphery has been shown to exert an important influence on the development of the cerebral cortex via the thalamic relay nuclei (Van der Laos and Woolsey, 1973; =]lackey et al., 1976; for reviews, see O’Leary, 1989; Killackey, 1990). A major challenge is to decipher how this extrinsic control meshes with developmenKennedy and Dehay, 1993). In the monkey, fetal removal of the retinae leads to a drastic reduction in the surface area of the striate cortex (Rakic, 1988; Dehay et al., 1989). There are a number of possible sources of areal reduction of striate cortex followAccepted Auflst 3, 1995, programs which are intrinsic to the cortex (Rakic, 1988; Address reprint requests to Henry Kennedy, INSERM U 371,18 Av Doyen Lepine, 69675 Bron Cedex, France. Q 1996 U’ILEY-LISS, INC. INDUCED RESPECIFICATION OF PRIMATE VISUAL CORTEX 71 TABLE 1. Dimensions and Relative Proportions of Lateral Geniculate and Pulvinar Nuclei Case enucleation observation LG" pulvinar' pulvinar' LGN (%)2 pulvinar (%I2 pulvinar ('%I2 Age at Age at Inferior Lateral Inferior Lateral E59 PO 10.7 14.3 56.3 27.9 34.0 67.0 BB46 BB95 BB104 E62 El37 5.7 6.8 32.7 23.2 26 5 63.4 E68 Pn 7.6 12.5 5.5.7 23.9 34 n 69.6 BB28 E77 PO 9.7 9.6 26.6 38 9 38.6 63.6 BB2 1 BB49 El09 PO 1Y.5 18.6 49.6 43.2 42 0 65 9 BB68 Normal Three months 34.2 16 2 70.4 53.9 35.6 70.6 E62 PO 11 4 15.3 58.7 28.2 34.5 66.9 BB3S E8 1 PO 12.6 14.3 67.1 34.6 37.6 73.8 BB34 Normal El33 29.6 8.4 36.8 79.7 52.7 83.0 Values in mm3. LGN: lateral geniculate nucleus; E: embryonic day; P: pnstnatal day 'Proportmns are expressed as percentages of glohus pallidus. ing enucleation. One is that the areal reduction reflects changes in cell size, because enucleation in rodents (Valverde, 1968) and in monkeys (Dehay, Horsburgh and Kennedy, unpublished observations) has been shown to lead to cortical cell atrophy. A second possibility is that enucleation leads to increased levels of naturally occurring cortical cell death in the developing striate cortex (Finlay and Slattery, 1983; Windrem and Finlay, 1985). A third possibility is that enucleation reduces cell production. This is supported by a large number of studies showing that af€erents modulate cell proliferation (Hamburger and LeviMontalcini, 1949; Kollros, 1953, 1982; Delong and Sidman, 1962; Currie and Cowan, 1974; Baptista et al., 1990; Selleck et al., 1992; Gong and Shipley, 1995; Dehay et al., 1995). A fourth possibility, which has received much attention over the last few years, is that enucleation leads to a change in areal specification of immature cortical neurons (Rakic, 1988). This amounts to a respecification of a cortical area; therefore, it is of considerable theoretical interest. Rakic's hypothesis postulates that the reduction in surface area of striate cortex is a secondary consequence of the reduction in numbers of ascending fibers from the lateral geniculate nucleus (LGN; Rakic, 1988). Accordingly, the portion of prospective striate cortex that is deprived of its normal input from the LGN, acquires a cytoarchitectonic identity that is distinct from that of striate cortex. This amounts to a developmental change in identity following a respecification. In support of this, Rahc and his collaborators have provided evidence that, in two fetuses enucleated on embryonic day 81 (E81) and E91 (gestation period, 165 days), enucleation appeared to induce regions within and bordering the striate field to acquire a cytoarchitecture combining striate and extrastriate features. This region was referred to as area X (Rakic et al., 1991). However, it has been shown that the reduction of striate cortex following enucleation depends critically upon the age at which the surgical manipulation is carried out (Dehay et al., 1991). Enucleation at E80 and later leads to a reduction in surface area of striate cortex of the order of 10-14%, which is quite modest compared to the reductions of 70% following enucleations prior to E77 (Dehay et al., 1991). However, in these early enucleates, we did not detect regions of area X showing hybrid cytoarchitectonic features that corresponded to the reduction of striate cortex. This would suggest either that respecification does not occur to any great extent or that the expanded extrastriate cortex is indistinguishable from normal area 18. The aim of the present study was to determine, by using an exhaustive series of tests, whether enucleation does cause a respecification of cortex and to what extent it accounts for the areal reduction of striate cortex. This involved determining whether enucleation causes substantial amounts of cortex destined to become striate cortex to take on the cytoarchitectonic features of extrastriate cortex. If this is the underlying mechanism, then the induced reduction of striate cortex should be accompanied by an expansion of extrastriate cortex. This amounts to a shift in the border between striate and extrastriate cortex. To determine whether this is the case, we made quantitative measurements of the areal dimensions of different cortical regions, including all of the neocortex, the extrastriate visual cortex, and the striate cortex following enucleation at different fetal ages. A striking feature of the primate visual cortex following early enucleation is the drastic change in cortical folding (Rakic, 1988; Dehay et al., 1989). The developmental determinants of cortical folding remain largely unknown (Welker, 1990). One possibility is that cortical folding permits the cortex to be confined in a restricted volume. Correlating the degree of cortical folding with changes in cortical volume should provide an insight into this interpretation. Another possibility is that cortical folding could serve to minimize distances between densely interconnected areas. Hence, one of the aims of the present study was to provide a description of cortical folding following enucleation and to examine the relationship between induced gyri and the dimensions and locations of cortical fields. MATERIALS AND METHODS Medication prior to surgery consisted of two injections of chlorpromazine (Largactyl; 2 mg/Kg, i.m.1 4 hours before anaesthesia and at the moment of anaesthesia. Before anaesthesia, animals were injected with atropine (1.25 mg, i.m.), dexamethasone (Soludecadron; 4 mg, i.m.). Eight timed, pregnant cynomolgus monkeys (Macaca irus) were prepared for surgery under Ketamine hydrochloride (20 mg/Kg, i.m.1 anaesthesia. The monkeys were then intubated, and anaesthesia was continued with halothane in a N,O/Oz mixture (70130). The heart rate was monitored, and the expired C02 was maintained between 5-6%. The body temperature was maintained by using a thermostatically controlled heating blanket. Using sterile surgical procedures, a midline abdominal incision was made, and a uterotomy was performed. The fetal head was exposed, bilateral eye removal was performed, and the fetus was replaced in the uterus after closing incisions. The mother was returned to her cage and was given an analgesic (Visceralgine; 1.25 mg, i.m.) twice daily for 2 days. In two cases (BB104 and BB34), the fetuses were delivered by caesarian section; otherwise, the fetuses were left until term on El65 (Table 1). The infant and fetal