Developmental Brain Research, 46 (1989) 303-308 303 Elsevier BRD 60305 Callosal projection neurons in area 17 of the fetal rhesus monkey Leo M. Chalupa 1, Herbert P. Killackey2, Cara J. Snider I and Barry Lia 1 1Department of Psychology, The Physiology and Neurobiology Graduate Groups, and The California Primate Research Center, University of California, Davis, CA 95616 (U.S.A.) and 2Departmentsof Psychobiology, Anatomy and Neurobiology, University of California, Irvine, CA 92717 (U.S.A.) (Accepted 20 December 1988) Key words: Visual cortex; Prenatal development; Cailosal projection We have studied the distribution of callosal projection neurons in area 17 of a fetal rhesus monkey which received large injections of horseradish peroxidase into the contralateral occipital cortex. In comparison to other cortical areas, area 17 contains few callosal projection neurons. Most of these cells are confined to a region extending tangentially about 2.5 mm from the 17/18 border, although a few neurons were noted as much as 5 mm from the border. Comparing the distribution of callosal projection neurons in the fetal monkey with what has been described in newborn and adult macaques, it is apparent that although some degree of refinement in striate caliosal connections may occur during in utero development, the prenatal development of callosal connections in the macaque is inherently adult-like. It has been demonstrated in a number of species and in a variety of cortical areas that the distribution of callosal projection neurons is relatively uniform early in development 5'9A1"13,16. With maturation a restricted pattern in the distribution of these cells is attained, the degree of restriction varying markedly within and across cortical fields. This restriction of callosal projection cells involves the elimination of callosal collaterals by cortical neurons which con- tinue to maintain ipsilateral cortical projections 2,8,1°. It has been commonly assumed that this develop- mental sequence reflects a general principle of callosal development. However, a notable exception was discovered recently by Dehay et al. 4. These investigators reported that area 17 of the macaque monkey 'is devoid of interhemispheric connections throughout prenatal development'. We have been studying the prenatal development of callosal connections in the somatosensory cortex of rhesus monkeys 2,13, where as late as embryonic day (E) 119 callosal projection neurons are contin- uously distributed throughout the postcentral gyrus. In this gyrus a mature pattern in the distribution of these cells becomes evident by E133, about a month before birth. Given our observations on the soma- tosensory cortex of the fetal rhesus monkey and the available literature on the early development of callosal connections of the visual cortex in other species, the findings of Dehay et al. 4 were unex- pected. It seemed worthwhile to re-examine the status of interhemispheric connections of area 17 in the fetal rhesus monkey using methods identical to those we employed previously to study the prenatal development of callosal projections in the postcen- tral gyrus. We injected horseradish peroxidase (50% HRP in 2% DMSO) into occipital cortex and the border region of areas 17 and 18 of a fetal rhesus monkey (Macaca mulatta) at 114 (+2) days of gestation. Detailed descriptions of the breeding protocol and the determination of gestational age, as well as surgical procedures, are provided in an earlier publication 13. After anesthetic induction with keta- mine hydrobromide (10 mg/kg), all surgery was conducted under halothane anesthesia, supple- mented with nitrous oxide. Following craniotomy, multiple injections (of about 0.5/A each) were made with a Hamilton syringe into occipital cortex poste- Correspondence: L.M. Chalupa, Department of Psychology, University of California, Davis, CA 95616, U.S.A. 0165-3806/89/$03.50 © 1989 Elsevier Science Publishers B.V. (Biomedical Division)