Morphometric effects of preweaning lead exposure on the hippocampal formation of adult rats.

We previously observed that lead exposure beginning at parturition reduced or retarded neuropil development and synaptogenesis in specific regions of the hippocampal formation in 15-day-old rats. To determine if morphologic effects of perinatal lead exposure persist into adulthood, the hippocampus of rats exposed to lead from parturition to weaning via the milk of dams drinking 0.2% lead acetate was analyzed by light and electron microscopy at 90-97 days of age. Preweaning lead exposure did not cause obvious hippocampal cytotoxicity, edema, damaged vasculature or altered numerical density (number per unit area) of dentate granule or hippocampal pyramidal neurons. However, lead exposure increased the area of the hilar portion of stratum pyramidale, increased the number of blood vessels per section in the dentate stratum granulosum and tended to increase the number of glial cells per section in the non-hilar CA3 stratum pyramidale. Effects of early postnatal lead exposure on synaptic profiles in the suprapyramidal mossy fiber zone (MFZ) were limited to the proximal (close to the dentate gyrus) region of the extrahilar zone. Both numerical density and areal density (proportion of neuropil covered) of complex invaginated (Ci) profiles, typical of mature mossy fiber boutons, were increased by lead exposure in the deep (close to stratum pyramidale) subfield of the proximal portion of the suprapyramidal MFZ. Lead exposure tended to increase the mean cross-sectional area of Cl profiles in the superficial (distant from stratum pyramidale) subfield of the proximal portion, and reduced the cross-sectional area of complex noninvaginated (CN) synaptic profiles in some subfields. Although lead exposure reduced or delayed neuropil development in selected late developing hippocampal regions examined at 15 days of age, it appeared to allow normal growth or to induce compensatory hypertrophy in these same regions in the adult.