ORGANIZATION AND DEVELOPMENT OF THE CINGULUM - LAMINAR ARRANGEMENT OF ACETYLCHOLINESTERASE-RICH COMPONENTS IN RAT

The cingulum of the rat is seen to be composed of several distinct fiber and neuronal somal components. In histochemical preparations, three acetylcholinesterase (AChE)-rich fiber zones are noted. The most dorsal originates in the cholinergic cell groups of the basal forebrain, (BF). There is a second, poorly reactive region subjacent to the BF afferent zone. The third, and most ventral zone, has not been described previously. It is operationally dubbed the entocingular fasciculus, ECF. It consists of a moderately dense plexus of AChE-reactive axons and dendrites. In contrast to the regularity and coherence of BF afferents, ECF fibers run in all directions in relation to adjacent cortical (cingulate, sensory-motor), subcortical and transcallosal sites. Knife cut lesions indicate the presence of a substantial proportion of fibers that travel either rostrally or caudally within the ECF itself. The small number of neuronal somata within the cingulum are mostly localized to the ECF. Ultrastructurally, these cells appear to synthesize AChE and form a single type of axo-somatic synapse; synapses are not common in the ECF. Altogether, these observations suggest that the ECF is a conduit for AChE-rich fibers directed towards targets outside of this structure. The AChE reactivity within the ECF first becomes apparent in the second postnatal week, lagging somewhat behind the enzymological maturation of BF afferents. It is concluded that in addition to BF afferents, other AChE-rich fibers travel in the cingulum. A distinctive population of fibers appears to bridge between various cerebral sites via the ECF. These latter observations raise the speculative possibility of a role for the ECF in limbic and sensorimotor cortical interactions.