Evidence for the early prenatal development of cortical cholinergic afferents from the nucleus of Meynert in the human foetus.

A combined histochemical and biochemical approach has shown that the cholinergic system in the nucleus of Meynert region of the substantia innominata is well defined both histochemically and neurochemically within the first 3 months of gestation in the human foetus. Thus, at between 12 and 22 weeks of development the most intense acetylcholinesterase (AChE) histochemical reactivity was observed in the neuropil, cell bodies and processes in the nucleus of Meynert. AChE-stained fibres were observed which coursed from the nucleus of Meynert towards the cortical mantle and within the mantle AChE-stained fibres were also present. Micropunch samples from within the nucleus of Meynert contained higher levels of choline acetyltransferase (ChAT) activity than any other area examined including the striatum, while in the cortical mantle the level of ChAT activity was comparable to that found in the adult cerebral cortex. These observations suggest that the cholinergic innervation from the nucleus of Meynert--considered to be the major source of cholinergic afferents in the adult cerebral cortex--may play a key role in the early development of the human neocortex.