Lectin staining in the basal nucleus (Meynert) and the hypothalamic tuberomamillary nucleus of the developing human prosencephalon.

Previous studies have demonstrated that extracellular matrix glycoconjugates, shown by lectin-histochemistry with Vicia villosa agglutinin (VVA) and peanut agglutinin (PNA) as so-called perineuronal nets, play an important role in brain maturation. Concanavalin A (ConA) binding to neuronal surface glycoconjugates may be a marker of synaptic junctions. The present study was done to demonstrate the binding sites of these lectins in two functionally related nuclei of the prosencephalon, the basal nucleus (Meynert) and the hypothalamic tuberomamillary nucleus. Fetal brains of 16-36 weeks of gestation were examined by using VVA, PNA, and ConA to determine appearance and distribution patterns of specific lectin-binding sites on glycoconjugates during fetal brain development. The basal nucleus and the tuberomamillary nucleus showed a characteristic "cellular staining" that may have been due to cytoplasmatic labeling, surface labeling, or both. Lectin-staining occurred much earlier in the basal nucleus than in the tuberomamillary nucleus. Although all three lectins were bound to neurons of the basal nucleus, only ConA-positive neurons were observed in the tuberomamillary nucleus. In conclusion, lectin-labeled cells most probably represent projection neurons that are GABAergic (tuberomamillary nucleus) or cholinergic (basal nucleus). Labeling with the three lectins demonstrated nuclear-specific staining patterns that occur early in fetal development and gradually increase. Binding sites for lectins characterizing perineuronal nets (VVA, PNA) occurred only in the basal nucleus, whereas binding sites for ConA on neuronal-surface glycoconjugates, which seem to play a role in early synaptogenesis, were present in the basal and the tuberomamillary nucleus. The basal nucleus, however, expressed ConA binding sites distinctly earlier, probably indicating early arriving afferents.

Development of the time coding pathways in the auditory brainstem of the barn owl.

The barn owl's head grows after hatching, causing interaural distances to more than double in the first 3 weeks posthatch. These changes expose the bird to a constantly increasing range of interaural time cues. We have used Golgi and ultrastructural techniques to analyze the development of the connections and cell types of the nucleus magnocellularis (NM) and the nucleus laminaris (NL) with reference to the growth of the head. The time coding circuit is formed but immature at the time of hatching. In the month posthatch, the auditory nerve projection to the NM matures, and appears adult-like by posthatch day (P)21. NM neurons show a late growth of permanent dendrites starting at P6. Over the first month, these dendrites change in length and number, depending upon rostrocaudal position, to establish the adult pattern in which high best frequency neurons have few or no dendrites. These changes are not complete by P21, when NM neurons still have more dendrites than in the adult owl. The neurons of NL have many short dendrites before hatching. Their number is greatly reduced by P6, and then does not change during later development. Like NM neurons, NL neurons and dendrites grow in the first month posthatch, and at P21, NL dendrites are longer than those in the adult owl. Thus, the auditory brainstem circuits grow in the first month after hatching, but are not yet mature at the time the head reaches its adult size.

The development of enkephalin and substance P neurons in the basal ganglia: insights into neostriatal compartments and the extended amygdala.

To study the comparative development of the two major neuropeptide genes of the striatum, we used immunocytochemistry to detect immunoreactivity (ir) for substance P and synenkephalin (the N terminus of proenkephalin), and in situ hybridization to detect proenkephalin mRNA. Earliest detection of substance P-ir was in the anlage of the bed nucleus of the stria terminalis (BST, at E15) and in the rostral-lateral caudate-putamen (CPu), at E16. Substance P in the BST was immediately subjacent to the medial ganglionic eminence, while immunoreactivity in the CPu was associated with the lateral ganglionic eminence. Earliest detection of synenkephalin-ir or proenkephalin mRNA was in the caudal-lateral CPu and the adjacent central nucleus of the amygdala (Ce), at E16. Over the next several days, expression of each neuropeptide spread toward the region of first expression of the other neuropeptide. The first overlap of expression of the two neuropeptides was at E18, at the level of the septum. Despite correspondence of substance P-ir and proenkephalin mRNA in patches at P0, very little co-expression of the two neuropeptides was evident in individual neurons. We propose a model in which the CPu develops primarily from the lateral ganglionic eminence, and the extended amygdala develops primarily from the medial ganglionic eminence. Within each structure, two poles of neuropeptide gene expression are established initially: substance P-ir in the rostral CPu and in the rostral-medial pole of the extended amygdala (represented by the BST), and synenkephalin/proenkephalin in the caudal CPu and in the caudal-lateral pole of the extended amygdala (represented by the Ce). A stream of substance P-ir cells connects the two poles of the extended amygdala, in the sublenticular substantia innominata.

Nucleus basalis magnocellularis and hippocampus are the major sites of FMR-1 expression in the human fetal brain.

The expression of the FMR-1 gene, which is implicated in fragile-X syndrome was investigated in human fetuses by in situ hybridization. In 8 and 9 week-old fetuses, FMR-1 mRNAs are expressed in proliferating and migrating cells of the nervous system, in the retina, and in several non-nervous tissues. In the brain of 25 week-old fetuses, FMR-1 mRNAs are produced in all nearly differenciated structures, with the highest level in cholinergic neurons of the nucleus basalis magnocellularis and in pyramidal neurons of hippocampus. The early transcription of FMR-1 gene and the distribution of FMR-1 mRNAs in human fetuses suggest that alterations of FMR-1 gene expression may contribute to the pathogenesis of fragile-X syndrome and especially the mental retardation.

Early development of the nucleus basalis-cortical projection but late expression of its cholinergic function.

The aim of this study was to examine the development of the basalocortical pathway by using choline acetyltransferase and nerve growth factor receptor immunocytochemistry, acetylcholinesterase histochemistry and retrograde axonal transport. The observations were made in the ferret because in this species brain development occurs over a much more protracted period than in the rat. Staining for choline acetyltransferase immunoreactivity in the brain was minimal before birth. Adult levels of staining for the enzyme were not seen in cell bodies until three weeks after birth and in axons up to six weeks after birth. This, however, did not mean that presumptive cholinergic pathways are absent early in development. There was strong staining for nerve growth factor receptor in basal forebrain neurons from at least two weeks before birth. Positive staining for acetylcholinesterase was found in axons that begin to invade the cerebral cortex a week before birth. The retrograde axonal transport technique showed that the basalocortical pathway has a normal organization in the neonate. The conclusion is that cholinergic pathways form early in the prenatal period in the ferret but express their transmitter function late in postnatal development.

Histogenesis of the striopallidal system in the rat. Neurogenesis of its neurons.

This study discusses the histogenesis of the structures of the extrapyramidal system. The first part based on [3H]thymidine autoradiography provides data on the time of origin of the various neuronal populations that characterize different structures of the extrapyramidal system. Such data are essential to any further study dealing with the localization of the different sites of origin of the neurons along the ependymal matrix and with their migration during the histogenetic sequences leading to their definitive pattern of adult distribution. The neurons of globus pallidus and of the entopeduncular nucleus are generated on days 12-15 and days 11-14, respectively. A peak of neurogenesis occurs on day 12 for the entopeduncular nucleus and on day 14 for the globus pallidus. In the pallidum, the first neurons to form on day 12 settle caudally, while neurons generated on day 15 settle in the rostral extremity. The genesis of the medium-sized neurons of the neostriatum extends from day 12 to at least postnatal day 2. A peak is obvious on day 15. Although the neurogenesis of these neurons follows a mild caudorostral gradient, a more careful examination reveals four different patterns of settlement according to the area involved and the period of gestation. At the level of the caudal neostriatum, the neurons display a clear mediolateral spatiotemporal gradient. More rostrally, the neurons generated on days 13, 14 and 15 show two patterns of settlement. On the one hand, many neurons settle rather densely along the external capsule on day 13, occupying more rostral levels on days 14 and 15. On the other hand, in the body of the neostriatum, clusters of isochronically generated neurons are obvious. Later, newly generated neurons display a rather homogeneous distribution in the structure. A parallel is drawn between these patterns of development and the patterns of distribution of afferent terminals or neurotransmitters. The large chromophilic neurons of the neostriatum appear exclusively during the early period. Two peaks of neurogenesis are apparent. The one on day 13 comprises neurons that settle caudally. It is contemporaneous to the neurogenesis of the adjacent basal nucleus. The second peak occurs on day 15 and corresponds to that of the medium-sized neurons.

Early cytoarchitectonic development of the anlage of the basal nucleus of Meynert in the human fetus.

The present study describes a short period of cytoarchitectonic development of the anlage of the basal nucleus of Meynert between 9 and 15 weeks of gestation (crown-rump length 42-120 mm). On the basis of temporo-spatial reconstruction of the cytoarchitectonic pattern within the basal telencephalon it was evident that magnocellular aggregations of the basal telencephalon contain the most differentiated cells in the whole prosencephalon of the 15-week-old human fetus. At this stage development many magnocellular islands can be observed in the sublenticular region. However, it seems that they are in antero-posterioral continuation and the real number of magnocellular islands is much smaller than observed in a single section. The most voluminous magnocellular aggregations are situated around the temporal limb of the anterior commissure and below the ventral pallidal surface in the 15th week of gestation. Between 12 and 15 weeks of gestation, at the most rostral levels, the distinct cell group appeared with unique cytoarchitectonic properties ('albino cell group'). This cell group is situated close to the ventral putaminal surface within the capsula externa fibres and it corresponds to the subputaminal nucleus of Ayala.

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.