Basic aspects of development and maturation of the brain: embryological contributions to neuroendocrinology.

The interpretation of studies on the development of neuroendocrine function presupposes a thorough knowledge of the complementary phenomena of morphogenesis and histogenesis of the brain. A short analysis of the morphogenesis of the diencephalic floor is given. The pituitary Anlage can be identified early in the neural plate stage. The hypophysis cerebri appears to be a key structure in the morphogenesis of both the head and the brain. The spatiotemporal pattern of histogenesis within the brain can be analysed by a study of the proliferative activity of the neuroepithelial matrix cell layer; a heterochrony of matrix (ventricular) layer mitotic activity and of mantle (intermediate) layer differentiation can be demonstrated. The process of neuron differentiation shows an articulate sequence of phenomena, among them migration, axon growth, dendrite growth, synapse formation and myelination. Dendritogenesis and the development of synapses in a particular area are strongly influenced by the ingrowth of axon nerve terminals from elsewhere. A number of structures observed in the developing central nervous system are only temporary phenomena that go into regression during subsequent stages; dendrites and synapses especially show a high degree of plasticity. Cell death occurs as a normal concomitant of development. Monoaminergic neuron systems originate early and show a positive histofluorescence shortly afterwards; their target areas are retarded as far as differentiation is concerned. The development of these target regions probably is influenced by the monoaminergic cells. The monoaminergic neurons are not subject to feedback regulation for some time, because of the typical late development of the dendritic receptive apparatus in these cells. Steroid receptors may play a role in the development of intersexual dimorphism of the brain. Probably a modulation of neurotransmitter synthesis is the intermediate between steroid receptor stimulation and a change in synaptogenesis in the target area of the neuron. Neuropeptide systems appear to possess a distribution beyond the limits of the hypophysiotropic area. The early presence of some of the neuropeptides within the embryonic brain suggests a role in histogenesis that is different from the usually presumed neurotransmitter or neuromodulator function of the neuropeptides.