The human brain at stages 18?20, including the choroid plexuses and the amygdaloid and septal nuclei

Abstract

The development of the human brain during the seventh embryonic week was studied in serial sections of 88 embryos, and graphic reconstructions were prepared. From stages 18 to 20 the cerebral hemispheres expand rapidly and become more and more distinct entities. The longitudinal fissure between them occupies approximately half of their rostrocaudal extent. In stage 20 they have progressed so far in organization that functional aspects (based on synapses in the primordial plexiform layer) are of importance. An advanced differentiation is also present in the amygdaloid body, which has at least four individual nuclei, and in the forebrain septum, which shows the nucleus of the diagonal band and the medial septal nucleus. This has a bearing on recent experimental studies that document the fundamental role of the septal nuclei with regard to behavioural and cognitive functions. Fibre connections between septal nuclei and hippocampus have appeared. A definitive internal capsule, however, is not yet present. The main connections with diencephalon and other parts of the brain are chiefly by fibres to or from the amygdaloid body by way of the lateral forebrain bundle. The olfactory areas are connected with the habenular nuclei by a well developed stria medullaris thalami. Globus pallidus externus, entopeduncular nucleus, and subthalamic nucleus are prominent features in the subthalamus. The main nucleus of the oculomotor nerve shows a dorsolateral and a ventromedial portion. The rhombic lip is mitotically active in all parts of the rhombencephalon, and seems to participate significantly in the formation of the intermediate layer of the cerebellum and of the cochlear nuclei. The sensory nucleus of the trigeminal nerve has appeared. In the cerebellum the cell layer thought to contain the future Purkinje cells develops. The cerebellar plate is organized into external and internal bulges, and is now connected with mid- and hindbrain through fibre bundles. The area thought to be the dentate nucleus and the supposed floccular region are especially rich in fibres. The accessory olivary nucleus appears in stage 19, and accessory nuclei of the abducent and hypoglossal nerves are evident in stage 20. The choroid plexuses of the fourth and lateral ventricles have appeared. In view of their advanced features, the study of embryos of stages 19-21 becomes increasingly relevant to questions of tissue transplantation.

Magnetic Resonance Imaging of Brain Anomalies in Fetal Alcohol Syndrome

“…The brain stem and cerebellum begin to develop earlier (in the sixth gestational week) than the commissural plate. 90 This may explain why we did not notice any severe brain stem or cerebellar anomalies in our patients.…”

Transient Architectonic Features in the Basolateral Amygdala of the Human Fetal Brain

“…Cytoarchitectonic studies in human embryos revealed that the amygdaloid nuclei first become visible in the 8th week of gestation and that the corticomedial nuclei appear earlier than the basolateral [Macchi, 1951;Humphrey, 1968;Müller and O'Rahilly, 1990;Setzer and Ulfig, 1992]. Unfortunately, little is known about the fetal development of the amygdaloid nuclei in man.…”

Studies of the cranial base in 23 patients with cri‐du‐chat syndrome suggest a cranial developmental field involved in the condition

“…The present study demonstrates in cri-du-chat patients malformation of that part of the cranial base which arose in the axial region where the cerebellum and the brainstem develop under normal conditions [Mü ller and O'Rahilly, 1986, 1990a, 1990bKessel, 1993]. The embryonic structure decisive for the development of the neural tube and giving rise to the brainstem, pons, and cerebellum is the notochord [Balling et al, 1996].…”