Relatively little is known about the structural development of the basal ganglia, and what is known is largely restricted to the embryonic period. Cell migration in the sub-ventricular zone and lateral migrations to and from the diencephalon beginning in the fifth week starts the process of forming the basal ganglia. Two weeks later (i.e., by the end of the embryonic period), the basal ganglia are clearly identifiable, and after which development of the cerebellum begins. The corpus striatum (caudate and putamen) develops from neuroblasts situated in the floor of the telelencephalic vesicle (i.e., the striatal ridge). As for the globus pallidus, it originates from neuroblasts in the third wall of the diencephalon, which subsequently migrate laterally to join the corpus striatum. Once the hemispheres have fused, and corticofugal fibers have divided the caudate nucleus from the putamen, the putamen merges with the globus pallidus to form the lentiform nucleus. Thus, much of the basic structural development of the basal ganglia, if not its connectivity, is already achieved during the embryonic period. In late fetal human brains, cells leave the forebrain (diencephalon and telencephalon), cross the internal capsule, and finally end up in the most posterior part of the thalamus (i.e., the pulvinar). Other mammals have no pulvinar (except for the rhesus monkey, which as a small pulvinar), and therefore this migration allowing thalamocortical connections between the basal ganglia and the cerebral cortex does not occur. Thus, it may be the case that the basal ganglia assume functions that are organised differently in most other mammalian species. Despite the basic structure, if not connectivity, of the basal ganglia appearing to be complete already in the embryonic period, their functional development seems to be a protracted process, with most of the evidence for the human being derived from PET scans. Glucose metabolism registered by these scans has been shown to have occasionally high levels in the basal ganglia of the newborn (perhaps state related) compared to the cerebral cortex. A major increase in glucose uptake by the basal ganglia is evident some two to three months later, as it is in the cerebellum (but perhaps slightly later) and the parietal, primary visual and temporal cortices. Interestingly, at this age (or shortly thereafter) a major transformation in behavioral development occurs such that the obligatory-like behavior of the newborn gives way to sensorimotor actions that have a voluntary appearance (e.g., the social smile, first attempts at visually-guided reaching). A pattern of glucose metabolism resembling that of the adult is evident by 8 months, another age at which a major transformation, more germane to cognitive functions, occurs in most areas of the brain, including the basal ganglia.
See Brain (neuro-imaging), Cell migration, Centrifugal/centripetal, Cerebellum (anatomy), Cerebellum and basal ganglia, Diencephalon, Internal capsule, Neuroblasts, Newborn, Parietal cortex, Primary visual cortex (V1), Pulvinar, Rubrospinal tract, Sensorimotor actions, Sub-ventricular zone, Synaptogenesis, Telencephalon, Thalamus