Each species exhibits particularities regarding gestation. Term parturition in women occurs around 280 days after the onset of their last menstrual period [
41]. In mouse and rat, gestation is shorter lasting between 20 and 22 days [
42]. Regarding the comparison between brain development of rodents and human, one has to consider different aspects, since they do not run proportionally in parallel [
32,
43,
44]. The developmental journey of the human cerebellum commences 30 days after conception and that of the rodent around E8.5 [
34,
35,
36]. Forty to forty-five days after conception in humans, the human cerebellar ventricular zone (VZ) slits into a VZ and a subventricular zone [
45]. Cerebellar maturation can be distinguished by a multifaceted, symbiotic cascade, encompassing gene expressions, electrical network interactions, and environmental factors, which are inherently interlinked with the evolution of the hindbrain and midbrain. For example, instances of cerebellar hypoplasia in humans often coincide with hypoplastic conditions of the pons, attributed at least in part to secondary effects, such as axonal degeneration [
46,
47]. Notably, genes regulating cerebellar development exhibit expression in various brain regions, next to that in other organs such as the skin [
48]. The transcription factors Otx2 and Gbx2, which are expressed anterior and posterior from the so-called isthmic organizer region between the midbrain (mb) and hindbrain (hb), are important for the general development of most of the rostral and caudal regions of the central nervous system [
49]. Originating from the caudal-most primary neural tube vesicles, the rhombencephalon (hindbrain) bifurcates into the metencephalon and myelencephalon. The foundational elements of the cerebellum emanate from the dorsal portions of the metencephalon (Alar plate) and the neural folds, termed rhombic lips (RLs) [
50]. Segmented along the rostral-caudal axis into seven rhombomeres, the cerebellum’s emergence is facilitated by transcription factors and signaling molecules, leading to the formation of specialized epithelium, RL1, through bilateral expansion of the Alar plate [
51] in the presence of Gbx2 and absence of Otx2 and Hoxa2 in rodents [
49,
52,
53]. Wnt family members, fibroblast growth factors (especially Fgf8 and Fgf17) En1-2, Lmx-1 and sonic-hedgehoc (shh) play a major role in defining the isthmic organizer region and the anterior-posterior as well as the dorso-ventral patterning of cerebellar development [
54,
55,
56]. Mutations in any of these genes lead to severe implications for cerebellar development or even death in rodents [
57,
58,
59,
60]. In rodents, around E9, cerebellar histogenesis starts and the cerebellar anlage develops from two symmetric bulges. The bulges grow and give rise to the unitary cerebellar plate comprising the vermis and hemispheres. The cerebellar medial regions expand, an orthogonal rotation happens, and the cerebellar wing-like anlagen transform into a homogeneous cylindric vermis at E15.5 (
Figure 1A). Just above the fourth ventricle the two germinative compartments of the RLs are formed, adjacent to the roof plate and the ventricular zone (VZ), placed in the inner side (
Figure 1). The RLs form the origin for all glutamatergic cerebellar neurons including GCs and DCN neurons, amongst others, whereas all GABAergic neurons as well as glia cells, oligodendrocytes and astrocytes originate from the VZ [
32,
33,
45]. Birthdating studies showed that the projection neurons are produced first, at the onset of cerebellar neurogenesis.
Glutamatergic neuron development
In rodents, the RLs are defined by the expression of the mouse homolog of Drosophila atonal (ATOH1) transcription factor [
61] forming the origin of all glutamatergic cerebellar neurons [
62]. Atoh1 expression begins at E9.5 in mice and from E10.5 to E12.5 progenitors leaving the rostral RLs give rise to the large DCN neurons, migrating to the surface of the cerebellar anlage where they aggregate in the nuclear transitory zone. From there they move inward from the PC plate to form the four DCN on both sides [
63]. Progenitors migrating between E14 and E21 give rise to UBCs [
64] and then GC progenitors exit the upper RLs moving tangentially along the cerebellar surface, by E16 eventually covering the entire cerebellar anlage [
65]. GCs start proliferating in response to shh signalling from PCs [
66]. The GC lineage arises already around E8.75. At least three transverse GC progenitor zones are identified by gene expression and birth-dating. Postmitotic GCs migrate from the external GL (eGL) to the inner GL guided by the Bergmann glia fibers, which are oriented in the same plane. Thereby eGL topography is projected into the nascent inner GL [
65]. After birth the GL is an 8 layered structure with another layer of proliferating granule precursor cells [
65,
67]. The proliferation window of murine GCs progenitors closes only at the end of the second postnatal week, when the last postmitotic GCs from the deepest portion of the eGL migrate inwardly to the nascent adult GL, marking the end of the eGL and ceasing Atoh-1 expression [
61,
68]. Interestingly, recent studies showed that the cell-fate specification among the cerebellar VZ and RL is not absolute. A so-called posterior transitory zone expresses genes to develop bipotent progenitors for cerebellar glutamatergic neurons and the choroid plexus [
69], (
Figure 1A,B).
GABAergic neuron development
The VZ is defined by the pancreas transcription factor 1-a (Ptf1a), giving rise to GABAergic neurons. A Ptf1a - neurogenin 1/2 (Neurog1/2)- early B-cell factor 2 (EBF2) regulatory network is implicated in PC subtype specification [
70]. In rodents, PCs are born between E10 and E13 and undergo terminal mitosis. Dividing VZ precursors emigrate into the cerebellar prospective white matter, via the cerebellar plate and form an array of clusters (E14-E18), which are suggested to aggregate into topographical organization centers (TOCs). These TOCs are not only specific for ingrowing afferent precerebellar MF and CF inputs as well as interneurons, but also for subsets of glia cells and migrating GCs [
71]. As the PC clusters disperse into parasagittal stripes, all components disperse with them, forming the adult cerebellar parasagittal architecture. MFs disconnect and form local connections with GCs within the zone. At least five molecularly distinct PC subgroups have been identified throughout development with distinctive levels of Foxp1 and Foxp2, respectively. Foxp1+/Foxp2+ PCs strongly express RELN receptors and lack Ebf2. RELN controls PC migration [
72,
73,
74,
75]. Probably early-born PCs become ZII+ in the adult, while late-born PCs adopt the ZII− phenotype, which is in line with the high ZII expression in the phylogenetically older vestibulocerebellum. The postnatal development of PCs can be divided into different stages. The first distinction can be made between intrinsic maturation by PCs themselves, and
guided maturation by stimulation of other cell types, such as GCs [
76]. In rodents, intrinsic growing starts with a fast somatic growth from P0-P9 followed by a rapid dendritic growth from P9-P18 [
77,
78] with more processes growing outwards from the soma. During the second postnatal week, the processes become more complex by growing rapidly and increasing the number of branches. This is the start of the dendritic tree, which will be
finished around postnatal week 4 in rodents [
79]. Different PC subtypes display distinct susceptibilities to environmental insults or genetic mutations [
80]. CFs are also involved in the dendritic arborisation during these stages, presumably by stimulating PCs. This may explain why higher-order mammals, including humans, show both a higher level of persistent multiple CF innervation and a higher complexity of the dendritic trees of their PCs [
81].
GABAergic interneurons in the CN are born between E10.5–E11.5, and Golgi cells at approximately E13.5–postnatally (peak around E14–E16) [
33,
82,
83]. Late-born GABAergic interneurons (INs), including stellate and basket cells, derive from secondary precursors in the prospective white matter at later stages (from E13 to P5 with a peak around birth) [
84]. Thus, cerebellar neuronal subtypes depend on when and where they are generated from neural progenitors. This leads to the idea that cerebellar progenitors with their own spatial and temporal identities produce specific neuronal subtypes. Additionally, the cerebellum accommodates astrocytes, glia, and oligodendrocytes, the origins of which remain subjects of ongoing discussions [
85].