Nociceptor subtypes are born continuously over DRG development

Sensory neurogenesis in the dorsal root ganglion (DRG) occurs in two waves of differentiation with larger, myelinated proprioceptive and low-threshold mechanoreceptor (LTMR) neurons differentiating before smaller, unmyelinated (C) nociceptive neurons. This temporal difference was established from early birthdating studies based on DRG soma cell size. However, distinctions in birthdates between molecular subtypes of sensory neurons, particularly nociceptors, is unknown. Here, we assess the birthdate of lumbar DRG neurons in mice using a thymidine analog, EdU, to label developing neurons exiting mitosis combined with co-labeling of known sensory neuron markers. We find that different nociceptor subtypes are born on similar timescales, with continuous births between E9.5 to E13.5, and peak births from E10.5 to E11.5. Notably, we find that thinly myelinated Aδ-fiber nociceptors and peptidergic C-fibers are born more broadly between E10.5 and E11.5 than previously thought and that non-peptidergic C-fibers and C-LTMRs are born with a peak birth date of E11.5. Moreover, we find that the percentages of nociceptor subtypes born at a particular timepoint are the same for any given nociceptor cell type marker, indicating that intrinsic or extrinsic influences on cell type diversity are occurring similarly across developmental time. Overall, the patterns of birth still fit within the classical "two wave" description, as touch and proprioceptive fibers are born primarily at E10.5, but suggest that nociceptors have a slightly broader wave of birthdates with different nociceptor subtypes continually differentiating throughout sensory neurogenesis irrespective of myelination.

Time in Neurogenesis: Conservation of the Developmental Formation of the Cerebellar Circuitry

The cerebellum is a conserved structure of vertebrate brains that develops at the most anterior region of the alar rhombencephalon. All vertebrates display a cerebellum, making it one of the most highly conserved structures of the brain. Although it greatly varies at the morphological level, several lines of research point to strong conservation of its internal neural circuitry. To test the conservation of the cerebellar circuit, we compared the developmental history of the neurons comprising this circuit in three amniote species: mouse, chick, and gecko. We specifically researched the developmental time of generation of the main neuronal types of the cerebellar cortex. This developmental trajectory is known for the mammalian cell types but barely understood for sauropsid species. We show that the neurogenesis of the GABAergic lineage proceeds following the same chronological sequence in the three species compared: Purkinje cells are the first ones generated in the cerebellar cortex, followed by Golgi interneurons of the granule cell layer, and lately by the interneurons of the molecular layer. In the cerebellar glutamatergic lineage, we observed the same conservation of neurogenesis throughout amniotes, and the same vastly prolonged neurogenesis of granule cells, extending much further than for any other brain region. Together these data show that the cerebellar circuitry develops following a tightly conserved chronological sequence of neurogenesis, which is responsible for the preservation of the cerebellum and its function. Our data reinforce the developmental perspective of homology, whereby similarities in neurons and circuits are likely due to similarities in developmental sequence.