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. 2020 Jun 9;14:29. doi: 10.3389/fnsys.2020.00029

FIGURE 1.

FIGURE 1

Timeline depicting major developmental milestones in the ferret visual cortex. The timeline of different developmental events in the ferret visual system, subdivided into five parts: (a) cortical neurogenesis and layer formation in area 17. The timing, progression and duration of neurogenesis and laminar differentiation are represented by horizontal bars, where VN and LD denote ventricular neurogenesis and laminar differentiation, respectively. Embryonic day 22 (E22) marks the earliest occurrence of neurogenesis in layer VI followed by other cortical layers over the next few weeks. In cortical layers II/III neurogenesis is not completed until postnatal day 14 (P14) and cortical folding is not complete until about postnatal day 30 (P30); (b) Formation and refinement of thalamocortical projections. LGN cells are born by E21 and axons first arrive in the visual cortex by E27. Ingrowth into layer IV occurs around P10 with the developmental projections of retino-geniculate fibers appearing as early as E27 and lasting until P14. Cortical patches are visible as early as P16. Formation of ocular dominance columns takes place around P37 and CP begins approximately around P35 (∼75 days after conception); (c) Formation of corticothalamic projections. Early projections from the cortex to the thalamus are formed by neurons in cortical layer V soon after birth, and subsequently followed by neurons in layer VI a week later; (d) Refinement of intracortical circuitry. At the onset of visual responsiveness in the cortex, one third of neurons show orientation selective responses, and the mature adult like pattern is not attained until around P45. Orientation map is formed by around P31 and clustered horizontal connections appear as early as around P27; (e) Cortical maturation. Developmental timecourse for several critical events such as neuronal migration, gliogenesis, synaptogenesis, oligodendrocyte formation and myelination in corticogenesis. The refinement of feedforward projections from area 17 to extrastriate cortex, and feedback projections from extrastriate areas to area 17 begins around P35, which is soon after eye opening (P30). Adapted from “Development and plasticity of cortical areas and networks” by Sur and Leamey (2001), Nature Reviews Neuroscience, 2(4), page 252. Copyright 2001 by Springer Nature.