Model of iLN development and stromal cell maturation from E15 to E17 highlighting LTα-independent and -dependent stages. E15 iLNs contain mostly single-positive V⁺ and I⁺ cells and very few V⁺I⁺ mature organizer cells. The CD45⁺ cells present at this stage are in their majority CD4⁻. Between E16 and E17, a marked increase in both LTic's and CD45⁺4⁻ cells takes place that correlates with a massive appearance of V⁺I⁺ cells. Cell-cell interactions between LTic's and stromal cells induce the maturation of the latter to V⁺I⁺ cells. LTα₁β₂/LTβR signals seem to be required from E15 onwards for the substantial emergence of V⁺I⁺ mature stromal cells and the recruitment of large numbers of LTic's and CD45⁺4⁻ cells seen at E17.