BMP-induced L-Maf regulates subsequent BMP-independent differentiation of primary lens fibre cells.

Bone morphogenetic protein (BMP) signals are essential for lens development. However, the temporal requirement of BMP activity during early events of lens development has remained elusive. To investigate this question, we have used gain- and loss-of-function analyses in chick explant and intact embryo assays. Here, we show that BMP activity is both required and sufficient to induce L-Maf expression, whereas the onset of δ-crystallin and initial elongation of primary lens fibre cells are BMP-independent. Moreover, before lens placode formation and L-Maf onset, but not after, prospective lens placodal cells can switch to an olfactory placodal fate in response to decreased BMP activity. In addition, L-Maf is sufficient to up-regulate δ-crystallin independent of BMP signals. Taken together, these results show that before L-Maf induction BMP activity is required for lens specification, whereas after L-Maf up-regulation, the early differentiation of primary lens fibre cells occurs independent of BMP signals.