In many regions of the developing CNS, distinct cell types are born at different times. The means by which discrete and stereotyped temporal switches in cellular identities are acquired remains poorly understood. To address this, we have examined how visceral motor neurons (VMNs) and serotonergic neurons, two neuronal subtypes, are sequentially generated from a common progenitor pool in the vertebrate hindbrain. We found that the forkhead transcription factor Foxa2, acting in progenitors, is essential for the transition from VMN to serotonergic neurogenesis. Loss-of-function and gain-of-function experiments indicated that Foxa2 activates the switch through a temporal cross-repressive interaction with paired-like homeobox 2b (Phox2b), the VMN progenitor determinant. This mechanism bears a marked resemblance to the cross-repression between neighboring domains of transcription factors that establish discrete progenitor identities along the spatial axes. Moreover, the subsequent differentiation of central serotonergic neurons required both the suppression of VMN neurogenesis and the induction of downstream intrinsic determinants of serotonergic identity by Foxa2.

The chick dermis is known to control the formation of feathers and interfeathery skin in a hexagonal pattern. The evidence that the segregation of two types of fibroblasts involves Delta/Notch signalling is based on three facts. Rings of C-Delta-1-expressing fibroblasts precede and delimit the forming feather primordia. C-Delta-1 is uniformly expressed in the dermis of the scaleless mutant, which is almost entirely devoid of feathers. Feather development is inhibited by overexpression of C-Delta-1 in wild type dermis using a retroviral construct. We also show that the distribution of C-Delta-1 in the mutant dermis can be rescued by its association with a wild type epidermis, which acts as a permissive inducer, or by epidermal secreted proteins like FGF2.

Most of the chick body is covered with feathers, while the tarsometatarsus and the dorsal face of the digits form oblong overlapping scales (scuta) and the plantar face rounded nonoverlapping scales (reticula). Feathers and scuta are made of β-keratins, while the epidermis of reticula and inter-appendage or apteria (nude regions) express α-keratins. These regional characteristics are determined in skin precursors and require an epidermal FGF-like signal to be expressed. Both the initiation of appendages, their outline and pattern depend on signals from the dermis, while their asymmetry and outgrowth depend on epidermal competence. For example, the plantar dermis of the central foot pad induces reticula in a plantar or feathers in an apteric epidermis, in a hexagonal pattern starting from the medial point. By manipulating Shh levels in the epidermis, the regional appendage type can be changed from scuta or reticula to feather, whereas the inhibition of Wnt7a, together with a downregulation of Shh gives rise to reticula and in extreme cases, apteria. During morphogenesis of plantar skin, the epidermal expression of En-1, acting as a repressor both of Wnt7a and Shh, is linked to the formation of reticula. Finally, in birds, the complex formation of feathers, which can be easily triggered, even in the extra-embryonic somatopleure, may result from a basic genetic program, whereas the simple formation of scales appears secondarily derived, as requiring a partial (scuta) or total (reticula) inhibition of epidermal outgrowth and β-keratin gene expression, an inhibition lost for the scuta in the case of feathered feet breeds.