Feather pattern stability and reorganization in cultured skin.

1. The formation of the feather pattern has been studied in skin explants obtained from the lumbar region of the spinal pteryla of 6\to 7^-day chick embryos. Explants were cultured in vivo on the chorioallantoic membrane (CAM) or in vitro, either on semi-solid natural media (containing whole chick embryo extract (J£) or chick brain extract (EC)), or in a liquid synthetic medium (199). 2. Feather pattern development was dependent on the culture method and also on the way the explants were excised and treated before cultivation. When the mid-dorsal initial row of rudiments was preserved at explantation, the feather pattern was stable on all media and initial feather rudiments progressively gave rise to feather buds. When the mid-dorsal initial row of rudiments was damaged at explantation, explants of younger stages (A, B or C) cultured on media JE or EC underwent a complete reorganization of their initial feather pattern; in other explants (of older stage D, or those of any stage cultured in medium 199 or on the CAM) the initial feather pattern was stable. When the initial rudiments were destroyed by dermo-epidermal dissociation and recombination, feather pattern reorganization was observed in explants of any stage cultured //; vitro (on medium EC or in medium 199) (Table 1, p. 625). 3. Feather pattern reorganization was characterized by the following events: during the first 24-30 h of culture the initial feather rudiments gradually vanished; dermal condensations and epidermal placodes disappeared; after which a new 'primary' row differentiated parallel to the longitudinal edges of the explant's dermis, its distance from the medial dermal edge was larger when the explant was obtained at a more advanced stage. The newly formed feathers were arranged in longitudinal rows parallel to the cephalo-caudal axis of the dermis, which was thus solely responsible for the polarity of the reorganized feather pattern. 4. The capacity of the feather pattern to reorganize shows that it is still labile at the time of explantation and that the site of subsequent feathers is not yet determined in the unpatterned dermis. The pattern is determined progressively from prospective row to prospective row starting from the primary row. In normal development in situ, the role of the primary row is supposedly played by the mid-dorsal row in the lumbar region of the spinal pteryla. Just prior to the formation of feather condensations the dermal cells appear to be the transitory seat of a passing peak of morphogenetic activity moving from the mid-dorsal line to the lateral edges of the spinal tract.