Regional influences on development of the single comb primordium

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Abstract
In an effort to resolve some of the developmental controls in single comb morphogenesis, comb primordia from New Hampshire Red and White Leghorn chick embryos were excised and grafted in atypical positions. The grafts were allowed to develop and recognizable structural details were later recorded and analyzed.Four different experimental series were made: (1) defect, i.e., the comb primordium was excised and the embryo developed with deficiency; (2) orthotopic, i.e., donor primordium grafted to frontonasal process of host; (3) heterotopic, i.e., donor primordium grafted to neck region of host; and (4) chorioallantoic, i.e., entire primordium grafted to chorioallantoic membrane of host chick. The promordia ranged in developmental age from stage 9 to stage 29.The results showed that the comb primordium can self‐differentiate. Single comb primordia of the White Leghorn breed are determined for comb by stage 15. The primary source of comb tissue appears to be local in origin. Regional influence of growing structures seems to affect comb form. When grafted in most regions and positions, the developing comb did not grow in length. Neck grafts which included some beak tissue developed elongate and typical comb. Rostral extension of the nasal and interorbital septum around seven days of development is thought to be important in normal elongation. Complex clusters of points developed in neck grafts that did not elongate. The normal distribution of comb components was not seen in such grafts. Form of blade and points appears to be predetermined.The conclusion is made that the capacity to differentiate comb form, e.g., ridge, axes, and regional divisions with or without serrations is fixed within the cell population that will form comb by stage 15; distribution of the structural components and typical elongation can be modified through tissue interactions in later stages.