Expression patterns of engrailed‐like proteins in the chick embryo

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Abstract
The protein products of both of the identified chick engrailed‐like (En) genes, chick En‐1 and chick En‐2, are localized in cells of the developing brain, mandibular arch, spinal cord, dermatome, and ventral limb bud ectoderm, as demonstrated by labeling with the polyclonal antiserum αEnhb‐1 developed by Davis et al. (Development 111:281–298, 1991). A subpopulation of cephalic neural crest cells is also En‐protein‐positive. The monoclonal antibody 4D9 recognizes the chick En‐2 gene product exclusively (Patel et al.: Cell 58:955–968, 1989; Davis et al., 1991) and colocalizes with chick En‐2 mRNA in the developing head region of the chick embryo as shown by in situ hybridization (Gardner et al.: J. Neurosci. Res. 21:426–437, 1988). In the present study we examine the pattern of αEnhb‐1 and 4D9 localization throughout the chick embryo from the first appearance of antibody (Ab)‐positive cells at stage 8 (Hamburger and Hamilton: J. Morphol. 88:49–92, 1951) through stage 28 (1–5.5 days). We compare the localization patterns of the two Abs to each other, as well as to the localization of the monoclonal Ab, HNK‐1, which recognizes many neural crest cells, using double‐and triple‐label fluorescence immunohistochemistry. Most En protein‐positive cells in the path of neural crest cell migration are not HNK‐1 positive. In detailed examination of αEnhb‐1 and 4D9 localization, we find previously undetected patterns of En protein localization in the prechordal plate, hindbrain, myotome, ventral body‐wall mesoderm, and extraembryonic membranes. Based upon these observations we propose: (1) that En expression in the mesoderm may be induced through interaction with En expressing cells in the neuroectoderm; (2) that En expression in the head mesenchyme is associated with somitomere 4; and (3) that En expression may be involved in epithelial‐mesenchymal cell transformations.