Differential expression of type II cytokeratin mRNA defines early developmental boundaries within the ectoderm, mesoderm and endoderm during chick development.

Using in situ hybridization we show that a chick type II cytokeratin (CKsel) is differentially expressed within the developing ectoderm, mesoderm, and endoderm as early as the time of germ layer formation. CKsel expression demarcates specific regions that in many instances can be correlated with distinct presumptive developmental fates or potencies. In the epiblast of definitive streak stage embryos, CKsel hybridization is detected only in regions destined to form extraembryonic ectoderm. After this stage expression occurs within embryonic ectoderm but is largely restricted to regions that will form epidermis. It is not detected in presumptive neural ectoderm or in most regions of head ectoderm. The area of non-expression of CKsel in head ectoderm delineates ectoderm which has a strong bias for lens formation, and gives rise not only to lenses but to other placodally derived structures such as the nasal epithelium and the otic vesicles. In the developing mesoderm, CKsel transcripts become restricted to regions of lateral plate and extraembryonic mesodermal tissues at later stages. CKsel is expressed throughout the developing endoderm except in the region of the presumptive foregut roof. There is a unifying pattern to the expression of CKsel transcripts in all three germ layers: it is expressed in more lateral areas which generally have more ventral fates, beginning at the time of germ layer formation and indicating the existence of medial/lateral boundaries early in development.