Co-culture of contiguous developmental fields in a serumless, chemically-defined medium: an in vitro model permissive for coordinate development of the mouse ear.

Pattern formation is intrinsically hierarchical, increasing in complexity from the first early embryonic inductive tissue interactions to the eventual integration of multiple organ systems. Viewed as a problem in pattern formation, the vertebrate ear is an exceedingly complex organ system in which normal morphogenesis requires multiple inductive interactions between a variety of adjacent tissues. In order to model the process of higher level pattern formation, we have developed a method for organ culture of the embryonic murine ear. E10.5 mouse embryos (38 to 42 somite pairs) were microdissected into explants that consist of the first and second branchial arches, the otocyst, and the adjacent neural tube. The growth of these explants in a serumless, chemically-defined medium was compared to medium supplemented with 10% fetal calf serum. After 6 days in culture using serumless medium, we observed that this environment was permissive for the formation of pinnae, rudimentary semicircular canals and cochlear ducts, chondrogenesis of the otic capsule and elongation of the endolymphatic ducts. Posterior elements of Meckel's and Reichert's cartilages were identified as ossicular anlagen. All of these structures maintained appropriate anatomic interrelationships during in vitro development. Furthermore, no significant differences were observed in explants grown in serum-supplemented medium. We conclude that during ear development several histogenetic and morphogenetic processes, including aspects of higher level pattern formation, are mediated primarily by paracrine and/or autocrine factors. The development of an organ culture model using serumless medium should facilitate the discovery of intrinsic factors which regulate the coordinate development of inner, middle and external ear structures.