[Ecto-mesodermal interactions and chick embryo limb chondrogenesis. Ultrastructural studies of cultures in the...

Stage 17 (Hamburger and Hamilton) limb mesoderm, isolated and cultured in vitro differentiates into cartilage (11.5% of the explants). However, chondrogenesis is considerably improved in the presence of ectoderm, whether the interacting tissues are in close contact or separated by the vitelline membrane. Stage 15-16 limb mesoderm cultured in vitro is unable to differentiate into cartilage. Previous experiments have shown that a contact between ectoderm and mesoderm is necessary for the induction of cartilage to take place. However, when the vitelline membrane separates the two tissues, cartilage differentiation occurs in 16.3% of the explants. Can a contact between the two interacting tissues be established through the vitelline membrane? Ultrastructural studies of the relationships between ectodermal and mesodermal cells and the vitelline membrane have shown that such a contact can be achieved by means of long mesodermal cell processes penetrating deeply into the vitelline membrane.