Development of the basal lamina and extracellular materials in the early chick embryo.

Chick embryos at developmental stages up to primitive streak formation were fixed in a mixture of tannic acid and glutaraldehyde. A basal lamina was present in the unincubated embryo and consisted of a lucent lamina interna and a lamina densa. At the primitive streak stage the lamina densa showed a periodicity of stained elements. Densely stained materials were present on the cell surfaces lining the cavity between the epiblast and endoblast, and on the mesoderm cells within this cavity. Considerable amounts of extracellular material were observed in the cavity. Hyaluronidase treatment removed the cell surface and extracellular material, indicating that hyaluronic acid is a major component. This enzyme disrupted the basal lamina, leaving a fibrillar remnant with no periodic structure. It is therefore suggested that the dense periodicities consist of glycosaminoglycan built on an enzyme-resistant framework which is probably collagen. Enzyme-resistant fibrils, presumably collagen precursors, are present elsewhere within the tissue spaces.