In vitro formation of the "S" layer, a unique component of the fertilization envelope in Xenopus laevis eggs

The extracellular matrix (ECM) of unfertilized Xenopus laevis eggs consists of an elaborate filamentous network in the perivitelline space (PS) and a thick fibrillar vitelline envelope (VE), with a thin layer of horizontal filaments (HF) separating the two. At fertilization this ECM is converted into the fertilization envelope (comprised of the fertilization (F) layer and altered VE), and a third layer, the smooth (S) layer, is formed at the upper boundary of the PS (Larabell and Chandler, 1988). In this report, we use quick-freeze, deep-etch, rotary-shadow electron microscopy to show that an intact S layer can be formed in vitro by incubation of unfertilized eggs in an exudate obtained from cortical granules. Within 5 min numerous 36-nm-diameter particles assemble in a highly ordered array at the microvillar tips. These particles appear to "melt" and to form patches of smooth material and within 10 min one continuous sheet has formed. The presence of the VE is required for formation of the S layer, and we suggest that the HF layer is the site of assembly.