The apical lamina and its role in cell adhesion in sea urchin embryos

The hyaline layer (HL) is an extracellular matrix surrounding sea urchin embryos which has been implicated in a cell adhesion and morphogenesis. The apical lamina (AL) is a fibrous meshwork that remains after removal of hyalin from the HL and the fibropellins (FP) are glycoproteins thought to be the principal components of the AL. Using anti-FP antibodies (AL-1 and AL-2) we report immunoprecipitations and affinity purifications yield a high molecular weight complex comprised of the FP glycoproteins. The three components form a complex, stabilized by disulphide cross-linking and have stochiometric ratios of 2 FPIa molecules to 1 each of FPIb and FPIII. Pulse chase experiments indicate all 3 FP's are synthesized throughout development with peaks in synthesis during cleavage and a sustained peak beginning at hatching. Using immunogold and immunoperoxidase localization, the FP localize to a fibrillar complex forming the innermost layer of the HL. In cell adhesion experiments, cells adhere to affinity purified FP in a temperature, time and concentration dependent manner. Cell adhesion to Fp is about 70% of that seen when hyalin is used as a substrate. Pretreating with AL-1 and AL-2 reduces in vitro cell adhesion by about 65%. We conclude FP's form a fibrillar complex, which is synthesized throughout early development and functions, with other components of the HL, as a substrate for cell adhesion.