Several peptide growth factors, including members of the fibroblast growth factor (FGF) superfamily, are potential inducers of mesoderm in vertebrates. Receptor binding of basic FGF (FGF-2) is promoted by cell surface or extracellular matrix proteoglycans. The substantial biosynthesis of proteoglycans by embryonic cells (called embryoglycans) and their potential role as ligands for growth factor receptors led us to examine the role of embryoglycans that carry the developmentally regulated oligosaccharide epitope TEC 1, in the binding of FGF-2 to cultured rabbit inner cell masses (ICMs). Culture of isolated ICMs in the presence of FGF-2 gave rise to well delimited colonies with migrating cells at the periphery. In these cells, TEC 1 staining shifts from a punctate pattern over the entire membrane, to an apical, finely granular distribution with some internalization. This shift occurs after 96 hours in culture. Here we show that: (1) migrating cells are mesoderm-like in phenotype; (2) antibodies against TEC 1 blocked FGF-2 mediated differentiation in vitro; (3) antibodies against TEC 1 selectively blocked binding of FGF-2 to ectodermal receptors and, vice versa, the binding of TEC 1-specific antibodies to ectodermal cells can be competed by excess FGF-2; (4) the same switch in TEC 1 staining patterns was observed in vivo, between the day 7 and the day 9 rabbit embryo. These data suggest the involvement of defined species of embryonic cell surface epitopes in the regulation of FGF-2 receptor binding. Moreover, this proposed binding activity is temporally restricted to ectodermal cells and disappears early during differentiation. Thus, the apical TEC 1 redistribution can be considered as the earliest indicator of mesoderm formation.
An acrosome reaction was induced in ejaculated ram spermatozoa by treatment with calcium and the ionophore A23187. Samples were fixed at different times after initiation of induction, and the morphological changes within the head membranes that took place as exocytosis occurred were studied in freeze-fracture replicas. Reacted acrosomes appeared in individual spermatozoa within the calcium/ionophore-treated population at different times after the start of treatment; the first cells had reacted by 10 min, whereas some took more than 40 min to react. No changes were observed in control populations. An early effect of treatment (seen in most cells within 10 min) was the appearance of particle-free ‘clearings’ in the plasma membrane over the entire acrosomal region, with aggregation of intramembranous particles between and around these ‘clearings’. At the same time, there was an increase in the number of large particles (greater than or equal to 10 nm) within the plasma membrane over the ‘lunula’ of the equatorial segment and the anterior part of the post-acrosomal region. Fusion of the plasma and outer acrosomal membranes began in a limited area at the border between the anterior and equatorial segments of the acrosome. It then spread, following arborescent pathways, sideways along this border and forwards towards the apex of the head. This labyrinthic propagation resulted in an ‘acrosomal cap’ increasingly fenestrated towards its posterior margin. Fusion propagation over the equatorial segment was inhibited, apparently as a result of the highly ordered structure of the membranes in this region.