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There are several species of alternatively spliced fibronec-tion (FN). One of these, FN EIIIB, is primarily present in embryonic and in proliferating and migrating cells and is believed to be important for cell maturation. We have studied the synthesis, localization, and secretion of this FN isoform in isolated guinea pig megakaryocytes, nonmegakaryocytic bone marrow cells, and platelets. There was 7.5 times more general FN in megakaryocytes than in nonmegakaryocytic cells based on the analysis of equivalent amounts of protein. FN EIIIB was detected by Western blotting in megakaryocytes but not in nonmegakaryocytic cells present in bone marrow. Neither megakaryocytes nor platelets secreted FN EIIIB, while megakaryocytes secreted 25.3% ± 4.6% general FN and platelets secreted about 61% general FN in response to thrombin. Analysis of immunostained cells by confocal microscopy revealed that FN EIIIB had been redistributed to the surface of megakaryocytes in response to thrombin. Synthesis was studied by metabolic labeling, and megakaryocytes were shown to synthesize FN and FN EIIIB. Thus, megakaryocytes and platelets are among a small number of adult cells and tissues that synthesize and contain FN EIIIB. The expression of FN EIIIB on the megakaryocyte surface may influence migration and maturation.

Fibronectin, present in the extracellular matrix of several tissues, is heterogeneous in structure. This heterogeneity is largely due to the alternative splicing of three exons (IIIB, IIIA, and V) during processing of the fibronectin primary transcript. Previously, we determined that the splicing patterns of fibronectin mRNAs change from B+A+ to B+A− during the differentiation of mesenchyme into cartilage (V. D. Bennett, K. M. Pallante, and S. L. Adams,J. Biol. Chem.266, 5918–5924, 1991). Therefore, the structure of fibronectin at the protein level most likely changes during chondrogenesis as well. In order to characterize the fibronectin protein in chick limb prechondrogenic mesenchyme and cartilage, we generated a polyclonal antibody specific for the region encoded by exon IIIB in the chick fibronectin gene. Immunoblot and immunohistochemistry analyses with this antibody and an antibody specific for the region encoded by exon IIIA indicate that both antibodies react with the fibronectin present in prechondrogenic mesenchyme. In contrast, only the exon IIIB antibody reacts with the fibronectin present in chick cartilage. Quantitative ELISA assays with these antibodies indicate that approximately 96% of the fibronectin in chick embryonic cartilage contains exon IIIB while less than 3% of the fibronectin contains exon IIIA. These results corroborate the structures of the fibronectin isoforms present in prechondrogenic mesenchyme and cartilage that were predicted from our previous characterization of the fibronectin mRNAs in these tissues and indicate that essentially all of the fibronectin in cartilage is synthesized and secreted by cartilage chondrocytes.