Two contrasting substrates, Drosophila laminin and human vitronectin, caused 
determined primary Drosophila embryo cells to follow alternate intermediate 
differentiation steps without affecting the final outcome of differentiation. 
Integrin alpha PS2 beta PS3 was essential for the initial spreading of myocytes 
on vitronectin: focal contacts rich in beta PS3 integrins formed and were 
connected by actin- and myosin-containing stress fibers. While alpha PS2 beta 
PS3 was unnecessary for myotube formation on laminin, it was required for the 
subsequent change to a sarcomeric cytoarchitecture. The differentiating primary 
cultures synthesized integrins and assembled them into detergent-insoluble, 
cytoskeleton-associated complexes. Collagen IV, laminin, glutactin, papilin, and 
other extracellular matrix proteins were made primarily by hemocytes and were 
secreted into the medium. Further differentiation within the cultures was 
influenced by secreted components and by later addition of vitronectin or bovine 
serum. Comparison of the differentiation of various cell types on the two 
substrates showed that vitronectin provided a selective advantage for the 
differentiation of myocytes, with enrichment over epithelia, epidermal cells, 
and neurites.