Biosynthesis of major plasma proteins in the primary culture of fat body cells from the silkworm, Bombyx mori

Plasma proteins termed "SP1" and "30K proteins" are synthesized by the fat body cells of the silkworm, Bombyx mori, in a sex- and stage-specific manne r during larval development. We successfully established a primary culture of the fat body cells in order to investigate the regulatory mechanisms of plasma protein gene expression. The primary cultures of fat body cells cont ained at least two cell types: small oval cells, and large spherical cells. The cells adhered to and migrated on the cultured dish after plating. By t he 7th day of cultivation, the cells clustered to form fat body-like struct ures, which were maintained for at least 3 months. Plasma proteins were act ively synthesized in the primary cultures of the fat body cells isolated fr om the final instar larvae only when the cells tightly adhered to and clust ered on the cultured dish. Immunocytochemical analysis revealed that only 1 0-15% of the clustered cells synthesized plas ma proteins in our culture sy stem, indicating that the primary culture comprises heterogeneous cells tha t are morphologically and functionally distinct. The patterns of SP1 synthe ses in primary cultures faithfully reproduced their sex-dependency in vivo.