Smooth muscle cells derived from the inner media and intima of immature guinea pig aorta were cultured for up to 8 weeks. The cells maintained their smooth muscle morphology throughout culture. After reaching confluency, they formed multiple overlapping layers. By 4 weeks, microfibrils (110 Å) appeared between the layers, and basement membrane-like material formed around the cells. Analysis showed these microfibrils had an amino acid composition similar to those in intact elastic fibers. These findings, combined with radioautographic evidence of extracellular protein synthesis, indicate that these cells are connective tissue synthetic cells. The study describes the growth of smooth muscle cells in culture, showing they retain their differentiated state. Subcultures of these cells formed elastic fibers. The cells were obtained from prepubertal guinea pigs, and segments of aorta were processed for culture. The cells were grown in culture dishes, and after several weeks, they formed multiple layers. The cells were observed under electron microscopy, showing myofilaments, dense bodies, and other organelles. The cells formed basement membrane-like material and microfibrils, which were similar to those in elastic fibers. The microfibrils appeared in the extracellular space, and by 3 weeks, they were found adjacent to the cells. These microfibrils were identical to those in elastic fibers. The cells also formed elastic fibers, and the microfibrils were similar to those in intact elastic fibers. The amino acid composition of the microfibrillar protein was similar to that of the microfibrillar protein in intact elastic fibers. The study shows that smooth muscle cells can form elastic fibers and other connective tissue components. The cells retained their smooth muscle characteristics, including the presence of myofilaments and dense bodies. The cells formed basement membrane-like material and microfibrils, which were similar to those in elastic fibers. The study also shows that smooth muscle cells can synthesize and secrete extracellular proteins, indicating their role in connective tissue formation. The findings suggest that smooth muscle cells are important in the formation of connective tissue and may be related to fibroblasts, osteoblasts, and chondroblasts. The study also highlights the potential of these cells for studying atherogenesis and the synthesis of elastic fiber proteins.Smooth muscle cells derived from the inner media and intima of immature guinea pig aorta were cultured for up to 8 weeks. The cells maintained their smooth muscle morphology throughout culture. After reaching confluency, they formed multiple overlapping layers. By 4 weeks, microfibrils (110 Å) appeared between the layers, and basement membrane-like material formed around the cells. Analysis showed these microfibrils had an amino acid composition similar to those in intact elastic fibers. These findings, combined with radioautographic evidence of extracellular protein synthesis, indicate that these cells are connective tissue synthetic cells. The study describes the growth of smooth muscle cells in culture, showing they retain their differentiated state. Subcultures of these cells formed elastic fibers. The cells were obtained from prepubertal guinea pigs, and segments of aorta were processed for culture. The cells were grown in culture dishes, and after several weeks, they formed multiple layers. The cells were observed under electron microscopy, showing myofilaments, dense bodies, and other organelles. The cells formed basement membrane-like material and microfibrils, which were similar to those in elastic fibers. The microfibrils appeared in the extracellular space, and by 3 weeks, they were found adjacent to the cells. These microfibrils were identical to those in elastic fibers. The cells also formed elastic fibers, and the microfibrils were similar to those in intact elastic fibers. The amino acid composition of the microfibrillar protein was similar to that of the microfibrillar protein in intact elastic fibers. The study shows that smooth muscle cells can form elastic fibers and other connective tissue components. The cells retained their smooth muscle characteristics, including the presence of myofilaments and dense bodies. The cells formed basement membrane-like material and microfibrils, which were similar to those in elastic fibers. The study also shows that smooth muscle cells can synthesize and secrete extracellular proteins, indicating their role in connective tissue formation. The findings suggest that smooth muscle cells are important in the formation of connective tissue and may be related to fibroblasts, osteoblasts, and chondroblasts. The study also highlights the potential of these cells for studying atherogenesis and the synthesis of elastic fiber proteins.