Scanning electron microscopy of fetal and neonatal lamb cardiac cells

Scanning electron microscopy (SEM) was employed to study the ultrastructural development of cardiac myocytes in fetal and postnatal sheep between 90 days gestational age and 36 days postnatal age. Development of the transverse tubular system is apparent as early as 90 days gestational age. Cells in the 90-day fetus reveal mitochondria to be elongated vermiform structures with numerous constrictions, and as the cells mature in later stages they become oval and assume their adult position in the perinuclear and interfibrillar regions, often retaining membranous connections with one another. Myofibrillar development is sparse at 90 days and is most apparent in the subsarcolemmal region. With time the lateral addition of fibrils results in central displacement of the older myofibrils, causing the sarcolemma to be drawn inward at its point of attachment at the Z-band to form the T-tubules. With progressive longitudinal addition of sarcomeres some of the T-tubules are pulled longitudinally when the sarcomeres are out of register resulting in the longitudinal component of the T-system. By the time of birth they resemble the adult configuration.