Heart development in the mexican salamander, Ambystoma mexicanum. II. Ultrastructure

Abstract

AbstractHeart development in the Mexican axolotl, Ambystoma mexicanum, from early embryonic stages to the adult was studied by electron microscopy. During early development, myocardial cells exhibit scanty cytoplasmic matrixes which contain numerous large yolk platelets and lipid droplets. As development progresses the yolk and lipid materials become reduced; the cytoplasmic matrixes increase in glycogen content and membranous organelles. Degradation of yolk platelets appears to take place by an “unraveling” process. The Golgi complex becomes well‐developed just prior to the first heart beats and its many associated vesicles are suggestive of secretory activity. Degeneration and death of certain myocardial cells are evident at the onset of trabeculation. Several mechanisms for myofibrillogenesis appear to operate both simultaneously and at different developmental stages. Pre‐heart‐beat myocardial cells display ribosome‐containing amorphous masses and unorganized 60 Å and 140 Å filaments in their cytoplasm. They also have electron‐opaque plaques on their plasma membranes. Sarcomeric myofibrils first appear parallel to and immediately beneath the sarcolemma. This occurs at stage 34, the heart‐beat initiation stage (6 days). During later development “isolated” Z band‐myofilament complexes, numerous loosely‐organized 60 Å filaments and a few 140 Å filaments are observable in the cell matrixes. This suggests that Z bands may form centers for myofibril organization at these stages. Also at later stages, polysomes are parallel to the already‐formed myofibrils. These polysomes are possibly synthesizing contractile proteins “in situ” and may represent a mechanism for myofibril diameter growth. In late embryonic and juvenile stages 100 Å—110 Å filaments appear to be continuous between Z bands of adjacent myofibrils. It is possible that such filaments of intermediate size are involved in aligning myofibrils into register during advanced developmental stages.