Cytoplasmic membrane transplants in early Xenopus embryos indicate cell-cycle-specific effects.

Fragments of rough endoplasmic reticulum or Golgi complex isolated from normal adult rat liver homogenates were injected into one cell of cleaving two-cell Xenopus laevis embryos and the effects on development were monitored during early cleavage by morphological analysis. Scanning electron microscopy revealed the formation of large cells on the injected side of the embryos. Such large cells were not present in controls and thus were considered to have been formed as a consequence of delayed cleavage. Delay of cleavage was obtained with as little as 1 ng of membrane protein giving a ratio of membrane protein to embryo protein of 1:10(5). Cytological observations of microinjected embryos confirmed the occurrence of delayed cytokinesis and suggested that nuclear division became asynchronous. Since rough microsomes from proliferating tissues (i.e., livers with primary tumors and livers undergoing regeneration) showed little or no effect on cytokinesis after microinjection into early embryos, we conclude that cytoplasmic membranes may exhibit cell-cycle-specific properties important for normal development.     

The identification of thyroglobulin within a rat thryroid subcellular fraction

A particulatte fraction obtained from a rat thyroid homogenate by centrifugation at 27 000 × g was compared to the soluble fraction used for the preparation of thyroglobulin. By sodium dodecyl sulfate polycrylamide gel electrophoresis analysis after reduction, the soluble fraction inculded, mainly, three high molecular weight componenents which became strongly radioactive after an in vivo radioiodine injection. The particulate fraction on the same gels after reduction inlcude several low molecular weight components and a single high molecular weight component containing little radioiodine and showing, by electron microscopy, at least three types of structure: rough surfaced and smooth surfaced vesicles and apical vesicles (the secretory granules of thyroid follicular cells). The substrate present in the particulate fraction behaved antigenically like thyrolobulin. It is concluded that the particulate fraction contained nascent thyroglobulin inits undegraded form and that this material is probably the precursor to the various components encountered in the soluble fraction.