Early Tip Research Articles

Developmental regulation of Dictyostelium discoideum plasma membrane proteins.

Developmental changes in the plasma membrane proteins of Dictyostelium discoideum have been studied using metabolic labeling with [35S]methionine and two-dimensional electrophoresis. Pulse labeling for 1 h at the early interphase, late interphase, aggregation, and tip formation stages of development showed that the profile of newly synthesized plasma membrane proteins changed dramatically over this interval. Only 14% of the polypeptide species were synthesized at all four stages at detectable levels; 86% of the species changed over this developmental interval according to the criterion that they were synthesized at some but not all of the four stages tested. Long-term labeling during vegetative growth followed by initiation of development showed that the "steady-state" levels of the plasma membrane proteins changed very little over the same period. The only changes were in minor species (33% overall change). Similar analyses of whole cell proteins showed 27 and 20% change, respectively. Cell surface radioiodination revealed 52 external proteins in the plasma membrane. Comparison with the uniform methionine labeling results showed that these proteins were, with one notable exception, minor membrane components. In these external proteins, also, developmental changes were limited and were observed in the less abundant species. These results demonstrate the existence of two general classes of plasma membrane proteins. The first is a population of high-abundance proteins that are present in vegetative cells and are largely conserved through development. These possibly serve "housekeeping" functions common to all stages. The second class consists of low-abundance species that are expressed in a highly stage-specific manner and which presumably participate in developmentally important functions.

Early stage crack tip dislocation morphology in fatigued copper

Observations of dislocation structures around the fatigue cracks in the early stage in polycrystalline copper were made through an ultrahigh voltage electron microscope operating at 2000 kV. Ladder like structures and initial cracks within them were fre-quently observed. These dislocation structures were formed prior to the crack initia-tion, and the crack was initiated and propagated within these structures accompanied by no appreciable dislocation entanglements in the earliest stage. On the other hand, cell structures were observed around the cracks which appeared to have grown into the sub-sequent stage (the stage between Stages I and II). Cell size ahead of the crack tip was not so small, and it seemed that the crack was able to propagate across the cells. On the basis of these observations, the mechanisms of fatigue crack initiation and propagation in the early stage in copper bulk-specimens are briefly discussed.

An ultrastructural study of the first cleavage of Xenopus embryos

During the first cleavage of Xenopus embryos, two morphologically distinct types of vesicles were frequently seen in close association with the membrane of the furrow. A large type (0.07–0.5 μm in diameter), with fibrous contents, fused in a localized area some distance behind the leading edge. This area showed increased lanthanum staining of the surface. Smaller vesicles (400–600 Å in diameter), with a moderately dense interior, were observed subjacent to the membrane at the early furrow tip, where lanthanum staining was either patchy or uniform. Both vesicle types probably originated from the Golgi bodies. A characteristic distribution of lanthanum-stained material on the furrow membrane suggested a translatory movement of this material in the plane of the membrane. In the deepest region of the furrow, lanthanum-stained material was sparse. The embryo surface close to the furrow showed stress folds and membrane protuberances by both scanning and transmission electron microscopy, and was accompanied by the presence of microfilaments in a corresponding area of the cortical cytoplasm. Microvilli were shown to accompany the leading edge of the furrow only in the early stages of development; thereafter they remained stationary and were present as a band (5–10 μm wide) that formed a complete circle in the furrow. In a restricted area of the animal pole furrow, interblastomeric contacts were demonstrated in which the membranes approached to within 30 Å.