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M R Dohmen

Publications and source records attributed to M R Dohmen.

14 recordsLinked to original sources
PubMed1992

Cell lineage in molluscan development.

Cell lineage specification in molluscs is brought about by two mechanism: the segregation of morphogenetic plasms and inductive cell interactions. The evidence for the existence of morphogenetic plasms is largely circumstantial, but in one species, Bithynia, such a plasm has been identified in the polar lobe that forms at first cleavage. Inductive cell interactions are thought to be a prerequisite for the development of a large number of tissues and organs. The most extensively studied example is the specification of the mesodermal stem cell in Lymnaea and Patella, which occurs between 5th and 6th cleavage through an interaction between one macromere and a large number of micromeres. Both segregation and induction are tuned to the animal-vegetal polarity of the egg, at least during early development. This polarity probably arises during oogenesis and is manifest in regional differentiations of the surface architecture of the egg, in the distribution of inner membrane particles in the plasma membrane, in membrane fluidity characteristics, in ionic conductance properties of the plasma membrane, etc. All these phenomena have in common that they represent properties of the egg surface, suggesting that the polarity of the egg is somehow imprinted into the plasma membrane and the cortex of the egg during oogenesis.

Animals↗
PubMed1985

Animal-vegetal polarity in the plasma membrane of a molluscan egg: a quantitative freeze-fracture study.

Using freeze-fracture electron microscopy, the numerical particle distribution in the fertilized Nassarius egg plasma membrane has been analyzed in four areas at different positions along the animal-vegetal axis of the egg. These areas can be distinguished by distinct microvilli patterns and differences in microvilli densities. In all areas, more IMPs (intramembrane particles) are present on the P face than on the corresponding E face. The ratio of the number of IMPs present on E and P face is similar in all areas (0.48-0.55) except for the most animal part of the vegetal hemisphere, where relatively more IMPs remain attached to the exterior half of the fractured membrane (E/P ratio = 0.88). The IMP density at the vegetal pole of the egg is considerably higher than in the animal hemisphere and in the animal part of the vegetal hemisphere. This difference is due to an increased number of IMPs in all size classes (4-18 nm). In the area adjacent to the vegetal pole the density of particles is also higher than in the two more animal areas, but here the difference is exclusively due to the smaller IMP size classes (4-8 nm). Statistical analysis of our data reveals that the area adjacent to the vegetal pole patch is significantly different from the other areas with respect to the distribution of the IMPs over the different IMP size classes. These results demonstrate the polar organization of the Nassarius egg plasma membrane. The possible role of this surface heterogeneity in the spatial organization of the egg cell and the later embryo is discussed.

Animals↗
PubMed1985

Regional differences in the lateral mobility of plasma membrane lipids in a molluscan embryo.

Regional and temporal differences in plasma membrane lipid mobility have been analyzed during the first three cleavage cycles of the embryo of the polar-lobe-forming mollusc Nassarius reticulatus by the fluorescence photobleaching recovery (FPR) method, using 1,1'-ditetradecyl 3,3,3',3'-tetramethylindocarbocyanine iodide (C14diI) as a fluorescent lipid probe. During this period of development the lateral diffusion coefficient of membrane lipids is consistently greater in the vegetal polar lobe area as compared to the animal plasma membrane area (on average 30%), demonstrating the existence of an animal-vegetal polarity in plasma membrane properties. At third cleavage, the differences between animal and vegetal plasma membrane region become even more pronounced; in the four animal micromeres the diffusion coefficient (D) and mobile fraction (MF) are 2.9 +/- 0.2 X 10(-9) cm2/sec and 51 +/- 2%, respectively, while in the four vegetal macromeres D = 5.0 +/- 0.3 X 10(-9) cm2/sec and MF = 78 +/- 2%. Superimposed upon the observed animal-vegetal polarity, the lateral diffusion in the polar lobe membrane area shows a cell-cycle-dependent modulation. The highest mean values for D are reached during the S phase (ranging from 7.0 to 7.8 X 10(-9) cm2/sec in the three cycles measured), while at the end of G2 phase and during early mitosis mean values for D have decreased significantly (ranging from 5.0 to 5.9 X 10(-9) cm2/sec). Diffusion rates in the animal membranes of the embryo are constant during the three successive cell cycles (D = 4.3-5.0 X 10(-9) cm2/sec), except for a peak at the S phase of the first cell cycle (D = 6.0 X 10(-9) cm2/sec). These results are discussed in relation with previously observed ultrastructural heterogeneities in the Nassarius egg plasma membrane. It is speculated that the observed animal-vegetal polarity in the organization of the egg membrane might play an important role in the process of cell diversification during early development.

Animals↗
PubMed1984

Localization of biotransformational enzymes along the crypt-villus axis of the rat intestine. Evaluation of two cell isolation procedures.

Rat intestinal epithelial cells were isolated by EDTA-chelation, combined with gentle shaking (modified Weiser procedure) or with strong longitudinal vibration (Harrison/Webster procedure). Both methods yield large numbers of viable cells and are relatively easy to use. Electronmicroscopical and biochemical data indicate that cell fractions from different levels of the villous region can be obtained only by the modified Weiser procedure. When strong mechanical forces are involved (Harrison-Webster procedure) the villus epithelium is released according to an all-or-nothing process. The biotransformational capacity of cell fractions, obtained from different levels of the villi by the modified Weiser procedure, was investigated. It was shown that the rate of metabolism of 7-ethoxycoumarin and 1-naphthol was substantially higher in lower villous cells than in cells isolated from the upper villous region. O-Deethylation of 7-ethoxycoumarin decreases from 145 +/- 13 pmole/min mg cell protein (72 +/- 4% conjugated) in lower villous cells to 62 +/- 12 pmole/min mg cell protein (37 +/- 6% conjugated) in tip cells. Glucuronidation of 1-naphthol decreased from 495 +/- 23 pmole/min mg cell protein (lower villous cells) to 137 +/- 13 pmole/min mg cell protein (tip cells).

Animals↗