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B H Nelson

Publications and source records attributed to B H Nelson.

24 records · Page 2Linked to original sources
PubMed1992

Cytoplasmic and cortical determinants interact to specify ectoderm and mesoderm in the leech embryo.

In leech embryos, segmental ectoderm and mesoderm are produced by a pair of sister cells located near the animal and vegetal poles, respectively. We have investigated the mechanism that localizes ectodermal and mesodermal fates along the animal-vegetal axis. The results of cleavage arrest and cell ablation experiments suggest that the full range of normal cell interactions are not required for this process. However, when the animal and vegetal hemispheres are separated by re-orientation of the first cleavage plane from meridional to equatorial, the ectodermal fate co-segregates with the animal hemisphere and the mesodermal fate with the vegetal hemisphere. Two pools of yolk-deficient cytoplasm, called teloplasm, are located at the animal and vegetal poles of the zygote, but separation of the animal and vegetal teloplasms is not sufficient for the segregation of ectodermal and mesodermal fates. Rather, complete segregation of fates requires an equatorial cleavage orientation that separates not only the two teloplasms, but also the animal and vegetal cortical regions. This, in conjunction with previous findings, indicates that ectodermal determinants are localized to the cell cortex in the animal hemisphere of the zygote. We propose that these determinants segregate to the ectodermal precursor and interact with factors in teloplasm to transform the fate of this cell from a mesodermal ground state to ectoderm.

Animals↗
PubMed1988

Effect of osteochondral defects on articular cartilage. Contact pressures studied in dog knees.

Full thickness osteochondral defects 6 mm in diameter were created in the weight-bearing regions of the femoral condyles in 5 adult mongrel dogs to study the contact pressure changes accompanying healing. A digital imaging technique employing Fuji Prescale film mapped contact pressures following 11 months of healing for comparison with contralateral normal knees, and knees with freshly made defects. Although all defects healed uneventfully with subchondral plate reconstitution and with growth restoration of the articular surface, the repair soft tissue appeared histologically to be primarily fibrous tissue, with varying degrees of a fibrocartilaginous component. The mean and peak stresses about fresh defects were not appreciably different from those about healed defects. Neither were there substantial differences in the total cartilage area making contact, except when very low loads were applied. The results suggest that the repair tissue is of poor mechanical quality, and does not contribute appreciably to weight bearing. The cartilage adjacent to the defect did not experience high stresses; neither gross nor light microscopic evidence of degeneration appeared at 11 months. If degeneration does occur following such defects, our data suggest that it is not because of elevated contact stresses.

Animals↗