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H Holtzer

Publications and source records attributed to H Holtzer.

142 records · Page 8Linked to original sources
PubMed1969

The loss of phenotypic traits by differentiated cells. VI. Behavior of the progeny of a single chondrocyte.

A single, functional, mitotically quiescent chondrocyte may be induced to reenter the mitotic cyde, and produce a progeny of over 10(11) cells. Sessile, adherent, polygonal cells deposit matrix, whereas amoeboid, dispersed, flattened fibroblastic cells do not. The prior synthetic history of a cell is of greater importance in determining whether the characteristic chondrogenic phenotype will be expressed, rather than growth in "permissive" or "nonpermissive" medium. Clonal conditions select for stem-like cells, some of whose progeny may become polygonal chondrocytes. The retention of the characteristic chondrogenic phenotype in vitro is favored by pruning the dedifferentiated chondrocytes which arise in these cultures. Dedifferentiated chondrocytes interfere with the deposition and synthesis of chondroitin sulfate by neighboring functional chondrocytes. Possible mechanisms are proposed to explain this type of cell-cell or cell exudate interference. If the progeny of a single, genetically programmed chondrocyte may or may not synthesize chondroitin sulfate, then extragenic sites in the cytoplasm or cell surface must influence the decision as to which cluster of "luxur" molecules the cell will synthesize.

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PubMed1969

Effect of 5-bromodeoxyuridine on expression of cultured chondrocytes grown in vitro.

Exposure of cultured cartilage cells to 5-bromodeoxyuridine results in a rapid loss of the ability to synthesize the chondroitin 4-sulfate-protein complex characteristic of the extracellular matrix of differentiated cartilage. UDP-glucose dehydrogenase, UDP-N-acetyl hexosamine 4-epimerase and the enzyme system responsible for catalyzing 3'-phosphoadenosine-5'-phosphosulfate synthesis all progressively decline in activity, while several other cellular processes seem unaffected. However, there are marked morphological changes which may be associated with plasma membrane components or the synthesis of the protein-polysaccharide complex.

Bromodeoxyuridine↗
PubMed1969

Mitosis and the processes of differentiation of myogenic cells in vitro.

The relation between the mitotic cycle and myoblast fusion has been studied in chick skeletal muscle in vitro. The duration of the cell cycle phases was the same in both early and late cultures. By tracing a cohort of pulse-labeled cells, it was found that myoblast fusion does not occur in S, G(2), or M. Cell surface alterations required for fusion are dependent upon the position of the cell in the division cycle. In early cultures, fusion takes place only after a minimum delay of 5 hr from the time the cell has entered G(1). The mitosis preceding fusion may condition the cell for the abrupt shift in synthetic activity that occurs in the subsequent G(1). In older cultures fusion of labeled cells is diminished. Two factors account for the cessation of fusion in older cultures. First, the number of myogenic stem cells declines, but these cells do not disappear as the cultures mature. Their persistence was demonstrated by labeling dividing mononucleated cells in older cultures and challenging them with nascent myotubes. Some of these labeled cells were incorporated into the forming myotubes. Second, a block to fusion develops during myotube maturation. Well developed myotubes challenged with labeled competent myogenic cells failed to incorporate the labeled nuclei.

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PubMed1969

Formation of arrowhead complexes with heavy meromyosin in a variety of cell types.

Heavy meromyosin (HMM) forms characteristic arrowhead complexes with actin filaments in situ. These complexes are readily visualized in sectioned muscle. Following HMM treatment similar complexes appear in sectioned fibroblasts, chondrogenic cells, nerve cells, and several types of epithelial cells. Thin filaments freshly isolated from chondrogenic cells also bind HMM and form arrowhead structures in negatively stained preparations. HMM-filament complexes are prominent in the cortex of a variety of normal metaphase and Colcemid-arrested metaphase cells. There is no detectable binding of HMM with other cellular components such as microtubules, 100-A filaments, tonofilaments, membranes, nuclei, or collagen fibrils. The significance of HMM-filament binding is discussed in view of the finding that arrowhead complexes form in types of cells not usually thought to contain actin filaments.

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PubMed1968

Mitosis and intermediate-sized filaments in developing skeletal muscle.

A new class of filaments intermediate in diameter between actin and myosin filaments has been demonstrated in skeletal muscle cells cultured from chick embryos. These filaments, which account for the majority of free filaments, average 100 A in diameter. They may run for more than 2 micro in a single section and can be distinguished in size and appearance from the thick and thin filaments assembled into myofibrils. The 100-A filaments are seen scattered throughout the sarcoplasm at all stages of development and show no obvious association with the myofibrils. The 100-A filaments are particularly conspicuous in myotubes fragmented by the mitotic inhibitors, colchicine and Colcemid. In addition, filaments similar in size and appearance to those found in myotubes are present in fibroblasts, chondrocytes, and proliferating mononucleated myoblasts. The 100-A filaments are present in cells arrested in metaphase by mitotic inhibitors. Definitive thick (about 150 A) or thin (about 60 A) myofilaments are not found in skeletal myogenic cells arrested in metaphase. Myogenic cells arrested in metaphase do not bind fluorescein-labeled antibody directed against myosin or actin. For these reasons, it is concluded that not all "thin" filaments in myogenic cells are uniquely associated with myogenesis.

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PubMed1966

The loss of phenotypic traits by differentiated cells. 3. The reversible behavior of chondrocytes in primary cultures.

Observations were made on the behavior of chondrocytes grown under various conditions in vitro. The chondrocytes in 10-day embryonic chick vertebrae were grown as cultures of intact vertebrae, as pellets of chondrocytes liberated from their matrix, and as monodispersed cells plated out on plasma clots. Cartilage matrix was stained metachromatically with toluidine blue. Radioautographs were made of incorporated H(3)-thymidine, H(3)-proline, and S(35)-sulfate to determine the extent of DNA synthesis, collagen synthesis, and chondroitin sulfate synthesis, respectively. Chondrocytes in intact vertebrae or in pellets are rounded and actively synthesizing chondroitin sulfate and collagen. There is little DNA synthesis by cells in either vertebrae or pellets. Chondrocytes grown as monodisperse cells rapidly cease synthesizing cytologically detectable chondroitin sulfate and are induced to synthesize DNA and divide. There is a change in the shape of these chondrocytes from a rounded to a more stellate condition which accompanies the shift in metabolic activity. Conversely, when the cells attain a certain cell density, they reacquire a rounded shape, cease dividing, and again synthesize chondroitin sulfate. Clusters of chondrocytes synthesize more chondroitin sulfate than isolated chondrocytes. It is concluded that most chondrocytes synthesizing chondroitin sulfate do not concurrently synthesize DNA. Interaction between associated chondrocytes is important in inducing and maintaining chondroitin sulfate synthesis in genetically determined chondrocytes. Failure of interaction between chondrocytes leads to DNA synthesis and cell multiplication.

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PubMed1965

Chondroitin sulfate: inhibition of synthesis by puromycin.

Puromycin reversibly inhibits synthesis of chondroitin sulfate by vertebral chondrocytes from 10-day-old chick embryos. Treatment of these cells with puromycin for 5 hours does not affect viability or capacity to multiply on subsequent release from their matrix. It is suggested that synthesis of this polysaccharide is coupled with that of protein; there may be a feedback control in its synthesis.

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PubMedDate not supplied

Generation of chick skeletal muscle cells in groups of 16 from stem cells.

The commonly accepted hypothesis explaining the control of skeletal muscle differentiation is that all myogenic precursor cells are equivalent and that they differentiate into post-mitotic muscle cells in response to exogenous signals, specifically low mitogen concentrations. Large clones derived from vertebrate myogenic cells, however, consist both of cycling precursors and of terminally differentiated, post-mitotic muscle cells. Here, we count the total number of cells and the number of terminally differentiated cells (or nuclei, in fused cells) in large myogenic clones. The number of terminally differentiated cells per clone was usually equal to or just below a multiple of 16. This finding is not expected from a model postulating a homogeneous population of muscle precursor cells. Rather, our results suggest that a self-renewing stem cell exists in the skeletal muscle lineage. This cell can generate committed precursors which then give rise to cohorts of 16 terminally differentiated muscle cells. This model of myogenesis provides a simple explanation for the protracted and asynchronous nature of muscle differentiation in vertebrate embryogenesis.

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