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A J Kahn

Publications and source records attributed to A J Kahn.

101 records · Page 6Linked to original sources
PubMed1968

Interference contrast and phase contrast microscopy of sporulation and germination of Bacillus megaterium.

The techniques of Nomarski interference contrast microscopy and phase-contrast microscopy were compared for their utility in monitoring sporulation and germination in Bacillus megaterium. The Nomarski technique permitted rapid and easy delineation of septation and engulfment during sporulation, whereas with phase contrast microscopy these stages were not detected at all. The later stages of sporulation were easily seen by either technique. Thus, of the seven stages of sporulation as recognized by the electron microscopy of thin sections, five can now be routinely detected quantitatively by optical microscopy: septation (stage II), engulfment (stage III), phase-dark forespore (corresponding to cortex formation, stage IV), phase-bright spore in a sporangium (corresponding to coat formation, stage V), and the free spore (stage VII). This means that now only stage I (axial filament) and stage VI (maturation of the refractile spore) require electron microscopy for routine detection. There was no advantage in using Nomarski optics for germination studies.

Bacillus megaterium↗
PubMedDate not supplied

Age-related bone loss in mice is associated with an increased osteoclast progenitor pool.

Age-associated osteopenia has been documented to occur in mice and, therefore, provides a model system whereby mechanisms of bone loss can be assessed in vivo and in vitro. One such mechanism, that could explain the increased resorptive activity seen in some forms of osteopenia, is an age-associated increase in the osteoclast precursor pool and osteoclastogenic formation. To test this hypothesis, we studied the bone marrow composition of aged (24 months) mice to determine if increased numbers of monocyte/macrophage/osteoclast precursor cells (MMOPC) were present when compared to young (4-6 months) animals. Our data show a moderate increase of 20-30% more hematopoietic cells obtained from the long bones of the aged animals. However, both liquid and semi-solid culture techniques demonstrate an approximately 2-3.5-fold increase in the numbers of plastic adherent macrophages or mononuclear colonies in bone marrow derived from the aged mice when stimulated by interleukin-3 (IL-3), granulocyte-macrophage colony stimulating factor (GM-CSF) or macrophage colony stimulating factor (GM-CSF), indicating a preferential increase in MMOPCs. In addition, cells derived from the aged mice show higher levels of cytokine stimulated incorporation of [3H]-thymidine and [3H]-leucine, with increased protein synthesis seen up to 7 days after cytokine stimulation, suggesting that these cells also have an enhanced sensitivity to cytokines.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗
PubMedDate not supplied

New concepts in bone remodeling: an expanding role for the osteoblast.

Until recently, the prevailing view regarding the function of osteoblasts and osteoclasts was to attribute bone formation to the former and bone resorption to the latter. While the capacity of the osteoclast to degrade bone matrix remains unquestioned, there is now provocative evidence indicating that the osteoblast plays a critical role in regulating osteoclast resorptive activity as well as in contributing directly to matrix dissolution. The first of these points follows from observations indicating that the osteoblast (but not the osteoclast) 1) exhibits receptors and/or responses to resorption-promoting agents (including parathyroid hormone and vitamin D), and 2) releases agents capable of stimulating bone resorption. The second point is derived from studies demonstrating that the osteoblast produces neutral collagenase (an enzyme specialized to degrade type I collagen, the principal organic constituent of bone matrix) and an inhibitor capable of blocking collagenase activity. The synthesis of both of these proteins is, in part, regulated by parathyroid hormone and other resorption-stimulating agents and appears to involve control at the transcriptional, translational, and secretory levels. Thus, in both physiologic bone remodeling and modeling, as well as the altered bone turnover associated with some disease states, it is the osteoblast rather than the osteoclast that may hold the key to understanding the mechanism of tissue form and function.

Bone Resorption↗
PubMedDate not supplied

Regulation of myc gene expression in HL-60 leukaemia cells by a vitamin D metabolite.

HL-60, a cell line established from a patient with promyelocytic leukaemia, responds to a variety of inducing agents by ceasing division and acquiring some of the characteristics of either granulocytes or monocytes. Among the agents so far tested, only a comparative few occur naturally in vertebrates and would appear to have significant clinical potential in the treatment of leukaemic patients. One of the most promising of these is the dihydroxymetabolite of vitamin D3, 1,25(OH)2D3. This compound circulates in normal man and has a major role in calcium homeostasis. Moreover, it has recently been reported that 1,25(OH)2D3 increases the survival time of mice injected with myeloid leukaemia cells. We and McCarthy et al. have previously shown that HL-60 cells respond to near physiological levels of 1,25(OH)2D3 by rapidly acquiring a number of monocyte-like features. Here we document that these phenotypic changes are preceded by a marked decrement in the expression of the c-myc oncogene. In fact, the diminution in the level of c-myc mRNA parallels the dose dependency and metabolite specificity shown by the various other indicators of phenotypic change. In addition, we demonstrate that removal of vitamin D3, after the onset of maturational change, results in the reappearance of elevated myc mRNA levels. We believe this to be the first demonstration of a sequential relationship between the application of an exogenous inducing agent, a reduction in myc mRNA levels and the development of characteristics associated with normal cell maturation.

Animals↗
PubMedDate not supplied

Chondrocyte-to-osteocyte transformation in grafts of perichondrium-free epiphyseal cartilage.

When perichondrium-free pieces of embryonic quail epiphyseal cartilage are incubated on the chorioallantoic membranes of chick embryos, 2 developmental changes are observed. First, most grafts develop a periosteum in which the osteoblasts and osteocytes are of donor, i.e., chondrocytic origin. No such periosteum is observed around explants of demineralized, inductive bone matrix. Second, the matrix surrounding some chondrocytes within the original graft became more bone-like with respect to staining pattern, birefringence and collagen morphology. We conclude that, under some conditions, the avian chondrocyte may in situ or subsequent to release from the cartilage lacuna synthesize a bone-like matrix and, in this sense, be thought to have undergone a "transformation" into an osteocytic or osteoblastic type of cell.

Allantois↗