Thursday, October 8, 2009

light and dark chondrocytes

How do light and dark chondrocytes effect height?  And does LSJL affect the ratio of light and dark chondrocytes?

Periostin expression distinguishes between light and dark hypertrophic chondrocytes.

"Hypertrophic chondrocytes exist in two forms detectable by electron microscopy, light and dark chondrocytes. The aims of the study were to establish a method for separating light from dark hypertrophic chondrocytes and to identify genes differentially expressed between the two populations. Three-dimensional pellet cultures of chondrocytes from cartilage of neonatal rats were induced to undergo hypertrophy by treatment with triiodothyronine. Cultures were dissociated and subjected to density gradient centrifugation. The cell fraction with the lowest density comprised predominantly light hypertrophic chondrocytes, and the fraction with the highest density comprised predominantly dark hypertrophic chondrocytes. An Affymetrix GeneChip rat expression array was used to compare expression between dark cell-containing pellets and the light cell-enriched fraction. Genes identified on the array as putative dark cell-selective genes included genes encoding extracellular matrix proteins and enzymic modulators thereof. Expression of a subset of genes (Col1a1{up in LSJL}, periostin{up in LSJL}, osteoglycin{down}, tPA/Plat, and Chst11) was confirmed as dark cell-selective using quantitative reverse transcriptase polymerase chain reaction. The most highly differentially expressed dark cell-selective gene was periostin. In immunocytochemical studies of light and dark cell-enriched fractions, periostin staining was detectable in dark, but not light hypertrophic chondrocytes. The results provide insight into molecular differences between light and dark hypertrophic chondrocytes."

"Dark and light chondrocytes can be observed at all levels of the growth plate"

"once the cells have started to undergo hypertrophy and can be distinguished as light or dark, the ratio does not change, that is, cells do not transform from light to dark or vice-versa with time in culture."

"In pellets, hypertrophic chondrocytes with electron-dense cytoplasm (i.e. dark cells) contained numerous cytoplasmic processes and abundant RER; their nuclei contained heterochromatin. Hypertrophic chondrocytes with electron-lucent cytoplasm (i.e. light cells), in contrast, had smooth rounded profiles and contained sparse RER; their nuclei contained euchromatin"

Other genes upregulated in dark versus light chondrocytes that are up(or -down) regulated in LSJL:

Genes downregulated in dark versus light chondrocytes:

"periostin is uniformly expressed by proliferative chondrocytes in the growth plate, and down-regulated in light but not dark cells with hypertrophy."<-periostin is also expressed by osteoblasts so we can't say for sure whether LSJL alters the ratio of dark to light chondrocytes.

Microarray analysis reveals age-related differences in gene expression during the development of osteoarthritis in mice.

"Surgical destabilization of the medial meniscus (DMM) was used to model OA in 12-week-old and 12-month-old male C57BL/6 mice. OA severity was evaluated histologically. RNA used for microarray and real-time polymerase chain reaction analysis was isolated from joint tissue collected from the medial side of the joint, including cartilage, meniscus, subchondral bone, and the joint capsule with synovium. Computational analysis was used to identify patterns of gene expression, and immunohistochemistry was used to evaluate tissue distribution of selected proteins.
OA was more severe in older mice than in young mice. Only 55 genes showed a similar expression with DMM-induced OA in the 2 age groups, while 493 genes showed differential expression, the majority having increased expression in older mice. Functional categories for similarly expressed genes included extracellular matrix- and cell adhesion-related genes; differentially expressed genes included those related to muscle structure and development and immune response genes. Comparison of expression in sham-operated control joints revealed an age-related decrease in matrix gene expression and an increase in immune and defense response gene expression. Interleukin-33 was present in multiple joint tissue cells, while CCL21 was more localized to chondrocytes and meniscal cells. Periostin was found in the extracellular matrix of cartilage and meniscus."

Genes with increased expression in older mice also upregulated in LSJL:

Genes with decreased expression:

"We also noted a decrease in [in older mice in the] expression of the gene for HMGB-2{down in LSJL}, consistent with a previous study in mice that showed that decreased levels may contribute to chondrocyte death in the superficial zone"

"Periostin is a vitamin K–dependent (Gla-containing) protein produced by osteoblasts and also found in the periosteum. We confirmed its presence in periosteum and osteoblasts and in addition found that it was present in the cartilage matrix and in low numbers of chondrocytes."

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