title: Physiological death of hypertrophic chondrocytes.
abstract: OBJECTIVE
  Post-proliferative chondrocytes in growth cartilage are present in two forms, light and dark cells. These cells undergo hypertrophy and die by a mechanism that is morphologically distinct from apoptosis, but has not been characterized. The aims of the current study were to document the ultrastructural appearance of dying hypertrophic chondrocytes, and to establish a culture system in which the mechanism of their death can be examined.
  
  
  METHODS
  Growth cartilage from fetal and growing postnatal horses was examined by electron microscopy. Chondrocytes were isolated from epiphyseal cartilage from fetal horses and grown in pellet culture, then examined by light and electron microscopy, and quantitative polymerase chain reaction.
  
  
  RESULTS
  In tissue specimens, it was observed that dying dark chondrocytes underwent progressive extrusion of cytoplasm into the extracellular space, whereas light chondrocytes appeared to disintegrate within the cellular membrane. Pellets cultured in 0.1% fetal calf serum (FCS) contained dying light and dark chondrocytes similar to those seen in vivo. Transforming growth factor-beta1 or 10% FCS increased the proportion of dark cells and induced cell death. Triiodothyronine increased the differentiation of dark and light cells and induced their death. Dark cells were associated with higher levels of matrix metalloproteinase-13 expression than light cells, and light cells were associated with higher levels of type II collagen expression.
  
  
  CONCLUSIONS
  Light and dark hypertrophic chondrocytes each undergo a distinctive series of non-apoptotic morphological changes as they die. Pellet culture can be used as a model of the two forms of physiological death of hypertrophic chondrocytes.
authors.affiliation: School of Veterinary Science, University of Melbourne, Parkville, Victoria 3010, Australia.
authors.forename: Y A
authors.initials: YA
authors.lastname: Ahmed
authors.forename: L
authors.initials: L
authors.lastname: Tatarczuch
authors.forename: C N
authors.initials: CN
authors.lastname: Pagel
authors.forename: H M S
authors.initials: HM
authors.lastname: Davies
authors.forename: M
authors.initials: M
authors.lastname: Mirams
authors.forename: E J
authors.initials: EJ
authors.lastname: Mackie
mesh_terms: D000818:Animals; D017209:Apoptosis; D016923:Cell Death; D019902:Chondrocytes / Q000648:ultrastructure*; D024043:Collagen Type II / Q000378:metabolism; D006132:Growth Plate / Q000648:ultrastructure*; D006736:Horses; D053509:Matrix Metalloproteinase 13 / Q000378:metabolism; D008854:Microscopy, Electron; D053773:Transforming Growth Factor beta1 / Q000378:metabolism
chemical_list: D024043:Collagen Type II; D053773:Transforming Growth Factor beta1; D053509:Matrix Metalloproteinase 13