Q. Chen et al., PROGRESSION AND RECAPITULATION OF THE CHONDROCYTE DIFFERENTIATION PROGRAM - CARTILAGE MATRIX PROTEIN IS A MARKER FOR CARTILAGE MATURATION, Developmental biology, 172(1), 1995, pp. 293-306
During endochondral bone formation, chondrocytes in the cartilaginous
anlage of long bones progress through a spatially and temporally regul
ated differentiation program before being replaced by bone. To underst
and this process, we have characterized the differentiation program an
d analyzed the relationship between chondrocytes and their extracellul
ar environment in the regulation of the program. Our results indicate
that, within an epiphyseal growth plate, the zone of proliferating cho
ndrocytes is not contiguous with the zone of hypertrophic chondrocytes
identified by the transcription of the type X collagen gene. We find
that the postproliferative chondrocytes which make up the zone between
the zones of proliferation and hypertrophy specifically transcribe th
e gene for cartilage matrix protein (CMP). This zone has been termed t
he zone of maturation. The identification of this unique population of
chondrocytes demonstrates that the chondrocyte differentiation progra
m consists of at least three stages. CMP translation products are pres
ent in the matrix surrounding the nonproliferative chondrocytes of bot
h the zones of maturation and hypertrophy. Thus, CMP is a marker for p
ostmitotic chondrocytes. As a result of the changes in gene expression
during the differentiation program, chondrocytes in each zone reside
in an extracellular matrix with a unique macromolecular composition. C
hondrocytes in primary cell culture can proceed through the same diffe
rentiation program as they do in the cartilaginous rudiments. In cultu
re, a wave of differentiation begins in the center of a colony and spr
eads to its periphery. The cessation of proliferation coincides with t
he appearance of CMP and eventually the cells undergo hypertrophy and
synthesize type X collagen. These results reveal distinct switches at
the proliferative-maturation transition and at the maturation-hypertro
phy transition during chondrocyte differentiation and indicate that ch
ondrocytes synthesize new matrix molecules and thus modify their preex
isting microenvironment as differentiation progresses. However, when '
'terminally'' differentiated hypertrophic chondrocytes are released fr
om their surrounding environment and incubated in pellet culture, they
stop type X collagen synthesis, resume proliferation, and reinitiate
aggrecan synthesis. Eventually they cease proliferation and reinitiate
CMP synthesis and finally type X collagen. Thus they are capable of r
ecapitulating all three stages of the differentiation program in vitro
. The data suggest a high degree of plasticity in the chondrocyte diff
erentiation program and demonstrate that the progression and maintenan
ce of this program is regulated, at least in part, by the extracellula
r environment which surrounds a differentiating chondrocyte during end
ochondral bone formation. (C) 1995 Academic Press, Inc.