THE GROWTH PLATE
The growth plate as depicted on the home page and here is where linear bone growth occurs in the mammalian skeleton. It is a complex and dynamic structure within which endochondral ossification occurs. The following illustrations are designed to acquaint the browser with the growth plate. They include its relationships to the growing bone and to the developing skeleton of a 17.5 day old mouse embryo, growth plate anatomy and growth plate dynamics.
 

 
Relationships

Growth plates arise as embryonic bones form. They reside near the ends of bones and are responsible for linear bone growth until the skeletal reaches its final size. 

 
 

Anatomy

Relevant cells are listed on the left and functional landmarks on the right. Chondrocytes arising at the leading edge synthesize cartilage template which is modified by hypertrophic cells, invaded by perivascular cells at the ossifiction front and replaced by bone at the trailing edge.
 
Dynamics

 If one takes a snapshot of the growth plate, stopping it in time, one finds the highly ordered structure typically seen in textbooks and highlighted here in white. However, in reality, the growth plate is comprised clones of chondrocytes at different stages of differentiation. The yellow cells are differentiated chondrocytes that serve as stem cells for clonal expansion (green cells), blue-green cells are postmitotic cells often called prehypertrophic cells and blue cells are hypertrophic or terminally differentiated chondrocytes. This illustration attempts to show the life histories of these cells during their tenure in the growth plate. Cell types are also identified in the growth plate anatomy illustration. 
FLUORESCENT REPORTER MICE

Transgenic mice have been engineered to "report" key events in endochondral ossification using autofluorescent proteins driven by relevant promoters.  The Col2-GFP mouse shows expression of collagen II,  marker of the differentiated chondrocyte phenotype.  GFP fluorescence proves a means to image skeletal development in live embryos, a way to study chondrogenesis in growing bones and a basis for chondrocyte analysis and purification.
 

 

Col2-GFP mouse embryos

The GFP fluorescent reporter is expressed in cells of cartilaginous skeleton in E14.5 (left) and E17.5 (right) mouse embryos.  Fluorescence is brightest in structures exhibiting the highest level of chondrogenesis, such as external ears and long bones of the extremities of the younger embryo.  Some structures in the older embryo, such as most of the spine, the posterior ribs and central portions of limb bones  no longer show fluorescence because the cartilage has been converted to bone. 
"Chondro-mouse"

 
lower extrm
Col2-GFP mouse embryo tibia
Confocal optical sectioning of tibia from E17.5 embryo shows epiphyseal cartilages and growth plates on right and higher magnification of proximal growth plate on left.

 

Chondrocyte analysis
and isolation by FACS
 
Cartilaginous ribs of newborn Col2-GFP pup (left) provides source of chondrocytes for FACS analysis of sorting into relatively pure populations (right) of chondrocytes. 
 ribs
 

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