Zones of the growth plate
- Reserve zone
- Proliferative zone
- Hypertrophic zone: Maturation, Degeneration, Provisional calcification
- Primary spongiosa
- Secondary spongiosa
Weakest link of the growth plate-need to decrease shearing stress
Mamillary processes - Microscopic irregularities
Undulations -Macroscopic contouring
Periphery of the physis
Groove of ranvier
- Wedge of cells laterally, supplies chondrocytes to the periphery of the growth plate for lateral growth [Increases the width of the growth plate]
Perichondral ring of lacroix
- Dense fibrous band at the periphery growth plate, anchors and supports the physis
Resting zone, involved in matrix production, storage lipids, glycogen and proteoclygan aggregates.
Contains germinal cells [stem cell population] existing singly or in pairs separated by an abundant extracellular matrix and not clearly ordered in columns
Cells are in a relatively quiescent state
Low PO2 tension as epiphyseal arteries pass through this region but do not form terminal capillaries.
Injury to this layer results in cessation of growth
Chondrocytes are highly ordered in columns directed along the axis of growth of the long bone.
Longitudinal growth occurs with stacking of chondrocytes
The top cell is the dividing mother cell or progenitor cell on which the entire growth in length of bone depends
This zone is involved with cell division and matrix production
High PO2 tension and high proteogylcan concentration [inhibits minerisation]
Failure of progenitor cells to thrive results in termination of growth at the long bone end
Zone involved in the maturation of cells
Cells abruptly increase in size [x5], accumulate calciumin their mitochondria and then die releasing calcium from matrix vesicles[ which allows calcification of the matrix]
Columns of cartilage cells extend toward the metaphysis being constantly lengthened by cell division occurring at the base[Proliferative zone]
Cells nearer the metaphysis begin to undergo changes that ultimately lead to their destruction
Low PO2 tension
The rate of chondrocyte maturation is regulated by systemic hormones and local growth factors
Marked increase in alkaline phosphatase enzyme activity
This enzyme increases the concentration of phosphate ions, which are required in the calcification process
In hypophosphatasia there is an absence of alkaline phosphatase and deficient mineralization of the matrix resulting in widening of the growth plate
Physeal fractures are classically believed to occur through the zone of provisional calcification [within the hypertrophic zone]
Although it complicates matters slightly it is probably best to learn the three zones of subdivision of this layer
Preparation of matrix for calcification
Cell deterioration and death
Provisional calcification zone
The last two or three cells in the column of cartilage cells are in the zone of provisional calcification.
Ingress of vascular channels and bone-marrow stromal cells
Calcified cartilage bars resorbed by osteoblasts, formation woven bone
Remodelling to lamellar bone
Growth plate signalling pathways
- The height of the growth plate is tightly controlled by a local feedback loop between parathyroid hormone-related protein (PTHrP) and Indian hedgehog (IHH).
- PTHrP produced at the resting zone suppresses the differentiation of chondrocytes while IHH produced at the maturation zone stimulates the differentiation of chondrocytes and positively regulates the production of PTHrP.
Fibroblast growth factor (FGF) signalling mediated by FGFR3 is a negative regulator for chondrocytic proliferation.
BMP-2 expressed within the perichondrium promotes proliferation of chondrocytes through BMP type I receptors
- IHH and BMP signalling pathways act in parallel to induce chondrocyte proliferation and differentiation. In contrast, FGF signalling inhibits BMP signalling and negatively regulates IHH expression, suggesting that BMP signalling coordinately works with PTHrP/IHH and FGF signalling to control the balance between proliferation and differentiation of chondrocytes.
- PTHrP also acts on osteoblasts and osteoclasts in the calcifying zone, thus also enhancing bone formation and remodelling the newly constructed metaphysis.