Anatomy of the Physis

Gross Anatomy

• Five regions characterize long bones: the bulbous, articular cartilage-covered ends (epiphyses) tapering to the funnel-shaped metaphyses, with the central diaphysis interposed between the metaphyses. 
• During growth, the epiphyseal and metaphyseal regions are separated by the organized cartilaginous physis, which is the major contributor to longitudinal growth of the bone.
• Epiphysis incorporates the growth plate or physis and the secondary ossification center
• The larger long bones (clavicle, humerus, radius, ulna, femur, tibia, and fibula) have physes at both ends, whereas the smaller tubular bones (metacarpals, metatarsals, and phalanges) usually have a physis at one end only.
• At birth, with the exception of the distal femur and occasionally the proximal tibia, all of the above-mentioned epiphyses are purely cartilaginous. 
• At various stages of postnatal growth and development, a secondary ossification center forms within the epiphysis. 
• This development helps define the radiolucent zone of the physis, which persists until the physis closes at skeletal maturation.

 Microscopic Anatomy

• Zones RPM DC = Resting, Proliferative, Maturation, Degeneration, Calcification 
• Growth plate chondrocytes are organised into different zones with each cell population being part of a different stage of maturation in the endochondral sequence 
• The physis is divided into four zones from the center of the epiphysis to the metaphysis: germinal, proliferative, hypertrophic, and provisional calcification (or enchondral ossification).

Microscopic anatomy of the physis

 Reserve Or Resting Zone 

• Located in a thin layer at the epiphyseal pole of the growth plate
• Show relatively little evidence of metabolic activity
• The functions have not been clearly established, but they may serve as stem cells for the proliferative zone.

Proliferative Or Columnar Zone

• Chondrocytes rapidly divide, synthesize new matrix, & assume a highly oriented flattened disc shape. 
• In rapidly growing bones they create long columns of highly ordered cells that resemble stacks of plates resulting in longitudinal growth. 
• The zone is the true germinal layer of the growth plate, with cells actively dividing 
• Type II collagen synthesis and mRNA expression increase in this zone.

Hypertrophic Zone 

• Cell size abruptly increases and the columnar arrangement is less regular.
• Increasing vacuolation of the cells.
• Chondrocytes become swollen and vacuolated in process of maturation leading to cell death.
• Overall matrix synthesis per cell increased approximately 3-fold, compared to the proliferative zone.
• The main matrix components synthesised are types II and X collagen and Aggrecan.
• The hypertrophic zone is divided into 3 zones: 

1. Maturation zone - preparation of matrix for calcification 
2. Degeneration zone - cell deterioration & death (glycogen from anaerobic glycolysis depleted) 
3. Provisional Calcification zone - chondroid matrix becomes impregnated with calcium salt from mitochondria from destroyed cartilage cells. 

• Widened in: Rickets, SCFE
• Physeal fractures occur through the zone of provisional calcification.