Cartilage Age Changes

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Immature articular cartilage is considerably different to adult cartilage, unlike many other tissues.

Gross Inspection

Colour: Immature articular cartilage is blue-white in colour, presumably due to reflection from the vascular structures in the underlying bone.

Thickness: It is relatively thick as it has two functions: serving as an articular surface, and as a microepiphyseal plate for endochondral ossification of the underlying bony epiphyseal nucleus.

Histology

Cellularity: Immature cartilage is more cellular with a higher cellular density than adult cartilage, which is fairly uniform throughout the tissue.

Zones: The zonal regions show changes as well, especially the deeper zones.

• The tangential layer is present in immature cartilage however the cells are larger and less discoid than adult tangential cells.
• The midzone is wider and has a larger number of randomly arranged cells. The lower zones have a different cellular orientation
• Lower zones: midway between the surface to underlying bone the chondrocytes are arranged in irregular columns. With further depth the columnation is more apparent. With even further depth the cells increase in size and have shrunken pyknotic nuclei along with large intracytoplasmic vacuoles containing glycogen. There are vascular buds that invade the cartilage from the underlying bone, resembling the zone of provisional calcification of the epiphyseal plate.

Mitosis: Immature cartilage has increased mitotic figures, with all stages being seen. This is not uniformly present, occuring in two zones

• Subjacent to the surface is where the growth of the cellular component of the articular portion of the cartilage occurs.
• Deep to the above there is a narrow band of cells that resemble the proliferative zone of the microepiphyseal plate of the bony nucleus deep to it.

With development to maturity, the mitotic activity becomes confined to the area just above the zone of vascular invasion in the deep most part of the cartilage. This area now demonstrates diffuse calcification. In the more superficial regions there is no cell replication.

In adulthood, mitosis ceases upon the development of a well-defined calcified zone, tidemark, and closure of the epiphyseal plate.

Composition

See also: Fibrous Connective Tissues, Synovial Joints

Water content: increased in immaturity, and this slowly reduces during growth, and by adulthood it remains relatively constant.

Collagen content: Reduced in immaturity. It increased to adult levels soon after birth, and this is maintained throughout life.

Proteoglycan content: The proteoglycan content is highest level at birth, reduces during growth. The protein core and GAG chains are longer in immaturity. The protein core length reduces likely due to enzymatic cleavage of the proteoglycans near the C-terminal end of the protein core.

The GAG chain lengths reduce, especially chondroitin sulfate. In immaturity there are very high levels of chondroitin 4-sulfate, which rapidly decreases, while chondroitin 6-sulfate increases. With increasing age and the total reduction of chondroitin sulphate, keratan sulfate increases until age 30 reaching 25-35% of the total GAG content. This value remains constant throughout life.

Aggregation reduces with increasing age, not due to a concentration of hyaluronate, but possibly due to alteration of the core protein or link protein. With maturity comes reduces proteoglycan synthesis. With increasing age the chondrocytes become less responsive to anabolic cytokines.

Further Reading

• Martin et al 2001
• Li et al 2013

References

Orthopaedic Basic Science. 2nd edition. Chapter 17.