fibrocartilage

**Histology/Composition:** Fibrocartilage is intermediate between dense regular connective tissue and hyaline cartilage. Its matrix has thick bundles of **type I collagen** (tensile strength) plus **type II collagen** and proteoglycans (aggrecan; compression resistance). Chondrocytes are sparse and arranged in rows/columns between collagen bundles. It is **acidophilic** on H&E because of the abundant collagen. It has **no perichondrium**, unlike hyaline and elastic cartilage. It is avascular (or poorly vascularized), so nutrients diffuse and healing is limited.

**Locations (high-yield):** - Annulus fibrosus of intervertebral discs (the nucleus pulposus is a remnant of the notochord, with a gelatinous matrix of proteoglycans and type II collagen) - Menisci of the knee (medial and lateral) - Pubic symphysis - Articular disc of the temporomandibular joint (TMJ) and sternoclavicular joint - Glenoid and acetabular labra - Triangular fibrocartilage complex (TFCC) of the wrist - Insertion sites of tendons and ligaments onto bone (entheses)

**Comparison with other cartilage:** - Hyaline cartilage: type II collagen, has a perichondrium (except articular surfaces); found in the trachea, costal cartilage, epiphyseal plate, and articular surfaces. It is the template for endochondral ossification. - Elastic cartilage: type II collagen plus elastin, has a perichondrium; found in the epiglottis, external ear, and Eustachian tube. - Fibrocartilage: type I and II collagen, **no perichondrium**.

**Clinical significance:** - **Meniscal tears:** Common in athletes after a twisting injury to a flexed knee. They cause joint-line tenderness, locking or catching, and a positive McMurray test. The medial meniscus is injured more often (it is less mobile and attached to the MCL). The peripheral "red zone" has blood supply and can heal or be repaired; the inner "white zone" is avascular and heals poorly. The "unhappy triad" involves the ACL, MCL, and medial meniscus. - **Disc herniation:** The nucleus pulposus herniates through a tear in the annulus fibrosus, usually posterolaterally at L4-L5 or L5-S1, because the posterior longitudinal ligament is weaker laterally. It compresses nerve roots and causes radiculopathy (for example, L5-S1 herniation affects the S1 root, causing a diminished ankle reflex). - **Pregnancy:** Relaxin loosens the pubic symphysis, which can cause separation and pain. - **Repair:** Injured fibrocartilage heals with scar-like fibrous tissue and typically does not regenerate normally. - **Osteoarthritis:** Meniscal degeneration contributes to joint degeneration. - **Collagen disorders:** Defects in type I collagen (osteogenesis imperfecta) affect tissues rich in type I collagen.

**Mnemonic:** Fibrocartilage = "**PIT**" for Pubic symphysis, Intervertebral disc (annulus), and TMJ/menisci. Type I collagen is "bone" and type II is "carTWOlage," so fibrocartilage is a mix of both.