elastic cartilage

**Histology** - Chondrocytes sit in lacunae within a matrix rich in type II collagen, proteoglycans, and a dense network of elastic fibers (elastin + fibrillin). - Elastic fibers stain with special stains: orcein, Verhoeff-van Gieson (VVG), or resorcin-fuchsin. They are not well seen on routine H&E. - A perichondrium is present (like hyaline cartilage, unlike articular cartilage and fibrocartilage). Growth is mainly appositional from the perichondrium, with some interstitial growth. - It is avascular. It receives nutrients by diffusion from perichondrial vessels.

**Locations (mnemonic: "EEE": Ear, Eustachian tube, Epiglottis)** - Auricle (pinna) of the external ear - External auditory canal (lateral portion) - Eustachian (auditory) tube - Epiglottis - Corniculate and cuneiform cartilages of the larynx. The arytenoid apex and vocal process are also elastic, but the rest of the arytenoid is hyaline.

**Comparison of cartilage types** - Hyaline: type II collagen; most common; fetal skeleton template, growth plate, articular surfaces, trachea, bronchi, nose, costal cartilages, most laryngeal cartilages (thyroid, cricoid). Can calcify with aging. - Elastic: type II collagen plus elastic fibers; flexible; generally does not calcify. - Fibrocartilage: type I (and some type II) collagen; no perichondrium; found in intervertebral discs (annulus fibrosus), pubic symphysis, menisci, and the TMJ disc. It resists tension and compression.

**Clinical significance** - Cartilage is avascular and has few chondrocytes, so healing is poor. Damage is often repaired with fibrous tissue or fibrocartilage. - Cauliflower ear: trauma to the auricle causes a subperichondrial hematoma. The hematoma separates the perichondrium from the cartilage and cuts off nutrient supply, causing necrosis and neocartilage formation. Treat with prompt drainage and a pressure dressing. - Relapsing polychondritis: autoimmune inflammation of cartilage. It typically involves the ears (spares the lobule, which has no cartilage), nose (saddle-nose deformity), and tracheobronchial tree. It can also involve the joints and the eyes. - Epiglottitis: infection and swelling of the epiglottis (classically H. influenzae type b in unvaccinated children). It causes a "thumbprint sign" on lateral neck X-ray and risks airway obstruction. - Chondrocytes are derived from mesenchymal stem cells. Chondroblasts secrete the matrix. - Elastic fibers are formed by elastin on a fibrillin microfibril scaffold. Fibrillin-1 mutation causes Marfan syndrome. Elastin defects are associated with cutis laxa and supravalvular aortic stenosis (Williams syndrome).