Cartilage

Key questions

1. Q1What kind of tissue is cartilage?

   A specialized, avascular connective tissue in which the amorphous ECM (GAGs, proteoglycans) predominates over the fibrous ECM (collagen, elastic fibers). It supports, lets joints slide, absorbs shock and drives long-bone growth.
2. Q2What are the four cell types in cartilage?

   Perichondrial fibroblasts, chondroblasts, chondrocytes and chondroclasts (the cartilage equivalent of osteoclasts).
3. Q3What is the order of cells from the surface inward?

   Perichondrium (perichondrial fibroblasts) → chondroblasts → chondrocytes → chondrocytes grouped in isogenic nests (several cells in one matrix "bubble").
4. Q4Which cell has a territorial matrix, and which does not?

   Chondrocytes are surrounded by a territorial matrix; chondroblasts are NOT. On H&E the territorial matrix is a pale ring around each chondrocyte.
5. Q5Territorial vs interterritorial matrix?

   Territorial (right around each chondrocyte) = mostly amorphous components (GAGs). Interterritorial (between chondrocyte groups) = more fibrous components.
6. Q6What is in the amorphous matrix?

   GAGs and proteoglycans (aggrecan, syndecan, fibroglycan) plus glycoproteins (chondronectin, chondrocalcin). Aggrecan molecules attach to a central hyaluronic acid filament to form huge proteoglycan aggregates that hold type II collagen fibrils together.
7. Q7Why can cartilage bend without breaking?

   Hyaluronic acid and the proteoglycans are hydrophilic — they trap water like a sponge, giving resilience. Papain digests the proteoglycans and the cartilage loses its rigidity (the rabbit ear flops).
8. Q8What is the main collagen of cartilage?

   Type II collagen — in all three cartilage types. Type I is added in fibrocartilage; elastic fibers are added in elastic cartilage. Type II deposition marks differentiation and maturation of cartilage.
9. Q9What do chondroclasts do?

   They are multinucleated cells that resorb calcified cartilage (for example during endochondral ossification); they enter the cartilage with invading blood vessels.
10. Q10What are the consequences of cartilage being avascular?

    Nutrients diffuse in from the perichondrium (or synovial fluid), so diffusion limits cartilage thickness, and damaged cartilage regenerates poorly. Microfracture surgery pokes holes into the underlying bone so marrow mesenchymal cells can fill the defect.

Key questions

1. Q11What is appositional growth?

   Growth at the surface by DIFFERENTIATION: perichondrial fibroblasts become chondroblasts, which become chondrocytes. It requires a perichondrium.
2. Q12What is interstitial growth?

   Growth from within by MITOSIS of existing chondroblasts and chondrocytes. It does not require a perichondrium.
3. Q13Which is the main type of cartilage growth?

   Appositional growth.
4. Q14What drives appositional growth?

   The perichondrium releases Sox transcription factors → perichondrial fibroblasts differentiate into chondroblasts → they secrete amorphous matrix → become chondrocytes with a territorial matrix → secrete type II collagen. (Isogenic nests come from interstitial growth, not appositional.)
5. Q15What drives interstitial growth?

   Growth hormone (somatotropin) from the anterior pituitary → somatomedin C (IGF-1) from the liver → mitosis of chondroblasts and chondrocytes.
6. Q16How do isogenic nests form?

   By interstitial growth — a chondrocyte divides within its lacuna, so the daughter cells stay clustered in one matrix bubble.
7. Q17Which hormones and vitamins affect cartilage growth?

   Stimulate: thyroxine, testosterone, GH. Inhibit: cortisol, estradiol. Vitamin A and C deficiency → growth-plate defects; excess vitamin A → accelerated ossification of the growth plate; vitamin D deficiency → cartilage fails to ossify (rickets).
8. Q18What does the perichondrium do besides appositional growth?

   It is vascular, so it supplies nutrients that diffuse into the cartilage, and it produces the Sox factors that make chondroblasts.

Key questions

1. Q19Where does interstitial growth occur?

   Only in cartilage that LACKS a perichondrium: the epiphyseal growth plate, articular cartilage and fibrocartilage.
2. Q20Which cartilage lacks a perichondrium?

   Epiphyseal growth plate, articular cartilage and fibrocartilage. All other cartilage has one.
3. Q21Where exactly in the growth plate does interstitial growth happen?

   The zone of proliferation — chondrocytes stacked in columns dividing under GH/somatomedin C.
4. Q22How is articular cartilage formed?

   By interstitial growth. A perichondrium on a joint surface would be rubbed off as bones slide against each other, so appositional growth is impossible there.
5. Q23A child with low GH is short. Which growth-plate zone is affected?

   The proliferative zone — chondroblasts and chondrocytes there are the target of GH (via somatomedin C).
6. Q24What is special about the collagen in articular cartilage?

   Its type II collagen fibers are arranged in arches, parallel to the surface, suited to resisting shear.

Key questions

1. Q25What is the most common type of cartilage?

   Hyaline cartilage.
2. Q26Where is hyaline cartilage found?

   Articular surfaces, epiphyseal growth plates, costal cartilage, trachea/bronchi, larynx (most), nose, and the fetal skeleton template.
3. Q27What does hyaline cartilage look like?

   Glassy, homogeneous pink-to-blue matrix (type II collagen fibrils are invisible) with chondrocytes in lacunae, often in isogenic nests; perichondrium present except at joints and growth plates.
4. Q28Where is elastic cartilage found and how do you recognize it?

   External ear (auricle), external auditory canal, eustachian tube, epiglottis. Type II collagen plus abundant dark elastic fibers visible around the chondrocytes (elastic stain); has a perichondrium.
5. Q29Where is fibrocartilage found?

   Intervertebral discs (annulus fibrosus), pubic symphysis, menisci, and tendon/ligament insertions.
6. Q30How do you recognize fibrocartilage?

   Chondroblasts and chondrocytes aligned in chains (parallel rows) along lines of stress, surrounded by bundles of type I collagen; no perichondrium.
7. Q31A ruptured disc fragment shows chains of chondrocytes and no perichondrium. Tissue?

   Fibrocartilage.
8. Q32Costal cartilage removed from an athlete shows clusters of chondrocytes in a glassy matrix. Best description?

   Isogenic nests of hyaline cartilage.