Cartilage and Bone histology Flashcards
Where does cartilage come from?
Cartilage derives from the lateral plate mesoderm, somites and neural crest. The tissues form mesenchyme before differentiating.
Discuss differentiated cartilage
Differentiated cartilage consists of chondrocytes embedded in lacunae in a firm, resilient avascular matrix. In life, each chondrocyte entirely fills its lacuna, but this may not appear to be the case in preserved tissue. The chondrocytes contain organelles typical of protein-secreting cells, as well as some lipid droplets and glycogen.
What does matrix consist of?
The matrix consists of collagen fibers (± elastic fibers) and a firm, well-hydrated ground substance. The collagen fibers (mostly Type II) are very fine and have the same refractive index as the ground substance, so are usually not discernible in LM.
What composes ground substance? What is the most important protein we find here?
The ground substance is composed of hyaluronic acid and proteoglycans composed of glycosaminoglycans (GAGs) (hylaluronan, chondroitin sulfates and keratan sulfate) linked to core proteins. Most important proteoglycan linking protein is aggrecan.
Discuss Hyaline cartilage and where we find it
Most widely distributed type of cartilage:
a. Serves as template for development of many bones,
b. Forms smooth articular surfaces of bones;
c. Provides firm, flexible support for respiratory passages.
How is Hyaline cartilage formed?
Formation:
a. Mesenchymal cells aggregate, round up, become chondroblasts.
b. These secrete fibers and ground substance until each is isolated (chondrocytes).
c. Connective tissue condenses around mass of cartilage, forming perichondrium.
Discuss growth of hyaline cartilage (both types)
a. Appositional: perichondrial cells differentiate into chondroblasts.
b. Interstitial: chondrocytes divide to form isogenous groups. Each member secretes pericellular matrix, separating it from sister cells.
Discuss how hyaline cartilage changes with time
a. Interstital growth slows.
b. Proportion of glycosaminoglycans shifts to more highly sulfated forms, flexibility declines.
c. Fibrosis, cavitation, calcification may occur. Osteoarthritis is a common degenerative joint disease.
Discuss elastic cartilage and where we find it
Elastic cartilage: contains high content of elastic fibers (made by chondrocytes); found in external ear, eustachian tube, epiglottis and larynx.
Discuss the structure and location of fibrocartilage
- Cells in lacunae, often in rows; high content of collagen, much of it Type I; bundles visible in LM; fibrocartilage has no perichondrium; merges into dense CT or hyaline cartilage.
- Found in intervertebral discs, interarticular cartilages and where some ligaments attach to bone.
What are the two ways collagen is organized in bone?
Primary (woven) and secondary (lamellar) – refer to organization of collagen fibers in bone matrix – unorganized in primary bone, organized in layers in secondary bone
Where do osteocytes come from and where do we find them?
Former osteoblasts that are trapped in lacunae within and between lamellae (layers) of bone matrix.
How do osteocytes communicate?
Communicate via canaliculi with each other (cytoplasmic extensions linked by gap junctions) and with perivascular space (never more than 0.1 to 0.2 mm away).
Discuss the structure of compact bone
Composed of lamellae in columns or sheets
Columns (osteons or Haversian systems) each have a central Haversian canal containing vessels and nerves surrounded by concentric lamellae; columns branch; canals communicate with the marrow cavity and the outer surface of the bone via narrow Volkmann’s canals.
Circumferential lamellae often form the outermost and innermost layers of gross bones.
Where are osteogenic cells?
Outer surface of compact bone covered by periosteum of dense connective tissue; outer layer fibrous, inner layer more cellular and contains osteogenic cells
Epiphyseal cartilage develops zones. Moving towards primary ossification center, these are the zones:
i. Resting zone – slowly growing hyaline cartilage; farthest from ossification center.
ii. Zone of proliferation – chondrocytes rapidly dividing, form elongated groups.
iii. Zone of hypertrophy – lacunae and cells enlarge.
iv. Calcification zone – calcium deposited in cartilage matrix, chondrocytes die.
v. Ossification zone – blood vessels invade and erode spaces in calcified cartilage; spicules of calcified cartilage are coated with bone matrix.
What are Sharpey’s fibers?
Attachment of tendons and ligaments to bone is by collagen fibers (Sharpey’s fibers) that are encased in bone as it grows
Growth hormone on bone growth
Growth hormone: acting both directly and through somatomedins, promotes cartilage growth; excess produces gigantism or acromegaly; absence, dwarfism
Thyroid hormone on bone growth
Thyroid hormones: needed for normal skeletal growth. Stimulate bone turnover for remodeling
Sex hormones on bone growth
Sex hormones: promote both cartilage growth and bone maturation, enhance production of active form of vitamin D. Estrogen inhibits bone resorption via indirect effect.
Parathyroid Hormone on bone growth
Parathyroid hormone: secreted by parathyroid chief cells in response to lowered blood calcium; promotes Ca++ release from bone, increases in activity, size and number of osteoclasts, and promotes Ca++ retention by kidney. Indirectly increases calcium absorption from gut.
Calcitonin on bone growth
Calcitonin (thyrocalcitonin): secreted in response to high blood calcium; inhibits bone resorption by osteoclasts and inhibits the development of new osteoclasts.
Vitamin C on bone growth
Vitamin C: essential for production of normal collagen; absence affects growth of both bone and cartilage and results in scurvy; bones thin and brittle.
Discuss Vitamin D and how it/products are related to bone growth.
Vitamin D (cholecalciferol): Vitamin D is formed in the skin by irradiation of 7-dehydrocholesterol. This is converted to 25-hydrocholecalciferol in the liver which is converted in the kidney to1,25 dihydroxycholecalciferol. Necessary for uptake and transport of calcium. Required for mineralization of new bone matrix
