Cartilage and bone

Perichondrium

Cellular dense connective tissue layer at surface which gives rise to new chondrocytes. Articular surface of joint cartilages lacks perichondrial layer (partly why poor repair after injury)

Growth of epiphyseal plate

Chondrocytes in deeper layers of epiphyseal cartilage can proliferate to produce elongation. As new cells are produced programmed to be replaced by bone. Series of zones. Growth controlled by genetics, hormones, growth factors.

Primary lamellae

Lamellae of complete osteons

Territorial matrix

Layer which surrounds chondrocytes in cartilage. Basophilic due to proteoglycans.

Osteons

Laying bone down within previous sites of erosion so that lamellae laid down from the outside in. Newest are immediately surrounding the Haversion canal and oldest at the perimeter.

Periosteum

Lines the outside of the long bone. Contains osteogenic layer.

Hyaline cartilage structure

Protein core with chondroitin sulfate GAGs and keratin sulfate GAGs which make an aggrecan monomer. Pale, hydrated homogeneous matrix with anionic (acidic) GAGs predominate and impart light basophilia. Collagen type II fibrils form network which is not visible with standard staining. Shock absorber due to hydration of GAGs.

Thyroid hormone in cartilage

Regulates cartilage hypertrophy and cell death

Osteocytes

Resident cells of bone. Reside in lacunae surrounded by bone matrix they produce. Send spider-like cytoplasmic processes into tiny canaliculi in matrix. Communicate in a network with other osteocytes via gap junctions. Function in turnover and maintenance of matrix by sensing mechanical stress on bone. Have ability to deposit or remove calcium from bone (for homeostasis).

Chondrocytes

Resident cells of cartilage

Secondary lamellae

Residual lamellae between fully formed primary osteons

Osteoporosis

Resorption exceeds new bone formation, bone develops resorption cavities, and loses structural integrity. Less bone density, loss of strength and fractures. Note: Exacerbated by exogenous steroids

Organic bone matrix

Rich in type I collagen and accounts for eosinophilia of demineralized preparations

How cessation of bone growth occurs

Secondary centers of ossification are a prelude to the cessation of growth when the entire growth plate is replaced by bone.

Diaphysis

Shaft of the long bone, connected to the epiphysis by the metaphysis region.

Endosteum

Single layer of osteogenic cells lining the marrow cavity

Resorption cavities

Sites of bone removal that create larger spaces intersecting the boundaries of osteons. Often cross sections of tunnels called cutting cones in which osteoclasts remove bone at the tip of the cone.

Lacunae

Spaces where chondrocytes and osteocytes are found

Trabecular bone

Thin anastamosing (branches that come together again) trabeculae which contain entrapped osteocytes. Layer of osteoblasts cover surface producing osteoids on the surface of mineralized trabeculae. Osteogenic cells resemble low cuboidal epithelium (but they aren't). Osteocytes nourished by diffusion from surface through canalicular channels b/c trabeculae thin and surrounded by sinusoidal capillaries of marrow cavity.

3 causes of osteoporosis

1. Decreased estrogen and bone formation 2. Decreased vitamin D and calcium absorption in alcoholic cirrhosis 3. Decreased vitamin D, hypercalcemia , secondary hyperparathyroidism in renal disease

2 forms of cartilage growth

1. Interstitial 2. Appositional

Process of long bone formation (3)

1. Long bones first formed from cartilage models in fetus 2. Epiphyseal cartilages permit joints to function while bones grow 3. Intramembranous bone formation in the shaft lends support during growth

5 zones of the epiphyseal plate

1. Reserve cartilage 2. Proliferation 3. Hypertrophy 4. Calcified cartilage 5. Vascular invasion, bone deposition, and resorption (osteoblasts in the marrow cavity lay down bone on the residual calcified cartilage)

Osteophytes

AKA bone spurs. Bony projections that form along joint margins in deteriorating bones (arthritis).

Osteoarthritis

Very common (85% by age 75). Chondrocyte death and loss of joint cartilage, inflammation of synovial membranes, narrowing of joint space, thickened subchondral bone, development of osteophytes. Painful, nonfunctioning joint.

Osteoblasts

Bone cells active in proliferating/producing new organic bone matrix (osteoid). Found on bone surfaces where they lay down new matrix during appositional growth.

Intramembranous bone formation/ossification

Bone formation in dense connective tissue membranes. Occurs from mesenchymal stem cells which differentiate into osteoprogenitor cells and begin to mineralize their surrounding bone matrix to form bone.

Development of fetal bones/growing epiphyses of long bones

Bone replaces cartilage in a process called endochondral ossification. Cartilage model provides flexible structure for growth (interstitial and appositional) and remodeling of immature skeleton.

Haversion canal

Contains neurovascular bundle of vein, artery, and nerve

Cutting cones

Cross sections of tunnels (resorption cavities) in which osteoclasts spearhead bone removal

Inorganic bone matrix

Crystalline matrix produced by mineralization composed of hydroxyapatite rich in Ca(PO4)

Haversion systems (osteons)

Cylindrical units of compact bone with concentric lamellae organized axis of Haversion canal

Fibrocartilage

Dense network of collagen type I fibers that stain with eosin. Looks like dense connective tissue with differences in lacunar arrangements and fiber arrays which are sometimes pinnate instead of linear.

Compact bone

Dense outer shell with subunits with calcified layers arranged in concentric layers or lamellae around a core of vessels and nerves. May lack regular Haversian systems (woven bone).

Osteoclasts

Develop from monocytes stimulated by cytokines (M-CSF and RANKL) and direct interaction with osteoblasts. Attach to bony surface, remove inorganic matrix, and digest organic matrix. Stimulated by parathyroid hormone through osteoblasts. Inhibited through binding by calcitonin. Create acidic microenvironment for demineralization and lysosomal digestion of organic matrx.

Yellow marrow

Fatty bone marrow (increases with age)

Fracture repair

Fibrin in blood clot at fracture site produces scaffold for organization of fibroblasts (like a scar) which generate cartilaginous callus or union. Soft callous undergoes transformation into vascularized bony callus. Remodeling can make fracture site indistinguishable.

Cartilage

Forms a flexible skeletal support which can grow and provide substrate for bone formation during development. Avascular-lacking capillary network and nourished by diffusion through cartilage matrix. 3 types: Hyaline, elastic, and fibrocartilage.

Hyaline cartilage

Forms the bulk of embryonic skeleton. Supporting role in adults in some locations. Resists compressive forces and provides smooth surfaces for articulation at diarthrodial joints.

IFG-1 and somatomedin

From excess growth and causes acromegaly

Paget's disease

Generalized skeletal disorder associated with thickening and weakening of bones related to disregulation of osteoclastic and osteoblastic activity

Osteomalacia

Group of disorders characterized by failure to form normal mineralized matrix. Unmineralized matrix forms soft bone. In growing skeleton called Rickets.

Isogenous groups

Groups of 2-8 cells within one lacunae suggesting that a cell divided but daughter cells have not yet produced enough matrix to be separated

Bone morphogenetic proteins, IGF-1, Wnt proteins

Growth factors and hormones which regulate the process of osteogenitor differentiation to osteoblasts and osteoblast secretion of inorganic matrix (osteoid)

Epiphyseal plate

Growth plate in the intermediate region of the long bone.

Epiphysis

Head of long bone, usually with expanded surface on which the articular cartilage is attached

Red marrow

Hematopoetic bone marrow

Spongy bone

Interconnected network of trabeculae or bone spicules which provide supporting framework beneath compact bone. Can remodel in response to stress. Marrow permeates between bony spicules.

Osteogenic layer

Inner cellular layer of periosteum which can give rise to new bone on the outside (single layer called endosteum)

Volkmann's canals

Interconnect Haversion canals and appear to be perpendicular to the longitudinal axis of osteons.

Elastic cartilage

Looks similar to hyaline but contains bundles of elastic fibers which need special stains to be visualized. Deformability and resilience.

What is the origin of cartilage and bone tissue?

Mesenchymal

Osteoclast structure

Multinucleated (3-8) giant cells. Found in erosion pits (Howship's lacunae) that they have created on bony trabecular surface of spongy bone or may be active in resorption cavitiies within compact bone.

Appositional growth in cartilage

Occurs from perichondrium where cells on inner surface give rise to new chondrocytes

Interstitial growth in cartilage

Occurs from within by division of chondrocytes and expansion of flexible matrix

Appositional growth in bone

Only form of growth in bone since matrix cannot expand from within. Addition of bone occurs on surfaces (e.g. trabeculae of spongy bone) or on walls of resorption cavities created by osteoclasts within compact bone.

Bone

Support, protection, and attachments for tendons and ligaments. Mineral storage depot, remodels in response to damage/stress, blood cell production. Highly vascular.

Metaphysis region

Where the epiphysis (head) and diaphysis (shaft) of the long bone connect