Cartilage and Bone Microanatomy

Cartilage, bone, and blood are all considered to be...

specialized connective tissues

Cartilage

an avascular tissue consisting of chondrocytes and extracellular matrix (ECM)

How do cartilage cells receive nourishment?

via diffusion of nutrients through the matrix from surrounding blood vessels

What makes cartilage solid yet pliable?

high glycosaminoglycan (GAG) to type II collagen in ECM

What are the three types of collagen?

Hyaline
Elastic
Fibrocartilage

What makes up hyaline cartilage?

type II collagen

What makes up elastic cartilage?

type II collagen and elastic fibers

What makes up fibrocartilage cartilage?

type II and I collagen

What is the most abundant cartilage type and appears glassy and bluish in life?

Hyaline cartilage

Where is hyaline cartilage located?

Articular ends of long bones
Respiratory tract
Ribs (ventral ends)
Temporary skeleton

The airway of the trachea is held open by what?

rings of hyaline cartilage

Perichondrium

a layer of dense irregular connective tissue that surrounds cartilage

What are the two laters of the perichondrium?

Outer fibrous layer
Inner chondrogenic layer

Outer Fibrous Layer

contains fibroblasts that produce the type I collagen on the outer surface of the perichondrium

Inner Chondrogenic Layer

- contains fibroblast-like cells that can differentiate into chondroblasts, initiate matrix production (type II collagen) and become immature chondrocytes

Appositional Growth

is the process by which old bone that lines the medullary cavity is reabsorbed and new bone tissue is grown beneath the periosteum, increasing bone diameter

Growth in width due to bone formation by osteoprogenitor cells within the inner periosteum around the diaphysis

Chondrocytes

cells within lacunae inside the cartilage that occur singularly or in clusters called isogenous groups. Mitotic division leads to isogenous groups – interstitial growth

The matrix of hyaline cartilage is composed mainly of what?

composed mostly of type II collagen and a ground substance of proteoglycans

Territorial matrix

basophilic region around chondrocytes

Interterritorial matrix

less intensely stained area between isogenous groups of chondrocytes

What is the predominant collagen type of the fibrils of the hyaline cartilage matrix?

Type II collagen

What percentage of the matrix is intercellular water?

60-80%

What three types of GAGs are found in the ground substance?

chondroitin sulfate, keratin sulfate, hyaluronan

Where is elastic cartilage found?

found in external ear (auricle and external auditory meatus), auditory (eustacian) tube, epiglottis and cartilage of the larynx

What is found in the ECM of elastic cartilage?

abundant elastic fibers in addition to collagen type II fibrils

What is present in elastic cartilage?

Perichondrium present

Where is fibrocartilage located?

Intervertebral disc (annulus fibrosus), pubic symphysis, meniscus of the knee, and may be found at the insertions of tendons and ligaments

Fibrocartilage is a hybrid between what?

dense connective tissue and cartilage

What does the ECM of fibrocartilage contain?

contains numerous collagen type I fibers in addition to type II, often forming parallel bundles in direction of stress in addition to the ECM components of hyaline cartilage

Fibrocartilage does NOT contain...

Perichondrium

What are the two modes of hyaline cartilage growth?

Appositional and Interstitial growth

Interstitial Growth

Growth in length due to growth of epiphyseal plate, i.e., interstitial growth of cartilage followed by endochondral ossification

Degeneration of hyaline cartilage

chondrocytes hypertrophy and die, the matrix calcifies

What is the function of bone tissue?

Forms system of levers for musculoskeletal movement
Serves as mineral reservoir for calcium and phosphorus
Bone is well-vascularized (unlike cartilage)
Red marrow is site of hematopoiesis
Composed of both cells and matrix

Bone is characterized by a _______ extracellular matrix which allows it to provide both support or the entire body and protection for internal organs.

mineralized

What is the predominant structural protein of bone?

type I collagen

What does the ground substance of bone contain?

non-collagenous proteins, proteoglycans and glycoproteins

Osteoid

collagen and the ground substance prior to mineralization

What do osteoblasts release?

release alkaline phosphatase and matrix vesicles

Osteoblasts

secrete osteoid (organic matrix); they are located at bone surfaces; they are cuboidal or columnar shaped cells

Osteocytes

mature bone maintaining cells trapped in lacunae with cytoplasmic processes in canaliculi

Osteoclasts

large multinucleated cells responsible for bone resorption

Osteoprogenitor cells

have a basophilic cytoplasm, have a fibroblast-like appearance and give rise to osteoblasts

What do osteocytes develop from?

osteoblasts

What cells resorb bone matrix, creating depressions in the bone surface?

osteoclasts

Resorption bays (Howship's lacunae)

depressions in the bone surface

Osteoclasts are derived from the fusion of what?

mononuclear hemopoietic cells

What dissolves the inorganic matrix?

H+ (via an H+ ATPase) and Cl-

What degrades the organic matrix (collagen and other proteins)?

Lysosomal enzymes

Diaphysis

shaft of a long bone

Epiphysis

proximal and distal ends

Metaphysis

flares out toward the epiphysis

Epiphyseal Line

between the metaphysis and the epiphysis

Marrow Cavity

the central cavity of bone shafts where red bone marrow and/or yellow bone marrow (adipose tissue) is stored

Articular Cartilage

the smooth, white tissue that covers the ends of bones where they come together to form joints

Periosteum

a fibrous connective tissue that surrounds the outer surface of the non-articulating parts of the bone and has 2 layers

What are the two layers of the periosteum?

An outer fibrous layer of dense C.T.
An inner cellular layer of osteoprogenitor cells, can differentiate into osteoblasts

What nerves are a source of pain from a bone fracture?

Periosteal nerves

Endosteum

lines internal surfaces as a thin layer of osteoprogenitor cells

Nutrient foramina

nutrient vessels for the bone

Compact (cortical) Bone

very dense area without cavities, provides strength to the outer portions of bones

Spongy (cancellous, trabecular) Bone

numerous trabeculae (bridges) and spicules separated by marrow cavities

Osteoporosis

characterized by progressive loss of bone density - loss of both organic matrix and the mineral component of bone

What two structures constitute the Osteon?

Concentric lamellae and the Haversion canal

Haversion canal

any of the minute tubes which form a network in bone and contain blood vessels

Osteon

the chief structural unit of compact (cortical) bone

Canaliculi

small canals permitting the extended cytoplasmic processes of osteocytes to contact other osteocytes and communicate via gap junctions

Volkmann's canals

provide communication between Haversian systems, and between Haversian systems and marrow cavity

Run perpendicular to Haversian systems

Interstitial lamellae

fill the spaces between osteons

What are the two modes of bone development?

Intramembranous ossification
Endochondral ossification

Intramembranous ossification

Most flat bones (face and skull) of the body are formed first from membranous sheaths that are replaced by bone

Endochondral ossification

Most long bones are formed in the embryo as cartilage; Bone replaces cartilage in long bones

What mode of bone development begins by the condensation of mesenchymal cells?

Intramembranous ossification

Epiphyseal Plate

a thin layer of cartilage that lies between the epiphyses and metaphyses, and is where the growth of long bones takes place

aka Growth Plate

Resting Zone (Zone of Reserve)

typical hyaline cartilage, no active matrix production

Proliferative Zone

chondrocytes proliferate, form columns, produce matrix

Zone of Hypertrophy

chondrocytes enlarge, become vacuolated, matrix is compressed into linear bands between chondrocyte columns

Zone of Calcification

remaining cartilage matrix calcifies, the chondrocytes degenerate; usually appears basophilic

Zone of Ossification/Resorption

nearest diaphysis, calcified cartilage in contact with marrow cavity, blood vessels invade

Internal Remodeling

Continuous resorption and deposition of bone

Primary (woven) Bone

Immature bone; characterized by an irregular array of collagen fibers, a low mineral content and a greater number of osteocytes; fewer osteocytes per unit area; osteons w/ concentric lamellae; interstitial lamellae

In adults, where is primary bone typically found?

in the tooth sockets, near the sutures of the flat bones of the skull and at the insertions of some tendons

Secondary (lamellar) Bone (Mature Bone)

More typical of adults; Collagen fibers are numerous and organized into parallel lamellae or concentric lamellae around blood vessels; more cells; matrix is not layers

What hormone raises serum Ca2+ levels by increasing bone resorption?

Parathyroid hormone (PTH)

What inhibits osteoclasts, leading to decreased bone resorption, preventing Ca2+ release from bone, and decreasing serum Ca2+?

Calcitonin

What increases Ca2+ absorption in the gut to raise serum levels?

Vitamin D

With osteoporosis, there is an increased risk of fracture of what body parts?

Hips, wrist, and vertebral column

What are what's to prevent and treat osteoporosis?

Weight-bearing exercise
Adequate vitamin D
Adequate dietary calcium

Osteomalacia

abnormal softening of bones caused by deficiencies of phosphorus or calcium or vitamin D

Osteomalacia leads to deficient absorption of what in the gut?

calcium and phosphorus

Rickets

a disease of children caused by vitamin D deficiency, characterized by imperfect calcification, softening, and distortion of the bones typically resulting in bow legs

How long does fracture repair take?

6-12 weeks

Vitamin C Deficiency (Scurvy)

a severe lack of vitamin C in the diet; w/o vitamin C matrix production stops which distorts cartilage columns in epiphyseal plates

Vitamin C is an enzyme cofactors required for the synthesis of what?

Collagen