CT SUPPORTIVE -- Cartilage and Bone

I. CARTILAGE - contains fibers, ground substance and cells - no vascularization, cells receive nutrients via long-range diffusion - cells = chondrocytes(blasts) produce ground substance and fibers

1. Hyaline - most common; found covering articular surfaces of most joints, costal cartilages, larger respiratory tubes, nasal cartilages; embryonically and in young it provides temporary model for endochondral bone development

a) Chondrocytes = present in lacunae (holes) in matrix (up to 40 micrometers in diameter)

- Sequence of Development: Mesenchymal cell Fibroblast Chondroblast Chondrocyte
- Chondroblasts at periphery of cartilage, mature to chondrocytes as they are enveloped in matrix; cells attain a more rounded state
- Young chondroblasts capable of cell division, daughter cells secrete a layer of matrix around themselves = isogenous nests - composed of 2-4 cells

b) Matrix = appears homogenous, glasslike, stains basophilic

- Matrix composed of fine collagen fibers (invisible w/o special staining) and amorphous gel of proteoglycans (GAGs + proteins)

c) Perichondrium = dense irregular CT that surrounds cartilage; composed of 2 layers:

1) Fibrous Zone = normal dense irregular CT outer layer
2) Chondrogenic Zone = inner layer, gives rise to chondroblasts

d) Cartilage Growth of 2 types:

1) Appositional = adding new layers to outside
2) Interstitial = formation of isogenous nests + secretion of matrix around nests (territorial cartilage)

2. Elastic = adapted to resist bending; found in external ear, epiglottis, some laryngeal cartilages

- similar to hyaline cartilage in appearance and composition except that matrix contains extensive network of elastic fibers
- not prone to undergo degeneration or calcification as sometimes occurs in hyaline cartilage
- surrounded by perichondrium

3. Fibrocartilage = structurally intermediate between dense regular CT and cartilage; contains collagen bundles + chondrocytes with minimum amount of matrix.

- occurs where tough support or high tensile strength is needed (e.g., intervertebral discs, certain joints)
- functions to attach dense regular CT to cartilage or bone
- lacks perichondrium
- chondrocytes tend to be grouped in short rows (capsules) between collagen bundles

II. BONE - rigid supportive tissue making up most of the skeleton in evol. advanced vertebrates - composed of cells and matrix with a dense CT periosteum covering

a) Cells - 4 general types:

1) Osteogenic Cells = precursor cells derived from mesenchyme; capable of mitotic division; found near bone surfaces and in inner portion of periosteum
2) Osteoblasts = deposit bone matrix, found at margins of growing bone
3) Osteocytes = occupy lacunae (holes) in bone matrix
4) Osteoclasts = multinucleated cells acting in dissolution of bone; derived from monocytes which collect at sites of bone resorption and fuse together

b) Matrix = contains collagen fibers in bundles which act as strengthening framework, ground substance of matrix becomes impregnated with calcium salts (mainly hydroxyapatite = crystalline calcium phosphate - 3Ca3[PO4]2Ca[OH]2)

- Bone is highly vascularized, unlike cartilage
- Canaliculi = tiny canals interconnecting osteocyte lacunae; eventually connect lacunae (directly or indirectly) to a fluid surface; contain cytoplasmic processes of osteocytes - so allow nourishment and gas exchange for osteocytes

fig 17

- Bone growth occurs by appositional growth only, since osteocytes become embedded in hard matrix, no interstitial growth possible

BONE STRUCTURE - 2 Types of Bony Tissue, differ in architecture only.

1) Spongy (Cancellous) Bone = consists of a latticework of slender trabeculae enclosing large numbers of marrow cavities; found in flat bones of the skull, sternum and epiphysis of long bones

fig 18

- Blood cells and red bone marrow found in marrow cavities

2) Compact Bone = lamellar bone made of osteons (Haversian Systems) = concentric layers of bone surrounding central Haversian canal; small amount of soft tissue present

fig 19

- Haversian Canals = carry nerves, blood and lymph vessels along longitudinal plane of bone
- Volkman's Canals = carry nerves, blood and lymph vessels between marrow cavity, Haversian systems, and periosteum

- Sharpey's Fibers = direct extensions of dense irregular CT from periosteum into compact bone. Functions to anchor tendon (with fibers penetrating periosteum to bone) to bone. Formation by appositional bone growth around original attachment site.
- Transition between compact bone and periosteum:

1) outer bone layers = outer circumferential lamellae, enclose entire bone
2) Periosteum outside of circumferential lamellae - 2 zones:

a) Osteogenic Zone = gives rise to osteogenic cells
b) Fibrous Zone = dense irregular CT

1. Intramembranous (Direct) - forms spongy bone and eventually may form compact bone

a) forms directly from mesenchyme
b) ossification centers (centers of osteogenesis) form from dense vascular regions of mesenchyme
c) Mesenchymal cells Osteoblast Secrete matrix which eventually becomes calcified (ossified) as thin needlelike spicules
d) when osteoblasts become embedded in calcified matrix --- osteocytes
e) new osteoblasts on surface of spicules continue to lay down bone by appositional growth, form trabeculae = radiating network of spongy bone. [1], [2]

2. Endochondral - bone forms replacing cartilage "model"

I. Formation of Primary Center of Ossification

a) hyaline cartilage model is formed
b) cells in the middle of model enlarge, absorb matrix, leaving irregular cartilage spicules which subsequently calcify
c) capillaries + osteogenic cells + mesenchyme cells invade disintegrating cartilage as osteogenic bud from periosteum (perichondrium)
d) simultaneous with "c" osteoblasts form from inner perichondrium and begin laying down a tube of periosteal bone encircling the middle third of the cartilage model. Periosteal bone forms as intramembranous (spongy) bone but fills
in to form compact bone.
e) ossification of calcified cartilage spicules internally to form trabeculae and primary marrow from osteogenic bud
f) with continued formation of periosteal bone, get trabecular resorption inside leaving primary marrow cavity (osteoclasts involved in resorption)

II. Expansion of Primary Center of Ossification

a) as bone increases in size by appositional growth, primary center of ossification expands toward ends of bone as adjacent cartilage regions become osteogenic
b) epiphyses at each end continue interstitial production of cartilage, but epiphyseal regions don't expand because cartilage is replaced on diaphyseal side by bone at same rate as it is produced
c) epiphyseal plate extends horizontally across bone and is the region where cartilage replacement during expansion occurs. This plate consists of 4 regions:

1)      Zone of Resting Cartilage = chondrocytes are not actively contributing to bone growth, acts to anchor epiphyseal plate to epiphysis

2)      Zone of Proliferation = chondrocytes proliferate to replace dying chondrocytes from calcification; chondrocytes are stacked into parallel columns

3)      Zone of Maturation = chondrocytes hypertrophy and accumulate glycogen and lipid, secrete alkaline phosphatase - involved in formation of calcified cartilage spicules

4)      Zone of Calcification = hypertrophied cells become surrounded by calcified cartilage and die; trabeculae form as osteoblasts secrete bone onto calcified cartilage spicules SEE HANDOUT FOR REVIEW OF ENDOCHONDRAL BONE FORMATION SEE HANDOUT

III. Formation of Secondary Center of Ossification = develop in epiphyses of long bones sometime after birth

- chondrocytes in middle of epiphyseal zone hypertrophy and mature, matrix between them calcifies and disintegrates, osteogenic bud then invades giving rise to osteoblasts which deposit bone on cartilage spicules

- bone growth stops at about 20 yrs. of age in humans - adult bone continuously adapts to prevailing stresses by appropriate deposition and resorption - deposition and resorption are under hormonal control - integrated with regulation of blood calcium levels

1) Parathyroid Hormone = promotes resorption, increases blood calcium
2) Calcitonin = reduces resorption, decreases blood calcium

- intense activity increases blood lactic acid levels (decreases pH), causes dissolution of bone and can increase blood calcium levels

To Lecture 9