Lecture 08

Skeleton

-organ system consisting of bones -includes cartilage, ligaments, and other connective tissues

Bones

-primary organs of the skeletal system -interior cavity of bone contains CT consisting of either red or yellow bone marrow -osseous tissue -marrow -2 types

Primary function of bones

-form the framework of the body

Compact bone (dense or cortical)

-relative dense, white, smooth, and solid -makes up 80% of total bone mass

Spongy bone (cancellous or trabecular)

-internal to compact bone -appears porous -remaining 20% of total bone mass

Periosteum

-covers the external surface of bone -consists of 2 layers

Outer fibrous layer

-layer of periosteum -dense irregular CT -protect the bone from surrounding structures -anchors blood vessels and nerves -attachment site for tendons and ligaments

Inner cellular layer

-layer of periosteum -osteoprogenitor cells, osteoblasts, and osteoclasts

Endosteum

-covers the internal surface of bone within the medullary cavity -an incomplete layer of cells containing osteoprogenitor cells, osteoblasts, and osteoclasts

Diaphysis (structure of long bone)

-structure of a long bone -long, hollow cylindrical shaft -provides leverage and weight support -an external, relatively thick layer of compact bone and an internal thin layer of spongy bone surrounds the central medullary cavity *(in children: cavity is filled with red marrow) *(in adults: cavity is filled with yellow marrow)

Epiphysis

-structure of a long bone -knobby ends of the long bone -an external, thin layer of compact bone and an internal core filled with spongy bone *(spongy bone resists stresses in all directions) -at the moveable joints, covered in articular cartilage to reduce friction and absorb shock

Metaphysis

-structure of a long bone -region of mature bone between the diaphysis and epiphysis -contains the epiphyseal (growth) plate

Unique structures of other bones

-short, flat, and irregular differ from long bones in structure -external surface is compact bone -interior is composed of spongy bone *(also called diploe in the flat bone of the skull) -no medullary cavity

Flat bones

-flat, thin surfaces -may be slightly curved -protect underlying soft tissue and serve as large surface area for muscle attachment -ex: cranial vault of skull, scapulae, sternum, and ribs

Sesamoid

-found within tendons -small, flat, and oval shaped (think of sesame seed) -increase muscle leverage -ex: patella

Long bones

-longer than they are wide -have an elongated cylindrical shaft (diaphysis) and two ends (epiphysis) -most common bone shape -found in upper and lower limbs (except wrists and ankles)

Short bones

-same length as width (apprx.) -ex: carpals and tarsals (wrist & ankle) and sesamoid bones, e.g. patella

Irregular bones

-elaborate, complex shapes -doesn't fit in other categories -ex: vertebrae, bones of skull, and hip bones

Bone markings

-surface features that characterize each bone in the body

Depressions

-clefts located when the bone meets another structure -allow blood vessels and nerves to travel along the bone -allows 2 bones to articulate

Openings

-holes in the body -indicate where blood vessels or nerves travel through the bone

Projections

-bony extensions -variety of shapes and sizes -points where muscles, tendons, and ligaments attach to the bone

Marrow

-soft connective tissue of the bone

Red marrow

-hematopoietic tissue -in adults, located within: spongy bone of the long bones diploe of flat bones -in children, located within: medullary cavities of long bones

Yellow marrow

-replaces red marrow in medullary cavities of adults -primarily adipocytes -may revert back to red marrow under extreme anemic conditions

4 major bone cell types

-osteoclasts (resorbs bone) -osteogenic (stem cell) -osteoblast (forms bone matrix) -osteocyte (maintains bone tissue)

Osteoprogenitor cells

-mitotic stem cells -can give rise to the osteoblasts -found in the periosteum and endosteum

Osteoblasts

-produce new bone matrix (osteoid) -become trapped within calcified osteoid -differentiate into osteocytes

Osteocytes

-mature bone cell -cannot make new bone -reside within lacuna within the bone matrix -connected to the other nearby osteocytes -maintain the bone matrix and detect mechanical stress in the bone matrix

Osteoclasts

-large, multinuclear phagocytic cell -derived from a hematopoietic progenitor -located within depressions on the bone surface called resorption lacuna -break down the bone matrix (during resorption)

Formation of bone matrix

-osteoblasts secrete bone matrix, osteoid -osteoid calcifies -occurs as a result of high plasma Ca2+ levels or when bone needs to respond to greater stress

Resorption of bone matrix

-osteoclasts destroy bone matrix -releases Ca2+ into blood plasma

Bone continues to renew and reshape

-recycle ~5-7% of our bone mass every week -occurs at the periosteal and endosteal surfaces of the bone -compact bone is replaced slower than spongy bone

Composition of bone matrix (Organic)

-osteoid (~1/3rd of the matrix) *(collagen fibers) *(semi-solid ground substance (proteoglycans and glycoproteins) -cells -gives bone tensile strength -resists stretching and twisting

Composition of bone matrix (Inorganic)

-mineral salts (~2/3rd of the matrix) *primarily Ca3(PO4)2 *Ca3(PO4)2 combines with other salts to form Hydroxyapatite crystals which deposit along the long axis of collagen -harden the matrix -accounts for rigidity and inflexibility -gives bone compression strength

Osteon of compact bone

-structural unit of compact bone -run parallel with the long axis -weight bearing pillars -resist torsion stress -when viewed in cross-section, has the appearance of a bullseye

Components of the osteon

-Central canals -Composed of concentric lamella -Osteocytes -Lacunae -Caniculi

Central canals

-contain small blood vessels & nerve fibers that serve the osteon's cells -run parallel to the long axis of the bone

Composed of concentric lamella

-collagen fibers running in alternating directions in each lamella -gives bone strength and resilience

Osteocytes

-mature bone cells that maintain the bone matrix -isolated in lacunae

Lacunae

-contain osteocytes -found at the junctions of the lamellae

Caniculi

-tiny canals between lacunae -osteocytes connect with one another (by gap junctions) to pass nutrients & waste and to communicate with one another

Structures of the compact bone

-Perforating (Volkmann) canals -Circumferential lamellae -Interstitial lamellae

Perforating (Volkmann) canals

-connect and supply small blood vessels and nerves to the central canals & medullary cavity -lie at right angles to the long axis of the bone

Circumferential lamellae

-rings of bone that run the entire circumference of the bone -protect against torsion -external circum. -internal circum.

External circum. lamellae

-found internal to the periosteum

Internal circum. lamellae

-found internal to the endosteum

Interstitial lamellae

-located between intact osteon -incomplete and have no central canal -remnants of resorbed osteons or fill-ins

Structures of spongy bone

-structure appears more haphazard -open lattice -trabeculae -no osteons *(possess parallel lamellae) *(no blood vessels) *(some canaliculi open to surface to allow the diffusion of nutrients and waste)

Trabeculae

-narrow plates and rods of bone -align along lines of stress -distribute stress throughout

Bone formation

-ossification or osteogenesis -begins in the embryo through childhood and adolescence -2 types resulting different kinds of bone formations *intramembranous *endochondral

Intramembranous

-yields flat bones -bone growth within a membrane -4 steps

4 steps of Intramembranous ossification

1: Formation of the ossification center -committed osteogenic cells divide to become osteoblasts secreting osteoid -several ossification centers can occur simultaneously 2: Calcification of the osteoid 3: Formation of woven bone and the periosteum -woven bone is immature and unorganized -mesenchyme condenses and forms periosteum -blood vessels form and branch 4: Replacement of woven bone by lamellar bone

Endochondral

-yields most bones of the skeleton -bone development that replaces cartilage (used as a model for construction) -long bones -bones of the pelvis, vertebrae, and clavicle -6 steps

6 steps of Endochondral ossification

1: Fetal hyaline cartilage model develops (week 8-12 development) 2: Cartilage calcification & formation of the periosteal collar 3: Primary ossification center forms in the diaphysis 4: Secondary ossification center forms in the epiphysis 5: Bone replaces almost all cartilage -articular cartilage remains on epiphyseal surfaces -cartilage at the epiphyseal plate, the junction between the diaphysis and the epiphysis 6: Lengthwise growth continues until epiphyseal plate closes -cartilage in epiphyseal plate ossify between age 10 and 25

Interstitial bone growth

-growth in length -dependent upon cartilage growth within the epiphyseal plate *(growth in epiphyseal plate of cartilage on the epiphysis side of the plate) *(chondrocytes become deprived of nutrients and become calcified) *(remodeling of calcified cartilage by the bone tissue on the diaphysis side of the plate -epiphyseal plate closure occurs after adolescence *(chondroblasts in epiphyseal plate divide less frequently and eventually are replaced by bony tissue) *(epiphysis and diaphysis fuse)

5 cartilaginous zones

-involved in interstitial bone growth -Zone of resting cartilage -Zone of Proliferating cartilage -Zone of hypertrophic cartilage -Zone of calcified cartilage -Zone of ossification

Zone of resting cartilage

-furthest from medullary cavity of diaphysis -resembles healthy cartilage -secures epiphysis to epiphyseal plate

Zone of proliferating cartilage

-rapid mitotic division -cells aligned in columns

Zone of hypertrophic cartilage

-no longer dividing -cells become hypertrophic and resorb matrix

Zone of calcified cartilage

-mineral deposition of the matrix -chondrocytes die

Zone of ossification

-vascularization and bone deposition

Appositional bone growth

-growth in width -bone is added to the external surface -bone is resorbed on the internal surface -rate of addition > rate of resorption *(thicker, stronger bones)