Bones and Skeletal Tissue

BIO 231

Anatomy and Physiology I

Skeletal Hyaline Cartilages

Milky white color

Articular cartilages - ends of bones at movable joints

Costal cartilages - connect ribs to sternum

Respiratory cartilages - form skeleton of the larynx and reinforce respiratory passageways

Nasal cartilages - support external nose

Elastic Cartilages

Similar to hyaline cartilage - more elastic fibers

Supports external ear

Forms the epiglottis

Fibrocartilages

Highly compressible & great tensile strength

Consists of roughly parallel roles of chrondrocytes -alternates with thick collagen fibers

Padlike cartilages of the knee (menisci)

Discs of cartilage between the vertebrae

Location of Cartilages

Classification of Bones

206 bones in the human skeleton

Axial skeleton - forms long axis of the body

Skull, vertebral column & rib cage

Appendicular skeleton - bones of the upper & lower limbs and girdles

Classification of Bones

Long bones - longer than wide

Short bones - roughly cube shaped

Bones of the wrist and ankles

Sesamoid bones - special type of short bone forms in a tendon

Patellae

Flat bones - thin flattened and usually a bit curved

Sternum, scapulae, ribs,& most skull bones

Irregular bones -complicated shapes - vertebrae & hip bones

Structure of a Long Bone

Diaphysis - shaft of the bone - collar of compact bone surrounding a central medullary cavity - contain syellow bone marrow

Epiphysis - ends of long bones - interior of spongy bone - exterior of compact bone

Joint surfaces are covered with articular cartilage

Epiphyseal line -remnant of epiphyseal plate - disc of cartilage that allows long bones to growin childhood

Structure of a Long Bone

Periosteum -double layer membrane surrounding the bone

Outer layer - fibrous connective tissue

Inner (osteogenic)layer - osteoblasts & osteoclasts

Richly supplied with nerve fibers, blood & lymphatic vessels - enter bone through nutrientforamen

Sharpey’s fibers -connect periosteum to the bone (collagen)

Provides anchor point for tendon & ligaments

Long Bones

Long Bones

Structure of Short, Flat & IrregularBones

Thin plates of periosteum-covered compact bone on the outside

Inside is spongy bone

No marrow cavity but do contain marrow in the honeycomb of the spongy bone - called trabeculae

Diploe - layer of spongy bone in the middle offlat bones

Red Marrow

Found in the cavities of spongy bone of long bones& diploe of flat bones

Called red marrow cavities

Compact Bone

Osteon or Haversiansystem - structural unit of compact bone

An elongate cylinder oriented parallel to the long axis of the bone

Lamella are layers that appear like rings of a tree trunk

Collagen of each lamella runs opposite directions

The center of each osteon is central or Haversian canal - blood vessels and nerves

Perforating or Volkmann’s canals run at right angles to the central canals

Compact Bone

Osteocytes - mature bone cells live in cavities called lacunae at the junctions of lamellae

Canaliculi - connect lacunae to each other and to the central canal

Each osteocyte is in contact with other via gap junctions & the canuliculi

Interstitial lamellae -lamellae between osteons

Circumferential lamellae are located just below the the periosteum - extend around the whole circumference of the bone

Compact Bone

Spongy Bone

Consists of trabeculae - looks poorly organized -however the trabeculae align precisely along lines of stress

Contain irregularly arranged lamellae & osteocytes interconnected by canaliculi

No oesteons present

Spongy Bone

Chemical Composition of Bone

Osteoid - organic part of the matrix

About 1/3 of the matrix

Proteoglycans, Glycoproteins and collagen

Hydroxyapatites or mineral salts make up the remaining 65%-largely calcium phosphates

The calcium salts form tiny crystals around the collagen

Bone Markings

The bulges, depressions and holes that serves asattachment sites and passage ways

Bone Development

Osteogenesis/ossification - process of bonetissue formation

Bony Skeleton - before 8 weeks - skeleton is fibrous membranes & cartilage

Intramembranous ossification - bone from a fibrous membrane - membrane bone

Endochondral ossification - bone formation that replaces hyaline cartilage - cartilage or endochondral bone

Intramembranous Ossification

Most bones of the skull & clavicles

Flat bones

Intramembranous Ossification

Endochondral Ossification

All bones from the base of the skull down - except clavicles

Endochondral Ossification

Growth of Long Bones

Epiphyseal plate cartilage on the shaft side - form tall columns - zone 1 - growth zone

This pushes the epiphysis away from the diaphysis - bone lenghtens

Oldest chondsrocytes in astack - closest to the top of the shaft form zone 2 - transformation zone -hypertrophy - lacunae erode & enlarge - matrix calcifies - chrondrocytes die

This leaves spicules of calcified cartilage - becomes zone 3 - osteogenic zone - eroded by osteoclasts then recovered by osteoblasts

Growth of Long Bones

Longitidinal growth accompanied by almost continuous remodeling of the epiphyseal ends to maintain proper proportions

Growth in thickness -appositional growth

Osteoblasts below the periosteum secrete a bone matrix on the external bone surface as osteoclasts on the endosteal surface of the diaphysis remove bone

Slightly less break downthan buildup - get thicker stronger bones over time

Growth of Long Bones

Controlled by growth hormone during infancy &childhood

Thyroid hormones modulate GH

At puberty sex hormones inc growth -get feminization and masculinization of specific parts

Later induce closure of epiphyseal plates - ending bone growth

Growth in Length of a Long Bone

Growth of Long Bones

Bone Remodeling

Involves adjacent “packets” of osteoblasts andosteoclasts called remodeling units

Total bone mass remains constant

Bone Remodeling

Bone depositoccurs - where bone is injured or added bone strength is needed

Osteoid seam - unmineralized bandof gauzy-looking bone matrix - is the area of new matrix

Calcification front- abrupt transition between old bone and the osteoid seam

Clacification requires certain levels of calcium and phosphates - then hydroxyapatite crystals begin to form

Alkaline phosphatase from osteoblasts is also essential

Bone Remodeling

Bone absorption - via osteoclasts

Osteoclasts - giant multinucleate cells from hematopoietic stem cells

Create resorption bays (pits & grooves)

The cells ruffled boarder secretes lysosomal enzymes and acids that convert calcium salts to soluble forms

Control of Bone Remodeling

Hormonal Mechanism - Interaction of PTH and calcitonin

PTH is released when blood Ca⁺⁺ declines

Stimulates osteoclasts to resorb bone - release into blood - neg feedback control

Calcitonin - secrteted when blood Ca⁺⁺ levels rise - inhibitsbone resorption & promotes Ca⁺⁺deposition - as blood Ca⁺⁺ falls - calcitonin release also wanes

Control of Bone Remodeling

Responses to mechanical stress

Muscle pull and gravity -promote skeletal remodeling

Wolff’s law - a bone grows in response to the forces or demands placed on it

Bones loaded when weight bears down on them or muscles pull on them

Loading is usually off center - bone bends - thus the bone on one side is compressed & then otherside is stretched

Loading may induce electrical currents that that direct remodeling

Common Fractures

Common Fractures

Stages in Healing

Stages in Healing