Skeletal system and tissues

Functions of skeletal system

• Provide shape• Protects• Supports (bears body weight)• Movement

– Muscle attachment– joints

• Storage (e.g., minerals, Ca+ & PO4-)• Hematopoesis (blood cell production)

Requires rigidity/ability

to resist compression

BONE TISSUECELLS• Osteoblasts: build/create matrix

– Periosteum & endosteum• Osteocytes: maintain matrix, provide

feedback on stress experienced– In lacunae

• Osteoclasts: destroy matrix– In periosteum and endosteaum

MATRIX• Collagen: provides tensile strength

(bends, twists, and pulls)– 1/3rd of matrix– organic

• Calcium Salts (Calcium Phosphate/hydroxyappitite): provide rigidity and compressive strength (pushes)– 2/3rds of matrix– inorganic

Cartilage:Firm enough to hold shape and resist compression, but softer, smoother, and more flexible than bone

General Characteristics

• Surrounded by supportive/reinforcing membrane of dense irregular CT: perichondrium

• Contains blood vessels

• Matrix– Ground substance of proteoglycans

• High water contentresiliency

– Elastic and collagen fibers• Also provides resiliency

– Avascular poor/slow repair

• Cells

– Chondroblasts: build/create matrix

• In perichondrium

– Chondrocytes: in lacunae

lacuna

chondrocyte

Matrix

Hyaline Cartilage

• Most common type

• Ground substance ≥ fibers

Elastic Cartilage

• Most flexible

• Ear & larynx• Elastic fibers ˃ ground substance

Fibrocartilage

• Strongest cartilage

• No perichondrium

• Collagen fibers ˃ ground substance

Dense Regular CT

Cells:

• Predominantly fibroblasts

Matrix:

• Many parallel arranged collagen fibers– Unidirectional strength

• Tightly packed fibers

• Variable amounts of elastic fibers

• Significant elastic fibers elastic tissue

• Often Poorly vascularized

Ligaments connect bone to bone

Elastic CT

• Dense Regular CT w/ enough elastic fibers to give appreciable amount of elasticity

• Different examples have different amounts of elasticity due to amount of elastic fibers

Reticular

• Mesh of weak reticular fibers

• Found in bone marrow

Long Bone AnatomyLong Bone as a model bone

Diaphysis: • Mostly compact bone• medullary cavity = hollow space

– saves weight without compromising strength

– Cavity contains yellow marrow – Yellow Marrow = mostly adipose tissue

Epiphyses: • Mostly spongy bone with “shell” of

compact bone

• save weight without compromising needed strength

– May contain yellow or red marrow,

– Red Marrow = blood cell formation

• Epiphyseal Plates: site of longitudinal growth bone growth

• Articular cartilage: – Hyaline cartilage at joint surface

w/ trabeculae

• Trabeculae of compact bone are oriented along the paths of structural stress placed on the bone so that bone material is only present where it is most needed.

• These trabeculae can be remodelled if stresses change

Locations of red marrow in adult

red marrow = site of blood cell production

Hematocytoblasts (hematopoietic stem cells)

Myeloid Stem CellsLymphoid Stem Cells

• Redblood cells•From erythroblasts

• Platelets•From megakaryoblast

• Granulocytes (WBC)• from myeloblasts

• Monocytes (WBC)•From monoblasts

•Lymphocytes (WBC)•From lymphoblasts

Periosteum:

Figure 6.5

Periosteum: Structure:

• Membrane surrounding outer surface of bone

– Except articular cartilage

• Outer layer of dense irregular CT– Rich in collagen

• Inner cellular layer – Includes osteoblasts, osteoclasts, &

osteoprogenitor (i.e., stem) cells

Function:

• Connects tendons and ligaments to bone

– Sharpeys/perforating fibers

• Site of bone formation/remodelling

• Contains blood vessels and nerves that serve the bone.

• Site of appositional growth

Endosteum• lines the inner surfaces of bone

• Contains osteoprogenitor (i.e., stem) cells, osteoblasts and osteoclasts

• Site of bone formation/remodelling

Structure of non-long bones

• Periosteum• Endosteum• Spongy bone with trabeculae and spaces all as in long bones• No medullary cavity

Histology of compact bone

Compact Bone

High power (close up)Mid-power

Osteon:• repeating structural units of compact bone• oriented the length of long bones (our primary model)

osteon

Figure 6.8

Lamellae= Layers of matrix (collagen + calcium phosphate/hydroxyapitite)• too dense for oxygen, nutrients or wastes to diffuse through (from a practical stand point)

Non-osteonic lamellae• Circumferential—around the outside of the entire bone• Interstitial—remnant lamellae found between existing, complete osteons

Central Canal:

•passageway for blood vessels and nerves

Lacunae:

• Spaces/cavities containing osteocytes

Canaliculi:• Passageways through matrix• interconnect lacunae and central canal• Filled with extensions of osteocytes that are joined by gap junctions• Transport of oxgyen, nutrients, and wastes

Perforating canals:• Passageways that interconnect central canals• filled with blood vessels and nerves

Spongy (Cancellous) Bone• No osteons• Trabeculae consisting of

– Parallel lamallae– Cells obtain nutrients from

vessels in contact with surface of trabeculae

BONE REMODELLING

• The process of constant creation and destruction of bone by osteoblasts and osteoclasts

• Bone Remodelling Continually:

1. reshapes bones in response to the specific stresses placed on them

1. Gravity

2. Muscular contraction

2. Adjust balance of bone creation/destruction to regulate Ca+ and PO4 levels

• Also part of bone development (e.g., creation of processes) and repair

• Bone is remodelled on its periosteal and endosteal surfaces• On average 20% of our skeleton is remodelled each year (but

rates vary widely in different parts of skeleton)

BALANCE BETWEEN OSTEOBLAST AND OSTEOCLAST ACTIVITY

• Osteoblast activity > osteoclast activity– Net increase in bone mass/amount of bone– Net uptake of calcium and phosphate from blood– Blood Ca and PO4 decreases

• Osteoblast activity < osteoclast activity– Net reduction in bone mass/amount of bone– Net release of calcium and phosphate from bone into blood– Blood Ca+ and PO4 increases

Fig. 11.02

Fig. 11.10a

Fig. 11.01b

Fig. 11.06

HORMONES ASSOCIATED WITH THE SKELETAL SYSTEM

Calcitonin (from Thyroid)• Inhibits osteoclast acitivity• Encourages bone deposition/creation• Reduces blood Ca+

Parathyroid hormone (from parathyroid gland)• Stimulates osteoclast activity• Encourages bone destruction/Ca+ release• Increases blood Ca+

Fig. 7.07

Blood Ca+ decrease

Blood Ca+ increase

Table 6.1