Tissues -four primary tissue types: 1. Epithelial 2. Connective 3. Muscle 4. Neural -however: all tissues in the body develop from three germ layers 1. ectoderm - epithelial + neural 2. mesoderm - connective + muscle + some epithelial 3. endoderm – organs + some epithelial

THE SKELETAL SYSTEM: OSSEOUS TISSUE AND SKELETAL STRUCTURE Part A C H A P T E R F I V E

Bones of the skeleton Cartilage Ligaments ◦ Stabilize and connect bones Skeletal System

Structural support ◦ framework for attachment Storage of minerals and lipids ◦ calcium bone salts (1-2 kg: 98% of it in bone) Blood cell production, triglyceride storage ◦ triglycerides stored in the yellow marrow ◦ RBCs, WBCs, platelets in red bone marrow Protection of delicate tissues and organs ◦ ribs - heart and lungs; skull - brain; pelvis - digestive and reproductive Leverage: change the magnitude and forces generated by skeletal muscles Functions of the Skeletal System

binds structures together provides support protects organs fills cavities produces blood Connective Tissue

components: matrix + cells -matrix: non-cellular support material -comprised of extracellular protein fibers + ground substance (fluid + sugar) -proteins - e.g. collagen fibers, elastic fibers, fibronectin -sugars – e.g. hyaluronan, glucosamine -cells: secrete the matrix -cells vary from tissue to tissue Connective Tissue

Connective tissues of the Body Loose – loose/areolar CT, fat & reticular Dense – dense (regular, irregular), elastic Supportive – bone & cartilage Fluid – blood & lymph

Supportive connective tissue ◦ Cells: osteoblasts, osteocytes, OPCs  cells only contribute total of 2% total bone tissue volume ◦ Solid extracellular matrix  framework of protein fibers: collagen type I ◦ other specialized proteins: osteocalcin, osteonectin, osteopontin  ground substance – sugar + water: e.g. hyaluronan  calcium phosphate crystals: hydroxyapatite Osseous Tissue

Bone matrix is contains a large amount of hydroxyapatite crystals (calcium phosphate) Ca 10 (PO 4 ) 6 (OH) 2 ◦ can withstand compression very well ◦ 2/3 weight of bone ◦ remaining 1/3 is collagen fibers, proteins & other salts ◦ collagen contributes flexibility Bone: Histological Organization

also known as osteogenesis osteogenesis involves calcification deposition of calcium salts within a tissue e.g. deposition of hydroxyapatite crystals among the “scaffolding” of collagen fibers and sugars mechanism?? controlled by the cells of bone Bone formation

Mature bone cells maintain and monitor the protein content of bone completely surrounded by hard bone matrix directs the release of calcium from bone to blood directs the deposition of calcium within the bone matrix do NOT undergo cell division Osteocytes

Immature bone-forming cells ◦ cuboidal in shape ◦ found on the inner and outer surfaces of bone ◦ secrete the protein extracellular matrix that will become surrounded by hydroxyapatite crystals (i.e. calcification) ◦ non-mineralized matrix = osteoid matrix  collagen I fibers + ground matrix + specialized bone proteins ◦ becomes surrounded by a calcifying matrix and matures to form an osteocyte ◦ do NOT divideOsteoblasts

derived from stem cells (found within the bone marrow) differentiate into osteoblasts undergo division play a role in fracture repair & bone remodelling Osteoprogenitor Cells (OPCs)

1. formation of OPCs from stem cells within bone marrow 2. differentiation of OPCs into osteoblasts (OBs) 3. OBs begins to produce the collagenous osteoid matrix 4. Osteoid calcifies into hard bone – formation of HA crystals around protein fibers of the osteoid 5. OBs mature into osteocytes 6. Bone maintenance by osteocytes Bone formation

Osteoclasts –large cells derived from macrophages –move into the bone – carving out tunnels by dissolving the bony matrix (osteolysis) –through the production of mineral dissolving acids + protein-degrading enzymes called proteases –the components of the digested bone are recycled into the bloodstream by the OC –regulates blood calcium levels

-new bone is made by osteoblasts -destruction of old bone is by osteoclasts - there must be balance between new bone production and old bone destruction to maintain bone volume -controlled by estrogen hormone action in females (increases activity of OBs) -also controlled in males and females by the hormones parathyroid hormone (stimulates OCs) and calcitonin (inhibits OCs) -disruption in this balance leads to disease e.g. osteoclast activity exceeds OB - loss of bone = osteoporosis e.g. too high OB activity – bone spurs Bone Remodelling

high turnover rate 1/5 of skeleton is remodelled every month takes 4 months to replace the femur completely Bone Remodelling

all bones are covered by periosteum (dense irregular connective tissue) ◦ important for muscle attachment ◦ outer fibrous layer ◦ inner cellular layer of OPCs Periosteum and Endosteum

organizational pattern of bone matrix can be described as: 1)compact bone 2) spongy bone Types of Bone

also known as dense bone makes up the bulk of the shaft of a long bone and the outer layers of short, flat and irregular bones arranged in units called osteons or Haversian systems osteons run the length of the bone’s shaft in long bone Compact Bone

central canal is the site for blood vessels, lymphatic vessels and nerves concentric circles of bone = concentric lamellaeOsteons matrix is organized as concentric circles around a central canal called a Haversian canal osteons are aligned along the bone in the same direction as stress e.g. shaft of a long bone - parallel to the long axis

within the lamellae are small spaces in the matrix containing osteocytes = lacunae lacunae are interconnected by canaliculi the canaliculi are connected to the central canal areas between osteons are filled with interstitial lamellae that also have lacunae and canaliculi are fragments of older osteons that are being degraded

Bone histology this is what calcified compact bone looks like under the microscope

Spongy Bone ◦ same matrix as compact bone ◦ BUT different arrangement of osteocytes, canaliculi and lamellae ◦ does not contain osteons ◦ consists of trabeculae = irregular lattice of thin bone columns ◦ spongy bone makes up the majority of the irregular, flat and short bones of the body (often covered by a thin layer of compact bone) ◦ e.g. hip, ribs, sternum, ends of long bones (femur) ◦ also found at the ends of long bones

the trabeculae of spongy bone are made up of lamellae with lacunae containing osteocytes and canaliculi but what are trabeculae missing? spaces between trabeculae are filled with red marrow spongy bone is the site of hematopoiesis in adults (blood cell formation)

spongy bone is lighter - reduces overall weight of the bone higher level of remodelling in spongy bone vs. compact (greater number of OBs and OCs within this type of bone) stronger than compact bone - the trabeculae are arranged along lines of stress – highly resistant to breakage Spongy. vs. Compact Bone

Based on anatomical classification ◦ Long bones = greater length than width ◦ Short bones = cube-shaped, spongy bone except at surface ◦ Flat bones = two parallel plates of compact bone sandwiching spongy bone layer ◦ Irregular bones = cannot be grouped ◦ Sesamoid bones = develop in tendons where there is considerable friction, tension and stress ◦ Sutural bones = located within joints between cranial bones Classification of bones

1) diaphysis: shaft of the bone 2) epiphysis: distal & proximal ends of bone 3) metaphysis: region where the diaphysis joins the epiphysis -in a growing bone it includes the epiphyseal growth plate (hyaline cartilage) 4) articular cartilage: thin layer of hyaline cartilage covering the epiphysis -reduces friction during joint movement 5) periosteum: dense irregular connective tissue that covers the outside of the bone where not covered with articular cartilage -site of OPCs for bone formation 6) medullary cavity: or marrow cavity -space within diaphysis - contains yellow marrow 7) endosteum: thin membrane that lines the medullary cavity of long bones -single layer of bone forming OPCs Anatomy of a Long Bone

all bones are covered by periosteum (dense irregular connective tissue) ◦ important for muscle attachment ◦ outer fibrous layer ◦ inner cellular layer of OPCs long bones are also lined by endosteum which also contains OPCs + numerous osteoclasts Periosteum and Endosteum

-bones have a rich blood supply -blood vessels are abundant in bone portions containing red marrow -arteries + nerves enter the diaphysis and the epiphyses through holes called foramina (singular = foramen) Circulatory Supply of a Bone