Bones and Skeletal Tissues Chapter 6-8

Skeletal Cartilages Initially, skeleton composed of cartilage and fibrous membranes Most are then replaced by bone in the adult skeleton Some places where flexibility is needed, remain cartilage

Basic Structure, Types, and Locations Skeletal Cartilage – made of some variety of cartilage tissue (mostly water) The water lets cartilage spring back to shape Perichondrium (a dense connective membrane) surrounds blood vessels and nerves

Basic Structure, Types, and Locations Three types of Cartilage: hyaline, elastic, and fibrocartilage (remember chapter 4?)

Basic Structure, Types, and Locations Hyaline Cartilages Articular – bone ends, movable ends Costal – connect ribs to the sternum Laryngeal – for the skeleton of the larynx Tracheal and bronchial – reinforce other passageways of the respiratory system Nasal – support the external nose

Basic Structure, Types, and Locations Elastic Cartilages Looks similar to hyaline (ref. ch. 4) More stretchy elastic fibers – better for bending Supports external ear Forms epiglottis

Basic Structure, Types, and Locations Fibrocartilages Parallel rows of chondrocytes and thick collagen fibers Compressible High tensile strength Places subjected to heavy pressure and stretch Pad-like cartilages (menisci) of the knee Discs between vertebrae

Growth of Cartilage Two growth patterns: Appositional Growth – cartilage-forming cells in perichondrium around matrix secrete new matrix against external face of existing cartilage Interstitial growth – the lacunae bound chondrocytes inside the cartilage divide and secrete new matrix expanding from within

Functions of Bones 1. Support – hard framework that supports the body and cradles soft organs 2. Protection – fused bones of skull enclose brain (vertebrae surround spinal cord) 3. Movement – used as levers for skeletal muscles attached by tendons

Functions of Bones 4. Mineral Storage – reservoir for minerals such as calcium and phosphate 5. Blood Cell Formation – hematopoiesis (formation of blood cells)

Classification of Bones Compact Bone – dense outer layer; appears smooth and solid Spongy Bone – honeycomb of small needle-like of flat pieces called trabeculae

Classification of Bones Bone Shape: 1. Long Bones: Longer than they are wide Shaft and two ends Mostly compact bone with some spongy bone All bones of limbs

Classification of Bones Bone Shape: 2. Short Bones: Roughly cube-like Mostly spongy bone; compact bone only forms thin outer layer Bones of wrist and ankle Sesamoid bones – short bone embedded within tendon - patella

Classification of Bones Bone Shape: 3. Flat Bones Thin, flat, and usually curved Two compact bone surfaces with spongy between Sternum, ribs, most skull bones

Classification of Bones Bone Shape: 4. Irregular Bones Complex shapes and mainly spongy bone Don’t fit the previous three categories Vertebrae, hip bones

Bone Structure Bones are organs and consist of a variety of tissues Osseous tissue (the most common) Nervous tissue Articular cartilages Fibrous connective tissue Muscle and epithelial tissue in their blood vessels

Structure of Typical Long Bone Diaphysis – long part of compact bone that surrounds the medullary cavity (yellow bone marrow cavity in adults) Epiphyses – ends of the bones; covered in articular cartilage; also has epiphyseal line (from before birth)

Structure of Typical Long Bone Membranes – bones are covered in shiny white membrane called periosteum composed of dense irregular tissue Inner layer: osteogenic layer made of osteoblasts (bone forming) osteoclasts (bone destroying)

Structure of Typical Long Bone Nutrient foramen richly vascularized; supplies nutrients Sharpey’s fibers – tufts of collagen fibers that extend from the fibrous layer; provides anchoring points for tendons Endosteum – delicate connective tissue; covers trabeculae of spongy bone in marrow cavities Fig. 6.3

Structure of Short, Irregular, and Flat Bones Thin plates of periosteum-covered compact bone Endosteum-covered spongy bone within Diploë – internal layer of spongy bone Fig. 6.4

Location of Hematopoietic Tissue in Bones Hematopoietic tissue – red marrow Red marrow cavities – cavities of spongy bone in long bones and diploë of flat bones Sites of red marrow: diploë of flat bones (sternum), some irregular (hip bone), long bones (femur and humerus)

Microscopic Structure of Bone Compact Bone Appears very dense Has many tunnels and passageways filled with blood vessels and nerves Osteon (or Haversian system) – structural part of bone Osteons are like pillars bearing weight

Compact Bone Lamella – matrix of tubes Compact bone called lamellar bone Central (Haversian) canal – runs through the core of each osteon Perforating (Volkmann’s) canals – second type of canals at right angles to long axis of bones; connects vascular and nerve supplies

Compact Bone Osteocytes occupy small cavities – lacunae Canaliculi – hair-like canals that connect lacunae to each other and to the central canal Osteoblasts secrete bone forming material into matrix trapping cells as it hardens

Compact Bone Interstitial lamellae – fill gaps between osteons Circumferential lamellae – just deep of the periosteum and extend around the circumference of the shaft Resist twisting

Spongy Bone Consists of trabeculae Form along lines of stress and not randomly to strengthen bone No osteons Few cell layers thick Irregular lamellae and osteocytes

Chemical Composition of Bone Organic components : Cells: osteoblasts, osteocytes, and osteoclasts osteoid Osteoid – proteoglycans, glycoproteins, and collagen fibers Inorganic components: 65% of mass, made of hydroxyapatites (mineral salts) Hydroxyapatites – calcium phosphates (makes bone hard)

Bone Markings Bones are rarely smooth on outside Display bulges, depressions, and holes Serve as sites for muscles, ligaments, and tendons to attach Or for blood vessels and nerves to enter

Bone Markings Projections – spines, trochanters Depressions – openings, fossae, sinuses, foramina, and grooves See Table 6.1

Bone Development (Osteogenesis) Osteogenesis (ossification) – process of bone formation Formation of the skeleton, bone growth, or remodeling

Formation of the Bony Skeleton Membrane bone – intramembranous ossification Bone formation that occurs by replacing hyaline cartilage structure is called endochondral ossification Resulting bone is cartilage (endochondral) bone

Intramembranous Ossification Produces all flat bones 1. Formation of ossification center in fibrous membrane 2. Formation of bone matrix within the fibrous membrane 3. Formation of woven bone and the periosteum 4. Formation of compact bone plates and red marrow -See Fig. 6.7

Endochondral Ossification Most bones of the skeleton formed by this process Late in second month of development Hyaline cartilage as model, later replaced by bone

Endochondral Ossification 1. A bone collar forms around the diaphysis of the hyaline cartilage model 2. Cartilage in the center of the diaphysis calcifies 3. The periosteal bud invades the internal cavities and spongy bone forms 4. the medullary cavity forms 5. the epiphyses ossify -See Fig. 6.8

Postnatal Bone Growth Most bones stop growing during adolescence or early adulthood Some facial bones (nose and jaw continue)

Growth in Length of Long Bones Mimics many of the events of endochondral ossification Causes bone to lengthen Hormones also regulate bone growth See Fig. 6.9 and Fig. 6.10

Bone Homeostasis: Remodeling and Repair Architecture is constantly changing 5-7% of bone mass is recycled At least half a gram of calcium is entering and leaving bones each day

Bone Remodeling Bone deposit and bone resorption occur Remodeling units - “packets” of osteoblasts and osteoclasts Occurs at different rates if various bones Bone deposit – when bone is injured or added bone strength is required

Bone Remodeling New matrix deposit occurs at an osteoid seam Controlled by osteoblasts Calcium salts are crystalized here Alkaline phosphatase is essential for mineralization These are contained in matrix vesicles which carry the phosphates

Bone Remodeling Bone resorption Accomplished by osteoclasts Giant multinucleate cells that are believed to arise from hematopoietic stem cells Osteoclasts secrete… 1. lysosomal enzymes 2. metabolic acids

Control of Remodeling: Hormonal Mechanism Reflected by the interaction of parathyroid hormone (PTH) and calcitonin Released when blood levels of ionic calcium decline Human body contains g of calcium – 99% in bones

Control of Remodeling Ca 2+ is needed for a wide variety of physiological processes Nerve impulses Muscle contraction Blood coagulation Secretion by glands and neurons Cell division

Mechanical Stress Wolff’s law – (not universally accepted) says that bones remodel or strengthen in response to where stress or force is acting upon it Bone anatomy reflects the common stresses it encounters

Mechanical Stess 1. long bones are thickest midway along the shaft (where bending stress is greatest) 2. Curved bones are thickest where they are most likely to buckle 3. trabeculae of spongy bone forms trusses or struts along compression 4. large, bony projections occur where heavy active muscles attach

Mechanical Stress Electrical impulses are created as bones grow It is suggested that as stress occurs, electrical signals direct the remodeling process