1. Chapter 7: Skeletal Tissues

2. FUNCTIONS OF BONE
Support: bones form the framework of the body and contribute to the shape, alignment, and positioning of body parts; ligaments help hold bones together (Figure 7-1)
Protection: bony “boxes” protect the delicate structures they enclose
Movement: bones and their joints constitute levers that move as muscles contract
Mineral storage: bones are the major reservoir for calcium, phosphorus, and other minerals
Hematopoiesis: blood cell formation is carried out by myeloid tissue

4. TYPES OF BONES Five major types of structural bones (Figure 7-2)
Long bones
Short bones
Flat bones
Irregular bones
Sesamoid bones
Bones serve various needs, and their size, shape, and appearance vary to meet those needs
Bones vary in the proportion of compact and cancellous (spongy) bone; compact bone is dense and solid in appearance, whereas cancellous bone is characterized by open space partially filled with needlelike structures

6. TYPES OF BONES (cont.) Parts of a long bone (Figure 7-3) Diaphysis
Main shaft of a long bone
Hollow, cylindrical shape and thick compact bone
Function is to provide strong support without cumbersome weight
Epiphyses
Both ends of a long bone; made of cancellous bone filled with marrow
Bulbous shape
Function is to provide attachments for muscles and give stability to joints

8. TYPES OF BONES (cont.) Articular cartilage Periosteum
Layer of hyaline cartilage that covers the articular surface of epiphyses
Function is to cushion jolts and blows
Periosteum
Dense, white fibrous membrane that covers bone
Attaches tendons firmly to bones
Contains cells that form and destroy bone
Contains blood vessels important in growth and repair
Contains blood vessels that send branches into bone
Essential for bone cell survival and bone formation

9. TYPES OF BONES (cont.) Medullary (or marrow) cavity
Tubelike, hollow space in the diaphysis
Filled with yellow marrow in adults
Endosteum: thin, fibrous membrane that lines the medullary cavity

10. TYPES OF BONES (cont.) Parts of a flat bone
Inner portion is cancellous bone covered on the outside with compact bone
Cranial flat bones have an internal and external table of compact bone and an inner cancellous region called the diploë (Figure 7-4)
Bones are covered with periosteum and lined with endosteum, such as in a long bone
Other flat bones, short bones, and irregular bones have features similar to the cranial bones
Spaces inside the cancellous bone of short, flat, irregular and sesamoid bones are filled with red marrow

12. BONE TISSUE Most distinctive form of connective tissue
Extracellular components are hard and calcified
Rigidity of bone gives it supportive and protective functions
Tensile strength nearly equal to that of cast iron at less than one third the weight

13. Bone tissue-between cells
Glycoprotein attachments also allow local communication within a tissue

14. CONNECTIVE TISSUE: BONE TISSUE
Uniquely hard and strong connective tissue type
Cells (osteocytes) embedded in a calcified matrix
Inorganic component of matrix accounts for 65% of total bone tissue
Functions
Support
Protection
Point of attachment for muscles
Reservoir for minerals
Supports blood-forming tissue

15. CONNECTIVE TISSUE: BONE TISSUE (cont.)
Compact bone (Figures 5-24 and 5-25)
Osteon (Haversian system)
Structural unity of bone
Spaces for osteocytes called lacunae
Matrix present in concentric rings called lamellae
Canaliculi are canals that join lacunae with the central Haversian canal
Cell types
Osteocyte: mature, inactive bone cell
Osteoblast: active bone-forming cell
Osteoclast: bone-destroying cell

18. CONNECTIVE TISSUE: BONE TISSUE (cont.)
Formation (ossification) (Figure 5-23)
In membranes (e.g., flat bones of skull)
From cartilage (endochondral) (e.g., long bones, such as the humerus)

19. BONE TISSUE (cont.) Composition of bone matrix Inorganic salts
Hydroxyapatite: crystals of calcium and phosphate contribute to bone hardness
Slender, needlelike crystals are oriented to most effectively resist stress and mechanical deformation
Magnesium, sodium, sulfate, and fluoride are also found in bone
Organic matrix
Composite of collagenous fibers and an amorphous mixture of protein and polysaccharides called ground substance
Ground substance is secreted by connective tissue cells
Adds to overall strength of bone and gives some degree of resilience to bone

20. MICROSCOPIC STRUCTURE OF BONE
Compact bone (Figure 7-5)
Contains many cylinder-shaped structural units called osteons, or haversian systems (Figure 7-6)
Osteons surround central (osteonal or haversian) canals that run lengthwise through bone and are connected by transverse (Volkmann) canals
Living bone cells are located in these units, which constitute the structural framework of compact bone
Osteons permit delivery of nutrients and removal of waste products

23. MICROSCOPIC STRUCTURE OF BONE (cont.)
Structures that make up each osteon
Lamellae
Concentric: cylinder-shaped layers of calcified matrix around the central canal
Interstitial: layers of bone matrix between the osteons; leftover from previous osteons
Circumferential: few layers of bone matrix that surround all the osteons; run along the outer circumference of a bone and inner circumference (boundary of medullary cavity) of a bone

24. MICROSCOPIC STRUCTURE OF BONE (cont.)
Structures that make up each osteon (cont.)
Lacunae: small spaces containing tissue fluid in which bone cells are located between hard layers of the lamella
Canaliculi: ultra-small canals radiating in all directions from the lacunae and connecting them to each other and to the central canal
Central (osteonal or Haversian) canal: extends lengthwise through the center of each osteon; contains blood vessels and lymphatic vessels

25. MICROSCOPIC STRUCTURE OF BONE (cont.)
Cancellous bone (Figure 7-6)
No osteons in cancellous bone; it has trabeculae instead
Nutrients are delivered and waste products removed by diffusion through tiny canaliculi
Bony branches (trabeculae) are arranged along lines of stress to enhance the bone’s strength (Figure 7-7)
Blood supply
Bone cells are metabolically active and need a blood supply, which comes from the bone marrow in the internal medullary cavity of cancellous bone
Compact bone, in addition to bone marrow and blood vessels from the periosteum, penetrates the bone and then, by way of transverse (Volkmann) canals, connects with vessels in the central canals of osteons

27. MICROSCOPIC STRUCTURE OF BONE (cont.)
Types of bone cells
Osteoblasts (Figure 7-8)
Bone-forming cells found in all bone surfaces
Small cells synthesize and secrete osteoid, an important part of the ground substance
Collagen fibrils line up in osteoid and form a framework for the deposition of calcium and phosphate

29. MICROSCOPIC STRUCTURE OF BONE (cont.)
Types of bone cells
Osteoclasts
Giant multinucleated cells
Responsible for the active erosion of bone minerals
Contain large numbers of mitochondria and lysosomes
Osteocytes: mature, nondividing osteoblasts surrounded by matrix and lying within lacunae (Figure 7-9)

31. BONE MARROW
Type of soft, diffuse connective tissue; called myeloid tissue
Site for the production of blood cells
Found in the medullary cavities of long bones and in the spaces of spongy bone

32. BONE MARROW (cont.)
Two types of marrow occur during a person’s lifetime
Red marrow
Found in virtually all bones in an infant’s or child’s body
Produces red blood cells
Yellow marrow
As an individual ages, red marrow is replaced by yellow marrow
Marrow cells become saturated with fat and are no longer active in blood cell production

33. BONE MARROW (cont.)
The main bones in an adult that still contain red marrow include the ribs, bodies of the vertebrae, humerus, pelvis, and femur
Yellow marrow can change to red marrow during times of decreased blood supply, such as anemia, exposure to radiation, and certain diseases

34. REGULATION OF BLOOD CALCIUM LEVELS
Skeletal system is a storehouse for about 98% of body calcium reserves
Helps maintain constancy of blood calcium levels
Calcium is mobilized and moves in and out of blood during bone remodeling
During bone formation, osteoblasts remove calcium from blood and lower circulating levels
During breakdown of bone, osteoclasts release calcium into blood and increase circulating levels

35. REGULATION OF BLOOD CALCIUM LEVELS (cont.)
Homeostasis of calcium ion concentration essential for the following:
Bone formation, remodeling, and repair
Blood clotting
Transmission of nerve impulses
Maintenance of skeletal and cardiac muscle contraction

36. REGULATION OF BLOOD CALCIUM LEVELS (cont.)
Mechanisms of calcium homeostasis (Figure 7-10)
Parathyroid hormone
Primary regulator of calcium homeostasis
Stimulates osteoclasts to initiate breakdown of bone matrix and increase blood calcium levels
Increases renal absorption of calcium from urine
Stimulates vitamin D synthesis

38. REGULATION OF BLOOD CALCIUM LEVELS (cont.)
Mechanisms of calcium homeostasis
Calcitonin
Protein hormone produced in the thyroid gland
Produced in response to high blood calcium levels
Stimulates bone deposition by osteoblasts
Inhibits osteoclast activity
Far less important in homeostasis of blood calcium levels than is parathyroid hormone

39. DEVELOPMENT OF BONES
Osteogenesis: development of bone from small cartilage model to adult bone (Figure 7-11)
Intramembranous ossification
Occurs within a connective tissue membrane
Flat bones begin when groups of cells differentiate into osteoblasts
Osteoblasts are clustered together in ossification center
Osteoblasts secrete matrix material and collagenous fibrils

41. DEVELOPMENT OF BONES (cont.)
Intramembranous ossification
Large amounts of ground substance accumulate around each osteoblast
Collagenous fibers become embedded in the ground substance and constitute the bone matrix
Bone matrix calcifies when calcium salts are deposited
Trabeculae appear and join in a network to form spongy bone
Appositional growth occurs by adding osseous tissue

42. DEVELOPMENT OF BONES (cont.)
Endochondral ossification (Figure 7-12)
Most bones begin as a cartilage model with bone formation spreading essentially from the center to the ends
Periosteum develops and enlarges to produce a collar of bone
Primary ossification center forms (Figure 7-13)
Blood vessel enters the cartilage model at the midpoint of the diaphysis
Bone grows in length as endochondral ossification progresses from the diaphysis toward each epiphysis (Figure 7-14)
Secondary ossification centers appear in the epiphysis, and bone growth proceeds toward the diaphysis
Epiphyseal plate remains between the diaphysis and each epiphysis until bone growth in length is complete

46. DEVELOPMENT OF BONES (cont.)
Epiphyseal plate is composed of four layers (Figures 7-15 and 7-16)
“Resting” cartilage cells: point of attachment joining the epiphysis to the shaft
Zone of proliferation: cartilage cells undergoing active mitosis, which causes the layer to thicken and the plate to increase in length
Zone of hypertrophy: older, enlarged cells undergoing degenerative changes associated with calcium deposition
Zone of calcification: dead or dying cartilage cells undergoing rapid calcification

49. DEVELOPMENT OF BONES (cont.)
Epiphyseal plate can be a site for bone fractures in young people (Figure 7-17)
Long bones grow in both length and diameter (Figure 7-18)

52. BONE REMODELING
Primary osteons develop within early woven bone (Figure 7-19)
Conelike or tubelike space is hollowed out by osteoclasts
Osteoblasts in the endosteum that lines the tube begin forming layers (lamellae) that trap osteocytes between layers
A central canal is left for the blood and lymphatic vessels and nerves
Primary osteons can be replaced later by secondary osteons in a similar manner
Bones grow in length and diameter by the combined action of osteoclasts and osteoblasts
Osteoclasts enlarge the diameter of the medullary cavity
Osteoblasts from the periosteum build new bone around the outside of the bone
Mechanical stress, such as physical activity, strengthens bone

54. REPAIR OF BONE FRACTURES
Fracture: break in the continuity of a bone
Fracture healing (Figure 7-20)
Fracture tears and destroys blood vessels that carry nutrients to osteocytes
Vascular damage initiates repair sequence
Callus: special repair tissue that binds the broken ends of the fracture together
Fracture hematoma: blood clot occurring immediately after the fracture, which is then resorbed and replaced by callus

56. CARTILAGE Characteristics Avascular connective tissue
Fibers of cartilage are embedded in a firm gel
Has the flexibility of firm plastic
No canal system or blood vessels
Chondrocytes receive oxygen and nutrients by diffusion
Perichondrium: fibrous covering of the cartilage
Cartilage types differ because of the amount of matrix present and the amounts of elastic and collagenous fibers

57. CARTILAGE (cont.) Types of cartilage (Figure 7-21) Hyaline cartilage
Most common type
Covers the articular surfaces of bones
Forms the costal cartilages, cartilage rings in the trachea, bronchi of the lungs, and the tip of the nose
Forms from special cells in chondrification centers, which secrete matrix material
Chondrocytes are isolated into lacunae

59. CARTILAGE (cont.) Types of cartilage Elastic cartilage Fibrocartilage
Forms external ear, epiglottis, and eustachian tubes
Large number of elastic fibers confers elasticity and resiliency
Fibrocartilage
Occurs in pubic symphysis and intervertebral disks
Small quantities of matrix and abundant fibrous elements
Strong and rigid

60. CARTILAGE (cont.) Functions
Tough, rubberlike nature permits cartilage to sustain great weight or serve as a shock absorber
Strong yet pliable support structure
Permits growth in length of long bones

61. CARTILAGE (cont.) Growth of cartilage
Interstitial or endogenous growth
Cartilage cells divide and secrete additional matrix
Seen during childhood and early adolescence while cartilage is still soft and capable of expansion from within
Appositional or exogenous growth
Chondrocytes in the deep layer of the perichondrium divide and secrete matrix
New matrix is deposited on the surface, thereby increasing its size
Unusual in early childhood, but once initiated continues throughout life

62. CYCLE OF LIFE: SKELETAL TISSUES
Skeleton fully ossified by mid-20s
Soft tissue may continue to grow; ossifies more slowly
Adults: changes occur from specific conditions
Increased density and strength from exercise
Decreased density and strength from pregnancy, nutritional deficiencies, and illness
Advanced adulthood: apparent degeneration
Hard bone matrix replaced by softer connective tissue
Exercise can counteract degeneration