04/18/23 1

Chapter 7: Skeletal Tissues

04/18/23 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

04/18/23 3

04/18/23 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

04/18/23 5

04/18/23 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

04/18/23 7

04/18/23 8

TYPES OF BONES (cont.)

Articular cartilage 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

04/18/23 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

04/18/23 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

04/18/23 11

04/18/23 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

04/18/23 13

BONE TISSUE (cont.)

Composition of bone matrix Inorganic salts

Hydroxyapatite: crystals of calcium and phosphate contribute to bone hardness

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 Chondroitin sulfate (compression) and glucosamine (growth

and repair) Adds to overall strength of bone and gives some degree of

resilience to bone

04/18/23 14

MICROSCOPIC STRUCTURE OF BONE Compact bone (Figure 7-5) – 80%

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

04/18/23 15

04/18/23 16

04/18/23 17

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

04/18/23 18

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

04/18/23 19

MICROSCOPIC STRUCTURE OF BONE (cont.) Cancellous bone (Figure 7-6) – 20%

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

04/18/23 20

04/18/23 21

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

04/18/23 22

04/18/23 23

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)

04/18/23 24

04/18/23 25

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

04/18/23 26

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

04/18/23 27

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

04/18/23 28

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

04/18/23 29

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 pH regulation

04/18/23 30

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 Increases Blood [Ca++] levels When blood passing through the parathyroid gland is

sufficient, PTH secretion is stopped

04/18/23 31

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 Decreases Blood [Ca++]

04/18/23 32

04/18/23 33

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

04/18/23 34

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

04/18/23 35

04/18/23 36

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

04/18/23 37

04/18/23 38

04/18/23 39

04/18/23 40

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

04/18/23 41

04/18/23 42

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 (interstitial growth) and diameter (appositional growth) (Figure 7-18)

Why care about an epiphyseal plate fracture?

04/18/23 43

04/18/23 44

04/18/23 45

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 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 Between 35-40 bone loss surpasses bone growth

Mechanical stress, such as physical activity, strengthens bone

04/18/23 46

04/18/23 47

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 Fracture hematoma: blood clot occurring

immediately after the fracture, which is then resorbed and replaced by callus

Callus: special repair tissue that stabilizes the bone so healing can occur and bone replaces callus

04/18/23 48

04/18/23 49

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

04/18/23 50

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

04/18/23 51

04/18/23 52

CARTILAGE (cont.)

Types of cartilage Elastic cartilage

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

04/18/23 53

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

04/18/23 54

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

04/18/23 55

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