3. Elastic cartilages are more flexible than hyaline, and are located only in the external ear and the epiglottis of the larynx.
4. Fibrocartilage is located in areas that must withstand a great deal of pressure or stretch, such as the cartilages of the knee and the intervertebral discs.
B. Growth of Cartilage (p. 173)
1. Appositional growth results in outward expansion due to the production of cartilage matrix on the outside of the tissue.
2. Interstitial growth results in expansion from within the cartilage matrix due to division of lacunae-bound chondrocytes and secretion of matrix.
II. Classification of Bones (pp. 173–175; Figs. 6.1–6.2)
A. There are two main divisions of the bones of the skeleton: the axial skeleton, consisting of the skull, vertebral column, and rib cage; and the appendicular skeleton, consisting of the bones of the upper and lower limbs, and the girdles that attach them to the axial skeleton (pp. 173–174; Fig. 6.1).
B. Shape (pp. 174–175; Fig. 6.2)
1. Long bones are longer than they are wide, have a definite shaft and two ends, and consist of all limb bones except patellas, carpals, and tarsals.
2. Short bones are somewhat cube shaped and include the carpals and tarsals.
3. Flat bones are thin, flattened, often curved bones that include most skull bones, the sternum, scapulae, and ribs.
A. Bones support the body and cradle the soft organs, protect vital organs, allow movement, store minerals such as calcium and phosphate, and house hematopoietic tissue in specific marrow cavities (pp. 175–176).
IV. Bone Structure (pp. 176–182; Figs. 6.3–6.7; Table 6.1)
A. Gross Anatomy (pp. 176–178; Figs. 6.3, 6.5; Table 6.1)
1. Bone markings are projections, depressions, and openings found on the surface of bones that function as sites of muscle, ligament, and tendon attachment, as joint surfaces, and as openings for the passage of blood vessels and nerves.
2. Bone Textures: Compact and Spongy Bone
a. All bone has a dense outer layer consisting of compact bone that appears smooth and solid.
b. Internal to compact bone is spongy bone, which consists of honeycomb, needle-like, or flat pieces, called trabeculae.
3. Structure of a Typical Long Bone
a. Long bones have a tubular bone shaft, consisting of a bone collar surrounding a hollow medullary cavity, which is filled with yellow bone marrow in adults.
c. The epiphyseal line is located between the epiphyses and diaphysis, and is a remnant of the epiphyseal plate.
d. The external surface of the bone is covered by the periosteum.
e. The internal surface of the bone is lined by a connective tissue membrane called the endosteum.
4. Structure of Short, Flat, and Irregular Bones
a. Short, flat, and irregular bones consist of thin plates of periosteum-covered compact bone on the outside, and endosteum-covered spongy bone inside, which houses bone marrow between the trabeculae.
5. Location of Hematopoietic Tissue in Bones
a. Hematopoietic tissue of bones, red bone marrow, is located within the trabecular cavities of the spongy bone in flat bones, and in the epiphyses of long bones.
b. Red bone marrow is found in all flat bones, epiphyses, and medullary cavities of infants, but in adults, distribution is restricted to flat bones and the proximal epiphyses of the humerus and femur.
B. Microscopic Anatomy of Bone (pp. 179–180; Figs. 6.3–6.7)
1. The structural unit of compact bone is the osteon, or Haversian system, which consists of concentric tubes of bone matrix (the lamellae) surrounding a central Haversian canal that serves as a passageway for blood vessels and nerves.
a. Perforating, or Volkmann’s, canals lie at right angles to the long axis of the bone, and connect the blood and nerve supply of the periosteum to that of the central canals and medullary cavity.
b. Osteocytes occupy lacunae at the junctions of the lamellae, and are connected to each other and the central canal via a series of hairlike channels, canaliculi.
c. Circumferential lamellae are located just beneath the periosteum, extending around the entire circumference of the bone, while interstitial lamellae lie between intact osteons, filling the spaces in between.
2. Spongy bone lacks osteons but has trabeculae that align along lines of stress, which contain irregular lamellae.
C. Chemical Composition of Bone (p. 180)
1. Organic components of bone include cells (osteoblasts, osteocytes, and osteoclasts) and osteoid (ground substance and collagen fibers), which contribute to the flexibility and tensile strength of bone.
2. Inorganic components make up 65% of bone by mass, and consist of hydroxyapatite, a mineral salt that is largely calcium phosphate, which accounts for the hardness and compression resistance of bone.
V. Bone Development (pp. 182–185; Figs. 6.8–6.11)
A. Formation of the Bony Skeleton (pp. 182–184; Figs. 6.8–6.9)
1. Intramembranous ossification forms membrane bone from fibrous connective tissue membranes, and results in the cranial bones and clavicles.
2. In endochondral ossification bone tissue replaces hyaline cartilage, forming all bones below the skull except for the clavicles.
a. Initially, osteoblasts secrete osteoid, creating a bone collar around the diaphysis of the hyaline cartilage model.
b. Cartilage in the center of the diaphysis calcifies and deteriorates, forming cavities.
c. The periosteal bud invades the internal cavities and spongy bone forms around the remaining fragments of hyaline cartilage.
d. The diaphysis elongates as the cartilage in the epiphyses continues to lengthen and a medullary cavity forms through the action of osteoclasts within the center of the diaphysis.
e. The epiphyses ossify shortly after birth through the development of secondary ossification centers.
B. Postnatal Bone Growth (pp. 184–185; Figs. 6.10–6.11)
1. Growth in length of long bones occurs at the ossification zone through the rapid division of the upper cells in the columns of chondrocytes, calcification and deterioration of cartilage at the bottom of the columns, and subsequent replacement by bone tissue.
2. Growth in width, or thickness, occurs through appositional growth due to deposition of bone matrix by osteoblasts beneath the periosteum.
3. Hormonal Regulation of Bone Growth
a. During infancy and childhood, the most important stimulus of epiphyseal plate activity is growth hormone from the anterior pituitary, whose effects are modulated by thyroid hormone.
b. At puberty, testosterone and estrogen promote a growth spurt, but ultimately induct the closure of the epiphyseal plate.
VI. Bone Homeostasis: Remodeling and Repair (pp. 185–190; Figs. 6.11–6.15; Table 6.2)
A. Bone Remodeling (pp. 185–188; Figs. 6.11–6.13)
1. In adult skeletons, bone remodeling is balanced bone deposit and removal; bone deposit occurs at a greater rate when bone is injured; and bone resorption allows minerals of degraded bone matrix to move into the blood.
2. Control of Remodeling
a. The hormonal mechanism is mostly used to maintain blood calcium homeostasis, and balances activity of parathyroid hormone and calcitonin.
b. In response to mechanical stress and gravity, bone grows or remodels in ways that allow it to withstand the stresses it experiences.
B. Bone Repair (pp. 188–190; Fig. 6.15; Table 6.2)
1. Fractures are breaks in bones, and are classified by: the position of the bone ends after fracture, completeness of break, orientation of the break relative to the long axis of the bone, and whether the bone ends penetrate the skin.
2. Repair of fractures involves four major stages: hematoma formation, fibrocartilaginous callus formation, bony callus formation, and remodeling of the bony callus.
VII. Homeostatic Imbalances of Bone (pp. 189–191, 194; Fig. 6.16)
A. Osteomalacia and Rickets (p. 189)
1. Osteomalacia includes a number of disorders in adults in which the bone is inadequately mineralized.
2. Rickets is inadequate mineralization of bones in children caused by insufficient calcium or vitamin D deficiency.
B. Osteoporosis refers to a group of disorders in which the rate of bone resorption exceeds the rate of formation (pp. 189–191, Fig. 6.16).
1. Bones have normal bone matrix, but bone mass is reduced and the bones become more porous and lighter, increasing the likelihood of fractures.
3. Other factors that contribute to osteoporosis include a petite body form, insufficient exercise or immobility, a diet poor in calcium and vitamin D, abnormal vitamin D receptors, smoking, and certain hormone-related conditions.
C. Paget’s disease is characterized by excessive bone deposition and resorption, with the resulting bone abnormally high in spongy bone. It is a localized condition that results in deformation of the affected bone (pp. 191, 194).
VIII. Developmental Aspects of Bones: Timing of Events (p. 194; Fig. 6.17)
A. The skeleton derives from embryonic mesenchymal cells, with ossification occurring at precise times. Most long bones have obvious primary ossification centers by 12 weeks gestation (p. 194).
B. At birth, most bones are well ossified, except for the epiphyses, which form secondary ossification centers (p. 194).
C. Throughout childhood, bone growth exceeds bone resorption; in young adults, these processes are in balance; in old age, resorption exceeds formation (p. 194).
1. Marieb, E. N., and S. J. Mitchell. Human Anatomy & Physiology Laboratory Manual: Main Version. Eighth Edition Update. Benjamin Cummings, 2009.
Exercise 9: Overview of the Skeleton—Classification and Structure of Bones and Cartilages
The following shows the organization of the Chapter Guide page in myA&P™. The Chapter Guide organizes all the chapter-specific online media resources for Chapter 6 in one convenient location, with e-book links to each section of the textbook. Students can also access A&P Flixanimations, MP3 Tutor Sessions, Interactive Physiology® 10-System Suite, Practice Anatomy Lab™ 2.0, PhysioEx™ 8.0, and much more.
Section 6.1 Skeletal Cartilages (p. 173)
Section 6.2 Classification of Bones (pp. 173–175)
Section 6.3 Functions of Bones (pp. 175–176)
Section 6.4 Bone Structure (pp. 176–182)
Art Labeling: Structure of a Long Bone (Fig. 6.3, p. 176)
Art Labeling: Microscopic Anatomy of Compact Bone (Fig. 6.7, p. 181)
Memory Game: The Architecture of Bone
Memory Game: Cartilage and Bone Structure
Section 6.5 Bone Development (pp. 181–185)
Section 6.6 Bone Homeostasis: Remodeling and Repair (pp. 185–190)
MP3 Tutor Session: Calcium Regulation
MP3 Tutor Session: How Bones React to Stress
Section 6.7 Homeostatic Imbalances of Bone (pp. 189–191)
Section 6.8 Developmental Aspects of Bones: Timing of Events (p. 194)
Crossword Puzzle 6.1
Crossword Puzzle 6.2
Art Labeling Quiz
Chapter Practice Test
Resources in the myA&P™ Chapter Guide are also available in the Chapter Contents section of the CourseCompass™. Students can also access A&P Flix animations, MP3 Tutor Sessions, Interactive Physiology® 10-System Suite, Practice Anatomy Lab™ 2.0, PhysioEx™ 8.0, and much more.
Answers to End-of-Chapter Questions
Multiple Choice and Matching Question answers appear in Appendix G of the main text.
Short Answer Essay Questions
15. Cartilage has greater resilience because its matrix lacks bone salts, but its cells receive nutrients via diffusion from blood vessels that lie external to the cartilage. By contrast, bone has a beautifully engineered system of canaliculi for nutrient delivery, and for that reason its regeneration is much faster and more complete. (pp. 173, 179–180)
16. a. A bone collar is laid down around the diaphysis of the hyaline cartilage model.
b. Cartilage in the center of the diaphysis calcifies and then develops cavities.
c. The periosteal bud invades the internal cavities and spongy bone forms.
d. The diaphysis elongates and a medullary cavity forms.
e. The epiphyses ossify. (pp. 183–184)
17. Volkmann’s canals radiate throughout the bone matrix, linking the central canal, periosteum, and all lamellae. Canaliculi interconnect all lacunae, allowing transfer of nutrients and wastes between the blood and osteocytes. (p. 179)
18. The increase in thickness of compact bone on its superficial face is counteracted by the resorption of bone by osteoclasts on its internal surface. (pp. 185–186)
19. An osteoid seam is an unmineralized band of bony matrix that is about 10–12 micrometers wide. Between this seam and older bone is an abrupt transition called a calcification front. The osteoid seam always stays the same width, indicating that osteoid tissue must mature before it can be calcified. This area then changes quickly from an unmineralized matrix to a mineralized matrix. (p. 186)
20. Two control loops regulate bone remodeling. One is a negative feedback hormonal mechanism that maintains blood Ca2+, and the other involves mechanical and gravitational forces acting on the skeleton. (p. 186)
21. a. First decade is fastest; fourth decade is slowest. (p. 194)
b. Elderly people usually experience bone loss, osteoporosis, and an increasing lack of blood supply. (p. 191)
c. Children have proportionally more organic matrix. (p. 190)
22. This bone section is taken from the diaphysis of the specimen. The presence of an osteon, the concentric layers surrounding a central cavity, indicates compact bone found in the diaphysis. The epiphyseal plate, the site of active bone growth, lacks osteons. (pp. 179, 181)
Critical Thinking and Clinical Application Questions
1. A bony callus represents the conversion of the fibrocartilaginous callus to a callus containing trabeculae of spongy bone. (p. 189)
2. Rickets. Milk provides dietary calcium and vitamin D. Vitamin D is needed for its uptake by intestinal cells; the sun helps the skin synthesize vitamin D. Thick epiphyseal plates indicate poor calcification of the growing area. Because of this lack of sufficient calcium, the bones will be more pliable, and weight-bearing bones, like those in the leg, will bend. (p. 189)
3. The compact lamellar structure of dense bone produces structural units designed to resist twisting and other mechanical stresses placed on bones. In contrast, spongy bone is made up of trabeculae only a few cell layers thick containing irregularly arranged lamellae. (p. 187)
4. Because the changes throughout adolescence involve changes in stress associated with muscle growth, and sex steroids that, in part, affect bone deposition, a lack of bone remodeling during adolescence would keep bones in more of a preadolescent form. Bones would retain a thinner, less dense appearance than would usually be expected, especially in areas that are under the greatest stress. (pp. 185–187)
5. According to Wolff’s law, bone growth and remodeling occur in response to stress placed on such bones. With disuse, the bones in the limbs not being used will begin to atrophy. (p. 187)
6. Presumably the epiphyseal plate-bone junction has separated. The same would not happen to the boy’s 23-year-old sister because by this age, epiphyseal plates have been replaced by bone and are no longer present. (p. 185)
7. Paget’s disease, which results in irregular thickening of bone tissue and often affects the skull and spine, causing pain and deformity. (p. 191)