Chapter 6 - Bones and Skeletal Tissues PDF

6 by first exploring Bones and Skeletal Tissues and asking In this chapter, you will learn that Bones and cartilages form the internal supports of the body...

6 by first exploring Bones and Skeletal Tissues and asking In this chapter, you will learn that Bones and cartilages form the internal supports of the body and next asking then asking 6.1 Skeletal 6.2 What functions 6.5 How do 6.8 What happens when cartilages do bones perform? bones develop? things go wrong? and and and finally, exploring 6.3 How are bones 6.6 How are Developmental Aspects classified? bones remodeled? of Bones looking closer at and 6.4 Bone structure 6.7 How are bones repaired? Bones appear to be the most lifeless of body organs, CAREER CONNECTION and may even summon images of a graveyard. However, the bones within you are not the dead, dry bones that you may have seen. Rather, they are living, dynamic organs, and bone tissue is formed and remodeled throughout life. Without bones to form our internal supporting skeleton, we would all creep along the ground like slugs, lacking any definite shape or form. Along with its bones, the skeleton contains resilient cartilages, which we briefly discuss at the beginning of this chapter. Play a video to learn how the chapter content is used in a real healthcare setting @ Mastering A&P > Study Area. 173  174 UNIT 2 Covering, Support, and Movement of the Body external ear and the epiglottis (the flap that bends to cover the 6.1Hyaline, elastic, and opening of the larynx each time we swallow). fibrocartilage help form the skeleton Fibrocartilages Learning Outcomes Both resistant to compression and having great tensile strength, N Describe the functional properties of the three types of fibrocartilages consist of roughly parallel rows of chondrocytes cartilage tissue. alternating with thick collagen fibers ( Figure 4.11i, p. 136). N Locate the major cartilages of the adult skeleton. Fibrocartilages occur in sites that are subjected to both pressure N Explain how cartilage grows. and stretch, such as the padlike cartilages (menisci) of the knee The human skeleton is initially made up of cartilages and and the discs between vertebrae (colored red in Figure 6.1). fibrous membranes, but bone soon replaces most of these early supports. The few cartilages that remain in adults are found Growth of Cartilage mainly in regions where flexible skeletal tissue is needed. Unlike bone, which has a hard matrix, cartilage has a flexible matrix that can accommodate mitosis. It is the ideal tissue to Basic Structure, Types, and Locations use to rapidly lay down the embryonic skeleton and to provide A skeletal cartilage is made of some variety of cartilage tissue for new skeletal growth. sculpted to fit its body location and function. Cartilage con- Cartilage grows in two ways. sists primarily of water. It is very resilient—it has the ability to Appositional growth. In appositional growth (ap″o- 11 6 spring back to its original shape after being compressed. zish′un-al), cartilage-forming cells in the surrounding peri- Cartilage contains no nerves or blood vessels. It is sur- chondrium secrete new matrix against the external face of the rounded by a layer of dense irregular connective tissue, the existing cartilage tissue. perichondrium (per″ĭ-kon′dre-um; “around the cartilage”). The Interstitial growth. In interstitial growth (in″ter-stish′al), perichondrium acts as reinforcement to resist outward expan- the lacunae-bound chondrocytes divide and secrete new sion when the cartilage is compressed. The perichondrium matrix, expanding the cartilage from within. also contains blood vessels that nourish the cartilage cells. The thickness of cartilage is limited by the distance nutrients can Typically, cartilage growth ends during adolescence, once the diffuse through the matrix to reach the cells. skeleton has stopped growing. As we described in Chapter 4 ( p. 134), the three types of Under certain conditions—during normal bone growth in cartilage tissue are hyaline, elastic, and fibrocartilage. All three youth and during older adulthood, for example—cartilage can types have the same basic components—cells called chondro- become calcified (hardened due to deposit of calcium salts). cytes, enclosed in small cavities (lacunae) within an extracel- Note, however, that calcified cartilage is not bone; cartilage and lular matrix containing a jellylike ground substance and fibers. bone are always distinct tissues. Table 6.1 compares some of the key characteristics of carti- Hyaline Cartilages lage and bone ( p. 174). This overview will help orient you as you discover more about bone in the rest of the chapter. Hyaline cartilages, which look like frosted glass when freshly exposed, provide support with flexibility and resilience. They Check Your Understanding are the most abundant skeletal cartilages. Their chondrocytes 1. Which type of cartilage is most plentiful in the adult body? are spherical ( Figure 4.11g, p. 135), and the only fiber type 2. What two body structures contain flexible elastic cartilage? in their matrix is fine collagen fibers (which are undetectable 3. Cartilage grows by interstitial growth. What does this mean? microscopically). Colored blue in Figure 6.1, skeletal hyaline cartilages include: For answers, see Answers Appendix. Articular cartilages ( artic = joint, point of connection ), which cover the ends of most bones at movable joints Table 6.1 Comparison of Cartilage and Bone Costal cartilages, which connect the ribs to the sternum CARTILAGE BONE (breastbone) Surrounded by perichondrium Surrounded by periosteum Respiratory cartilages, which form the skeleton of the lar- No blood vessels or nerves (only Blood vessels and nerves ynx (voice box) and reinforce other respiratory passageways in perichondrium) throughout Nasal cartilages, which support the external nose Chondrocytes in lacunae Osteocytes in lacunae Flexible extracellular matrix Rigid extracellular matrix (due Elastic Cartilages to inorganic calcium salts) Elastic cartilages resemble hyaline cartilages ( Figure 4.11h, Extracellular matrix made by Extracellular matrix (organic chondroblasts part) made by osteoblasts p. 135), but they contain more stretchy elastic fibers and so are better able to stand up to repeated bending. They are found in Appositional growth and Appositional growth only interstitial growth only two skeletal locations (shown in green in Figure 6.1)—the  Chapter 6 Bones and Skeletal Tissues 175 Elastic cartilages Cartilage in external ear Epiglottis Axial skeleton Hyaline cartilages Appendicular Cartilages in nose Larynx skeleton Thyroid cartilage Cricoid cartilage Trachea Lung Fibrocartilages Cartilage Respiratory tube in intervertebral cartilages in neck disc and thorax Costal cartilage Articular cartilage of a joint 11 6 Pubic symphysis Articular cartilage of a joint Meniscus (padlike cartilage in knee joint) Figure 6.1 The bones and cartilages of the human skeleton. The cartilages that support the respiratory tubes and larynx are drawn separately at the right. Bones perform several 6.2 act as pillars to support the body trunk when we stand, and the important functions rib cage supports the thoracic wall. Learning Outcome Protection. The fused bones of the skull protect the brain. The vertebrae surround the spinal cord, and the rib cage N Describe the functions of the skeleton and of bone tissue. helps protect the vital organs of the thorax. Our bones perform seven important functions: Anchorage. Skeletal muscles, which attach to bones by ten- Support. Bones provide a framework that supports the body dons, use bones as levers to move the body and its parts. As and cradles its soft organs. For example, bones of lower limbs a result, we can walk, grasp objects, and breathe.  176 UNIT 2 Covering, Support, and Movement of the Body Mineral storage. Bone is a reservoir for minerals, most Long bones, as their name suggests, are considerably longer importantly calcium and phosphate. The stored minerals than they are wide (Figure 6.2a). A long bone has a cylindrical are released into the blood in their ionic form as needed shaft with two distinct ends, which are often expanded. A cav- for distribution to all parts of the body. Indeed, “deposits” ity (the medullary cavity described later) extends the length and “withdrawals” of minerals to and from the bones go on of the shaft along its center. All limb bones except the patella ­almost continuously. (kneecap) and the wrist and ankle bones are long bones. Blood cell formation. Most blood cell formation, or hemat- Notice that these bones are named for their elongated shape, opoiesis (hem″ah-to-poy-e′sis), occurs in the red bone mar- not their overall size. The three bones in each of your fingers row of certain bones. are long bones, even though they are small. Triglyceride (fat) storage. Fat, a source of energy for the Short bones are roughly cube shaped. The bones of the body, is stored as yellow bone marrow in the cavities of long wrist and ankle are examples (Figure 6.2d). Sesamoid bones bones. (ses′ah-moid; “shaped like a sesame seed”) are a special type of short bone that form in a tendon (for example, the Hormone production. Bones produce osteocalcin, a hor- patella). They vary in size and number in different individu- mone that helps to regulate insulin secretion, glucose home- als. Some sesamoid bones act to alter the direction of pull ostasis, and energy expenditure (see Chapter 16). of a tendon. Others reduce friction and modify pressure on tendons to reduce abrasion or tearing. Check Your Understanding Flat bones are thin, flattened, and usually a bit curved. The 4. What is the functional relationship between skeletal muscles sternum (breastbone), ribs, and most cranial bones of the skull 11 6 and bones? are flat bones (Figure 6.2c). 5. List two types of substances stored in bone and state where each is stored. Irregular bones have complicated shapes that fit none of the For answers, see Answers Appendix. preceding classes. Examples include the vertebrae and the hip bones (Figure 6.2b). Check Your Understanding 6.3Bones are classified by their 6. What are the components of the axial skeleton? location and shape 7. Contrast the general function of the axial skeleton to that of the appendicular skeleton. Learning Outcomes 8. What bone class do the ribs and skull bones fall into? N Name the major regions of the skeleton and describe For answers, see Answers Appendix. their relative functions. N Compare and contrast the four bone classes and provide examples of each class. The 206 named bones of the human skeleton are divided into two groups: axial and appendicular. All bones consist of outer 6.4 The axial skeleton forms the long axis of the body and compact bone and inner spongy bone includes the bones of the skull, vertebral column, and rib cage Learning Outcomes (shown in orange in Figure 6.1). Generally speaking, these N Describe the gross anatomy of a typical flat bone and a bones protect, support, or carry other body parts. long bone. Indicate the locations and functions of red and The appendicular skeleton (ap″en-dik′u-lar) consists of the yellow bone marrow, articular cartilage, periosteum, and bones of the upper and lower limbs and the girdles (shoulder endosteum. bones and hip bones) that attach the limbs to the axial skel- N Indicate the functional importance of bone markings. eton (colored gold in Figure 6.1). Bones of the limbs help us N Describe the histology of compact and spongy bone. move from place to place (locomotion) and manipulate our N Discuss the chemical composition of bone and the environment. advantages conferred by its organic and inorganic components. Bones come in many sizes and shapes. For example, the pisi- form bone of the wrist is the size and shape of a pea, whereas the Because they contain different types of tissue, bones are femur (thigh bone) is nearly 2 feet long in some people and has organs. (Recall that an organ contains several different tissues.) a large, ball-shaped head. The unique shape of each bone fulfills Although bone (osseous) tissue dominates bones, they also con- a particular need. The femur, for example, withstands great pres- tain nervous tissue in their nerves, cartilage in their articular sure, and its hollow-cylinder shape provides maximum strength cartilages, dense connective tissue covering their external sur- with minimum weight to accommodate our upright posture. face, and muscle and epithelial tissues in their blood vessels. Bones are classified by their shape as long, short, flat, or We will consider bone structure at three levels: gross, micro- irregular (Figure 6.2). scopic, and chemical.  Chapter 6 Bones and Skeletal Tissues 177 (c) Flat bone (sternum) (a) Long bone (humerus) 11 6 (b) Irregular bone (vertebra), right lateral view (d) Short bone (talus) Figure 6.2 Classification of bones on the basis of shape. Gross Anatomy Compact and Spongy Bone Every bone has a dense outer layer that looks smooth and solid to the naked eye. This external layer is compact bone (Fig- ures 6.3, 6.4, and 6.5). Internal to this is spongy bone (also called trabecular bone), a honeycomb of small needle-like or flat pieces called trabeculae (trah-bek′u-le; “little beams”). In Spongy bone living bones the open spaces between trabeculae are filled with has a mesh of red or yellow bone marrow. bony spines called trabeculae. Compact bone looks smooth and solid. Figure 6.3 Compact and spongy bone. Photo of a sectioned proximal femur (thigh bone).  178 UNIT 2 Covering, Support, and Movement of the Body Diaphysis A tubular diaphysis (di-af′ĭ-sis; dia = through, physis = growth ), or shaft, forms the long axis of the bone. It is constructed of a relatively thick collar of compact bone that surrounds a central medullary cavity (med′u-lar-e; “middle”), or marrow cavity, that contains no bone tissue. Instead, the med- ullary cavity contains yellow bone marrow (fat) in adults and so is called the yellow marrow cavity. Between the marrow and the compact bone, there is often a thin layer of spongy bone. Epiphyses The epiphyses (e-pif′ĭ-sēz; singular: epiphysis) are the bone ends ( epi = upon ). An outer shell of compact bone forms the epiphysis exterior and the interior contains spongy bone. A thin layer of articular (hyaline) cartilage covers the Compact joint surface of each epiphysis, cushioning the opposing bone bone ends during movement and absorbing stress. Spongy Between the diaphysis and each epiphysis of an adult long bone bone is an epiphyseal line, a remnant of the epiphyseal plate. (diploë) The epiphyseal plate, commonly called the growth plate, is a disc of hyaline cartilage that grows during childhood to Compact 11 6 bone lengthen the bone. The flared portion of the bone where the diaphysis and epiphysis meet, whether it is the epiphyseal plate or line, is called the metaphysis ( meta = between ). Membranes A glistening white, double-layered membrane called the periosteum (per″e-os′te-um; peri = around, osteo = bone ) covers the external surface of the entire bone except the joint surfaces. The outer fibrous layer of the periosteum Trabeculae of spongy is dense irregular connective tissue. The inner osteogenic layer bone next to the bone surface contains osteoprogenitor cells (primi- tive stem cells that give rise to most bone cells). It also has bone- destroying cells (osteoclasts) and bone-forming cells (osteoblasts). The periosteum is richly supplied with nerve fibers and blood vessels, which is why broken bones are painful and bleed profusely. Perforating fibers—bundles of collagen fibers that extend into the bone matrix—secure the periosteum to the underlying bone (Figure 6.5c). The periosteum also provides anchoring points for tendons and ligaments. At these points the Figure 6.4 Structure of a flat bone. Flat bones consist of a layer perforating fibers are exceptionally dense. of spongy bone sandwiched between two thin layers of compact A delicate connective tissue membrane called the endos- bone. (Photomicrograph at bottom, 25×) teum (en-dos′te-um; “within the bone”) covers internal bone surfaces (Figure 6.5). The endosteum covers the trabeculae of Structure of Short, Irregular, and Flat Bones spongy bone and lines the canals that pass through the compact bone. The endosteum contains the same cell types as the inner Short, irregular, and flat bones share a simple pattern: They layer of the periosteum. all consist of thin plates of spongy bone (diploë) covered by compact bone. The compact bone is covered outside and inside Blood Vessels and Nerves Unlike cartilage, bones are well by connective tissue membranes, respectively the periosteum vascularized. The main vessels serving the diaphysis are a nutri- and endosteum. However, these bones are not cylindrical and so ent artery (Figure 6.5c) and a nutrient vein. Together these run they have no shaft or expanded ends. They contain bone mar- through a hole in the wall of the diaphysis, the nutrient foramen row (between their trabeculae), but no well-defined marrow (fo-ra′men; “opening”). The nutrient artery runs inward to sup- cavity. Where they form movable joints with their neighbors, ply the bone marrow and the spongy bone. Branches then extend hyaline cartilage covers their surfaces. outward to supply the compact bone. Several epiphyseal arteries Figure 6.4 shows a typical flat bone arrangement: compact and veins serve each epiphysis in the same way. However, recent bone–spongy bone–compact bone, which resembles a stiffened evidence suggests that most of the blood flow in long bones sandwich. takes a different course. Both arterial and venous blood goes through an extensive network of very fine vessels that extend Structure of a Typical Long Bone between the periosteum and the medullary cavity. (These vessels With few exceptions, all long bones have the same general are so tiny that they are not shown in Figure 6.5.) Nerves accom- structure: a shaft, bone ends, and membranes (Figure 6.5). pany blood vessels through the nutrient foramen into the bone.  Chapter 6 Bones and Skeletal Tissues 179 Hematopoietic Tissue in Bones sternum) and in some irregular bones (such as the hip bone) is Hematopoietic (blood-forming) tissue is also called red bone much more active in hematopoiesis. When clinicians suspect marrow. It is found in different locations in infants and adults. problems with the blood-forming tissue, they obtain red mar- In infants, the medullary cavity of the diaphysis and all areas row samples from these sites. However, yellow marrow in the of spongy bone contain red marrow. In adults, much of the red medullary cavity can revert to red marrow if a person becomes marrow, particularly in long bones, has been replaced by yel- very anemic (has low oxygen-carrying capacity) and needs low marrow. In most adult long bones, the fat-containing yellow more red blood cells. marrow extends well into the epiphysis, and little red marrow is present in the spongy bone cavities. As a result, red marrow Bone Markings in adults is only found in the cavities between trabeculae of The external surface of a bone is rarely smooth and featureless. spongy bone in: Instead, it has distinct bone markings that provide a wealth The flat bones of the skull, as well as the sternum, ribs, clavi- of information about how that bone and its attached muscles cles, scapulae, hip bones, and vertebrae and ligaments work together. Bone markings fit into three cat- egories: (1) projections that are sites of muscle and ligament The heads of the femur (thigh bone) and humerus (arm bone) attachment, (2) surfaces that form joints, and (3) depressions Compared to long bones (femur and humerus), the red and openings for blood vessels and nerves. Table 6.2 on p. 180 marrow found in the spongy bone of flat bones (such as the describes the most important bone markings. These will help 11 6 Articular cartilage Compact bone Proximal epiphysis Spongy bone Endosteum Epiphyseal line Periosteum Endosteum Compact bone Medullary cavity (lined by endosteum) (b) Yellow Diaphysis bone marrow Compact bone Nutrient foramen Periosteum Perforating fibers Nutrient artery Distal epiphysis (a) (c) Figure 6.5 The structure of a long view of spongy bone and compact bone of periosteum, but the articular surface of the bone (humerus of arm). (a) Anterior view the epiphysis of (a). (c) Enlargement of the epiphysis is covered with hyaline cartilage. with bone sectioned frontally to show the diaphysis from (a). Note that the external interior at the proximal end. (b) Enlarged surface of the diaphysis is covered by  180 UNIT 2 Covering, Support, and Movement of the Body Table 6.2 Bone Markings NAME OF BONE MARKING DESCRIPTION ILLUSTRATIONS Projections That Are Sites of Muscle and Ligament Attachment Tuberosity Large rounded projection; may be Iliac (too″bĕ-ros′ı̆-te) roughened crest Crest Narrow ridge of bone; usually prominent Trochanter Intertrochanteric line Trochanter Very large, blunt, irregularly shaped (tro-kan′ter) process (the only examples are on the femur) Ischial Line Narrow ridge of bone; less prominent than spine a crest Tubercle Small rounded projection or process Hip Ischial (too′ber-kl) bone tuberosity Adductor Epicondyle Raised area on or above a condyle tubercle (ep″ı̆-kon′dīl) Vertebra Femur of Medial Spine Sharp, slender, often pointed projection thigh epicondyle Process Any bony prominence Condyle 11 6 Spinous process Surfaces That Help to Form Joints Head Bony expansion carried on a narrow neck Head Facet Smooth, nearly flat articular (joint) surface Condyle Condyle (kon′dīl) Rounded articular projection; often articulates with a corresponding fossa Facets Rib Mandible    Depressions and Openings For Passage of Blood Vessels and Nerves Groove Furrow Fissure Narrow, slitlike opening Inferior Meatus orbital Foramen Round or oval opening through a bone Sinus fissure (fo-ra′men) Fossa Foramen Notch Indentation at the edge of a structure Others Notch Meatus Canal-like passageway Groove (me-a′tus) Skull Sinus Cavity within a bone, filled with air and lined with mucous membrane Fossa (fos′ah) Shallow, basin-like depression in a bone, often serving as an articular surface you understand the detailed structure of the individual bones cells is what makes bone a dynamic living tissue because these you will study in Chapter 7. cells continuously resorb (break down) and deposit bone in a process called bone remodeling. Microscopic Anatomy of Bone Osteoprogenitor Cells Osteoprogenitor cells, also called Cells of Bone Tissue osteogenic cells, are mitotically active stem cells found in the Five major cell types populate bone tissue: osteoprogenitor membranous periosteum and endosteum. In growing bones they cells, osteoblasts, osteocytes, bone lining cells, and osteoclasts are flattened or squamous cells. When stimulated, these cells dif- (Figure 6.6). All of these except for the osteoclasts originate ferentiate into osteoblasts, while others persist as osteoprogeni- from embryonic connective tissue cells. The presence of bone tor cells.  Chapter 6 Bones and Skeletal Tissues 181 (a) Osteoprogenitor cell (b) Osteoblast (c) Osteocyte Some Some osteoprogenitor osteoblasts cells become become Stem cell Matrix-synthesizing cell Mature bone cell Responsible for Monitors and maintains the bone growth mineralized bone matrix (d) Osteoclast (e) Nuclei of osteoclast 11 6 Ruffled border of osteoclast Osteocyte (within a Bone-resorbing cell lacuna) From white blood Bone cell lineage matrix Figure 6.6 Types of bone cells and their derivation. (e) Photomicrograph of an osteoclast destroying bone tissue ( 447×). Bone lining cells (not illustrated) are derived from osteoprogenitor cells and look very similar. Osteoblasts Osteoblasts are bone-forming cells that secrete Bone Lining Cells Bone lining cells are flat cells found on the bone matrix. Like their close relatives, the fibroblasts and bone surfaces where bone remodeling is not occurring. Like chondroblasts, they are actively mitotic. The unmineralized osteocytes, they are thought to help maintain the matrix. bone matrix they secrete includes collagen (90% of bone pro- Osteoclasts Osteoclasts (Figure 6.6d, e) are giant multinucle- tein) and calcium-binding proteins that make up the initial ate cells located at sites of bone resorption. They are derived unmineralized bone, or osteoid. As described later, osteoblasts from the same white blood cell lineage that gives rise to macro­ also play a role in matrix calcification. phages. When actively resorbing (breaking down) bone, the oste- When actively depositing matrix, osteoblasts are cube oclasts lie in a shallow depression they have carved out. They shaped. When inactive, they resemble the flattened osteopro- have a distinctive ruffled border that directly contacts the bone. genitor cells or may differentiate into bone lining cells. When The deep plasma membrane infoldings of the ruffled border tre- the osteoblasts become completely surrounded by the matrix mendously increase the surface area for enzymatically degrading being secreted, they become osteocytes. the bones and seal off that area from the surrounding matrix. Osteocytes The spidery osteocytes (Figure 6.6c) are mature Microscopic Anatomy of Compact Bone bone cells that occupy spaces (lacunae) that conform to their shape. Osteocytes monitor and maintain the bone matrix. They Although compact bone looks solid, a microscope reveals that also act as stress or strain “sensors” and respond to mechani- it is riddled with passageways that serve as conduits for nerves cal stimuli (bone loading, bone deformation, weightlessness). and blood vessels (see Figure 6.8). (Remember the concentric Osteocytes communicate this information to the cells respon- “tree rings” of hard matrix that allowed you to identify bone sible for bone remodeling (osteoblasts and osteoclasts) so that tissue in Chapter 4, p. 137.) bone matrix can be made or degraded as mechanical stresses Osteon The structural unit of compact bone is called the dictate. Osteocytes can also trigger bone remodeling to main- osteon (os′te-on), or the Haversian system (ha-ver′zhen). Each tain calcium homeostasis, as we will see shortly. osteon is an elongated cylinder oriented parallel to the long  182 UNIT 2 Covering, Support, and Movement of the Body Artery with The manner in which canaliculi are formed is interesting. As capillaries bone forms, the osteoblasts maintain contact with one another Structures in the Vein and existing osteocytes by gap junctions ( p. 68) in their ten- central Nerve fiber tacle-like extensions. The osteoblasts secrete bone matrix, and canal are trapped within it as it hardens, becoming osteocytes. This leaves a system of tiny canals—the canaliculi—filled with tis- Lamellae sue fluid and containing the osteocyte extensions. The canali- Collagen fibers run culi tie all the osteocytes in a mature osteon together, allowing in different directions them to communicate and permitting nutrients and wastes to be in adjacent lamellae. This helps resist twisting relayed from one osteocyte to the next throughout the osteon. (torsional force). It’s as if the osteocytes are holding hands and passing notes to each other. Although bone matrix is hard and impermeable to nutrients, the canaliculi and gap junctions allow bone cells to be well nourished. Interstitial and Circumferential Lamellae Not all the lamel- lae in compact bone are part of complete osteons. Lying between intact osteons are incomplete lamellae called interstitial lamellae Twisting (in″ter-stish′al) (Figure 6.8c, right). They either fill the gaps 11 6 force between forming osteons or are remnants of osteons that have been cut through by bone remodeling (discussed later). Figure 6.7 A single osteon. The osteon is drawn so that the Circumferential lamellae are found in two locations: just individual lamellae are illustrated. deep to the periosteum and just superficial to the endosteum. They extend around the entire circumference of the diaphysis (Figure 6.8a) and effectively resist twisting of the long bone. axis of the bone. Functionally, osteons are tiny weight-bearing pillars. Microscopic Anatomy of Spongy Bone As shown in the expanded view in Figure 6.7, an osteon is In contrast to compact bone, spongy bone looks like a poorly a group of hollow tubes of bone matrix, one placed outside the organized, even haphazard, tissue (see Figures 6.4 and 6.5b). next like the growth rings of a tree trunk. Each matrix tube is a However, its trabeculae are as carefully positioned as the lamella (lah-mel′ah; “little plate”), and for this reason compact cables on a suspension bridge. Trabeculae in spongy bone bone is often called lamellar bone. Although all of the collagen align precisely along lines of stress and help the bone resist fibers in a particular lamella run in a single direction, the col- that stress. lagen fibers in adjacent lamellae always run in different direc- Only a few cells thick, trabeculae contain irregularly tions. This alternating pattern beautifully withstands torsional arranged lamellae and osteocytes interconnected by canali- stresses—the adjacent lamellae reinforce one another to resist culi. No osteons are present. Nutrients reach the osteocytes of twisting. You can think of the osteon’s structure as a “twister spongy bone by diffusing through the canaliculi from capillaries resister.” in the endosteum surrounding the trabeculae. Collagen fibers are not the only part of bone lamellae that is beautifully ordered. The tiny crystals of bone salts usually align Chemical Composition of Bone between the collagen fibers and also alternate their direction in adjacent lamellae. Bone contains both organic and inorganic substances. When these two substances are present in the right proportions, bone Canals and Canaliculi Running through the core of each is extremely strong and durable without being brittle. osteon is the central canal, or Haversian canal, containing Its soft organic components—including bone cells and osteoid— small blood vessels and nerve fibers that serve the osteon’s allow it to resist tension (stretch). cells. Canals of a second type called perforating canals, or Volkmann’s canals (folk′mahnz), lie at right angles to the long Its hard inorganic components—mineral salts—allow it to axis of the bone and connect the blood and nerve supply of the resist compression. medullary cavity to the central canals (Figure 6.8a). Unlike Healthy bone is half as strong as steel in resisting compression the central canals of osteons, the perforating canals are not sur- and fully as strong as steel in resisting tension. rounded by concentric lamellae, but like all other internal bone cavities, these canals are lined with endosteum. Organic Components Spider-shaped osteocytes (Figures 6.6c and 6.8b) occupy The organic components of bone include its cells (osteopro- lacunae (lac = hollow ; una = little ) at the junctions of the genitor cells, osteoblasts, osteocytes, bone lining cells, and lamellae. Hair-width canals called canaliculi (kan″ah-lik′u-li) osteoclasts) and osteoid (os′te-oid), the organic part of the radiate from the lacunae, connecting them to each other and to matrix. Osteoid, which makes up approximately one-third of the central canal. the matrix, includes ground substance ( p. 128) and collagen  Chapter 6 Bones and Skeletal Tissues 183 Compact bone Spongy bone Central Perforating canal canal Endosteum lining bony canals and covering trabeculae Osteon 11 6 Circumferential lamellae (a) Perforating fibers Lamellae Periosteal blood vessel Periosteum Nerve Vein Lamellae Artery Central Canaliculi canal Osteocyte Lacunae in a lacuna (b) (c) Interstitial Lacuna (with osteocyte) lamella Figure 6.8 Microscopic anatomy of compact bone. (a) Diagram of a section of long bone diaphysis. (b) Close-up of a portion of one osteon. (c) SEM (left) of cross-sectional view of an osteon ( 410×). Light photomicrograph (right) of a cross-sectional view of an osteon ( 400×). fibers, both of which are secreted by osteoblasts. These organic from rising to a fracture value. In the absence of continued or substances, particularly collagen, contribute both to a bone’s additional trauma, most of the sacrificial bonds re-form. structure and to the flexibility and tensile strength that allow it to resist stretch and twisting. Bone’s resilience is thought to come from sacrificial bonds Inorganic Components in or between collagen molecules. These bonds stretch and The balance of bone tissue (65% by mass) consists of inorganic break easily on impact, dissipating energy to prevent the force hydroxyapatites (hi-drok″se-ap′ah-tītz), or mineral salts, largely  184 UNIT 2 Covering, Support, and Movement of the Body calcium phosphates present as tiny, tightly packed, needle- Ossification and osteogenesis (os″te-o-jen′ĕ-sis) are both like crystals in and around collagen fibers in the extracellular names for the process of bone tissue formation (os = bone, matrix. The crystals account for the most notable characteris- genesis = beginning ). In embryos this process leads to the tic of bone—its exceptional hardness, which allows it to resist formation of the bony skeleton. Later, another form of ossifica- compression. tion known as bone growth goes on until early adulthood as the Because of the mineral salts they contain, bones last long body increases in size. Bones are capable of growing thicker after death and provide an enduring “monument.” In fact, skel- throughout life. However, ossification in adults occurs mainly etal remains many centuries old reveal the shapes and sizes of during bone remodeling and repair. ancient peoples, the kinds of work they did, and many of the ailments that affected them, such as arthritis. Formation of the Bony Skeleton Check Your Understanding Before week 8, the embryonic skeleton consists only of fibrous 9. In the figure below, name the parts of the bone indicated by connective tissue membranes and hyaline cartilage. Bone tis- a and b, and the membranes found at c and d. What kind of sue begins to develop at about this time and eventually replaces cartilage is found at e? Is this bone from an adult or a child? most of the existing fibrous or cartilage structures. How can you tell? In endochondral ossification (endo = within, chondro = cartilage), a bone develops by replacing hyaline cartilage. The a resulting bone is called an endochondral bone. 11 6 In intramembranous ossification, a bone develops from a fibrous membrane and the bone is called a membranous c bone. The beauty of using flexible structures (membranes and cartilages) to fashion the embryonic skeleton is that they can d accommodate mitosis. If the early skeleton was composed of b calcified bone tissue from the outset, growth would be much more difficult. e Endochondral Ossification Except for the clavicles, essentially all bones below the base of the skull form by endochondral ossification (en″do-kon′dral). Beginning late in the second month of development, this pro- 10. WHAT IF? What would happen to a bone if you removed cess uses hyaline cartilage “bones” formed earlier as models, or all of the inorganic components (for example, by soaking patterns, for bone construction. It is more complex than intra­ it in vinegar for several days)? Would it be able to resist membranous ossification because the hyaline cartilage must be compression? Tension? broken down as ossification proceeds. 11. DRAW Draw a cross section midway along the diaphysis of a Let’s examine the formation of a large long bone as an typical long bone in an adult (omit any spongy bone). Label example of endochondral ossification (Figure 6.9). the medullary cavity and indicate the type of bone marrow 1  A bone collar forms around the diaphysis of the hyaline found in this cavity. Label the compact bone. Draw and label the periosteum and endosteum. cartilage model. Like all cartilage, the cartilage model is initially surrounded by perichondrium. Ossification begins 12. MAKE CONNECTIONS Which cell has a ruffled border and acts to break down bone matrix? From your knowledge of organelles when the underlying mesenchymal cells (cells of the (Chapter 3), state which organelle would be the likely source embryonic connective tissue, p. 129) in the deep layer of of the enzymes that can digest bone matrix. the perichondrium specialize into osteoblasts. As a result, For answers, see Answers Appendix. the perichondrium becomes periosteum. Osteoblasts of the newly converted periosteum secrete osteoid against the hyaline cartilage diaphysis, encasing it in a collar of bone. As the bone collar forms, chondrocytes within the shaft hy- 6.5 Bones develop either by pertrophy (enlarge). This is the primary ossification center. intramembranous or endochondral 2  Cartilage calcifies in the center of the diaphysis and then ossification develops cavities. The hypertrophied chondrocytes calcify the surrounding cartilage matrix. Then, because calcified Learning Outcomes cartilage matrix is impermeable to diffusing nutrients, the N Compare and contrast intramembranous ossification and chondrocytes die and the matrix begins to deteriorate. This endochondral ossification. deterioration opens up cavities, but the bone collar stabi- N Describe how epiphyseal plates allow long bones to grow. lizes the hyaline cartilage model. Elsewhere, the cartilage  Chapter 6 Bones and Skeletal Tissues 185 remains healthy and continues to grow briskly, causing the erodes, and then is replaced by little bony spikes on the cartilage model to elongate. epiphyseal surfaces facing the medullary cavity. 3  The periosteal bud invades the internal cavities and At birth, most of our long bones have a bony diaphysis spongy bone forms. In month 3, the forming cavities are surrounding remnants of spongy bone, a widening medul- invaded by a collection of elements called the periosteal lary cavity, and two cartilaginous epiphyses. bud, which contains a nutrient artery and vein, nerve fib- Shortly before or after birth, secondary ossification ers, red marrow elements, osteoprogenitor cells, and osteo- centers develop in one or both epiphyses. Typically, the clasts. The entering osteoclasts partially erode the calcified large long bones form secondary centers in both epiphyses, cartilage matrix. The osteoprogenitor cells become osteo- whereas the small long bones form only one secondary os- blasts that secrete osteoid around the remaining calcified sification center. (In short bones, only the primary ossifica- fragments of hyaline cartilage. In this way, bone-covered tion center is formed. Most irregular bones develop from cartilage trabeculae, the earliest version of spongy bone, is several distinct ossification centers.) formed. 5  The epiphyses ossify. Secondary ossification reproduces 4  The diaphysis elongates and a medullary cavity forms. As almost exactly the events of primary ossification, except the primary ossification center enlarges, osteoclasts break that the spongy bone in the interior is retained and no med- down the newly formed spongy bone and open up a medul- ullary cavity forms in the epiphyses. lary cavity in the center of the diaphysis. Throughout the When secondary ossification is complete, hyaline cartilage fetal period (week 9 until birth), the rapidly growing epi- remains only at two places in a long bone: physes consist only of cartilage, and the hyaline cartilage On the epiphyseal surfaces, as the articular cartilages 11 6 models continue to elongate by division of viable cartilage cells at the epiphyses. Ossification “chases” cartilage for- At the junction of the diaphysis and epiphysis, where it forms mation along the length of the shaft as cartilage calcifies, the epiphyseal plates Week 9 Month 3 Birth Childhood to adolescence Articular cartilage Secondary ossification Spongy center bone Epiphyseal Area of blood vessel deteriorating Epiphyseal cartilage matrix plate cartilage Hyaline cartilage Medullary Spongy bone cavity formation Bone Blood collar vessel of Primary periosteal ossification bud center 1 Bone collar forms 2 Cartilage calcifies 3 The periosteal 4 The diaphysis 5 The epiphyses ossify. around the diaphysis in the center of the bud invades the elongates and a When ossification is of the hyaline diaphysis and then internal cavities and medullary cavity forms. complete, hyaline cartilage model. develops cavities. spongy bone forms. Secondary ossification cartilage remains only in centers appear in the the epiphyseal plates epiphyses. and articular cartilages. Figure 6.9 Endochondral ossification in a long bone.  186 UNIT 2 Covering, Support, and Movement of the Body Mesenchymal Intramembranous Ossification cell Intramembranous ossification forms most of the bones of Collagen the skull (for example, frontal, parietal, occipital, and temporal fiber bones) and the clavicles. Most bones formed by this process are Ossification flat bones. At about week 8 of development, ossification begins center within fibrous connective tissue membranes formed by mesen- Osteoid chymal cells. This process involves four major steps, depicted Osteoblast in Figure 6.10. 1 Ossification centers develop in the fibrous connective tissue membrane. Mesenchymal cells cluster and differentiate into osteoblasts, Postnatal Bone Growth forming an ossification center. During infancy and youth, long bones lengthen entirely by inter- stitial growth of the epiphyseal plate cartilage and its replace- ment by bone, and all bones grow in thickness by appositional Osteoblast growth. Most bones stop growing during adolescence. However, Osteoid some facial bones, such as those of the nose and lower jaw, con- tinue to grow almost imperceptibly throughout life. Osteocyte 11 6 Newly calcified Growth in Length of Long Bones bone matrix Longitudinal bone growth mimics many of the events of endo- chondral ossification and depends on the presence of an epiphy- seal cartilage consisting of five zones (Figure 6.11). 2 Osteoid is secreted and calcifies. Osteoblasts continue to secrete osteoid, which calcifies. 1  Resting zone: The cartilage is relatively inactive on the side Trapped osteoblasts become osteocytes. of the epiphyseal plate farthest from the medullary cavity. Mesenchyme 2  Proliferation zone: Below the resting zone, the cartilage condensing cells form tall columns, like coins in a stack, that allow fast, to form the periosteum efficient growth. The cells at the “top” (epiphysis-facing) side of the stack next to the resting zone comprise the pro- Trabeculae of immature spongy liferation or growth zone. These cells divide quickly, push- bone ing the epiphysis away from the diaphysis and lengthening Blood vessel the entire long bone. 3  Hypertrophic zone: The older chondrocytes in the stack, which are closer to the diaphysis, hypertrophy (enlarge). 3 Immature spongy bone and periosteum form. Their lacunae erode and enlarge, leaving large intercon- Accumulating osteoid is laid down between embryonic blood necting spaces. vessels, forming a honeycomb of immature spongy bone. Vascularized mesenchyme condenses on the external face of the 4  Calcification zone: The surrounding cartilage matrix calci- bone and becomes the periosteum. fies, the chondrocytes die, and the matrix begins to dete- riorate, allowing blood vessels to invade. This leaves long, Fibrous periosteum slender spicules of calcified cartilage at the epiphysis-­ Osteoblast diaphysis junctions, which look like stalactites hanging from the roof of a cave. Plate of 5  Ossification zone: The calcified spicules are invaded by compact bone marrow elements from the medullary cavity. Osteoclasts Spongy bone partly erode the cartilage spicules, then osteoblasts cover cavities contain red marrow them with new bone. Ultimately, spongy bone replaces them. Eventually, as osteoclasts digest the spicule tips, the 4 Compact bone replaces immature spongy bone, just deep to medullary cavity also lengthens. the periosteum. Red marrow develops. Trabeculae just deep to the periosteum are remodeled and During growth, the epiphyseal plate maintains a constant replaced with compact bone. thickness because the rate of cartilage growth on its epiphysis- The immature spongy bone in the center is remodeled into facing side is balanced by its replacement with bony tissue on mature spongy bone that is eventually filled with red marrow. its diaphysis-facing side. Figure 6.10 Intramembranous ossification. Diagrams 1 and Longitudinal growth is accompanied by almost con- 2 represent much greater magnification than diagrams 3 and 4. tinuous remodeling of the epiphyseal ends to maintain the  Chapter 6 Bones and Skeletal Tissues 187 1 Resting zone Epiphysis Epiphyseal plate 2 Proliferation Resting zone zone Cartilage cells Proliferation zone undergo mitosis. Hypertrophic zone Calcification zone Ossification zone 3 Hypertrophic zone Diaphysis Older cartilage cells enlarge. (a) Epiphyseal plate 4 Calcification zone Matrix calcifies; cartilage cells die; 11 6 matrix begins Calcified deteriorating. cartilage spicule Osseous tissue 5 Ossification zone Figure 6.11 Growth in length New bone is of a long bone occurs at the forming. epiphyseal plate. The epiphyseal plate consists of five zones 1 – 5 (b) Photomicrograph of (c) Diagram of the zones that extend from the epiphysis to cartilage in the epiphyseal within the epiphyseal the diaphysis. plate (1253). plate. proportion between the diaphysis and epiphyses. Bone Before growth After growth remodeling involves both new bone formation and bone and remodeling and remodeling resorption ­(Figure 6.12). As the bone grows in length, remodeling (resorption As adolescence ends, the chondroblasts of the epiphyseal and deposition) maintains the bone’s shape. plates divide less often. The plates become thinner and thinner Articular and until they are entirely replaced by bone tissue. Longitudinal epiphyseal plate bone growth ends when the bone of the epiphysis and diaphy- cartilage grows here… sis fuses. This process, called epiphyseal plate closure, hap- …and is replaced Outline of bone pens at about 18 years of age in females and 21 years of age by bone before growth (endochondral and remodeling. in males. Once this has occurred, only the articular cartilage ossification) here. remains in bones. However, an adult bone can still widen by appositional growth if stressed by excessive muscle activity or body weight. Articular Growth in Width (Thickness) cartilage Bone that was Growing bones widen as they lengthen. As with cartilages, Epiphyseal here has been bones increase in thickness or, in the case of long bones, plate resorbed. diameter, by appositional growth. Osteoblasts in the peri- osteum secrete bone matrix on the external bone surface as Bone was added here osteoclasts on the endosteal surface of the diaphysis remove by appositional growth. bone (see Figure 6.12). Normally there is slightly more build- ing up than breaking down. This unequal process produces a thicker, stronger bone but prevents it from becoming too Figure 6.12 Long bone growth and remodeling during heavy. youth. Two points in time during growth.  188 UNIT 2 Covering, Support, and Movement of the Body Hormonal Regulation of Bone Growth Remodeling does not occur uniformly. For example, the dis- The bone growth that occurs until young adulthood is exqui- tal part of the femur (thigh bone) is fully replaced every five to sitely controlled by a symphony of hormones. During infancy six months, whereas its shaft is altered much more slowly. and childhood, the single most important stimulus of epiphyseal plate activity is growth hormone released by the anterior pitui- Bone Resorption tary gland. Thyroid hormones modulate the activity of growth As noted earlier, the giant osteoclasts accomplish bone resorption. hormone, ensuring that the skeleton has proper proportions as Osteoclasts move along a bone surface, digging depressions or it grows. grooves as they break down the bone matrix. The ruffled border At puberty, sex hormones are released in increasing amounts. of the osteoclast clings tightly to the bone, sealing off the area Estrogens have a critical role in bone development. (In males, of bone destruction and secreting acid (H + ) that dissolves the some circulating testosterone is converted to estrogens.) At low bone minerals and lysosomal enzymes that digest the organic levels, estrogens stimulate the growth spurt typical of adoles- matrix. The digested matrix end products and dissolved miner- cence, but as estrogen levels increase over time, they induce als are then endocytosed, transported across the osteoclast by epiphyseal closure, ending longitudinal bone growth. The rela- transcytosis ( p. 76), and released at the opposite side. There, tive levels of estrogens and testosterone determine the extent they enter the interstitial fluid and then the blood. Osteoclasts to which specific parts of the skeleton have male or female may also phagocytize dead osteocytes and any undigested characteristics. demineralized matrix. When resorption of a given area of bone Excesses or deficits of any of these hormones can result is completed, the osteoclasts undergo apoptosis (cell death). 11 6 in abnormal skeletal growth. For example, hypersecretion of growth hormone in children results in excessive height (gigan- tism), and deficits of growth hormone or thyroid hormone pro- Bone Deposition duce characteristic types of dwarfism. Following behind the osteoclasts, osteoblasts deposit new bone matrix. This begins as an osteoid seam—an unmineralized band Check Your Understanding of gauzy-looking bone matrix 10–12 micrometers ( µm ) wide. 13. Bones don’t begin with bone tissue. What do they begin with? Between the osteoid seam and the older mineralized bone, there 14. When describing endochondral ossification, some say “bone is an abrupt transition called the calcification front. chases cartilage.” What does that mean? How does the osteoid become calcified? The proteins of the 15. Where is the primary ossification center located in a long newly deposited osteoid bind calcium ions (Ca 2+ ) , raising the bone? Where is (are) the secondary ossification center(s) local concentration of Ca 2+. This in turn triggers osteoblasts located? to release matrix vesicles studded with the enzyme alkaline 16. As a long bone grows in length, what is happening in the phosphatase. This enzyme cuts phosphate ( Pi ) ions off of the hypertrophic zone of the epiphyseal plate? osteoid proteins, raising the local concentration of Pi. When For answers, see Answers Appendix. the concentrations of Ca 2+ and Pi are high enough, tiny cal- cium phosphate crystals form. These crystals then act as the Bone remodeling involves bone 6.6 seed around which hydroxyapatites (the mineral salts in bones) form. The result is that calcium salts are deposited throughout deposition and removal the osteoid, creating calcified bone matrix. Learning Outcomes N Compare the locations and remodeling functions of the Control of Remodeling osteoblasts, osteocytes, and osteoclasts. Remodeling goes on continuously in the skeleton, primarily N Explain how hormones and physical stress regulate bone regulated by two control loops that serve different purposes: remodeling. Maintaining Ca 2 + homeostasis. A hormonal negative feed- As you have just learned, bone is a dynamic and active tissue, back loop involving parathyroid hormone maintains Ca 2+ and small-scale changes in bone architecture occur continually. homeostasis in the blood. Every year, remodeling replaces about 5–10% of our skeleton, Keeping bone strong. Mechanical and gravitational forces and our entire skeleton is replaced about every 10 years. Spongy bone is replaced every three to four years; compact bone, every acting on a bone drive remodeling where it is required to 10 years or so. This is essential because when bone remains in strengthen that bone. place for long periods, more of the calcium salts crystallize and the bone becomes more brittle—ripe conditions for fracture. Hormonal Controls In the adult skeleton, bone is deposited and removed pri- To understand the hormonal control of bone remodeling, you marily at the endosteal surface. Together, bone resorption need to understand calcium’s importance in the body. Maintain- and deposit constitute bone remodeling. Bone remodeling is ing extracellular fluid calcium levels within homeostatic lev- coordinated by cohorts of adjacent osteoblasts and osteoclasts, els is absolutely critical for maintaining the resting membrane which respond to signals from hormones and stress-sensing potential of all cells. Without normal levels of blood calcium, osteocytes (discussed shortly). nerves cannot fire as needed and muscles are unable to contract.  Chapter 6 Bones and Skeletal Tissues 189 H OMEOSTATIC Control by PTH acts to preserve blood calcium homeosta- CLINICAL IMBALANCE 6.1 sis, not the skeleton’s strength or well-being. Osteoclasts are no Minute changes from the homeostatic range for blood cal- respecters of bone age: When activated, they break down both cium can lead to severe neuromuscular problems. For exam- old and new bone. In fact, if blood calcium levels are low for ple, hypocalcemia (hi″po-kal-se′me-ah; low blood Ca 2+ levels) an extended time, the bones become so demineralized that they causes hyperexcitability. In contrast, hypercalcemia (hi″per-kal- develop large holes. We will discuss blood calcium regulation se′me-ah; high blood Ca 2+ levels) causes nonresponsiveness by PTH further in Chapter 16. and inability to function. In addition, sustained high blood levels Another hormone, calcitonin (kal″sĭ-to′nin), is released of Ca 2+ can lead to the formation of kidney stones or undesir- from the thyroid gland in response to elevated blood concen- able deposits of calcium salts in other organs, which may hamper trations of calcium. Calcitonin was once thought to regulate their function. blood Ca 2+ levels as well. However, in adult humans, the effect of normal calcitonin levels on calcium homeostasis is negligi- ble. When administered at pharmacological (abnormally high) The human body contains 1200–1400 g of calcium. More doses, it does lower blood calcium levels briefly. than 99% of this calcium is contained in bone minerals, 0.9% Various other hormones also affect bone resorption and dep- is inside cells, and less than 0.1% is in the extracellular fluid. osition by either stimulating or inhibiting osteoclast activity. Hormones use the vast amount of calcium in bone as a “storage Like PTH, glucocorticoids (hormones from the adrenal cortex) bank” from which they can make withdrawals (resorption) or and vitamin D (calcitriol) indirectly stimulate osteoclast activ- deposits as needed to maintain extracellular fluid calcium lev- ity by increasing the synthesis of RANK-L and decreasing the els. Hormonal control normally maintains blood Ca 2+ within 11 6 synthesis of its antagonist (osteoprotegerin). Sex hormones, on the narrow range of 9–11 mg per dl (100 ml ) of blood. Cal- the other hand, have the opposite effect—they increase the syn- cium is absorbed from the intestine under the control of the thesis of RANK-L’s antagonist. active form of vitamin D, called calcitriol. The hormonal controls primarily involve parathyroid hormone (PTH), produced by the parathyroid glands. When Response to Mechanical Stress blood levels of Ca 2+ decline, PTH is released (Figure 6.13). The second set of controls regulating bone remodeling, bone’s The increased PTH level stimulates osteoclasts to resorb bone, response to mechanical stress (muscle pull and gravity), keeps releasing Ca 2+ into the blood. The PTH stimulation of osteo- the bones strong where stressors are acting. clasts is indirect—various cells (e.g., osteoblasts) respond to Wolff’s law holds that a bone grows or remodels in response PTH secretion by producing another protein (RANK-L) that to the demands placed on it. The first thing to understand is stimulates the formation and activity of osteoclasts. that a bone’s anatomy reflects the common stresses it encoun- As blood concentrations of calcium rise, the stimulus for ters. For example, a bone is loaded (stressed) whenever weight PTH release ends. The decline of PTH reverses its effects and bears down on it or muscles pull on it. This loading is usually causes blood Ca 2+ levels to fall. off-center and tends to bend the bone. Bending compresses the IMB AL AN CE Calcium homeostasis of blood: 9–11 mg/100 ml BALANCE BALANCE Stimulus: Falling blood IMB AL Ca21 levels AN CE Thyroid gland Osteoclasts degrade bone Parathyroid matrix and release glands Parathyroid Ca21 into blood. glands release parathyroid hormone (PTH). PTH Figure 6.13 Parathyroid hormone (PTH) control of blood calcium levels.  190 UNIT 2 Covering, Support, and Movement of the Body How do mechanical forces communicate with the cells responsible for remodeling? Deforming bone pushes fluid con- Load here (body weight) threatens to bend this taining ions through the canaliculi. This creates an electrical bone along this arc. current. This electrical current is detected by osteocytes, which release chemical messengers that promote the formation of additional bone. This process u