Chapter 6

Protection. The fused bones of the skull protect the brain. The vertebrae surround the spinal cord, and the rib cage helps protect the vital organs of the thorax.

Anchorage. Skeletal muscles, which attach to bones by tendons, use bones as levers to move the body and its parts. As a result, we can walk, grasp objects, and breathe. The design of joints determines the types of movement possible.

Hyaline cartilages, which look like frosted glass when freshly exposed, provide support with flexibility and resilience. They are the most abundant skeletal cartilages. Their chondrocytes are spherical, and the only fiber type in their matrix is fine collagen fibers (which are undetectable microscopically). skeletal hyaline cartilages include:

Articular cartilages (artic = joint, point of connection), which cover the ends of most bones at movable joints Costal cartilages, which connect the ribs to the sternum (breastbone) Respiratory cartilages, which form the skeleton of the larynx (voice box) and reinforce other respiratory passageways Nasal cartilages, which support the external nose

Mineral storage. Bone is a reservoir for minerals, most importantly calcium and phosphate. The stored minerals are released into the bloodstream in their ionic form as needed for distribution to all parts of the body. Indeed, "deposits" and "withdrawals" of minerals to and from the bones go on almost continuously.

Blood cell formation. Most blood cell formation, or hematopoiesis (hem″ah-to-poy-e′sis), occurs in the red marrow of certain bones.

The spidery osteocytes (Figure 6.6c) are mature bone cells that occupy spaces (lacunae) that conform to their shape. Osteocytes monitor and maintain the bone matrix. They also act as stress or strain "sensors" and respond to mechanical stimuli (bone loading, bone deformation, weightlessness). Osteocytes communicate this information to the cells responsible for bone remodeling (osteoblasts and osteoclasts) so that bone matrix can be made or degraded as mechanical stresses dictate. Osteocytes can also trigger bone remodeling to maintain calcium homeostasis as we will see shortly.

Bone lining cells are flat cells found on bone surfaces where bone remodeling is not going on. Like osteocytes, they are thought to help maintain the matrix.

Which of the following accurately describes a key difference between cartilage and bone tissue?

Cartilage grows in an interstitial manner; bone does not. That's correct. Chondrocytes may divide and secrete new matrix, expanding the cartilage interstitially (from within). The hard calcified matrix of bone cannot grow in this manner.

Hematopoietic (blood-forming) tissue is also called red marrow. It is found in different locations in infants and adults. In infants, the medullary cavity of the diaphysis and all areas of spongy bone contain red bone marrow. In adults, much of the red marrow, particularly in long bones, has been replaced by yellow marrow. In most adult long bones, the fat-containing yellow marrow extends well into the epiphysis, and little red marrow is present in the spongy bone cavities. As a result, red marrow in adults is only found in the cavities between trabeculae of spongy bones in: The flat bones of the skull, as well as the sternum, ribs, clavicles, scapulae, hip bones, and vertebrae The heads of the femur (thigh bone) and humerus (long bone of the arm)

Compared to long bones (femur and humerus), the red marrow found in the spongy bone of flat bones (such as the sternum) and in some irregular bones (such as the hip bone) is much more active in hematopoiesis. When clinicians suspect problems with the blood-forming tissue, they obtain red marrow samples from these sites. However, yellow marrow in the medullary cavity can revert to red marrow if a person becomes very anemic (has low oxygen-carrying capacity) and needs more red blood cells.

Triglyceride (fat) storage. Fat, a source of energy for the body, is stored as yellow marrow in the cavities of long bones.

Hormone production. Bones produce osteocalcin, a hormone that helps to regulate insulin secretion, glucose homeostasis, and energy expenditure (see Chapter 16).

Minute changes from the homeostatic range for blood calcium can lead to severe neuromuscular problems. For example, hypocalcemia (hi″po-kal-se′me-ah; low blood Ca2+Ca2+levels) causes hyperexcitability.

In contrast, hypercalcemia (hi″per-kal-se′me-ah; high blood Ca2+Ca2+levels) causes nonresponsiveness and inability to function. In addition, sustained high blood levels of Ca2+Ca2+can lead to the formation of kidney stones or undesirable deposits of calcium salts in other organs, which may hamper their function.

In endochondral ossification (endo = within, chondro = cartilage), a bone develops by replacing hyaline cartilage. The resulting bone is called an endochondral bone

In intramembranous ossification, a bone develops from a fibrous membrane and the bone is called a membranous bone

Cartilage grows in two ways. Appositional growth. In appositional growth, cartilage-forming cells in the surrounding perichondrium secrete new matrix against the external face of the existing cartilage tissue.

Interstitial growth. In interstitial growth, the lacunae-bound chondrocytes divide and secrete new matrix, expanding the cartilage from within. Typically, cartilage growth ends during adolescence when the skeleton stops growing.

Flat bones are thin, flattened, and usually a bit curved. The sternum (breastbone), scapulae (shoulder blades), ribs, and most cranial bones of the skull are flat bones (Figure 6.2c).

Irregular bones have complicated shapes that fit none of the preceding classes. Examples include the vertebrae and the hip bones (Figure 6.2b).

Remodeling goes on continuously in the skeleton, primarily regulated by two control loops that serve different purposes: Maintaining Ca2+Ca2+homeostasis. A hormonal negative feedback loop involving parathyroid hormone maintains Ca2+Ca2+ homeostasis in the blood.

Keeping bone strong. Mechanical and gravitational forces acting on a bone drive remodeling where it is required to strengthen that bone.

However, the trabeculae in spongy bone align precisely along lines of stress and help the bone resist stress. These tiny bone struts are as carefully positioned as the cables on a suspension bridge.

Only a few cells thick, trabeculae contain irregularly arranged lamellae and osteocytes interconnected by canaliculi. No osteons are present. Nutrients reach the osteocytes of spongy bone by diffusing through the canaliculi from capillaries in the endosteum surrounding the trabeculae.

Osteoprogenitor cells, also called osteogenic cells, are mitotically active stem cells found in the membranous periosteum and endosteum. In growing bones they are flattened or squamous cells. When stimulated, these cells differentiate into osteoblasts, while others persist as osteoprogenitor cells.

Osteoblasts are bone-forming cells that secrete the bone matrix. Like their close relatives, the fibroblasts and chondroblasts, they are actively mitotic. The unmineralized bone matrix they secrete includes collagen (90% of bone protein) and calcium-binding proteins that make up the initial unmineralized bone, or osteoid. As described later, osteoblasts also play a role in matrix calcification. When actively depositing matrix, osteoblasts are cube shaped. When inactive, they resemble the flattened osteoprogenitor cells or may differentiate into bone lining cells. When the osteoblasts become completely surrounded by the matrix being secreted, they become osteocytes.

Growth in long bones

STEP 1 Proliferation zone: The cells at the "top" (epiphysis-facing) side of the stack next to the resting zone comprise the proliferation or growth zone. These cells divide quickly, pushing the epiphysis away from the diaphysis and lengthening the entire long bone. STEP 2 Hypertrophic zone: The older chondrocytes in the stack, which are closer to the diaphysis, hypertrophy (enlarge). Their lacunae erode and enlarge, leaving large interconnecting spaces. STEP 3 Calcification zone: The surrounding cartilage matrix calcifies, the chondrocytes die, and the matrix begins to deteriorate, allowing blood vessels to invade. This leaves long, slender spicules of calcified cartilage at the epiphysis-diaphysis junctions, which look like stalactites hanging from the roof of a cave. STEP 4 Ossification zone: The calcified spicules are invaded by marrow elements from the medullary cavity. Osteoclasts partly erode the cartilage spicules, then osteoblasts cover them with new bone. Ultimately spongy bone replaces them. Eventually, as osteoclasts digest the spicule tips, the medullary cavity also lengthens.

Long bones, as their name suggests, are considerably longer than they are wide (Figure 6.2a). A long bone has a shaft plus two ends, which are often expanded. All limb bones except the patella (kneecap) and the wrist and ankle bones are long bones. Notice that these bones are named for their elongated shape, not their overall size. The three bones in each of your fingers are long bones, even though they are small.

Short bones are roughly cube shaped. The bones of the wrist and ankle are examples (Figure 6.2d). Sesamoid bones (ses′ah-moid; "shaped like a sesame seed") are a special type of short bone that form in a tendon (for example, the patella). They vary in size and number in different individuals. Some sesamoid bones act to alter the direction of pull of a tendon. Others reduce friction and modify pressure on tendons to reduce abrasion or tearing.

Our bones perform seven important functions:

Support. Bones provide a framework that supports the body and cradles its soft organs. For example, bones of lower limbs act as pillars to support the body trunk when we stand, and the rib cage supports the thoracic wall.

The axial skeleton forms the long axis of the body and includes the bones of the skull, vertebral column, and rib cage (shown in orange in Figure 6.1). Generally speaking these bones protect, support, or carry other body parts.

The appendicular skeleton (ap″en-dik′u-lar) consists of the bones of the upper and lower limbs and the girdles (shoulder bones and hip bones) that attach the limbs to the axial skeleton (colored gold in Figure 6.1). Bones of the limbs help us move from place to place (locomotion) and manipulate our environment.

Control by PTH acts to preserve blood calcium homeostasis, not the skeleton's strength or well-being. Osteoclasts are no respecters of matrix age: When activated, they break down both old and new matrix.

The hormonal controls primarily involve parathyroid hormone (PTH), produced by the parathyroid glands. When blood levels of Ca2+Ca2+decline, PTH is released (Figure 6.14). The increased PTH level stimulates osteoclasts to resorb bone, releasing Ca2+Ca2+into the blood. The PTH stimulation of osteoclasts is indirect—various cells (e.g., osteoblasts) respond to PTH secretion by producing another protein (RANK-L) that stimulates the formation and activity of osteoclasts.

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 is dense irregular connective tissue. The inner osteogenic layer next to the bone surface contains osteoprogenitor cells (primitive 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 perforating fibers are exceptionally dense. A delicate connective tissue membrane called the endosteum (en-dos′te-um; "within the bone") covers internal bone surfaces (Figure 6.5). The endosteum covers the trabeculae of spongy bone and lines the canals that pass through the compact bone. The endosteum contains the same cell types as the inner layer of the periosteum.

Every year, remodeling replaces about 5-10% of our skeleton, and our entire skeleton is replaced about every 10 years. Spongy bone is replaced every three to four years; compact bone, every 10 years or so.

This is essential because when bone remains in place for long periods, more of the calcium salts crystallize and the bone becomes more brittle—ripe conditions for fracture.

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 perforating fibers are exceptionally dense. A delicate connective tissue membrane called the endosteum (en-dos′te-um; "within the bone") covers internal bone surfaces (Figure 6.5). The endosteum covers the trabeculae of spongy bone and lines the canals that pass through the compact bone. The endosteum contains the same cell types as the inner layer of the periosteum.

Unlike cartilage, bones are well vascularized. The main vessels serving the diaphysis are a nutrient artery (Figure 6.5c) and a nutrient vein. Together these run through a hole in the wall of the diaphysis, the nutrient foramen (fo-ra′men; "opening"). The nutrient artery runs inward to supply the bone marrow and the spongy bone. Branches then extend outward to supply the compact bone. Several epiphyseal arteries and veins serve each epiphysis in the same way. Nerves accompany blood vessels through the nutrient foramen into the bone.

Which of the following is found within compact bone, but not within spongy bone? central canal trabeculae lamellae osteocytes

central canal That's correct. A central canal is found within each cylindrical osteon comprising compact bone.

The material that initially connects the broken ends of bones together is formed from _________.

collagen fibers That's correct. Collagen fibers produced by fibroblasts contribute to the formation of a fibrocartilaginous callus that spans the break and connects the broken bone ends.

Intramembranous ossification results in the formation of _________.

cranial bones That's correct. Intramembranous ossification forms the flat cranial bones of the skull and the clavicles.

Which of these structures consists of hyaline cartilage?

epiphyseal plate That's correct. The epiphyseal plate, commonly called the growth plate, is a disc of hyaline cartilage that grows during childhood to lengthen the bone. This cartilage is replaced by bone in the epiphyseal line found in adults.

Which of the following is a function of red marrow?

hematopoiesis That's correct. Blood cell formation, or hematopoiesis, occurs in the red marrow of certain bones.

Which type of cartilage is most plentiful in the adult body?

hyaline cartilage That's correct. Hyaline cartilage is present widely throughout the body. It is found on the ends of long bones, in the rib cage, respiratory passageways, and the proximal region of the external nose.

8. Which of the following does not occur during the events leading to the lengthening of a long bone?

osteoblasts produce osteoid, which calcifies within fibrous connective tissue membranes. That's correct. This event occurs during the intramembranous development of flat bones, not during the growth in length of long bones.

Which cell type is found within lacunae? osteocytes osetoblasts osteoprogenitor cells osteoclasts

osteocytes That's correct. Osteocytes are mature bone cells derived from osteoblasts as they become completely surrounded by the matrix being secreted.

Which bones are incorrectly paired with its classification? shoulder blades: irregular bones The bones of the wrist and ankle are short bones. bones of the arm and forearm: long bones sternum: flat bones

shoulder blades: irregular bones That's correct. The scapulae (shoulder blades) are classified as flat bones.

Bone remodeling in response to mechanical stress is triggered by _________.

signals produced by osteocytes in the affected bone That's correct. Osteocytes detect stresses in the surrounding bone matrix and release chemical messengers that promote the formation of additional bone.

Which of the following results from the effects of parathyroid hormone (PTH)?

stimulation of osteoclast activity That's correct. PTH is released in response to a calcium deficiency. The breakdown of bone matrix by osteoclasts results in an increase in blood calcium levels.