NOTE: Lecture notes are intended to help the student organize their notes and facilitate assimilation of the material. They are in no way a substitute for the actual lectures; however, this material will be covered on exams!
Text Reference: CHAPTER 6
The framework of bones and cartilage
that protects our internal organs and allows us to move is called the SKELETAL
SYSTEM. Functions of the skeletal system include:
1. Bones provide a hard framework that supports the body.
2. Bones provide protection to internal organs. The cranium protects the
brain, the vertebrae protect the spinal cord, the rib cage protects the
thoracic cavity organs, and the hip bones protect pelvic cavity organs.
3. Skeletal muscle uses the bones as levers for movement.
4. Bone serves as a reservoir of minerals, especially calcium.
5. Red bone marrow manufactures blood cells and platelets.
6. Fat is stored in the yellow bone marrow as an energy
reserve.
Read about the functions of the skeletal system on page 145.
Structurally, the skeletal system consists of 3 types of connective tissue: BONE, CARTILAGE & LIGAMENTS.
CARTILAGE is an important part of the skeleton. What type of cartilage is the most common type? HYALINE CARTILAGE makes up most of the EMBRYONIC SKELETON, but eventually is replaced by bone during fetal and childhood development. HYALINE CARTILAGE is also found at the ends of long bones at joints, connects the ribs to the breastbone, and forms the end of the nose. ELASTIC CARTILAGE gives shape to the outer ear. FIBROCARTILAGE forms the intervertebral discs, between the vertebrae. Please review the structure of cartilage from your tissues notes and TABLE 3.3 (pp. 79-80).
CARTILAGE resists compression (pushing forces) & tension (pulling forces) due to its rubbery ground substance (chondroitin sulfate) and collagen. Cartilage is also very resilient, able to spring back to its original shape following compression. Unfortunately, cartilage is weak in resisting shear forces (twisting & bending). Because of this weakness, torn cartilage is a common sports injury.
Most LIGAMENTS are cords of DENSE REGULAR CONNECTIVE TISSUE (see your tissue notes & TABLE 3.3, p. 77) that attach bone to bone at joints. The ligaments between the vertebrae, however, are made of ELASTIC CONNECTIVE TISSUE (Table 3.3, p. 78).
Like all connective tissue, BONE TISSUE contains a great deal of extracellular matrix. The extracellular matrix of bone consists of 25% water, 50% mineral salts & 25% collagen. The mineral salts include primarily calcium salts, like calcium phosphate and calcium carbonate. There are also small amounts of magnesium and fluoride. The mineral salts give bone its hardness, which allows bone to resist compression. Collagen contributes to the bone's great tensile strength, making the bone more resilient and pliable, and less brittle.
MICROSCOPIC STRUCTURE OF BONE TISSUE: There are 2 types of bone tissue: compact and spongy.
COMPACT BONE forms the outer layer of bone. Compact bone appears dense and smooth, but if you look at compact bone under the microscope, you will see tiny canals permeating the bone tissue, carrying blood vessels and nerves into the bone. See TABLE 3.3, p. 80 & FIG. 6.3.
The structural unit of compact bone is called the OSTEON or HAVERSION SYSTEM. Each OSTEON consists of the hard bone matrix arranged in concentric rings around a central canal. This matrix is called the CONCENTRIC LAMELLAE. The lamellae form concentric rings around a central canal, called the HAVERSIAN CANAL. Each Haversian canal is oriented along the long axis of the bone and contains blood vessels and nerves. The blood vessels of the Haversian canals carry nutrients and oxygen to the bone cells and carry waste products away. The PERFORATING CANALS run at right angles to the Haversian canals and they connect the blood vessels and nerves from outside the bone to the Haversian canals.
Spider-shaped mature bone cells, called OSTEOCYTES, live in small spaces in the lamellae called LACUNAE. Tiny canals, called CANALICULI, connect the lacunae to each other and to the Haversian canal. The canaliculi contain slender, tentacle-like cellular processes of the osteocytes. The canaliculi provide routes through which nutrients and oxygen from the blood can reach the osteocytes and waste products (ammonia, carbon dioxide) can be removed and eventually carried away by blood vessels in the Haversian canals.
To summarize: Compact bone tissue contains many osteons. Each OSTEON is a structural unit that contains one HAVERSIAN CANAL with its surrounding CONCENTRIC LAMELLAE, LACUNAE, OSTEOCYTES, and CANALICULI.
The matrix areas between osteons contain INTERSTITIAL LAMELLAE. These lamellae do not form concentric rings around a Haversian canal, but they do contain osteocytes living in lacunae. Canaliculi are also present. The interstitial lamellae fill in the gaps between osteons and represent the remnants of older osteons that have been partially destroyed during bone replacement, which is an ongoing process. Circumferential lamellae are lamellae on the outer surface of bone under the periosteum.
Compact bone is found at the bone surface and is deposited over spongy bone tissue. In contrast to compact bone tissue, SPONGY BONE TISSUE does not contain osteons. Instead, spongy bone is composed of small needlelike pieces of bone that form an irregular latticework called TRABECULAE ("little beams"). Trabeculae are like the struts and beams of a tall office building, producing a lightweight framework inside the bone. Between the trabeculae of some bones are spaces that are filled with RED BONE MARROW. The cells of the red bone marrow function in BLOOD CELL PRODUCTION.
The trabeculae of spongy bone contain LACUNAE (spaces) that house osteocytes. See FIG. 6.3b & c. No Haversian canals are present and the bony matrix (INTERSTITIAL LAMELLAE) is not arranged in concentric rings. Notice that there are CANALICULI. Blood vessels from outside the bone penetrate the compact bone to the spongy bone through the PERFORATING CANALS. Blood circulates through blood vessels in the spaces between the trabeculae, providing nutrients and oxygen to the osteocytes living in the trabeculae of the spongy bone.
ANATOMY OF A LONG BONE:
As you can see from studying the bone's histology, a bone is an ORGAN, consisting of bone tissue, blood vessels and blood, and nerves. Now let us look at the structure of a typical LONG BONE. All long bones have the same general structure (see FIGURE 6.1). A typical long bone consists of the following parts:
1. The DIAPHYSIS (die-a'-fih-sis) is long, main portion of the long bone. It is also called the SHAFT. It consists of a thick layer of compact bone that surrounds a space called the MEDULLARY CAVITY or MARROW CAVITY. The medullary cavity contains the YELLOW BONE MARROW. Yellow bone marrow consists primarily of adipose tissue, so it functions in fat (energy) storage.
2. The PROXIMAL & DISTAL EPIPHYSES (ih-pih'-fih-seez) are the ends of the long bone (singular is EPIPHYSIS). The epiphyses have a thin layer of compact bone on the outside, and the interior consists of spongy bone.
3. A thin layer of glassy-smooth HYALINE CARTILAGE covers each epiphysis where the bone forms a joint with another bone (where the bones ARTICULATE). The hyaline cartilage that covers the ends of long bones is called ARTICULAR CARTILAGE. This cartilage cushions the ends of bones and absorbs stress during joint movements.
4. The EPIPHYSEAL LINE is located at the junction of the epiphysis and diaphysis (a region called the “metaphysis”) in adult bone. The epiphyseal line is a remnant of the EPIPHYSEAL PLATE, which is a cartilage plate that serves as a growth area for long bone lengthening. The epiphyseal plate allows the diaphysis of the bone to increase in length until early adulthood. When growth stops, the epiphyseal plate cartilage is replaced with bone, then becoming the epiphyseal line. See FIGS. 6.1, 6.4, 6.6, & 6.7.
5. The outer surface of the bone is covered and protected by a glistening white, fibrous membrane called the PERIOSTEUM (except where there is articular cartilage). The periosteum consists of 2 layers. The outer FIBROUS LAYER of the periosteum is composed of DENSE IRREGULAR CONNECTIVE TISSUE. Tendons and ligaments attach to the fibrous layer of the periosteum. The fibrous layer of the periosteum is richly supplied with blood vessels and nerves that pass into the bone via the perforating canals. See FIG. 6.3.
The inner OSTEOGENIC LAYER of the periosteum (see FIG. 6.3), which touches the bone surface, contains primarily bone-forming cells, called OSTEOBLASTS, which secrete the extracellular matrix of the bone. The osteogenic layer also contains OSTEOGENIC CELLS. Osteogenic cells are BONE STEM CELLS derived from MESENCHYMAL CELLS. They divide and differentiate into osteoblasts. The periosteum also contains the OSTEOCLASTS, which are bone-degrading cells. Osteoclasts breakdown the bone tissue and release calcium to the bloodstream. Osteoclasts are specialized macrophages that are important in bone reabsorption, which is important for bone repair and replacement. See FIG. 6.2.
6. The MEDULLARY CAVITY, the space within the diaphysis that contains the fatty yellow bone marrow, is lined with a thin membrane called the ENDOSTEUM. The endosteum contains OSTEOGENIC CELLS, OSTEOBLASTS, and OSTEOCLASTS. These cells also cover the surface of the trabeculae of spongy bone (see FIG. 6.3c).
OSSIFICATION
Note that there are 4 types of cells associated with the osseous tissues (see
FIG. 6.2) : OSTEOGENIC CELLS (bone stem cells),
OSTEOBLASTS (bone-forming cells), OSTEOCYTES (mature bone cells), and
OSTEOCLASTS (bone-destroying cells). OSTEOCYTES are the only bone cells that
actually LIVE IN BONE TISSUE. Osteocytes
are actually osteoblasts that have become isolated in
the bony extracellular matrix that the osteoblasts secrete. Osteocytes
maintain the daily cellular activities of bone tissue, but they no longer
produce new bone.
Ossification, bone growth, maintenance, and repair depends on the activities of the OSTEOBLASTS & OSTEOCLASTS, which are found in the periosteum and endosteum of bone.
The process by which bone forms is called OSSIFICATION. Most of the "skeleton" of the human embryo is composed of HYALINE CARTILAGE. The FLAT BONES of the SKULL & the mandible are the exception - they consist of FIBROUS CONNECTIVE TISSUE MEMBRANES. Ossification begins around the 6th or 7th week of embryonic development and continues on through childhood into adulthood. Ossification involves the conversion of cartilage and fibrous connective tissue into bone. SEE FIG. 6.5, 6.6, 6.7, & 6.8.
After the baby is born, there are still sites of cartilage and fibrous connective tissue, which have not yet ossified. FONTANELS (fibrous connective tissue membranes called "softspots") between the flat bones of the skull completely ossify by 24 months after birth. See FIG. 7.14. EPIPHYSEAL PLATES of long bones, which consist of HYALINE CARTILAGE, are completely ossified by age 25 (FIGS.6.6 & 6.7).
Bone is continually remodelled as it grows in children and young adults. Even after bones have reached their adult shapes and sizes, old bone tissue is constantly being destroyed and new bone tissue is formed in its place. Bone is never at rest; osteoclasts and osteoblasts constantly remodel the bone to adjust the bone to whatever stress it experiences. Example: bone remodelling due to weight-bearing exercise (weightlifting, running, aerobics).
On page 158, "Exercise and Bone Tissue" may be of interest to those of you who do or don't exercise. Weight-bearing exercise increases bone mass, making bones stronger. This is important for women in preventing osteoporosis. Osteoporosis, a disorder that depletes bone mass and causes porous bones, primarily affects middle-aged and elderly women. Osteoporosis is discussed under "Aging and Bone Tissue" and “Applications to Health”, p. 159-160. Bone loss from osteoporosis can be prevented with a calcium-rich diet and weight-bearing exercise (running, dancing, weightlifting). In menopausal and postmenopausal, estrogen replacement therapy can also help prevent osteoporosis.
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