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Skeletal System
Essential Information and Problems

Student Performance Objectives - for the lecture
1.   Describe the following functions of the skeletal system in a few, brief sentences: support, protection, movement, hemopoiesis and mineral storage.
2.   Give an example of one specific bone, by its name, in each of the following categories: long, short, flat, irregular, and sesamoid.

3.   State the functions of collagen and apatite salts in the structure of bone matrix.
4.   Label the following parts of a typical long bone: articular cartilage, cancellous bone, compact bone, diaphysis, endosteum, epiphyses, epiphyseal cartilage, medullary cavity, periosteum, and marrow spaces.
5.   Label the following parts of a typical flat bone: cancellous bone, compact bone, marrow spaces, dipl e.
6.   Explain the function of each of the parts of the bones labeled in objectives 3 and 4.
7.    Explain the differences and similarities of endochondral and intramembranous ossification.
8.   List the functions of each of the following cell types found in human bones: chondroblasts, chondrocytes, osteoblasts, osteocytes, osteoclasts, fibroblasts, fibrocytes.
9.   Define: trabeculae and lamellae.
10. Explain how cancellous bone regions are transformed into compact bone.
11.  Describe the regions of bones typically transformed from cancellous to compact.
12. Given a diagram on an Haversian system of compact bone, label these parts: Haversian canal, lacunae, osteocytes, canaliculi, lamellae and bone matrix.
13. Explain the effects of normal, everyday physical stresses on bone structure and the effects of breaks.
14. Explain the role of the following nutrients in bone formation: calcium, phosphate, vitamin D and vitamin K.
15. Explain the interplay of the following hormones in bone formation: parathormone, calcitonin, growth hormone, thyroxine, sex hormones (estrogens and androgens).
16. Describe how long bones grow in length including discussion of the terms: primary ossification center and secondary ossification centers.
17. Explain how long bones and other shaped bones grow in width and overall size.

Student Required Bone and Bone Parts - for Laboratory Practical Examinations (bones and bone parts will be added or removed at your laboratory instructor's discretion).

Student Required Joints - for Laboratory Practical Examinations (joints will be added or removed at your laboratory instructor's discretion).

Lesson Outline
A. Functions of the Skeletal System
    1. Support - as an endoskeleton, the weight of all body parts is supported by bones. All parts of the body attach to bone directly or indirectly through tendons, ligaments or other fibrous connective tissue.
    2. Protection - the cranium, thoracic cage and pelvic girdle are three clear examples of bones surrounding and protecting soft body parts.

     3. Movement - the rigid bones can move to varying degrees depending on the type of joint articulating adjacent bones. The skeletal muscles pull on the bones causing movements at the joints.
    4. Hemopoiesis - red marrow is a tissue found in the spaces within cancellous bone substance. This tissue produces the body's red blood cells and some of the white blood cells.
    5. Mineral Storage - the bones are reservoirs for calcium, phosphates, magnesium, sodium and other minerals.

B. Bone Classification by Shape
    1. Long bones - These are the bones of the upper arms, the forearms, the thighs and the lower legs. They possess a long shaft (diaphysis) that has a hollow core and is composed of a dense type of bone called compact bone in their cylindrical walls. The ends of the long bones (epiphyses) form some of the most complex articulations: the shoulders, elbows, wrists, hips, knees and ankles.
    2. Short bones - These are bones of the wrists, palms, fingers, ankles, insteps and feet. These bones are solidly composed of spongy bone and possess an outer covering of compact bone.
    3. Flat bones - These are the bones of the calvarium (upper part of the skull) and also the clavicle. These bones' outer and inner surfaces are composed of compact bone. Their interior is cancellous bone. This sandwich like arrangement of compact-cancellous- compact bone is called dipl e. skeletonbones
    4. Irregular bones - These are the vertebrae and many other bones of the body including several skull bones. They are composed of cancellous bone within and covered by a veneer of denser, harder compact bone.
    5. Sesamoid bones - These are irregularly shaped bones that form within tendons and serve to protect and strengthen them. A good example is the patella - the bone found within the patella tendon of the knee joint.

C. Osseus tissue - the structure of bone as a substance
     1. The origin of osseus tissue. Our original "skeleton", which we form during the first 8 weeks of gestation, is composed of hyaline cartilage (sometimes called cartilage models,, slide 35) and, in our cranial area, fibrous membranes. So as an embryo, we have a soft, pliable skeleton that has no bone substance.
    2. From embryo to fetus. Eight weeks, or 2 months, into gestation, marks an important turning point for us in that the first bone-forming cells, osteoblasts, make their appearance in the embryo and begin ossifying cartilage and fibrous membranes into osseus tissue. At this time the embryo changes its name to "fetus".
    3. Osseus tissue, or bone substance, is a combination of organic and inorganic materials.
        a. Organic material: the major organic substance of osseus tissue is collagen which is formed in and secreted from osteoblasts into the fluid surrounding them. The collagen polymerizes into rods that begin to surround and enclose the osteoblasts.
        b. Inorganic material: ions of calcium and phosphates combine into a complex called apatite salts. These salts also contain ions of magnesium, fluoride, and sodium in lesser concentrations.
        c. The apatite salts adhere to the outer surface of the collagen rods making them more rigid. What we call mature osseus tissue is this combination of collagen and apatite salts.
        d. The collagen brings a tough pliability to bone; the apatite salts bring rigidity and hardness to bone. In a healthy bone, this combination is just about as strong as reinforced concrete.
        e. Osteoblast Living Space: The osteoblasts gradually become encased in a bony armor that they have secreted around themselves. These living cells are immersed in a fluid space just within the boundaries of the bone substance. This space the osteoblasts are in is called a lacuna., slide 43 and 44.  The fluid in each of the many lacunae is interstitial fluid - the same fluid that is found in all capillary beds of the body.
        f. Cellular Communication: Adjacent osteoblasts communicate with each other through fine canals - canaliculi - filled with interstitial fluid. Nutrients, wastes, and hormones easily pass through each canaliculus., slide 42.
        g. Red Marrow: If you picture the osseus tissue formed like a honeycomb, then what is the honey? It is red marrow in those bones that accumulate this tissue. It is yellow marrow in other bones. The main red marrow accumulating bones are the frontal, sternum, vertebrae, ribs and iliac crests.
    4. The cells of osseus tissue. After osteoblasts have formed bone substance and are now enclosed within it, their metabolism changes them into bone-maintaining cells called osteocytes.  If a bone breaks or additional bone substance is required, osteocytes can change back into osteoblasts. It is just the same cell called by two different names depending on what it is doing - synthesizing bone substance, or maintaining it. Another named cell type in osseus tissue is the osteoclast.   This cell can dissolve bone substance and is important in bone remodeling. For example, as a long bone grows in length, the hollow space within the bone's shaft is expanded. Osteoclasts help to dissolve osseus tissue to widen the area of this medullary cavity. It has been observed that osteoblasts - the bone-forming cells - can, under certain circumstances, also dissolve bone substance the way osteoclasts do.
    5. Other cells found in bones. The outer surfaces of living bones are covered by a tightly adhering membrane called the periosteum. This tough connective tissue covering is formed by fibroblasts and is maintained by fibrocytes. Also, cartilage is found at several locations on and in bone and the cartilage-forming cells are called chondroblasts; the cartilage-maintaining cells are called chondrocytes: - slide 33.
        a. Cartilage on bone surfaces is at freely movable joints. This hyaline or articular cartilage is only a few millimeters thick but cushions the joints and helps to make joint motions smooth and frictionless (working with the lubricating gel - synovial fluid).
        b. Cartilage in bones occurs in all individuals still growing in height. The interior growing regions of the long bones of the arms and legs are regions composed of hyaline cartilage. The cartilage disappears and is replaced by osseus tissue when growth in height for that individual has ended.

D. Types of Osseus Tissue
    1. Cancellous bone substance - This is the type of tissue described in formation in the above section. Pictured as a honeycomb, the osseus tissue is hard and possesses internal marrow spaces that are filled with either red or yellow marrow.
        a. Cancellous bone formation results from the ossification of either cartilage models of bone, or by the ossification of fibrous membranes.
            (1). Endochondral ossification - this is the conversion of cartilage models found in the early embryo into bone. Formation occurs as described above, part C.
            (2). Intramembranous ossification - this is the conversion of the fibrous membranes composing the bones of the upper part of the skull, the calvarium, and the clavicle, into bone. Formation occurs as described above, part C.
Compact bone substance - This harder and more dense type of bone substance forms at the surface of all bones to give them resistance to potentially being damaged, and is also found composing the walls of the shafts (diaphyses) of the long bones., slides 40-42. The formation of compact bone follows the following steps:
        a. Picture the honeycomb pattern of cancellous bone and imagine the osteocytes in the bone substance surrounding the marrow cavities.
        b. Imagine some of these osteocytes, now called osteoblasts because they are ready to lay down new bone substance, moving into the marrow cavities and laying down new bone substance. As they do this they reduce the size of the marrow cavity. Since they are coming into the marrow cavity from the edges, the cavity gets smaller.
            A way to understand what is happening is to imagine a group of 100 children holding hands in a large circle. The children stop holding hands and 75 of them take 1 step forward deeper into the circle, moving toward the center. Then 50 take another step forward moving even deeper toward the circle's center. This continues until the once large circle now is filled with children forming smaller circles (concentric circles) all the way to the theoretical center of the original large circle. Eventually there is only a small space at the circle's center that has no children in it. The children are arranged in circular layers around the circle's tiny center.
        c. The marrow cavity is reduced in size gradually as each wave of bone migration inward converts the marrow space into bone substance. Each new layer of bone that forms is called a lamella.
        d. As the osteocytes migrate inward, lamella by lamella, the cells on the periphery maintain contact with those migrating inward through canaliculi. , canaliculi.  The inwardly migrating cells also maintain contact with each other through canaliculi.
        e. At the end of the process, what used to be a honeycomb pattern of bone substance and marrow spaces is now bone substance in layers (lamellae) encircling small central regions that are still marrow spaces and which are now called Haversian canals. The entire structure of the Haversian canal surrounded by the concentric lamellae populated with interconnected bone cells (osteocytes) is called a Haversian System. Each system looks something like a bull's-eye target although Haversian systems are not generally perfectly circular.      

E. The Structure of Bones
1. The long bones of growing children possess the following structures:
        a. Diaphysis - the bone shaft - hollow with walls made of compact bone substance.
        b. Epiphyses - the ends of the bone, composed of cancellous bone substance with compact bone as a veneer on the surface except where there is articular cartilage.
        c. Epiphyseal growth discs - the growing regions composed of hyaline cartilage inside bones at the region between the diaphysis and each epiphysis.
        d. Medullary cavity - the hollow region free of bone substance in the center of the diaphysis.
        e. Endosteum - the tissue lining the medullary cavity.
        f. Articular cartilage - the hyaline cartilage covering the surface of the bones at the joints.
        g. Periosteum - the fibrous connective tissue covering the bone's outer surface, except where there is articular cartilage.
        h. Nutrient foramen - blood vessels and nerves can enter the bone through this opening.
    2. The long bones of adults who are no longer growing possess all the parts of the long bone above except that the epiphyseal growth discs are gone. They are replaced by an epiphyseal line which is made of bone substance. All interior cartilage areas of the bone are now ossified.
    3. Flat bone structure was discussed previously and is referred to as dipl e.
    4. Short and Irregular bone structure consists of a core of cancellous bone and a covering veneer of compact bone.

F. The Growth of Bones  
1. During gestation, the initial laying down of bone substance in the bone models made of cartilage occurs at primary ossification centers located at the center of the shaft of each long bone, and which are roughly in the center of other types of bones. The process involves many cell types and is a marvel of cellular interactions.
    2. Growth of long bones in length.
        a. Remember that at the beginning of the growth of a long bone, the bone is made out of cartilage and is a tiny model of the bone.
        b. Osteoblasts enter the bone at the center of the diaphysis and begin ossifying the central region. This is the primary ossification center.
        c. The osteoblasts migrate from the center of the long bone, in both directions, toward the bone's ends - the epiphyses. As the osteoblasts migrate they ossify each successive area they encounter: regions formerly made of cartilage are turned into bone.
         d. As the osteoblasts migrate toward the epiphyses, chondroblasts composing the cartilage areas not yet ossified lay down new cartilage in the direction of the bone's ends, and the bone grows in length.
         e. By the time a baby is born, ossification has been going on for 30 weeks: from the 8th week of gestation through the end of week 38 when the baby is full term and ready to be born. In this period of time the diaphyses of the long bones have been ossified but the epiphyses are unossified.
        f. By age 2, secondary ossification centers have appeared in the centers of the epiphyses of the long bones and the epiphyses completely ossify thereafter.
        g. As long as the child is still growing in height, a region of cartilage remains between the diaphyses and each epiphysis - the epiphyseal growth plate (or disc). It is here that cartilage growth toward the epiphyses elongates the bone, followed by ossification of non-growth plate cartilage regions. Growth is not uniform: spurts in growth occur based on factors including the next item - sex hormones.
       h. Cartilage growth in the plates is stimulated by sex hormones - girls' estrogens secreted during the several years prior to and during puberty stimulate plate growth and therefore bone elongation. Girl's growth in early teen is often rapid. Boy's androgens (mainly testosterone) also stimulate plate growth and bone elongation, and androgen influence lasts longer than estrogen influence. The result is that female growth in height in the early teen years is generally followed by complete ossification of the epiphyseal growth plate and cessation of growth in height. Male growth in height continues through late teen years, until the growth plate is ossified, as in the female. A completely ossified epiphyseal plate is called an epiphyseal line. Its presence indicates that further growth in length is not possible.
    3. Growth of long bones in width.
       a. Bones thicken as they get longer through growth in width. Osteoblasts under the periosteum lay down new bone substance, thickening the bone, as growth in length occurs.        b. Bone growth in width can occur even after bone lengthening is no longer possible since bone growth in width is now dependent on regions of cartilage. Bones can thicken in response to exercise against gravity (walking, running, skiing, weight training) at any age.
    4. Role of Osteoclasts in bone growth.  
       a. As long bones grow in length and width, the medullary cavity is maintained mostly free of bone substance. The medullary cavity contains either red or yellow marrow.  The hollow medullary cavity contributes to bones' lightness.
       b. Osteoclasts dissolve bone within the medullary cavity keeping it hollow in proportion to the bone's overall length and width.
    5. Bone growth after injury.
If a bone is fractured, connective tissue, known as a callus, grows around and into the broken region.
 Phagocytic cells within the callus absorb and digest the damaged tissue and osteoclasts dissolve broken bone fragments. Osteoblasts restore bone to the region over a period of 8-12 weeks. After healing, the formerly broken region may be stronger than it was prior to the break, in a healthy person.
    6. Bone remodeling throughout life.   
      Bones are continually maintained throughout life through the combined actions of the osteoblasts and the osteoclasts. These cells work under genetic control and are influenced by nutrition, hormones and drugs.
      a. Role of Hormones in Bone Growth: In children, growth hormone from the pituitary gland, parathormone from the parathyroid glands, and calcitonin and thyroxine from the thyroid gland contribute to bone growth and have an influence on the skeletal system's final size and degree of hardness. During and after puberty, the sex hormones, as indicated earlier, contribute to the growth of bone. Changes in sex hormone concentration after menopause in women and in older age groups in men can result in bone weakening due to loss of bone's protein matrix. In the disease osteoporosis, merely taking calcium supplements may have little effect on the progression of bone deterioration: reduced hormonal levels combined with as yet poorly identified genetic factors reduce the ability of bones to bind calcium that may be present either from food or supplements. For this reason, some doctors recommend hormone replacement therapy, although this practice may make women more prone to cardiovascular disease and other problems.
     b. Role of Dietary factors in Bone Growth : Sun exposure and vitamin D synthesis along with dietary intake of protein, and minerals (mostly calcium and phosphates) contributes to bone strength. Osteomalacia is a condition of adults in which the bones are weakened based on dietary deficiency of vitamin D and/or calcium and phosphates. This condition is reversible based on improvements in the diet and/or better sun exposure in the case of vitamin D deficiency. Rickets is a disease of children in which the bones are weakened and sometimes deformed based on poor dietary practices of the mother during gestation and often poor diet during childhood.


Biomedical Terminology:   Define each term.

Apatite salts
Articular cartilage
Cancellous bone
Compact bone
Endochondral ossification
Epiphyseal line
Epiphyseal plate
Haversian canal
Haversian system
Hemopoietic tissue
Intramembranous ossification
Medullary cavity
Primary ossification center
Red marrow
Secondary ossification center
Yellow marrow

Skeletal System Problems

 1. Choose one of the problems described below.
 2. Prepare your solution as a word document.
 3. Send it to your professor as an email attachment. You will receive an email response.

Problem #1: A woman of 50 years begins experiencing symptoms that her physician tells her are the beginning of menopause. The physician suggests she consider taking estrogen replacement therapy (ERT) to relieve these symptoms and to prevent, or slow down, bone demineralization over time. Utilize the Internet or other sources to research the pros and cons of estrogen replacement therapy. 
     Your report should include
          1. A definition of menopause and a description of menopausal symptoms.
          2. An explanation of ERT.
          3. The benefits of ERT.
          4. The potential side-effects of ERT, both short and long-term.
          5. Your decision, based on your research, whether or not the lady should consider ERT as an option in her life.

Problem #2: A man of 48 years experiences chronic low-back pain based, in part, on a history of running and football injuries. The physician suggests non-steroidal anti-inflammatory drugs for immediate pain relief. Utilize the Internet to answer the following questions:
         1. List and describe possible causes of low-back pain.
         2. What is a non-steroidal anti-inflammatory drug? Give 5 specific examples listing both              generic and brand names.
         3. From the 5 drug examples you have chose, choose one and list its possible side effects.
         4. List and describe 3 alternatives to drug therapy for relief of chronic low-back pain.
         5. If you were the individual experiencing this pain, what treatment(s) would you pursue to obtain relief, based on your research.

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