In Chapter 6, we will examine osseous tissue and bone structure. First, let's get an introduction to the bones of the skeletal system. The functions of the skeletal system are listed here. The skeletal system provides support, structural support for the entire body.
The individual bones or groups of bones provide a framework for the attachment of soft tissues. and or organs. Delicate tissues and organs are often surrounded by skeletal elements, thus providing protection. The ribs protect the heart and lungs, the skull encloses the brain, the vertebrae shield the spinal cord, and the pelvis cradles delicate urinary and reproductive organs. Another function of the skeletal system is leverage.
Many bones of the skeleton function as levers that can change the magnitude and direction of the forces generated by skeletal muscles. The movements produced range from the delicate motions of a fingertip to powerful changes in the position of the entire body. The skeletal system also provides storage of both minerals and energy. The calcium salts of bone, represent a valuable mineral reserve that maintains normal concentrations of calcium and phosphate ions in the body fluids.
Calcium is the most abundant mineral in the human body. A typical human body, for example, contains 1 to 2 kilograms of calcium, with more than 98% of it deposited in the bones of the skeleton. There is also an energy reserve.
As we age, Our bones accumulate adipose tissue in the form of yellow bone marrow. This tissue can be used as an energy source during times of high energy demand. And finally, the skeletal system provides blood cell production, also known as hematopoiesis.
Red blood cells, white blood cells, and platelets are produced in the red bone marrow, which fills the internal cavities many bones. So here you can see examples of the various functions that we just discussed. The red bone marrow is shown there along with some yellow marrow.
Now this slide shows the divisions of the skeleton. The adult skeleton system includes approximately 206 separate bones and a number of associated cartilages. The body system is divided into the axial skeleton and appendicular skeleton.
The axial skeleton, which encompasses approximately 80 bones, consists of the bones of the skull, hyoid, sternum, rib cage, vertebral column, sacrum, and coccyx. The appendicular skeleton, which includes approximately 126 bones, includes bones of the limbs and the pectoral and pelvic girdles that attach the limbs to the axial skeleton. Bones are classified according to shape and structure. Shape and structure encompasses flat bones, long bones, short bones, irregular bones, and sesamoid bones. Flat bones have a thin, roughly parallel surface.
Flat bones form the roof of the skull, sternum, the ribs, and the scapula. They provide protection from underlying soft tissues and offer an extensive surface for the attachment of skeletal muscles. Long bones are relatively long and slender. They are located in the arm, forearm, thigh, lower leg, palms, soles, fingers, and toes. The femur, the long bone of the thigh, is the largest and heaviest bone in the body.
Short bones are small and boxy. Examples of short bones include bones of the wrist, and bones of the ankles. Irregular bones have complex shapes with short, flat, notched, or ridged surfaces. The spinal vertebrae, the bones of the pelvis, and facial bones are examples of irregular bones. Sesamoid bones are generally flat, small, and shaped somewhat like a sesame seed.
They develop inside of tendons and are most commonly located near joints at the knees, the hands, and the feet. Everyone has a sesamoid patella or kneecap, but individuals vary in the location and abundance of other sesamoid bones. This variation, as well as others, accounts for disparities in the total number of bones in the skeleton.
And here you can see some of the examples of classifications of bones. Flat bones like the parietal bone in the skull, sutural bones like the sutures within the sutures of the skull, the humerus is an example of a long The vertebrae, an example of a regular bone. The patella, a sesamoid bone. And the carpal bones are examples of short bones.
And this table also shows some of the features and functions of the different bone classifications. Now bones also have surface features known as bone markings. Depressions and openings, for example, allow blood vessels and nerves to pass.
Other projections are for muscle and ligament attachment sites. Shown here are some of those. A tuberosity is a large rounded projection.
that may cover a broad area. A crest is a narrow ridge of bone and is usually prominent. Trochanter is a very large irregularly shaped projection.
Line is a narrow ridge of bone and is less prominent than a crest. Turbical is a small rounded projection. Epicondyle is a raised area above a condyle. A spine is a sharp, slender, and often pointed process.
And a process is any bony prominence. And you can see examples of those bone markings in the bones listed on the slide. Now other bone markings are depressions or openings. A groove is a furrow. A fissure is a narrow slit-like opening or an elongated cleft or gap.
A foramen is a round or oval opening through the bone. A notch is an indentation. at the edge of a structure. A meatus is a canal-like passageway.
Sinus is a chamber within a bone filled with air and lined with a mucous membrane. And a fossa is a shallow depression or recess in the surface of a bone. Other projections help form joints. A head is an expanded proximal end of a bone carried on a narrow neck. A facet is a smooth articular surface.
A condyle is a smooth rounded articular surface. An aramus is an arm-like bar of a bone. Knowing the bone markings, the definitions of the bone markings, will help you identify the various features and bone markings as you study the bones of both the axial and appendicular skeleton.
And here are some examples of the various bone markings that we just talked about. So, for example, if you're looking for the foramen. or the foramen ovale you know you're looking for a round or oval opening through a bone so it can help you identify the bone marking that you're looking for here's some additional bone markings and some more here are continued on this slide Other bone markings like a crest, fossa are shown here. Now we're going to examine the anatomy of bones.
And we're going to begin with the long bone. Long bones are designed to transmit forces along the shaft and have a rich blood supply. There are several anatomical features.
noted here that you should be familiar with, such as the diaphysis, which is a long tubular shaft that forms the axis of a typical long bone. The walls of the shaft are made primarily of compact bone. The epiphesis is the ends of the bones, and the ends are composed primarily of spongy bone surrounded by the epiphyseal bone. by a thin layer of compact bone. There is the proximal end, which is closest to the origin of attachment, and the distal end, which furthers from the origin of the attachment.
The medullary cavity is within the shaft of a long bone, and it is a cavity where bone marrow is located. In childhood, The medullary cavity is filled with red bone marrow, but as we age, fat accumulates within the red marrow, transforming it to yellow bone marrow. Red bone marrow is important for hematopoiesis, but yellow bone marrow is no longer hematopoietic and instead stores fat as an important energy source. There is some other anatomical features shown here which we will examine in further detail, like the membranes associated with the bones, the periosteum and the endosteum. The periosteum is the outermost covering of bone and is primarily made of dense irregular tissue.
The endosteum is the internal membrane of bone also made of connective tissue, and it also lines the many canals that pass through bone to supply blood and nerves to the bone. The next slide shows some of the additional features here as well as others, like the nutrient foramen, which is the opening for the nutrient artery and vein. and you can see that here. The nutrient foramen allows bones to grow and be maintained, and in order to do that they need an extensive blood supply. So the nutrient foramen is a tunnel that penetrates the shaft of the bone and provides access for the blood vessels.
The nutrient artery transports oxygenated nutrient-rich blood to the bone, and the nutrient vein transports deoxygenated, waste-laden blood from the bone. You also have the metaphyseal artery and vein, which carry blood to and from the area of the metaphyse and to the epiphyse. Articular cartilage covers portions of the epiphysis that articulates with other bones.
The cartilage is avascular hyaline cartilage and it relies primarily on diffusion. from the synovial fluid to obtain oxygen and nutrients as well as to eliminate waste. This slide shows the membranes of the bone that we discussed previously, the periosteum, which you can see here has both an outer fibrous layer and an intercellular layer, and the endosteum. Now, bone is associated with four cells that account for approximately 2% of the weight of bones.
Osteoblasts are immature bone cells located on the surface of bone, and they build new bone matrix in a process called osteogenesis or ossification. Osteoblasts make and release the proteins and other organic components of the matrix. Before calcium salts are deposited, this organic matrix is called osteoid. When osteoblasts have become completely surrounded by bone matrix and trapped within the lacunae, they transform into osteocytes.
Osteocytes are mature bone cells that maintain the protein and mineral content of the surrounding matrix. through the turnover of matrix components. Osteocytes live within tiny voids called the cunei. Osteocytes secrete chemicals that dissolve the adjacent matrix and the release of minerals enter the circulation. The osteocytes then rebuild the matrix stimulating the deposition of mineral Osteocytes also participate in the repair of damaged bones.
Osteogenic cells are mesochemal cells located in the periosteum and endosteum. These stem cells divide to produce daughter cells that differentiate into osteoblasts and they are important in the formation of osteocytes. Osteoclasts are bone digesting cells that remove and recycle the bone matrix.
These are giant cells with 50 or more nuclei. Osteoclasts are not related to osteogenic cells or their descendants. Instead, they are derived from the same stem cells that produce phagocytic white cells, white blood cells called monocytes. acids and proteolytic enzymes secreted by osteoclasts dissolve the matrix and release stored minerals.
This process is called osteolysis or resorption and is important in bone remodeling. So here are the four different cell types, their function and location. Now we're going to examine the microscopic anatomy of compact bone. Compact bone consists of the osteon, which is the basic structural and functional unit of bone consisting of bone cells organized around a central canal and separated by concentric lamellae. Osteons form cylindrical shaped rods that run parallel to the axis of the bone to protect against compression forces.
The central canal, also known as the Haversian canal, runs parallel to the axis of bone and is located in the middle of each osteon. Each central canal possesses an artery and vein, a lymph vessel, and a nerve. The perforating canals, which are also known as Volkmann's canals, are passageways that extend perpendicular to the axis of the bone and connect the central canals of adjacent osteons. Lamellae are nested concentric rings of matrix surrounding the central canal.
Lacunae are where the mature bone cells called osteocytes are located. Osteocytes cannot divide and therefore each lacuna contains only one osteocyte. And the caniculi, which is a little canal, are the narrow crevices that penetrate the lamellae and connect the lacunae to the central canal.
Now the microscopic anatomy of spongy bone is shown here, and as you can see it is different from compact bone. Spongy bone is also called trabecular bone. Spongy bone consists of an open network of struts and plates called trabeculae that resemble a latticework with red bone marrow filling in the spaces between. The spongy bone is then covered by a thin layer of compact bone and articular cartilage. Within flat bones, spongy bone is sandwiched between two layers of compact bone, forming diplo.
The chemical composition of bone consists of an organic osteoid and an inorganic hydroxyapatite. The osteoid is roughly one-third of the weight of bone and is contributed by collagen fibers. Collagen fibers are strong and flexible, but if they are compressed, they bend. The inorganic component of bone consists of mineral salts, which account for almost two-thirds of the weight of bone. Calcium phosphate interacts with calcium hydroxide to form crystals of hydroxyapatite.
As they form, these crystals incorporate other calcium salts, such as calcium carbonate, and ions such as sodium, magnesium, and fluoride. The combination of the salts and the collagen fibers gives bone a strong, somewhat flexible material. Furthermore, this protein rich crystal combination is highly resistant to shattering.
In fact, bone is far more superior to concrete and is more in line with steel reinforced concrete. Now let's examine the formation and growth of the formation of bone. called osteogenesis or ossification begins during embryonic development.
There are two types of osteogenesis that occur in the embryo, intramembranous ossification and endochondral ossification. Let's first look at intramembranous ossification. The formation of bones in this ossification process is without a cartilage model. Intramembranous ossification is typical in flat bones of the skull, the mandible, clavicles, and patella.
It begins approximately eight weeks after fertilization and mesochem cells differentiate into osteoblasts within a fibrous connective tissue. This type of ossification normally occurs in the deeper layers of the dermis or in the connective tissues of tendons. First, the formation of a bone matrix begins within a fibrous membrane. As you can see in this slide, mesochemal cells cluster and secrete organic components of the matrix. The location of this activity is the ossification center.
The resulting osteoid mineralizes and the mesochymal cells differentiate into osteoblasts. As ossification proceeds, the osteoblasts get trapped within the lacunae and differentiate into osteocytes. Next.
the formation of woven bone and the periosteum occurs. The osteoid accumulates and fuses together forming struts called trabeculae around the blood vessels. The overall structure as you can see in the slide is very similar to spongy bone.
Now we have the formation of the compact bone plate. Initially the intramembranous bone consists of spongy bone only. Subsequent remodeling around the trapped blood vessels can produce osteons typical of compact bone.
And as the rate of growth slows at the surface, the connective tissue around the bone becomes organized into the fibrous layer of the periosteum. And here you can see the osteoblast layer, the osteoid. the osteocytes in the lacunae, as well as the blood vessels, bone matrix, and mesenchymal cells. Endochondral ossification is shown here. An endochondral ossification is how most bones of the body are formed, and it uses a high-linked cartilage model.
Endochondral ossification begins approximately six weeks after fertilization. The hyaline cartilage does not turn into bone. Instead, it is broken down as ossification occurs.
Let's examine the steps. First, you have the cavitation of the hyaline shaft. The chondrocytes within the shaft hypertrophy or enlarge as the surrounding matrix begins to calcify.
The impermeable matrix causes chondrocytes to die from lack of nutrients leaving the matrix that starts to deteriorate. Blood vessels grow around the edges of the cartilage. The cells of the perichondrium convert to osteoblasts producing a superficial layer of bone, sometimes called the bony collar.
Next, we have invasion of the internal cavities. Blood vessels penetrate the cartilage and invade the central region. This area within the shaft of hyaline cartilage is called the primary ossification center, and you can see that on the slide.
Migrating with the blood vessels are fibroblasts, which differentiate into osteoblasts, lymph vessels, nerve fibers, and red marrow elements. Effectively, these are called the periosteal bud. The osteoblasts secrete osteoid around remaining fragments of hyaline, forming trabeculae or spongy bone. Next, we have formation of the medullary cavity.
So as the primary ossification center enlarges, osteoclasts break down newly formed spongy bone. and opens up a medullary cavity in the center of the shaft of the bone. The osseous tissue of the outer shaft becomes thicker, forming compact bone. And then we have formation of the epiphysis.
Secondary ossification centers appear in the area at the opposite ends of the bone. The cartilage in this region calcifies and deteriorates forming cavities that allow entry of the periosteal bud. Soon the epiphysis are filled with spongy bone. This spongy bone is not broken down during the remodeling process.
And here you can see further the steps of endochondral ossification. Now, growth of bones occurs by two primary processes as well. We have longitudinal growth, which is lengthwise growth in a bone, and we also have appositional growth, or width or diameter growth.
In longitudinal growth, hyaline cartilage cells form tall columns at the growth plate and within the articular cartilage. The cells at the top of the stack divide quickly, forming a zone of proliferation, increasing the thickness of the epiphyseal plates and causing the entire long bone to lengthen. Older chondrocyte cells closer to the shaft enlarge. This area is called the zone of maturation and hypertrophy.
The matrix surrounding the chondrocytes becomes calcified resulting in the death of the chondrocytes and deterioration of the cartilage called the zone of calcification. Osteoblasts in the medullary cavity then ossify the cartilage forming spongy bone within the zone of ossification. The hyaline cartilage at the growth plate is eventually replaced entirely by bone.
Once completely replaced with bone, the epiphyseal plate is now called the epiphyseal line. This typically occurs in the person's early 20s and as a result the person stops growing. height.
And here you can see some of those steps that we just discussed. Now appositional growth is growth that occurs in width or diameter. In this case osteoprogenitor cells beneath the periosteum differentiate into osteoblast and form new osteons on the external surface of the bone.
While bone is being added to the outer surface through appositional growth, osteoclasts are removing and recycling lamellae at the inner surface. As a result, the medullary cavity gradually enlarges as the bone increases in diameter. Appositional growth is important in increasing the diameter of existing bones, but it does not form the original bone. Now we'll examine fractures and bone tear.
A fracture is a crack or a break in a bone. Fractures are classified according to several categories. They can be classified whether the bone penetrates the skin.
A simple or closed fracture is when the bone breaks cleanly but does not penetrate the skin. A compound or open fracture is when the broken ends of the bone protrude through the tissue and skin. Fractures can also be classified by the orientation of the break. A transverse fracture is when a break occurs perpendicular to the long axis of a bone. A linear fracture is when a break occurs parallel to the long axis of the bone.
Fractures can also be classified by the position of the bones after the fracture. Non-displaced fractures are when the bone ends retain their position. Displaced fractures or when the bone ends are out of their normal alignment. And here you can see examples of open versus closed fractures.
Different types of fractures are shown here. There is some common fractures that can be observed in a medical setting, such as a spiral fracture, where there is a break as a result of excessive twisting of the bone. A green stick fracture is also noted here, which is common in children, and that is when the bone breaks in completely. A compression fracture is when the bone is crushed from upward or downward forces, and you can see is typical in the vertebrae.
Other types of bone fractures commonly observed in medical settings are shown here, like a POTS fracture, and that can be where the tibia and fibula break or as seen in the figure occurs at the ankle and affects both bones of the leg. Now the process of repair of broken bones is outlined here. And when bones are broken, the first step in the repair process is a formation of a hematoma.
The blood vessels in the bone tear and hemorrhage occurs. Over a period of several hours, a large blood clot or hematoma develops. Then a fibrocartilage callus forms.
Capillaries grow into the hematoma and phagocytic cells invade the area. Within about 48 hours after the fracture, Chondrocytes from the endosteum have created an internal callus by secreting a fibrocartilage matrix between the two ends of the broken bone. Meanwhile, the periosteal chondrocytes create an external callus of hyaline cartilage around the outside of the break.
An internal callus connects the bone ends and an external callus protrudes from the outer bone surface. Then we have formation of the bony callus. Osteoblast and osteoclast continue to migrate inward and multiply rapidly in the fibrocartilage callus. As the material calcifies, the tissue becomes a bony callus.
Finally, there is remodeling. Eventually, the internal and external callus unite, compact bone replaces spongy bone at the outer margins of the fracture, and healing is complete. A slight swelling may remain on the outer surface of the bone, but quite often, that region undergoes remodeling and no external evidence of the fracture remains. Now we're going to discuss calcium homeostasis.
Bones are constantly undergoing the deposition and resorption in a process known as remodeling. This is coordinated by activity of the osteoblast and osteoclast that regulate both processes. And as you can see here, our bones are mineral reservoirs.
Our bones contain most of the body's calcium as well as other important compounds noted on the slide. Calcium and phosphate are the two predominant minerals found within our bones. Now, bone deposition occurs where bone is injured or added bone strength is needed and is accomplished by osteoblast. Bands of new matrix are deposited in the area and are referred to as an osteoid seam.
Bone resorption, which is also known as osteolysis, is accomplished by osteoclasts. Osteoclasts secrete lysosomal enzymes that digest the organic matrix and then secrete metabolic acids that convert calcium salts into soluble forms. The remodeling of bones is under negative feedback hormonal control.
So there's a balance going on between osteoblast and osteoclast activity within our bones and calcium and phosphate ions can be absorbed in the intestines and can also be lost in our urine. All three of these processes help to maintain normal calcium levels in our blood. So let's look at how bone resorption is regulated.
Changes in the levels of blood calcium will trigger the release of either parathyroid hormone or calcitonin. If we have low blood or calcium levels known as hypocalcemia, The chief cells of the parathyroid gland secrete parathyroid hormone or PTH. PTH has three effects, all leading to an increase in blood calcium levels. The first is within the bones. So within the bones, osteoclasts are stimulated, and they can accelerate the erosion of bone matrix, which leads to the release of stored calcium ions in the blood.
Our intestines can also respond. PTH enhances the calcium absorbing effects of calcitriol on the intestine. As a result, the rate of intestinal calcium absorption increases and the kidneys can play a role.
PTH increases the production of the hormone calcitriol, which is continuously secreted by the kidneys at low levels. This hormone in turn stimulates calcium reabsorption at the kidney tubules. And those three combined activities can all help bring the calcium levels in the plasma back up.
Now, bone deposition is when blood calcium levels are too high. So we would be hypercalcemic. And the cells of the thyroid gland secrete. Calcitonin.
Calcitonin also has three effects, all leading to blood calcium levels being lowered. First, the bones. Calcitonin inhibits osteoclasts but does not affect osteoblasts so that they continue to deposit calcium ions into the matrix of bone.
Our intestines can also assist. Calcitonin the rate of calcium absorption from foods in the digestive tract. And finally, the kidneys.
Calcitonin inhibits the absorption of calcium in urine so that more calcium is excreted from the body. And all three of these effects would lead to the calcium levels in the plasma declining. By providing a calcium reserve, the skeleton plays the primary role in the homeostatic maintenance of normal calcium ion concentration of body fluids. The skeleton is also important in the homeostatic balance of other ions as well.