The femur thigh bone is the largest bone in the body and extends from the hip to the knee where it ends in structures known as condyles that are covered in cartilage. The condyles of the femur form the upper part of the knee joint that articulates bends and glides against the top of the tibia shin bone in a region termed the tibial plateau that is also covered in cartilage. The patella knee cap , the backside of which is covered in cartilage, sits on the front of the knee and articulates against the femoral condyles.
Between each of the femoral condyles and the tibial plateau sits a crescent shaped soft tissue structure called the meniscus. The meniscus is made from fibrocartilage and acts as a shock absorbing, stabilizing pad between the femur and the tibia. Both meniscus and cartilage tissues lack blood vessels and nerve endings that make repair of these tissues difficult once they are damaged. Ligaments are another key component of the knee that hold the joint in a stable and properly aligned position as it articulates through a normal range of motion.
Cartilage is a tissue that can provide structure, act as a shock absorber, and provide a smooth, friction-free surface that allows our joints to work and our bones to painlessly move against each other. Cartilage is made up of special cells, called chondrocytes, that lay down a tissue made of a specialized protein and collagen fibers around themselves.
The outer walls of the diaphysis cortex, cortical bone are composed of dense and hard compact bone, a form of osseous tissue. Red bone marrow fills the spaces between the spongy bone in some long bones. Each epiphysis meets the diaphysis at the metaphysis.
During growth, the metaphysis contains the epiphyseal plate, the site of long bone elongation described later in the chapter. When the bone stops growing in early adulthood approximately 18—21 years , the epiphyseal plate becomes an epiphyseal line seen in the figure.
These bone cells described later cause the bone to grow, repair, and remodel throughout life. On the outside of bones there is another layer of cells that grow, repair and remodel bone as well.
The cellular layer is adjacent to the cortical bone and is covered by an outer fibrous layer of dense irregular connective tissue see Figure 6. The periosteum also contains blood vessels, nerves, and lymphatic vessels that nourish compact bone.
Tendons and ligaments attach to bones at the periosteum. The periosteum covers the entire outer surface except where the epiphyses meet other bones to form joints Figure 6. In this region, the epiphyses are covered with articular cartilage , a thin layer of hyaline cartilage that reduces friction and acts as a shock absorber. The two layers of compact bone and the interior spongy bone work together to protect the internal organs. If the outer layer of a cranial bone fractures, the brain is still protected by the intact inner layer.
Four types of cells are found within bone tissue: osteoblasts, osteocytes, osteogenic cells, and osteoclasts Figure 6. The osteoblast is the bone cell responsible for forming new bone and is found in the growing portions of bone, including the endosteum and the cellular layer of the periosteum. Osteoblasts, which do not divide, synthesize and secrete the collagen matrix and other proteins.
As the secreted matrix surrounding the osteoblast calcifies, the osteoblast become trapped within it; as a result, it changes in structure and becomes an osteocyte , the primary cell of mature bone and the most common type of bone cell. Each osteocyte is located in a small cavity in the bone tissue called a lacuna lacunae for plural. Osteocytes maintain the mineral concentration of the matrix via the secretion of enzymes.
Like osteoblasts, osteocytes lack mitotic activity. Osteocytes are connected to one another within the canaliculi via gap junctions. If osteoblasts and osteocytes are incapable of mitosis, then how are they replenished when old ones die? The answer lies in the properties of a third category of bone cells—the osteogenic osteoprogenitor cell.
These osteogenic cells are undifferentiated with high mitotic activity and they are the only bone cells that divide. Immature osteogenic cells are found in the cellular layer of the periosteum and the endosteum. They differentiate and develop into osteoblasts. The dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or for calcium release.
The cells responsible for bone resorption, or breakdown, are the osteoclasts. These multinucleated cells originate from monocytes and macrophages, two types of white blood cells, not from osteogenic cells. Osteoclasts are continually breaking down old bone while osteoblasts are continually forming new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of bone.
Table 6. Although compact and spongy bone are made of the same matrix materials and cells, they are different in how they are organized. Compact bone is dense so that it can withstand compressive forces, while spongy bone also called cancellous bone has open spaces and is supportive, but also lightweight and can be readily remodeled to accommodate changing body needs.
Compact bone is the denser, stronger of the two types of osseous tissue Figure 6. It makes up the outer cortex of all bones and is in immediate contact with the periosteum. Structural and functional differences distinguish synovial joints from cartilaginous joints synchondroses and symphyses and fibrous joints sutures, gomphoses, and syndesmoses.
Several movements may be performed by synovial joints. Abduction is the movement away from the midline of the body. Adduction is the movement toward the middle line of the body. Extension is the straightening of limbs increase in angle at a joint. Flexion is bending the limbs reduction of angle at a joint. Rotation is a circular movement around a fixed point. Body Movements I : Image demonstrating the various joint movements. There are six types of synovial joints.
Some are relatively immobile but more stable than mobile joints. Others have multiple degrees of freedom, but at the expense of greater risk of injury. The six types of joints include:.
Six Types of Synovial Joints : Image demonstrating the six different types of synovial joints. There are six different types of synovial joint based on their shapes, each allowing a different kind of movement. There are six basic types of synovial joints. Anatomical joints may consist of a combination of two or more joint types.
Some synovial joints are relatively immobile but stable. The types of the synovial joints are based on their shapes and can be classified as plane, hinge, pivot, condyloid, saddle, and ball-and-socket. The following descriptions are in ascending order of mobility:. In this example, the condyles of the femur join with condyles of tibia and the saddle joint, where the lower end of the femur joins with the patella.
Plane Joint : The left shoulder and acromioclavicular joints, and the proper ligaments of the scapula. Ball and Socket Joint : Hip joint: the ball of the femur head fits in the socket of the acetabulum of the pelvis.
Saddle Joint : Sternoclavicular articulation. Anterior view. Privacy Policy. Skip to main content. Search for:. Synovial Joints. Structure of Synovial Joints A synovial joint or diarthrosis occurs at articulating bones to allow movement. Learning Objectives Identify the structures of the synovial joint that allow it to move freely. Key Takeaways Key Points The bones of a synovial joint are surrounded by a synovial capsule, which secretes synovial fluid to lubricate and nourish the joint while acting as a shock absorber.
The ends of the joint bones are covered with smooth, glass-like hyaline cartilage which reduces friction during movement. A synovial joint contains a synovial cavity and dense, irregular connective tissue that forms the articular capsule normally associated with accessory ligaments. Key Terms articulation : A joint or the collection of joints at which something is articulated, or hinged, for bending. With its yolk-like consistency, its principal role is to reduce friction between the articular cartilage of synovial joints during movement.
A major constituent of the embryonic and young vertebrate skeleton, converted largely to bone with maturation. Synoviocytes The intimal cells are termed synoviocytes and can be either fibroblastic type B synoviocytes and macrophagic type A synoviocytes.
Structure of Synovium The surface of a synovium may be flat or covered with finger-like projections villi to allow the soft tissue to change shape as the joint surfaces move on one another. Synovial Bursa The synovial bursa is a small, fluid-filled sac lined by synovial membrane containing synovial fluid. Nerve and Blood Supply Synovial joints are highly innervated but vascularized indirectly by nearby tissues.
Learning Objectives Identify the nerve and blood supply of synovial joints. Key Takeaways Key Points Although the articular capsule is innervated with the nerves necessary for movement, it lacks blood vessels because the arteries wrap around the joint in an anastomosis, bypassing direct capillary contact with the capsule.
The articular and epiphyseal branches given off by the neighboring arteries form a periarticular arterial plexus. Exchange of gases oxygen and carbon dioxide and nutrients is achieved, albeit slowly, via diffusion or more efficiently during exercise via convection.
Key Terms convection : The movement of groups of molecules within fluids such as liquids or gases. Bursae and Tendon Sheaths Joints are cushioned by small fluid-filled sacs called bursae and stabilized by tough bands of fibrous connective tissue called tendons.
Learning Objectives List the components of a joint. Key Takeaways Key Points Synovial joints are made up of five classes of tissues. These include bone, cartilage, synovium, synovial fluid, and tensile tissues composed of tendons and ligaments. Tendons are tough bands of fibrous connective tissue that connect muscles to bones.
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