Immature osteogenic cells are found in the deep layers of the periosteum and the marrow. 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 cell responsible for bone resorption, or breakdown, is the osteoclast.
They are found on bone surfaces, are multinucleated, and 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.
The differences between compact and spongy bone are best explored via their histology. Compact bone is dense so that it can withstand compressive forces, while spongy cancellous bone has open spaces and supports shifts in weight distribution. Compact bone is the denser, stronger of the two types of bone tissue Figure 6. It can be found under the periosteum and in the diaphyses of long bones, where it provides support and protection.
The microscopic structural unit of compact bone is called an osteon , or Haversian system. Running down the center of each osteon is the central canal , or Haversian canal, which contains blood vessels, nerves, and lymphatic vessels. As described earlier, canaliculi connect with the canaliculi of other lacunae and eventually with the central canal. This system allows nutrients to be transported to the osteocytes and wastes to be removed from them.
Like compact bone, spongy bone , also known as cancellous bone, contains osteocytes housed in lacunae, but they are not arranged in concentric circles. The trabeculae may appear to be a random network, but each trabecula forms along lines of stress to provide strength to the bone.
The spaces of the trabeculated network provide balance to the dense and heavy compact bone by making bones lighter so that muscles can move them more easily. In addition, the spaces in some spongy bones contain red marrow, protected by the trabeculae, where hematopoiesis occurs. Watch this video to see the microscopic features of a bone. Skip to main content. Chapter 6: The Skeletal System. Search for:. Bone Classification and Structure Learning Objectives Classify bones according to their shapes Describe the function of each category of bones.
Identify the anatomical features of a bone Describe the histology of bone tissue Compare and contrast compact and spongy bone Identify the structures that compose compact and spongy bone.
Bone Classifications Bone classification Features Function s Examples Long Cylinder-like shape, longer than it is wide Leverage Femur, tibia, fibula, metatarsals, humerus, ulna, radius, metacarpals, phalanges Short Cube-like shape, approximately equal in length, width, and thickness Provide stability, support, while allowing for some motion Carpals, tarsals Flat Thin and curved Points of attachment for muscles; protectors of internal organs Sternum, ribs, scapulae, cranial bones Irregular Complex shape Protect internal organs Vertebrae, facial bones Sesamoid Small and round; embedded in tendons Protect tendons from compressive forces Patellae.
Gross Anatomy of Bone. Many facial bones, particularly the ones containing sinuses, are classified as irregular bones. A sesamoid bone is a small, round bone that, as the name suggests, is shaped like a sesame seed. These bones form in tendons the sheaths of tissue that connect bones to muscles where a great deal of pressure is generated in a joint.
The sesamoid bones protect tendons by helping them overcome compressive forces. Sesamoid bones vary in number and placement from person to person but are typically found in tendons associated with the feet, hands, and knees.
Figure reviews bone classifications with their associated features, functions, and examples. Bones can be classified according to their shapes. Long bones, such as the femur, are longer than they are wide. Short bones, such as the carpals, are approximately equal in length, width, and thickness. Flat bones are thin, but are often curved, such as the ribs. Irregular bones such as those of the face have no characteristic shape.
Sesamoid bones, such as the patellae, are small and round, and are located in tendons. What are the structural and functional differences between a tarsal and a metatarsal? Structurally, a tarsal is a short bone, meaning its length, width, and thickness are about equal, while a metatarsal is a long bone whose length is greater than its width.
Human Anatomy Atlas offers thousands of models to help understand and communicate how the human body looks and works.
X-rays of the hand , hip , knee , and ankle. Stony Brook University School of Medicine. The Body Online. An Overview of the Skeletal System. Appendicular Skeleton. Joints and Ligaments. Skeletal System Pathologies.
When you select "Subscribe" you will start receiving our email newsletter. Use the links at the bottom of any email to manage the type of emails you receive or to unsubscribe. See our privacy policy for additional details. Learn Site. So, what are the different types of bones? How are they categorized? There are five types of bones in the skeleton: flat, long, short, irregular, and sesamoid.
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