Anatomy and Physiology of Domestic Animals

Anatomy and Physiology of Domestic Animals 6 Bones and skeletal system Contents Bones Markings on Bones Introduction Skeleton Classifi cation of Bones Skeletal Cartilage Bone Structure Skeleton Classifi cation Gross Anatomy Axial Skeleton Microscopic Anatomy of Bone The Skull Chemical Composition of Bone The Vertebral Column Hematopoietic Tissue in Bone Appendicular Skeleton Bone Development Thoracic Limb Intramembranous Ossifi cation Pelvic Limb Endochondral Ossifi cation Avian Skeleton Bone Growth, Remodeling, and Repair Joints Bone Growth Types of Joints Bone Remodeling and Repair Synovial Joints Repair of Fractures Specifi c Joints Homeostatic Imbalances of Bone Intervertebral Articulations Osteomalacia and Rickets Thoracic Limb Parturient Paresis (Milk Fever) Pelvis Egg-Laying Fatigue in Birds Pelvic Limb Bones and Skeleton Bones Classifi cation of bones Introduction The human skeleton contains 206 major bones whereas the number of bones in different animals varies. The Osteology is the study of bones. The skeleton pro- bones can be classifi ed into fi ve categories including vides the basic scaffolding for the body. The skeletal long bones, short bones, fl at bones, irregular bones, system includes the bones, and the cartilage, liga- and sesamoid bones (Fig. 6.1). ments, and connective tissues that hold everything • Long bones. These are bones that are longer than together. they are wide. Some of the bones of the limbs are 133 134 Anatomy and physiology of domestic animals yellow bone marrow. Spongy bone predominates in short, fl at, and irregular bones, as well as in the epiphyses of long bones. It is also found as a narrow lining Irregular bone of the medullary cavity of the diaphysis of long bones. (vertebrae) Long bone The epiphyses consist mostly of cancellous bone with (femur) a thin outer coat of compact bone. In developing long bones, the shaft is called the diaphysis and each extremity is called an epiphysis (pl. = epiphyses) (Fig. 6.2). The epiphysis consists mostly of cancellous bone with a thin outer coat of compact bone. It is generally enlarged relative to the diaphysis. The metaphysis is the joining point of the Sesamoid bone diaphysis and epiphysis. Between the diaphysis and (patella) epiphysis of growing bones is a fl at plate of hyaline Short bone Flat bone cartilage called the epiphyseal plate. After growth is (carpal) (rib) complete, the plate is replaced by the epiphyseal line. Fig. 6.1. Types of bones. Examples of the various types of bones The medullary cavity (medulla, “innermost part”) is as found in the pig skeleton. the space in the diaphysis containing bone marrow. At the joint surface on the bone is an articular surface consisting of a smooth layer of hyaline cartilage that long bones. Long bones are characterized by an covers the epiphysis where one bone forms a joint elongated shaft and somewhat enlarged extremi- with another bone. ties that bear articular surfaces. Examples of long The fi brous covering surrounding that part of the bones include the humerus, radius, femur, tibia, bone not covered with articular cartilage is called the metacarpals, and metatarsals. periosteum. It consists of dense irregular connective • Short bones. Short bones are generally cube- tissue. Its innermost layer consists of an osteogenic shaped, and examples include the carpal and layer containing osteoblasts (bone germinators) that tarsal bones. make new bone, and osteoclasts that break down • Flat bones. Flat bones, as the name implies, are bone. The periosteum contains nerve fi bers, lymphatic thin and fl attened. They include two plates of vessels, and blood vessels that supply the bone. The compact bone separated by cancellous or spongy periosteum is attached to the underlying bone by bone. Examples include the sternum, ribs, scapula, Sharpey’s fi bers extending from the fi brous layer into and certain skull bones. the bone matrix. There is a high density of Sharpey’s • Irregular bones. These are complex and irregu- fi bers where tendons and ligaments attach to the larly shaped bones. Examples include the verte- periosteum. brae and certain facial bones. The internal surfaces of the bone are covered with • Sesamoid bones. Sesamoid bones are small bones the endosteum. The endosteum lines the medullary embedded in a tendon and resemble the shape cavity in long bones and covers the trabeculae of of a sesame seed. Examples include the patella, spongy bone. and proximal and distal sesamoid bones of the Short, irregular and fl at bones vary in the propor- digits. tion of compact and cancellous (Fig. 6.3). Furthermore, these bones also do not have a shaft or epiphyses. They contain bone marrow between their trabeculae, Bone structure but there is no bone marrow cavity. The internal spongy layer in fl at bones is called the diploë Gross anatomy (folded). Each bone consists of compact bone and cancellous bone. Compact bone, also called dense or cortical Microscopic anatomy of bone bone, is a term describing solid-looking bone. Compact bone is found on the surface of bones forming a pro- There are four major cell types found in bone (Fig. 6.4). tective outer coating; cancellous bone is found on the Osteocytes are the mature cells within bone that interior. account for most of the population of bone cells. They Cancellous bone, also called spongy bone, consists are found within a lacuna (see next section, “Compact of a network of pieces of bone called trabeculae or Bone”). Osteoblasts are cells that secrete the extracel- spicules, interspersed with spaces fi lled with red or lular matrix on bone. They secrete collagen and ground Bones and skeletal system 135 Articular Spongy bone cartilage Red Marrow Epiphysis Metaphysis Diaphysis Medullary cavity Compact bone Yellow marrow Metaphysis Epiphysis Periosteum Articular A Cartilage Metaphysis Metaphysis Epiphysis Diaphysis Epiphysis Spongy bone Spongy bone Compact bone Medullary cavity B Fig. 6.2. Anatomy of long bones. A) Using the femur as an example of a long bone, the epiphysis is the enlarged area at either end of the bone while the diaphysis is the long shaft in the middle portion of the bone. The metaphysis is the joining point between the epiphysis and diaphysis. The periosteum is the fi brous covering around the outside of the bone not covered with articular cartilage. The endosteum is the fi brous and cellular tissue lining the medullary cavity of the bone. B) Cross section of an equine humerus showing exterior and interior anatomy. substance that makes up unmineralized bone, called The structural unit of compact bone is the osteon, or osteoid. Once these cells get embedded within the Haversian system (Fig. 6.5). Each osteon appears as a matrix, they become osteocytes. Osteoclasts are cells cylindrical unit consisting of 3–20 concentric lamellae involved in resorption of bone, and are therefore of bone matrix surrounding the central osteonal canal present in areas where bone is being removed. Osteo- (Haversian canal, or central canal) that runs parallel clasts are giant multinucleated cells. Bone also con- to the long axis of the bone. The lamellae are like paper tains a small number of mesenchymal cells known as towels wrapped around a cardboard roll (i.e., osteonal osteoprogenitor cells. These are stem cells that can canal). The osteonal canal contains the vascular and produce osteoblasts, and are therefore important in nerve supply of the osteon. The osteonal canals run fracture repair. They are located in the inner, cellular parallel to the long axis of the bone, and they carry layer of the periosteum, the endosteum that lines the small arteries and veins. marrow cavity, and the lining of vascular passage- A second group of canals, called perforating or Volk- ways in the matrix. mann’s, or lateral, canals, run at right angles to the long axis of the bone. These canals connect the blood vessel and nerve supply of the periosteum with that in the Compact bone osteonal canal. These canals are lined with endosteum. Although compact bone appears solid to the unaided During bone formation, osteoblasts secrete the bone eye, microscopically it contains considerable detail. matrix. However, osteoblasts maintain contact with 136 Anatomy and physiology of domestic animals one another via connections containing gap junctions. and these cells cannot divide. Numerous processes As the matrix hardens, the osteoblasts become trapped extend from each osteocyte into little tunnels running within it, thus forming the lacunae and canaliculi. The through the mineralized matrix called canaliculi, osteoblasts become osteocytes, or mature bone cells. which connect adjacent lacunae. Therefore, there is a Osteocytes, the spider-shaped mature bone cells, are continuous network of canaliculi and lacunae contain- found in lacunae, the small cavities at the junctions of ing osteocytes and their processes running throughout the lamellae. Only one osteocyte is found per lacunae, the mineralized bone. Canaliculi are important because they provide a route by which processes from one osteocyte can contact those of adjacent osteocytes. Therefore, via the canalicular system, all osteocytes are potentially in communication with one another. They pass information, nutrients, and/or wastes from one place to another. Osteocytes can synthesize or absorb bone matrix. If the osteocyte dies, bone matrix resorption occurs due to osteoclast activity, which is later followed by repair or remodeling by osteoblast activity. While mature compact bone has a lamellar structure in which the fi bers run parallel, immature bone, also Compact bone called woven bone, has a nonlamellar structure. Woven bone is put down rapidly during growth or repair, and its fi bers are aligned at random resulting in lower Spongy bone strength. Woven bone is generally replaced by lamel- (diploë) lar bone as growth continues. Compact bone Cancellous or spongy bone Fig. 6.3. Internal anatomy of fl at bone. Flat bones consist of an Unlike compact bone, spongy bone does not contain outer layer of compact bone that sandwiches an inner layer of osteons.

Load more