Anatomy and Physiology of Domestic Animals

By R. Michael Akers Donald Denbow

John Wiley & Sons

Copyright © 2008 R. Michael Akers
All right reserved.

ISBN: 978-0-8138-0329-6

Chapter One

Bones and skeletal system


Bones Introduction Classification of Bones Bone Structure Gross Anatomy Microscopic Anatomy of Bone Chemical Composition of Bone Hematopoietic Tissue in Bone Bone Development Intramembranous Ossification Endochondral Ossification Bone Growth, Remodeling, and Repair Bone Growth Bone Remodeling and Repair Repair of Fractures Homeostatic Imbalances of Bone Osteomalacia and Rickets Parturient Paresis (Milk Fever) Egg-Laying Fatigue in Birds Bones and Skeleton Markings on Bones Skeleton Skeletal Cartilage Skeleton Classification Axial Skeleton The Skull The Vertebral Column Appendicular Skeleton Thoracic Limb Pelvic Limb Avian Skeleton Joints Types of Joints Synovial Joints Specific Joints Intervertebral Articulations Thoracic Limb Pelvis Pelvic Limb



Osteology is the study of bones. The skeleton provides the basic scaffolding for the body. The skeletal system includes the bones, and the cartilage, ligaments, and connective tissues that hold everything together.

Classification of bones

The human skeleton contains 206 major bones whereas the number of bones in different animals varies. The bones can be classified into five categories including long bones, short bones, flat bones, irregular bones, and sesamoid bones (Fig. 6.1).

Long bones. These are bones that are longer than they are wide. Some of the bones of the limbs are long bones. Long bones are characterized by an elongated shaft and somewhat enlarged extremities that bear articular surfaces. Examples of long bones include the humerus, radius, femur, tibia, metacarpals, and metatarsals.

Short bones. Short bones are generally cube-shaped, and examples include the carpal and tarsal bones.

Flat bones. Flat bones, as the name implies, are thin and flattened. They include two plates of compact bone separated by cancellous or spongy bone. Examples include the sternum, ribs, scapula, and certain skull bones.

Irregular bones. These are complex and irregularly shaped bones. Examples include the vertebrae and certain facial bones.

Sesamoid bones. Sesamoid bones are small bones embedded in a tendon and resemble the shape of a sesame seed. Examples include the patella, and proximal and distal sesamoid bones of the digits.

Bone structure

Gross anatomy

Each bone consists of compact bone and cancellous bone. Compact bone, also called dense or cortical bone, is a term describing solid-looking bone. Compact bone is found on the surface of bones forming a protective outer coating; cancellous bone is found on the interior.

Cancellous bone, also called spongy bone, consists of a network of pieces of bone called trabeculae or spicules, interspersed with spaces filled with red or yellow bone marrow. Spongy bone predominates in short, flat, and irregular bones, as well as in the epiphyses of long bones. It is also found as a narrow lining of the medullary cavity of the diaphysis of long bones. The epiphyses consist mostly of cancellous bone with 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 diaphysis and epiphysis. Between the diaphysis and epiphysis of growing bones is a flat plate of hyaline cartilage called the epiphyseal plate. After growth is complete, the plate is replaced by the epiphyseal line. The medullary cavity (medulla, "innermost part") is 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 covers the epiphysis where one bone forms a joint with another bone.

The fibrous covering surrounding that part of the bone not covered with articular cartilage is called the periosteum. It consists of dense irregular connective tissue. Its innermost layer consists of an osteogenic layer containing osteoblasts (bone germinators) that make new bone, and osteoclasts that break down bone. The periosteum contains nerve fibers, lymphatic vessels, and blood vessels that supply the bone. The periosteum is attached to the underlying bone by Sharpey's fibers extending from the fibrous layer into the bone matrix. There is a high density of Sharpey's fibers where tendons and ligaments attach to the periosteum.

The internal surfaces of the bone are covered with the endosteum. The endosteum lines the medullary cavity in long bones and covers the trabeculae of spongy bone.

Short, irregular and flat bones vary in the proportion 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, but there is no bone marrow cavity. The internal spongy layer in flat bones is called the diploŽ (folded).

Microscopic anatomy of bone

There are four major cell types found in bone (Fig. 6.4). Osteocytes are the mature cells within bone that account for most of the population of bone cells. They are found within a lacuna (see next section, "Compact Bone"). Osteoblasts are cells that secrete the extracellular matrix on bone. They secrete collagen and ground substance that makes up unmineralized bone, called osteoid. Once these cells get embedded within the matrix, they become osteocytes. Osteoclasts are cells involved in resorption of bone, and are therefore present in areas where bone is being removed. Osteoclasts are giant multinucleated cells. Bone also contains a small number of mesenchymal cells known as osteoprogenitor cells. These are stem cells that can produce osteoblasts, and are therefore important in fracture repair. They are located in the inner, cellular layer of the periosteum, the endosteum that lines the marrow cavity, and the lining of vascular passageways in the matrix.

Compact bone

Although compact bone appears solid to the unaided eye, microscopically it contains considerable detail. The structural unit of compact bone is the osteon, or Haversian system (Fig. 6.5). Each osteon appears as a cylindrical unit consisting of 3-20 concentric lamellae of bone matrix surrounding the central osteonal canal (Haversian canal, or central canal) that runs parallel to the long axis of the bone. The lamellae are like paper towels wrapped around a cardboard roll (i.e., osteonal canal). The osteonal canal contains the vascular and nerve supply of the osteon. The osteonal canals run parallel to the long axis of the bone, and they carry small arteries and veins.

A second group of canals, called perforating or Volkmann'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 osteonal canal. These canals are lined with endosteum.

During bone formation, osteoblasts secrete the bone matrix. However, osteoblasts maintain contact with one another via connections containing gap junctions. As the matrix hardens, the osteoblasts become trapped within it, thus forming the lacunae and canaliculi. The osteoblasts become osteocytes, or mature bone cells.

Osteocytes, the spider-shaped mature bone cells, are found in lacunae, the small cavities at the junctions of the lamellae. Only one osteocyte is found per lacunae, and these cells cannot divide. Numerous processes extend from each osteocyte into little tunnels running through the mineralized matrix called canaliculi, which connect adjacent lacunae. Therefore, there is a continuous network of canaliculi and lacunae containing osteocytes and their processes running throughout 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 fibers run parallel, immature bone, also called woven bone, has a non lamellar structure. Woven bone is put down rapidly during growth or repair, and its fibers are aligned at random resulting in lower strength. Woven bone is generally replaced by lamellar bone as growth continues.

Cancellous or spongy bone

Unlike compact bone, spongy bone does not contain osteons. As mentioned earlier, it consists of an irregular lattice network of bone called trabeculae. Red bone marrow can be found in the space between the trabeculae. Osteocytes are found in lacunae within the trabeculae, and canaliculi radiate from the lacunae.

Chemical composition of bone

Bone consists of both organic and inorganic components. The major inorganic component is calcium phosphate, [Ca.sub.3][(P[O.sub.4]).sub.2], accounting for two-thirds of the weight of bone. Calcium phosphate interacts with calcium hydroxide, Ca[(OH).sub.2], to form hydroxyapatite, [Ca.sub.10][(P[O.sub.4]).sub.6][(OH).sub.2]. As the crystals of hydroxyapatite form, they also incorporate other inorganic materials including calcium carbonate, sodium, magnesium, and fluoride.

The remaining organic portion of the bone is made up of cells (osteoblasts, osteocytes, and osteoclasts) and osteoid, which includes collagen fibers and ground substance (proteoglycans and glycoproteins). The osteoid is secreted by osteoblasts.

Hematopoietic tissue in bones

Red bone marrow, which is hematopoietic (i.e., blood forming), is found in the spongy bone of long bones and the diploŽ of flat bones. Red bone marrow consists of mature and immature red blood cells, white blood cells, and stem cells that produce them. In newborns, the medullary cavities of spongy bones contain red bone marrow. In adult long bones, the medullary cavities of spongy bone become large fat-filled medullary cavities containing yellow bone marrow and extending into the epiphysis. Yellow marrow functions in fat storage, and contains mostly fat cells. Therefore, blood cell production in adult long bones is restricted to the head of the femur and humerus. However, if an animal is anemic, the yellow marrow can revert to red marrow to supplement red blood cell production. In contrast, the spongy bone found in flat bones, such as those in the hips, remains hematopoietic and therefore a good source when needing to sample bone marrow.

The osteonal and lateral canals are also the way in which blood cells formed in the marrow enter circulation. The sinuses of the bone marrow connect with the venous vessels running through these channels, and newly formed blood cells are released into them. From there they can leave the confines of the bone and enter the general circulation.

Bone development

Osteogenesis, or ossification, is the process of bone formation. Calcification, the process of calcium salt deposition, occurs during ossification. While calcification is associated with bone formation, it can occur in other tissues.

There are two general classes of bone formation. Intramembranous ossification occurs when bone develops from a fibrous membrane. The flat bones of the skull and face, the mandible, and the clavicle if present, are formed by this method. Intramembranous ossification can also result in the formation of bones in abnormal locations such as testes or whites of the eyes. Such bones are called heterotopic bones (hetero = different; topos = place). If cartilage serves as the precursor for the bone, formation is called endochondral ossification. Because of remodeling that occurs later, the initial bone laid down by either method is eventually replaced.

Intramembranous ossification

Early in embryonic development, elongate mesenchymal cells migrate and aggregate in specific regions of the body. Remember, mesenchyme is tissue from which all connective tissue develops. As these cells condense, they form the membrane from which the bone will develop (Fig. 6.6). This presumptive bone site becomes more vascularized with time, and the mesenchymal cells enlarge and become rounder. As the mesenchymal cells change from eosinophilic (i.e., stained with eosin dyes) to basophilic (affinity for basic dyes), they differentiate into osteoblasts. These cells secrete the collagen and proteoglycans (osteoid) of the bone matrix. As the osteoid is deposited, the osteoblasts become increasingly separated from one another, although they remain connected by thin cytoplasmic processes.

The site where the matrix begins to calcify is called the ossification center. Eventually, as the matrix becomes calcified, the osteoblasts become osteocytes. The osteocytes are contained in canaliculi. Some of the surrounding primitive cells in the membrane proliferate and give rise to osteoprogenitor cells. These cells come in opposition to the spicules, and become osteoblasts, thus adding more matrix. This results in appositional growth in which the spicules (areas of calcification extending from the ossification center) enlarge and become joined into a trabecular network having the shape of bone.

Endochondral ossification

Endochondral ossification begins similar to intramembranous ossification, with mesenchymal cells migrating and aggregating (Fig. 6.7). However, these cells now become chondroblasts, instead of osteoblasts, and begin making a cartilage matrix. Once made, the cartilage matrix grows by both interstitial and appositional growth. Interstitial growth is responsible for most of the increase in length of the bone, whereas the increase in width is produced by new chondrocytes that differentiate from the chondrogenic layer of the perichondrium surrounding the cartilage mass.

Bone formation begins when perichondrial cells in the midregion give rise to osteoblasts rather than chondrocytes. At this point, the connective tissue surrounding the middle of the cartilage changes from perichondrium to periosteum. A thin layer of bone begins forming around the cartilage model. This bone can be called either periosteal bone because of its location, or endochondral bone because of its method of development. This periosteal bone is sometimes termed the bony collar.

As the chondrocytes in the midregion become hypertrophic, the matrix becomes compressed. These cells begin to synthesize alkaline phosphatase, and the surrounding matrix begins to calcify. As the chondrocytes die, the matrix breaks down and the neighboring lacunae become interconnected. At the same time, blood vessels begin to enter this diaphyseal area vascularizing the developing cavity.

Cells from the periosteum migrate inward with the blood vessels and become osteoprogenitor cells. Other cells also enter to give rise to the marrow. The breakdown of the matrix leaves spicules that become lined with osteoprogenitor cells that then differentiate into osteoblasts. Osteoblasts then begin to produce the osteoid on the spicule framework. Bone formed in this manner is called endochondral bone, and this region becomes the primary ossification center. As the cartilage is resorbed (i.e., broken down), the bone deposited on the calcified spicules becomes spongy bone.

Eventually, a secondary ossification center develops in each epiphysis. Bone develops in these regions similarly to that in the primary ossification center. As the secondary ossification develops, the only cartilage remaining is that at the ends of the bones, and a transverse region known as the epiphyseal plate separating the diaphyseal and epiphyseal cavities.

As the cavity in the diaphyseal marrow enlarges, there is a distinct zonation that develops in the cartilage at either end of the diaphysis (Fig. 6.8). The following five regions develop beginning most distal from the diaphysis:

1. Zone of reserve cartilage. This region contains no cellular proliferation or matrix production. It contains small, scattered chondrocytes.


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