BIOLOGY Volume 2 - A Guide to General Biology - 2004

18. THE ANIMAL MUSCULOSKELETAL SYSTEM

18.2. Skeletal Tissues

The vertebrate Skeleton is constructed of bone or Cartilage tissue. In both cases, it serves as the internal structural framework of the body. An endoskeleton composed entirely of cartilage is found only in elasmobranchs, which include sharks and rays. All other vertebrates possess a bony skeleton in adulthood, although cartilage persists in certain regions—such as in joints or intervertebral discs. During Embryogenesis, the skeleton of these vertebrates first forms as hyaline cartilage (sec. 6.4.2). This feature is of significant biological importance because cartilage is capable of interstitial growth, allowing different PARTS OF THE skeleton to increase proportionally as the Organism develops. In this respect, Bone tissue differs from cartilage, since bone growth occurs through The addition of new material to its surfaces.

Class="center">18.2.1. Cartilage

There are Three types of cartilage: hyaline (white), fibrocartilage (collagenous), and yellow elastic. Their histological features are discussed in detail in sec. 6.4.2. All types of cartilaginous tissue consist of a dense matrix (ground substance) penetrated by numerous Connective Tissue fibers. The matrix is secreted by living Cells called chondroblasts. Later, these cells become trapped in microscopic cavities (lacunae) scattered throughout the matrix. In this state, they are referred to as chondrocytes.

The most common type of cartilage is hyaline cartilage; for example, it covers the articular surfaces of bones. Its matrix, composed of chondroitin sulfate, is compressible and elastic, enabling it to withstand heavy loads and absorb sudden mechanical shocks experienced by the joint. The matrix derives its resistance to such stresses from fine collagenous fibers running through it. The entire surface of such cartilage, except for the areas facing the interior of the Joint Capsule, is covered by a Cytology/practical/45.html">Dense connective tissue layer called the perichondrium.

Fibrocartilage contains a dense network of Collagen fibers. It forms the intervertebral discs and is a component of tendons. It is an extremely tough yet moderately flexible tissue. Yellow elastic cartilage contains numerous elastic fibers and forms the outer ear, epiglottis, and laryngeal cartilages.

18.2.2. Bone Tissue

Bone is a dense, hard connective tissue that largely consists of calcified elements. Details of its Structure are given in sec. 6.4.2. Living bone cells (osteocytes) are embedded in a hard matrix (harder than that of cartilage). The structure of the matrix provides high mechanical strength. The mineral portion of the matrix (about 70%) is composed of calcium phosphate, which provides resistance to compressive forces. The organic portion (approximately 30%) includes numerous collagen fibers that are highly resistant to tension. Forces acting on bone are discussed in more detail in sec. 18.2.3. Bone can be envisioned as a system of nested cylinders (sec. 6.4.2), which enhances the mechanical strength of the overall structure.

In a Longitudinal section of a long bone (such as the Femur), its structural regions are clearly distinguishable. Such a bone consists of a hollow shaft—the diaphysis—with two expanded extremities at its ends—the epiphyses. The entire bone is covered on the outside by a dense connective tissue membrane called the periosteum. The diaphysis is composed of compact bone, whereas the epiphyses consist of spongy (cancellous) bone surrounded by a thin layer of compact bone (sec. 6.4.2). The Organization of bone tissue provides maximum strength in the directions where loads are applied (Fig. 18.2).

Fig. 18.2. Longitudinal section of the HEAD of the femur, showing the arrangement of trabeculae in the spongy bone.

The medullary cavity of the diaphysis is occupied by yellow Bone Marrow, while Red bone marrow is located in the epiphyses between the bone plates (trabeculae). The bone surface is penetrated by numerous small foramina through which nerve fibers and Blood Vessels supply the bone tissue and red bone marrow.

In addition to the Functions noted previously (sec. 18.1), the skeleton also participates in The production of red and white Blood Cells. Furthermore, it helps maintain constant Calcium and phosphorus levels in the blood (ch. 17). Calcium and phosphate ions stored in the bones can be released under the action of Calcitonin and parathyroid hormone, produced by the thyroid and Parathyroid glands, respectively.

18.2.3. Structure and function Relationship

Using the femur, which is involved in locomotion, we can demonstrate how its constituent Tissues are adapted to perform their functions.

The femur is a long bone representing a hollow cylinder. When compressive forces act on one side of it, the opposite side undergoes tension. Along the central axis, these loads neutralize each other. Consequently, the material located inside the bone (soft marrow) does not affect the strength of the entire structure. At the same time, the presence of the internal cavity reduces the bone's mass, facilitating overall animal mobility. The diaphysis wall consists almost entirely of compact bone, which possesses high resistance to compression and tension (sec. 8.4.4). Spongy bone in the epiphyses is formed by a network of bone partitions oriented along lines of stress. This ensures bone strength, while the spaces between them (containing light bone marrow) ensure a relatively low mass.

Cartilage covering the contacting articular surfaces of bones acts as a Shock absorber. Its matrix deforms under compression but easily recovers its original shape due to high elasticity. In addition, the smooth surface of the cartilage reduces joint friction.

Tendons consist of dense white Fibrous connective tissue. The relatively weak pulling force of a Muscle that moves a bone is transmitted to it via a thin tendon—that is, it is concentrated over a small area, providing a leverage effect. At the same time, heavy external loads are redistributed across the entire muscle by tendinous fibers extending into it, protecting the Muscle tissue from tearing.

Ligaments are structurally almost identical to tendons, but they connect various skeletal elements to each other primarily in the region of joints. Not only do they stabilize joints, but they also determine the specific direction and range of motion, thereby enhancing the efficiency of movements.

18.2.4. The Vertebrate Musculoskeletal System

The first land-dwelling vertebrates were amphibians. They evolved from fish, and with their transition from Water to land, a problem arose associated with the force of gravity and the need to support their bodies above the ground. As a result, their vertebrae evolved into complex structures articulating with one another via processes. Together, they form a strong yet sufficiently flexible Column—THE Vertebral Column—which serves as structural body support.

In ancient amphibians, the limbs projected sideways from the trunk; consequently, these animals moved across the ground with their bodies barely elevated above it. This same type of limb attachment and locomotion was characteristic of primitive reptiles (Fig. 18.3, A). With this mode of locomotion, muscular energy is expended primarily on keeping the body suspended above the ground, and these energy costs are so high that the animal spends most of its time on land resting motionless with its belly on the ground.

Fig. 18.3. Types of limb posture in vertebrates. A. Primitive amphibian — limbs splayed to the sides and then directed downward. B. Modern reptile — an intermediate position between amphibians and mammals. C. Mammal — limbs straightened and positioned directly beneath the trunk.

During further evolution, reptiles exhibited a trend toward shifting their limbs downward, so that the trunk was raised noticeably above the ground (Fig. 18.3, B). This posture facilitated locomotion, distributing body weight more evenly across four relatively straight limbs. This trend reached its extreme expression in mammals (Fig. 18.3, C).

Some reptiles and mammals transitioned to bipedal locomotion. Their hind limbs are used for walking, running, or jumping, which frees the forelimbs for manipulation tasks such as feeding, building activities, and grooming. Certain monkeys are characterized by a specific mode of locomotion known as brachiation, in which the animals swing from tree to tree by suspending themselves from long arms and grasping branches with elongated hands. However, many arboreal canopy dwellers are unable to move in this manner due to their small size and can only leap from branch to branch. The most specialized form of aerial locomotion is flight. It emerged during the Jurassic period simultaneously in flying reptiles (pterosaurs) and the first birds, which evolved from reptiles. The forelimbs underwent modification to become wings. Flying reptiles eventually went extinct, whereas birds evolved into a diverse array of forms during subsequent evolution.