Skeletal System

The skeletal system is an incredible design by nature. Not only does the skeleton provide a rigid, support framework for the soft tissues of the body to attach to, it provides for the ability to move rapidly by producing a series of levers through the articulations that exist. Also, the skeletal system serves the physiological functions of storing certain minerals and producing blood cells.

The organs that comprise the skeletal system are the bones and their associated tissues and articulations (joints). The largest component of bone is the osseous tissue, a specialization of connective tissue. This specialized connective tissue produces a unique matrix that provides for bones that have both a degree of rigidity and a degree of flexibility. This combination allows for bones to withstand great stresses prior to reaching the breaking point. Osseous tissue is a particularly active tissue. Through the growth and reabsorption of osseous tissue, bones can be constantly remodeled throughout life.

The bones of the skeletal system are joined together at the articulations (joints). These articulations vary from being immovable, such as in the sutural joints of the skull, to freely movable, such as in the ball and socket joint of the shoulder. This variety of different types of articulations illustrates the varied mechanical functions of the skeletal system.

Gross Anatomy

Microscopic Anatomy

There are four different types of bone cells:

1. Osteoprogenitor - unspecialized mesenchymal cells that can develop into osteoblasts.

2. Osteoblasts - cells that form osseous matrix by secreting collagen and mineral salts.

3. Osteocytes - developed from osteoblasts that have become trapped in the matrix. These cells maintain existing osseous matrix.

4. Osteoclasts - function in the reabsorption of the osseous matrix.

The matrix of bone is similar to other connective tissues in that it is produced primarily of heavy collagen fibers. This tough, structural protein provides the tensile strength to bones. Bone matrix differs from other connective tissues in its rigidity (hardness). The rigidity is produced by the calcification (mineralization) of bone matrix due to the secretion of mineral salts by the bone producing cells (osteoblasts). These calcium based salts, primarily hydroxyapatite and calcium carbonate, accumulate in the spaces between the collagen fibers and crystallize producing the rigidity of bone.

Compact bone appears to be very organized, consisting of cylinders of bone growth around a central canal containing blood vessels. Each cylinder of bone growth is called an osteon (Haversion system). Compact bone is particularly dense and rigid, providing a long external layer in all bones and the bulk of thediaphysis in long bones. Spongy bone appears to grow in irregular patterns or plates called trabeculae leaving many open spaces. These spaces are filled with red bone marrow, a soft heamatopoietic tissue. The inclusion of these macroscopic spaces provides a "spongy" quality to this matrix. Spongy bone makes up the majority of the tissue of short, irregular, and flat bones as well as most of the epiphyses of the long bones.


Mechanically, bones provide protection for certain organs, a rigid framework for supporting the soft tissues, and a system of levers to allow for movement. Physiologically, bones serve to help maintain mineral homeostasis by storing minerals, especially calcium and phosphorus, produce blood cells in the red bone marrow (hematopoiesis), and store some energy in the yellow bone marrow.

Bone Formation

Bones are formed by ossification, which may be intramembranous or endochondral. Intramembranous ossification will produce most of the flat and irregular bones of the body while endochondral ossification will produce the long and short bones. Intramembranous ossification occurs as the replacement of fibrous connective membranes. Endochondral ossification occurs as the replacement of cartilage.

Bone osteoprogenitor cells begin to accumulate near the midpoint of the cartilage and begin to ossify the tissue forming a bone collar. The invagination of a blood vessel into the developing cavity (cavitation) beneath the collar is called the periosteal bud which brings about the primary ossification center as the capillaries induce growth of osteoblasts to replace the cartilage, The invagination of additional blood vessels develops secondary centers of ossification at the ends of the bones producing pads of cartilage between the primary and secondary center which become the epiphyseal disks, resulting in the mature form of the bone.

Bone Growth

The epiphyseal disk is the growth plate in long bones that is found between the two epiphyses and the diaphysis. The plate is composed primarily of hyaline cartilage. The cells closest to the epiphysis divide rapidly and the production of new cartilage matrix causes the elongation of the bone. The cells nearest the diaphysis are typically replaced at approximately the same rate, The decreased production of growth hormone at the end of adolescence results in a decreased production of somatomedin causing a slower growth of cartilage cells. The faster growing bone cells eventually replace all the epiphyseal disk resulting in a "closure" of the growth plate producing an epiphyseal line.

Appositional growth is the increase in the diameter of bones. Although elongation is limited by the closure of epiphyseal disks at maturity, the remodeling of bones due to stress alterations continues throughout life. Bones continue to grow in response to stresses by the production of new matrix from the osteoblasts beneath the periosteum.

Calcium Homeostasis

In the event that the blood calcium level in the blood begins to drop below acceptable levels, the parathyroid gland responds by the release of parathyroid hormone (parathormone) wich acts to stimulate the activity of osteoclasts. The osteoclasts will reabsorb bone matrix, releasing the calcium to the blood stream raising the calcium concentration. In the event that the calcium level in the blood becomes too high, the C cells of the thyroid gland release calcitonin which inhibits the activity of osteoclasts (and thus inhibits the release of calcium from the matrix).

Types of Bones

    There are four principal types of bones:

1. Long - considerably longer than they are wide with a shaft composed primarily of compact bone and distal ends composed primarily of spongy bone.

2. Short - cuboidal in shape and are composed primarily of spongy bone.

3. Flat - have a spongy center sandwiched between two plates of compact bone (diploe) and typically are thin and curved.

4.. Irregular - bones which do not fit into the other categories.

Bone Markings

    The following are common markings found on bone:

  • Condyle - large, rounded articular prominence.
  • Epicondyle - prominence superior to a condyle.
  • Facet - an opening through which blood vessels or nerves pass.
  • Fissure - cleft-like opening between bones, typically for the passage of blood vessels or nerves.
  • Foramen - opening through which blood vessels or nerves pass.
  • Fossa - depression in a bone.
  • Head - rounded articular structure supported by a constricted portion (neck) of a bone.
  • Meatus - tube-like passageway through a bone
  • Sinus - air filled cavity within a bone
  • Trochanter - large projection found on the femur.
  • Tubercle - small, rounded process.
  • Tuberosity - large, rounded and roughened process.

Divisions of the Skeletal System

The skeletal system is divided into the axial and appendicular components. The axial are the bones along the central axis of the body and include the cranium, vertebral column and ribcage. The appendicular skeleton includes the upper and lower appendages (limbs) along with the pelvic and pectoral girdles.

Axial Skeleton

Cranial bones form some part of the cranial cavity that houses the brain. The facial bones do not. Cranial bones include: frontal, parietal (2), temporal (2), occipital, sphenoid, and ethmoid. The facial bones include: nasal (2), maxillae (2), mandible, lacrimal (2), palatine (2), inferior nasal conchae (2), vomer, and zygomatic (2). A suture is a fixed joint between the cranial bones. The four primary sutures are the coronal, sagittal, lambdoid, and squamous.

The vertebral column is divided into cervical, thoracic, lumbar, and sacral vertebrae (along with the vestigial coccygeal). The cervical are the smalles in mass and include two features found in no other group; transverse foramen and bifurcated spiny processes. The thoracic are larger in mass and include costal facets (points of rib articulation). The lumbar are the largest vertebrae with a massive body. The sacral vertebrae are fused into a single plate called the sacrum.

Ribs are classified according to their attachment to the sternum. The first seven pairs of ribs attach directly to the sternum and are called true ribs. Ribs 8-10 have cartilage that attaches to the cartilage of the seventh rib and are called false ribs. The eleventh and twelth ribs do not attach to the sternum and are called floating ribs.

    The pectoral girdle consists of the clavicle and scapula. Its function is to serve as the site of attachment of the upper limbs. The clavicle has two articulations: the sternoclavicular joining it to the sternum of the axial skeleton, and the acromioclavicular joining it to the acromion of the scapula. The scapula forms the humeroscapular joint for the point of attachment to the upper limb.

Appendicular Skeleton

Each upper limb consists of a humerus, radius, ulna, eight carpals, five metacarpals, and fourteen phalanges.

The pelvic girdle consists of two fused coxal bones which form a common joint at the pubic symphysis. The coxal bone is comprised of three fused bones, the ilium, ischium, and pubis. In addition to the joint at the pubic symphysis, the coxal bone forms two more articulations: the sacroiliac joint for attachment to the axial skeleton, and the coxafemoral joint for attachment to the lower limb.

Each lower limb consists of a femur, tibia, fibula, patella, seven tarsals, five metatarsals, and fourteen phalanges.

Sex Differences in the Skeletal System

The female pelvis is specialized for pregnancy and childbirth. The female pelvis is more shallow and more oval that the male pelvis. It is slightly larger with a pubic arch angle of greater than 90 degrees. It is less vertical and produces a slightly greater angle of attachment for the femurs. Due to the larger and more massive muscular development in males, the male skeleton tends to be larger and heavier with more prominent bone markings at points of tendon attachment.

Classification of Joints

An articulation is the point of contact between two or more bones (or between bone and cartilage or teeth). Functionally, joints are classified as synarthrotic, which typicall are immovable, amphiarthrotic, which are slightly movable, and diarthrotic, which are freely movable.

The synarthrotic joints include the synostosis, gomphosis, and synchondrosis. Synostoses are joints connected by a thin fibrous connective sheath that often are completely replaced by bone such as a suture. Gomphoses are specialized fibrous joints such as the articulations between the mandible or maxillae and the teeth. Synchondroses are joints connected by cartilage such as the articulation of the ribs to the sternum or within a long bone as the diaphysis connects to the epiphysis by the growth plate. The amphiarthrotic joints include the syndesmoses and the symphyses. The syndesmoses are joints connected by a relatively thick fibrous connective tissue sheath allowing some movement such as in the joints between the shafts of tibia and fibula. The symphyses are fibrocartilagenous jonts such as that found between the vertebrae.

Structure of Synovial Joints

A synovial joint consists of an articular capsule that completely encapsulates the ends of the articulating bones. The capsule is composed of two layers, the fibrous capsule and the synovial membrane. The external fibrous layer is primarily dense, irregular connective tissue and includes the external ligaments of the joint. The internal synovial membrane is composed primarily of specialized areolar connective tissue that secretes synovial fluid which fills the synovial cavity, or space inside the capsule. The ends of articulating bones in synovial joints are covered with articular cartilage (hyaline). The synovial fluid acts to lubricate the joint and provides nutrients to the chondrocytes of the articular cartilage. The hyaline cartilage serves as an exvellent shock absorber and provides a low friction surface for these freely movable joints.

Accessory ligaments are ligaments that help maintain joint stability but are not continuous with the articular capsule, either extracapsular or intracapsular. Examples of both are found in the knee joint, with the cruciate ligaments found inside the joint capsule and the popliteal found outside. Articular discs (menisci) are fibrocartilage pads attached to the bone surfaces (atop the hyaline cartilage) that acts to provide additional shock absorption and channel the synovial fluid to the sites of the greatest friction.

As a general rule, the greater the mobility of the joint, the less structurally stable it becomes. Mobility is measured as the number of directions (planes) of movement available at a joint as well as the range of movement in each direction. Mobility is limited by shape of the articulation surfaces of the bones as well as by the soft tissues, such as the arrangement of ligaments and tendons surrounding the joint. Typically, the more limiting the anatomical structure, the more stable the joint becomes, and thus, the less mobile.

Classification of Movements

Angular movements include extension, flexion, hyperextension, circumduction, abduction, and adduction.

The ten special movements are those that occur only at particular joints. At the ankle, these movements include plantar flexion, dorsiflexion, inversion, and eversion. At the shoulder girdle and mandibular joint, these include protraction, retraction, elevation, and depression. At the elbow, supination and pronation is possible.

Review Questions:

1. What are the mechanical functions of bone? Give at least one example.

2. What are the physiological functions of bones?

3. What are the names and functions of the four types of bones?

4. What is the difference between bone matrix and the matrix of connective tissues?

5. What are the two methods bone is formed? What is the difference between them?

6. List and describe the stages of endochondral ossification.

7. How does bone elongation occur?

8. What is appositional bone growth? How is it significant?

9. Identify and describe the four principal types of bones.

10. What is an articulation? What are the classifications of joints based upon their structure and function?