Skeletal and Articular Physiology - Anatomy

Bone growth has multiple steps that allow growth in both length and width. One thing to remember is the functions of the bone cells during growth and development. Osteoclasts are responsible for "hollowing out" the center of long bones, which makes for larger cavities within the diaphysis. Osteoblasts, on the other hand, are responsible for laying down additional bone matrix on the outsides of the bones.

As bone cells mature, they become further embedded within subsequent layers of the bony matrix. Osteogenic cells, which give rise to osteoblasts, are located in the outer periosteum of the bone. When damage occurs to the bone, osteogenic cells differentiate and begin repairing the bony matrix from the outside.

Bone is a dynamic tissue that remodels under mechanical stress, or orthodonture. Mechanical stress in bone generates electric potential via the piezoelectric effect. Negative potential results in bone deposition (bone is laid down) whereas positive potential results in bone resorption (bone is broken down).

Histologically, the epiphyseal growth plate is divided into five zones. From epiphysis to diaphysis they are the resting zone, zone of proliferation, zone of maturation, zone of calcification, and zone of ossification. At the growth plate, cartilage is constantly being developed into the bone of the diaphysis. The stages of this process align with the regions of the epiphyseal plate. The resting zone houses quiescent chondrocytes that are not yet active in bone synthesis. The zone proliferation is characterized by chondrocyte mitosis and replication. These cells then develop and grow with in the zone of maturation. Eventually the cells reach their maximum growth and undergo apoptosis to release cell contents in the zone of calcification. This prevents cartilage from infiltrating the bony region of the diaphysis. The chondrin matrix begins to calcify in this zone as well. As calcification progresses and the organic cartilage matrix is replaced by bony hydroxyapatite mineral in the zone of ossification, the epiphyseal plate completely replaces the original chondrocytes with bone.

As more bone is produced, the epiphyseal plate is pushed farther and farther away from the midpoint of the bone. The lengthening of the bone ends when the zones of the epiphyseal plate fuse and further growth becomes impossible.

There are three primary types of bone cell: osteoblasts, osteocytes, and osteoclasts. Osteoblasts are responsible for creating new bone by sequestering minerals and generating new hydroxyapatite matrix. Osteoclasts break down this matrix, releasing the minerals into the blood. Osteocytes are mature osteoblasts that have become embedded in the matrix of the bone and serve primarily for communication purposes.

Satellite cells are located at the periphery of muscle cells and are capable of dividing and giving rise to new myoblasts. Satellite cells are, essentially, adult muscle stem cells. Common lymphoid progenitor cells are another type of adult stem cell, housed in red bone marrow, and are responsible for regenerating the erythrocyte population of the body, as well as producing lymphocytes.

The endosteum is the layer that is found deep to all layers. It is found in the inner lining of the bone. Meanwhile, the periosteum is the most superficial layer of the bone which compromises the outer covering of bones.

Osteoblasts will lay down bone matrix around previously formed bone tissue. This forms concentric rings of bone tissue referred to as lamellae.

Lacunae are small gaps in the hydroxyapatite matrix that house the osteocytes. Trabeculae are thin bony structures that span and branch within the region of spongy bone. Canaliculi are small channels between lacunae that allow for cellular nourishment and communication. The epiphyses are the ends of the bone (as opposed to the diaphysis, or bone shaft).

The only bone listed that is a sesamoid bone is the pisiform. The ethmoid is an irregular bone, the talus is a short bone, and the rib is a flat bone.

Osteoblasts are differentiated non-dividing cells that secrete osteoid, an organic matrix material, that becomes mineralized forming bone. These are the cells primarily responsible for building bone.

Osteocytes are osteoblasts that become trapped in lacunae by bony matrix. These cells still maintain the bone environment, but do not actively build or modify bone a significant amount.

Osteoclasts resorb bone by breaking down the crystalline matrix.

Deposition of the calcium phosphate salt hydroxyapatite leads to mineralization of the bony matrix.

A synarthrosis is a type of joint that permits little or no movement. Syndesmoses, synostoses, and symphyses are all examples of synarthroses. Syndesmoses are joined by an interosseous ligament. Joints between carpals and tarsals are mostly syndesmoses. Synostoses is a joint formed from the fusion of two bones, generally in an atypical fashion. Symphyses can be synarthroses or amphiarthorses and are characterized by a fibrocartilage band between bones, such as in the pubic symphysis.

A diarthrosis, also known as a synovial joint, is the most common joint type in humans and allows free bone movement. The knee and elbow are examples of a diarthroses or synovial joints.

The joints, or articulations, of the body can be classified according to several different criteria. They can be named for their structure, type of motion, or range of motion.

Synovial joints are determined by the joint structure. All synovial joints are housed within a joint capsule and contain synovial fluid. Joints between long bones are almost always synovial joints.

Syndesmoses are defined by their range of motion, and have very small, if any, motility. Syndesmoses are joined by interosseous ligaments, such as those between the carpals of the wrist.

Saddle joints are defined the type of motion allowed at the joint. Saddle joints are biaxial, allowing flexion, extension, adduction, and abduction, but no axial rotation. The pollical (thumb) joint is a saddle joint. While a "swinging joint" would seem to indicate a joint defined by its type of motion, no such joint exists in anatomical terms.

Metaphysis: has the greatest metabolic activity, and contains the epiphyseal plate, which is replaced by the epiphyseal line

Diaphysis: mechanical support, site of bone marrow and muscle attachments

Apophysis: functions as a site for attachments of ligaments and tendons

Epiphysis: at the end of long bones

Periosteum: covers the diaphysis and parts of metaphysis.

The medullary cavity is the bone marrow cavity contains red and/or yellow bone marrow; red bone marrow is the site of hematopoiesis. The end of a long bone is known as the epiphysis. Compact bone (cortical bone) is the dense outer layer of bone. The diaphysis is the shaft of a long bone.

There are three main joint categories: synovial, fibrous, and cartilaginous. Synovial joints have the largest range of movement, and are characterized by the presence of articular cartilage and synovial fluid. Fibrous joints lack cartilage and allow for little to no movement. Cartilaginous joints are formed by bands of cartilage between bones, and generally have some limited movement. The shoulder is a synovial joint, the sutures of the skull are fibrous joints, and the intervertebral joints are cartilaginous joints.

Diarthrosis is used to describe joints with wide ranges of movement. Amphiarthrosis describes joints with limited movement, while synarthrosis describes joints with no movement. Almost all fibrous joints are synarthrotic.

Your fingers are able to flex and extend. This type of motion is bidirectional, but along only one axis. This type of joint is called a hinge joint, at which you are able to flex and extend along an axis, with no side to side motion.

The pelvis is considered to be part of the appendicular skeleton. The sacrum, mandible, and seventh thoracic vertebra are part of the axial skeleton.

The nucleus pulposus is a gelatinous, spongy material located in the core of the vertebral disk. This material responds directly to the pressures of the spinal joints, allowing for twisting, compressing, and stretching forces acting upon the spinal joints and vertebrae. This material can become less resilient with age, but generally does not disintegrate or die as other structures do.

When the rhomboids contract, they pull on the scapula, causing the inferior angle of the scapula to swing up toward the spine. This causes the glenoid fossa to orient downward. Thus, the scapula is downwardly rotated by the rhomboids at the scapulocostal joint.

When the psoas minor contracts to flex the lower trunk relative to the upper trunk, this posterior tilting of the pelvis occurs at the lumbosacral joint.

Functions of the skeletal system include support, protection, mineral storage, and hematopoiesis. The structure of bones and their coordinated function with muscles, ligaments, and tendons allow for movement and support of the body. They also provide protection of vital organs, such as the rib cage surrounding the heart and pelvis protecting reproductive organs. Calcium and phosphate are stored in the hydroxyapatite matrix of the bone and hematopoiesis, the generation of red blood cells, takes place in red bone marrow, which houses hematopoietic stem cells.

Several systems coordinate thermoregulation, but ultimately this function is performed by the muscles. Muscles are responsible for most ATP usage in the body. Break down of ATP releases heat, which is why we shiver when we are cold. Regulation of blood flow and actions of the integumentary system also contribute to thermoregulation in other manners.