Physiology and pathology of the musculoskeletal system

Question 1

Physiology

Types of bone

Answer 1

• The skeleton contains two types of bone.
• These are cortical (or compact) bone, which makes up most of shafts (diaphysis) of the long bones, such as the femur and tibia, and trabecular (also known as medullary) bone, which makes up most of the vertebral bodies and the ends of the long bones

Question 2

Physiology

Bone composition

Answer 2

• organic component of bone matrix comprises mainly type I collagen, which is a fibrillar protein formed from three protein chains, wound together in a triple helix
• Type I collagen is laid down by bone-forming cells (osteoblasts) in organized parallel sheets (lamellae) and, subsequently, the collagen chains become cross-linked by specialized covalent bonds, which help to give bone its tensile strength
• Mineralization confers upon bone the property of mechanical rigidity, which complements the tensile strength and elasticity derived from bone collagen.

Question 3

Bone formation, modelling and remodelling

Answer 3

• Bone is formed through the process of osteogenesis early during embryonic life and is modelled thereafter as the skeleton grows until early adulthood, when peak bone mass is attained.
• Thereafter, bone constantly undergoes remodelling.
• The function of remodelling is to repair and renew damaged bone, but in later life the net amount of bone removed during remodelling exceeds that which is replaced, resulting in bone loss. In old age, subtle changes in bone shape occur with remodelling, particularly in the long bones, leading to a slight increase in cross-sectional area (cortical expansion) with cortical thinning.

Endochondral bone growth and modelling
Skeletal growth occurs as the result of endochondral bone formation with subsequent modelling of the newly formed bone during childhood and adolescence. This results in longitudinal bone growth, changes in bone shape and an increase in cross-sectional area. Endochondral bone growth continues until fusion of the epiphysis occurs – mediated by an increase in circulating levels of sex hormones – at the end of puberty. Longitudinal bone growth occurs primarily as the result of proliferation of chondrocytes in the growth plate. These cells then undergo hypertrophy and the surrounding cartilage matrix becomes calcified. The calcified cartilage is then removed by osteoclast activity and replaced by new bone as a result of osteoblast activity, as in the developing bone.

Question 4

Cartilage and its functions

Answer 4

• Elastic cartilage plays an important role in nasal septa, the ears, larynx and trachea, where it provides support and shape
• Fibrocartilage is found in intervertebral discs and menisci
• Hyaline cartilage covers articular surfaces, which provide a low friction surface allowing bones to slide over one another. It also acts as a shock absorber.
• Growth plate cartilage is found at the epiphyses of long bones

Question 5

Synovial joints

Answer 5

Joints are the sites at which two or more bones are united, regardless of whether there is movement between them. Synovial (diarthrodial) joints are especially important because they account for most of the body’s articulations and are characterized by a wide range of movement. Different joints vary in their degree of movement and are classified as multi-axial, bi-axial and mono-axial, depending upon how many planes of movement they support. Examples of multi-axial joints, which support a wide range of movement, include the shoulder and hip; other joints, such as the wrist, ankle, knee and elbow, have more limited planes of movement. The stability and range of movement is achieved by ligaments and muscles.

Question 6

Inflammation and healing

Answer 6

• The hallmark signs of inflammation are tumour (swelling), rubor (redness), calor (heat), dolor (pain) and functio laesa (loss of function).
• In response to pathogen invasion, activated cells of the immune system release biologically active proteins, including plasma enzyme mediators such as bradykinin and fibrinopeptides.
• Complement products act as opsonins (to bind foreign proteins) and chemotactic agents (to attract foreign proteins). Chemokines and cytokines are released, with upregulation of adhesion molecules on activated cells.
• Together, these chemical signals cause dilation and increased permeability of blood vessels, which allows fluid and a large number of blood cells to enter the tissues.
• During an acute inflammatory response, neutrophil migration usually begins within 1 hour of injury and is subsequently followed by an influx of monocytes and lymphocytes

Question 7

Immune system

Answer 7

• A fundamental principle of the immune response is the differentiation of self from non-self.
• When this capacity fails, autoimmune diseases, such as RA and systemic lupus erythematosus (SLE), may develop
• The host defence system consists of two cooperative components: the innate immune system and the adaptive immune system.
• Their main distinction lies in the mechanisms and receptors used for immune recognition. They also differ in speed, defence specificity and involvement of effector cells.