Musculoskeletal System AND Muscle Lab Flashcards
Structure of a Long Bone
Epiphysis: Makes up the round ‘knob’ at the end of bones. Covered with articular cartilage. Consists of an outer layer of compact bone and an inner layer of trabeculated spongy bone.
Metaphysis: Interface between the epiphysis and the diaphysis of the long bone. Gives rise to the epiphyseal plate- a region of cartilage where growth occurs by interstitial growth before it is calcified.
Diaphysis: The shaft of the lone bone, with a thick layer of compact bone along the wall, enclosing a thin layer of spongy bone, with a hollow medullary space in the middle.
Periosteum and Endosteum
Consists of a protective fibrous outer layer and a cellular inner layer made of osteogenic cells, which arise from embryonic mesenchyme. The endosteum and periosteum is usually attached to the bone via Sharpey’s fibres, which blend into the fibres of the two types of tissue it connects, making it a very strong connection.
Periosteum and endosteum is the same type of tissue, except the endosteum is found INSIDE the medullary cavity.
The periosteum is vascularised and innervated, and move through into the bone via the nutrient foramen.
Woven Bone Structure
Found only in neonatal skeletons, or regions of the skeleton undergoing rapid bone reconstruction. This is when collagen is rapidly and randomly laid down during bone construction, forming a callous.
The collagen isn’t in any specific arrangement, hence it is weak.
Lamellar/Mature bone structure
Consists of organic collagen fibres, in an inorganic ground substance of hydroxyapatite.
Fibres resist tension while hydroxyapatite resists compression. They combine to resist torsion, which is a twisting force.
Formed in lamellae/sheets, where the fibres are arranged to be normal to each other to allow resistance to tension in more direction.
Spongy Bone
Consists trabeculae made of concentric lamellae, with blood vessels moving in between to vascularise the osteocytes via diffusion.
Very large SA, so osteoclasts can easily attach and break down the bone for Ca2+ or more space to attach more Ca2+.
Only covered on the outer layer of the trabeulae by endosteum, so it only grows by appositional growth from the outside up to 0.4mm max at it’s most narrow point, at which point it splits in half due to osteocytes near the centre dying off.
Trabeculae allow force to be spread out by arranging along lines of stress.
Always protected by a layer of compact bone to receive the force.
Very light as it contains a lot of air, and makes up more irregularly shaped ones.
Compact Bone
Made of many osteons consisting of concentric lamellae, as well as intersitial; lamellae. Osteons have a Haversian canal in the middle where blood vessels and vasculature goes.
Osteocytes are found in spaces between the lamellae, and they are joined to each other by small channels called canaliculi. Blood vessels move out from the Haversian canal through Volkman’s channels.
The outer edge of compact bone is made of concentric layers of lamellae surrounding the entire compact bone.
Usually found as the ‘walls’ of the diaphysis of long bones.
Appositional Growth of Osseous Tissue
1) Cellular layer of periosteum activates after osteogenic cells become osteoblasts. This occurs through mitotic divisions, resulting in the formation of many layers of cuboidal cells.
2) Osteoblasts lay down osteoid, which is a precursor to bone consisting of proteoglycans, proteins, water, and collagen. Osteoblasts maintain contact to each other through cytoplasmic projections. So when some osteoblasts become trapped in the osteoid, they are still connected to the active periosteum.
3) Osteoid is slowly dehydrated and calcified as hydroxyapatite replaces water, which slows down the rate of growth as materials cannot easily diffuse into the bone. More osteoid is laid down above the sunken osteoblast, which is converted to osteocyte.
4) After growth completes, the osteoblasts either die by apoptosis, or return to their dormant osteogenic form.
Bone resorption
Monocyte progenitors are attracted to region to be absorbed by cell signalling.
They fuse on the surface of the bone, after being deposited on the surface from veins, to form a syncytium called osteoclasts, which contain vesicles of acid and protease.
Acid–> Hydroxyapatite.
Protease–>Collagen
These are released into Howship’s Lacunae- a region of bone to be dissolved, sealed off at the edges by clear zones, which are projections of the osteoclast.
The region of osteoclast in Howship’s lacunae is ruffled- increases surface area.
Dissolved materials are endocytosed by the osteoclast, then exocytosed out the other end.
Once the signal for resorption is removed, osteoclasts undergo apoptosis, and a new layer of periosteum/endosteum is laid down. Angiogenesis occurs in the cleared out space.
Relationship between bone deposition and resorption and how it can be influenced
Bone deposition is faster in youth and slower in elderly.
In adults osteoblast activity= osteoclast activity- bone remodelling to counteract dominant stress acting on bone.
Weight bearing exercise causes more rapid deposition of bone.
Oestrogen can reduce osteoclast activity- postmenopausal women more likely to experience osteoporosis due to overactive osteoclasts.
How do Long Bones grow?
Endochondral ossification.
Epiphyseal plate is a layer of cartilage found between epiphysis and metaphysis- called a synchrondrosis. Since it is cartilage, it can undergo interstitial growth, causing the epiphysis to be pushed away from the metaphysis. As the cartilage on the metaphyseal side dies, it is calcified and becomes a new surface of calcification. By the end of development, the rate of cartilage growth slows and the ossification front ‘catches up’ to completely calcify it into the epiphyseal plate.
Primary Osteon formation
1) As bone grows via appositional growth, concentric lamellae will be laid down. However, when it encounters a blood vessel, the region of bone beneath it will stop growing, while the regions around it will grow at the normal rate.
2) The periosteal ridges will fuse over the vessel to form a Haversian canal. The same will occur for any connective vessels, which will become Volkmann’s channels.
3) Periosteum is converted to endosteum, which becomes active and grows appositionally to shrink the Haversian canal.
4) This will repeat as long as there are blood vessels interfering with appositional growth.
Secondary Osteon Formation
Created inside existing bone which has grown too large- too much distance between Haversian canals.
1) Monocyte progenitors form a syncytium at a region where osteocytes are dying, due to malnutrition, or bone is damaged due to stress. The formed osteoclast will cut through existing bone in a cutting cone.
2) Osteoblasts move onto the walls of the cutting cone to form an active endosteum. This interface between old and new bone is known as the cement line. Bone forms via appositional growth at the endosteum.
3) Vasculature enter the cutting cone, which release more monocyte progenitors to replace osteoclasts lost to apoptosis.
4) The closing cone will continue to grow until a Haversian canal is formed. The osteoblasts are then converted back to inactive endosteum.
Functional Classification of Joints
Synarthrosis: Immovable joints in the axial skeleton. Responsible for the transmission of force or growth. Can undergo ankylosis.
Amphiarthrosis: Somewhat moveable- useful in the vertebral column. Can also transmit force.
Diathrosis: Very flexible joint that only restricts movement at the extremes of joint motion. Allows for wide range of appendicular movement but easily damaged.
Anatomy of a Simple Synovial Joint
Bone within the joint is covered by articular cartilage, which, when combined with synovial fluid, has a very low coefficient of friction.
Outer membrane of the articular capsule is the fibrous layer, which is collagenous and made of irregular and regular CT. It is fused to bone via Sharpey’s fibres. Highly innervated but poorly vascularised. Contains proprioceptors to detect position and nociceptors to detect damage. Excess regular CT deposition will form a capsular ligament which is highly resistant to tension.
Inner membrane of the articular capsule is the synovial membrane. Intima is an irregular layer of synoviocytes which secrete hyaluronic acid. Subtima contains areolar connective tissue, significant vasculature, adipocytes for force attenuation and fibroblasts.
Synovial fluid is made of hyaluronic acid and ultrafiltrate from subtintima arteries, and is responsible for absorbing force or providing a medium for exchange at the cartilage.
Joint cavity: Very small. Reduces distance for diffusion.
Fibrous cartilage: Sometimes, plates of fibrous cartilage called menisci can be found between the articular cartilage and attached to the fibrous capsule. Can provide shock absorption.
Components of articular cartilage
5%: Chondrocytes living inside lacunae.
Water: 75% by wet weight. Can move in and out of cartilage and responsible for transport.
GAGs and PGs: Solid component that provides the hydrating and swelling mechanisms of the cartilage.
Collagen: 75% dry weight. Maintains structural integrity. Orientation varies depending on the region of cartilage.
