Musculoskeletal System III - LECTURE NOTES

Musculoskeletal System III - LECTURE NOTES -...

Info iconThis preview shows pages 1–9. Sign up to view the full content.

View Full Document Right Arrow Icon
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 2
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 4
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 6
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 8
Background image of page 9
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Musculoskeletal System 111 Bone development and Joints A. Bone Development 1. Bone Formation Bones first appear as condensations of mesenchymal cells. These condensations define not only the position of the skeletal elements but also represent their basic shape. Mesenchymal condensations ultimately form bone by: Intramembranous bone formation; direct transformation of mesenchymal cells into osteoblasts. Endochondral bone formation; formation of bone via a cartilagenous model. a. Intramembranous bone formation (ossification). During intramembranous bone formation, osteoblast cells derived from differentiating mesenchyme lay down loose spicules or trabeculae that interconnect to form the primary trabecular bone. Compact bone is formed when holes between trabeculae become filled. The collagen fibers, cells and mineral crystals are randomly arranged and the tissue is described as woven bone. This bone will eventually undergo remodeling and be replaced with highly ordered lamellar bone. Bones formed by intramembranous ossification include: Bones of the cranial vault Some facial bones Parts of the mandible and clavicle. b. Endochondral bone formation (ossification). During endochondral ossification cartilage models are replaced by bone. 1. A small model of the bone is formed which is an elongated dumbbell shaped mass of cartilage consisting of a shaft (diaphysis) and future articular portions (epiphyses). This mass is enveloped by perichondrium which serves as a source of chondroblasts. 2. Chondroblasts secrete hyaline cartilage and mature into chondrocytes. . Blood vessels invade the perichondrium surrounding the future diaphysis and transform it into periosteum. 4. Osteoblastic cells within the periosteum differentiate, mature and secrete bone matrix which mineralizes and gives rise to a bony collar (cortical bone) surrounding the diaphysis. U.) 5. In the central diaphysis chondrocytes proliferate, mature, hypertrophy and ultimately calcify the cartilage matrix whilst undergoing apoptotic death. 6. Following calcification blood vessels originating in the perisosteal collar invade the central diaphysis. Vascular invasion permits migration of osteoblast precursors into the cartilage which mature and deposit bone on the calcified cartilage scaffold. This region of ossification is called the primary ossification center. 7. Primary growth plates are formed and serve as a continual source of cartilage for conversion to bone and growth in bone length. 8. Secondary centers of ossification develop later within the cartilagenous 'epiphyses by vascular invasion. 9. Eventually the growth plate ossifies (fuses) and growth in bone length ceases. 10. Cartilage is retained at the joint surface as articular cartilage. Shaft ' Hyaiine Osteopmgenitor cells cannage is me 01 the perictmnmium tum {emptale 01 a the porlollaal collar. ion; bone Bbod vessels, tormth me periomll bud. branch in 09:05:19 dIfECIQflS Endochondral ossification A cartilage model is formed by the diflerentiation of mesenchymal cells into chondrocytes which secrete a cartilage matrix. Subsequently a periosteum forms and generates a bony collar surrounding the diaphysis. Vascular invasion initiates the formation of the primary ossification center. Kierszenbaum, Histology and Cell Biology, 2002, Mosby Inc. Secondary ossmcamn center in one of the epiphyses. Epiphyseal growth Ttwwbhymlplatehasbeenreplaned by m epiphyml Sunnis process newts gmdmltyfrompubedytomlumwm longhonecanmlawmwinlengm Endochondral ossification. Vascular invasion of the epiphyses initiates the formation of the secondary ossification centers. Growth in length occurs at the growth plate. Growth in width is achieved by oppositional bone deposition. Eventually, the growth plate fuses and growth in length ceases. Kierszenbaum, Histology and Cell Biology, 2002, Mosby Inc. II. Growth Plate ’ The growth plate has 4 zones: Reserve Zone: chondrocytes are nearly spherical, arranged randomly and separated by large amounts of matrix. Proliferative Zone: Cells in this zone actively proliferate and become discoid and form columns of chondrocytes parallel to the long axis of the cartilage model. Proliferation is maximal here and this is the region responsible for growth in length of the newly developing bone. Zone of Maturation or Prehypertrophic Zone: Chondrocytes from the proliferation zone cease proliferating, enlarge and become prehypertrophic chondrocytes. Growth in this region is due to increases in cell size and not cell division. Chondrocytes continue to mature and increase in size. Hypertrophic Zone: In this zone, cells have increased in size so that their vertical height has increased 5 times. At this point they undergo programmed cell death (apoptosis) leaving behind longitudinal calcified septae and largely uncalcified transverse septae. Blood vessels penetrate the transverse septae and osteoclasts remove parts of the calcified cartilage. Osteoblasts deposit bone on the remnants of the longitudinal septae. This bone is disorganized woven bone. In time a second wave of osteoclasts resorb the woven bone and replace it with lamellar bone. This completes the process of endochondral ossification. { l c Epiphyu’ l , Reserve zone ‘ Growth pm. I‘ ! i Prolflerafive if g g zone E - e g ‘ Diaphysls g thypemphlc r zone 9% ; ' b g % ypemophlc r. a 7 Q m zone Growth plate F ' Cilafimtlon . _ ‘ 0‘ j dwtilaga Epiphysla l a . I; Imading capillary i“ !V [{l‘f‘lill: '..::"' 3!} gmchm Organization of the mammalian growth plate. Bilezikian, Bone Biology 2002 Academic press. III. Bone Growth Growth in Length Growth in length is dependent on the interstitial growth of cartilage in the growth plate. At the end of puberty, when the growth plates are replaced by bone, growth in length ceases. Growth plates close by ossification, connecting the epiphysis to the metaphysis with bone. Blood circulatory systems of the epiphysis and metaphysis formerly independent, also unite. This region of bone remodels over the ensuing years and the growth plate scar becomes undetectable. Growth in Diameter Growth in diameter is achieved by periosteal intramembranous ossification or appositional growth. The outer surface of the bone is invested by a layer of condensed fibrous tissue, the periosteum, which contains numerous osteoprogenitor cells. These cells differentiate into mature osteoblasts which can lay down bone by addition to the periosteal surface, thus effecting bone growth in diameter. During appositional growth, bone deposition occurs in layers around surface blood vessels, which then become incorporated into the matrix. These blood vessels and the bone that is laid down around them comprise the primary osteons. In a process referred to as bone modeling, osteoclasts simultaneously remove bone on the endosteal surface of the diaphysis which results in enlargement of the marrow cavity. Shape changes which involve the removal of bone at some sites and the addition of bone at others are also examples of bone modeling. IV. Bone remodeling In addition to modeling, throughout life the internal architecture of bone must be adjusted to changing loading conditions and bone damage must be repaired. This process involves the removal and replacement of bone at particular sites and is termed remodeling. The remodeling process is governed by hormonal and mechanical stimuli. Bone remodeling produces and maintains bone that is biomechanical and metabolically competent. Remodeling is achieved by osteoclasts and osteoblasts that work together in teams termed basic multicellular units or BMUs. A BMU generally consists of about 10 osteoclasts and several hundred osteoblasts. The life cycle of a BMU can be divided into 6 consecutive stages: 1. 9‘5" Mineralization l Osteo lasts I - A J\ o. m Osteoblasts Resting: typically 80-95% of the trabecular and cortical bone surfaces are resting or inactive with respect to remodeling at any given time. They are covered by a thin layer of bone lining cells. Activation: occurs when a chemical or mechanical signal initiates the recruitment of osteoclast precursor cells to the site. Bone lining cells retract and expose the bone surface. Resorption: osteoclast precursors fuse and form mature osteoclasts which proceed to erode the bone, forming cavities or resorption pits. Reversal: refers to the completion of resorption and the commencement of formation. ‘ Formation: osteoblastic cells are attracted to the region and deposit osteoid. Mineralization: The osteoid mineralizes and osteoblasts are either incorporated into the new bone as osteocytes or become lining cells. The bone surface then returns to its resting condition. Bone lining cell: i\ Resting Osteocla st precursors ® 636) fi Activation l /' Formation \ .. / Resorption Reversal Diagram of the six phases of bone remodeling. In cortical bone, remodeling results in the formation of osteons which give bone its characteristic microarchitecture. The difference between cortical and trabecular bone remodeling is that in cortical bone, the BMUs actually tunnel through the bone creating new osteons termed secondary osteons. Mature osteoclasts tunnel through the bone eroding the matrix as they go forming cylindrical tunnels called cutting cones. The osteoclasts are followed by osteoblast progenitor cells which differentiate into mature osteoblasts in the reversal zone and deposit osteoid, which then mineralizes. The cutting cone contains a capillary to supply nutrients, and most likely osteoprogenitor cells. Osteoblasts arrive in waves down the cutting cone. The first wave deposits the outermost lamellae and osteoblasts will be incorporated as future osteocytes. A second wave of osteoblasts arrives which lay down the next lamellae and so on until the entire tunnel is refilled with the exception of the very center (the haversion canal) through which the blood supply is retained. The resulting structure is known as a secondary osteon or haversion system. Oateodasts Q/ erodlnq a cavity E 5 day, 6 men (3 Iamotlae) Havetslan canat Haversian at m widesl canal mpm (5 lamellae) Diagram of a forming secondary osteon (haversion system) in longitudinal and cross sectional views. The system is extending towards the left. The times give, very roughly, the time course of the process in humans. At 5 days the osteoclasts are still widening the cavity of bone. At3 weeks the cavity is at its widest. By 6 weeks the cavity is half filled in by osteoblasts and by 10 weeks or so the process is competed although it will take a long time for the bone to become completely mineralized. In the cross sections, the central cavity is shown in black From Curry, 2002 Bones, Princeton University Press. B. Joints 1. Joint Classification 7 Joints can be defined by the material or structures that hold the bones together and the degree of movement allowed at the joint. Articulations are divided into three classes: 0 Fibrous or immovable (synarthroses) o Cartilaginous or slightly movable (amphiarthroses) o Synovial or freely movable (diarthroses) a. Fibrous Joints 0 Bones are united by dense collagenous connective tissue 0 The degree of movement permitted depends on the length of the collagen fibers, and on the shape and extent of the bone surface at the joint: most of the fibrous joints are immovable - a few are slightly movable. 0 There is NO JOINT CAVITY There are three subtypes of fibrous joints: Sutures Bones of the skull are held together by a thin layer of dense fibrous tissue and also by interlocking projections of the bones (serrations). The connecting fibers holding bones together are short and complex, which together with the serrations, inhibit movement. This type of joint occurs only in the skull; cranial sutures. During adulthood this joint commonly ossifies. Syndesmosis Bones are held together by a cord or sheet of dense fibrous connective tissue. The connecting fibers holding bones together are long. The tibia/fibula joint and the interosseous membrane connecting the radius and ulna along their length are examples of syndesmoses. Gomphosis The only examples of this type are the articulations of teeth with their alveolar sockets in the mandible or the maxilla. The thin fibrous membrane that holds teeth inside their alveolar sockets is called the periodontal ligament. b. Cartilagenous Joints 0 Bones are united by cartilage o Joints are slightly moveable 0 There is NO JOINT CAVITY There are two subtypes of cartilaginous joints i. Primaa cartilaginous [oints (synchondroses) A plate of hyaline cartilage connects the bones at the joint. Only hyaline cartilage is involved, and the joints are immoveable. An example is the cartilaginous epiphyseal plate which separates the epiphysis from the diaphysis in long bones during growth. A second example is the joint between the first rib and the sternum. ii. Secondary cartilaginous l'oints (symphyses) These joints involve both hyaline and fibrocartilage. The articular surface of each bone is covered with a thin layer of hyaline cartilage, and fibrocartilage unites these two layers. Limited movement is permitted and depends on the thickness of the fibrocartilage pad which can be compressed or stretched. Examples are the pubic symphysis and the intervertebral discs. c. Synovial Joints 0 Form the majority of articulations between bones 0 They are freely moveable joints. They are characterized by a JOINT CAVITY (synovial cavity). 0 Articulating surfaces are covered by a thin layer of hyaline (articular) cartilage o Surfaces are lubricated by synovial fluid which reduces friction. o The joint cavity is enclosed by a double layered membrane: the articular capsule. Components of the Synovial Joint i. Articular Cartilage Articular cartilage has unique chemical properties allowing it to serve as the load bearing material of diarthrodial joints. It has excellent friction, lubrication and wear characteristics. It helps to transfer loads from one bone to another, distribute joint loads more evenly across the underlying bony surfaces and allows the load bearing surfaces to articulate (roll and slide over one another) with very low friction. The role of cartilage is to provide a self renewing, well lubricated load bearing surface. Chondrocytes organize the collagens, proteoglycans, and other non-collagenous proteins into a unique, highly ordered structure which is dismissed in VMD 427 Lecture 10. ii. Meniscus The meniscus is a half moon shaped piece of fibrocartilage that lies between the weight bearing joint surfaces of the femur and the tibia. It is triangular in cross section and is attached to the lining of the knee joint along its periphery. Generally there are two menisci in a knee, the lateral and medial menisci. The menisci function to fill the space left by the lack of congruity between the femoral condyles and the tibial plateau. This aids the distribution of load over a wider surface area and provides some degree of stabilization. Complete removal of a meniscus can result in progressive arthritis in the joint as contact pressure in the cartilage is increased. The meniscus is mostly avascular and, as a result, atom meniscus has limited ability to self repair. iii. Capsule and Ligaments ‘ The articular capsule is a thin, strong, fibrous membrane which encloses the entire joint. It is made up of an outer fibrous layer and inner synovial membrane that is in contact with the synovial fluid. It is strengthened by fibers from the tendons and ligaments surrounding the joint which merge with the capsular tissue. Synovial joints are reinforced and stabilized by a number of ligaments. Ligaments are bands of dense regular connective tissue that interconnect bones. Ligaments may be found outside of, and blend with, parts of the fibrous articular capsule and are called extracapsular ligaments. Some ligaments lie within the joint cavity and are called intracapsular ligaments. Examples of extracapsular ligaments are the collateral ligaments and the cruciate ligaments are intracapsular. iv. Synovial membrane The synovial membrane lines the entire joint capsule. It is composed of a synovial cell layer and a loose, highly vascularized connective tissue layer which adheres closely to the inner surface of the joint capsule and supplies nutrients to the cellular layer. Synoviocytes are exocrine cells and produce many of the important proteins in the synovial fluid, including hyaluronan (hyaluronic acid, HA). v. Synovial Fluid Synovial fluid is an ultrafiltrate of blood plasma. The proximity of the capillaries in the synovial membrane to the joint cavity facilitates the exchange of solutes. The fluid also contains HA secreted by synoviocytes. HA is a gigantic glycosaminoglycan which gives synovial 'fluid its high viscosity. Synovial fluid functions to nourish articular cartilage and provide lubrication for effective joint movement. II. Muscle action at a Joint F lexion: makes the angle of the joint more acute (closes it). Extension: makes the angle of the joint more obtuse (opens it). Abduction: moves the limb away from the median plane. Adduction: moves the limb toward the median plane. Supination: rotate palmar/plantar surface upwards. Pronation: rotate palmar/plantar surface downwards. ...
View Full Document

This note was uploaded on 04/05/2011 for the course APC 100 taught by Professor Kelliewhited during the Spring '07 term at UC Davis.

Page1 / 9

Musculoskeletal System III - LECTURE NOTES -...

This preview shows document pages 1 - 9. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online