Ossification and joints Flashcards
Learning outcomes
- Discuss the process of ossification as “the replacement of connective tissue by bone”
- Describe the microanatomical differences between intramembranous and endochondral ossification
- Explain the processes of bone remodelling and growth, noting the role of bone cells
- Define a joint
- List and describe the different types of joint based on histological architecture
Osteoblasts- formation of bone tissue
- Osteoblasts produce osteoid, organic component of bone (Type 1 collagen + GAG gel containing glycoproteins which binds Ca2+)
- Osteoblasts release matrix vesicles containing enzymes e.g. alkaline phosphatase which increases local concentrations of Ca2+
- Matrix vesicles act as the focus for mineralisation
Osteomalacia- failure of mineralisation
- Lack of Ca2+ in the diet (e.g. vegans ) or malabsorption from small intestinal disease results in poor mineralisation of bones (vit D absorption of calcium)
- Bone softer with increased tendency to fracture
- Osteomalacia in children produces rickets
Osteoporosis (porous bone)
- Decrease in amount of bone when bone resorption exceed renewal, or renewal slows down
- Older white females most frequently affected (18 – 28%)- lack of oestrogen, increased osteoclast activity and bone absorption increased and osteoblast bone deposition reduced
- Half of all postmenopausal women over the age of 40 will have an osteoporosis related fracture (oestrogen inhibits bone resorption)
Ossification
- In foetus bone formation is by replacement of pre-existing CT by bone
2 types of ossification-
Endochondral ossification- hyaline cartilage, mesodermal germ layer
Intramembranous ossification- primitive mesenchymal ( 1 of 3 germ layers, outer ectoderm, middle mesoderm and inner endoderm) tissue
Osteoblasts, osteocytes and osteoclasts are involved in both types of ossification
Intramembraneous ossification
-Forms the flat bones such as those of the cranium
-Ossification occurs within a membrane of primitive mesenchymal cells
-The process involves 4 main stages:
Stage 1- primitive CT becomes highly vascularised- osteoprogenitor cells differentiate to form osteoblasts
- A cluster of osteoblasts form an ossification centre and lay down osteoid (unmineralised bone, no calcium from blood or phosphate)
Stage 2- Mineralisation of osteoid occurs within a few days to produce an island of bone
-Trapped osteoblasts become osteocyte
Stage 3- Islands of bone join to form trabeculae of woven bone with blood vessels inbetween
- Mesenchyme cells condense at edges to form a bony collar
Stage 4- increased bone formation on outer and inner surfaces to produce plates of bone (compact)
- Spongy bone persists internally and rerd marrow develops ( from mesenchymal cells) in the trabecular spaces
Endochondral ossification
Hyaline cartilage replaced by bone- takes place in appendicular cartilage
- Cartilage template is deposited in the shape of the required bone within a mass of mesenchymal tissue
- The “bone” template will have a shaft (diaphysis) and club-shaped ends (epiphysis)
- Begins in the second month of foetal development
- Forms the long bones of the body e.g. femur, humerus
Stage 1- Cartilage matrix cells in the central diaphysis die to leave cavities
-Periosteum produces osteoblasts which lay down a bony collar by Intramembranous ossification
Stage 2- Cartilage matrix in centre continues to deteriorate
- Diaphysis diameter increases by bony deposition on the outside and resorption on the inside
- Chondrocytes in the rest of the bone multiply increasing the length and breadth
Stage 3- Blood vessels penetrate the diaphysis carrying osteoprogenitor cells from the periosteum with them
- Osteoprogenitor cells transform to osteoblasts and set up a primary ossification centre
- Bone is laid down in replacement of lost cartilage in centre
Stage 4- At birth (approx.) capillaries penetrate the epiphyses
- Osteoprogenitor cells dragged in, differentiate to osteoblasts and produce osteoid where bone is laid down in place of cartilage
- Formation of medullary cavity
Stage 5-
When ossification of the epiphyses is complete, cartilage remains only in the:
-Epiphyseal plate and the Articular cartilage
Hypoxic gradient set up after loss of chondrocytes- blood vessels drawn (nutrient artery into cartilagenous core of hyaline core, osteoblasts and osteoclasts enter diaphysis)
in epiphysis, secondary ossification centre set up due to arterial introduction- osteoprogenitors differentiate into o.cytes and clasts move in, remodelling)
Epiphyseal plate
In long bone, length is increased through processes in EP- cartilage is replaced by bone
Chondrocytes form cartilage continuously while bone is constantly layed down by osteoblasts (lengthens bone)
Resting zone
Proliferative zone- chondrocytes mitose, inc. number of chondrocytes
Hypertrophic zone- chondrocytes apoptose and secrete alkaline phosphotase
Calcification zone- A.phosphotase allows for calcification of cartilage (breaks down matrix)
Ossification zone- osteoprogenitor cells invade this area (brought from epiphyseal artery) and differentiate into osteoblasts- lay down bone beside cartilage
Calcified cartilage and newly laid down bone = MIXED SPICULE: osteoclast resorbs bone, attracts osteoblasts which lay down new osteoid
Osteoid becomes mineralised, new bone, diaphysis inc. length and therefore length of bone.
- Fibrous joints
- Bones concerned are bound by tough, fibrous tissue- strength joints.
Can be divided into sutures, gomphoses and syndemoses
SUTURES
-Immovable joints, only found between the flat, plate like bones of the skull- limited movement until about 20 years of age (become fixed)
SYNDEMOSES
slightly movable joints, comprised of bones held by interosseous membrane- key in providing strength to long bones e.g middle radio-ulnar and middle tibiofibular joints
GOMPHOSES
also immovable joints, found where teeth articulate with sockets in mandible (lower jaw) and maxilla (upper jaw). Fibrous connection that binds the tooth and socket is periodontal ligament
- Cartilagenous joints
Bones attached by fibrocartilage or hyaline cartilage
SYNCHONDROSES
- Primary cartilaginous joints: only involve hyaline cartilage- can be IMMOVABLE or SLIGHTLY MOVABLE
e.g Epiphyseal cartilage and costochondral joints
SYMPHYSES
-Secondary cartilaginous joint: can involve hyaline or fibrocartilage- slightly movable, e.g intervertebral disc or pubic symphysis
- Synovial joints
Most common and mobile joint in the body- ends of 2 bones coated in hyaline articular cartilage, separated by synovial cavity and bound together by fibrous capsule lined on inside by synovial membrane. The synovial cavity/joint is filled with synovial fluid.
Many, but not all, Syn.js contain: - - - - - -
