Bone development

Question 1

bone classification

Answer 1

• Long bones, as their name suggests, are considerably longer than they are wide. A long bone has a shaft plus two ends which are often expanded.
• Short bones are roughly cube shaped. The bones of the wrist.
• Flat bones are thin, flattened, and usually a bit curved. The sternum (breastbone), scapulae (shoulder blades), ribs, and most skull bones are flat bones
• Irregular bones have complicated shapes that fit none of the preceding classes. Examples include the vertebrae and the hip bones

Question 2

structure of a long bone

Answer 2

• diaphysis/shaft: thick collar of compact bone that surrounds a medullary cavity that contains yellow/fat marrow
• epiphysis: the bone ends, that have compact bone shells covered with hyaline cartilage and spongy bone interior that houses red bone marrow
• membranes: periosteum= external cover of dense irregular connective tissue right underneath this layer are the osteogenic cells
  endosteum= covers the trabeculae of spongy bone and compact bone canals (also has osteogenic cells)

Question 3

bone functions

Answer 3

1. Support: bones provide a framework that supports the body and cradles its soft organs.
2. Protection: the fused bones of the skull protect the brain. The vertebrae surround the spinal cord, and the rib cage helps protect the vital organs of the thorax.
3. Movement: skeletal muscles, which attach to bones by tendons, use bones as levers to move the body and its parts.
4. Mineral and growth factor storage: bone is a reservoir for minerals, most importantly calcium and phosphate. Additionally, mineralized bone matrix stores important growth factors.
5. Blood cell formation: most blood cell formation (haematopoiesis) occurs in the red marrow cavities of certain bones.
6. Triglyceride (fat) storage: fat is stored in bone cavities
7. Hormone production: bones produce osteocalcin, a hormone which not only helps regulate bone formation, but also protects against obesity, glucose intolerance, and diabetes mellitus

Question 4

cartilage types

Answer 4

1. Hyaline: most abundant, spherical chondrocytes, only have fine collagen fibers and examples are articular cartilages (cover bone ends in joints), costal cartilages (connects ribs to sternum), respiratory cartilages and nasal cartilages
2. Elastic: contain stretchy fibers and are found in the external ear and epiglottis
3. Fibrocartilage: highly compressible with great tensile strength, thick collagen fibers, found in the menisci of the knees and discs between vertebrae

Question 5

bone cells

Answer 5

1. osteogenic cells –> stem cells (osteoblast and bone lining) in periosteum and endosteum
2. osteoblasts –> bone matrix secreting cells
3. osteoclasts –> giant multinucleated bone matrix breaking cells (derived from haemotopoeitic stem cells
4. osteocytes –> old osteoblasts that function as mechanosensors
5. bone lining cells –> found on bone surface to maintain bone matrix

Question 6

bone development in embryos

Answer 6

• Before week 8, the skeleton of a human embryo is constructed entirely from fibrous membranes and hyaline cartilage. Bone tissue begins to develop at about this time and eventually replaces most of the existing fibrous or cartilage structures.
• Except for the clavicles, essentially all bones below the base of the skull form by endochondral ossification:
  • Beginning late in the second month of development, this process uses hyaline cartilage “bones” formed earlier as models, or patterns, for bone construction.
  • The hyaline cartilage must be broken down as ossification proceeds

Question 7

endochondral ossification

Answer 7

1. mesenchymal cells condensate and become chondrocytes
2. the cente of this chondorcyte mass become hypertrophic (terminal differntiation of chondrocytes)
3. blood vessels infiltrate the perichondrium that covers the hyaline cartilage
4. mesenchymal cells now differentite into osteoblasts
5. a bone collar forms around the hyaline cartilage
6. cartilage in the centre of this collar calcifies (impermeable for nutrients) causing cavities to form
7. the periosteal bud invades these cavities and forms spongy bone (primary ossification centre)
8. as this ossification centre grows the centre spongy bone is broken down to form the medullary cavity
9. secondary ossification occurs in the epiphyses
10. the epiphyseal plate forms

Question 8

postnatal bone development

Answer 8

1. The cartilage is relatively inactive on the side of the epiphyseal plate facing the epiphysis, a region called the resting zone. –> Sox9 (collagen type 2)
2. The epiphyseal plate cartilage bordering on the diaphysis organizes into a pattern that allows fast, efficient growth. –> proliferation zone
   • Cells here divide quickly, pushing the epiphysis away from the diaphysis and lengthening the entire long bone.
3. The older chondrocytes in the stack, which are closer to the diaphysis (hypertrophic zone), hypertrophy, and their lacunae erode and enlarge, leaving large interconnecting spaces. –> Runex2
4. The surrounding cartilage matrix calcifies and these chondrocytes die and deteriorate, producing the calcification zone
5. The calcified cartilage matrix now becomes part of the ossification zone –> osteoclasts partly erode the cartilage spicules, then osteoblasts quickly cover them with new bone, and ultimately spongy bone replaces them

Question 9

fracture healing

Answer 9

1. A hematoma forms
   • When a bone breaks, blood vessels in the bone and periosteum, and perhaps in surrounding tissues, are torn and haemorrhage –> a haematoma forms
   • Soon, bone cells deprived of nutrition die
2. Fibrocartilaginous callus forms
   • Capillaries grow into the hematoma and phagocytic cells invade the area and begin cleaning up the debris.
   • Meanwhile, fibroblasts (collagen) and cartilage and osteogenic cells (from the cambium layer of the periosteum) invade the fracture site and begin reconstructing the bone.
3. Some precursor cells differentiate into chondroblasts that secrete cartilage matrix. –> Within this mass of repair tissue, osteoblasts begin forming spongy bone.
4. Bony callus forms
   • Within a week, new bone trabeculae appear in the fibrocartilaginous callus and gradually convert it to a bony (hard) callus of spongy bone. –> endochondral ossification
5. Bone remodelling occurs
   • Excess material on the diaphysis exterior and within the medullary cavity is removed, and compact bone is laid down to reconstruct the shaft walls.