1. Objectives: At the end of this pair of practical classes you should be able to : 1. Recognise and distinguish between hyaline, fibro- and elastic cartilage and : a) identify stages of chondrocyte maturation and b) decide whether a perichondrium is present on the surface of the cartilage. 2. Appreciate where different types of cartilage are found and how structure relates to their function. 3. Distinguish between cartilage and bone in histological sections. 4. Identify various cell types of bone and understand differences in cancellous and cortical bone structure. 5. Recognise lamellar bone structure and understand its arrangement. 6. Appreciate the differences between the development of cartilage and bone as well as the differences between intramembranous and endochondral bone formation. 7. Understand long bone structure and the concept of bone remodelling in adult and growing bones. This should include an appreciation of how ‘whole’ bone geometry is maintained during growth. 008 Bone & Cartilage – 1/2 Histology of Bone & Cartilage -2 INTRAMEMBRANOUS OSSIFICATION SLIDE 129 Developing skull (20 day rabbit foetus) Look at the slide by eye and identify the head region. Under low magnification find the roof of the skull and find the site of bone development, outside the meninges covering the brain. Examine (at low magnification) the full length of this bone structure and sketch the site (at one end of this forming bone) where committed osteogenic and fibrogenic cells are closely associated. This a region undergoing intramembranous bone formation. Increase magnification and draw (no detail) : bone matrix, active osteoblasts, preosteoblasts (committed progenitors) and undifferentiated fibroblasts. Indicate the presence of blood vessels. Q 1. Why is this region devoid of lining cells and osteoclasts? Q 2. How is this bone subsequently strengthened? 1 Returning to low magnification, identify a more developed adjacent region of this skull bone, where bony trabeculae are taking shape. Increase magnification and make a detailed drawing illustrating all stages of the intramembranous ossification process fibroblasts, preosteoblasts, osteoblasts, osteocytes, matrix and blood vessels of this region.

2. SLIDE 10 Developing mandible (17 day rat foetus) This is a transverse section through a developing head. At low magnification examine and identify the developing oral cavity (which will have the forming tongue on its ventral floor). Scan the area ventral to the oral cavity and identify the developing mandible (do not confuse with the round structure of Meckel’s cartilage). Osteogenesis is more advanced than in the skull bone (see above). Look carefully at the bone close to the tooth primordia and identify osteoclasts. At high magnification, draw this region of mandible, highlighting osteoclasts. Q 3. What is the function of these cells? eye mandible brain oral cavity Meckel’s cartilage

3. 2 ENDOCHONDRAL OSSIFICATION SLIDE 127 (Early changes in limb’s cartilage model LS) Inspect the slide at low magnification to establish the position of the individual developing cartilage models (elements or bones) of the limbs. In addition, confirm your view by successfully recognising the developing articular surfaces and joint spaces. These individual bones develop in a proximal-distal sequence (most mature is most proximal). Using this information, scan each model and select one of the more mature to examine at higher magnification. Examine the various stages of chondrocyte maturation and draw a representative example of the matrix and cells in each zone. Articular surface RESTING PROLIFERATING MATURING HYPERTROPHIC CALCIFYING DYING Examine the mid-diaphyseal region and draw calcifying and dying chondrocytes and the collar of bone formation surrounding the cartilage.

4. SLIDE 136 Growing rat femur (ossification of epiphysis, longitudinal section) Make a naked eye inspection of this slide and establish in which region the femur’s epiphysial growth plate is to be found. Under low magnification: i) examine this region carefully. ii) increase the magnification (x10 objective). and draw to illustrate the following sequence of events. (if necessary use x40 objective) Proliferation of cartilage cells. Hypertrophy of chondrocytes. Calcification of cartilage matrix (purple). Deposition of bone (pink) on cartilage ‘template’ (blood vessels, osteoblasts and osteocytes). Organisation of bony trabeculae and their osteoclast resorption. Q 4. How much bone derived from the growth plate do you think is incorporated into the bone’s cortex?

5. GROWTH IN GIRTH SLIDE 140 Growing rabbit bone (TS through diaphysis) In this slide examine the bone’s periosteal surface. Note its’ irregularity and consider how the contour of this bone forming surface influences the character of the subsequent bone cortex. Examine the section and consider where sites of osteoclastic bone resorption might be appropriate (try to identify osteoclasts at these sites). Note the inclusion of osteoblasts into the forming osteoid layer and consider the process of consolidation by which this bone may be strengthened. 4 3 GROWTH IN LENGTH SLIDE 135 Growing rat femur (longitudinal section) This is similar to slide 136 (above) Make a sketch of the growth plate and the bony trabeculae projecting into the bone’s metaphysis. Show the remnants of cartilage.