Endochondral Ossification: Step-By-Step Microscopic Changes

Endochondral Ossification: Step-By-Step Microscopic Changes

Question 1. Microscopic picture of endochondrial ossification
Answer:

• Following changes occurs during endochondrial ossification
• Mesenchymal cells are closely packed to form Mesenchymal condensation
• Some of them differentiate into chondrocytes & lay down hyaline cartilage
• Cells of this cartilage are
  1. Initially-small & irregular
  2. Later-Enlarges
     • Next, due to lack of nutrition, cells die forming empty spaces called primary areolae
     • This is later destroyed by the blood vessels of perichondrium & results in secondary areolae
     • Osteoprogenitor cell invade it & becomes osteoblasts
     • Osteoblasts lay down a layer of osteoid
     • Osteoid is calcified & forms lamellus of bone
     • Similarly, osteoblasts lay down another layer of lamellus which gets calcified
     • Some osteoblasts are entrapped between the two lamellae & becomes osteocytes Osteocyte

Question 2. Histology of skeletal muscle
Answer:

• The muscle present in relation to bony skeleton is called skeletal muscle
• Histology of skeletal muscle It is made up of:

1. 1. Muscle fibres
      • They are long and cylindrical
      • They are arranged in bundles called fasciculi
      • Each muscle fibre is covered by a plasma membrane called sarcolemma
   2. Connective tissue
      • It supports and unites muscle fibres

Histology of skeletal muscle Striations:

• Skeletal muscle section on staining shows alternate dark and light band

1. 1. A band dark band
   2. I band-light band
   3. Z band-dark line between I band
   4. H band-light line between A band
   5. M band dark line in center of H band

• Part present between two consecutive Z bands is called sarcomere.