Figure 2. Histology of the os penis and the distal ligament of the glans penis. (A) In rats, type I collagen with a reddish appearance is the major component of the os penis, which can be regarded as a woven bone (upper). The interlocking type III collagen with a greenish appearance is uncommon in the os penis proper but is substantial in its envelope (lower) (Picrosirius red stain, 400x). (B) In dogs, the appearance of a lacuna (arrow) is characteristic. Osteocytes (curved arrow) as well as chondrocytes (arrowhead) are observed (Picrosirius red, 100x). (C) The distal ligament in human beings is similar to the os penis of a dog. Type I collagen is also the major component, with type III collagen as an inter-locking ingredient. However, there are neither osteocytes nor chondrocytes (Picrosirius red, 400x).