Slide 19
This section is taken from the foot of the 19-month-old white male patient which you have seen or are about to see in a gross session. Under low power, we see aggregates of atypical cells dispersed in a fibrous stroma.

Slide 20
Under higher power, it can be seen that the cells are round or oval, of moderate size and seem to be unattached to each other. They appear to be contained within small irregularly-shaped spaces, which to the imaginative student, are reminiscent of alveoli. Hence the descriptive name.

Examination of the area with excessive numbers of cells reveals that their nuclei vary in size and shape and in their chromatin patterns. Many of these cells have definite brown or tan staining of their cytoplasm. The normal striated muscle cells, the smooth muscle cells present in the walls of blood vessels and these atypical tumor cells all contain cytoplasmic actin by an immunohistochemical stain. The section has been incubated first with a mouse monoclonal antibody to actin, then with a second antibody made in a rabbit against mouse IgG and labelled with peroxidase enzyme. Upon the addition of peroxide and a chromogen, the brown color indirectly reveals the presence of actin, thereby identifying the origin of the tumor to be muscle.