Gleason grade 3 is the most common grade by far and is also considered well differentiated (like grades 1 and 2 ). This is because all three grades have a normal “gland unit” like that of a normal prostate; that is, every cell is part of a circular row which forms the lining of a central space (the lumen).
The lumen contains prostatic secretion like normal prostate, and each gland unit is surrounded by prostate muscle which keeps the gland units apart. In contrast to grade 2, wandering of glands into (invading) the stroma (muscle) is very prominent and is the main defining feature. The cells are dark rather than pale and the glands often have more variable shapes.
Figure 3 shows invasion under low magnification; Figure 4 is part of the same area at higher magnification and shows the separate gland units and the dark cells.
Figure 3: Grade 3 carcinoma with individual glands arranged randomly (invading), seen at low magnification.
Figure 4: Grade 3 carcinoma (same as shown in Figure 3) showing the usual single layer of cells around each lumen and showing almost all glands separated by muscle (stroma), seen at higher magnification.
Illustrations courtesy of the late John E. McNeal, MD, Department of Urology, Stanford University School of Medicine.