Apoptosis is the process of planned cell death, or cell removal. It may occur if a cell is damaged beyond reclaim, or simply if it has outlived its usefulness.

Some cells with dense, darkly stained chromatin in tissues with high rates of cell turnover are undergoing programmed cell death (apoptosis) rather than mitosis.

• The two processes can be difficult to distinguish in routinely stained slides.
• As shown in the adjacent figure, cells undergoing apoptosis have their chromatin clumped into a uniform dense mass (pyknotic nuclei) with no indication of chromosomes as in mitosis.
• Apoptosis and mitosis generally occur in different regions of tissues.
• Examine the small intestine (slide 4), looking for pyknotic cells near the tips of the projecting villi.
• The sequence of events leading to apoptosis is also initiated when a cell’s nuclear DNA is damaged beyond repair.
  • Slide 156 is skin from a mouse treated with ultraviolet radiation and shows apoptotic cells in the epidermis.

Pathology includes the study of cell death both by apoptosis and by necrosis.

• What is the major difference between these two processes with regard to the roles of other cells involved?

Let's now take a look at specific tissue types and their cellular constituents.