Atrophy, Hypertrophy, Hyperplasia, Metaplasia, and Dysplasia, are all cellular adaptations to the demands of function or the effects from environmental stimuli or damaging disease. All cells die--they are pre-programmed genetically to do this (what is called "apoptosis"). Before this final "adaptation," other adaptations are part of the cell's story. For illustrative purposes, one organ system, the female genitourinary system, will be used to demonstrate each of these.

• ATROPHY

"Reverse growth": atrophy refers to a decrease in the size of a body part, cell, organ, or other tissue. A decrease in cell size causes shrinkage in organ size that can be part of a normal healthy function based on aging or as a result of reduced workload, use, metabolic activity, blood supply, nutrition, disease, etc.

An example of atrophy is when the muscles of the uterus--which enlarge during pregnancy, atrophy after delivery.

• HYPERTROPHY

"Excessive growth" in size: cells and tissues can increase in size causing an increase in the size of their respective organs. This is often based on increased demand. In hypertrophy, the increased cellular size is caused by an increased accumulation of protein in the cellular components.

Again, in the pregnant uterus, the size of each muscle cell increases, which also increases the size of the entire uterus--up to 15 times the non-pregnant size.

• HYPERPLASIA

"Excessive growth" in numbers of cells, hyperplasia is characterized by an increase in the number of cells due to a higher rate of cell division and is a response to severe and prolonged injury, as well as under the direction of hormones or as a compensatory mechanism for dysfunctional cells in the same tissue.

A pregnant uterus, as above, increases in size (hypertrophy) due to cell enlargement (and thereby) causes organ enlargement. But the sheer numbers of cells increase, too, which is hyperplasia (distinct from the hypertrophy).

NOTE: Hypertrophy and hyperplasia are often confused:

• Hypertrophy = increase in size of an organ/tissue due to cellular enlargement. 
• Hyperplasia = increase in the size of an organ/tissue due to the increase in numbers of cells. 

• METAPLASIA

Metaplasia is an adaptation in which one cell type can convert to another cell type, based on a changing environment (stimulus).

In the uterus (pregnant or not), the cervix is the lowest part that is exposed to the vaginal environment (vaginal birth involves passage of the baby through the cervix as its exit from the uterus). Vaginal pH can cause the cells to convert from glandular (more "uterus-like") to squamous (external, or more "vaginal"). Metaplasia is considered a reversible process.

• DYSPLASIA

Whereas the above are normal processes, dysplasia is abnormal. It refers to abnormal changes in the shape, size, and organization of mature cells. It is usually associated with cancerous or pre-cancerous growth and is often reversible by removal of the inciting stimulus or (more drastically) eliminated by removing the entire diseased organ. Since it is considered a neoplastic process, it can have features of all the types of cellular types of growth. Dysplasia has a whole science of ratings and grades that describes it in detail for medical professionals.

An example is, again, the uterine cervix. Due to the stimulus of human papillomavirus (HPV), the cervical cells can become dysplastic (dysplasia), the most benign conversion in the continuum toward invasive cancer. Dysplasia can remain unchanged or can progress toward the higher degrees of malignancy, including invasive cancer. For the cervical cells (as well as--more famously--the lungs), smoking has been found to act as a stimulus for dysplasia in the cervix (a "co-carcinogen").

• NEOPLASIA

Neoplasia is an abnormal growth of tissue that if large enough to be a mass, is called a tumor. The words "mass," "tumor," and "neoplasia" are not always malignant, as a common wart is considered a neoplasia, a mass, or a tumor, technically. Therefore, not all neoplasias are malignancies, but all malignancies are neoplasia.

In the uterus, abnormal proliferation of fibrotic tissue can form swirls of tissue large enough to require surgical removal. These "fibroids" (leiomyomata) are benign, but their sheer size can become dangerous. Yet, it is a benign neoplasm.

On the other hand, if the muscle cells of the uterus undergo malignant transformation (myosarcoma), they are considered malignant neoplasms.