Atrophy
Atrophy is decrease in the size of a cell that has at one time been of normal size.
Types of atrophy
- Physiologic atrophy: Occurs due to a normal stressor. For example, decrease in the size of the uterus after pregnancy.
- Pathologic atrophy: Occurs due to an abnormal stressor. In general, atrophy is due to the loss of stimulus to the organ. Specific types of loss of stimulus include loss of blood supply or innervation, loss of endocrine stimulus, disuse, mechanical compression, decreased workload, or aging.
Gross morphology of atrophy (Figure 1-2): The organ is smaller than usual. Atrophy occurs in a once normally developed organ. If the organ was never a normal size (i.e., because it did not develop normally), the condition is called hypoplasia.
Kidneys from two different patients. The kidney on the left is normal in size, whereas the kidney on the right is atrophic. The kidney on the right was from a patient who had severe atherosclerosis of the renal artery, which led to ischemia (i.e., decreased perfusion) of the organ. Due to an insufficient supply of oxygen and nutrients, the cells of the kidney decreased in size to adapt. An incidental renal cell carcinoma is visible near the pole of the atrophic kidney (arrow).
Basic description: Change of epithelium at a site, or location, from one type of epithelium to another type. In metaplasia, the epithelium is normal in appearance but in an abnormal location.
Mechanism of metaplasia: The epithelium normally present at a site cannot handle the new environment so it converts to a type of epithelium that can adapt.
Examples: Barrett esophagus is due to reflux of gastric contents into the esophagus, which causes the epithelium type to convert from squamous to glandular (Figure 1-3 A and B). Squamous metaplasia in the lungs is due to exposure of respiratory epithelium to toxins in cigarette smoke.
FIGURE 1-3.
Barrett esophagus (glandular metaplasia). A, This specimen is taken from the region of the gastroesophageal junction and includes a segment of proximal stomach (on the left side) in continuity with the distal esophagus (on the right side). A small patch of mucosa with an appearance similar to the gastric mucosa extends proximally (circle), above the gastroesophageal junction. In this area, the normal stratified squamous epithelium of the esophagus has been replaced by glandular epithelium. Glandular metaplasia of the esophagus occurs in response to gastric acid reflux. B, The right side of the image shows stratified squamous epithelium, and the left side shows glandular epithelium, with goblet cells present (arrow). Transformation of one type of tissue to another type of tissue is termed metaplasia; in this case, stratified squamous epithelium was transformed to intestinal-type epithelium. Hematoxylin and eosin, 200×.