Hypertrophy is an increase in the size of the cell.

 

  • Physiologic hypertrophy: Occurs due to a normal stressor. For example, enlargement of skeletal muscle with exercise.
  • Pathologic hypertrophy: Occurs due to an abnormal stressor. For example, increase in the size of the heart due to aortic stenosis. Aortic stenosis is due to a change in the aortic valve, which obstructs the orifice, resulting in the left ventricle working harder to pump blood into the aorta.

Mechanisms by which hyperplasia and hypertrophy can occur:

 

Up regulation or down regulation of receptors and induction of new protein synthesis.

 

The two processes can occur together. For example, up regulation of receptors results in the induction of new protein synthesis; or up and down regulation of receptors and induction of new protein synthesis can occur as independent processes. The types of new proteins induced include transcription factors (e.g., c-Jun, c-Fos), contractile proteins (e.g., myosin light chain), and embryonic proteins (e.g., -myosin heavy chain).Both hyperplasia and hypertrophy result in an increase in organ size; microscopic examination is required to distinguish them.

Atrophy is a decrease in decrease in size, number, and function; can be applied to tissues, organs, or cells. If enough cells in an organ atrophy the entire organ will decrease in size. Thymus atrophy during early human development is an example of physiologic atrophy. Skeletal muscle atrophy is a common pathologic adaptation to skeletal muscle disuse. Tissue and organs especially susceptible to atrophy include skeletal muscle, cardiac muscle, secondary sex organs, and the brain.

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.

 

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.

1b. Hypoplasia: failure to grow to normal size; usually applied to a tissue or organ and indicates that there are fewer or smaller cells.

1c. Aplasia: failure to grow; usually meaning a complete absence of a cell type, organ or tissue (agenesis).

 

Metaplasia is a 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. Metaplasia occurs when a differentiated cell of a certain type is replaced by another cell type, which may be less differentiated. It is a reverse process thought to be caused by stem cell reprogramming. Stem cells are found in epithelia and embryonic mesenchyme of connective tissue.1

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.

 

A prominent example of metaplasia involves the changes associated with the respiratory tract in response to inhalation of irritants, such as smog or smoke. The bronchial cells convert from mucus-secreting, ciliatedcolumnar epithelium to non-ciliated, squamous epithelium incapable of secreting mucus. These transformed cells may become dysplasic or cancerous if the stimulus (e.g., cigarette smoking) is not removed. 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. Squamous metaplasia in the lungs is due to exposure of respiratory epithelium to toxins in cigarette smoke.

Image not available.

Image not available.

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×.