Cellular Adaptation III: Hyperplasia

Hyperplasia is an increase in the number of cells in a tissue or organ due to enhanced cell division. It is an adaptive, controlled response to stimuli such as injury, hormones, or stress, involving mitosis to produce genetically identical cells and support tissue repair and regeneration.

Tissue Capacity

Certain tissues, including the epidermis, intestinal epithelium, bone marrow, and fibroblasts, have a high potential for hyperplasia. Others, such as bone, cartilage, and smooth muscle, show limited capacity for hyperplasia.

Mechanisms

Hyperplasia usually arises through two main mechanisms:

• Stimulation by Growth Factors and Hormones: Following tissue damage or cell loss, growth factors and hormones prompt surviving cells to re-enter the cell cycle and proliferate to restore tissue integrity.
• Activation of Tissue Stem or Progenitor Cells: In regenerative organs, resident stem or progenitor cells generate new mature cells to replace damaged ones. For instance, in the liver, regeneration primarily occurs through the proliferation of existing hepatocytes, which can re-enter the cell cycle after injury. Liver progenitor cells (LPCs) mainly contribute during severe or chronic injuries when hepatocyte division is insufficient.

Types of Hyperplasia

Physiologic hyperplasia supports normal function. Compensatory hyperplasia restores tissue after loss, such as liver regeneration after partial removal, callus formation, wound healing, and increased bone marrow activity. Hormonal hyperplasia occurs in response to hormones, such as breast growth during puberty and pregnancy or endometrial proliferation during the menstrual cycle.

Pathologic hyperplasia results from excessive stimulation and may increase cancer risk. Examples include endometrial hyperplasia from unopposed estrogen, benign prostatic hyperplasia (BPH) due to dihydrotestosterone effects, and thyroid hyperplasia (goiter) from chronic TSH stimulation.