Hyperplasia and hypertrophy

HYPERPLASIA
  • Hyperplasia is the increase in the size of the tissue or an organ or a part of an organ due to quantitative increase in the number of cells.
  • Hyperplasia is classified into physiological hyperplasia and pathological hyperplasia

Physiological hyperplasia

  • Physiological hyperplasia may be the result of hormonal influence as in the case of increase in the size of mammary gland due to glandular epithelial cell proliferation in puberty and pregnancy.
  • Compensatory hyperplasia: It occurs due to partial loss of hepatocytes in liver. Hepatic regeneration occurs following partial hepatectomy by the proliferation of surviving cells. These cells are primed from the matrix degradation products followed by proliferation under the influence of growth factors (HGF) and cytokines (TNF-α, IL-6 etc.) and aided by adjuvants like norepinephrine and growth inhibition influenced by TGF-β, with reduction in the growth factors and adjuvants. Compensatory hyperplasia can also be observed in abraded epidermis in which basal layer proliferates to form the superficial layers.

Pathological hyperplasia

This is most commonly caused by excessive hormonal stimulation. e.g. endometrial hyperplasia or effects of growth factors on target cells. In canine uterus, cystic endometrial hyperplasia occurs in prolonged progesterone secretion; in wound healing, hyperplasia of connective tissue (e.g. fibroblast and blood vessels) occurs under the influence of growth factors; hyperplasia also occurs in viral infections involving the epithelium i.e. epidermis or mucosal epithelium. e.g. papilloma virus infections. Pathological hyperplasia may also lead to cancerous growth.

  • Pathological hyperplasia may be localized or generalized/diffused.
  • Localized hyperplasia - e.g. Nodular hyperplasia in liver, spleen of aged dogs.
  • Generalized/diffused hyperplasia- e.g. diffuse enlargement of an organ, prostatic hyperplasia in dogs and thyroid hyperplasia in case of goitre.
  • Hyperplastic ability depends on different adult cell types. Accordingly three cell populations are identified:

1. Labile cells: These cells can proliferate normally. e.g. Epidermis, bone marrow cells

2. Stable cells: These cells proliferate when need arises. e.g. Liver, bone, cartilage, smooth muscle

3. Permanent cells: These cells have lost their ability to regenerate/ become hyperplastic. e.g. Neurons, cardiac and skeletal myocytes.

HYPERTROPHY

  • It is the increase in the size of the cells or the organ. The number of the cells doesnot increase. The hypertrophic changes are seen in the permanent/stable cells. Striated muscles are most commonly affected.
  • In microscopic view, the organ will be normal but the cells are bigger. The number and the size of the organelles will be increased due to the increase in the functional demand. e.g. smooth endoplasmic reticulum in hepatocytes are enlarged in chronic alcoholism and increase in the size of the rough endoplasmic reticulum and Golgi apparatus as a need for increased synthesis of proteins (e.g. collagen and immunoglobulin); the mitochondrial number varies with ATP requirements.

Types of hypertrophy

  • Physiologic hypertrophy: It occurs following work or exercise/specific hormonal stimulus. e.g. Muscles in race and draft horses; in pregnancy with increased estrogen stimulation hypertrophy of uterus occurs and in lactation mammary gland development occurs under the influence of prolactin and estrogen.
  • Compensatory hypertrophy: It occurs due to the loss of a part of the organ or loss of one of the paired organs (One kidney undergoes hypertrophy with the loss of the other) or due to the obstruction of the lumen in hollow muscular organ (Right ventricular hypertrophy in pulmonary stenosis).With the continued haemodynamic overload, the compensatory mechanisms fail, resulting in the decompensation and cardiac failure.

Mechanism of hypertrophy involves many signal transduction pathways with induction of a number of genes and synthesis of cellular protein. So there will be increase in growth factors, its receptors (TGF-β, fibroblast growth factor), transcriptional factor (C- fos) and vasoactive agents especially endothelin-1.
Last modified: Friday, 16 December 2011, 4:28 AM