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Cellular Adaptive Response To Injury


Cellular adaptation is the result of a persistent stress or injury

Adaptive responses are:


A. Atrophy:


1. Decreases in cell size and functional ability

   Causes of Atrophy:

  1. Decreased work load/ disuse e.g. Limb in a plaster cast
  2. Decreased blood supply eg: Ischemia (atherosclerosis)
  3. Lack of hormonal/neural stimulation e.g. muscle paralysis in Polio
  4. Malnutrition
  5. Aging

2.  Microscopy: small shrunken cells with Lipofuscin granules - Brown atrophy


3.  EM: Decreased intracellular components and autophagosomes


B. Hypertrophy:

An increase in size and functional ability due to increased synthesis of intracellular components.


1. Causes of hypertrophy:

    Increased mechanical demand

    Eg. Physiological - striated muscle of weight lifters,

    Pathological - Cardiac muscle in hypertension


2. Increased endocrine stimulation

Hypertrophy and hyperplasia can occur together e.g. Gravid uterus Mediators of hypertrophy

Growth factors, cytokines and other trophic stimuli

Increased expression of genes and increased protein synthesis


Example : Puberty (growth hormones, androgens/estrogens etc.) Lactating breast (Prolactin and estrogen)


3. Hyperplasia

  1. Definition: an increase in the number of cells in a tissue or organQ
  2. Some cell type are unable to exhibit hyperplasia (eg. Nerve, cardiac, skeletal muscle cells).
  3. Physiologic causes of hyperplasia
    1. Compensatory (e.g. after the partial hepatectomy)
    2. Hormonal stimulation (e.g. breast development at puberty)
    3. Antigenic stimulation (e.g. lymphoid hyperplasia)

4. Pathologic causes of hyperplasia

  1. Endometrial hyperplasia
  2. Prostatic hyperplasia of aging

5. Hyperplasia is mediated by:

  1. Growth factors, cytokines, and other trophic stimuli
  2. Increased expression of growth promoting genes (proto-onco-genes)
  3. Increased DNA synthesis and cell division

6. Metaplasia: A reversible change of one cell type two another, usually in response to irritation.

  1. It has been suggested the replacement cell is better able to tolerate the environmental stresses Q e.g Bronchial epithelium undergoes squamous metaplasia in response to the chronic irritation of tobacco smoke.Q
  2. Proposed mechanisms: the reserve cells (stem cells) of the irritated tissue differentiate into a more protective cells type due to the influence of growth factors, cytokines and matrix components. .

7. Dysplasia: An abnormal proliferation of cells, Q i.e. characterized by change in cell size, shape and loss of cellular organization.

  1. However basement membrane is intact.
  2. Dysplasia is not cancer, but may progress to cancer (pre-neoplastic lesion) E.g. cervical dysplasia

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