We know that cells are the basic units of tissues and under normal conditions, these cells must constantly adapt to changes in their environment. Cellular adaptation is the ability of the cells to respond to various types of stimuli and adverse environmental changes.

  • Physiologic adaptation – Can return to its form
  • Pathologic adaptation – Injurious

These adaptations occur either due to the increase/decrease in cell size( atrophy or hypertrophy), due to the increase in the number of cells(hyperplasia), or due to changing the type of phenotypic differentiation(dysplasia or metaplasia). These adaptations are explained in detail.

ATROPHY OF CELLS

REDUCTION IN CELL SIZE IS CALLED ATROPHY

Shrinkage of the cell size by the loss of the cell substances. Here the entire tissue or organ diminishes in size and function. This may occur due to the aging process, lack of blood supply, inadequate nutrition, and loss of nerve innervation. The causes of atrophy are classified into two major types,

  1. PHYSIOLOGIC ATROPHY
    • Here atrophy is a normal process of aging in the tissues, due to the loss of endocrine stimulation or arteriosclerosis.
    • Examples:
      • Atrophy of the brain with age.
      • Atrophy of lymphoid tissues in lymph nodes, appendix, and thymus.
  2. PATHOLOGIC ATROPHY
    • The causes are under various types
      • Starvation atrophy – Depletion of carbohydrates and fats followed by protein catabolism.
        • General weakness
      • Ischaemic atrophy – Gradual decrease in blood supply due to atherosclerosis.
        • Cerebral atherosclerosis
      • Disuse atrophy – Prolonged diminished function is associated with disuse atrophy of organs.
        • Obstruction of pancreas
      • Neuropathic atrophy – Interruption in the nerve supply
        • Motor neuron disease
      • Endocrine atrophy – Loss of endocrine regulatory mechanism
        • Hypothyroidism
      • Pressure atrophy – Prolonged pressure from benign tumors
        • Erosion of spine by tumor in nerve root
      • Idiopathic atrophy – No obvious cause
        • Testicular atrophy
Atrophy

HYPERTROPHY

INCREASE IN THE CELL SIZE IS CALLED HYPERTROPHY

Hypertrophy is the increase in the size of the parenchymal cells. It results in the enlargement of organs or tissue without any change in the number of cells. Hypertrophy without accompanying hyperplasia affects the muscles mainly. Here there are two basic types of hypertrophy that are listed below:

  1. PHYSIOLOGIC HYPERTROPHY
    • It reflects an increase in the workload that is not caused by the disease.
      • Enlarged size of the uterus in pregnancy.
  2. PATHOLOGIC HYPERTROPHY
    • This is a response to a disease.
      • Hypertrophy of cardiac muscle – Occur in a number of cardiovascular diseases
        • Systemic hypertension
        • Aortic valve disease
      • Hypertrophy of smooth muscle
        • Pyloric stenosis
        • Muscular arteries in hypertension
      • Hypertrophy of skeletal muscle
        • Hypertrophied muscles in athletes
      • Compensatory hypertrophy – It takes place when cell size increases to take over for nonfunctioning cells.
        • Adrenal hyperplasia following removal of one adrenal gland.

HYPERPLASIA

INCREASE IN THE NUMBER OF CELLS IS HYPERPLASIA

It is the increase in the number of parenchymal cells in an organ or tissue. Here hyperplasia and hypertrophy occur together. This results in the enlargement of the organ or tissue. This hyperplasia persists so long as the stimulus is present. Again hyperplasia is also classified under two types.

  1. PHYSIOLOGIC HYPERPLASIA
    • The two common types are:
      • Hormonal hyperplasia – Occurs under the influence of hormonal stimulation
        • Hyperplasia of the pregnant uterus
      • Compensatory hyperplasia – Hyperplasia occurs following the removal of an organ
        • Regeneration of epidermis after skin abrasion
  2. PATHOLOGIC HYPERPLASIA
    • Occurs due to excessive stimulation of hormones or growth cancers.
      • Endometrial hyperplasia following estrogen excess.
      • Intraductal epithelial hyperplasia in the breast in fibrocystic breast tissue.
Hyperplasia vs Hypertrophy

DYSPLASIA OF CELLS

DYSPLASIA IS DISORDERED CELLULAR DEVELOPMENT ALSO KNOWN AS ATYPICAL HYPERPLASIA

Dysplasia is disordered cellular development and is often accompanied by metaplasia or hyperplasia. It occurs most often in epithelial cells. Epithelial dysplasia is characterized by cellular proliferation and cytologic changes. Here most commonly uterine, cervix, and bronchial mucosa are affected. The cytologic changes include:

  • Loss of basal polarity
  • Increased number of layers of epithelial cells
  • Disorderly arrangement of cells from the basal layer to the surface layer
  • Increased nucleocytoplasmic ratio

METAPLASIA

ONE CELL TYPE IS REPLACED BY AN ANOTHER CELL TYPE

It is a reversible change where the one cell type is replaced by the another cell type. Metaplasia is broadly classified into two types.

  1. EPITHELIAL METAPLASIA
    • It is the most common type
    • Depending upon the types of epithelial transformation, 2 types of epithelial metaplasia exist.
      • Squamous metaplasia – Various types of specialized epithelium undergo this change due to chronic irritation that may be mechanical, chemical, or infective in origin.
        • Changes in bronchus, gall bladder, etc.,
        • In Vitamin A deficiency, squamous metaplasia in nose, bronchi, urinary tract.
      • Columnar metaplasia – Transformation occurs in the columnar epithelium.
        • Intestinal metaplasia
        • Columnar metaplasia in Barrett’s oesophagus
  2. MESENCHYMAL METAPLASIA
    • Transformation of one adult-type mesenchymal tissue to another.
    • Osseous metaplasia – Formation of bone in fibrous tissue, cartilage, and myxoid tissue.
      • In the arterial wall in old age
      • In the fibrous stroma of the tumor
    • Cartilaginous metaplasia – This occurs where there is undue mobility.
      • In the healing of fractures

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