Cell Growth Disorders

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Aplasia also known as agenesis is the complete failure of an organ or part to form during embryogenesis.


  • inherited genetic defects
  • absence of limbs or amelia


Atrophy is a decrease in the size of the cells and organ, occurring after the organ has reached normal size.


  • Atrophic tissues and organs appear smaller and perhaps paler than usual.
  • Microscopy of atrophic tissue shows:
    • Cells of a smaller size.
    • Inactive appearance.
    • A relative increase in the supportive connective tissue.


  • Starvation
  • Malabsorption
  • Compression
    • E.g by a nearby lesion.
  • Immobilisation
    • Immobilisation of a limb results in atrophy of the muscles.
  • Denervation
  • Lack of trophic hormones
  • Chronic inflammation
    • May be idiopathic.


Serous Atrophy of Adipose Tissue

  • Also known as gelatinous atrophy of adipose tissue.
  • The fat becomes transparent, watery and severely depleted.
  • Occurs as a result of severe debilitation and weight loss.

Brown Atrophy

  • A senile change in muscles where they appear brownish rather than reddish in colour.
  • Due to the intracytoplasmic accumulation of lipofuscin within the muscle fibres.
    • The "wear and tear" pigment.


  • Hypertrophy is an increase in the size of an organ due to an increase in size of the individual cells.
    • The organ also gains in weight.


Functional Hypertrophy

  • Occurs in response to:
    • An increased physiological need.
      • E. g. muscles of the heart and limbs in training.
    • An increased demand because of organ dysfunction.
      • E.g in cardiac hypertrophy due to a progressively failing heart.

Compensatory Hypertrophy

  • Occurs when one of a paired organ is damaged or lost.
    • E.g. the kidney.

Obstructional Hypertrophy

  • Hollow organs may become thickened around an obstruction.

Hormonal Mediated Hypertrophy

  • Anabolic steroids produce hypertrophy of muscle.
  • Thyroid hormones have a general hypertrophic effect on tissues.
    • Increase protein synthesis within them.
    • The heart can become quite hypertrophied in thyroid excess.
      • Commonly seen in older cats which often develop hyperthyroidism.


  • Hypoplasia is a reduction in the size of cells and tissues,.
  • Due to a failure to grow to normal size.
  • Ranges from mild hypoplasia to almost complete absence.
    • Almost complete absence is also called vestigial or rudimentary.
  • Aplasia and agensis refer to complete absence of tissue.
    • Generally refer to the gross appearance rather than the microscopic appearance.
      • Some rudimentary tissue can be seen if searched for carefully.


  • Hyperplasia is an increase in the size of an organ due to an increase in the numbers of cells present within it.
  • Hypertrophy and hyperplasia may occur concurrently.
  • The hyperplastic response stops when the inciting agent ceases.
  • Hyperplastic tissue is more prone to injury by chemicals, and also may be more prone to undergo neoplastic change in some cases.


Benign prostatic hyperplasia (Courtesy of BioMed Archive)
  • Hormonal stimulation.
    • Parathyroid hyperplasia in chronic renal failure.
    • Prostatic hyperplasia in older dogs.
  • Hyperplasia may also occur as a regenerative response to
    • Irritation
    • Cell loss
    • Injury

Gross Appearance

  • Hyperplastic nodules can be seen in a variety of organs in older dogs and cats.
    • Particularly the thyroid in cats, and the spleen and liver of dogs.

Histological Appearance

  • Hyperplasia is characterised by an increase in the numbers of cells.
  • Mitotic activity is not always seen.
  • There is some increase in cellular basophilia.
  • The cells are well differentiated and tissue structure is normal.


  • Metaplasia is a transformation of one type of tissue into another.
  • Occurs solely in:
    • Connective tissue
      • The metaplastic change id to cartilage and bone in damaged tissue.
      • Caesarean scars in the pig are especially prone to osseous metaplasia.
    • Epithelium
      • Squamous metaplasia of cuboidal or columnar epithelium is quite common.
        • Seen in the prostate of dogs under the influence of oestrogens.
          • Oestrogens are present in Sertoli cell tumours and in Vit. A deficiency.
        • The most striking example is the squamous metaplasia of the oesophageal glands in the chicken.
  • Mixed tumours of the mammary gland of the dog are so called because there is proliferation of both the glandular element and the surrounding myoepithelium.
    • The myoepithelium can transform into cartilage and bone.
      • Some regard this as a metaplastic change.
      • The bone formed may even show marrow formation within the spaces between the bony trabeculae.


  • Dysplasia is abnormal growth within a tissue.
  • The normal arrangement and pattern of the tissue may be lost.
  • The most common example is the renal dysplasia in some breeds of dogs.
    • Causes fibrous tracts.
    • Because of the reduction in functional tissue, gives a predisposition to renal failure early in life in the more severely affected cases.


  • Anaplasia is a marked and irreversible loss of cellular differentiation with return to a more primitive state.
  • Afeature of highly malignant tumours.
  • There is no pattern, just a diffuse sheet of cells.
  • Both cells and nuclei are of differing sizes.
  • There are prominent nucleoli in some nuclei.