Cell Growth Disorders

Aplasia

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

Atrophy

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

Appearance

• 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.

Causes

• 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.

Examples

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

• 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.

Types

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.
  • E. g. intestine, bladder, gall bladder.

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

• 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

• 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.

Causes

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

• 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

• 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

• 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.

Neoplasia

Neoplasia