Cellular Adaptations

1. Hypertrophy
2. Hyperplasia
3. Atrophy
4. Metaplasia
5. Dysplasia
6. Proliferation
7. Quiz

• In response to various stimuli, tissues can have a number of responses including tissue growth (by either increasing cell size or cell number), tissue shrinkage and cell transformation. These changes can lead to:
  • Re-establishment of the normal function of the affected tissue.
  • Increased efficiency of the affected tissue.
  • Decreased efficiency of the affected tissue.
  • Malignancy.

In response to physiological and pathological stimuli, cells can undergo changes which make them more suited for their environment. Chemical mediators are able to signal cells to undertake a number of actions. These chemicals can prompt apoptosis, cell division and cell differentiation, or resist apoptosis.

Hypertrophy

Hypertrophy is the increase in the size of a tissue via the enlargement of cells. The intracellular organelles, cytoplasm and structural proteins all increase in these enlarged cells, but the number of cells in the tissue remains the same.

Hypertrophy can occur both physiologically and pathologically. A common cause of physiological hypertrophy is increased skeletal muscle mass gained through anabolic metabolism following exercise. Smooth muscle hypertrophy occurs in the uterus, stimulated by hormonal changes.

In hypertension there is often pathological hypertrophy of the cardiac tissue, due to the increased strain on the ventricles and atria of the heart. Excessive cardiac hypertrophy can lead to a situation where the muscular walls of the heart become too thick, potentially impairing effective blood pumping and contributing to heart failure, especially if the underlying causes are not addressed.

Hyperplasia

Hyperplasia is the increase in the size of a tissue by an increase in the number of cells in that tissue.

Physiological hyperplasia occurs in the endometrium of the uterus upon stimulation of oestrogen during the menstrual cycle. Hyperplasia is also the method by which the liver restores itself from a partial resection.

Pathological hyperplasia can occur in overstimulation of endometrial tissue with oestrogen, for example in women taking unopposed oestrogen (without progesterone) for hormone replacement therapy after the menopause. The increased cell replication increases the chance of dysplasia (discussed later), and then neoplasia, taking place.

Diagram - The differences between hyperplasia and hypertrophy

Creative commons source by SchwarzeMelancholie [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

Atrophy

Atrophy is the opposite of hyperplasia and hypertrophy, in essence, it is the shrinkage of tissue. This can happen by a decrease in cell number and/or cell size.

Atrophy occurs physiologically in involution, when a tissue reduces in size after having increased in size. For example, the uterus and breasts both reduce in size post-pregnancy. Atrophy will also occur with old age. When an individual gets older, most systems and tissues undergo some level of atrophy.

Pathological atrophy can happen through catabolic metabolism due to malnutrition, whether this be caused by diseases such as cancer or by nutritional deficit. It can also occur due to inactivity, pressure, ischaemia, and chronic inflammation.

Diagram - The difference between normal and atrophied muscle

Creative commons source by OpenStax [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]