Hypertrophy and Hyperplasia

Definitions

Hypertrophy — increase in cell size, no change in cell number. Hyperplasia — increase in cell number. Non-dividing tissue (cardiac/skeletal muscle) can only hypertrophy; dividing tissue can do either or both.

Hypertrophy

Causes: increased functional demand or hormonal stimulation.

• Physiologic: pregnant uterus (hypertrophy + hyperplasia).
• Pathologic cardiac: hypertension, aortic valve disease, mitral insufficiency.
• Pathologic smooth muscle: achalasia, pyloric stenosis, intestinal strictures, hypertensive arteries.
• Pathologic skeletal muscle: athletes, manual labourers.
• Compensatory: remaining kidney/adrenal after contralateral removal.

Morphology: organ enlarged, heavy (hypertrophied heart 700–800 g vs normal 350 g). ↑DNA/RNA/protein synthesis, ↑organelle number.

Hyperplasia

Mechanism: recruitment of cells from G0 into mitosis.

• Labile cells (skin, mucosa, marrow, lymph node) — readily hyperplastic.
• Stable cells (liver, pancreas, kidney, adrenal, thyroid) — quiescent but capable.
• Permanent cells (neurons, cardiac/skeletal muscle) — cannot undergo hyperplasia.

Hyperplasia vs neoplasia: hyperplasia needs the stimulus to persist and stays under normal growth control; neoplasia is autonomous.

Causes:

• Physiologic hormonal: breast (puberty/pregnancy/lactation), pregnant uterus, proliferative endometrium, prostate (old age).
• Physiologic compensatory: liver after hepatectomy, epidermis after abrasion, remaining kidney nephrons.
• Pathologic: endometrial hyperplasia (oestrogen excess), granulation tissue, skin warts (HPV), pseudocarcinomatous hyperplasia (ulcer margin), fibrocystic breast change.

Morphology: enlarged organ, increased cell number, ↑DNA synthesis/mitoses.

Comparison

Clinical relevance

Cardiac hypertrophy from hypertension/valve disease is compensatory at first, then decompensates into heart failure (fixed capillary supply can’t match enlarged muscle mass). Permanent cells hypertrophy only — never compensatory hyperplasia — which is why MI and CNS injury heal by scar, not regeneration.

Definitions

Hypertrophy and hyperplasia are the two adaptive responses by which a tissue increases in size, and they are frequently found together in the same organ, though the mechanism behind each is distinct.

• Hypertrophy is an increase in the size of individual parenchymal cells, enlarging the organ or tissue without any increase in cell number.
• Hyperplasia is an increase in the number of parenchymal cells, enlarging the organ or tissue through cell proliferation rather than cell enlargement.

Which of the two occurs in a given tissue depends on whether its cells retain the capacity to divide. Hypertrophy without accompanying hyperplasia is the rule in tissues built of non-dividing cells — chiefly muscle, both cardiac and skeletal — since these cells cannot supplement their numbers no matter how strong the stimulus. Tissues capable of mitosis can respond with hyperplasia, hypertrophy, or a combination of both.

Hypertrophy

Causes

Hypertrophy arises from increased functional demand or from hormonal stimulation, and may be physiologic or pathologic.

• Physiologic hypertrophy. Enlargement of the uterus in pregnancy is the classic example, and here hypertrophy occurs together with hyperplasia.
• Pathologic hypertrophy of cardiac muscle, seen in a range of cardiovascular disease: systemic hypertension, aortic valve disease (stenosis or incompetence), and mitral insufficiency all impose an increased workload on the myocardium.
• Pathologic hypertrophy of smooth muscle, as in cardiac achalasia of the oesophagus, pyloric stenosis of the stomach, intestinal strictures, and the muscular arteries in hypertension.
• Pathologic hypertrophy of skeletal muscle, exemplified by the hypertrophied muscles of athletes and manual labourers.
• Compensatory hypertrophy, occurring in an organ when its paired counterpart is removed or lost — hypertrophy (together with hyperplasia) of the nephrons of the remaining kidney after contralateral nephrectomy, or hyperplasia of the remaining adrenal gland after removal of the other.

Morphology

The affected organ is enlarged and heavy — a heart hypertrophied from systemic hypertension may weigh 700 to 800 g against an average normal adult weight of 350 g. Individual muscle fibres and their nuclei are enlarged. At the ultrastructural level, hypertrophy is accompanied by increased synthesis of DNA and RNA, increased protein synthesis, and an increased number of organelles — mitochondria, endoplasmic reticulum, and myofibrils.

Hyperplasia

Mechanism

Hyperplasia results from increased recruitment of cells out of the resting G0 phase of the cell cycle into active mitosis when an appropriate stimulus is present. Not all cell populations are capable of this response.

• Labile cells — epithelium of the skin and mucous membranes, and cells of the bone marrow and lymph nodes — divide continuously and readily undergo hyperplasia.
• Stable cells — parenchymal cells of the liver, pancreas, kidney, adrenal, and thyroid — are normally quiescent but retain the capacity to re-enter the cycle and undergo hyperplasia when stimulated.
• Permanent cells — neurons, and cardiac and skeletal muscle — have little or no capacity for regenerative, hyperplastic growth, which is why these tissues respond to increased demand by hypertrophy alone.

Hyperplasia is distinguished from neoplasia by its dependence on the inciting stimulus: hyperplastic growth persists only so long as the stimulus is present and remains under the tissue’s normal growth-regulatory control, whereas neoplastic growth continues autonomously because the genetic composition of the cell itself has changed.

Causes

Physiologic hyperplasia, of which the two most common patterns are hormonal and compensatory.

• Hormonal: hyperplasia of the female breast at puberty, in pregnancy, and during lactation; hyperplasia of the pregnant uterus; the proliferative phase of the normal endometrial cycle; and prostatic hyperplasia in older men.
• Compensatory: regeneration of the liver after partial hepatectomy; regeneration of the epidermis after an abrasion; and hyperplasia of the nephrons of the remaining kidney after contralateral nephrectomy.

Pathologic hyperplasia, most often driven by excessive hormonal or growth factor stimulation.

• Endometrial hyperplasia following oestrogen excess.
• Formation of granulation tissue in wound healing, from proliferation of fibroblasts and endothelial cells.
• Skin warts, from epidermal hyperplasia driven by human papillomavirus.
• Pseudocarcinomatous hyperplasia of the skin at the margin of a non-healing ulcer.
• Intraductal epithelial hyperplasia in fibrocystic change of the breast.

Morphology

The affected organ or tissue is enlarged, with an increased number of cells reflecting an increased rate of DNA synthesis and mitosis.

Hypertrophy Compared with Hyperplasia

Clinical Relevance

• Cardiac hypertrophy from long-standing hypertension or valve disease is initially compensatory but eventually decompensates into heart failure, since hypertrophied myocardium has a fixed capillary supply that cannot keep pace with the enlarged muscle mass.
• Recognising that permanent cells (neurons, cardiac and skeletal muscle) can only hypertrophy, never undergo compensatory hyperplasia, explains why myocardial infarction and central nervous system injury heal by scarring rather than by regeneration of functional tissue.
• Distinguishing hyperplasia from early neoplastic change matters clinically: hyperplasia is a reversible, stimulus-dependent process, while dysplasia or neoplasia arising within a hyperplastic focus represents a step toward autonomous, irreversible growth.

Diagram 1: Hypertrophy vs Hyperplasia at the Cellular Level

Draw three small grids of cells side by side, each grid representing the same patch of tissue.

Panel 1 — normal: a 3×3 grid of nine identical, normal-sized cells.

Panel 2 — hypertrophy: the same 3×3 arrangement of nine cells, but each cell drawn visibly larger than in panel 1, with a correspondingly larger nucleus in each.

Panel 3 — hyperplasia: a denser grid of smaller cells at normal individual size but greater in number (for example a 4×4 or 5×5 grid) packed into the same overall tissue area.

Labels required

• Each panel titled: normal, hypertrophy, hyperplasia.
• Panel 2 must show unchanged cell number with increased individual cell size — count the cells and keep the count identical to panel 1.
• Panel 3 must show unchanged individual cell size with increased cell number — the individual cells should match panel 1’s size, not panel 2’s.

Errors commonly made

• Drawing hyperplasia with enlarged cells instead of more numerous cells of normal size — this is the single most common confusion between the two processes.
• Changing the cell count in the hypertrophy panel, which contradicts the definition being tested.

Diagram 2: Which Tissues Can Do Which (classification table as a diagram)

Draw three labelled columns headed “Labile cells,” “Stable cells,” and “Permanent cells,” each containing a short list of example tissues, with a response capability tag beneath each column.

Labile cells: skin and mucosal epithelium, bone marrow, lymph nodes — tag: “hyperplasia readily.”

Stable cells: liver, pancreas, kidney, adrenal, thyroid — tag: “hyperplasia when stimulated.”

Permanent cells: neurons, cardiac muscle, skeletal muscle — tag: “hypertrophy only, no hyperplasia.”

Labels required

• All three category tags must use the words “readily,” “when stimulated,” and “hypertrophy only” respectively, since the exact distinction in degree and mechanism is what is tested.
• Cardiac and skeletal muscle must both be listed under permanent cells, since compensatory hypertrophy of muscle (a frequently asked topic) depends on this classification.

Errors commonly made

• Placing liver under permanent cells rather than stable cells — the liver’s marked regenerative capacity is a frequently tested exception students get backwards.
• Suggesting permanent cells can hyperplasia under strong enough stimulus — they cannot, regardless of stimulus strength, and this absolute distinction is the point of the diagram.

Multiple Choice Questions (2)