Atrophy

Definition

Reduction in size and number of parenchymal cells of a previously normal organ or tissue — an adaptive, reversible response, not cell death. Distinct from hypoplasia (organ never reached normal size) and aplasia (organ never developed).

Physiologic causes

Lymphoid tissue with age · thymic involution · gonadal atrophy after menopause · brain atrophy with age · osteoporosis.

Pathologic causes

• Starvation atrophy — depletes carbohydrate/fat first, then protein; produces cachexia.
• Ischaemic atrophy — gradual arterial insufficiency (atherosclerosis) — small kidney, atrophic brain.
• Disuse atrophy — immobilised limb, duct-obstructed pancreas.
• Neuropathic atrophy — poliomyelitis, motor neuron disease, nerve section.
• Endocrine atrophy — hypopituitarism (thyroid/adrenal/gonads), hypothyroidism (skin and adnexae).
• Pressure atrophy — tumour, cyst, or aneurysm compressing adjacent tissue (spine, skull, sternum).
• Idiopathic atrophy — some myopathies, testicular atrophy.

Morphology

Organ small, shrunken. Cells smaller but viable (not dead). Loss of organelles — mitochondria, myofilaments, endoplasmic reticulum. Increased autophagic vacuoles, some persisting as residual bodies containing lipofuscin — the basis of brown atrophy (e.g. heart).

Clinical relevance

Reversible if the cause is removed early (remobilisation, hormone replacement). Brown atrophy of the heart signals chronic wasting or advanced age.

Definition

Atrophy is a reduction in the size and number of parenchymal cells of an organ or tissue that had previously attained a normal size. It is an adaptive response — the cell shrinks to a level of function it can sustain with a reduced supply of nutrients, blood, or trophic stimulation, rather than dying outright. Atrophy is distinguished from two related but distinct concepts: hypoplasia, the failure of an organ to reach its normal size during development, and aplasia, the extreme form in which an organ fails to develop at all and only rudimentary tissue is present.

Causes

Physiologic Atrophy

Atrophy is a normal, expected part of ageing in certain tissues, generally reflecting withdrawal of a hormonal stimulus or the effect of arteriosclerosis.

• Atrophy of lymphoid tissue with advancing age.
• Involution of the thymus in adult life.
• Atrophy of the gonads after menopause.
• Atrophy of the brain with ageing.
• Osteoporosis, in which the bony trabeculae reduce in size with age.

Pathologic Atrophy

• Starvation atrophy. Nutritional deprivation depletes carbohydrate and fat stores first, followed by protein catabolism, producing the generalised weakness, emaciation, and anaemia known as cachexia, seen in cancer and other chronic severe illness.
• Ischaemic atrophy. A gradual reduction in blood supply, typically from atherosclerosis, causes gradual shrinkage of the affected organ — a small, atrophic kidney from renal artery atherosclerosis, or atrophy of the brain in cerebral atherosclerosis.
• Disuse atrophy. Prolonged reduction in functional activity produces atrophy of the corresponding tissue — wasting of a limb’s muscles when immobilised in a cast, or atrophy of the pancreas following obstruction of the pancreatic duct.
• Neuropathic atrophy. Interruption of the nerve supply to a muscle leads to its wasting, as in poliomyelitis, motor neuron disease, or a severed nerve.
• Endocrine atrophy. Loss of a hormone’s regulatory influence lowers the metabolic activity of its target tissue and produces atrophy — hypopituitarism leading to atrophy of the thyroid, adrenal, and gonads, or hypothyroidism causing atrophy of the skin and its adnexal structures.
• Pressure atrophy. Sustained pressure from an adjacent benign tumour, cyst, or aneurysm compresses and atrophies nearby tissue — erosion of the spine by a nerve root tumour, erosion of the skull by a meningioma, or erosion of the sternum by an aneurysm of the aortic arch.
• Idiopathic atrophy. Some instances of atrophy have no identifiable cause, such as certain myopathies or testicular atrophy.

Morphology

The pathologic changes of atrophy are similar regardless of the underlying cause.

• The affected organ is small, often visibly shrunken.
• Individual cells become smaller, but they remain viable — atrophy is a state of reduced cell mass, not cell death.
• The reduction in cell size reflects loss of cell organelles, chiefly mitochondria, myofilaments, and endoplasmic reticulum.
• Autophagic vacuoles containing digested cell debris increase in number within the shrunken cell. Some of these vacuoles persist as residual bodies — the lipofuscin pigment granules responsible for the brown discolouration seen in brown atrophy of organs such as the heart.

Clinical Relevance

• Atrophy is reversible if the underlying cause is corrected early — disuse atrophy of an immobilised limb recovers with remobilisation, and endocrine atrophy responds to hormone replacement — which is why identifying the specific mechanism matters more than simply recognising that atrophy is present.
• Brown atrophy of the heart, identified by lipofuscin-laden myocardial fibres, is a marker of chronic wasting or advanced age and is often found alongside other evidence of cachexia at autopsy.
• Because atrophic cells are shrunken but alive, atrophic organs can regain size and function once the causal stimulus is withdrawn, unlike tissue that has undergone necrosis.

Diagram 1: Normal Organ Compared with an Atrophic Organ

Draw two paired outlines of the same organ (a kidney works well), normal on the left and atrophic on the right, each with a small inset showing three individual cells at higher magnification.

Left — normal: organ at expected size and contour; inset cells of normal size with a full complement of organelles (nucleus, several mitochondria, endoplasmic reticulum) drawn schematically.

Right — atrophic: organ visibly smaller with a wrinkled or reduced capsule, but the same basic shape retained (not destroyed or replaced by scar); inset cells noticeably smaller than the normal ones, with fewer mitochondria and a scattering of small vacuoles inside the cytoplasm.

Labels required

• Label the vacuoles in the atrophic cell as autophagic vacuoles, and note that some persist as residual bodies containing lipofuscin.
• A caption under the atrophic organ stating that the cells are reduced in size but remain viable, distinguishing this from necrosis.
• Label the overall organ size difference with approximate proportions, since examiners look for the organ being drawn convincingly smaller, not just labelled as such.

Errors commonly made

• Drawing the atrophic organ as distorted, necrotic, or replaced by scar tissue rather than simply smaller — atrophy preserves the organ’s basic architecture at reduced scale.
• Omitting the autophagic vacuoles and lipofuscin from the atrophic cell inset, which is the specific ultrastructural detail examiners expect.
• Drawing atrophic cells as dead or fragmented — atrophy is a state of reduced but living cell mass.

Diagram 2: Causes of Atrophy (classification chart)

Draw a simple two-branch chart with “Atrophy” at the top, splitting into “Physiologic” and “Pathologic,” and list the causes as short branches under each.

Physiologic branch: lymphoid tissue (age) · thymus (adult involution) · gonads (menopause) · brain (ageing) · bone (osteoporosis).

Pathologic branch: starvation · ischaemic · disuse · neuropathic · endocrine · pressure · idiopathic — each with a one-word example alongside it (starvation → cachexia; ischaemic → atherosclerotic kidney; disuse → immobilised limb; neuropathic → poliomyelitis; endocrine → hypopituitarism; pressure → aneurysm erosion; idiopathic → testicular atrophy).

Labels required

• Both branches must be explicitly headed “Physiologic” and “Pathologic,” since this division is the organising principle examiners expect.
• Every one of the seven pathologic causes must appear individually — a partial list is a common way marks are lost on this topic.

Errors commonly made

• Omitting one or more of the seven pathologic causes, most often idiopathic or pressure atrophy, since they are less intuitive than starvation or disuse.
• Mixing a physiologic and a pathologic cause under the same branch, such as placing osteoporosis (physiologic, age-related) under the pathologic list.