Tissue Growth: Hyperplasia and Atrophy

Study Notes

Increase in stress leads to increased organ size.
Occurs through increased cell size (hypertrophy) and/or number (hyperplasia).
Hypertrophy involves gene activation, protein synthesis, and organelle production.
Hyperplasia involves production of new cells from stem cells.
Hyperplasia and hypertrophy typically occur together, like in the uterus during pregnancy.
Permanent tissues, like cardiac muscle, skeletal muscle, and nerves, only undergo hypertrophy due to their inability to produce new cells.
Cardiac myocytes experience hypertrophy, not hyperplasia, in response to systemic hypertension.
Pathologic hyperplasia can progress to dysplasia and potentially cancer.
Benign Prostatic Hyperplasia (BPH) is an exception and doesn't increase prostate cancer risk.

Decrease in stress leads to decreased organ size.
Occurs due to decreased cell size and number.
Cell number decreases via apoptosis.
Cell size decreases via ubiquitin-proteosome degradation and autophagy.
Ubiquitin-proteosome degradation involves tagging intermediate filaments with ubiquitin, leading to their destruction by proteosomes.
Autophagy involves the formation of autophagic vacuoles that fuse with lysosomes for breakdown of cellular components.

Stress causes a change in cell type.
Mostly involves changing one type of surface epithelium to another.
Metaplastic cells are better equipped to handle the new stress.
Barrett esophagus is a prime example.
Can progress to adenocarcinoma.
Apocrine metaplasia of the breast is an exception and poses no cancer risk.
Vitamin A deficiency can cause metaplasia.
Vitamin A is crucial for differentiating specialized epithelial surfaces, like those in the eye.
In vitamin A deficiency, squamous lining undergoes metaplasia into keratinizing squamous epithelium, known as keratomalacia.
Connective tissues can also undergo metaplasia.
Myositis ossificans, where connective tissue in muscle turns into bone during healing, is an example.

Disordered cellular growth.
Primarily refers to precancerous cell proliferation.
Examples include Cervical Intraepithelial Neoplasia (CIN) which is a precursor to cervical cancer.
Arises from longstanding pathologic hyperplasia or metaplasia.
Reversible with stress alleviation.
Progresses to carcinoma (irreversible) if stress persists.