Cell and Tissue Biology: Injury, Adaptation, Death, and Basic Histology
Core concepts
A candidate must be fluent in the reversible-versus-irreversible injury threshold. Reversible injury shows cellular swelling (hydropic change), fatty change, mitochondrial swelling, and ribosomal detachment. Irreversible injury is defined by membrane integrity loss and mitochondrial permeability transition, with calcium influx, reactive oxygen species, and lysosomal enzyme release. Know the four adaptive responses: hypertrophy (increase in cell size, often in permanent tissues such as myocardium), hyperplasia (increase in number, requiring a proliferative compartment), atrophy (shrinkage via ubiquitin-proteasome and autophagy pathways), and metaplasia (a change from one mature cell type to another, usually a response to chronic irritation, e.g. squamous metaplasia of bronchial epithelium in smokers).
Cell death pathways
Distinguish apoptosis (energy-dependent, caspase-mediated, intact membranes, no inflammation, DNA laddering) from necrosis (passive, membrane rupture, inflammation). Recognise oncosis, pyroptosis (inflammasome and gasdermin pores), and ferroptosis (iron-dependent lipid peroxidation) as named entities that appear in stems. Necroptosis is a programmed necrotic pathway via RIPK1/RIPK3/MLKL.
Basic histology
Be able to identify the four tissue classes and their subtypes: epithelia (simple, stratified, pseudostratified; squamous, cuboidal, columnar), connective tissue (collagen types I–IV, elastic fibres, ground substance), muscle (skeletal, cardiac with intercalated discs, smooth), and nervous tissue. Know collagen type I in bone, skin, and mature scar; type II in cartilage; type III (reticulin) in early wound healing and lymphoid organs; type IV in basement membrane.
Traps
Questions often conflate metaplasia with dysplasia; metaplasia is reversible and not pre-malignant per se, whereas dysplasia denotes disordered maturation with atypia. Another trap is attributing anthracosis or lipofuscin to pathological injury when both are accumulations of endogenous pigment. Dystrophic calcification occurs in damaged tissue with normal serum calcium; metastatic calcification occurs with hypercalcaemia in normal tissue.
Rule of thumb
If the stem describes cell swelling with intact membranes, choose reversible injury; if membranes are breached with enzyme leakage (troponin, transaminases) and inflammation, choose necrosis.