Histology and Physiology
This domain tests the microscopic structure and function of the dental hard and soft tissues, because almost every clinical decision - bonding, cavity preparation, pulp diagnosis, periodontal response, desensitising - rests on it.
Enamel and dentine
- Enamel is about 96% mineral by weight, formed by ameloblasts that are lost at eruption, so enamel cannot regenerate. Rods run from the dentino-enamel junction to the surface; Hunter-Schreger bands reflect rod decussation. Superficial enamel is aprismatic and more resistant to acid; the inner enamel is more porous.
- Dentine is roughly 70% mineral, 20% organic (mainly type I collagen) and 10% water, permeated by tubules about 1–2.5 micrometres wide whose density is greatest near the pulp. Peritubular (intratubular) dentine is highly mineralised; intertubular dentine is collagen-rich.
- Dentine types: primary (formed before eruption), secondary (slow, physiologic, continued), tertiary/reactionary (rapid, in response to irritants), plus sclerotic dentine and dead tracts.
Pulp, cementum and periodontal ligament
The pulp contains odontoblasts, fibroblasts, undifferentiated mesenchymal cells and defence cells, and becomes smaller with age through secondary dentine and pulp stones. Cementum is avascular and less mineralised than dentine; acellular extrinsic fibre cementum dominates the cervical half and anchors Sharpey fibres. The periodontal ligament is about 0.2 mm wide, with principal fibres (alveolar crest, horizontal, oblique, apical, interradicular) - oblique fibres resist axial load best.
Mucosa and saliva
Masticatory mucosa (gingiva, hard palate) is keratinised; lining mucosa is not; specialised mucosa covers the tongue dorsum. Junctional epithelium attaches via hemidesmosomes to the internal basal lamina, forming the epithelial seal. Resting saliva flow is roughly 1–1.5 litres daily, buffered mainly by bicarbonate, containing amylase, mucins, secretory IgA, lysozyme and lactoferrin.
Worked example
A patient has cervical abrasion with sharp cold sensitivity but a vital, normal pulp on testing. Reasoning: loss of cementum and enamel exposes dentine; fluid shifts within open tubules stimulate A-delta fibres - the hydrodynamic theory. Management is tubule occlusion (bonding agent, desensitiser, glass ionomer) plus addressing the cause, not endodontics.
Traps that cost marks
- Claiming enamel can remineralise after cavitation; only early subsurface lesions can.
- Confusing secondary with tertiary dentine.
- Describing cementum as highly mineralised or vascular.
- Forgetting that the smear layer is created by instrumentation and must be handled deliberately in bonding and endodontics.