Pulpal and Periapical Biology, Pathogenesis, and Histopathology
Pulpal biology
The pulp occupies a rigid, low-compliance chamber. Its peripheral organisation is orderly: odontoblasts, the cell-free zone of Weil, the cell-rich zone, and the pulp core containing vessels and nerves. Nerves are mainly A-delta fibres (sharp, fast, low threshold - the fibres that respond to cold and to electric pulp testing) and C fibres (dull, aching, higher threshold, sensitised by inflammation). Because the pulp cannot swell, even modest oedema raises tissue pressure, collapses venules and drives ischaemia - the reason pain escalates in a closed chamber and the reason local anaesthetic often fails in irreversible pulpitis.
Pathogenesis and histopathology
Bacteria within the canal, and their products, are the cause. Pulpitis begins as a focal inflammatory lesion that spreads; histologically it is described as acute, chronic, suppurative or granulomatous, and as partial or total. Necrosis follows. Periapical disease is a host-defence lesion: bacterial antigens diffuse through the apical foramina and stimulate macrophages and lymphocytes to release IL-1, TNF and RANKL, driving osteoclastogenesis and bone resorption. The classic presentations - granuloma, radicular cyst and abscess - cannot be reliably distinguished on a radiograph, and a large periapical radiolucency may still be a granuloma.
Traps
Examiners love the mismatch between clinical and histologic diagnosis: 'irreversible pulpitis' is a clinical treatment decision, not a microscopic finding, and near-normal histology can coexist with severe symptoms. Remember that a true cyst (epithelium-lined cavity not communicating with the canal) is less common than a pocket or bay cyst, and that condensing osteitis is a reaction in young teeth with vital pulps, not a sign of necrosis.
Rule of thumb: radiographic size does not predict histology; treat the pulp, and let the periapex heal.