Paper II
Question
Draw a labelled diagram of hydatid cyst
Answer
A labelled diagram cannot be rendered in this text-based answer; the description below covers the key structural features that would appear in the diagram, sufficient for labelling purposes.
Hydatid cyst — the larval (metacestode) stage of Echinococcus granulosus, developing in the intermediate host (commonly liver or lungs).
Structure (labelled from outside in):
- Pericyst — outermost, host-derived fibrous capsule (a reactive inflammatory response of surrounding host tissue).
- Laminated (ectocyst) membrane — an acellular, layered, whitish, elastic membrane secreted by the parasite, permeable to nutrients.
- Germinal (endocyst) layer — innermost, thin, nucleated, metabolically active layer, from which:
- Brood capsules bud internally, each containing numerous protoscolices (immature scolices — “hydatid sand” when free-floating).
- Daughter cysts may also form, either within (endogenous) or outside (exogenous) the primary cyst.
- Cyst (hydatid) fluid — clear, colourless to pale yellow, filling the cyst cavity, highly antigenic (risk of anaphylaxis on rupture).
Clinical labelling note: the pericyst is host tissue; the laminated membrane and germinal layer (plus brood capsules/protoscolices/daughter cysts) are all parasite-derived structures — this distinction is often specifically tested, since surgical management aims to remove the parasite-derived components without rupturing them into host tissue.