Scolices, which are infectious miniature adult tapeworms, possess a dual potential. If they are ingested by a primary host the apical region of the scolex (suckers, rostellum, and hooklets) evaginates and the organism attaches within a crypt of Lieberkuhn in the host's proximal small bowel, where it develops into the adult worm. However, if the scolex is deposited in a favorable milieu such as the peritoneal cavity or tracheobronchial tree after cyst rupture it may form a new hydatid cyst.

The hydatid's laminated membrane, which is white and slimy, is not elastic, yet its surface area increases and it becomes thicker as the hydatid sphere enlarges. It is not clear how such an acellular structure accomplishes this but presumably laminations are laid down to accommodate growth and repair. The host tissue adjacent to the cyst mounts a chronic inflammatory reaction. As this subsides, scar tissue forms arounds the cyst. This is the pericyst (ectocyst) and it protects the fragile hydatid much like a tire protects an inner tube. The pericyst, which is not demonstrable by ultrasound or CT unless it is thickened or calcified (Figs. 3.12A, 3.35D), becomes thicker as the cyst enlarges. The pericyst around lung lesions is always quite thin, probably because the lung is less dense than liver. The thin pericyst allows the typical umbilication or notching of enlarging lung hydatids to occur where they contact bronchi and blood vessels. Blood vessels are obliterated within the developing pericyst, but bile ducts and bronchi are not. Eventually the increasingly substantial pericyst becomes a hindrance to further enlargement of the endocyst, forcing it to become redundant within the confines of the pericyst.

Those parts of the endocyst that become isolated from the nourishing hydatid fluid degenerate, forming a gelatinous amber-colored structure called matrix which may have a pseudotumoral appearance on ultrasound or CT (Fig. 3.12). Because matrix is amorphous and yellow, it can be confused with pus by the surgeon. Matrix is most common in older patients who on average have harbored the parasite longer than the young. Lesions in the lung usually do not develop matrix, probably because enlargement of the cysts is relatively unrestricted until they become large enough that further growth is inhibited by the chest wall, mediastinum, or diaphragm.

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