The inner germinal layer of the endocyst is the active part of the cyst, producing brood capsules which release protoscolices (after ingestion by the primary host the head of the protoscolex evaginates and the larva is then called a scolex) (Fig. 3.10). Most of the brood capsules and scolices pinch off from the germinal layer and precipitate to the bottom of the fluid to become hydatid sand, which is just visible to the naked eye. The hydatid sand floating within the cyst may contain as many as 400,000 protoscolices per ml and an average cyst contains up to 500 ml of such sand (Fig. 3.11).

...

...

Fig. 3.10 (A) Microscopic section of a hydatid cyst showing brood capsules and scolices. This cross-section of a liver hydatid clearly reveals the acellular, hyaline, outer layer of the cyst (arrow 1) and the delicate inner layer or germinal membrane (arrow 2). A large brood capsule (arrow 3) arises from the germinal membrane as a vesicle one cell layer thick. Invaginated scolices (arrow 4) arise as buds from the inner surface of the brood capsule. AFIP 55-4985. (B) Hydatid cyst in the lung of a dog showing the thick pericyst, the thin germinal membrane of the endocyst, and several brood capsules containing multiple scolices. AFIP 56-11365. (C) A hydatid cyst of the liver showing the thick, acellular, adventitial layer, the thin germinal layer, and brood capsules with scolices. (Hematoxylin and eosin, X 72). AFIP 65-2462. (D) Partially distintegrated brood capsules with scolices floating in the fluid of a hydatid cyst. Within the scolices can be seen the rostellum containing a double row of hooklets by which the worm attaches itself to the intestinal mucosa of a canine in its normal life cycle. The scolices arise as buds from the inner surface of the brood capsule, which in turn arises from the germinal epithelium of the endocyst (right) (H & E, X 560). AFIP 65-2464.

..

..

Fig. 3.11 Hydatid cysts contain "sand" or sediment with multiple scolices and hooklets. (A) The typical appearance of hydatid sand containing several scolices and hooklets floating free within the cyst fluid. The hooklets are birefringent and thus can be seen under polarized light. The germinal membrane lining the endocyst is on the right. AFIP 68-4760-2. (B) The sediment from a hydatid cyst containing numerous invaginated scolices with their hooklets, as well as free-floating, detached hooklets. (C) An invaginated scolex (left) and an evaginated scolex (right) showing the suckers and rostellum with its hooklets. AFIP 53-20053-6 (D) Numerous scolices, some free, some in cysts in the sediment at the bottom of a hydatid cyst. When the free scolices evert, the suckers may protrude to give a cross-like appearance.