The principal causative factor in pearl formation in a pearl oyster is the presence of a nucleus. It can be of organic or inorganic origin, such as parasites adults or larvae, molluscan eggs, decaying parts of plants, sand grains, epithelium or blood cells of the same animal, etc. These tiny particles or organisms enter the oyster when the shell valves are open for feeding and respiration. These foreign bodies may become embedded between the shell and mantle. In response to this stimulus, the foreign body is invaginated by the outer epithelium of the mantle and a pearl-sac is formed around it (Fig. 7 A).
Pearls are not produced without the formation of the pearl-sac. The pearl-sac is derived from the internal or external layer of the apithelium of the mantle or of the gill plates. The epithelial cells of the pearl-sac secrets the nacre which becomes deposited over the foreign body, forming a pearl in due course of time. These pearls are produced either within the mantle, in other soft tissues of the oyster, or between the mantle, and the interior surface of the shell. Such pearl production is accidental and occurs very rarely. They are generally small and irregular. Large and spherical pearls are still rarer to find. When the extraneous matter becomes fixed to the shell, only the exposed portion becomes covered by the pearl-sac resulting in a blister pearl.
Cultured pearls are formed in a pearl oyster, thanks to human interference. In any pearl formation, two things are required, the outer epithelium of the mantle lobe and core substance or nucleus. It was found that cut pieces of the mantle epithelium would provide the pearl secreting cells and that processed shell beads would be accepted by the oyster as the foreign body. Through careful surgery, the mantle piece graft tissue and the shell bead nucleus are implanted together, side by side, into the gonad of the oyster.
The oysters are then returned to sea for further growth. The outer epithelial cells of the graft tissue proliferate and rearrange themselves over the shell bead nucleus, forming a pearl-sac. The inner epithelium and connective tissue of the mantle disintegrate and become absorbed by the surrounding tissue. The cells of the pearl-sac derive their nourishment from the surrounding tissues and soon reassume their function of nacre (mother-of-pearl) secretion which is deposited over the nucleus in the form of concentric micro-layers (Fig. 7 C). The nacreous matter consists of thin alternate layers of aragonite and conchiolin deposited around the nucleus. The conchiolin is organic in nature and consists of mucopolysaccarides. It forms the binding layer for the aragonite crystals. The aragonite layers are 0.29–0.60 mm thick and are made of calcium carbonate in the form of highly laminated crystals. In cultured pearls the nacre quality and the process of pearl formation are the same as in the formation of natural pearls. Cultured half-pearls (Fig. 7 B) are produced by affixing many nuclei on the inner surface of the shell valves. The outer epithelium of the mantle forms the pearl-sac on the free surface of the nucleus and the halfpearl is formed.
FIGURE 7. Process of pearl formation. (A) round and half-natural pearls; (B) half-cultured pearl; and (C) round cultured pearl with an artificially implanted nucleus.