The Pattern of Cell Proliferation during Pearl Sac Formation in the Pearl Oyster

For cultured pearl production, a mantle allograft from a donor is implanted into a recipient gonad together with a small inorganic bead. Encapsulation of the grafts by hemocytes follows. The mantle allograft's outer epithelial cells subsequently emigrate along the inside of the hemocyte capsule, and eventually form a follicle, called a pearl sac, surrounding the bead. To clarify the cell types involved and timing of mitotic activity in the process of pearl sac formation, pearl oysters at various phases of pearl sac formation were injected with a thymidine analog, 5-bromo-2'-deoxyuridine (BrdU), and dividing cells labeled with BrdU were visualized with anti-BrdU antibody. At 4 days after implantation, the epidermal cells (dominant cells in the outer epithelium) of the allograft initiated active proliferation and emigration. Their mitoses continued during emigration and after establishment of the pearl sac. No incorporation of BrdU into the other two types of mantle epithelial cells (mature mucous cells and cells with large granules) was observed. In the gonad surrounding the pearl sac, a large number of connective tissue cells among gonadal follicles and around the intestine showed peak mitotic activity around one week after the operation, which implied activation of hemocyte production.