is the pallial line, which is an impression of the pallial muscles lined in an arc in the middle of a mantle. Several types of epithelial cells are recognized in the outer epithelium of a pearl oyster mantle (Tsujii 1960;Wada 1966). The most abundant type is named as outer epithelial cells. They are regarded to play a major role in shell formation and regeneration, and their functions are a main theme of this article.A shell of pearl oyster Pinctada fucata, characterized by its beautiful luster over the inner surface, consists of two layers: periostracum and ostracum. The periostracum is a thin, pliable proteinaceous sheet covering the outer surface of the shell. Components of the periostracum are synthesized in a periostracum gland located at the bottom of a periostracal groove between an outer and a middle fold of a mantle. The periostracum is thought to serve as a barrier that iso- Fundamental Studies on in vivo and in vitro AbstractOuter epithelial cells, which constitute a monolayer epithelium covering the outer surface of pearl oyster mantle, play principal roles in shell and pearl formation. In pearl culture, a fragment of the mantle prepared from a donor is implanted into the recipient's gonad together with a small inorganic bead. Histological studies using pearl oyster Pinctada fucata have revealed that the outer epithelial cells emigrate from the allograft, proliferate, and form a pearl sac surrounding the bead. Following the pearl-sac formation, the pearl-sac epithelia start to form calcium carbonate crystals, such as nacre, on the bead showing morphological characteristics closely related with the crystal structures.To investigate cellular mechanisms of the pearl formation, organ and cell culture methods for the outer epithelial cells of pearl oyster mantle were developed. In the organ culture, crystal formation, deposition of shell matrix-like structure, and DNA synthesis of the outer epithelial cells were observed. The outer epithelial cells separated from the mantle started DNA synthesis in co-culture with hemocytes that revealed a part of cellto-cell interactions during the pearl-sac formation processes. Substitution of the cultured outer epithelial cells for a mantle allograft in pearl culture was tested by injection of the cultured cells; the results of which implied future possibilities for the application of the cultured outer epithelial cells for pearl production.

Three major organic matrix components, nacrein, MSI60 and N16 have been reported from the nacreous layer of Japanese pearl oyster, Pinctada fucata. Though several in vitro experiments have been carried out to elucidate the functions of these molecules details have not yet been clarified. In this report, we tempt to clarify the gene expression levels encoding the above three proteins between samples of 1) summer and winter seasons and 2) ocean and aquarium environments by using realtime polymerase chain reaction (PCR).It was confirmed that the biomineralization process of P. fucata is mainly influenced by the circatidal rhythm of the ocean environment. The gene expressions coding for N16 and MSI60 increased at the time of high tide, while that of nacrein increased at the time of low tide. The similar tendency observed in N16 and MSI60 showed the possibility that both components are secreted simultaneously, supporting a hypothesis that N16 forms crosslinkage with MSI60 to form the membrane. The expressions of MSI60, N16 and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes were remarkable in winter season, while no variation was found in the expression level of the nacrein gene in summer and winter season.The study is the first attempt regarding the seasonal and circadian rhythms observed on gene expressions incorporated into molluscan shell formation. The results will give a new insight into the relationship between molluscan physiology and the mechanism of shell formation.

No abstract

No abstract