Abstract
In bivalves, the mantle tissue secretes organic matrix and inorganic ions into the extrapallial space (EPS) to form the shells. In addition, more and more evidences indicate the participation of hemocytes in shell mineralization, but no direct evidence has been reported that verifies the presence of hemocytes in the EPS, and their exact roles in biomineralization remain uncertain. Here, we identified hemocytes from the EPS of Pinctada fucata. Numerous components involved in cellular and humoral immunity were identified by proteome analysis, together with several proteins involved in calcium metabolism. The hemocytes exerted active phagocytosis and significantly upregulated the expression of immune genes after immune stimulation. A group of granulocytes were found to contain numerous calcium-rich vesicles and crystals, which serve as a calcium pool. During shell regeneration, some genes involved in calcium metabolism are upregulated. Strikingly, most of the shell matrix proteins were absent in the hemocytes, suggesting that they might not be solely responsible for directing the growth of the shell. Taken together, our results provided comprehensive information about the function of hemocytes in immunity and shell formation.
Highlights
Bivalves are among the most successful invertebrates living in a broad range of aquatic habitats
Our group successfully identified hemocytes from P. fucata and showed their potential roles in shell regeneration[6]. To further analyse their physiological features during shell formation, we studied the hemocytes in the extrapallial space (EPS) of P. fucata. We demonstrated that these hemocytes in the EPS originate from the circulation system, and their comprehensive functions in immunity and biomineralization were investigated on multiple levels
Granulocytes could be identified by the pink eosin cytoplasm, and the granules in the cytoplasm refracted light (Fig. 1c).When exposed in vitro, these hemocytes in the EPS tended to agglutinate and adhere to the bottom, consistent with those reported in other bivalves[15]
Summary
Bivalves are among the most successful invertebrates living in a broad range of aquatic habitats. Mount AS, et al.[5] proposed that in the eastern oyster, a special type of refractive hemocyte could mediate the calcification process and respond to shell damage These hemocytes seemed to fill the vacancy of the large non-exchangeable calcium pool left by mantle tissue. Our group successfully identified hemocytes from P. fucata and showed their potential roles in shell regeneration[6] To further analyse their physiological features during shell formation, we studied the hemocytes in the EPS of P. fucata. We demonstrated that these hemocytes in the EPS originate from the circulation system, and their comprehensive functions in immunity and biomineralization were investigated on multiple levels
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