1374 



Neff, Jerry M. 1972. 



Ultrastructural studies of periostraeum formation in the hard shelled clam 

 Mereenaria mereenaria (L) . Tissue Cell 4(2): 311-326. 



The inner surface of the outer fold of the mantle of hard clam contains 

 5 cell types. The outer surface of the second fold contains only flattened 

 cells, all of similar morphology. Intermediate cells lie between the cells 

 of the 2 folds. Periostraeum has 2 layers, an outer mineral-rich pellicle 

 and an inner homogeneous protein layer. The pellicle arises at the base of 

 the periostracal groove between the basal cell and the 1st intermediate cell. 

 It has a membrane-like layer and a granular layer. Periostraeum is secreted 

 by columnar epithelium lining the inner surface of the outer mantle fold. 

 Cells of the outer surface of the 2nd fold support the periostracal sheet 

 and move it out of the groove as it is synthesized. All phases of formation 

 of periostraeum are going on simultaneously, but each phase is the result of 

 activities of particular groups of cells at different positions in the groove. 

 Those components which appear at the base of the groove are secreted first and 

 subsequent layers are added in the sequence in which they first appear moving 

 from the base toward the mouth of the groove. In Mereenaria the 1st component 

 formed is the membrane-like layer of the pellicle. Then electron-dense 

 granules are secreted onto the outer surface of the pellicle membrane. 

 Finally, the homogeneous periostraeum proper is added to the inner surface 

 of the pellicle as it moves distally in the periostracal groove. The 2nd 

 cell of the outer fold probably has 2 functions: to guide the pellicle out 

 of the base of the periostracal groove, and to protect the pellicle from 

 being torn when the mantle contracts upon closure of the valves. The 

 function of the mineral-rich granule-filled cells of the outer fold is 

 obscure. The remaining cells of the outer fold are typical of cells engaged 

 in protein synthesis and excretion. The variable number of lysosome-like 

 bodies probably are capable of resorbing components of the periostraeum. 

 The gland cells underlying the epithelial cells of the inner surface of the 

 outer fold contain proteins rich in phenolic groups. The rounded vacuoles 

 contained in these cells probably contain phenolic precursors of the tanning 

 agent for the periostraeum. Fine fibrous material between the inner surface 

 of the periostracal protein and the apex of the columnar cells of the inner 

 surface of the outer fold is probably an uncondensed precursor of the 

 homogeneous protein layer of the periostraeum. The low epithelial cells of 

 the outer surface of the 2nd fold support the periostraeum and are probably 

 responsible for moving it out of the periostracal groove as it is formed. 

 The periostraeum appears to be tightly bonded to the microvilli. Vacuoles 

 in these cells may contain the bonding substance that attaches the 

 periostraeum to these cells. The slender outer plica of the 2nd fold 

 undoubtedly manipulates the fully formed periostraeum and applies it to the 

 growing edge of the shell. (Abstracter's note: this paper contains a brief 

 discussion of the functions of the periostraeum. It provides a waterproof 

 covering for the shell, protecting it from dissolution by acids in the 

 environment. It provides the original substrate for mineral deposition at 

 the edge of the shell. In some mollusks, elements or derivatives of the 

 periostraeum may actually be incorporated into the outer part of the 

 prismatic layer of the shell. It encloses the extrapallial space ventrally 

 and isolates it from the external environment.) - J.L.M. 



1375 



Neff, Jerry M. 1972. 



Ultrastructure of the outer epithelium of the mantle in the clam Mereenaria 

 mereenaria in relation to calcification of the shell. Tissue Cell 4(4): 

 591-600. 



The general outer epithelium of the mantle of hard clam is composed of tall 

 narrow columnar cells with densely packed apical microvilli, convoluted 

 lateral and basal cell membranes, and abundant mitochondria in the apical and 

 basal cytoplasm. Cells are joined apically by a zonula adherens followed by 



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