Checa A
Departamento de Estratigrafía y Paleontología, Facultad de Ciencias, Universidad de Granada, Spain.
Tissue Cell. 2000 Oct;32(5):405-16. doi: 10.1054/tice.2000.0129.
The periostracum in Unionidae consists of two layers. The outer one is secreted within the periostracal groove, while the inner layer is secreted by the epithelium of the outer mantle fold. The periostracum reaches its maximum thickness at the shell edge, where it reflects onto the shell surface. Biomineralization begins within the inner periostracum as fibrous spheruliths, which grow towards the shell interior, coalesce and compete mutually, originating the aragonitic outer prismatic shell layer. Prisms are fibrous polycrystalline aggregates. Internal growth lines indicate that their growth front is limited by the mantle surface. Transition to nacre is gradual. The first nacreous tablets grow by epitaxy onto the distal ends of prism fibres. Later growth proceeds onto previously deposited tablets. Our model involves two alternative stages. During active shell secretion, the mantle edge extends to fill the extrapallial space and the periostracal conveyor belt switches on, with the consequential secretion of periostracum and shell. During periods of inactivity, only the outer periostracum is secreted; this forms folds at the exit of the periostracal groove, leaving high-rank growth lines. Layers of inner periostracum are added occasionally to the shell interior during prolonged periods of inactivity in which the mantle is retracted.
蚌科动物的贝壳外层由两层组成。外层是在壳皮沟内分泌形成的，而内层则由外套膜外褶的上皮细胞分泌。贝壳外层在壳边缘处达到最大厚度，并在此处反射到贝壳表面。生物矿化在内部贝壳外层内以纤维球霰石的形式开始，这些纤维球霰石向贝壳内部生长、合并并相互竞争，从而形成文石质的外部棱柱层。棱柱是纤维状多晶聚集体。内部生长线表明它们的生长前沿受到外套膜表面的限制。向珍珠层的转变是渐进的。第一批珍珠质片通过外延生长在棱柱纤维的远端。随后的生长则在先前沉积的片层上进行。我们的模型涉及两个交替阶段。在贝壳活跃分泌期间，外套膜边缘延伸以填充外套膜外空间，壳皮传送带开启，随之分泌壳皮和贝壳。在不活跃期，仅分泌外部壳皮；这在壳皮沟出口处形成褶皱，留下高阶生长线。在长时间不活跃且外套膜回缩的期间，内部贝壳外层的层偶尔会添加到贝壳内部。
