The mantle of mussels is stimulated by the invasion of foreign bodies (sand and parasites). The epidermal cells in the stimulation area take foreign bodies as the core and fall into the connective tissue of the mantle. Some of the trapped epidermal cells divide themselves to form nacre, which secretes nacre and the nucleus is wrapped layer by layer to form pearl. Pearls with foreign bodies as the core are called "nucleated pearls".

2. Internal factors

After pathological stimulation, a part of the outer skin of the mantle of mussel undergoes cell division, then it is separated, coated with organic substances secreted by itself, and gradually falls into the connective tissue of the mantle, forming pearl sacs, and then pearls are formed. Because there is no foreign body as the core, it is called "seedless pearl".

Extended data:

Mytilus forms the structure of pearl;

The mantle is an organ that all shellfish have to protect their bodies. It covers the internal organs, but it is thin and soft and can't protect mussels. But it can secrete a liquid and form a hard calcareous shell.

The stratum corneum and prismatic layer of the shell are formed by the edge of the mantle. They can increase with the growth of mussels, but the thickness cannot be increased.

The nacre is formed by the whole epithelial cells of the mantle, so it can not only increase, but also thicken during the growth of mussels. The older the mussel, the thicker and brighter the nacre, which is why.

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