What is a Raphe?
Raphes are commonly found in pennate diatoms, including those diatoms within the Pseudo-nitzschia genus. It is visualed as two longitudinal slits that appear on the face of the valve. In this genus, the raphe can be seen as two slits near the median of the valve and does not contain silica like the other parts of the frustule. Between the two slits, there is a bridge of silica called the "central nodule."
What is the purpose of a Raphe?
The raphe is thought to serve a few purposes for this diatom. It is thought that transmembrane proteins, attached to both motors and mucopolysaccharides, move within the raphe in order to aid in cell movement (see Motility ). The slits are also hypothesized to confer flexibility to the cell, guarding against splitting of the valve under stress.
How Does a Raphe Form in Pseudo-nitzschia ?
After the cell cycle has been completed, raphe formation takes place along with valve formation. One side of the raphe forms first and is thus designated the primary raphe. Arms are then laid down at the poles and at the center of the primary raphe. Since arm formation occurs at the center of the primary raphe, the central nodule is formed. The arms extend towards each other and, upon fusion, form a raphe devoid of silica. See figure below.
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copyright Jennifer Shin 1999.