In freshwater tilapia (O. mossambicus), Lee and
co-workers (’96) have defined three MRC types
according to the apical membrane appearance by
SEM and categorized them as either wavy-convex,
shallow-basin, or deep-hole. The ‘‘wavy-convex’’
type has a convex apically exposed area with
variable ornamentation with microvilli and a
relatively large two-dimensional area. The surface
of the ‘‘shallow-basin’’ type MRCs is flat but
recessed below neighboring PVCs. The density of
the ‘‘wavy-convex’’ type MRCs increases in low
NaCl and the ‘‘shallow-basin’’ type MRCs increase
in low Ca2þ water, suggesting roles in NaþCl and
Ca2þ uptake, respectively (Chang et al., 2001).