Pisam et al. (’88, ’89, ’90) aided by their specific
heavy metal stains have also characterized the
accessory cell type in some freshwater euryhaline
species. These cells are associated with the a-cell
type in tilapia and Atlantic salmon but b-cell type
in brown trout (S. trutta) and are similar to the AC
described in the seawater fish gill, but they are less
common and do not send cytoplasmic processes
into the apical cytoplasm of the CC. Upon transfer
to sea water or in anticipation of transfer (smolting),
their numbers increase and interdigitations
are present. The AC are presumably present in
fishes living in fresh water in order to facilitate the
rapid switch to NaCl elimination. In the tilapia
(Alcolapia grahami) of extremely alkaline (pH 10)
Lake Magadi in Kenya, Laurent et al. (’95) have
found active seawater ACs associated with larger
CC type MRCs. The lake water has very high
HCO3
and CO3 2 concentrations of 40 and 265
mEq L1, respectively, with Naþ and Cl at 342
and 108 mEq L1, respectively. In the proposed
model, the CC–AC complex facilitates paracellular
Naþ efflux via the leaky tight junction while HCO3
is pumped out by the CC.