1967 ; Barrington m d thorp^, 19658,b ; Thorpe and Barrington, 1965 ;
Thorpe et al., 1974) using l 3 I 1 and 1261 as tracers. Iodine is actively
taken up and bound in the (+ells of zone 7 of the endostyle, a simple
epithelium lying between the dorsal glandular tract and the ciliated
lip (zone 8 ) of the endostylr (see Fig. 8). According to Thiebold and
Illoul (1 967) t,he uptake of iodine by zone 7 is under the control of the
neural gland and rcmoviil of the gland increases the amount of iodine
taken up in II given time. Prolonged immersion in radioactive iodide
shows that iictive secretion of iodinated compounds takes place from
these cells and is carried up thr wall of the pharynx by the cilia of
zone 8 and the longitiidinal b t ~ s
of the pharynx. The cells of zone 8
and of the longitudinal bars also serrete smsll quantities of iodinated
compounds, b u t nowhere is the intensity of activity so great as it is in
zone 7. Thc relatively high conrcntrations of iodinated compounds
reported from the branchial ~ a c
by Suzuki and Kondo (1971) may be
due to the incorporation of portions of filtration membrane in their
homogenates. Histochemical tests show that the glandular secretions
of the endostylcx contain only relatively small quantities of acid mucopolysaccharides but are rich in protein, and Barrington and Thorpe
have suggested that not only would this account for the remarkable
filtering properties of the membrane, but that the iodine secreted by the
endostyle is associated with the production of a specialized protein
for the membrane. Thiehold (1972) deduced that the proteins secreted
by the glandular tracts of the endostyle arc iodinated at the apical
surfare of thc wlls of zone 7 and thnt while most of the resulting compound is passrd out of tho cndo.&plc ns the filtration membrane, some
of it is rcinrorporated into vcsides of zone 7 cells. Clearly there are
complex biocliemiral prorcsses being carried out at the level of zone 7
and these tertninatc in the production o f the filtration membrane.
The ascidian endostyle is the cvolutionary forerunner of the
vertebrate thyroid gland and it, in thereforr not surprir4ng that it
should be engaged in the secretion of iodinated compounds. However,
Barrington and Thorpe have found that the principal iodinated compounds in the endostyle are 3-mono-iodotyrosine and 3,5-di-iodotyrosine; there is no evidence for the presence of tri-iodothyronine and
only ver.y small amounts of thyroxine are identifiable (cj. also Suzuki
and Kondo, 1971). This is in marked contrast to the ammocoete larva
and to Brnnchiostornn (Amphioxus) both of which are known to produce
the hornional iodothyronines from the endostyle (see Barrington and
Thorpe, I!jMb, for rdierences). On the other hand, the protochordah
S&cc~loss.r~s only produces mono-iodotyrosine and this appears to be an
epidermal secretion (Barrington and Thorpe, 1963).
Thorpe et al., 1974) using l 3 I 1 and 1261 as tracers. Iodine is actively
taken up and bound in the (+ells of zone 7 of the endostyle, a simple
epithelium lying between the dorsal glandular tract and the ciliated
lip (zone 8 ) of the endostylr (see Fig. 8). According to Thiebold and
Illoul (1 967) t,he uptake of iodine by zone 7 is under the control of the
neural gland and rcmoviil of the gland increases the amount of iodine
taken up in II given time. Prolonged immersion in radioactive iodide
shows that iictive secretion of iodinated compounds takes place from
these cells and is carried up thr wall of the pharynx by the cilia of
zone 8 and the longitiidinal b t ~ s
of the pharynx. The cells of zone 8
and of the longitudinal bars also serrete smsll quantities of iodinated
compounds, b u t nowhere is the intensity of activity so great as it is in
zone 7. Thc relatively high conrcntrations of iodinated compounds
reported from the branchial ~ a c
by Suzuki and Kondo (1971) may be
due to the incorporation of portions of filtration membrane in their
homogenates. Histochemical tests show that the glandular secretions
of the endostylcx contain only relatively small quantities of acid mucopolysaccharides but are rich in protein, and Barrington and Thorpe
have suggested that not only would this account for the remarkable
filtering properties of the membrane, but that the iodine secreted by the
endostyle is associated with the production of a specialized protein
for the membrane. Thiehold (1972) deduced that the proteins secreted
by the glandular tracts of the endostyle arc iodinated at the apical
surfare of thc wlls of zone 7 and thnt while most of the resulting compound is passrd out of tho cndo.&plc ns the filtration membrane, some
of it is rcinrorporated into vcsides of zone 7 cells. Clearly there are
complex biocliemiral prorcsses being carried out at the level of zone 7
and these tertninatc in the production o f the filtration membrane.
The ascidian endostyle is the cvolutionary forerunner of the
vertebrate thyroid gland and it, in thereforr not surprir4ng that it
should be engaged in the secretion of iodinated compounds. However,
Barrington and Thorpe have found that the principal iodinated compounds in the endostyle are 3-mono-iodotyrosine and 3,5-di-iodotyrosine; there is no evidence for the presence of tri-iodothyronine and
only ver.y small amounts of thyroxine are identifiable (cj. also Suzuki
and Kondo, 1971). This is in marked contrast to the ammocoete larva
and to Brnnchiostornn (Amphioxus) both of which are known to produce
the hornional iodothyronines from the endostyle (see Barrington and
Thorpe, I!jMb, for rdierences). On the other hand, the protochordah
S&cc~loss.r~s only produces mono-iodotyrosine and this appears to be an
epidermal secretion (Barrington and Thorpe, 1963).