~ t r ~
PHYSIOI,OO(IY OF ASCIDIANS
27
Zone 6 is ti broad bmd of' secwtory cells and does not exhibit the
tnarked fan-like nrrltngcwient of z o m s 2 and 4. A few cilia also occur
on these cells and prwmaI)ly assist in latcral transport across the
surface. All authors nre ltgrccd that! the primary secretion from zone 6
is protein, but Orsi and Relini (1960b) and Thorpe et al. (1972) point
out that the cells are rich in tyrosine. According to Orsi and Relini it
also containa a large amount of tryptophan.
Zone 7 has low columnar aclls with an extensive rough endoplasmic
reticulum which produaos grnnulei of a secretion subsequently " reworked " by the Golgi-body (Gorbunova, 1971 ; Thorpe et al., 1972).
This is the site of iodine bindiiig asid secretion of iodinated compounds
into the endostyle and is discussed further in the next section.
From all of these details it, is p:)ssible to rc-consider the functional
role of the endostyle a8 tho sit(% of production of a complex iodinated
protein sheet which is passed out to the pharynx to serve as a filtration
membranc. There i~ only limitod cvidence of the secretion of mucopolysacchnridcs i d the ctwlier conorpt of a purely mucus-bared
membrane produced by the endostyle would appear to have been
incorrect. Nevertheless, the final tnembrane may be in the form of an
iodinated muco-protein. Thc primnry products appear to be produced
in the two lower secretory ZOIICH, 2 and 4, and these are mixed by the
flagella of zone 3 prior to being p s c d laterally to zonc 5. The fanshaped arrangement of cells in zones 2 and 4 probably ensures that
their secretions are concentrated at either margin of zone 3 and thereby
adequately mixed. This mixt lire must then b.? passed laterally by the
cilia of zone 5 to combine with furt'her protein secretions from zone 6
prior to the addition of iodinatcd compounds from zone 7. The mixture
is finally spread out by the combined activity of the flagella of zone 1
and the cilia of zone 8. The precise role of thct flagella of zone I i~ not
at all clear but they probably serve a dual role of maintaining: a Rteady
flow of material out of the endost,,ylar groove and keeping it from
" clogging ", while at the same time assisting in an even spread of
material to either side of the branchial sac.
v. IODINE BLNDING
The organic binding of iodine in animals is almost entirely due to
simple linkages with the amino acid tyrosine to form either 3-monoiodotyrosine (MIT) or 3,6-di-iodotyrosine (DIT). The iodine involved
in this linkage is derived from the oxidation of iodides taken from the
animal's environment. The iodotyrosines may be linked with other
amino acids .in the formation of scleroproteins such as gorgonin,
antipathinn and spongins and can be identified as such in hydrolymtes
PHYSIOI,OO(IY OF ASCIDIANS
27
Zone 6 is ti broad bmd of' secwtory cells and does not exhibit the
tnarked fan-like nrrltngcwient of z o m s 2 and 4. A few cilia also occur
on these cells and prwmaI)ly assist in latcral transport across the
surface. All authors nre ltgrccd that! the primary secretion from zone 6
is protein, but Orsi and Relini (1960b) and Thorpe et al. (1972) point
out that the cells are rich in tyrosine. According to Orsi and Relini it
also containa a large amount of tryptophan.
Zone 7 has low columnar aclls with an extensive rough endoplasmic
reticulum which produaos grnnulei of a secretion subsequently " reworked " by the Golgi-body (Gorbunova, 1971 ; Thorpe et al., 1972).
This is the site of iodine bindiiig asid secretion of iodinated compounds
into the endostyle and is discussed further in the next section.
From all of these details it, is p:)ssible to rc-consider the functional
role of the endostyle a8 tho sit(% of production of a complex iodinated
protein sheet which is passed out to the pharynx to serve as a filtration
membranc. There i~ only limitod cvidence of the secretion of mucopolysacchnridcs i d the ctwlier conorpt of a purely mucus-bared
membrane produced by the endostyle would appear to have been
incorrect. Nevertheless, the final tnembrane may be in the form of an
iodinated muco-protein. Thc primnry products appear to be produced
in the two lower secretory ZOIICH, 2 and 4, and these are mixed by the
flagella of zone 3 prior to being p s c d laterally to zonc 5. The fanshaped arrangement of cells in zones 2 and 4 probably ensures that
their secretions are concentrated at either margin of zone 3 and thereby
adequately mixed. This mixt lire must then b.? passed laterally by the
cilia of zone 5 to combine with furt'her protein secretions from zone 6
prior to the addition of iodinatcd compounds from zone 7. The mixture
is finally spread out by the combined activity of the flagella of zone 1
and the cilia of zone 8. The precise role of thct flagella of zone I i~ not
at all clear but they probably serve a dual role of maintaining: a Rteady
flow of material out of the endost,,ylar groove and keeping it from
" clogging ", while at the same time assisting in an even spread of
material to either side of the branchial sac.
v. IODINE BLNDING
The organic binding of iodine in animals is almost entirely due to
simple linkages with the amino acid tyrosine to form either 3-monoiodotyrosine (MIT) or 3,6-di-iodotyrosine (DIT). The iodine involved
in this linkage is derived from the oxidation of iodides taken from the
animal's environment. The iodotyrosines may be linked with other
amino acids .in the formation of scleroproteins such as gorgonin,
antipathinn and spongins and can be identified as such in hydrolymtes