THE Pl€YHIOIA)C)Y OF ASCIDIAhS
111
bouring blood sinun not only by its own basement membrane but alsu
by an outer connective lamella i i i which fine fibres may be discerned.
Swan and C h o n reported cells, in the lamella, containing numerous
membrane bound granules with u mean diameter of about 300 mp, and
small aggregations of ribosome-like partides. They considered that
these cells might be secretory.
There is general agreement hetwcben Qcorges and Lam that the
intact epithcliid cclls arc about 5 p high with a rounded nucleus and are
held together laterrilly by tlesmoncmes. Basally the cell membrane
lacks folding, is not microvillous and is not associated with dense
aggregations of mitochondria &A might be expected in excretory cells.
There are numerou8 small vesicles in tho cell, some optically clear to the
electron microscopc, others more dense arid apparently enclosing fibrillar
materid. Similar vesicles were noted by Swan and Olsson who believed
that the contained material was due to phagocytosis. The Golgi-body is
relatively smull with a few flattened saccules and some small vesicles,
thus suggesting that the cell at this stage is not in active secretion.
The endoplasmic reticulum has irregular vesicles.
When the gland enters the compact phase the epithelial cells swell
and becoma vacwolated itnd by loosening of the desmonemes are
separated from one l~nother and freed into the lumen of the gland.
The content of the vticuo1c:s is uncertain but according to Lane (1971)
thiamine pyropliosphatase can b c s demonstrated in some of the Golgi
saccules aid Golgi vesicles while wid phosphutase is found in these as
well as in othcr vacuoles. The pwsenoe of acid phosphataac suggests a
lysosomal origin for at least somv of the vacuoles. Lane also reported
that in the highly vaciiolated phase distension of the rough endoplasmic
reticulum might indicate avtive protein synthenie producing hydrolytic enzymes in the vacuoles.
The evidence so far confirms the fact that the epithelium ‘of the
neural canal undergoes periodic changes from R simple cuboiddl state
(the reticulated phase of the gland) to an enlarged highly vacuolar
state (the compact phase of the gland) culminating in sloughing of the
epithelium and breakdown of the cells which must thus release in the
lumen of the gland the contents of their vesicles. Several authors
(P6r&, 1943; Godeaux, 1964a; Georges, 1971) report that large phagocytic cells, probably of vascular origin, also invade the lumen of the
gland at this time, passing through the connective lamella and insinuating themselves between the epithelial cells. Both PBrhs (1943) and
Georges (1071) have found u circadian rhythm in this cycle but while
PBr&s found a single cycle in 24 hours in the Mediterranean, korges
I
111
bouring blood sinun not only by its own basement membrane but alsu
by an outer connective lamella i i i which fine fibres may be discerned.
Swan and C h o n reported cells, in the lamella, containing numerous
membrane bound granules with u mean diameter of about 300 mp, and
small aggregations of ribosome-like partides. They considered that
these cells might be secretory.
There is general agreement hetwcben Qcorges and Lam that the
intact epithcliid cclls arc about 5 p high with a rounded nucleus and are
held together laterrilly by tlesmoncmes. Basally the cell membrane
lacks folding, is not microvillous and is not associated with dense
aggregations of mitochondria &A might be expected in excretory cells.
There are numerou8 small vesicles in tho cell, some optically clear to the
electron microscopc, others more dense arid apparently enclosing fibrillar
materid. Similar vesicles were noted by Swan and Olsson who believed
that the contained material was due to phagocytosis. The Golgi-body is
relatively smull with a few flattened saccules and some small vesicles,
thus suggesting that the cell at this stage is not in active secretion.
The endoplasmic reticulum has irregular vesicles.
When the gland enters the compact phase the epithelial cells swell
and becoma vacwolated itnd by loosening of the desmonemes are
separated from one l~nother and freed into the lumen of the gland.
The content of the vticuo1c:s is uncertain but according to Lane (1971)
thiamine pyropliosphatase can b c s demonstrated in some of the Golgi
saccules aid Golgi vesicles while wid phosphutase is found in these as
well as in othcr vacuoles. The pwsenoe of acid phosphataac suggests a
lysosomal origin for at least somv of the vacuoles. Lane also reported
that in the highly vaciiolated phase distension of the rough endoplasmic
reticulum might indicate avtive protein synthenie producing hydrolytic enzymes in the vacuoles.
The evidence so far confirms the fact that the epithelium ‘of the
neural canal undergoes periodic changes from R simple cuboiddl state
(the reticulated phase of the gland) to an enlarged highly vacuolar
state (the compact phase of the gland) culminating in sloughing of the
epithelium and breakdown of the cells which must thus release in the
lumen of the gland the contents of their vesicles. Several authors
(P6r&, 1943; Godeaux, 1964a; Georges, 1971) report that large phagocytic cells, probably of vascular origin, also invade the lumen of the
gland at this time, passing through the connective lamella and insinuating themselves between the epithelial cells. Both PBrhs (1943) and
Georges (1071) have found u circadian rhythm in this cycle but while
PBr&s found a single cycle in 24 hours in the Mediterranean, korges
I