T H E I’tlYSIOI~OUY O P ASCIDlINY
15
to be thinner arid tougher than those of’ the phlebobranchiates. The
differenccs between phlebobranchiates and stolidobranchiates are
further emphasized in studics o i l gross cahemical composition in four
Hpecies of’asciciitbii (Smith mid I)chnrl, 1!)71 ). In tlwo species of phlebobranchiate thc “ tunirin ’’ frtwt ioit yieldcd approximately I 00% of it8
dry weight ax c*ellulosc: equivulrilts as opposed to 50% equivalents in two
species of st,olidobranchiates. The tmino-acid composition in the two
groups is similar and Hugpsts t h t the protein fraction of the test may
be uniform throughout thcb Ascidiacca. If this in the case the differing
physical properties of the rtsciditrn test rnay be attributable to differences
in carbohydrato composition arid watw content. A comparative ,study
of the architecture and mechaiiioal properties of‘the test would be of
great intorest.
The carbohydrate material for test formation appears to be secreted
primarily from the epithelium of the mantle wall, the cells of which have
an extensive development of endoplasmic reticulum and Golgi bodies.
Radio-trtwer studies with liibellcd glucose show that the carbohydrate
is accumulating in this region and passing out to the test (Deck et al.,
1966; Wardrop, 1970). In Didemnidae the epidermal cells of the
terminal bulbs of the stolonic vessels also secrete test materid (Millar,
1951) and this rnay occur in all nscidiann which possess such vascular
bulbs.
The role of morula cells, whether vanadocytcs or ferrocytes,
remains uneertairi but there is sufficiont cvidence to show that their
primary role is c*oncerned with trst formation. Most workers are
agreed that morula cells migratcb into t h e tcnt, either directly through
the mantle wall or through the cbpithclial wiills of test vessels, and that
they discharge their vacuolar contents and finally disintegrate in the
test matrix (see especially Emlean, 195Ac, 1961 ; Deck et al., 1966;
Wardrop, 1970; Smith, 1970a). Entlcaii and Smith have both demonstrated that the moriila cells arc ae.qociated with sites of fibre production. Endean believed that the rnorula cells actively secreted the fibres
but this is probably incorrect. The evidence from tracer studies that
carbohydrate is secreted by the mantle and the chemical evidence on the
nature of the rontents of the morula cell both suggest a different role
for these cells. The metal chromogen in the morula cells is a powerful
reducing substance maintained in the reduced state by strong sulphuric
acid (see p. 75). When the vesicles of the cell are discharged in the test
and the chrornogen is freed from the iicid these reducing properties will
berornc effrctivr. The fact that fibres ttre produtvd at the same time
suggests that the reducing propcrticBs tiw in mine way used to polymerize thr ciirbohydrrites in th(1 t(*st t m forrri r~ fihril of ti~niciii.
15
to be thinner arid tougher than those of’ the phlebobranchiates. The
differenccs between phlebobranchiates and stolidobranchiates are
further emphasized in studics o i l gross cahemical composition in four
Hpecies of’asciciitbii (Smith mid I)chnrl, 1!)71 ). In tlwo species of phlebobranchiate thc “ tunirin ’’ frtwt ioit yieldcd approximately I 00% of it8
dry weight ax c*ellulosc: equivulrilts as opposed to 50% equivalents in two
species of st,olidobranchiates. The tmino-acid composition in the two
groups is similar and Hugpsts t h t the protein fraction of the test may
be uniform throughout thcb Ascidiacca. If this in the case the differing
physical properties of the rtsciditrn test rnay be attributable to differences
in carbohydrato composition arid watw content. A comparative ,study
of the architecture and mechaiiioal properties of‘the test would be of
great intorest.
The carbohydrate material for test formation appears to be secreted
primarily from the epithelium of the mantle wall, the cells of which have
an extensive development of endoplasmic reticulum and Golgi bodies.
Radio-trtwer studies with liibellcd glucose show that the carbohydrate
is accumulating in this region and passing out to the test (Deck et al.,
1966; Wardrop, 1970). In Didemnidae the epidermal cells of the
terminal bulbs of the stolonic vessels also secrete test materid (Millar,
1951) and this rnay occur in all nscidiann which possess such vascular
bulbs.
The role of morula cells, whether vanadocytcs or ferrocytes,
remains uneertairi but there is sufficiont cvidence to show that their
primary role is c*oncerned with trst formation. Most workers are
agreed that morula cells migratcb into t h e tcnt, either directly through
the mantle wall or through the cbpithclial wiills of test vessels, and that
they discharge their vacuolar contents and finally disintegrate in the
test matrix (see especially Emlean, 195Ac, 1961 ; Deck et al., 1966;
Wardrop, 1970; Smith, 1970a). Entlcaii and Smith have both demonstrated that the moriila cells arc ae.qociated with sites of fibre production. Endean believed that the rnorula cells actively secreted the fibres
but this is probably incorrect. The evidence from tracer studies that
carbohydrate is secreted by the mantle and the chemical evidence on the
nature of the rontents of the morula cell both suggest a different role
for these cells. The metal chromogen in the morula cells is a powerful
reducing substance maintained in the reduced state by strong sulphuric
acid (see p. 75). When the vesicles of the cell are discharged in the test
and the chrornogen is freed from the iicid these reducing properties will
berornc effrctivr. The fact that fibres ttre produtvd at the same time
suggests that the reducing propcrticBs tiw in mine way used to polymerize thr ciirbohydrrites in th(1 t(*st t m forrri r~ fihril of ti~niciii.