1063-0740/04/3005- © 2004
Russian Journal of Marine Biology, Vol. 30, No. 5, 2004, pp. 314–322.
Original Russian Text Copyright © 2004 by Biologiya Morya, Mashanov, Frolova, Dolmatov.
serves as a
model organism for studies of gut reparative regenera-
tion . However, prior to studying regeneration, one
should have obtained a profound knowledge of the
morphofunctional organization of the digestive tract in
intact animals, since this knowledge is necessary for an
adequate understanding of the mechanisms of repair.
Histological examinations of the gut wall in other
holothurian species showed signiﬁcant interspeciﬁc
variation in the cellular composition of the digestive
epithelium and in the interrelationships among the tis-
sue layers [2, 3, 7, 12–14]. It would also be of consid-
erable interest to compare the structure of the digestive
tube in adults with that in the previously studied juve-
. The aim of this study was to
characterize the structure of the alimentary canal in the
using electron microscopy
and histochemical techniques.
MATERIALS AND METHODS
Adult individuals of
collected from Vostok Bay (Sea of Japan). For light
microscopy, the parts of the digestive tract were ﬁxed in
Bouin’s ﬂuid, embedded in parafﬁn wax, and cut into
sections 5 to 6
m thick. For routine histology, the sec-
tions were stained with Carazzi’s haematoxylin fol-
lowed by eosin, methylene blue, Mallory’s azocarmin,
or toluidine blue. The histochemical staining proce-
dures were as follows: potassium periodate with
Schiff’s reagent for neutral polysaccharides; fast green
at pH 2.2 for proteins; alcian blue at pH 1 and pH 2.5
for glycosaminoglycans; and sudan black for lipids .
For transmission electron microscopy, the tissues
were ﬁxed in 2.5% glutaraldehyde in seawater for 1 h at
C and then postﬁxed for 1 h with 1% OsO
. The spec-
imens were decalciﬁed with 1% ascorbic acid ,
dehydrated in a graded series of ethanol and acetone,
and embedded in a mixture of araldite and upon 812
resins. The sections were obtained with an Ultracut E
(Reichert) ultramicrotome. Semithin sections were
stained with 1% methylene blue. Ultrathin sections
were stained with 0.5% aqueous uranyl acetate fol-
lowed by Reynolds lead citrate, and examined in a Jeol
JEM-100 B transmission electron microscope.
As in other holothurian species, the digestive tube in
consists of a pharynx that passes
through the aquapharyngeal bulb, a short esophagus, an
enlarged stomach, a bulbus, three parts of an intestine
(i.e., a ﬁrst descending part, an ascending part, and a
second descending part), and an enlarged cloaca (Fig. 1).
The gut wall consists of three layers: an inner digestive
epithelium, a middle connective tissue layer, and an
outer mesothelium (coelomic epithelium). The height
and structure of each layer vary along the digestive tube.
The pharyngeal digestive epithelium is covered by a
thin (about 0.2
m) cuticle and, together with the
underlying connective tissue, forms small folds. Folded
areas of the inner surface alternate with ﬂat regions.
The predominant cell type in the epithelium is the
Structure of the Digestive Tube in the Holothurian
V. S. Mashanov, L. T. Frolova, and I. Yu. Dolmatov
Institute of Marine Biology, Far East Division, Russian Academy of Sciences,
Vladivostok, 690041 Russia
Received November 26, 2003
—In the holothurian
the gut wall exhibits trilaminar organization. It consists
of an inner digestive epithelium, a middle layer of connective tissue, and an outer mesothelium (coelomic epi-
thelium). The pharynx, esophagus, and stomach are lined with a cuticular epithelium composed of T-shaped
cells. The lining epithelium of the intestine and cloaca lacks a cuticle and consists of columnar vesicular entero-
cytes. Mucocytes are also encountered in the digestive epithelium. The connective tissue layer is composed of
a ground substance, which houses collagen ﬁbers, amoebocytes, morula cells, and ﬁbroblasts. The gut mesothe-
lium is a pseudostratiﬁed epithelium, which is dominated by peritoneal and myoepithelial cells and also
includes the perikarya and processes of the neurons of the hyponeural plexus and vacuolated cells.
digestive tube, ultrastructure, histochemistry, holothurian