1. The gut of adult rainbow trout, Salmo galrdnerl, was studied by
dissection, scanning electron microscopy, light microscopy and
transmission electron microscopy
2. The mucosa exhibited the following topographical characteristicss
longitudinal ridges in the oesophagus and stomach, villi in the
intestine, fine longitudinal ridges in the caeca and annular folds
in the rectum, arranged like a stack of caudally-directed
funnels starting from the intestino-rectal valve to the vent.
Detailed scanning KM studies revealed ta3te pores in the anterior
oesophagus and the sculpting of the luminal surface of the surface
cells into microridges with complicated patterns. The surfaces
of the posterior oesophagus and stomach were demarcated into
polygons by rows of 3tubby microvilli - each polygon representing
the luminal surface of an epithelial cell. Each rectal fold
consisted of a smooth, caudally-directed apex and a base which was
supported by perpendicular buttress-like secondary folds.
3. The gut wall consisted of mucosa, submucosa, iausculari3 externa
and serosa, showing appropriate regional differences. A
muscularis mucosae was present only in the stomach and rectum,
a subraucosa only in the oesophagus and stomach, while the
muscularis externa consisted of a circular layer of striated
muscle in the oesophagus, and two coats of smooth muscle in the
remainder of the gut.
4. The oesophageal epithelium showed a progressive posterior
reduction in height, ranging from an anterior stratified
epithelium to a simple columnar mucous epithelium in the distal
third. The epithelium was associated with globular mucous cells
and taste buds in the anterior portion, acinar mucous glands in
the middle portion, and tubular serous glands in the distal third.
5. Cytological features of the columnar mucous cells lining the
surface and pits of the gastric mucosa, oxyntic cells lining the
glands and the endocrine cells of the gastric mucosa were studied
Microfilaments were prominent in the basal cytoplasm of the
surface cells, where they formed annular bands around the
convolutions of the basal lamina complex, which Impressed the
basal plasma membrane. Release of the mucous vesicles was
achieved by exocytosis. The oxyntic cells possessed apical and
basal microvillous processes, a well-developed tubulo-vesicular
system, zymogen granules, extensive granular endoplasmic
reticulum and many large mitochondria. When stimulated by
distension of the stomach, the apical cytoplasm was converted
into a labyrinth of cytoplasmic processes, while annular lamellae
each of which showed a short peripheral linear density,
appeared in the basal cytoplasm. The endocrine cells showed
such apical modifications as microvilli, cilia, centrioles, small
lucent vesicles, microtubules and a reduced glycocalyx covering.
Three types were distinguished on the basis of their granular
morphology.
6. The absorptive cells of the intestine, caeca and rectum were
columnar, bore apical microvilli and possessed lamellar
vi
structures la their supra-nuclear and basal cytoplasm. Active
cells of the intestine contained accumulations of fat particles
in the Golgi zone, within the lamellar structures and in the
intercellular spaces. Even larger accumulations were observed
in the lamellar structures and intercellular spaces of the caecal
epithelium. The rectal cells displayed tubular invaginations of
the plasma membrane between the microvilli, and many vacuoles in
their supra-nuclear cytoplasm.
Other cells present included endocrine cells, of which two types
were distinguished, goblet cells, pear-shaped cells and intra¬
epithelial leucocytes.
7. The stratum compactum was composed of densely-packed collagenous
fibrils arranged in layers. The fibrils within each layer were
unidirectional and alternated within adjacent layers, resulting
in an overall plywood or orthogonal pattern. Highly active
fibroblasts were associated with its internal surface. Granule
cells were associated generally with the connective tissue of
the gut wall, but the majority were arranged in a definitive
layer, the stratum granulesum. Whatever location however, they
maintained physical contact with fixed connective tissue cell
types, referred to here as 'ensheathing1 cells. Phospholipid,
acid mucopolysaccharides and the activities of alkaline
phosphatase, acid phosphatase, arylsulphatase, 5-nacleotidase and
proxidase were demonstrated in the granule cells.
8. The oesophageal serosa contained vagal nerve trunks, which
became incorporated in the myenteric plexus of the stomach.
Each trunk was an encapsulated unit containing nerve cell bodies,
myelinated and unmyelinated axons and supporting cells. The same
elements, with the exception of myelinated axons, constituted the
myentaric plexus outside the trunks in the stomach, and in the
remaining parts of the gut. The submucous plexus consisted of
two tiers in the stomach and only one tier in the post-gastric
gut. Contrary to reports in the literature, nerve cell bodies
were demonstrated in this plexus. Six types of nerve cell bodies
and three groups of axon profiles were distinguished on the
basis of their ultrastructure. "En-grappe" and "en-plaque"
types of motor end-plates were demonstrated in the muscularis
externa of the oesophagus. Amongst the smooth muscle generally,
that of the stomach was most sparsely innervated, while that of
the rectal muscularis mucosae showed the densest innervation of
all, at the same time, containing nerve cell bodies.
9. The co-existence of intraluminal and intracellular digestion of
proteins is discussed in relation to diet and the presence of the
stratum compactura. The stratum compactum, 'ensheathing' cells
and granule cells are hypothesised as constituting part of the
bodily defence system. The demonstration of nerve cell bodies
in the submucous plexus is 3hown to invalidate the existing schema
for telaost gut innervation
