1. The aim of the thesis was to contribute new knowledge on the
intestinal nerve plexuses of birds using the domestic fowl (Gallus
gallus) as the subject.
2. The literature on the histology of the intestinal nerve
plexuses was analysed and the gaps in our present knowledge of the
structure of the plexuses were outlined.
3. The detailed objectives were:
(a) to establish the appearance and distribution of
the enteric plexuses using both histochemical and empirical
(b) to provide quantitative data on the myenteric
plexus by estimating the number and size of the perikarya;
(c) to provide information on the fine structure of
the myenteric and submucosal plexuses;
(d) to interpret the findings in the light of the
available information on intestinal motility in birds.
4. The appearance and distribution of the nerve plexuses was
investigated in male and female, immature and adult birds, by means
of the cholinesterase and glyoxylic acid fluorescence histochemical
methods and the osmic acid and silver empirical methods. The nervous
tissue was examined in strip preparations, whole mount stretch
preparations and frozen sections.
(a) Cholinesterase-positive and fluorescent fibres were distributed
at all levels of the intestines as myenteric, submucosal, muscle,
mucosal and perivascular plexuses. The nerve cell bodies were
restricted to the myenteric and submucosal plexuses and were mainly
cholinesterase-positive. None of the perikarya was fluorescent.
Treatment with reserpine, nialamide and L-DOPA suggested that the
fluorescent fibres were probably adrenergic. The density of
innervation varied at different levels of the intestinal tract and
was best developed in the rectum.
(b) The myenteric plexus remained attached to the longitudinal
muscle layer and consisted of a primary meshwork of relatively thick
nerve bundles within which was a secondary meshwork of finer nerve
bundles. The appearance of the myenteric plexus varied along the
intestines, the primary meshwork being best developed in the rectum.
The perikarya occurred mainly in well-defined ganglia at the nodes
of the plexus, a small number of cells occurring in the internodal
nerve bundles and the nodes of the secondary meshwork. The majority
of the fluorescent fibres in the nodes were strongly fluorescent and
varicose, whereas in the internodal bundles most of the fibres were
weakly fluorescent and non-varicose. Pretreatment with NADH-Nitro BT
to stain the nerve cell bodies showed that the strongly fluorescent
varicose fibres formed dense pericellular networks around the majority
of the ganglion cells.
(c) The position of the submucosal plexus varied in the different
regions of the intestines. In the small intestine the plexus lay close
to the inner surface of the circular muscle layer, whereas in the rectum
and caeca the plexus was situated deeply within the submucosa. The
appearance of the submucosal plexus also varied in the different parts
of the intestines. Thus, in the small intestine and rectum the plexus
was arranged in one plane, whilst in the caecum it consisted of outer
and inner parts. The nerve cell bodies occurred in both the ganglia
and internodal bundles, but in the small intestine the perikarya were
almost equally distributed between the nodal and internodal regions.
Many fluorescent varicose fibres surrounded the non-fluorescent ganglion
cells. The number of the internodal varicosities in the submucosal
plexus appeared to be far less than in the myenteric plexus.
(d) The circular muscle layer, especially in the rectum, contained
a substantial number of cholinesterase-positive and fluorescent fibres.
The longitudinal layer of muscle was sparsely innervated except in the
rectum. The fluorescent fibres in the longitudinal muscle layer of
the small intestine and caeca were associated with blood vessels.
(e) The mucous membrane was innervated by fine cholinesterasepositive and fluorescent nerve bundles from the submucosal plexus.
These nerve fibres formed dense meshworks beneath the intestinal glands
and within the villi. None of the fibres entered the epithelium.
The muscularis mucosae was sparsely innervated.
(f) The intestinal arteries were accompanied by thick anastomosing
bundles of cholinesterase-positive fibres. A few nerve cell bodies
occurred along the course of the periarterial nerves. Thick bundles
of non-varicose fluorescent fibres also ran close to the arteries and
gave off fine strongly fluorescent varicose fibres which entered the
arterial wall. The intestinal veins were sparsely innervated.
(g) With the empirical staining methods the appearance and
distribution of the enteric plexuses was essentially similar to that
demonstrated by the histochemical methods. A wide variation in the
staining reactions of the enteric perikarya with the silver technique
was observed, argyrophobic and argyrophilic multipolar neurons and
argyrophobic unipolar neurons being demonstrated. The argyrophilic
nerve cell bodies corresponded to Dogiel's type I, II and III neurons.
(h) These observations were discussed in the light of the available
histological evidence of the innervation of the gut in birds and other
classes of vertebrates.
5. The number and size of the perikarya in the myenteric plexus
of three immature and three mature birds was estimated in strip and
whole mount stretch preparations using the histochemical technique for
detecting NADH-diaphorase activity. (a) In the chicks and adults the mean neuron density per cm and
the total number of the cells in each region increased distally. The
difference between the counts in adjacent regions were generally
significant. With age the neuron density per cm decreased and the
absolute number of neurons increased. The mean neuron density in
each region in the chick was two to three times higher than in the
adult. The neuron density per cm was significantly greater in the
mesenteric zone of the plexus. The calculated total number of cells
in the adult was significantly higher than in the chick.
(b) The size of the neurons varied in the different regions of
the intestines and increased with age. In both the chick and adult
the largest neurons were present in the distal part of the caecum.
Small-sized neurons were present in the chick and adult although they
were especially numerous in the chick.
(c) The differences in neuron density and estimated cell sizes
between the chicks and adults were discussed and compared with similar
data in other vertebrates.
6. The ultrastructure of the myenteric and submucosal plexuses
was investigated in male and female, immature and adult birds.
(a) The ganglia consisted of a dense neuropil consisting of nerve
cell bodies, myelinated and unmyelinated axons, Schwann cells and
satellite cells. At the outside of the ganglia was a basal lamina
and dense connective tissue containing fibroblasts, interstitial cells
and blood vessels. Whilst most of the nerve cell bodies were covered
by satellite cells, a part of some of them lay directly under the basal
lamina; (b) The perikarya displayed the basic structural features of nerve
cell bodies. The majority of them had a small number of randomlydistributed granular vesicles.
(c) Small and large axon profiles were identified. Small axon
profiles contained mainly microtubules and neurofilaments, whilst
larger profiles contained mainly granular and agranular vesicles.
Three types of varicosity were described. One type of varicosity
contained numerous small agranular vesicles which were sometimes
intermingled with medium-sized granular vesicles. This axon profile
was probably cholinergic. A second type of varicosity contained small
granular vesicles and small agranular vesicles and was probably
adrenergic. The third type of axon profile contained numerous small
agranular vesicles, many large granular vesicles and a few small
granular vesicles. The possibility was considered that this type of
varicosity was adrenergic. All three types of varicosity formed
typical motor synapses with the neurons. At the synaptic junction
only agranular vesicles were associated with the presynaptic membrane
of the axon.
(d) The structure of the Schwann cells and satellite cells was
essentially similar. The perikarya of the Schwann cells gave rise to
long, attenuated processes which ensheathed many axons. Structurally,
the interstitial cells resembled fibroblasts.
(e) The findings were discussed in relation to the available
ultrastructural information in other classes of vertebrates.
7. The difficulties in interpreting the present observations in
the light of the available information on intestinal motility were
outlined. The findings emphasized the urgent need for electro
physiological studies on the avian enteric plexuses.