Revista chilena de anatomía
Print version ISSN 0716-9868
Rev. chil. anat. vol.19 n.3 Temuco Dec. 2001
MORPHOLOGIC CONSIDERATIONS ABOUT THE WALL OF THE
GLANDULAR STOMACH OF YOUNG RABBITS (Oryctolagus cuniculus)
* Departament of Anatomy, Universidade Paranaense, Cascavel, Paraná, Brazil.
** Departament of Morphophysiological Science, Universidade Estadual de Maringá, Paraná, Brazil.
*** Departament of Zootecnia, Universidade Estadual de Maringá, Paraná, Brazil.
SUMMARY: The morphology of the stomach may vary between species as a function of feeding habits, differences on the cellular composition of the mucosa of the stomach wall and the different functions it carries out. Morphofunctional variations of the stomach of several animals along phylogeny led us to investigate the morphology of the wall of the glandular stomach of rabbits. We used the stomachs of 32 young rabbits (Oryctolagus cuniculus) from the White New Zealand strain, coming from the Experimental Farm of Iguatemi, owned by the State University of Maringá, Brazil. The stomachs were collected and fixed in Bouin solution; sections of 7 and 12 µm were made from the greater to the lesser gastric curvatures, and subjected to histological routine, for staining with either Hematoxilin-Eosin, P.A.S. or Azan. Next the sections were analyzed under microscope and photographed. Differences in the stratigraphy of the gastric wall were observed when the sections from the greater gastric curvature of the glandular stomach of rabbits were compared with those from the lesser curvature. We verified that the wall of the glandular stomach of rabbits shows differences on the morphology of the mucosa, submucosa, muscle and serosa layers, so that the wall is not uniform around the stomach circumference.
KEY WORDS: 1. Stomach; 2. Glandular stomach, 3. Rabbit.
The stomach wall has all the layers of a typical tubular organ. For mammals it is described the mucosa, composed of simple cylindrical epithelium, lamina propria with numerous tubular glands and the muscularis mucosae; the submucosa network containing collagen fibers, adipose cells and blood vessels; the outer muscle layer possessing three layers with different orientations of the smooth muscle fibers: an inner oblique layer, an intermediate circular layer and an outer longitudinal layer; and the serosa composed of mesothelium over a layer of loose connective tissue. These tunicas are innervated by intramural nerve plexuses and cholinergic fibers from the parasympathetic, and adrenergic fibers from the sympathetic systems (Ham, 1983; Junqueira & Carneiro, 1999).
According to Getty (1986), the stomach morphology may vary between species because of feeding habits, this distinction occurring specially in the mucosa, which allows the classification of the stomach as glandular or aglandular. A stomach completely lined by a glandular mucosa with a single layer of cylindrical epithelium is found in carnivores, rodents and in man; stomachs devoid of glands and lined by a pavimentous stratified epithelium are present in the first half of the stomach of swine, horses, ruminants and rats.
The differences in the cellular composition of the mucosa along the stomach wall and the different functions it carries out allows a morphofunctional division of the stomach (Grossman, 1958).
The stomach of rabbits is similar to a thin-walled pouch and has similarities of external morphology with the stomach of rats and men. The secretions of the stomach, coming from glands in the wall, are chloride acid, digestive enzymes such as pepsin, and mucine. The stomach of rabbits acts as a storage organ regulating the delivery of food to the small intestine. In these animals the stomach is never completely empty, so that after a 24-hour fast it still contains food (Griffiths & Davies, 1963). The pH of the stomach content of rabbits, as compared to others, is remarkably low.
The morphofunctional variations of the rabbit stomach, led us to investigate whether the microscopic morphology of the tunicas of the glandular stomach of rabbits is also similar to that of other rodents and man. This morphologic analysis took into consideration the glandular region extending from the greater to the lesser gastric curvatures.
MATERIAL AND METHOD
It was used the stomachs of 32 young (50-75 days old) rabbits (Oryctolagus cuniculus) of the White New Zealand strain, coming from the Cuniculture Sector of the Experimental Farm of Iguatemi, owned by the State University of Maringá. These animals were fed with diet composed of corn, wheat and soy flour, alfalfa and oat hay, and supplementation of minerals and vitamins, according to the requirements for growing rabbits (Lebas, 1989).
During laparotomy, the stomachs were collected and opened at the greater gastric curvature, washed in tap water, and fixed in Bouin solution for 24 hours. After displacement to 70% ethylic alcohol, transverse samples of the glandular stomach (body) spanning from the greater to the lesser gastric curvatures, were removed following the histological routine for paraffin sections.
The staining techniques employed were Hematoxilin-Eosin and P.A.S. (Periodic Acid of Schiff counter-stained with hematoxilin) in the 7 µm-thick sections and Azan, in the 12 µm-thick sections.
The laminae were analyzed under optic microscope and the photographic documentation was carried out in an Olympus BX50 Photomicroscope.
The organization of the stomach wall of rabbits mantains the stratigraphy made up of: mucosa, sub-mucosa, muscle layer and serosa. Differences in this pattern were observed when the sections from the greater gastric curvature were compared with those from the lesser curvature (Fig. 1A and B).
|Fig. 1. 12-µm transverse section of the stomach wall of rabbit showing in A) greater gastric curvature and in B) lesser gastric curvature. Observe the mucosal infoldings in A and their absence in B. Mucosa (c), submucosa (b), muscle layer (a) and serosa (arrow). AZAN, 60X. Fig. 1. 12-µm transverse section of the stomach wall of rabbit showing in A) greater gastric curvature and in B) lesser gastric curvature. Observe the mucosal infoldings in A and their absence in B. Mucosa (c), submucosa (b), muscle layer (a) and serosa (arrow). AZAN, 60X.|
In the greater gastric curvature the mucosa shows well-developed folds accompanied by evaginations of the submucosa (Fig. 1A); in the lesser gastric curvature the folds gradually disappear and give way to a characteristic mucosa (Fig. 1B).
This tunica, regardless of the region, had clearly outlined regions of gastric pits lined by simple cylindrical epithelium with mucus-secreting cells, and tubular gastric glands widely spread where the cell types showed uneven distribution, thus forming distinct regions (Fig. 2A and B).
|Fig. 2. 7-µm transverse section of the stomach wall of rabbit. A) Mucosa with gastric pits (arrow). H.E. 306X B) Distribution of the cells in the mucosa. P.A.S. 306X.|
Just below the gastric pits it was noted a region of small thickness having greater incidence of parietal acid-secreting cells, and next a region of parietal cells, mucous neck cells, chief cells, enteroendocrine cells and stem cells. The mucous cells observed in the gastric glands showed greater intensity in the P.A.S. reaction than those found at the glandular base (Fig. 2B). The muscularis mucosae was reduced both in the greater and in the lesser gastric curvatures.
Under the Azan staining it was possible to observe the bundles of collagen fibers and the blood vessels arranged differently in the regions. In the greater gastric curvature the collagen fibers ran to the apex of the folds, the blood vessels were large and were located at the basal portion of the submucosa (Fig. 1A). In the lesser gastric curvature the collagen fibers were irregularly arranged below the mucosa with shallow folds and the blood vessels were small (Fig. 1B).
As for the muscle tunica of the stomach, we verified that in the lesser gastric curvature it had three layers of smooth muscle fibers: an inner oblique, an intermediate circular and an outer longitudinal layer, with many collagen fibers among them and large blood vessels as well. In the greater gastric curvature the smooth muscle fibers formed only two layers, an inner circular and an outer longitudinal layer (Fig. 3A and B). This change occurs gradually from one region to the other so that it is possible to note a transitional region (Fig. 1A).
|Fig. 3. 12-µm transverse section of the stomach wall of rabbit. Observe muscle layer. A) greater gastric curvature and B) lesser gastric curvature. Oblique layer (a), circular layer (b), and longitudinal layer (c). AZAN, 151X.|
The serosa from the greater gastric curvature was reduced, containing few collagen fibers lined by simple pavimentous epithelium, while in the lesser curvature this tunica was well developed and exhibiting many adipose cells and large blood vessels. These cells were permeated by many collagen fibers lined by simple pavimentous epithelium (Fig. 1A and B).
Also composing the stomach wall in both curvatures there were the ganglia of the submucous plexus in the submucosa tunica and of the myenteric plexus between the layers of the muscle tunica.
The stomach of rabbits bears similarities to those of rodents and man concerning its external morphology, but the similarity no longer holds for the stomach wall, which can vary between species and according to feeding habits, this distinction being more clearly seen in the mucosa (Getty). Nevertheless, we verified that, in a single species, a regional morphological variation in the stomach wall may occur.
Despite the typical organization of the tunicas in the glandular stomach fitting the classical descriptions (Dellmann & Brown, 1982 and Banks, 1991; Junqueira & Carneiro), our results showed that the glandular stomach of rabbits must have its morphology described in terms of the greater and the lesser gastric curvatures due to the differences observed.
As for the type of epithelium, the uneven distribution of the cells forming the gastric pits and the glands present in the lamina propria, there is no difference between the regions, in this way conforming to the descriptions found in the literature. Thus, in rabbits as in rodents and man, the mucosa is characterized by a simple cylindrical epithelium in the region of the gastric pits containing mucus-producing cells. In the gastric glands we emphasize the presence of parietal cells, mucous neck cells, chief cells, enteroendocrine cells and stem cells.
Variations in the organization of these cells occur in swine, horses and ruminants, which have a stratified pavimentous epithelium in the first half of the stomach (Dellmann & Brown).
Nevertheless, the high level of mucosal infoldings in the greater gastric curvature as opposed to their almost complete absence in the lesser gastric curvature due to the deep invagination of the collagen fibers points to the necessity of describing the morphology of the organ in terms of the region observed, instead of making generalized assumptions for the stomach as a whole.
Well developed folds are found in the glandular stomach of cats and dogs, while pigs have less evident infoldings (Dellmann & Brown and Banks). These differences could be related to the degree of distension of the stomach; however, the observations were made in rabbits whose stomachs showed the same degree of distension.
Descriptions of the external muscle tunica invariably point to a three-layer organization of the smooth muscle: an inner oblique, an intermediate circular and an outer longitudinal layer (George et al. 1998 and Junqueira & Carneiro). We noted that this morphologic pattern is matched only by the lesser gastric curvature of rabbits.
According to George et al. fish, amphibians, reptiles and birds are the animals showing smooth muscle fibers disposed in a two-layer arrangement. Yet this description holds only for the greater gastric curvature in rabbits.
The serosa reveals the same behaviour. Clear morphologic differences between the two regions are seen, so that the descriptions of Dellmann & Brown, Banks, George et al. and Junqueira & Carneiro show similarities only with the greater gastric curvature of rabbits.
All the differences observed revealed the existence of a compartmentalization within the glandular stomach of rabbits, taking into consideration that the morphologies found in the greater and lesser gastric curvatures are involved in motility and gastric acid/enzyme secretion, functions carried out by this organ.
The studies made showed that the wall of the glandular stomach of rabbits have morphological differences in the mucosa, submucosa, muscle and serosa tunicas, when the greater and lesser gastric curvatures are analyzed, so that this organ does not have an even wall around the gastric circumference.
RESUMEN: La morfología del estómago puede variar de una especie a otra, en función de sus hábitos alimenticios, constitución celular de la mucosa de la pared estomacal y de las funciones realizadas por las células constituyentes. Variaciones morfofuncionales del estómago existentes entre los animales de la escala filogenética, nos llevaron a investigar la morfología de la pared del estómago glandular de los conejos. Utilizamos estómagos de 32 conejos (Oryctolagus cuniculus), jóvenes, raza Neozelandés Blanco, provenientes de la Estancia Experimental de Iguatemí, de la Universidad Estadual de Maringá, Brasil. Los estómagos fueron recolectados y conservados en solución de Bouin; se les practicaron cortes de 5 y 12 µm, alcanzando la región que va desde la curvatura gástrica mayor hasta la curvatura gástrica menor, verificación histológica de rutina y coloración con Hematoxilina-Eosina, P.A.S. y Azan. Se observaron las muestras a través del microscopio óptico y luego fueron fotografiados. Observamos diferencias de estratisgrafía en la pared del estómago al comparar los cortes practicados en las curvaturas gástricas mayor y menor del estómago glandular de los conejos. Verificamos que la pared de la región glandular del estómago de los conejos presenta diferencias con relación a la morfología de las túnicas mucosa, submucosa, muscular y serosa, no presentando, por lo tanto, pared uniforme a lo largo de la circunferencia estomacal.
PALABRAS CLAVE: 1. Estómago; 2. Estómago glandular; 3. Conejo.
Dirección para correspondencia:
Prof. Dra. Maria Raquel Marçal Natali
Universidade Estadual de Maringá.
Departamento de Ciências Morfofisiológicas.
Av. Colombo, 5790
Recibido : 18-06-2001
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