Chapter 3 (page 15)

The Pyloric Sphincteric Cylinder in Health and Disease

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Chapter 3 (page 15)

Development of musculature

According to Torgersen (l949) the developmental anatomy of the pyloric region is related to asymmetrical development of the viscera. The fact that the circular muscle fibres of the canalis egestorius radiate fan-like from the lesser curvature to embrace the greater curvature, is essentially an expression of structural and topographical asymmetry of the stomach.

Welch (l921) studied the development of the musculature of the stomach in the foetus and in newborn infants. The stomach first appeared as an expansion of the primitive gut at a foetal crown-rump length of 6.0 mm. The first evidence of stratification of the gastric wall was seen at this stage. At 11.0 mm differentiation of the layers had begun and three primitive layers, namely entodermal epithelium, mesenchyma including a myoblastic layer, and peritoneal epithelium were discernible; the myoblasts were seen to be arranged circularly. At l7.0 mm there was further condensation of the mesenchyma with actual transformation to circular fibres. Welch stated that the circular layer was complete at 24.0 mm and was definitely thickened over the pylorus. At 33.0 mm the gastric wall showed a general increase in circular fibres. The 41.0 mm embryo showed a high degree of organization with the circular fibres becoming continuous. At 65.0 mm there was a well formed circular coat with a thickness of approximately 0.01 mm; at the pyloric "sphincter" the thickness of the circular layer was 0.03 cm.

At birth the circular layer was well developed and constituted the major part of the gastric musculature. Its fibres were arranged in parallel rings approximately at right angles to the lumen, the rings anastomosing freely with each other. During the first year of life a marked thickening of the circular layer occurred.

Unlike the circular layer, which showed a simultaneous differentiation over the entire stomach, the longitudinal coat first appeared as discrete, scattered bundles at the l7.0 mm stage. Compared with the circular layer, its subsequent development was much delayed. At the 41.0 mm stage there was an increase in the number and distribution of the groups of muscle cells, this being particularly apparent at the pylorus, where an intermingling of cells of the longitudinal and circular layers was seen. At 65.0 mm a layer of muscle was evident.

Welch (l92l) stated that the development of the longitudinal coat was not complete until the first year of postnatal life; after birth there was an increase in these fibres. At the "pyloric antrum" the longitudinal fibres converged to form a complete cylinder.

Welch (l921) found that the oblique fibres became separated from the circular layer at the 24.0 mm crown-rump stage. At 33.0 mm they were seen to continue almost to the pylorus. In the 65.0 mm embryo they formed a band which terminated by joining the circular layer near the greater curvature. In the newborn the oblique fibres extended to the pylorus and in some cases reached the proximal portion of the "sphincter of the pylorus". On the lesser curvature some of the oblique fibres invariably joined the circular layer.

The muscularis mucosae was identified at the 65.0 mm stage, although it was still incomplete.

According to Welch, Forsell's description of the musculature of the adult stomach resembled the appearances seen in the foetus and newborn infant. The stomach of the newborn is characterized by a very thick circular layer; the longitudinal layer is more continuous than it is in the foetus, and invests the entire organ. The oblique fibres extend to the proximal part of the "sphincter of the pylorus". At birth, due to swallowing of air and feeds, a marked dilatation of the stomach, with a general thinning of the musculature, occurs.

Discussion

The anatomists Cunningham (1906), Forsell (l913), Welch (l921), Cole (l928) and Torgersen (l942) showed that the muscularis externa in the distal 3.0 to 4.0 cm of the stomach is thicker than that in the remainder of the stomach. The region involved is longer on the greater than on the lesser curvature, i.e. it has a roughly triangular or fan- like shape. It was called the pyloric sphincteric cylinder by Cunningham (l906), the canalis egestorius by Forssell (l913) and Torgersen (l942), and the fan-shaped muscle by Cole (l928). At its aboral end an additional, ring-like thickening of this muscular cylinder forms the muscular part of the pyloric ring; the ring is not a separate anatomical structure, its musculature being an inherent part of the cylinder. At its oral end the musculature of the cylinder merges imperceptibly into that of the remainder of the stomach.

Normally the entire pyloric sphincteric cylinder contracts in a concentric or systolic way, with obliteration of the lumen, to form a tightly contracted canal approximately 2.0 to 3.0 cm in length. In this way the entire cylinder acts as a sphincter, closing the whole length of the pyloric canal against the entrance of luminal contents from the proximal stomach. Contraction of the cylinder results in formation of the pyloric canal, which is a temporary, physiological structure to be differentiated from the pyloric aperture.

Forssell (l913) and Torgersen (l942) showed that the circular musculature of the canalis egestorius (i.e. the pyloric sphincteric cylinder) is arranged into a system of rings or loops. The right canalis loop is the muscular part of the pyloric ring. The left canalis loop is located at the oral end of the cylinder; it is less well developed than the right and corresponds to the sulcus intermedius on the greater curvature. The two loops meet and interlace on the lesser curvature in a muscle torus or knot, from which they diverge to encircle the greater curvature. The loops are connected by intervening curcular as well as by overlying longitudinal fibres; many of the latter dip into the right canalis loop.

Cunningham (l906), Forssell (l913), Welch (l921) and Torgersen (l942) looked upon the sphincteric cylinder as an anatomically preformed structure. Torgersen (l942) termed it an anatomical reality with a sound foundation in comparative anatomy.