Talk:Paper - The early stages of the development of the ileo-colic sphincter (1924)

THE EARLY STAGES OF THE DEVELOPMENT OF THE ILEO-COLIC SPHINCTER

By JOHN BEATTIE, M.B., B.Cu., M.Sc. The Department of Anatomy, Queen’s University of Belfast

A perusat of the text-books of anatomy on the ileo-colic region of the gut shows that there is a divergence of opinion on the manner of action of the ileo-colic valve. There are, it is evident, two views. In the first, it is considered that at the junction of the small and large intestines the arrangements constitute a mechanica] device, acting locally and automatically, to prevent regurgitation of the contents of the colon into the ileum. Some who uphold this view consider that the Valvula Coli acts much in the same way as the mitral valve of the heart, its two flaps being pressed together by the contents of the colon, and pulled together by the stretching of the frenula as the colon is filled (Birmingham, Cunningham); others regard the valve action to be due more to the obliquity of the entrance of the terminal ileum into the colon, in a manner analogous to the valve action of the intra-mural part of the ureter (Symington1, Waterson?). The second view, advanced chiefly by Keith’, regards the ileo-colic junction as a sphincteric mechanism which controls the exit of the ileal content and depends for its efficiency on a sphincter muscle surrounding the opening. Rutherford‘ has brought forward a series of observations on the histological structure of the junction in the human adult which supports this view and Elliott ® has worked out the innervation of the sphincteric band in the cat, which also confirms it. There may also be said to be a third view, a combination of the first and second. In it, it is held that while a sphincter muscle is present, the efficiency of the junction is enhanced by the arrangement of its parts (Robinson, Howden). This view is, in all probability, the correct one, at least so far as human anatomy is concerned; but in order to determine the morphological value of the sphincter muscle it is necessary that the early stages of its development should be studied.

The observations on which the findings of this paper are based were made on a series of human embryos of the following lengths, for the loan of some of which I am indebted to Prof. T. H. Bryce, F.R.S.:—8 mms., 9-5 mms., 14-5 mms., 16-1 mms., 19 mms., 21 mms., 22 mms., 82 mms., and 40 mms.

At the 8 mm. and 9-5 mm. stages the mesoblastic covering of the gut in the ileo-colic region consists of a mass of undifferentiated cells, closely packed

1 Quain’s Anatomy, 11th ed. 2 Cunningham’s Anatomy, 4th ed. 8 Journ. Anat. and Phys. 1903. 4 “The Ileo-caecal valve,” 1914. 5 Journ. Phys. vol. xxx, 1904. Early Stages of the Development of the Ileo-colic Sphincter 57

together and rapidly proliferating. At the 14-5 mm. stage two layers can be defined in the mesoblast: (a) the serous coat which throughout the series remains as a layer one cell thick, the cells being cubical in the early stages and more flattened in the later, and (b) a middle? layer of the gut wall which will differentiate to form the submucous, the muscular, and the subserous coats. At the 16-1 mm. stage it is possible to identify in the ileo-colic area the layer of cells from which will be developed the circular muscle coat of the gut. This layer differentiates from both the cranial and caudal ends of the gut tube, but the definition takes place at an earlier period and at a more rapid rate from the cranial end; and it is at this stage of development and from the cranial end that the process reaches the ileo-colic junction. At 19 mms. the myoblasts of this region have taken on the characteristic shape of embryonic muscle cells and are becoming well differentiated from the other cells of the mesenchymal coat of the bowel. After the 20 mm. stage it is possible to recognise at the ileo-colic junction a definite thickening of the

MESO-APPENDIX.

- ILEUM. CRECUM.~

SPHINCTER.

EPITHELIUM.| | ILEO-COLIC JUNCTION. CIRCULAR _] MUSCLE. SUP, MES. ART. SPHINCTER.

COLON.

Fig. 1. The ileo-colic junction in a 40 mm. human embryo. (x 150)

circular muscle layer. In embryos which have been sectioned horizontally the ileo-colic junction presents a Y-shaped appearance (fig. 1). The small limb on the left side of the Y represents the base of the caeco-appendicular anlage. The right limb represents the terminal ileum while the “leg” of the Y is the proximal colon. The angle between the small limbs can be called the ileocaecal angle while the angle between the right limb and the “leg” of the Y can be called the ileo-colic angle (fig. 1). At the 21 mm. stage the myoblast layer becomes very much thickened in the region of the ileo-caecal angle, and forms almost one-half of the total thickness of the mesenchymal coat of the gut. The myoblast layer at the ileo-colic angle also becomes thickened but not to the same amount as at the ileo-caecal angle. In reconstructions this thickening of the circular muscle coat can be traced round the gut at the line

1 The three layers of the gut wall at this stage are:—(a) the outer serous layer, (6) the middle mesoblastic layer, and (c) the inner. endodermic epithelial layer. 58 J. Beattie

of the junction of the ileum and the colon. The ileo-colic angle at this stage is about 140°.

After the 20 mm. stage the gut tube at the ileo-colic junction rapidly increases in diameter and the circular muscle coat becomes reduced in relative thickness in the terminal ileum and proximal colon; but the thickening of the circular muscle sphincter at the junction maintains its relative size. In the 82 mm. stage the ileo-colic angle has become a right angle, the terminal ileum now entering the colon from above and on the right. There is now at the junction (fig. 2) a well marked ring of muscle tissue which has invaded the sub-mucous layer of the gut and reduced it at the junction to a thin layer of cells; the outer or sub-serous layer remains of the same thickness as in the terminal ileum and proximal colon. [The sub-mucous layer develops later into the muscularis mucosae and the sub-mucous coat of adult anatomy and

NSS S888 5 rest

a7 8

V ILEO-COLIC JUNCTION. P EPITHELIUM.

SPHINCTER.

CIRCULAR MUSCLE.

- VACVOLE,

2c ILEUM,

L. MESENTERY.

Fig. 2. The ileo-colic junction in a 32 mm, human embryo. ( x 200)

the sub-serous layer differentiates into the sub-serous coat and the longitudinal muscle layer; but at this stage in the ileo-colic region these differentiation processes have not commenced. The epithelial layer has proliferated rapidly and at this stage is becoming thrown into the ridges described by Johnston1.] In the 40mm. embryo (fig. 1) practically the same stages of development are seen, but there is a great increase in the size of the gut. The anlage of the sphincter is quite sharply marked off from the remainder of the gut wall and evidently it is growing at the same rate as the gut itself.

SUMMARY

1. The first coat of the adult intestine to differentiate is the serous coat. After this the circular muscle coat becomes differentiated in the “middle” layer of the mesenchymal coat of the gut wall.

1 Amer. Journ. of Anat. vol. xtv. Early Stages of the Development of the Ileo-colic Sphincter 59

2. The myoblasts can be recognised in the mesenchyme at the ileo-colic junction at the 14-5 mm. stage. They form a definite layer at the 19 mm. stage and from then onwards they form a thickening which can be recognised without difficulty.

8. As the ileo-colic angle diminishes and the gut wall further differentiates, between the 20 and 82 mm. stages, the anlage of the sphincter muscle becomes more defined and ring-like.

4, The early presence of the sphincter anlage, even while the ileo-colic angle is greater than a right angle, would seem to show that the sphincter is the essential element in the Valvula Coli and that the later arrangements in the terminal ileum and proximal colon are secondary in nature.