Paper - The urachus - its anatomy, histology and development
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Begg RC. The urachus - its anatomy, histology and development (1930)
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The Urachus - Its Anatomy, Histology and Development
By R. Campbell Begg, M.C., M.A., M.Sc., M.D., Ch.B., F.R.C.S. (El)In.), M.R.C.S. (Eng.), L.R.C.P. (Lond.), F.A.C.S., F.C.S.A.
Senior Urological Surgeon, Wellington Hospital, Wellington, New Zealand
A casual study of the current descriptions of the urachus, some of which form the basis of erroneous conceptions of pathology, will disclose a singular lack of harmony in essential points. It was considered advisable in order to elucidate the etiology of certain rare conditions encountered that a systematic study of the organ should be undertaken. This was carried out by means of an examination of twenty—eight consecutive specimens derived from the postmortem room, from still-born foetuses and from preserved cadavers. Previous investigations of the same nature had been carried out by Luschka in 1862(9) and by Wutz(19) in 1883. The ﬁndings of these anatomists have not been followed by the writers of English anatomy textbooks, so that the time seemed opportune for conﬁrming or refuting them. .
Without referring in ewtenso to the investigations quoted, it may be said that the authors "denied that the epithelial lining of the urachus became obliterated in adult life to form a solid cord, but that the central canal with its epithelial lining persisted and sometimes communicated with the bladder. In addition, Luschka held that the urachus rarely reached the umbilicus in the adult, but on the contrary, “petered out in a series of ﬁne thread-like cords before it reached ‘that point.” It is obviously of importance in order to understand urachal pathology to know whether one has to deal with a ﬁbro-muscular cord or with an epithelially-lined canal and whether the structure developed from the foetal allantois or from the ventral cloaca.
Umbilical Arteries and the Urachus at Birth
These structures emerge from the umbilicus and it will be useful to recall the exact anatomy of the part. The anterior rectus sheath becomes continuous with the posterior sheath of its own side. The contact of the two rounded borders, so formed, fuse or are joined together by ﬁbrous bands forming the linea alba which may be compared to the stitches uniting the seams of a. coat. The umbilicus may be likened to a gape in the seam and leads to the space between the posterior sheath and the transversalis fascia. The latter is protruded like the ﬁnger of a glove through this gape or deﬁciency into the ﬁrst centimetre of the umbilical cord. At this point, the glove ﬁnger is deﬁcient at the tip, so that the protrusion of fascia forms a tunnel by which the umbilical vessels and the ﬁbrous remains of the allantois are conducted through skin, subcutaneous fascia and linea alba into the space between the transversalis fascia and the peritoneum. Viewed from within, the urachus at birth is a tiny structure hugging the anterior abdominal wall and is overlapped and almost concealed by the relatively enormous umbilical arteries. The latter project markedly inward, their diameter being 5 to 7 mm. At this stage the length of the urachus is 2 to 2-5 cm. from the apex of the bladder to the umbilicus, to which must be added half a centimetre for its intramural course in the bladder wall. The diameter at the bladder end averages 3 mm. but in the greater part of its course only 1 mm. The two umbilical arteries and the urachus, forming a kind of tripod, are met by the large umbilical vein 1 cm. in diameter, all passing through the tunnel of transversalis fascia already mentioned.
Fig. 1. The structures about the umbilicus in a full-time foetus viewed from the peritoneal aspect. B gives a general View of the bladder and urachus, and A illustrates the upper termination of the urachus. B, bladder; Ep, a branch of the epigastric artery; Ur, urachus; S1), urachal artery; Hy, umbilical artery; U, umbilical vein.
Of considerable importance in connection with the descent of the urachus to be described later, is the method of its termination at the upper end. Here it divides into a number of ﬁbrous strands usually three (ﬁg. 1 A). One of these is attached to, and continuous with, the adventitia of each umbilical artery; the third or middle strand passes through into the umbilical cord and breaking up into numerous ﬁne strands is lost at a point 1 cm. from the body surface. These strands represent the only remains of the obliterated allantois. The attachments to the umbilical arteries are almost invariably distinguishable in the adult and serve as landmarks to indicate the upper limits of the urachus proper.
Descent of the Urachus
The common conception of the change that takes place in the urachus after birth, is that the canal is obliterated from above downwards, but that in the adult it still maintains its attachment to the umbilicus. This theory cannot be made to ﬁt in with the embryological or anatomical facts. On the contrary, the urachus, developing as the upper part of the bladder, descends with this organ so that its apex leaves the umbilicus and later is found far down towards the symphysis pubis. The process that occurs is apparently the following: at birth the apex of the bladder is 4 cm. above the symphysis, and as it rapidly descends it pulls the ‘urachus with it. The latter being attached to the umbilical arteries drags the obliterated ends of these with it so that an anchor-like arrangement is formed about 3 cm. above the bladder apex.
Fig. 2. The peritoneal aspect of the urachus in an adult male, age 56. Note the ﬁbrous strands of “Luschkafs plexus.” The urachus has been pulled over during its descent and is attached to the left umbilical artery only. Bl, bladder; Ur, urachus; Umb, umbilicus.
In one specimen examined there had apparently been no such attachment, and the round occluded ends of the arteries lay free, one on either side of the upper end of the urachus. The adventitia of the arteries and the apex of the urachus usually become involved in the extensive ﬁbrous tissue which marks the closure of the umbilical fascial tunnel, and the production of the umbilical scar. As the arteries and the urachus are dragged down by the bladder descent, this tissue is drawn out into a ﬁbrous cord. More commonly the adventitia of the arteries is teased into a multitude of ﬁbrous strands which may be seen in most adult subjects radiating from the umbilicus. This was observed by Luschka who described it as a “fascial plexus.” A common variation is where the attachment to one artery has been stronger than that to the other so that the urachus is pulled markedly to one side (ﬁg. 2). This “plexus” which is much commoner than a single ﬁbrous cord, becomes much more marked in cases where abdominal distension due to pregnancy or ascites has occurred, and reaches a lateral spread of as much as 3 inches. The individual threads frequently reassemble just below the umbilicus forming a central cord which has been mistaken for the urachus, the latter really terminating as much as 10 cm. lower down.
In this teasing out process, the urachus itself being attached on either side is torn asunder and doubtless the epithelial cells which always exist in its centre are separated and give rise to the cystic adenomatous masses which sometimes occur in this situation, in a similar manner to the formation of “inclusion” cysts by the burial of dermal epithelium (Plate I, ﬁgs. 1, 2). Further than this, the splitting in the 33 per cent. of cases where the urachal canal communicates with the bladder may liberate tiny drops of urine into the transversalis-peritoneal space and give rise to septic effusions. These are strictly localised between the ﬁbrous strands of Luschka’s plexus which gain attachment to both peritoneum and fascia shutting off the space into a large number of compartments.
ANATOMY OF THE ADULT URACHUS
The true upper extremity of the urachus was determined not only by noting the point of junction with the obliterated umbilical arteries or some portion of them, but also by cutting sections at various levels. For some distance above the bladder the urachal canal, as observed in sections, is so characteristic as to be unmistakable (Plate I, ﬁg. 3). Even at higher levels, and, at birth right up to the umbilicus, the canal, even if partially obliterated, has a characteristic whorled arrangement, while once the ﬁrst attachment to the artery is passed, the sections show merely normal ﬁbrous tissue, clearly proving that at this point the urachus ends. The ﬁbrous strands that carry out the partial obliteration which is always conﬁned to a small segment at the upper end are longitudinal in direction and appear in cross-sections as bundles cut transversely. Each of these bundles corresponds to one of the tiny cell columns into which the main block of central epithelium breaks up as a prelude to obliteration (Plate II, ﬁg. 2). The adult urachus extends upwards from the anterior border of the bladder, usually arising 5 to 10 mm. below the apex, very occasionally from the apex itself, and still more rarely from the superior surface close to the apex. Its extreme limits in length are 3-10 cm. but in the vast majority of cases it is 5-5-5 cm. (The average distance from umbilicus to bladder apex is 16-18 cm.) Where it leaves the bladder it is broad and somewhat ﬂattened from before backwards. Here its breadth is 8 mm. and it tapers to 2 mm. at the apex. It lies between the transversalis fascia and the peritoneum in the space of Retzius and is surrounded by loose areolar tissue, being freely mobile in its lowest part to allow it to move with the expansion of the bladder. At its apex it is more ﬁrmly attached and closely applied to both peritoneum and fascia. In no case was a mesentery present though Binnie(1) maintained that one always existed at an early stage of development.
The blood supply of the urachus is deﬁnite and has a bearing on certain pathological features of congenital umbilical ﬁstula. It can best be observed in the foetus (ﬁg. 1). One of the superior vesical arteries, usually the left, passes up the lateral aspect of the bladder to the apex. After giving off its branches to the bladder it courses along" the ventral aspect of the urachus, being closely applied to it and supplying it with branches. This vessel is so constant that it may be described as the urachal artery (ﬁg. 1, So). It can be traced right up to the apex of the urachus and along the allantoic remains as far as the junction of the umbilical cord with the skin and in some instances even into the cord itself. In cases of congenital urinary umbilical ﬁstula this artery causes increased vascularisation of the ﬁrst part of the cord which prevents the onset of dry gangrene in virtue of which the cut cord normally separatesthe separation does not take place—and the large red tumour which forms such a remarkable feature of these cases is left projecting from the navel. The urachal artery anastomoses in the region of the umbilicus with the terminal branches of the superior vesical artery of the other side and also with branches of the epigastric arteries which re-pierce the transversalis fascia from without inwards.
Histology of the Urachus
This account is based on a study of specimens by means of serial sections and the dissecting microscope. They have also been compared with the observations of Luschka (9) and Wutz (19). On one point all observations harmonise, viz. that the epithelial canal persists throughout life and is never completely obliterated. The lumen is, however, subject to interruption by ﬁlling with desquamated cells from the inner layer. These cells unlike those which are always being desquamated into hollow viscera such as the bladder have no outlet and so accumulate in the canal at intervals, and by their degeneration form a cast which, in parts, completely ﬁlls the lumen (Plate II, ﬁg. 1). The canal is, as a rule, sacculated at various points (Plate II, ﬁg. 3), and tiny cysts due to cell proliferation and degeneration formed. The epithelial cells, however, far from being destroyed display a remarkable activity in reproduction and multiplication, pushing out into the connective tissue when they can no longer ﬁnd room in the canal. As the result of this process small adenoma-like structures and cysts are the rule rather than the exception, and this phenomenon is more marked after the age of ﬁfty. This point requires emphasis, as in such an authoritative textbook as Cunm'ngham’s Anatomy (p. 1314, 1922 ed.) the following statement occurs: “This part of the cloaca (the urachus) loses its lumen about the ﬁfth wee ,” and again: “The epithelium of the urachus completely disappears.” In reality, it is doubtful if there is in the body any better example of the remarkable persistence of epithelium in rudimentary structures, or of its renewed activity in adult life. The urachus grows Very little after birth,- the average length being in the adult and the child 5 cm. and 2-5 cm. respectively. It appears also to undergo but slight obliteration of its canal after birth—as it is in the new-born child so it will remain in the adult. In only one specimen examined was actual capillary invasion with
Fig. 3. A diagrammatic cross-section of the urachus in situ. T, transversalis fascia; R, rectus abdominis; Adv, connective tissue layer or adventitia; M, muscle layer showing longitudinal bundles in section with ﬁbrous tissue between them; 0, loose connective tissue surrounding the urachal canal; D, dense ﬁbrous tissue forming outer layer of the urachal canal; E, epithelium; L, lumen of urachal canal; P, peritoneum; Ar, loose areolar tissue surrounding urachus; V, urachal vein; N, urachal artery.
the formation of ﬁbroblasts seen in progress and that close to the upper. extremity. The urachus (ﬁg. 3) has a serous coat (peritoneum) on its dorsal surface only, and this is deﬁcient where it pursues an intramural course of 1 cm. in the wall of the bladder. Next to the serous lining from without inwards is a loose areolar and then a ﬁne connective tissue layer. The comparatively thick adventitial coat of the bladder is not prolonged to cover the urachus. It is a somewhat striking circumstance that the outer coat of the bladder forms a collar-like deﬁciency through which the urachus ascends. Here there is a relatively weak point in the bladder wall, and several examples of diverticula have been observed. The next layer of the urachus is formed of muscle ﬁbres which arise in a very deﬁnite fashion from the bladder. Two bands of longitudinal muscle ﬁbres sweep like broad straps from the prostate in front to the prostate behind (Schafer, Qua1In’s Anatomy, vol. XI, pt 1). They pass on either side past the urachus contributing no ﬁbres to it. When the bladder is being emptied the contraction of these bands brings their medial margins ﬁrmly together forming a strong support in the moment of danger for the weak point of the urachal insertion, just as the contraction of the recti support the weak middle line of the abdominal Wall. Superﬁcial to the longitudinal bands are other fasciculi which run more or less obliquely. Many of their ﬁbres pass up the urachus, giving it a complete investment in the lower part, but gradually thinning out as they ascend. By their attachment to the upper ﬁbrous part of the structure they form a kind of musculo-tendinous apparatus. From this point of View the urachus may be considered as a muscle whose tendon gains attachment to the umbilical region through the medium of ﬁbrous cords derived from the umbilical arteries. That its purpose is not that of a mere passive suspender of the bladder apex is suggested by the tortuosity of the canal in its intramural part (ﬁg. 4«), indicating that the structure undergoes lengthening and shortening in life. It is probably analogous to the musculi papillares of the heart. It takes in the slack as the bladder empties just as in emptying a rubber hot water bottle one lifts up the bottom end to prevent it ﬂapping over and so hindering the outﬂow, or as the Arab attaches a string to the closed end of his water skin to facilitate the outﬂow of its contents.
Another function of the urachus is suggested by the manner in which its muscular fasciculi spread out in cone-shaped fashion over the bladder gaining an attachment to the prostate all round. Thus the urachus may give a point d’appu'£ for the contraction of these muscle bundles, whereby the bladder is compressed uniformly from without and steadied while the strap-like bands of the detrusor urinae are acting.
Deep to the muscle wall of the urachus and intimately connected with it is a dense stratum of ﬁbrous tissue, and internal to that again a layer of somewhat loosely arranged connective tissue. So loose is the latter, that the central urachal canal can easily be dissected out and appears as a. white tubular structure of uniform bore. The pyramidal appearance of the urachus as a whole is in no way reﬂected in this tube which resembles a small thoracic duct. Its usual diameter is 1 mm. It can’ be traced almost from the upper end of the urachus, and as stated before pursues a tortuous course through the muscular wall of the bladder, till it reaches the external surface of the mucous membrane To this point it can always be traced with the greatest facility, but its manner of ending is subject to considerable variation (ﬁg. 4). In a certain number of cases it communicates directly with the bladder, its epithelium becoming continuous with that of the latter organ. In others it ends in a number of ﬁbrous strands which lose themselves in the submucosa, while again it may terminate in a cystic dilatation. The urachal artery does not follow the canal in its intramural courses but joins the urachus external to the bladder wall.
The Minute Structure of the Urachal Canal
The urachal canal proper (ﬁg. 3) is but loosely connected with the muscular and ﬁbrous portion of the structure by areolar tissue, and consists of two layers——an outer coat of condensed ﬁbrous tissue (ﬁg. 3, D) and an inner one of epithelium (ﬁg. 3, E). The outer coat displays evidence of proliferation where it abuts on the lining of epithelium, one or two rows of young ﬁbroblasts forming a whorled arrangement around the latter (Plate II, ﬁg. 1). These are in direct contact with the outermost layer of epithelial cells, no basement membrane intervening. The epithelium, is as a rule six or seven layers deep, though there may be no more than two. It has rarely the regular structure of transitional epithelium, the deep cells being of the same type as the superﬁcial. Occasionally pearshaped cells may be seen or a single cell with more than one nucleus is present. Sometimes the nuclei of the outermost layer of cells lie with their long axes at right angles to the diameter of the canal, but more commonly are in the same line. The cells next to the lumen show a tendency to desquamate and become free, and sometimes they accumulate and almost plug the lumen. It is noteworthy that in those cases in which there is a communication with the cavity of the bladder, the lower part of the lumen remains free, as the desquamated cells pass into the bladder and escape with the urine.
The nuclei are vesicular, oval, and stain lightly. They average 12 ,1. in length and 9 p. in breadth. There is a deﬁnite nucleolus, sometimes two, and a loosely arranged chromatin network. Division by an amitotic process is frequently observable but there is an absence of clearly deﬁned mitotic ﬁgures. The thickness of the epithelial lining is rarely uniform, the cells being heaped up at one segment to ten or twelve layers, while at others there are only two or three (Plate II, ﬁg. 1). The size of the lumen varies from a mere slit to a canal fully half a millimetre in diameter and appears to bear no constant ratio to the age of the subject. The general thickness of the epithelial part increases slightly with age, and the canal is longer in the adult than the child, proving that a certain amount of proliferation is taking place. As Luschka previously observed (lac. cit.) the epithelial tube does not remain central in relation to its ﬁbrous covering throughout the whole course, but deviates from side to side as it ascends.
It gives rise also to numerous small pockets or diverticula. These ‘latter, however, owe their rise not to simple dilatation but to the degeneration of masses of epithelial cells which have become detached from the central epithelium. These give rise to small adenomatous formations or by central degeneration to tiny cysts. The canal is between 3 and 7 cm. in length or practically coterminous with the urachus as a whole. At the lower part it is suﬂiciently pervious to receive a ﬁne bristle but higher up its potency is interrupted by accumulation of desquamated cells, by the epithelium of one side meeting that of the other, or by the central core of epithelium splitting up into several smaller cores (Plate II, ﬁg. 2). It may be patent above and below and impervious in the middle, or be shut off into numerous pervious sections like a bamboo cane.
Contents of the Urachal Canal
Luschka described the contents of the canal as consisting of pigmentary, fatty, and amyloid substances, the products of cell degeneration, also of fat molecules and corpora amylacea. The latter bodies are of great interest as tumours of the urachus resemble prostatic tissue, and Nicholson (11) has observed amyloid bodies in them. There seems little evidence that the epithelial cells of the urachal canal can give rise to a true secretion. I have seen numerous cells of the goblet type in a malignant tumour of the urachus but never in a simple tumour or a normal organ. I believe that cysts due to such a cause never arise. The only true type of urachal cyst is that formed by the proliferation of epithelial cell masses and their breaking down in the central parts. A certain amount of obliteration of the urachal canal from above downwards appears to take place occasionally throughout life as witnessed by the discovery even in subjects of over ﬁfty years of vascular granulation tissue invading and replacing epithelium. This process however is rare, never reaches very far down and is accompanied in many instances by increased epithelial growth. The detachment of cells during this process leads to those curious adenomatous formations referred to above, and found so commonly in the adult urachus (Plate I, ﬁg. 2).
Method of Termination of the lower end of the Urachus and its relation to the Bladder
The constant presence of a pervious urachal canal at all ages having been established, it is of great interest to ascertain in what percentage of cases there is a direct communication between it and the bladder and how in these cases the urine is prevented from passing up through the oriﬁce.
The question is not new. Peyer at the end of the seventeenth century maintained in opposition to Caspar and others that there was a communication between the urachal canal and the bladder during the foetal period. J. G. Walker (17) stated in 1775 that this communication was present at all stages of life and would admit a ﬁne probe. Wutz (19) considered that the opening existed in most cases and described a small transverse fold of mucous membrane which prevented regurgitation of urine. Later writers, notably Doran (3), have discussed the presence of this “valve of Wutz” in relation to vesicoumbilical ﬁstula. The subject was approached in the present investigation by making a minute examination of the uracho-vesical junction and by attempting to pass ﬁne bristles or inject methylene blue under pressure, with subsequent examination of longitudinal and serial transverse sections. In seven out of twenty-one specimens examined in this way there was a connection between the urachal canal and the bladder through which a ﬁne bristle could be passed (i.e. 33;]; per cent.). In the remaining fourteen (or 66% per cent.) microscopic sections showed that there was an absence of this communication. In only one of the ﬁrst series was there a “Wutz valve” guarding the oriﬁce (see ﬁg. 4.-).
A careful examination was also made in most of the surgical museums of Edinburgh and London of all bladder specimens. A large proportion of these cases had suffered from obstruction to outﬂow of urine, and about the same percentage (33% per cent.) showed the vesico-urachal oriﬁce, frequently much dilated. .
A third method of approach has been by examination in some hundreds of cystoscopic examinations, but in these, possibly from the difficulty in placing the cystoscope so as to get a close-up View and the necessary magniﬁcation, not nearly the same proportion of oriﬁces was observed. The urachovesical oriﬁce could be divided into four main types. In the commonest (three dissection cases) there was no depression or papilla, the opening being ﬂush with the mucous surface (ﬁg. 4B). The second method (two cases) was Where the mucous membrane of the bladder was protruded in the form of a diverticulum, between the fasciculi of the bladder muscle, presenting much the same puckering of mucous membrane at the bladder oriﬁce as is seen in a pathological diverticulum. Into the sacculus the canal of the urachus opens. I have seen this form several times in otherwise normal bladders with the cystoscope and a beautiful specimen of it exists in the Royal College of Surgeons, Edinburgh (Spec. 28, 50).
A case where such a diverticulum was pathologically enlarged and contained a calculus was reported by Campbell Dykes (4).
In the Royal College of Surgeons Museum, London, specimens 3728, 3721 a, 2232 and 2223 all show a protruding pouch of this nature.
The third method (one case, ﬁg. 4.4) was where the oriﬁce opened at the apex of a pyramidal projection. A beautiful illustration of this is shown in specimen 3730 in the museum of the Royal College of Surgeons, London, described in the catalogue as “a papillary projection of unknown origin.”
The fourth method (one case) was where the opening was at the bottom of a dimple 3 mm. in depth and diameter. In all these cases the continuity of urachal and bladder epithelium was shown by sections. In none of them would a bristle or a blue injection pass in more than 1 cm. owing to the blocking of the otherwise patent canal with epithelial debris at this point, though microsopic examination showed continuity of epithelium and patency at the higher level. It is noteworthy that this is about the extreme depth of the dilated canal in cases of urinary obstruction, an observation which shows the improbability of the re-opening of the whole length of the canal in adult life. Such an acquired umbilical ﬁstula is an impossibility on anatomical grounds. In several cases where no actual communication existed a slight depression (ﬁg. 4- D) or red spot on the bladder mucosa marked the point opposite the urachal insertion. In all cases the ﬁbro-epithelial canal could be traced as far as the outer surface of the bladder mucosa.
Fig. 4. Different methods of termination of the lower end of the urachus. In A it communicates directly with the bladder ending on the summit of a papilla; in B the terminal opening is ﬂush with the bladder mucosa; in C’ and D it reaches the outer surface of the bladder mucosa but does not communicate with the lumen. In D a dimple in the vesical mucosa indicates the point of attachment of the urachus. Note the tortuous course of the urachal canal.
Development of the Urachus
It appears now to be universally conceded that the allantois takes no share in the development of the bladder which is partially cloacal and partially Wolfﬁan in origin. Similar agreement has not hitherto been reached in regard to the urachus. Sir Arthur Keith (8) maintains its origin from the allantois. W. Felix(7), however, writing in Keibel and Mall’s Human Embryology maintains that the apex of the ventral cloaca is carried in a cephalic direction with the umbilicus and that its upper part is obliterated to form the urachus. He says: “ In man a ventro-cloacogenic and mesodermic bladder develops, an allantoidiogenic origin such as was formerly supposed to occur does not exist. A View of the anterior abdominal wall from behind (in embryos of 10-24 mm.) shows that the two umbilical arteries bound a triangular area—the vesical plate Whose apex is of course formed by the umbilicus (see ﬁg. 1 A). Within this area the bladder lies in such a way that its cranial apex reaches the umbilicus where it is connected with the remains of the allantoic stalk. This position the bladder retains at birth except that its apex diminishes more and more and ﬁnally becomes the urachus, this structure being accordingly a product of the bladder, i.e. of the cloaca and not of the allantoic stalk.” Prof. Arthur Robinson (20) supports this view.
No doubt in some animals, such as the horse, the apex of the bladder reaches to the umbilicus and so-called urachal (really Vesical ﬁstula) is not uncommon in foals. There seems little doubt that this is the correct view, and the question is of considerable importance in refuting the common explanation of umbilical urinary ﬁstulae, which are mostly in reality due to the lack of formation of the urachus and not to its patency. The essential similarity of the epithelia of the urachus and bladder and the arrangement of the muscle and ﬁbrous coats of the two organs strongly support a common origin.
Further evidence is available also from the pathological side which need not be elaborated here. G. W. Nicholson (11) as the result of an exhaustive research laid down the rule that all the normal glands of the cloaca as well as those formed in pathological conditions are of one type and similar to prostatic glands. The type of so-called adenomatous tumour that develops in the bladder wall is amply illustrated in miniature in the urachus in a large proportion of adult subjects over ﬁfty years of age as well as in several more gross tumours, simple and malignant, that have been observed. Thus on all grounds the conclusion is inevitable that the urachus is the modiﬁed—not obliterated—upper end of the foetal bladder. Only on this supposition can a rational explanation be given of umbilical urinary ﬁstula. The only remains of the allantois in the full-time foetus is the central of the three ﬁbrous bands attached to the upper end of the urachus. This band has been referred to early in the present article. It passes out between the umbilical arteries to be lost in the tissue of the umbilical cord (ﬁg. 1 A).
Anomalies of the Urachus
The most important anomalies are: (a) of formation; (b) of descent.
(a) In the anomalies of formation, the upper part of the ventral cloaca does not narrow, so that a true functional bladder reaches the umbilicus at birth. A gross example of this occurs in cases of extroversion of the bladder. In rare cases with the separation of the umbilical cord, this bladder discharges urine at the umbilicus. In others, the bladder descends so that in the adult its apex is conical and reaches as far as the normal apex of the urachus. This condition is familiar to gynaecological surgeons, and endangers the bladder in low laparotomy. Imperfect formation without descent may give rise to permanent umbilical urinary ﬁstula, or the urine may break through at the umbilicus in adult life owing to obstruction to urethral outﬂow. This is one of the forms erroneously called “acquired urachal ﬁstula‘.”
(b) The urachus is normally formed but the urachus and bladder do not undergo descent. In this case also the apex of the bladder is close to the umbilicus and connected to it by the urachus. I believe this form is excessively rare, and is not a cause of umbilical ﬁstula as the normal urachal canal is so narrow and so blocked with debris that no pressure of urine would ever penetrate its length.
The relation of these anomalies to pathological conditions is dealt with elsewhere and is not a suitable subject for an anatomical paper.
The urachus is developmentally the upper part of the bladder, both being derived from the ventral cloaca. This part of the bladder narrows more and more, but retains in miniature its essential structure and at birth is still attached to the umbilicus. It is then known as the urachus, and in about two individuals out of three its lumen is separated from that of the bladder proper by a thin partition of mucous membrane which has grown in. In the remainder no such partition has developed. The urachus shares in the post-partum descent of the bladder, and in the adult is a cone-shaped structure 5-5 cm. in length passing up from the bladder apex, and attached to the umbilicus only by adventitial bands derived from the umbilical arteries which it has pulled down in its descent. Like the bladder it has a peritoneal and an extra-peritoneal surface and has the transversalis fascia on its outer and the peritoneum on its inner surface. The lumen of the urachus remains patent throughout life, though it may be plugged in places by masses of epithelial cells which have desquamated from the walls of its canal.
Being a vestigial structure, though of use as a musculo-tendinous adjunct to the bladder, it does not grow in proportion to the development of the body or bladder. It is only twice the length in the adult that it is in the newly born. There is little or no tendency towards the obliteration of its canal by ﬁbrous tissue. The epithelium of the canal is an irregular form of transitional epithelium and displays great activity in cell division. On this account, groups of cells are pushed out into the connective tissue lining the canal, and by their degeneration form detached cysts or cystic diverticula of the lumen. After the age of ﬁfty the continuance of the process causes the formation of quasipathological, and in rare cases grossly pathological, phenomena in the form of cystic adenomata of the prostatic type. Anomalies in the development of the urachus are responsible for occasional cases of urinary ﬁstula at the umbilicus. These are all congenital in nature though symptoms may not develop till late in life.
1 A case was observed in a child of two in which it appeared that the urachus had not formed and yet the bladder had descended to the normal level. The most exhaustive search within and without the bladder failed to discover any traces of a urachus.
Description of Plates
Fig. l. A low power view of cystic formation found in the midline two centimetres below the umbilicus in a child of four years dying of tuberculosis meningitis. The urachus had descended and formed with the two obliterated umbilical arteries an anchor-like arrangement 3 cm. above the bladder apex. Possibly the structure illustrated which is of the adenomatous type was derived from epithelial cells shed from the urachus during its descent. Similar types of structure may be found lateral to the urachus near the bladder.
Fig. 2. A high power view of the upper part of the cystic structure.
Fig. 3. A low power view of a cross-section of the urachus of a male of 62 years, 2 cm. above the bladder apex. The thin epithelium of the canal with its distinct lumen can be clearly seen. (x60.)
Fig. 1. A high power view of the urachus showing the many layers and irregular distribution of the epithelial cells, some of which are lying free in the lumen. '
Fig. 2. A cross-section of the upper end of the urachus in a male aged 56. The section is a transverse one 5 cm. above the bladder apex. The urachal canal which had a single lumen in its lower part has now broken up into seven separate canals. Serial sections above the level shown showed that the lumen became still further divided and was ﬁnally represented by small solid columns of cells, which gradually became replaced by young fibroblasts.
Active capillary invasion and fibroblast formation was taking place leading to obliteration of the extreme upper end of the urachus, 7 cm. from the bladder.
Fig. 3. Section through a cystic dilatation 2 cm. above the bladder in a male subject 60 years of age. The cyst is lined by two layers of epithelial cells and the lumen is ﬁlled with homogeneous material which has shrunk away from the walls during the preparation of the specimen. The presence of faintly staining nuclei suggests that the content is formed from broken down cell detritus. ( x 60.)
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Cite this page: Hill, M.A. (2021, April 19) Embryology Paper - The urachus - its anatomy, histology and development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_urachus_-_its_anatomy,_histology_and_development
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