*Corresponding author:K Nicoll, Surgical Department, Speciality Registrar General Surgery, Ninewells Hospital and Medical School, Ward 7, Dundee, Scotland, DD2 1SG, UK
How to cite this article:K Nicoll. Recent Advancements in the Morphologic Description of Cloacal Development Have Implications for Oncologic
Resection of Tailgut Cysts. Adv Res Gastroentero Hepatol, 2020;15(1): 555902. DOI: 10.19080/ARGH.2020.15.555902.
Tailgut cysts are rare para-rectal lesions thought to arise from incomplete involution of the tailgut during ontogenesis. They commonly present to gastroenterologists with tenesmus, extra-luminal compression, disturbance of bowel habit or rectal bleeding and carry significant threat of malignant transformation; particularly adenocarcinoma.
Guidelines on treatment of colonic adenocarcinoma are well established internationally. However, there are no guidelines on tailgut cyst management which can make onward referral for consideration of resection challenging. This problem is compounded by the inaccessibility of these cysts to visualisation or biopsy by endoscopy.
A significant barrier to consensus on the approach to these lesions is that there is presently scant explanation on their morphogenesis. Ontogenesis is the guiding principle in resection of colonic adenocarcinoma, where the lesions are resected en bolc with their embryonic mesentery, which contains their draining lymphatics. Recent developments in the description of the embryology of the anorectal region have resolved old morphogenic controversies and point the way to a rational approach to resection of tail gut cysts.
Recent systematic evaluation of tailgut cysts cases has suggested that more than a quarter of these undergo malignant transformation . Technique in modern colorectal cancer resections is built on the belief that total mesocolonic and mesorectal excision (TME) gives the best chance of curative resection. The basis for this practice is the orthodoxy that the large bowel and its mesentery constitute a single embryonic entity, intact but veiled by zygosis in the adult . In the adult colonic and rectal lymphatics respect embryologic boundaries and can be clearly surgically delineated from the retroperitoneum. However, while they are presumed to represent tailgut remnants, there is no precise description of the morphogenesis of tailgut cysts. This makes it impossible to form a rational approach to their resection with oncologic margins, a concerning problem given their rate of malignant transformation.
Tailgut cysts are typically found anterior to the sacrum, in close association with the mesorectum [1,3]. In contrast, the embryonic
tailgut is described as extending caudally from the cloaca or nascent anus. This casts doubt on the claim that peri-rectal cysts in the infant and adult actually represent tailgut cysts. How can their putative origins and their adult anatomy be married?
While classical morphological descriptions have a phylogenetic dimension, in gut rotation this is often lacking. The empirical success of the TME approach notwithstanding, direct embryological evidence for enteric rotational processes is poor [4,5]. The description of visceral rotation prevalent in textbooks represents an expedient construct which was largely arrived at a priori based on the ‘malrotations’ observed in infants. Description of the ontogenesis of tailgut cysts specifically has been further retarded by controversy over the means of division of the cloaca. In particular, the classical cloacal septation models of Retterer & Tourneux [6,7]; which were based on concrescence of infolding of the cloaca or a descending curtain of mesenchyme respectively. These models have not withstood contemporary interrogation by three dimensional techniques; specifically scattering electron microscopy (SEM)  or high resolution episcopic microscopy
(HRES)  in a murine model. The classical descriptions, and
disagreement between them, seem to arise from artefacts caused
by the plane through which histological sections were taken.
A newer model is emerging [8-10] which views the septation
of the cloaca as largely passive and contributed to mostly by
unfolding of the caudal embryonic curve and distortion of the
cloaca by marked growth of the genital tubercle (Figure 1). There
is also no evidence from these recent studies that the cloacal
septum ever reaches the cloacal membrane [8,9,11]. Rather, the
anal and urethral openings are formed by apoptosis of the cloacal
Of note, the mesenchyme at the caudal extreme of the cloaca
(i.e. the cephalic extreme of the tailgut, dot in (Figure 1) seems to
act as a fixed point around which the remainder of genitourinary
structures are distorted by the relative growth of mesenchyme
and caudal unfolding . Embryonic unfolding creates a sigmoidal
relationship between cloaca and tailgut. It is at approximately this
point (Carnegie stage 14/15) that the tailgut typically receeds
Were the tailgut to persist, an L like relationship would
presumably be created between tailgut and hindgut as the genital
tubercle distorts the cloaca with its marked outward growth [8,9];
causing the proximal part of any tailgut remnant to come to lie in
the same horizontal plane as the cloacal membrane. This would
explain how a tailgut remnant could be retained in the absence of
a tail; how it comes to lie anteriorly to the sacrum but posteriorly
to rectum; and why it is occasionally closely associated with the
anal muscles. It also explains why the tailgut cysts are so often
found embedded within the mesorectum.
Tailgut cysts are incredibly rare lesions. Outside of quaternary
referral centres, a gastroenterologist counselling a patient
is unlikely to have treated such a lesion before. There is an
increasing volume of literature on their malignant threat but
sparse description of if, or how, excision should be approached.
This makes onward referral for surgical management problematic.
The latest models of anorectal development [8-12] are able to
provide an explanation for the position of tailgut cysts observed
in the adult; though this has not been explicitly stated before.
This explanation has as a weakness that it takes an a priori form,
based on current sparse knowledge of anorectal morphogenesis
and clinical descriptions of tailgut cysts. This flaw is in common
with much of the anatomical theory supporting TME excision.
Importantly, such an explanation allows for a rational approach
to the oncologic resection of tailgut cysts, since their associated
mesentery will be inseparable from that of the rectum. Similarly,
lymphatics can be argued to trace to the inferior mesenteric
pedicle and a prophylactic or curative excision can rationally be
achieved by use of the TME plane.