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36 Mesenteric and peritoneal anatomy
Left mesosigmoidal peritoneal reflection
White line
of Toldt
White line
of Toldt
Sigmoid colon
Sigmoid colon
Left iliac
fossa
(b)
Left iliac
fossa
(a)
Mesentery
Fascia
Colon
Peritoneum
Legend
Figure 2.25 (a) (See also QR 2d/8 and 9.) 2.5D snapshot of a 3D digital model demonstrating the peritoneal reection at
the lateral aspect of the mesosigmoid, looking from below up. (b) (See also QR2d/8 and 9.) View of peritoneal reection at
the lateral aspect of the mesosigmoid, looking from above downward.
Flexural anatomy 37
Mesorectum
Toldt’s
fascia
W
aldeyer’s
Fascia
Pelvic side wall
connective tissue
Mesorectum
(b) (c)
Pararectal peritoneal reflections
Peritoneal reflection
in pouch of Douglas
Left lateral
peritoneal
reflection
Right pararectal
peritoneal reflection
(a)
Legend
Mesentery
Fascia
Colon
Peritoneum
Figure 2.26 (a) (See also QR 2/8 and QR 3/2.) 2.5D snapshot of 3D digital model demonstrating the rectum, associated
peritoneal reections, mesorectum, and fascia. (b) 2.5D snapshot demonstrating the same model as in (a) but sectioned in
the sagittal plane to demonstrate anatomic relationships. (c) (See also QR 2/8 and QR 3/2.) 2.5D snapshot demonstrating
the same model as in (a) but sectioned in the transverse plane to demonstrate anatomic relationships.
38 Mesenteric and peritoneal anatomy
bearing continuity in mind. At the intestinal margin of the
conuence, the intestinal tract is seen to round the ex-
ure, centered on the mesentery [10,16,20,21]. is complex
of anatomic structures and pattern of attachment is again
obscured from direct visualization by the right peritoneal
and hepatocolic regions of the peritoneal reection.
Splenic exure
e reverse occurs at the splenic exure where the colon
changes from being mobile (transverse colon) to attached
(descending colon) to the posterior abdominal wall. e mes-
enteric component of the conuence changes from having a
mobile component at the mesenteric border of the intestinal
tract to fully attaching to the posterior abdominal wall across
the transverse extent of the left mesocolon. is anatomic
arrangement is obscured from direct view by the splenocolic
and left eritoneal regions of the peritoneal reection.
Colosigmoid and rectosigmoid exures
At the junction between the descending and sigmoid colon,
the intestinal margin of the mesentery separates from the
posterior abdominal wall and elongates considerably in tan-
dem with the colon itself. At the exure between the rectum
and sigmoid, the reverse occurs as the intestinal margin
of the mesentery tapers distally, becomes adherent, and
continues caudally as the mesorectum.
MESENTERIC CONFORMATION
INGENERAL
All anatomic structures have limits (i.e., a beginning and an
end). e mesentery distal to the duodenum emerges from
the superior mesenteric root region. e anatomic end, or
termination, occurs at the distal end of the mesorectum.
In between these, the mesentery is best understood as fan-
ning out and adopting a spiral conformation. In addition, it
undergoes an extensive elongation at the intestinal margin.
is combination, in conjunction with plication, ensures
that despite extensive elongation at the intestinal margin,
the entire gastromesenteric complex can be neatly packaged
into a limited intraperitoneal space.
FUTURE DIRECTIONS
An improved understanding of mesenteric and peritoneal
anatomy enables one to propose areas for future research, of
which there are many. To the surgeon involved in excising the
rectum, it remains unclear as to how, structurally, vessels and
nerves interface with the mesorectum. Some propose the pres-
ence of lateral ligaments or the “zone of adhesion,” while oth-
ers demonstrate that the mesorectum remains separate from
surrounding structures, throughout its full circumference.
Splanchnic nerves supply postganglionic parasympa-
thetic and sympathetic nerve bers to the intestinal tract.
Although the distribution of these nerves is of enormous
clinical relevance, little is known regarding their trajectory
within the mesenteric organ [21]. Most descriptions men-
tion three ganglia from which postganglionic bers arise,
but none accurately delineate the course of these, once
they leave the ganglia. It is not unreasonable to suggest that
the mesentery provides a crucial platform in this regard
and that the lack of comprehensive descriptions of enteric
neuroanatomy stems from the inaccurate appraisals of mes-
enteric anatomy in the fi st instance. It will be important to
redress this in future studies and accurately determine the
course of postganglionic nerves once these have left the
celiac, superior or inferior mesenteric ganglia.
SUMMARY
e mesenteric organ, distal to the duodenojejunal fle -
ure, fans out from its origin at the superior mesenteric
artery to span the intestinal tract from DJ flexu e to ano-
rectal junction[2]. It is continuous, as are the associated
intestinal tract and peritoneal refl ctions. Anatomic conti-
nuity has implications for all related basic and clinical dis-
ciplines. Clarifi ation of mesenteric, peritoneal and fascial
anatomy now enables a systematic (i.e., scientific) study of
each [21].
REFERENCES
1. Treves, F., Lectures on the anatomy of the intesti-
nal canal and peritoneum in man. Br Med J, 1885.
1(1264): 580–583.
2. Standring, S., Gray’s Anatomy: The Anatomical Basis
of Clinical Practice. Elsevier Health Sciences, London,
U. K ., 2015, pp. 10 9 8 –1111, 1124 –1160.
3. Blackburn, S.C. and M.P. Stanton, Anatomy and
physiology of the peritoneum. Semin Pediatr Surg,
2014. 23(6): 326–330.
4. Sinnatamby, C.S., Last’s Anatomy: Regional and
Applied. Elsevier Health Sciences, London, U.K.,
2011, pp. 234–238, 247–259.
5. Snell, R.S., Clinical Anatomy by Regions. Lippincott
Williams & Wilkins, 2008, pp. 216, 226, 228, 233, 237,
240.
6. Schoenwolf, G.C. etal., Larsen’s Human Embryology.
Elsevier Health Sciences, Philadelphia, PA, 2014,
pp.341–375.
7. Cochard, L.R., Netter’s Atlas of Human Embryology:
Updated Edition. Elsevier Health Sciences, London,
U.K., 2012, pp. 133–134.
8. Sadler, T.W., Langman’s Medical Embryology. Wolters
Kluwer Health, Philadelphia, PA, 2011, pp.208–232.
9. Sehgal, R. and J.C. Coffey, The development of
consensus for complete mesocolic excision (CME)
should commence with standardisation of anatomy
and related terminology. Int J Colorectal Dis, 2014.
29(6): 763–76 4.
References 39
10. Culligan, K. etal., The mesocolon: A prospective
observational study. Colorectal Dis, 2012. 14(4):
421–428; discussion 428–430.
11. Coffey, J.C., Surgical anatomy and ana-
tomic surgery—Clinical and scientic mutualism.
Surgeon, 2013. 11(4): 177–182.
12. Treves, F., Discussion on the subsequent course and
later history of cases of appendicitis after operation.
Med Chir Trans, 1905. 88: 429– 610.
13. Standring, S., Gray’s Anatomy: The Anatomical
Basis of Clinical Practice. Churchill Livingstone/
Elsevier, Edinburgh, Scotland, 2008, pp. 1069–1083,
10 9 9 –1111, 1125 –1163.
14. Moore, K.L., A.F. Dalley, and A.M.R. Agur, Clinically
Oriented Anatomy. Wolters Kluwer Health,
Philadelphia, PA, 2013, pp. 219–221, 239–263.
15. McConnell, A.A. and T.H. Garratt, Abnormalities of x-
ation of the ascending colon: The relation of symptoms
to anatomical ndings. Br J Surg, 1923. 10: 532–557.
16. Culligan, K. etal., The mesocolon: A histological and
electron microscopic characterization of the mesen-
teric attachment of the colon prior to and after surgi-
cal mobilization. Ann Surg, 2014. 260(6): 1048–1056.
17. Coffey, J.C. etal., An appraisal of the computed
axial tomographic appearance of the human
mesentery based on mesenteric contiguity from
the duodenojejunal exure to the mesorectal level.
EurRadiol, 2016. 26(3): 714 –721.
18. Coffey, J.C. etal., Mesenteric-based surgery exploits
gastrointestinal, peritoneal, mesenteric and fascial
continuity from duodenojejunal exure to the
anorectal junction—A review. Dig Surg, 2015. 32(4):
291–300.
19. Culligan, K. etal., Review of nomenclature in colonic
surgery—Proposal of a standardised nomencla-
ture based on mesocolic anatomy. Surgeon, 2013.
11(1):1–5.
20. Coffey, J.C. etal., Terminology and nomencla-
ture in colonic surgery: Universal application of
a rule-based approach derived from updates on
mesenteric anatomy. Tech Coloproctol, 2014. 18(9):
789–794.
21. Coffey, J.C. and D.P. O’Leary, The mesentery:
structure, function, and role in disease. Lancet
Gastroenterol Hepatol, 2016. 1(3 ) : 2 3 8 – 24 7.
41
3
Embryologic development of the mesentery,
peritoneal reection, and Toldt’s fascia
J. CALVIN COFFEY, RISHABH SEHGAL, AND JOEP KNOL
Being entirely honest with oneself is a good
exercise.
Sigmund Freud
AIM
e current interpretation of mesenteric anatomy means
that the embryologic development of mesentery and associ-
ated structures must be reappraised. e aim of this chapter
is to conduct such an appraisal and attempt to reconcile cur-
rent anatomic thinking with a plausible series of embryo-
logic events.
INTRODUCTION
e embryologic development of the mesentery is a truly
astonishing biologic process. e relative lack of pub-
lished reports on mesenteric abnormalities indicates that
it is highly conserved in man. One of the earliest identi-
able forms of mesentery is the dorsal mesentery, and this
undergoes a remarkable series of changes in size, shape, and
orientation during development in and ex utero. e fol-
lowing chapter is divided into two parts. In the rst, classic
teaching and the current one are contrasted and the pro-
cesses involved in the development of mesentery discussed.
e second component attempts to develop an explanation
as to how the dorsal mesentery might develop into the shape
observed in the adult human.
PART I: CLASSIC TEACHING ON
MESENTERIC ANATOMY
Classic teaching holds that the mesentery elongates con-
siderably and then rotates counterclockwise around the
superior mesenteric artery. Rotation is a concept used to
explain how the right colon and associated mesentery ulti-
mately reach their position in the adult, in the right ank.
It is also used to explain how, in the adult, the small intes-
tine and associated mesentery end up centrally positioned.
When rotation does not occur (i.e., mal or nonrotation), the
duodenum and small bowel continue vertically down the
right ank. e left colon lies in the left ank, and the right
colon (and mesentery) takes up a position between both.
Aim 41
Introduction 41
Part I: Classic teaching on mesenteric anatomy 41
Current teaching on mesenteric anatomy 42
Mesenteric rotation, anchorage, andelongation 42
Attachment: The peritoneal reection 42
Attachment: Toldt’s fascia 43
Summary 43
Part II: Reverse engineering gastromesenteric
embryology 43
Mesentery: Reverse engineered 43
Peritoneal reection 43
Reverse engineering the reection: Detachment 43
Hindgut: Shortening and detachment ofthe
mesorectum 44
Reverse engineering the mesorectum 44
Hindgut: Shortening and detachment ofthe
mesosigmoid 44
Reverse engineering the mesosigmoid 44
Hindgut: Shortening and detachment ofthe
leftmesocolon 44
Reverse engineering the left mesocolon 44
Midgut 44
Reverse engineering mesentery between
theduodenojejunal and splenic exure 44
Summary 44
Future directions 45
Summary 45
References 45
42 Embryologic development of the mesentery, peritoneal reection, and Toldt’s fascia
As it is not known why rotation occurs in the rst instance,
the cause of nonrotation is similarly obscure. Interestingly,
a study of porcine mesentery demonstrates a similar gas-
tromesenteric conformation to that observed in humans
with mal or nonrotation. In the porcine context, the small
intestinal gastromesenteric context continues vertically
downward in the right side of the abdominal cavity. e
left colon and mesocolon are positioned on the left side of
the abdomen, and the right colon and mesentery are cen-
trally positioned between both [1,2].
Following rotation, the gastromesenteric complex
returns to the peritoneal cavity where components attach
to the posterior abdominal wall. Attachment is dened as
attening of the mesentery against the posterior abdomi-
nal wall. It does not mean “insertion” of the mesentery into
the posterior abdominal wall. Attachment is mediated by
the development of Toldt’s fascia and the peritoneal reec-
tion. In this manner, the intestine and mesentery take up
the adult conformation (see earlier discussion). According
to classic teaching, the small intestinal, transverse, and sig-
moid mesentery persists into adulthood. However, the right
and left mesocolon regress and the mesentery becomes dis-
continuous. Although the sliding and regression theories
were developed to explain right and left mesenteric disap-
pearance, neither gained broad acceptance [3–6].
Current teaching on mesenteric anatomy
is is detailed in Chapter 2; however, a brief summary will be
provided here. Current teaching holds that the mesentery dis-
tal to the duodenojejunal exure is continuous to the meso-
rectal level [7–9]. It fans out from the root region to span the
intestinal tract from jejunum to anorectal junction. Broadly
speaking, it has attached (see denition given above) and
nonattached regions. e conformation of the attached region
resembles a question mark. e curve of the question mark is
made up by zones of attachment of the right, transverse, and
left mesocolon. e stem of the question mark is formed by
attachment of the left mesocolon, mesosigmoid, and meso-
rectum. Nonattached mesentery elongates considerably at the
intestinal (nonattached) margin. Together, the intestinal and
mesenteric components of the gastromesenteric complex are
compactly plicated into the peritoneal cavity.
Crucially, this relatively simplistic shape forms a new
structural end point toward which embryologists must now
work [9].
Mesenteric rotation, anchorage,
andelongation
Relatively little is known regarding the cellular events
involved in mesenteric development. Is it reasonable to
suggest, however, that rotation, anchorage, elongation, and
attachment are important and interrelated processes. ese
will be briey discussed as follows.
During development, the mesorectum and rectum
are anchored within the developing bony pelvis. is is
crucially important as it means that the developing hindgut
is positioned to the left of the center. e vitello-intestinal
duct anchors the intestine (and by denition the mesentery)
in the midline (i.e., center) anteriorly. e superior and infe-
rior mesenteric arteries anchor the mesentery (but not the
intestine) in the midline (center) posteriorly. Anchorage of
the hindgut to the left and the midgut in the center means
that these are slightly oset in anatomic terms. e eff cts
of anchorage at both ends means elongation between these
leads to the adoption of a spiral conformation. It is illus-
trated by taking a tube, xing it at both ends, then elongat-
ing at between. e tube will curl in a spiral conformation.
When this curling is viewed front on (i.e., anteriorly), it
leads to the impression of “rotation.” As a result of elonga-
tion between points of anchorage, the right colomesenteric
(distal midgut) complex takes up position on the right side
of the peritoneal cavity, the small bowel and associated mes-
entery (proximal midgut) are centrally positioned, and the
left olomesenteric (hindgut) complex remains on the le.
Elongation of the mid and hindgut occurs at diffe ent
rates. is means that the adult midgut remnant (i.e., small
intestine, right colon, and proximal transverse) is consider-
ably longer than the hindgut remnant (distal transverse and
left ide of large bowel).
Abnormalities in the process of elongation and the adop-
tion of a spiral conformation are referred to as mal and
non-rotation and can have devastating consequences (see
Chapter 7).
Attachment: The peritoneal reection
A further process must occur once the gastromesenteric
complex has assumed its position within the abdominal
cavity, that is, attachment. At present, this is understood to
comprise two major events, the development of the perito-
neal reection and Toldt’s fascia.
When the mesentery returns to its nal adult position
a gap is present between the surface of the mesentery and
the abdominal wall. In the adult, this gap is bridged by an
extensive peritoneal reection, a layer of mesothelium that
extends from the mesenteric surface (or intestinal surface)
to the abdominal wall. e peritoneal reection occurs at
the base of the small intestinal region of mesentery, where
it attaches to the posterior abdominal wall. It continues
around the ileocecal junction and back up around the
right colon as the right peritoneal reection. From there, it
continues around the upper surface of the hepatic exure.
Itthen continues across the upper surface of the transverse
colon, between this and the greater omentum. At the splenic
exure, it continues as the splenocolic reection, which in
turn continues along the lateral aspect of the left colon as
the left peritoneal reection. From here, the reection con-
tinues into the pelvis as the left pararectal pelvic reection.
A peritoneal reection also occurs on the right side of the
mesosigmoid and continues into the pelvis as the right
pararectal reection. e right and left peritoneal reection
coalesce as the anterior reection in the pouch of Douglas.
Part II: Reverse engineering gastromesenteric embryology 43
Little is known regarding the embryologic development
of the peritoneal reection. ere are no reports in the liter-
ature, of congenital absence of the reection. Even in nonro-
tation, it is still present (albeit in a diffe ent conformation).
Given the lack of valid data related to reection develop-
ment, any statements made on the topic are highly specu-
lative. It seems reasonable, however, to suggest how the
reection develops once the gastromesenteric complex has
adopted a nal intraperitoneal position and that it serves
to secure the gastromesenteric complex in position. A sec-
ondary and perhaps coincidental benet is that it limits the
spread of future disease.
Attachment: Toldt’s fascia
Where the mesocolon and mesentery are attached to the
retroperitoneum a fascial layer arises (i.e., Toldt’s fascia).
e cellular and molecular basis of this process have not
been determined although histologic composition provides
important clues to both [10]. e fascia extends beneath the
colon where this is apposed to the retroperitoneum (i.e., on
the left and right side). It occupies a potential space between
the mesentery and retroperitoneum. e peritoneal refl c-
tion is the anatomic limit of this space and the fascia [8].
Whenever two mesothelial surfaces come into direct
and prolonged contact, Toldt’s fascia develops between
and bridges both. Movement between peritoneal surfaces
(or lack of) may be a key determinant in the process and
may explain the lack of fascial adhesion between pleural
mesothelial surfaces and between intestinal mesothelial
surfaces (outwith the context of previous abdominal sur-
gery or trauma).
Pronounced similarities occur between Toldt’s fascia and
intra-abdominal adhesions and may point to overlapping
cellular and molecular processes. Adhesions can be areolar
in appearance. Toldt’s fascia is similarly areolar and imsy
within the pelvis. Alternatively, adhesions can be lmy, thus
closely resembling Toldt’s fascia underlying the right and
left mesocolon. Finally, adhesions can be dense and vascu-
lar in nature, as can Toldt’s fascia in disease settings such as
Crohn’s disease [7]. us, it is feasible that similar develop-
mental processes underpin adhesion and fascia formation
and that prolonged direct contact between two mesothelial
surfaces is an important determinant.
Summary
A question also arises as to why the gastromesenteric com-
plex attaches to the retroperitoneum in the rst place.
Impaired attachment (coupled with other factors) predis-
poses to volvulus formation. Volvulus is where the intes-
tine and associated mesentery twist, possibly resulting in
occlusion of their blood supply, obstruction, or perfora-
tion. In nonrotation, attachment of the right mesocolon is
considerably less than that seen in normality. As a result,
the small intestine (and associated mesentery) is prone to
twist around the superior mesenteric artery. us, it appears
that attachment via the root region alone is potentially life
threatening and that additional mechanisms of attachment
are necessary in general.
A further point is also important in relation to attach-
ment. If attachment did not occur, then mesenteric elonga-
tion would likely follow intestinal elongation. Th s would
occur across the radial breadth of the mesentery and not
just at the intestinal margin. An enormous gastromesenteric
tissue mass would result [9–11]. It would be hard to see this
mass as functional, as if one were to stand upright the non-
attached mass would collapse into the pelvis and peristalsis
impaired. us, it is feasible that the attached mesenteric
conformation observed in humans aids in upright mobi-
lization, while a nonrotated and nonattached mesenteric
conformation is suffici t for species on all fours.
PART II: REVERSE ENGINEERING
GASTROMESENTERIC EMBRYOLOGY
Current theory on mesenteric anatomy diffe s markedly
from classic teaching. Mesenteric continuity presents a far
more simplistic shape as well as a new structural end point
toward to which embryologists must now work. In the fol-
lowing, we aim to adopt the processes described in part one,
in identifying a sequence of mesenteric-based events that
could lead to the adult shape. Although it is highly specula-
tive, it represents a rst step in reconciling current anatomic
and embryologic thinking.
It would be possible, albeit extremely dicult, to concep-
tualize mesenteric development from start to nish. Given
that we already know the shape of the nished product
(i.e., the adult mesentery), then it may be easier to work
backward from the adult to the embryologic shape.
e following is experimental in so far as we will com-
mence with the adult mesentery and work backward to the
embryologic dorsal mesentery. Events will be described
in terms of the processes explained above, i.e., anchorage,
elongation, rotation/curling, and attachment.
Mesentery: Reverse engineered
PERITONEAL REFLECTION
e peritoneal refl ction bridges the space between mesen-
teric surfaces and the posterior abdominal wall. e refl ction
continues laterally between the colon and lateral abdominal
wall. e refl ction is also present between the colon and
omentum. In the pelvis, it attaches the rectum to the lateral
pelvic side wall and terminates at the anterior refl ction.
REVERSE ENGINEERING THE REFLECTION:
DETACHMENT
e peritoneal reection regresses. is means that perito-
neal connections between the surface of the mesentery and
the posterior abdominal wall are lost. e reection between
the right and left colon and lateral abdominal wall is lost.
e reection between the omentum and transverse colon
is also lost. A similar phenomenon occurs in the pelvis.
44 Embryologic development of the mesentery, peritoneal reection, and Toldt’s fascia
e net eff ct is to release the mesentery, colon, and
rectum from the anatomic mainframe provided by the
posterior wall of the abdomen. At this point, the mesen-
tery remains attached via (1) Toldt’s fascia and (2)vascular
points of suspension.
HINDGUT: SHORTENING AND DETACHMENT
OFTHE MESORECTUM
e adult mesorectum is a downward extension of the meso-
sigmoid. e major vessel of the mesorectum is the superior
rectal artery (a caudal extension of the inferior mesenteric
artery once the latter has given off the left colic artery).
Atthis point, the mesorectum is attached to the pelvis via
the mesorectal fascia (i.e., Toldt’s fascia). Waldeyer’s fascia
is a distal condensation of Toldt’s fascia.
REVERSE ENGINEERING THE MESORECTUM
Dissolution of Waldeyer’s and Toldt’s fascia allows complete
detachment of the mesorectum from its bony surroundings.
e mesorectum and rectum then contract back to meso-
sigmoid level. ey continue to occupy a central position in
the midline.
HINDGUT: SHORTENING AND DETACHMENT
OFTHE MESOSIGMOID
e mesosigmoid is continuous proximally and distally with
the left mesocolon and mesorectum, respectively. Intrans-
verse section, it is attached to the retroperitoneum medially,
while laterally it is nonattached and mobile. Atthis point,
in the overall sequence, Toldt’s fascia maintains attach-
ment. e major vessel is the superior rectal and associated
sigmoidal branches.
REVERSE ENGINEERING THE MESOSIGMOID
Toldt’s fascia beneath the mesosigmoid dissolves allowing
complete detachment of the mesosigmoid from the retro-
peritoneum. e intestinal margin of the sigmoid mesentery
shortens in tandem with the sigmoid itself. e mesosig-
moid contracts along vertical and transverse axes and the
now mobile mesosigmoid rotates medially to adopt a central
position in line with the mesorectum. e inferior mesen-
teric artery and vein remain in position as the nal point of
attachment of the hindgut.
HINDGUT: SHORTENING AND DETACHMENT
OFTHE LEFT MESOCOLON
is is continuous proximally and distally with the trans-
verse and sigmoid mesocolon, respectively. e major vessel
is the left colic, a side branch of the inferior mesenteric
artery. e left mesocolon is attached throughout its full
extent, to the retroperitoneum via Toldt’s fascia.
REVERSE ENGINEERING THE LEFT MESOCOLON
Dissolution of Toldt’s fascia beneath the left mesocolon
allows detachment of the mesocolon and colon from the
retroperitoneum. e left mesocolon and colon can now
rotate medially to take up a central position in line with the
mesosigmoid and mesorectum.
At this point, the hindgut has been fully returned to a
central and midline position, entirely detached from sur-
rounding structures with exception of at points of vascular
suspension.
MIDGUT
e midgut gastromesenteric complex will be considered as
a single unit. e root region of the mesentery, where the
superior mesentery artery emerges from the pancreas, is a
crucially important anatomic landmark. e concept that
the mesentery fans out from its root region to span the intes-
tinal tract is also important. Finally, the central position of
the middle colic and inferior mesenteric artery are relevant
as they provide fi ed points of anchorage. e left side of
the mesentery (from splenic flexu e to the mesorectum) has
these, while the right does not.
We propose that during normal mesenteric development,
the mesentery fans out from its root region. As it does so, it
adopts a spiral conformation centered on the root region.
e overall impression (when viewed from in front) is that
of rotation. Rotation stops where the middle colic and infe-
rior mesenteric vessels exert a break-like eff ct (i.e., at the
level of the splenic exure).
REVERSE ENGINEERING MESENTERY BETWEEN
THEDUODENOJEJUNAL AND SPLENIC FLEXURE
Toldt’s fascia, attaching the midgut to the posterior abdomi-
nal wall, dissolves. is means the midgut is now free to
unwind. If viewed from the front on, unwinding gives
the impression of straightening in a clockwise direction.
Unwinding occurs as the midgut shortens (i.e., reverse of
elongation). It unwinds in a clockwise direction because of
the mechanical eff ct of the middle colic and inferior mes-
enteric vessels. As it unwinds, the vitello-intestinal duct
reforms and connects the small intestine with the umbilicus
in the midline. e process of unwinding clockwise means
that the gastromesenteric complex of the midgut takes up a
central position, above (but continuous with) the hindgut.
Summary
e following is the sequence of events which, if reversed,
explain how the laminar dorsal mesentery transforms into
the adult shape:
1. e peritoneal reection regresses.
2. Toldt’s fascia regresses beneath the hindgut mesentery
leaving it entirely detached with the exception of at
vascular points of suspension.
3. e hindgut then shortens and adopts a central position
in the midline.
4. Toldt’s fascia regresses beneath the small intestinal
mesentery and right mesocolon, thereby fully freeing
References 45
the midgut mesentery with exception of at points of
vascular suspension (i.e., the superior mesenteric root
region).
5. e midgut mesentery and intestine shortens.
6. Shortening causes to the midgut to appear to unwind
clockwise under the mechanical inuence of the hindgut.
7. e midgut mesentery and intestine take up a position
above the hindgut mesentery and intestine.
8. e embryonic dorsal mesentery has reformed.
e reverse of this process may occur as follows:
1. e laminar dorsal mesentery is positioned in the
midline, connected anteriorly via the vitello-intestinal
duct and posteriorly via the superior and inferior
mesentery vessels.
2. e hindgut mesentery and intestine elongate and then
attaches to the left f the midline.
3. e midgut elongates and because of the le-
sided position of the hindgut, it takes up a spiral
conformation.
4. e midgut adopts a position whereby the right colon
and mesentery are located on the right side, with the
small bowel and mesentery centrally positioned.
5. Toldt’s fascia attaches the mesentery to the posterior
abdominal wall.
6. e peritoneal reection develops to bridge any space
between the intestine, mesentery, and abdominal wall
(including also the greater omentum).
FUTURE DIRECTIONS
e mesentery and associated intestine are integrated at
anatomical and histological levels. e cellular and extracel-
lular developmental events that underpin such integration
must also be tightly coordinated at multiple levels. is is
supported by ndings related to neural crest cells, as well as
by vascular, lymphatic, and connective tissue contiguity. As
mentioned earlier, the incidence of embryologic mesenteric
abnormalities is extremely low, indicating the process is
highly conserved. It is remarkable to think that at the intesti-
nal margin of the mesentery, a multilayered intestine devel-
ops involving as of yet uncharacterized interactions between
intestinal endoderm and mesenteric mesoderm. Future
studies should aim to better characterize the embryologic
development of the intestine, as well as the manner in which
this is integrated with that of the mesentery in general.
Future studies should determine the conformation of the
gastromesenteric complex in other species. In particular, com-
parisons should be made between species that mobilize on all
fours with those that stand upright for prolonged periods.
Finally, online archives of images that depict human embry-
ologic development should be reviewed in the context of the
current understanding of mesenteric and peritoneal anatomy.
SUMMARY
Classic embryologic appraisals of mesenteric development
were based on the concept of it being fragmented and
complex, in the adult. As the mesenteric organ distal to the
third part of the duodenum is now regarded as continu-
ous, the embryologic development of this and associated
structures (i.e., peritoneum and intestinal tract) must be
reappraised.
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