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Surg Radiol AnatDOI 10.1007/s00276-014-1301-y

AnAtOmIc BASeS Of meDIcAl, RADIOlOgIcAl AnD SuRgIcAl technIqueS

Preliminary embryological study of the radiological concept of retroperitoneal interfascial planes: what are the interfascial planes?

Kazuo Ishikawa · Shota Nakao · Gen Murakami · Jose Francisco Rodríguez‑Vázquez · Tetsuya Matsuoka · Makoto Nakamuro · Takeshi Shimazu

Received: 13 february 2014 / Accepted: 12 April 2014 © Springer-Verlag france 2014

connective tissue. A hypothesis for the formation of the interfascial planes was generated from the developmental study and analysis of retroperitoneal fasciae in computed tomography images from 224 patients.Results Whereas the loose connective tissue was uni-formly distributed in the retroperitoneum by the 9th week, the primitive renal and transversalis fasciae appeared at the 10th–12th week, as previous research has noted. By the 23rd week, the renal fascia, transversalis fascia, and primi-tive adipose tissue of the flank pad emerged. In addition, the primitive lateroconal fascia, which runs parallel to and close to the posterior renal fascia, emerged between the renal fascia and the adipose tissue of the flank pad. con-versely, pre-existing loose connective tissue was sand-wiched between the opposing fasciae and was compressed and narrowed by the developing organs and fatty tissues.Conclusion through this developmental study, we pro-vided the hypothesis that the compressed loose connective tissue and both opposed fasciae compose the interfascial planes. Analysis of the thickened retroperitoneal fasciae in computed tomography images supported this hypothesis. further developmental or histological studies are required to verify our hypothesis.

Keywords Retroperitoneal space · Interfascial planes · fetal development · multidetector computed tomography

Introduction

the structure of the retroperitoneum has not been entirely clarified. Radiologically, meyers et al. [18, 19] built a famous tricompartmental theory for the retroperitoneal structure based on gerota’s anatomical study [9]. how-ever, the theory was unable to fully explain the dynamic

Abstract Purpose Recently, the radiological concept of retroperito-neal interfascial planes has been widely accepted to explain the extension of retroperitoneal pathologies. this study aimed to explore embryologically based corroborative evi-dence, which remains to be elucidated, for this concept.Methods using serial or semi-serial transverse sections from 29 human fetuses at the 5th–25th week of fetal age, we microscopically observed the development of the retro-peritoneal fasciae and other structures in the retroperitoneal

K. Ishikawa (*) · S. nakao · t. matsuoka Senshu trauma and critical care center, Rinku general medical center, 2-23 Rinku-Ourai-Kita, Izumisano, Osaka 598-0048, Japane-mail: ishikawa@sccmc.izumisano.osaka.jp; k-ishikawa@seikeikai.or.jp

Present Address: K. Ishikawa Department of Surgery, Seikeikai hospital, 4-2-10, Kouryou-nakamachi, Sakai-ku, Sakai, Osaka 590-0024, Japan

g. murakami Division of Internal medicine, Iwamizawa Kojin-kai hospital, 297-13 Shibun-cho, Iwamizawa, hokkaido 068-0833, Japan

J. f. Rodríguez-Vázquez Department of Anatomy and embryology II, faculty of medicine, universidad complutense, ciudad universitaria, 28040 madrid, Spain

m. nakamuro Department of Surgery, Seikeikai hospital, 4-2-10, Kouryou-nakamachi, Sakai-ku, Sakai, Osaka 590-0024, Japan

t. Shimazu Department of traumatology and Acute critical medicine, Osaka university graduate School of medicine, Suita, Osaka 565-0871, Japan

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changes of retroperitoneal pathologies because it con-sidered the retroperitoneal fasciae as mere boundaries of three compartments in the retroperitoneum and as barri-ers which confine the retroperitoneal pathologies to one compartment. Recently, however, the radiological concept of interfascial planes, which considers the retroperitoneal fascia as pathways within which retroperitoneal pathol-ogy can extend beyond the original compartment [1, 2, 10, 20], has been widely accepted not only by radiologists [15, 21] but also by general physicians and surgeons [13, 14]. the concept nicely explains the extension of retrop-eritoneal lesions [1, 2, 10, 13–15, 20, 21], and the severity of retroperitoneal pathologies can be stratified according to this concept [13, 14]. now, this concept appears likely to be essential for imaging diagnosis of retroperitoneal pathologies [15, 21].

however, the concept remains incomplete. for exam-ple, the relations between the interfascial planes and the anterior pararenal space [25] and other potential spaces [13, 14] are left unsolved. Additionally, the concept has a serious defect in that there is no supporting evidence for the concept anatomically or embryologically, except for the colored latex injection experiment in cadavers [20]. Although there are many classic studies on retroperito-neal fascia [3, 5, 6, 9, 12, 26], none of them described fascial structures that can explain the interfascial planes. the advocators of the concept [1, 2, 10, 20] considered the planes as potential spaces within the fusion fasciae, according to the Dodds et al. [7] paper, and their consid-eration is continuing to be cited and to be taught widely without any verification [15, 21]. the advocators recog-nized the boundary between the primary retroperitoneum and the secondary retroperitoneum, which originally belonged to the peritoneal cavity, as the retromesenteric plane, and they considered the origin of the retromesen-teric plane as the fusion fascia with the visceral perito-neum and the posterior parietal peritoneum, that is, the fusion fascia of toldt and that of treitz. If their consid-eration is valid, however, the retromesenteric plane must escape from the retroperitoneum into the peritoneal cavity and not continue into the pelvic retroperitoneum. Some anatomists deny the existence of interfascial planes and consider the planes as artificial contaminations [28]. even if the concept of interfascial planes can reasonably explain the extension of retroperitoneal lesions, the absence of the anatomical evidence might threaten the existence of the planes and prevent further development of the concept.

therefore, we re-examined the embryological devel-opment of the retroperitoneal fasciae by microscopically observing the transverse sections of human fetuses in light of the interfascial planes and proposed a hypoth-esis for the development of the planes. In addition, we reviewed computed tomography (ct) images in a clinical

study to support the hypothesis. the main purpose of this study was to establish the embryological basis of the interfascial planes and not to analyze their structure in detail.

Materials and methods

embryological study

We examined under a light microscope retroperitoneal tissue in paraffin-embedded transverse sections stained with hematoxylin and eosin from 29 human fetuses: 19 embryonic-stage fetuses with a crown-rump length (cRl) of 10–31 mm (5th–8th week of fetal life), 6 early-stage fetuses with a cRl of 40–68 mm (9th–12th week), and 4 middle-stage fetuses with a cRl of 155–180 mm (23rd–25th week). All sections were a part of the large collec-tion maintained for medical education at the embryology Institute, universidad complutense, madrid, Spain, and the fetuses were products of miscarriages and ectopic pregnan-cies managed at the Department of Obstetrics at the uni-versity. Almost all were serial sections made at 50–100-μm intervals with a slice thickness of 10 μm, whereas four specimens from the middle-stage fetuses were semi-serial sections made at 100–200-μm intervals with a slice thick-ness of 10–20 μm. the present study was performed in accordance with the provisions of the Declaration of hel-sinki 1995 (as revised in edinburgh 2000). Approval of the study was granted by the university ethics committee (no. B08/374).

clinical study

medical records and abdominal ct images were reviewed retrospectively for a consecutive 303 non-trauma patients admitted to Senshu trauma and critical care center in Japan in the three most recent years. Of the 303 patients comprising this study, 67 patients with a history of lapa-rotomy and 12 patients suffering from pancreatitis were excluded from the precise review because of the possibility that their retroperitoneal fasciae might be injured by exter-nal forces or proteolytic pancreatic juice. ct scans were obtained with a 64-detector helical ct scanner (Aquilion; toshiba medical Systems, tokyo, Japan). Sectional images of 5-mm thickness were obtained at 5-mm intervals with the scanner. In the ct images, retroperitoneal fasciae cor-responding to the interfascial planes, to the combined interfascial plane, and to the subfascial plane [14], which is a possible potential space between the posterior para-renal space and the transversalis fascia, were identified. Additionally, thickened fasciae of more than 3-mm thick-ness, as defined by Parienty et al. [22], corresponding to

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fluid collection in the interfascial planes and in the related planes, were identified.

Results

microscopic observation of embryonic-stage fetuses

the definitive kidney was induced at the 5th week of fetal life and was located adjacent to the mesonephros in the pel-vic retroperitoneum. At the 5th or 6th week, point-like mes-oderm cells were scattered randomly around the kidneys (fig. 1a). At the 7th or 8th week, loose mesenchymal tis-sue composed of cells and constituent fibers was observed around the kidney (fig. 1b). At this stage, no primordial structures of the retroperitoneal fasciae could be identified.

microscopic observation of early-stage fetuses

In this stage, mesonephroi begin to regress, in contrast with cranial migration and rapid growth of the definitive kidney. At the 10th week of fetal age, subtle density differences occur in the loose retroperitoneal connective tissues. tight connective tissues were identified around the kidney and on the inside of the abdominal muscles, which individu-ally looked like the renal fascia or the transversalis fascia (fig. 2a). At the 12th week, the primitive near-circumfer-ential renal fascia was identified around the kidney, and the clear transversalis fascia was also observed (fig. 2b).

furthermore, vascular- and cell-rich structures, in which no vacuolated cells were identified (fig. 2c), appeared between the primitive renal fascia and the primitive trans-versalis fascia in the loose connective tissue. the latero-conal fascia was not yet identified in this stage. the anterior part of the primitive renal fascia fused with the peritoneum, and the posteromedial part of the fascia was indistinct. the mesocolon did not yet fuse with the peritoneum.

microscopic observation of middle-stage fetuses

At the 23rd–25th week of fetal age, other fascial structures were observed in addition to the evident renal and trans-versalis fasciae. fasciae were evident on the surface of both the psoas muscle and quadratus lumborum muscle (fig. 3a). In addition, thick fasciculation of a multilaminar structure, which was obscure near the psoas muscle, was observed on the outer side of the renal fascia (fig. 3a). Obvious adipocytes including lipid droplets were observed in this cell-rich structure (fig. 3b). thus, this structure, which was also identified at the 12th week, was definitely immature fatty tissue. At this stage, the colon was attached to the retroperitoneal tissue, and both visceral and parietal peritoneums were fusing (fig. 3a).

clinical study of ct images

the 224 study patients had a broad range of underlying diseases: digestive diseases in 104 patients, cardiovascular

Fig. 1 Retroperitoneal structure in embryonic-stage fetuses. a the left definitive kidney and mesonephros are seen in a transverse sec-tion of a fetus at the 6th week of fetal life (cRl 14 mm). they are embedded in the point-like mesenchymal cells. b the lower pole of the right kidney is seen in a transverse section of a fetus at the

8th week (cRl 27 mm). Reticular arrangement in the loose connec-tive tissue is seen in the retroperitoneum. K kidney, M mesonephros, P psoas muscle, Q quadratus lumborum muscle, R urogenital ridge, SV subcardinal vein, U ureter, V vertebra

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diseases in 35, respiratory diseases in 24, urogenital dis-eases in 23, and others in 38. Of the 224 patients, 118 (52.7 %) suffered from infectious diseases; in particular, septic shock was observed in 52 of them. Retroperito-neal fasciae were identified in 215 (96.0 %) of the study patients, as shown in table 1. moreover, the combined interfascial plane and the subfascial plane [14] were identi-fied in 34 (15.2 %) and in 30 (13.4 %) patients, respectively (fig. 4a). Additionally, thickened fascia of more than 3-mm thickness was identified in 54 (24.1 %) patients. the com-bined interfascial plane or the subfascial plane depicted as thickened fascia (fig. 4b) was identified in 24 (10.7 %) or in 16 (7.1 %) patients, respectively, accompanied by other thickened retroperitoneal fasciae. In particular, the image of fluid collection within the subfascial plane was called the checkmark sign from its characteristic appearance [13, 14]. Of the 54 patients with any thickened fasciae, only 14

(25.9 %) had a disorder in the retroperitoneal organs or ves-sels, 6 in the urogenital organs, 5 in the colon, and 3 in the aorta. Only one patient had retroperitoneal hemorrhage, which resulted from rupture of an abdominal aortic aneu-rysm (AAA).

Discussion

this is the first study, to our knowledge, to examine the fetal retroperitoneal fasciae microscopically in light of the radiological concept of interfascial planes even though the number of specimens, especially those from the middle-stage fetuses, was small. We found the following phenom-ena: In the embryonic stage, the retroperitoneal organs were embedded in the homogeneous loose mesenchymal tissues. At the 10th or 12th week of fetal life, dense parts

Fig. 2 Retroperitoneal structure in early-stage fetuses. a fascia-like structures were identified around the kidney (arrows) and on the inside of the abdominal muscles (arrowheads) in a transverse section of a fetus at the 10th week of fetal life (cRl 50 mm). b the primitive renal fascia (arrows) and the primordial transversalis fascia (arrow-heads) are observed in a transverse section of a fetus at the 12th week (cRl 67 mm). the cell-rich area (open arrow) appears between the

primitive renal and transversalis fasciae. c higher magnification of the cell-rich area in b. there are many vessels (thin arrows) in the part sandwiched between the renal fascia (arrows) and the transver-salis fascia (arrowheads). note the existence of red blood cells in the vessels. C ascending colon, I ilium, K kidney, L liver, P psoas muscle, Q quadratus lumborum muscle, V vertebra

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were observed in the homogeneous loose connective tissue, and furthermore, the dense connective tissue differentiated into fibrous structures, considered as the primordial renal or transversalis fascia, and into the cell-rich area. At the 23rd week, both the renal and transversalis fasciae became clearly identifiable, and primitive adipose tissue developed between the renal fascia and the transversalis fascia. In addition, another fascia-like structure, considered to be the lateroconal fascia, appeared between the renal fascia and the primitive adipose tissue of the flank pad. conversely, the primitive structure of the homogeneous loose connec-tive tissue was compressed and narrowed by the developing organs and fatty tissues and was sandwiched between the opposing fasciae or peritoneum.

the above findings are based neither solely on the par-ticular fetal specimens examined nor on a biased view-point. Some earlier researches reported findings similar to ours. many researchers [3, 12, 26] reported that the kid-neys and adrenal glands are embedded in loose connective

tissue in the embryonic stage. Baumann [3] reported in a microscopic study the emergence of the primordial renal and transversalis fasciae in the loose connective tissues of fetuses in the third month of gestation. hayes [12] also reported both the presence of condensed connective tis-sue around the kidney at the 11th week of fetal life and the development of renal and transversalis fasciae at the 13th–14th week by microscopic observation of human fetuses. the presence of the fasciae has been macroscopically con-firmed by many researchers such as gerota [9] and tobin [26] in late-stage fetuses. In the classic theory, it was rec-ognized that there are three types of development of the retroperitoneal fasciae [12]: migration fasciae, which are less-known; fusion fasciae, developed by fusion of oppos-ing peritoneums, which are well-known and are cited in the Dodds et al. [7] paper; and parietal fasciae, such as the transversalis fascia. According to hayes [12], the continu-ous migration and growth of organs gradually brings pres-sure on the spongy loose connective tissues to produce a linear orientation of the loose connective tissue fibers, which are further compressed to be fascia-like morpho-logical patterns. Such mechanism provides a basis for the concept of “migration fascia”. the renal fascia, formed by compression resulting from ascent and growth of the kid-ney, is considered as the representative migration fascia.

Development of the adipose tissue has been studied since the nineteenth century [8, 23, 27]. Adipose tissue is derived from the embryologic mesenchyme, and the first indication of adipogenesis is mesenchymal condensation with proliferation of vessels termed a “primitive organ” [23, 27] around the 14th week of fetal life. next, mesen-chymal cells around the capillary network in the primitive

Fig. 3 Retroperitoneal structure in a middle-stage fetus. a the primi-tive lateroconal fascia (stars) is observed in a transverse section of a fetus at the 25th week of fetal age (cRl 180 mm). fasciae on the sur-face of the psoas muscle and quadratus lumborum muscle (open cir-cles) are evident. the dense fasciculation (stars) runs parallel to the posterior renal fascia (arrows) and the fusing of the parietal perito-neum with the visceral one (thin arrows) but is obscure near the psoas

muscle (open triangle). Between the primitive lateroconal fascia and the transversalis fascia, a massive cell-rich structure is identified. b higher magnification of the lateroconal fascia (stars) adjacent to the primitive adipose tissue and renal fascia (arrows). note the immature adipocytes including lipid droplets (thin arrows). C ascending colon, K kidney, P psoas muscle, Q quadratus lumborum muscle, fat fatty tissue

Table 1 Identification of the retroperitoneal fasciae

Part of the retroperitoneal fasciae Visible fascia (%) thickened fascia (%)

Anterior renal fascia 116 (51.8) 27 (12.1)

Posterior renal fascia 215 (96.0) 51 (22.8)

lateroconal fascia 159 (71.0) 29 (12.9)

One or more parts of fasciae 215 (96.0) 54 (24.1)

Other fascia or fascia-like structures

combined interfascial plane 34 (15.2) 24 (10.7)

Subfascial plane 30 (13.4) 16 (7.1)

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organ differentiate into preadipocytes and form primitive adipose tissue. these preadipocytes, which do not initially contain lipid droplets, ultimately form definitive fat tissues together with vessels, lymphatics, nerves, and other con-nective components. According to Poissonnet et al. [23], fatty tissue around the kidney appears at the 20th week. matsubara et al. [17] reported fatty tissue forming the flank pad of the posterior pararenal space in the fetus at 20 weeks of gestation, and fatty tissue in the perirenal space formed later, at 23 weeks of gestation.

As mentioned above, development of the retroperitoneal structure has already been researched extensively [3, 5, 6, 9, 12, 26], but the classic studies did not clarify the origin of the lateroconal fascia. condon and edson [5] closely exam-ined the fascia and gave it the name of “lateroconal fascia”. they explained that both the anterior and the posterior renal fascia fuse mutually and form the lateroconal fascia and the fascia terminated by blending with the peritoneal reflection, which forms the paracolic gutter. even after their study, it has long been thought that the anterior and pos-terior renal fasciae unite to form the lateroconal fascia [3, 6, 12, 18, 19, 26]. Raptopoulos et al. [24] and marks et al. [16], however, demonstrated that the lateroconal fascia is distinct from the renal fascia by macroscopic dissection in cadavers and by histological study. they concluded that the posteromedial part of the lateroconal fascia runs close to the posterior renal fascia, and this combination looks like a single-layered fascia. the embryologic development of the lateroconal fascia was first reported in the present century by matsubara et al. [17]. they identified fasciculation of the multilaminar structure microscopically in the retroperi-toneal connective tissue between the posterior renal fascia

and the developing fatty tissue of the flank pad in the fetus after 20 weeks of gestation. they confirmed that the fas-ciculation is the lateroconal fascia because after leaving the renal fascia, it extended anterolaterally into a narrow space between the thin colonic adventitia or peritoneum and the flank pad. they concluded that the lateroconal fascia origi-nates from retroperitoneal loose connective tissue outside the renal fascia and that the fascia was a migration fascia resulting from mechanical compression due to rapid growth of the kidney and the flank pad.

In the present study, we were unable to find adequate sections in the prepared slides large enough to overview the retroperitoneal spaces in later-stage fetuses. We noticed, however, that the primitive loose connective tissues con-sistently exist in the retroperitoneum and adipose tissues, which occupy a great part of the retroperitoneum in the adult, do not develop until the middle fetal period. We pos-tulated that the content of the interfascial planes might be a remnant of the primitive loose mesenchymal tissue. If these connective tissues do not vanish altogether, there will be no places other than the interfascial planes for them to remain. Additionally, as shown in fig. 5b, we hypothesized that the interfascial planes originate from a combination of pre-existing loose connective tissue, which might be rudimen-tary in the adult, and the later-developed opposed fasciae sandwiching the former. Our hypothesis is coincident with the findings by Raptopoulos et al. [24] and marks et al. [16]. they demonstrated that the posterior renal fascia is composed of three layers: the inner layer of true posterior renal fascia, the outer layer of proper lateroconal fascia, and the intervening connective tissue. this fact means that loose connective tissues between the proper lateroconal

Fig. 4 Retroperitoneal fasciae identified in ct images. a contrast-enhanced ct image from a 71-year-old man with septic shock. All of the anterior renal fascia (white arrow), posterior renal fascia (black arrow), and the lateroconal fascia (white triangle) are clearly identi-fied. note that curved line between the posterior renal fascia and inner body wall (short white arrows), which is considered as the fas-cia corresponding to the subfascial plane [14], is not identified in the left retroperitoneum. b ct image from a 52-year-old man in septic shock complicated by acute renal failure. In the right retroperito-

neum, a thickened subfascial plane (white short arrows), forming a checkmark sign [13] (as illustrated in the inset square), is clear, as are the thickened posterior renal fascia (black arrows), anterior renal fas-cia (white arrow), and lateroconal fascia (white triangle). In the left retroperitoneum, thickened posterior renal fascia (black arrow) is identified as well as the clear anterior renal fascia (white arrow), lat-eroconal fascia (white triangle), and fascia corresponding to the sub-fascial plane (white short arrow). Ao aorta, IVC inferior vena cava, K kidney

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and posterior renal fasciae remain in the adult. Although their studies were confined to only the posterior renal fas-cia and did not touch on embryology, our hypothesis has developed their consideration into the universal theory that the interfascial planes are composed of three layers. they interpreted intervening tissues between the proper renal and lateroconal fasciae as pathways by which pan-creatic inflammation spread. however, we take issue with their consideration of the pathway as the anterior pararenal space [25] because the other spaces, the posterior pararenal and perirenal spaces, are basically composed of adipose tis-sue and not of loose connective tissue.

Our hypothesis was clinically supported by ct images. In this study, fasciae were identified more frequently (96.0 %) than in another study [22]. A small amount of fluid collection in the interfascial planes might make the retroperitoneal fasciae clear, as shown in fig. 4a. thick-ened fasciae were identified in almost one quarter of the patients. With the exception of one patient with retroperi-toneal hematoma, no mechanisms can explain the pathol-ogy of fascial thickness other than fascial edema resulting from volume overload associated with heart failure, renal failure, or septic shock with enhanced permeability. In addition, fluid collection in the subfascial plane, forming a checkmark sign [13], was identified in 7 % of patients. the sign was often seen in patients with severe retroperitoneal

trauma or with severe acute pancreatitis [13, 14], but in this study, there was no possibility of leakage from the damaged interfascial planes, except in the one patient with a ruptured AAA, which means that another potential space continuing into the interfascial planes must exist. more surprisingly, as shown in fig. 6, the loose connective tissue identified in the cross-sectional fetus at the 12th week closely resembles fluid collection in the interfascial planes and in the subfas-cial plane in ct images if we consider the loose connec-tive tissue as the retroperitoneal fluid collection. It might be natural that the expanded potential space of the interfascial planes resembles the loose connective tissues before being compressed.

Recently, cho et al. [4] reported embryological develop-ment of the retropancreatic fascia, which is known as the fusion fascia of treitz. According to their histological study, immediately after the retroperitoneal fixation of the pancreas at around 10 weeks of gestation, no distinct fascia behind the pancreas can be detected; instead, only a loose connective tissue layer is observed. they found that the retropancreatic fascia emerged after 20 weeks of gestation and concluded that the fascia is one of the migration fasciae formed by mechanical compression due to development of the pancreas and duodenum. the commonly accepted theory believes that the interfascial planes are potential spaces within the fusion fascia by considering the retropancreatic fusion fascia as a

Fig. 5 Diagram illustrating of our findings and our hypothesis. a (our findings) the clear renal and transversalis fasciae and the developing lateroconal fascia are located in the loose connective tissues (stip-pled area). Immature adipose tissue is also developing in the connec-tive tissues between the lateroconal fascia and the transversalis fas-cia and in those within the perirenal space. b (our hypothesis) the pre-existing loose connective tissues (stippled area) are narrowed by developing fatty tissue and organs. We consider the combination of the peritoneum, anterior renal fascia, and loose connective tissue as the retromesenteric plane; the combination of the peritoneum, latero-conal fascia, and the loose connective tissue as the lateroconal plane; and the combination of the posterior renal fascia, lateroconal fascia

or fasciae of the psoas muscle and quadratus lumborum muscle, and the loose connective tissue as the retrorenal plane. Although these structures are not yet identified, we consider the combination of the transversalis fascia, the fascia formed behind the flank pad of the pos-terior pararenal space (gray area), and the loose connective tissue as the subfascial plane and the combination of the fasciae around the fat clusters in the perirenal space and the remnant connective tissue as the perinephric bridging septa. the subfascial plane communicates with the interfascial planes through the obscure part of the lateroconal fascia near the psoas muscle (double-headed arrow; see fig. 3a). P psoas muscle, PBS perinephric bridging septa, Q quadratus lumbo-rum muscle

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part of the retromesenteric plane [2, 15, 20, 21]. however, the theory must be denied if observations by cho et al. [4] are true. Instead, the findings in this study should serve as a basis for complete clarification of the interfascial planes. In addition, we believe that the findings are helpful in clarifying the other potential spaces [11] because they might be formed in the same manner as shown with our hypothesis.

Conclusions

We presented the hypothesis that the interfascial planes develop as a combination of the pre-existing retroperito-neal loose connective tissue and later-developed migra-tion fasciae by examining the fetal retroperitoneal tissues microscopically. the hypothesis can be supported by the analysis of ct images. further studies will be required to verify our hypothesis and to clarify the precise structure of the planes.

Acknowledgments this work was supported by a grant from marumo Research foundation for emergency medicine in Japan.

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Fig. 6 Our viewpoint of the microscopic morphology in a fetus. the microscopic morphology is from the fetus shown in fig. 2b. the loose connective tissue looks like the fluid collection in the interfas-cial and subfascial planes as shown in fig. 4b. the posterior pararenal space is colored in gray and the perirenal space in yellow. LCP latero-conal plane, RMP retromesenteric plane, RRP retrorenal plane, SFP subfascial plane (color figure online)

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