[Intraoperative anatomical observation of mesentery morphology of colonic splenic flexure].

Objective: At present, surgeons do not know enough about the mesenteric morphology of the colonic splenic flexure, resulting in many problems in the complete mesenteric resection of cancer around the splenic flexure. In this study, the morphology of the mesentery during the mobilization of the colonic splenic flexure was continuously observed in vivo, and from the embryological point of view, the unique mesenteric morphology of the colonic splenic flexure was reconstructed in three dimensions to help surgeons further understand the mesangial structure of the region. Methods: A total of 9 patients with left colon cancer who underwent laparoscopic radical resection with splenic flexure mobilization by the same group of surgeons in Union Hospital of Fujian Medical University from January 2018 to June 2019 were enrolled. The splenic flexure was mobilized using a "three-way approach" strategy based on a middle-lateral approach. During the process of splenic flexure mobilization, the morphology of the transverse mesocolon and descending mesocolon were observed and reconstructed from the embryological point of view. The lower margin of the pancreas was set as the axis, and 4 pictures for each patient (section 1-section 4) were taken during middle-lateral mobilization. Results: The median operation time of the splenic flexure mobilization procedure was 31 (12-55) minutes, and the median bleeding volume was 5 (2-30) ml. One patient suffered from lower splenic vessel injury during the operation and the bleeding was stopped successfully after hemostasis with an ultrasound scalpel. The transverse mesocolon root was observed in all 9 (100%) patients, locating under pancreas, whose inner side was more obvious and tough, and the structure gradually disappeared in the tail of the pancreatic body, replaced by smooth inter-transitional mesocolon and dorsal lobes of the descending colon. The mesenteric morphology of the splenic flexure was reconstructed by intraoperative observation. The transverse mesocolon was continuous with a fan-shaped descending mesocolon. During the embryonic stage, the medial part (section 1-section 2) of the transverse mesocolon and the descending mesocolon were pulled and folded by the superior mesenteric artery (SMA). Then, the transverse mesocolon root was formed by compression of the pancreas on the folding area of the transverse mesocolon and the descending mesocolon. The lateral side of the transverse mesocolon root (section 3-section 4) was distant from the mechanical traction of the SMA, and the corresponding folding area was not compressed by the tail of the pancreas. The posterior mesangial lobe of the transverse mesocolon and the descending mesocolon were continuous with each other, forming a smooth lobe. This smooth lobe laid flat on the corresponding membrane bed composed of the tail of pancreas, Gerota's fascia and inferior pole of the spleen. Conclusions: From an embryological point of view, this study reconstructs the mesenteric morphology of the splenic flexure and proposes a transverse mesocolon root structure that can be observed consistently intraopertively. Cutting the transverse mesocolon root at the level of Gerota's fascia can ensure the complete resection of the mesentery of the transverse colon.

Morphologic alterations of the genital mesentery implicated in testis non-descent in rats prenatally exposed to flutamide.

BACKGROUND: There is an endless debate on whether androgens mediate testis descent through developmental changes in the gubernacular or the cranial suspensory ligament. OBJECTIVE: To investigate the relation of any possible morphologic changes in the genital mesentery, that is, the system of genital peritoneal folds including the gubernacular and cranial suspensory ligaments, with the event of testis non-descent in rats prenatally exposed to the antiandrogen flutamide. MATERIALS AND METHODS: Time-pregnant Sprague Dawley rats received flutamide (100 mg/kg/d) or vehicle subcutaneously on gestational days 16-17. Flutamide-treated male offspring (n = 67), and vehicle-treated male (n = 34) and female (n = 28) offspring were surgically explored under microscope on postnatal day 50. Testicular position was examined bilaterally. Dimensions of genital mesentery parts were also assessed bilaterally. Association of flutamide-induced morphologic changes with descended (n = 61) and undescended (n = 50; 33 cryptorchid and 17 ectopic) testes was investigated with logistic regression analysis. RESULTS: The male genital mesentery comprised a cranial and a caudal fold converging on the vas deferens. Flutamide resulted in enlarged cranial and reduced caudal folds. Of all flutamide-induced alterations, the increased length of the posterior fixation of the cranial fold and the decreased length of the gubernacular ligament of the caudal fold were found to independently increase the odds of testis non-descent. Testicular ectopy, unlike cryptorchidism, was associated with a short gubernacular ligament only. The female genital mesentery consisted of a cranial fold only. CONCLUSION: Our findings showed a combined contribution of both cranial and caudal folds of the genital mesentery to testis non-descent, through an abnormally long mesentery root and an abnormally short gubernacular ligament, respectively. Inhibition of male-specific development of the genital mesentery with flutamide did not result in a feminized architecture.

Variations in Laminar Arrangements of the Mesocolon and Retropancreatic Fascia: a Histological Study Using Human Fetuses Near Term.

OBJECTIVE: The embryonic mesentery of the ascending and descending colons as well as the pancreas disappears due to peritoneal fusion, but there might be no or few photographic demonstrations of the intermediate morphologies during the process. The aims of this study were to characterize the morphological relationship of the interface between the renal fascia and peritoneum. METHODS: Fourteen late-stage fetuses with crown rump lengths (CRLs) of 250-325 mm (gestational age: 30-38 weeks) were histologically examined. RESULTS: The renal fascia, a thick or thin layer consisting of densely-distributed abundant fibers, was consistently separated from the renal capsule by a perirenal space containing fat. The transverse colon carried a typical mesocolon histologically different from the renal fascia. The ascending and descending mesocolons were irregularly divided into multiple laminae and the colic external longitudinal muscle appeared to directly contact the renal fascia. There was a spectrum of variations from multiple laminae to a single thick fascia between the pancreatic body and the left kidney or adrenal. CONCLUSIONS: A fascial development after retroperitoneal fusion of the mesentery showed great individual and site-dependent differences in proportion of 1) a complete fusion with the renal fascia and 2) a multilaminar structure including the remnant peritoneum. These variations masked the likely stage-dependent change.

Mapping of Extrinsic Innervation of the Gastrointestinal Tract in the Mouse Embryo.

Precise extrinsic afferent (visceral sensory) and efferent (sympathetic and parasympathetic) innervation of the gut is fundamental for gut-brain cross talk. Owing to the limitation of intrinsic markers to distinctively visualize the three classes of extrinsic axons, which intimately associate within the gut mesentery, detailed information on the development of extrinsic gut-innervating axons remains relatively sparse. Here, we mapped extrinsic innervation of the gut and explored the relationships among various types of extrinsic axons during embryonic development in mice. Visualization with characterized intrinsic markers revealed that visceral sensory, sympathetic, and parasympathetic axons arise from different anatomic locations, project in close association via the gut mesentery, and form distinctive innervation patterns within the gut from embryonic day (E)10.5 to E16.5. Genetic ablation of visceral sensory trajectories results in the erratic extension of both sympathetic and parasympathetic axons, implicating that afferent axons provide an axonal scaffold to route efferent axons. Coculture assay further confirmed the attractive effect of sensory axons on sympathetic axons. Taken together, our study provides key information regarding the development of extrinsic gut-innervating axons occurring through heterotypic axonal interactions and provides an anatomic basis to uncover neural circuit assembly in the gut-brain axis (GBA).SIGNIFICANCE STATEMENT Understanding the development of extrinsic innervation of the gut is essential to unravel the bidirectional neural communication between the brain and the gut. Here, with characterized intrinsic markers targeting vagal sensory, spinal sensory, sympathetic, and parasympathetic axons, respectively, we comprehensively traced the spatiotemporal development of extrinsic axons to the gut during embryonic development in mice. Moreover, in line with the somatic nervous system, pretarget sorting via heterotypic axonal interactions is revealed to play critical roles in patterning extrinsic efferent trajectories to the gut. These findings provide basic anatomic information to explore the mechanisms underlying the process of assembling neural circuitry in the gut-brain axis (GBA).

[Scientific interpretation of membrane anatomy theory and standardized application of membrane anatomy terms].

There has been an upsurge of the theory of membrane anatomy in China, but it is still in the initial stage of establishing preliminary framework. The concept of fasciae in membrane anatomy actually refers to the fasciae constituting the particular plane or the 'holy plane'. Therefore, the membrane anatomy can't simply be defined as the anatomical relationship among fascia. The application of the membrane anatomy is also not just to pursue the avascular plane in the surgical field. Nowadays, nonstandard anatomical terms and diversification of views impede the development of the theory of the membrane anatomy. Fasciae occur in embryonic stage, undergo a series of changes in rotation and fusion, and lose the original features, which bring difficulties in understanding the anatomy of fasciae. In this paper, we restore the origin and continuity of fasciae related to the colorectal surgery by cadaveric study, surgical observation and literature review. Taking the TME for example, we also discuss the core content about the fasciae and plane related to 'mesenteric envelope' and complete mesorectal excision. From the perspective of the fasciae integrity, we illustrate the definitions of important anatomical structure and standardized the terminology of fasciae. To study the origin and architecture of fasciae in the view of embryology, integrity and continuity will contribute to establish the standard theoretical system of membrane anatomy.

[Application of membrane anatomy in key technologies during gastric cancer surgery: guidance and compromise].

Different from classical surgical anatomy which only pays attention to the morphology and structure of human organs, modern membrane anatomy focuses on not only the relationship between morphology and structure, but also the biological behavior characteristics of tumors. Membrane antomy is a theoretical system with interpretation on both the structural and disease function, so it has been accepted by more and more gastrointestinal surgeons. However, the theoretical system of gastric membrane anatomy is not mature yet. The stomach and its mesentery have undergone complex rotation and fusion in the process of embryonic development, so that surgeons have different understandings of the gastric membrane anatomy. Therefore, it is easy to cause various confusion and misunderstanding, resulting in deviations between the theory of membrane anatomy and the practice of surgery. In the present study, the mesentery of the stomach is divided into different regions, and the embryonic development process is traced back. The application and compromise encountered in the radical gastrectomy of gastric cancer will be expounded according to the membrane anatomy theory combined with the author's experience of operation.

'Testis-epididymis dissociation' in cryptorchidism and hydrocele: the tip of the iceberg of a persistent genital mesentery.

PURPOSE: To investigate whether testis-epididymis dissociation encountered in boys with cryptorchidism/hydrocele is related with an abnormal persistence of the fetal mesentery of testis and associated ducts. METHODS: We examined the morphology of peritoneal folds of the testis, epididymis, and vas deferens in 25 boys operated for unilateral cryptorchidism [inguinal (n = 20), intrabdominal (n = 5)] and 20 boys operated for unilateral communicating hydrocele. Findings were compared with the normally persisting genital mesentery of rats (n = 30, both sides), a known animal model of the genital mesentery of human fetuses, as well as with the normal mature pattern of genital peritoneal folds in adult male cadavers (n = 12, both sides). Rats before testis descent [aged 18 days (n = 15)] served for comparison with boys with cryptorchidism, while rats after testis descent [aged 50 (n = 15)] known to retain patent processi vaginales for comparison with boys with hydrocele. RESULTS: A well-developed genital mesentery, identical to the fetal-type genital mesentery in the rat, was documented in cryptorchidism and hydrocele. The peritoneum enveloped the testis, epididymis, and vas deferens, and formed wide ligaments between testis-epididymis, epididymis-vas deferens, and vas-posterior wall; processus vaginalis was patent in all cases. The testis-epididymis ligament was related with testis-epididymis distancing, the so-called testis-epididymis dissociation. On the contrary, genital mesentery had involuted in the adult male cadavers, except for a small portion of testis-epididymis ligament corresponding to the so-called sinus epididymis. CONCLUSION: The testis-epididymis dissociation encountered in cryptorchidism/hydrocele is part of an anomalously persisting fetal-type genital mesentery.

Development of mesenteric tissues.

Mesothelial, neurovascular, lymphatic, adipose and mesenchymal tissues make up the mesentery. These tissues are pathobiologically important for numerous reasons. Collectively, they form a continuous, discrete and substantive organ. Additionally, they maintain abdominal digestive organs in position and in continuity with other systems. Furthermore, as they occupy a central position, they mediate transmission of signals between the abdominal digestive system and the remainder of the body. Despite this physiologic centrality, mesenteric tissue development has received little investigatory focus. However, recent advances in our understanding of anatomy demonstrate continuity between all mesenteric tissues, thereby linking previously unrelated studies. In this review, we examine the development of mesenteric tissue in normality and in the setting of congenital abnormalities.

"A narrow bridge home": The dorsal mesentery in primordial germ cell migration.

Specification of primordial germ cells (PGCs) in all vertebrates takes place in extragonadal sites. This requires migration of PGCs through embryonic tissues towards the genital ridges by both passive and active types of migration. Commonly, colonization in the genital ridges follows migration of the PGCs along the thin tissue of the dorsal mesentery. Here we review the anatomy of the dorsal mesentery, the role it plays in migration of PGCs, and the interactions of PGCs with different cell types, extracellular matrix and signaling pathways that are all essential for attraction and orientation of PGCs along the dorsal mesentery towards the gonad anlage.

Epithelial-to-mesenchymal transition-based morphogenesis of dorsal mesentery and gonad.

Dorsal mesentery and gonad (ovary and testis) are formed in distinct regions of the body and have different characteristics. Recent studies using chicken embryos showed that progenitors of these two organs are derived from the coelomic lining region, a ventral part of the medial lateral plate mesoderm (M-LPM). Furthermore, both types of progenitors develop in a similar manner, concomitant with morphological changes termed the epithelial-to-mesenchymal transition (EMT). EMT processes in both dorsal mesentery and gonad formation are regulated by BMP signaling. Interestingly, EMT-based morphogenetic events occur repetitively at M-LPM specification before dorsal mesenteric and gonadal formation, at ovary formation later in embryogenesis, and even during adult ovary repair. We review recent findings related to EMT-based morphogenesis and the governing molecular mechanisms, mainly in early dorsal mesenteric and gonadal formation, as well as in their anlages and derivatives.

The development of the dorsal mesentery in human embryos and fetuses.

The vertebrate intestine has a continuous dorsal mesentery between pharynx and anus that facilitates intestinal mobility. Based on width and fate the dorsal mesentery can be subdivided into that of the caudal foregut, midgut, and hindgut. The dorsal mesentery of stomach and duodenum is wide and topographically complex due to strong and asymmetric growth of the stomach. The associated formation of the lesser sac partitions the dorsal mesentery into the right-sided "caval fold" that serves as conduit for the inferior caval vein and the left-sided mesogastrium. The thin dorsal mesentery of the midgut originates between the base of the superior and inferior mesenteric arteries, and follows the transient increase in intestinal growth that results in small-intestinal looping, intestinal herniation and, subsequently, return. The following fixation of a large portion of the abdominal dorsal mesentery to the dorsal peritoneal wall by adhesion and fusion is only seen in primates and is often incomplete. Adhesion and fusion of mesothelial surfaces in the lesser pelvis results in the formation of the "mesorectum". Whether Toldt's and Denonvilliers' "fasciae of fusion" identify the location of the original mesothelial surfaces or, alternatively, represent the effects of postnatal wear and tear due to intestinal motility and intra-abdominal pressure changes, remains to be shown. "Malrotations" are characterized by growth defects of the intestinal loops with an ischemic origin and a narrow mesenteric root due to insufficient adhesion and fusion.

Distinct roles of VE-cadherin for development and maintenance of specific lymph vessel beds.

Endothelial cells line blood and lymphatic vessels and form intercellular junctions, which preserve vessel structure and integrity. The vascular endothelial cadherin, VE-cadherin, mediates endothelial adhesion and is indispensible for blood vessel development and permeability regulation. However, its requirement for lymphatic vessels has not been addressed. During development, VE-cadherin deletion in lymphatic endothelial cells resulted in abortive lymphangiogenesis, edema, and prenatal death. Unexpectedly, inducible postnatal or adult deletion elicited vessel bed-specific responses. Mature dermal lymph vessels resisted VE-cadherin loss and maintained button junctions, which was associated with an upregulation of junctional molecules. Very different, mesenteric lymphatic collectors deteriorated and formed a strongly hyperplastic layer of lymphatic endothelial cells on the mesothelium. This massive hyperproliferation may have been favored by high mesenteric VEGF-C expression and was associated with VEGFR-3 phosphorylation and upregulation of the transcriptional activator TAZ Finally, intestinal lacteals fragmented into cysts or became highly distended possibly as a consequence of the mesenteric defects. Taken together, we demonstrate here the importance of VE-cadherin for lymphatic vessel development and maintenance, which is however remarkably vessel bed-specific.

Genetic Lineage Tracing of Lymphatic Endothelial Cells in Mice.

Lineage tracing allows for identification of all progeny produced by a single cell or groups of cells and can thus be used to assess developmental fate of cells. Here we focus on one of the most widely used lineage tracing approaches that utilize the Cre/loxP system for site-specific genetic recombination in studying the developmental origins of lymphatic endothelial cells (LECs) in the mouse embryo. We discuss general considerations for a successful Cre/loxP based lineage tracing experiment and provide information about strains that are available for genetic lineage tracing of LECs. A protocol for lineage tracing analysis of the lymphatic vasculature by whole-mount immunofluorescence in two embryonic tissues, the skin and the mesentery, is also provided.

Expression of Pref-1 and Related Chemokines during theDevelopment of Rat Mesenteric Lymph Nodes.

OBJECTIVE: The aim of this study was to investigate the ability of Pref-1+ adipocyte progenitor cells to mobilize into mesenteric lymph nodes (MLNs) and the dynamic expression of related chemokines during the development of rat MLNs. METHODS: Immunohistochemical analyses were used to detect the expression of Pref-1 and related chemokines. Transmission electron microscopy (TEM) was used to observe the changes in ultrastructure of MLNs. RESULTS: Cells containing lipid droplets were found in all rat MLNs at embryonic day (E) 18.5, 2 and 6 weeks (w) after birth, and they were similar to fibroblastic reticular cells (FRCs) or follicular dendritic cells (FDCs) under TEM. Pref-1+ adipocyte progenitor cells were found in all MLNs. The expression level of Pref-1 was significantly increased at 2 w after birth and decreased at 6 w after birth. The tendency of Cxcl12 expression was consistent with that of Pref-1 and was positively correlated with the expression of Pref-1 (P < 0.01; r = 0.897). At E18.5, Cxcl13, and Ccr7 were significantly expressed in the MLN anlage, but the expression level of Ccl21 was low. The expression level of Cxcl13, Ccr7, and Ccl21 in MLN were significantly increased at 2 w after birth (P < 0.05), while the expression of Ccr7 and Ccl21 were significantly decreased at 6 w after birth (P < 0.05). CONCLUSION: Adipocyte progenitor cells are involved in the rat MLNs development through differentiation into FRC and FDC. The expression of the relevant chemokines during the development of MLNs is dynamic and may be related to the maintenance of lymph nodes self-balance state.

Peripheral PDGFRα+gp38+ mesenchymal cells support the differentiation of fetal liver-derived ILC2.

Group 2 innate lymphoid cells (ILC2s) are derived from common lymphoid progenitors (CLPs) via several specific precursors, and the transcription factors essential for ILC2 differentiation have been extensively studied. However, the external factors regulating commitment to the ILC lineage as well as the sites and stromal cells that constitute the optimal microenvironment for ILC2-specific differentiation are not fully defined. In this study, we demonstrate that three key external factors, the concentration of interleukin 7 (IL-7) and strength and duration of Notch signaling, coordinately determine the fate of CLP toward the T, B, or ILC lineage. Additionally, we identified three stages of ILC2 in the fetal mesentery that require STAT5 signals for maturation: ILC progenitors, CCR9+ ILC2 progenitors, and KLRG1- immature ILC2. We further demonstrate that ILC2 development is supported by mesenteric platelet-derived growth factor receptor α (PDGFRα)+ glycoprotein 38 (gp38)+ mesenchymal cells. Collectively, our results suggest that early differentiation of ILC2 occurs in the fetal liver via IL-7 and Notch signaling, whereas final differentiation occurs in the periphery with the aid of PDGFRα+gp38+ cells.

Fetal sigmoid colon mesentery - In relevance in fetal ultrasound application. A pilot study.

INTRODUCTION: Ultrasound examinations during pregnancy are routine procedures used to detect fetal congenital malformations. Ultrasound monitoring of sigmoid colon mesenterial development could be useful for early detection of subjects at risk of sigmoid colon volvulus. OBJECTIVE: The aim of our paper was to assess the sigmoid colon length, and sigmoid colon mesentery width and height in the late fetal period, and, using the results, to estimate the surface area of the mesocolon (in mm2) in living fetuses. Moreover, we attempted to repeat some of these measurements in living fetuses using ultrasound imaging. METHODS: The study was carried out on 209 formalin fixed human fetuses (100 female and 109 male) aged from 4th to 7th gestational months (102-203 days), with a crown-rump length of 132-342mm. The length of the sigmoid colon, as well as the height and width of its mesentery were measured. The surface area of the mesocolon was estimated. Correction for formalin induced shrinkage was applied. Pilot ultrasound examinations of live fetuses were performed. RESULTS: Mean values of sigmoid colon length, mesenteric width and height (formalin fixed fetuses) for respective gestational ages were: month 4: 21.46±6.7mm, 6.80±2.1mm, 5.5±1.49mm; month 5: 27.32±1.2mm, 7.62±2.01mm, 7.33±2.17mm; month 6: 47.56±9.57mm, 11.68±3.8mm, 10.3±3.05mm; month 7: 56.92±17.48mm. 15.32±8 mm, 12.81±3.16mm. The surface area ranges of the sigmoid colon mesentery found for respective gestational months (intrauterine fetuses) were as follows: month 4: 33.24-51.95mm2; month 5: 49.63-77.6mm2; month 6: 106.89-167.15mm2 and month 7: 145.69-272.53mm2. CONCLUSION: The surface area of the sigmoid colon mesentery can be used as a simple parameter applied in fetal ultrasonographic evaluation. The development of the sigmoid colon accelerates in the 6th gestational month, and decelerates in the 7th gestational month. The sigmoid colon mesentery width was larger than its height between the 4th and 7th gestational months.

Defective lymphatic valve development and chylothorax in mice with a lymphatic-specific deletion of Connexin43.

Lymphatic valves (LVs) are cusped luminal structures that permit the movement of lymph in only one direction and are therefore critical for proper lymphatic vessel function. Congenital valve aplasia or agenesis can, in some cases, be a direct cause of lymphatic disease. Knowledge about the molecular mechanisms operating during the development and maintenance of LVs may thus aid in the establishment of novel therapeutic approaches to treat lymphatic disorders. In this study, we examined the role of Connexin43 (Cx43), a gap junction protein expressed in lymphatic endothelial cells (LECs), during valve development. Mouse embryos with a null mutation in Cx43 (Gja1) were previously shown to completely lack mesenteric LVs at embryonic day 18. However, interpreting the phenotype of Cx43-/- mice was complicated by the fact that global deletion of Cx43 causes perinatal death due to heart defects during embryogenesis. We have now generated a mouse model (Cx43∆LEC) with a lymphatic-specific ablation of Cx43 and show that the absence of Cx43 in LECs causes a delay (rather than a complete block) in LV initiation, an increase in immature valves with incomplete leaflet elongation, a reduction in the total number of valves, and altered lymphatic capillary patterning. The physiological consequences of these lymphatic changes were leaky valves, insufficient lymph transport and reflux, and a high incidence of lethal chylothorax. These results demonstrate that the expression of Cx43 is specifically required in LECs for normal development of LVs.

Angiopoietin receptor Tie2 is required for vein specification and maintenance via regulating COUP-TFII.

Mechanisms underlying the vein development remain largely unknown. Tie2 signaling mediates endothelial cell (EC) survival and vascular maturation and its activating mutations are linked to venous malformations. Here we show that vein formation are disrupted in mouse skin and mesentery when Tie2 signals are diminished by targeted deletion of Tek either ubiquitously or specifically in embryonic ECs. Postnatal Tie2 attenuation resulted in the degeneration of newly formed veins followed by the formation of haemangioma-like vascular tufts in retina and venous tortuosity. Mechanistically, Tie2 insufficiency compromised venous EC identity, as indicated by a significant decrease of COUP-TFII protein level, a key regulator in venogenesis. Consistently, angiopoietin-1 stimulation increased COUP-TFII in cultured ECs, while Tie2 knockdown or blockade of Tie2 downstream PI3K/Akt pathway reduced COUP-TFII which could be reverted by the proteasome inhibition. Together, our results imply that Tie2 is essential for venous specification and maintenance via Akt mediated stabilization of COUP-TFII.

Extravascular endothelial and hematopoietic islands form through multiple pathways in midgestation mouse embryos.

The de novo generation of hematopoietic cells occurs during midgestation when a population of endothelial cells called hemogenic endothelium transitions into hematopoietic progenitors and stem cells. In mammalian embryos, the newly formed hematopoietic cells form clusters in the lumens of the major arteries in the embryo proper and in the vascular plexus of the yolk sac. Small clusters of hematopoietic cells that are independent of the vasculature (referred to here as extravascular islands) were shown to form in the mesentery during vascular remodeling of the vitelline artery. Using three-dimensional imaging of whole mouse embryos we demonstrate that extravascular budding of hematopoietic clusters is a more widespread phenomenon that occurs from the vitelline and the umbilical arteries both proximal to the embryo proper and distal in the extraembryonic yolk sac and placenta. Furthermore, we show that there are several mechanisms by which hematopoietic clusters leave the arteries, including vascular remodeling and extrusion. Lastly, we provide static images suggesting that extravascular islands contribute to the formation of new blood vessels. Thus, extravascular islands may represent a novel mechanism of vasculogenesis whereby established vessels contribute endothelial and hematopoietic cells to developing vascular beds.

The growth pattern of the human intestine and its mesentery.

BACKGROUND: It remains unclear to what extent midgut rotation determines human intestinal topography and pathology. We reinvestigated the midgut during its looping and herniation phases of development, using novel 3D visualization techniques. RESULTS: We distinguished 3 generations of midgut loops. The topography of primary and secondary loops was constant, but that of tertiary loops not. The orientation of the primary loop changed from sagittal to transverse due to the descent of ventral structures in a body with a still helical body axis. The 1st secondary loop (duodenum, proximal jejunum) developed intraabdominally towards a left-sided position. The 2nd secondary loop (distal jejunum) assumed a left-sided position inside the hernia before returning, while the 3rd and 4th secondary loops retained near-midline positions. Intestinal return into the abdomen resembled a backward sliding movement. Only after return, the 4th secondary loop (distal ileum, cecum) rapidly "slid" into the right lower abdomen. The seemingly random position of the tertiary small-intestinal loops may have a biomechanical origin. CONCLUSIONS: The interpretation of "intestinal rotation" as a mechanistic rather than a descriptive concept underlies much of the confusion accompanying the physiological herniation. We argue, instead, that the concept of "en-bloc rotation" of the developing midgut is a fallacy of schematic drawings. Primary, secondary and tertiary loops arise in a hierarchical fashion. The predictable position and growth of secondary loops is pre-patterned and determines adult intestinal topography. We hypothesize based on published accounts that malrotations result from stunted development of secondary loops.