Posterior Sagittal Approach Provides Optimal Exposure for Urethral Reconstruction in Children With a History of Anorectal Malformations.

OBJECTIVE: To present a unique series of children with previously repaired anorectal malformations (ARM) with subsequent urethral pathology repaired via a posterior sagittal exposure and highlight the associated technical advantages. METHODS: Using a retrospective review of all procedures performed in our pediatric colorectal and pelvic reconstruction program from January 2020 through December 2022, we compiled a case series of patients with a history of ARM and prior posterior sagittal anorectoplasty (PSARP) who had urethral pathology and concurrent indication for redo-PSARP. Clinical features, operative details, and postoperative outcomes were collected. RESULTS: Six male patients presented at a median age of 4.3 years, all born with an ARM of recto-urinary fistula type, of which 3 were recto-prostatic, 1 recto-bladder-neck, and 2 unknown type. In addition to redo-PSARP, 2 underwent remnant of the original fistula excision and 4 had urethral stricture repair. One required post-operative Heineke-Mikulicz anoplasty. Patients underwent cystoscopy 4-6 weeks post-reconstruction, and none showed urethral stricture requiring treatment. Post-procedurally, 5 patients were able to void urethrally and 1 required additional bladder augmentation/Mitrofanoff. CONCLUSION: Redo-PSARP completely mobilizes the rectum, thereby providing excellent exposure to the posterior urethra for repair. This approach also allows the option of a rectal flap for augmented urethroplasty as well as harvest of an ischiorectal fat pad for interposition.

Reconstructing the anal sphincters to reverse iatrogenic overstretching following a pull-through for Hirschsprung disease. One-year outcomes.

BACKGROUND: In patients with Hirschsprung disease (HSCR), soiling may be related to anal sphincter damage following the initial pull-through. No optimal treatment has been developed for such patients, although enemas (rectal or antegrade) have been applied with some success. We present the one-year outcomes of a new technique for anal sphincter reconstruction. METHODS: All patients with HSCR referred from other institutions for post pull-through soiling were studied. Seven patients with patulous sphincters underwent sphincter reconstruction. Six had a full preoperative evaluation and were included in the study. Their 12-month outcomes were assessed. RESULTS: All six patients had soiling without voluntary bowel movements (VBMs). One patient was clean on Malone flushes when referred. Three underwent pre- and post-reconstruction non-sedated three-dimensional anorectal manometry, and objectively were able to close their sphincters following the reconstruction. All patients without Down syndrome (4 of 6) showed improvement in the abbreviated Baylor Continence Scale (4.5 vs. 0.75). One patient has achieved total bowel control without antegrade flushes, three now have VBMs which they did not have before but have occasional accidents and use antegrade flushes intermittently. They reported higher productivity, the ability to participate in sports and be away from home with confidence in their regimen. Two of 6 patients have Down syndrome and required a redo pull-through for other indications and underwent empiric sphincter reconstruction. For these two patients we do not have an outcomes assessment. CONCLUSIONS: A new technique for sphincter reconstruction shows promising results in improvement of bowel control at one year. LEVEL OF EVIDENCE: IV.

Functional constipation refractory to medical management: The colon is the problem.

BACKGROUND: Management of refractory constipation in children has not been standardized. We propose a protocolized approach which includes a contrast enema, anorectal manometry and exclusion of Hirschsprung disease (HD). For those without HD or with normal sphincters, an assessment of the colonic motility may be needed. The subgroups of dysmotility include (1) slow motility with contractions throughout, (2) segmental dysmotility (usually the sigmoid), or (3) a diffusely inert colon. We offered a Malone appendicostomy in all groups with the hope that this would avoid colonic resection in most cases. METHODS: Patients with medically refractory constipation were reviewed at a single institution (2020 to 2021). For patients without HD or an anal sphincter problem, assessment of colonic motility using colonic manometry was performed followed by a Malone appendicostomy for antegrade flushes. RESULTS: Of 196 patients evaluated for constipation refractory to medical management, 22 were felt to have a colonic motility cause. These patients underwent colonic manometry and Malone appendicostomy. 13 patients (59%) had a slow colon but with HAPCs throughout, 5 (23%) had segmental dysmotility, and 4 (18%) had a diffuse colonic dysmotility. 19 (86%) responded well to antegrade flushes with 17 reporting no soiling and 2 having occasional accidents. 3 patients (14%) failed flushes and underwent a colon resection within 6-month following Malone procedure. CONCLUSION: We propose a protocol for medically refractory constipation which provides a collaborative framework to standardize evaluation and management of these patients with antegrade flushes, which aids in avoidance of colonic resection in most cases. LEVEL OF EVIDENCE: Level IV.

Where Is the Vagina? A Rectal Stricture after a Presumed Cloacal Repair Turns Out to be the Mobilized Vagina and a Missed High Rectovaginal Fistula.

We present a case of a rare complication in a 10-month-old female referred to our institution for an anal stricture after primary cloacal repair as an infant. Multimodal imaging, careful physical exam, and endoscopic evaluation revealed her vagina had been pulled through to the location of her anal sphincter muscle complex. We describe the correction of this problem, including identification of her rectum.

Fetal and Newborn Management of Cloacal Malformations.

Cloaca is a rare, complex malformation encompassing the genitourinary and anorectal tract of the female in which these tracts fail to separate in utero, resulting in a single perineal orifice. Prenatal sonography detects a few cases with findings such as renal and urinary tract malformations, intraluminal calcifications, dilated bowel, ambiguous genitalia, a cystic pelvic mass, or identification of other associated anomalies prompting further imaging. Multi-disciplinary collaboration between neonatology, pediatric surgery, urology, and gynecology is paramount to achieving safe outcomes. Perinatal evaluation and management may include treatment of cardiopulmonary and renal anomalies, administration of prophylactic antibiotics, ensuring egress of urine and evaluation of hydronephrosis, drainage of a hydrocolpos, and creation of a colostomy for stool diversion. Additional imaging of the spinal cord and sacrum are obtained to plan possible neurosurgical intervention as well as prognostication of future bladder and bowel control. Endoscopic evaluation and cloacagram, followed by primary reconstruction, are performed by a multidisciplinary team outside of the neonatal period. Long-term multidisciplinary follow-up is essential given the increased rates of renal disease, neuropathic bladder, tethered cord syndrome, and stooling issues. Patients and families will also require support through the functional and psychosocial changes in puberty, adolescence, and young adulthood.

Prenatal diagnosis of intestinal nonrotation using magnetic resonance imaging: Is it possible?

BACKGROUND: Malrotation of the bowel refers to any variation in the rotation and fixation of the gastrointestinal tract during the first trimester and is most commonly detected postnatally. Nonrotation of the bowel and incomplete rotation of the bowel are subtypes of malrotation. OBJECTIVE: To determine if the nonrotation subtype of malrotation of the bowel can be detected on prenatal magnetic resonance imaging (MRI). MATERIALS AND METHODS: Cases from 2012 to 2018 with nonrotation of the bowel without obstruction confirmed by imaging, surgery and/or autopsy were compared to prenatal imaging. Prenatal imaging was retrospectively reviewed to determine if prenatal diagnosis of malrotation could be made. Exclusion criteria included diaphragmatic hernia, omphalocele and gastroschisis. RESULTS: Ten cases of nonrotation diagnosed postnatally by upper gastrointestinal series (upper GI)/small bowel follow-through (SBFT) or autopsy had prenatal MRI. Prenatal MR studies were performed for assessment of heterotaxy syndrome with congenital heart disease (6/10), congenital heart disease with additional anomalies (suspected VACTERL [vertebral, anorectal, cardiac, tracheoesophageal, renal, limb] and suspected lung agenesis, ventriculomegaly) (3/10) and skeletal dysplasia (1/10). Eight upper GI/SBFT cases demonstrated nonrotation of the bowel without obstruction with the small bowel completely on one side of the abdomen contralateral to the stomach and the colon ipsilateral to the stomach; four cases were confirmed by surgery. The small bowel in one upper GI/SBFT case was unilateral contralateral to the stomach with a meandering colon. One case had nonrotation diagnosed at autopsy. There were no cases of postnatal midgut volvulus. Retrospective review of the 10 cases had prenatal MRI performed between 23 and 37 weeks of gestation. The coronal plane was the most optimal plane to assess the position of the stomach, small bowel and colon in relationship to each other. The small bowel was best assessed on T2-weighted images while the colon was best assessed on T1-weighted images. A nonrotated position of the small bowel was present in all 10 fetal MRI cases mirroring postnatal findings, with the small bowel contralateral to the stomach in 9/10 cases and ipsilateral to the stomach (in the right abdomen) in 1/10 cases. The colon was visualized by prenatal MRI in 9/10 cases, with 1 case limited due to a lack of T1-weighted imaging. A nonrotated position of the colon contralateral to the small bowel was present in 7/9 cases. In 2/9 cases, the colon was wandering, positioned on both sides of the midline. Colonic position in all nine cases matched postnatal findings. No cases presented with prenatal bowel obstruction. CONCLUSION: Detection of nonrotation of the bowel is possible on prenatal MRI.

Lasting impact on children with an anorectal malformations with proper surgical preparation, respect for anatomic principles, and precise surgical management.

Optimal outcomes in the management of children with Anorectal Malformation (ARM) require careful surgical preparation and detailed understanding of the anatomic principles and operative setup. A clear understanding of operative anatomy and surgical principals guides decision making. Adherence to the principles of ARM repair, as well as the application of operative and imaging adjuncts, will yield the safest and most successful approach to ARM. In this review, we detail the surgical preparation, anatomic principles, and surgical management issues unique to ARM.

Qualities and characteristics of applicants associated with successful matriculation to pediatric surgery fellowship training.

BACKGROUND: Pediatric surgery (PS) is among the most competitive fellowship opportunities for general surgery residents. Prior investigations into factors associated with successful matriculation to PS have relied on surveys, which have inherent biases. STUDY DESIGN: Data were extracted from the Electronic Residency Application System for applicants from 2012 to 2017 and analyzed after stratification by matriculation status. RESULTS: Data were gathered on a total of 444 applicants, of which 238 matriculated. The applicant pool was predominantly Caucasian (63.5%), largely graduated from US allopathic school (81%) and had a slight male predominance (54%), although the number of female applicants increased significantly over the study period. Attendance at a US allopathic medical school (OR=4.55, p <0.001), university-based general surgery training (OR=3.02, p <0.001) and training at institutions that offer PS fellowships (OR=3.36, p<0.001) were associated with matriculation. Matriculants had a higher quantity of peer reviewed publications (median 11 vs. 6, p <0.001) and published in high impact factor journals (p<0.001). A total of 65 applicants reapplied at least once, of whom 32% successfully matriculated. CONCLUSIONS: PS applicants' medical school, residency, and research data points correlated with successful matriculation. These data may help guide general surgery residents and medical students aspiring to become pediatric surgeons. TYPE OF STUDY: Prognosis study (retrospective) LEVEL OF EVIDENCE: Level II.

Prenatal magnetic resonance and ultrasonographic findings in small-bowel obstruction: imaging clues and postnatal outcomes.

BACKGROUND: Prenatal small-bowel obstruction can result from single or multiple atresias, and it can be an isolated abnormality or part of a syndrome. It is sometimes the first manifestation of cystic fibrosis. Accurate prediction of the level of obstruction and length of bowel affected can be difficult, presenting a challenge for counseling families and planning perinatal management. OBJECTIVE: To review the prenatal US and MRI findings of small-bowel obstruction and to assess whether fetal MRI adds information that could improve prenatal counseling and perinatal management. MATERIALS AND METHODS: We retrospectively reviewed 12 prenatally diagnosed cases of small-bowel obstruction evaluated by both US and MRI from 2005 to 2015. We analyzed gestational age at evaluation, US and MRI findings, gestational age at delivery and postnatal outcomes. RESULTS: The final diagnoses were jejunal atresia (7), ileal atresia (1), cystic fibrosis (3) and combined jejunal and anal atresia (1). Four of the eight with jejunal atresia were found to have multiple small-bowel atresias. Prenatal perforation was noted in three. We identified a trend of increasing complexity of bowel contents corresponding to progressively distal level of obstruction, as indicated by increasing US echogenicity and high T1 signal on MRI. Seven cases of jejunal atresia and one case of ileal atresia demonstrated small ascending, transverse and descending colon (microcolon) with filling of a normal-diameter rectum. In contrast, all three fetuses with cystic fibrosis and the fetus with jejunal-anal atresia demonstrated microcolon as well as abnormal paucity or absence of rectal meconium. Polyhydramnios was present in nine. Eight were delivered prematurely, of whom seven had polyhydramnios. The fetus with jejunal and anal atresia died in utero. Postnatally, three had short gut syndrome, all resulting from multiple jejunal atresias; these three were among a subset of four fetuses whose bowel diameter measured more than 3 cm. Eight infants had no further gastrointestinal complications. The presence of multiple atresias was not predicted by prenatal US or MRI. CONCLUSION: MR provides useful additional information regarding meconium distribution in the small bowel, which helps to clarify the level of obstruction. MR was additionally useful in the assessment of colon and rectal contents, serving as a fetal enema. Abnormally diminished meconium in the rectum suggests cystic fibrosis or combined small-bowel and colonic obstruction, information that is useful in counseling and preparing for postnatal care.

Stromal progenitor cells promote leukocyte migration through production of stromal-derived growth factor 1alpha: a potential mechanism for stromal progenitor cell-mediated enhancement of cellular recruitment to wounds.

BACKGROUND/PURPOSE: Stromal progenitor cells (SPC) enhance tissue repair in a variety of injury models. However, the mechanisms by which SPCs facilitate tissue repair remain poorly understood. We hypothesized that SPC-enhanced tissue repair is, in part, because of SPC-mediated recruitment of circulating cells to areas of tissue injury. To test this, we examined the migration of leukocytes in response to SPC in vitro. METHODS: Leukocyte migration was assessed in response to SPC, SPC + transforming growth factor (TGF)-beta1, or SPC + AMD3100 using a Transwell assay system (Corning, distributed by Fisher Scientific, Pittsburgh, PA). Supernatants were collected from lower chambers and analyzed for leukocyte content, leukocyte viability, and stromal-derived growth factor (SDF)-1alpha concentration. RESULTS: Stromal progenitor cells increased leukocyte migration compared to media alone (450 +/- 70 vs 112 +/- 17 cells/microL; P < .05). SPC treatment with TGF-beta1 resulted in a 36% increase in leukocyte migration and correlated with an increase in SDF-1alpha production. Treatment with AMD3100 resulted in inhibition of leukocyte migration. CONCLUSIONS: Stromal progenitor cells promote leukocyte migration, and this appears to be mediated through SDF-1alpha production. The SPC production of SDF-1alpha may be modulated by other cytokines present in the microenvironment during wound healing. Together, these observations provide a potential mechanism by which SPC may augment healing through enhanced recruitment of inflammatory cells and tissue progenitor cells to areas of tissue injury.

Prenatal ultrasonographic gastrointestinal abnormalities in fetuses with gastroschisis do not correlate with postnatal outcomes.

PURPOSE: In the setting of gastroschisis, the clinical significance of prenatal ultrasound findings of secondary changes in bowel appearance remains unknown. The purpose of this study was to correlate prenatal identification of additional gastrointestinal sonographic abnormalities with postnatal clinical outcome. METHODS: A retrospective review was conducted on 64 fetuses with a prenatal diagnosis of gastroschisis treated at the Children's Hospital of Philadelphia from 2000 to 2007. Postnatal outcomes were compared between newborns with additional sonographic gastrointestinal abnormalities and those without secondary changes to the bowel appearance. RESULTS: Thirty (47%) patients had at least one gastrointestinal abnormality (eg, bowel dilatation, echogenic bowel, thickened bowel, matted bowel, herniation of the stomach through the abdominal wall defect, or segmental loss of bowel peristalsis) on prenatal ultrasound. There were no significant differences between groups with respect to the time to initial and full enteral nutrition, total hospital stay, requirement for ventilator support, central line infection rates, reoperation rates, or mortality. CONCLUSIONS: In the setting of gastroschisis, isolated findings of gastrointestinal abnormalities on prenatal ultrasound do not correlate with adverse postnatal outcome.

Murine bone marrow derived stromal progenitor cells fail to prevent or treat acute graft-versus-host disease.

Human and murine stromal progenitor cells (SPCs) can suppress alloresponse in vitro, suggesting that SPCs may have clinical application toward prevention or treatment of graft-versus-host disease (GVHD). However, th eresults of in vivo studies have been conflicting. This study utilized an established murine model of acute GVHD to assess the ability of bone marrow derived murine SPCs (mSPCs) to prevent or treat GVHD. GVHD was established by transplantation of B6 bone marrow and spleen cells into lethally irradiated (900 cGy) B6 x BALB/c F(1) recipients. mSPCs were administered using various dose and timing protocols designed to either prevent or treat GVHD. After transplantation, mice were monitored daily for weight and survival. Differences in symptom severity were compared using a clinical GVHD scoring system. All GVHD control mice died of lethal GVHD. All groups treated with mSPCs for the prevention of GVHD went on to develop clinical GVHD with no alteration of the disease course or severity compared to controls. Administration of mSPC after the development of GVHD failed to improve the disease course. We conclude that in this model, the ability of SPCs to suppress alloresponse in vitro does not correlate with in vivo prevention or treatment of acute GVHD.

IL-10 overexpression decreases inflammatory mediators and promotes regenerative healing in an adult model of scar formation.

Adult wound healing is characterized by an exuberant inflammatory response and scar formation. In contrast, scarless fetal wound healing has diminished inflammation, a lack of fibroplasia, and restoration of normal architecture. We have previously shown that fetal wounds produce less inflammatory cytokines, and the absence of IL-10, an anti-inflammatory cytokine, results in fetal scar formation. We hypothesized that increased IL-10 would decrease inflammation and create an environment conducive for regenerative healing in the adult. To test this hypothesis, a lentiviral vector expressing IL-10 and green fluorescent protein (GFP) (Lenti-IL-10) or GFP alone (Lenti-GFP) was injected at the wound site 48 hours before wounding. We found that both Lenti-IL-10 and Lenti-GFP were expressed in the wounds at 1 and 3 days post wounding. At 3 days, Lenti-IL-10-treated wounds demonstrated decreased inflammation and decreased quantities of all proinflammatory mediators analyzed with statistically different levels of IL-6, monocyte chemoattractant protein-1, and heat-shock protein 47. At 3 weeks, Lenti-GFP wounds demonstrated scar formation. In contrast, wounds injected with Lenti-IL-10 demonstrated decreased inflammation, a lack of abnormal collagen deposition, and restoration of normal dermal architecture. We conclude that lentivirus-mediated overexpression of IL-10 decreases the inflammatory response to injury, creating an environment conducive for regenerative adult wound healing.

Lentiviral gene transfer of SDF-1alpha to wounds improves diabetic wound healing.

BACKGROUND: Chronic wounds continue to be a major clinical problem and novel therapeutic approaches are needed. We have previously demonstrated that treatment of diabetic mouse wounds with local application of stromal progenitor cells results in improved healing and increased production of stromal-derived growth factor-1alpha (SDF-1alpha). We hypothesized that lentiviral-mediated increased production of SDF-1alpha in the wound environment could also improve diabetic wound healing. MATERIALS AND METHODS: Full-thickness excisional wounds were created in Db-/Db- mice and immediately treated with 10(6), 10(8), or 10(9) plaque-forming units of a lentiviral construct containing GFP-SDF-1alpha or GFP alone. At 7 and 14 days post wounding, wounds were harvested for histological and molecular analysis. RESULTS: At 7 days, Db-/Db- wounds treated with lenti GFP-SDF-1alpha exhibited a decrease in wound surface area for all doses tested. Morphologically, SDF-treated wounds were more cellular with increased granulation tissue volume compared to controls (P < 0.05). GFP expression was maintained in treated tissue at 7 days post wounding, but little expression was observed at 14 days. While we did not observe a difference in the gross wound surface area at 14 days, histological analysis revealed that SDF-treated wounds were fully epithelialized (n = 6) compared to only one of six controls. CONCLUSIONS: Lentiviral-mediated overproduction of SDF-1alpha is sufficient to correct the pathophysiologic abnormalities in diabetic wound healing resulting in complete epithelialization at 2 weeks. SDF-1alpha-mediated improvement in diabetic wound healing has significant implications for the development of novel therapeutic strategies to facilitate wound closure which target progenitor cell mobilization and recruitment.

Enhanced epithelial gap closure and increased angiogenesis in wounds of diabetic mice treated with adult murine bone marrow stromal progenitor cells.

The direct application of bone marrow (BM) can accelerate the healing of chronic wounds. We hypothesized that this effect is due to the presence of stromal progenitor cells (SPCs) found within whole BM preparations. To test this hypothesis, we isolated adult murine SPCs from whole BM and examined their ability to enhance impaired wound healing compared with ficoll separated BM cells in the diabetic (db/db) mouse model. SPCs significantly enhanced reepithelialization, granulation tissue formation, and neovascularization compared with control wounds treated with BM or PBS alone. Higher frequencies of donor SPC cells compared with donor BM cells were observed in treated wounds at 7 days. Transdifferentiation into GFP-positive mature endothelial cells was not observed. These observations suggest that SPCs improve wound healing through indirect mechanisms which lead to enhanced vascularization rather than through direct participation and incorporation into tissue. We conclude that topical application of BM-derived SPCs may represent an effective strategy for the treatment of chronic diabetic wounds.

Treatment of diabetic wounds with fetal murine mesenchymal stromal cells enhances wound closure.

Diabetes impairs multiple aspects of the wound-healing response. Delayed wound healing continues to be a significant healthcare problem for which effective therapies are lacking. We have hypothesized that local delivery of mesenchymal stromal cells (MSC) at a wound might correct many of the wound-healing impairments seen in diabetic lesions. We treated excisional wounds of genetically diabetic (Db-/Db-) mice and heterozygous controls with either MSC, CD45(+) cells, or vehicle. At 7 days, treatment with MSC resulted in a decrease in the epithelial gap from 3.2 +/- 0.5 mm in vehicle-treated wounds to 1.3 +/- 0.4 mm in MSC-treated wounds and an increase in granulation tissue from 0.8 +/- 0.3 mm(2) to 2.4 +/- 0.6 mm(2), respectively (mean +/- SD, P < 0.04). MSC-treated wounds also displayed a higher density of CD31(+) vessels and exhibited increases in the production of mRNA for epidermal growth factor, transforming growth factor beta 1, vascular endothelial growth factor, and stromal-derived growth factor 1-alpha. MSC also demonstrated greater contractile ability than fibroblast controls in a collagen gel contraction assay. The effects of locally applied MSC are thus sufficient to improve healing in diabetic mice. Possible mechanisms of this effect include augmented local growth-factor production, improved neovascularization, enhanced cellular recruitment to wounds, and improved wound contraction.

Murine bone marrow stromal progenitor cells elicit an in vivo cellular and humoral alloimmune response.

Stromal progenitor cells (SPC) exhibit immunosuppressive effects in vitro that have led to speculation regarding their capacity to evade host immune recognition and to treat autoimmune diseases and gravt-versus-host disease. However, there is little in vivo experimental data to support these immunologic claims. To assess immune recognition of SPC in vivo, we evaluated the immune response of animals transplanted with SPC. C57BL/6 (B6) or Balb/c adult, murine, bone marrow-derived SPC (AmSPC) were administered by intraperitoneal injection into B6 recipients. T cell proliferation and alloantibody response was assessed from spleens and peripheral blood harvested from transplanted animals and analyzed by cell proliferation assay and flow cytometry. To assess tolerance induction, transplanted animals also received allogeneic skin grafts. Animals injected with allogeneic AmSPC mounted an accelerated CD4 response to alloantigen compared to syngeneic AmSPC injected and uninjected controls. Allogeneic AmSPC-injected animals also demonstrated high titers (> or =1:1000) of antibody directed against allogeneic AmSPC targets. Animals primed with donor or host-matched AmSPC also failed to induce tolerance, and all animals exhibited rejection of allogeneic skin grafts (n = 7, P < .0001). In contrast to their in vitro behavior, our data demonstrate that AmSPC are recognized by the host immune system in vivo, elicit a cellular and humoral immune response, and fail to induce tolerance. These findings have significant implications for all allogeneic SPC-based therapeutic strategies.

Wilms' tumor arising within a mediastinal teratoma.

The majority of Wilms' tumors present in young children as an intraabdominal mass of renal origin. The use of multimodal treatment protocols according to internationally recognized treatment guidelines has resulted in extremely high cure rates even in the setting of advanced disease. By comparison, Wilms' tumors in older patients or Wilms' tumors located entirely outside the kidney are extremely rare. Because of the rarity of these tumors and the lack of established treatment guidelines specific to these lesions, older patients with atypical Wilms' tumors are at risk for under treatment and poorer outcomes. We report the case of an 18-year-old man with a Wilms' tumor arising within a mediastinal teratoma and discuss the management of this extremely rare lesion.

The regulatory role of stromal microenvironments in fetal hematopoietic ontogeny.

Fetal hematopoietic development occurs through the successive expansion and differentiation of hematopoietic stem cells in distinct anatomic sites. The temporal pattern of fetal hematopoietic ontogeny suggests a coordinated developmental sequence whereby the preceding organ sustains the basic, immediate hematopoietic needs of the embryo allowing time for the development of niches within the subsequent organ with more complex supportive functions. We examine the hypothesis that there is a period of stromal genesis and circulating mesenchymal precursor cells, which gives rise to specialized niches within each of the definitive fetal hematopoietic organs, and these niches regulate hematopoietic stem cells fate determination. This article reviews fetal hematopoietic and stromal development and the current understanding of the development, composition, and regulation of the fetal stem cell niche.