Parametric transition from deflagration to detonation in stellar medium

The nature of thermonuclear explosions of white-dwarf stars is a fundamental astrophysical issue, the first principle interpretation of which is still commonly regarded as an unresolved problem. There is a general consensus that stellar explosions are a manifestation of the deflagration-to-detonation transition of an outward propagating self-accelerating thermonuclear flame subjected to instability-induced corrugations. A similar problem arises in unconfined terrestrial flames where a positive feedback mechanism leading to the pressure runaway has been identified. The present study is an application of this finding to the stellar environment. Notwithstanding a substantial modification of the equation of state the runaway effect endures. Approaching the runaway point the pretransition flame may stay perfectly subsonic, which challenges the view that to ensure the transition the flame should cross the threshold of Chapman-Jouguet deflagration.

The complete blood cell count in a refined cohort of preterm NEC: the importance of gestational age and day of diagnosis when using the CBC to estimate mortality

OBJECTIVE

The value of the white blood cell count (WBC) in necrotizing enterocolitis (NEC) is controversial. One reason for this confusion may be that the various WBC lineages change substantially with increasing gestational age and thereby age of NEC onset. This study postulated that if a data set was large enough and the diagnosis of NEC clean enough, absolute WBC counts would facilitate prediction of NEC mortality. The objective of this study was to determine whether absolute WBC counts enhance the prediction of NEC mortality.

STUDY DESIGN

A de-identified data subset from the Pediatrix national data set specific to the diagnoses of NEC in patients who had a CBC drawn on the day of diagnosis (exclusive of the diagnoses of spontaneous intestinal perforations and congenital anomalies) was the target for analysis. Values of primary interest included: gestation, day of diagnosis, absolute WBC count, platelet count, hematocrit, mortality and the day of diagnosis. Stepwise regression analysis was used to predict mortality.

RESULT

A total of 4059 (79%) survivors and 1107 (21%) infants who died with a diagnosis of medical or surgical NEC were included in the data set. Associations with mortality were found with low gestational age, low platelet count, low hematocrit, high band/segmented neutrophil ratio, earlier day of diagnosis, high birth weight z-score, non-white race, no antenatal steroids in gestations above 24 weeks, absolute lymphocyte count adjusted for gestational age, and absolute monocyte count high and low values. A stepwise regression analysis yielded a receiver-operator curve of 0.819 with a sensitivity of 65% and specificity of 84%.

CONCLUSION

Absolute WBC values enhance prediction of NEC survival when used in combination with readily available data on the day of NEC diagnosis.

An elementary model for autoignition of laminar jets

In this paper, we formulate and analyse an elementary model for autoignition of cylindrical laminar jets of fuel injected into an oxidizing ambient at rest. This study is motivated by renewed interest in analysis of hydrothermal flames for which such configuration is common. As a result of our analysis, we obtain a sharp characterization of the autoignition position in terms of the principal physical and geometrical parameters of the problem.

Gelfand-type problem for two-phase porous media

We consider a generalization of the Gelfand problem arising in Frank-Kamenetskii theory of thermal explosion. This generalization is a natural extension of the Gelfand problem to two-phase materials, where, in contrast to the classical Gelfand problem which uses a single temperature approach, the state of the system is described by two different temperatures. We show that similar to the classical Gelfand problem the thermal explosion occurs exclusively owing to the absence of stationary temperature distribution. We also show that the presence of interphase heat exchange delays a thermal explosion. Moreover, we prove that in the limit of infinite heat exchange between phases the problem of thermal explosion in two-phase porous media reduces to the classical Gelfand problem with renormalized constants.

Local accumulation times for source, diffusion, and degradation models in two and three dimensions

We analyze the transient dynamics leading to the establishment of a steady state in reaction-diffusion problems that model several important processes in cell and developmental biology and account for the diffusion and degradation of locally produced chemical species. We derive expressions for the local accumulation time, a convenient characterization of the time scale of the transient at a given location, in two- and three-dimensional systems with first-order degradation kinetics, and analyze their dependence on the model parameters. We also study the relevance of the local accumulation time as a single measure of timing for the transient and demonstrate that, while it may be sufficient for describing the local concentration dynamics far from the source, a more delicate multi-scale description of the transient is needed near a tightly localized source in two and three dimensions.

Neonates presenting with bloody stools and eosinophilia can progress to two different types of necrotizing enterocolitis

OBJECTIVE

We hypothesized that neonates with bloody stools and concomitant eosinophilia are likely to have atopic enteropathy rather than necrotizing enterocolitis (NEC).

STUDY DESIGN

This was a retrospective cross-sectional study using electronic medical records and paper charts. Records of neonates admitted to any Intermountain Healthcare NICU between 1 January 2005 and 30 June 2010 were eligible if 'bloody stools' were listed in any archive. Qualifying records were divided into two groups depending on whether or not within 72 h of passing bloody stool eosinophil counts were above the 95th percentile reference range limit for age.

RESULT

Bloody stools were identified in 275 predominantly Caucasian neonates. Fifty-four of these had eosinophilia and 221 had normal eosinophil counts. Those with eosinophilia were born at a slightly younger gestational age (31.3 ± 4.6 vs 32.6 ± 4.0 weeks, mean ± s.d., P=0.032). Contrary to our hypothesis, those with eosinophilia did not have a lower rate of pneumatosis or bowel resection, or death ascribed to NEC. Eosinophilia was more common among those who had a red blood cell (RBC) transfusion within 48 h before passing bloody stools (P<0.001). Those with a recent RBC transfusion were the only neonates to have NEC surgery or to die from NEC. Preceding the bloody stools, those with no antecedent transfusion had been fed a larger volume (P=0.014), and had trends toward receiving calorically enriched feedings (P=0.055) and recent addition of human milk fortifier (P=0.060). Eosinophil counts following RBC transfusion tended to increase for 3-6 days, but when bloody stools were not preceded by transfusion the eosinophil counts were more static over that period.

CONCLUSION

In this predominantly Caucasian group of neonates with bloody stools, the presence of eosinophilia did not identify a benign condition distinct from NEC. A total of 44% of these neonates had transfusion-associated NEC. Eosinophils could have a previously unrecognized role in the pathogenesis of this NEC subtype.

Self-similar dynamics of morphogen gradients

Morphogen gradients are concentration fields of molecules acting as spatial regulators of cell differentiation in developing tissues and play a fundamental role in various aspects of embryonic development. We discovered a family of self-similar solutions in a canonical class of nonlinear reaction-diffusion models describing the formation of morphogen gradients. These solutions are realized in the limit of infinitely high production rate at the tissue boundary and are given by the product of the steady state concentration profile and a function of the diffusion similarity variable. We solved the boundary value problem for the similarity profile numerically and analyzed the implications of the discovered self-similarity on the dynamics of morphogenetic patterning.

Necrotizing enterocolitis during the first week of life: a multicentered case-control and cohort comparison study

OBJECTIVE

Necrotizing enterocolitis (NEC) is rare during the first week of life; most cases occur after 2 to 4 weeks. We hypothesized that when NEC develops in the first week, certain predisposing factors and feeding practices are identifiable. To test this, we sought to identify every case of NEC diagnosed during the first week within the Intermountain Healthcare system during the most recent 6-year period.

STUDY DESIGN

Data were collected from neonates admitted to any Intermountain Healthcare neonatal intensive care unit (NICU) with a date of birth from 1 January 2001 through 31 December 2006. Electronic and paper records were obtained for all with a diagnosis of NEC (Bell stage >or=II) within the first 168 h. X-rays, physician notes, nursing records, laboratory reports and operative reports were subjected to critical review to reexamine the diagnosis of NEC. Among those with confirmed NEC, we recorded underlying conditions and every feeding given prior to the diagnosis of NEC. Comparisons were made with patients that did not develop NEC, yet were cared for in the same NICUs, during the same period of time, and of the same gestational ages.

RESULT

A total of 28 neonates were identified electronically as having NEC during the first week. Critical review confirmed this in 21, but 5 were determined at laparotomy to have had spontaneous intestinal perforation, and 2 others were found on surgical reports to have had a congenital infarction of the colon. Total 20 of the 21 confirmed cases developed NEC while in a NICU being treated for another condition. The exception was a small-for-gestational-age neonate in a well baby nursery. Compared to 6100 controls, the 21 with early NEC were more likely to have had a meconium-positive test for illicit drug exposure (P<0.005), early onset sepsis (P<0.034) and respiratory distress (P<0.039). They were less likely than case-controls to have been fed human milk (P=0.003) and were more likely to have been fed formula exclusively (P=0.019). None who were fed human milk exclusively developed early NEC. Twelve of the twenty-one were fed (by gavage or bottle) amounts exceeding the upper limit of volumes taken by breastfed neonates.

CONCLUSION

We speculate that the prevalence of NEC during the first week could be reduced by identifying at-risk patients, feeding them human milk exclusively for the first week and using feeding volumes that do not exceed that taken by healthy breastfed neonates.

Emerging trends in acquired neonatal intestinal disease: is it time to abandon Bell's criteria?

In the last decade, it has become increasingly clear that necrotizing enterocolitis (NEC) is neither a uniform nor a well-defined disease entity. There are many factors that are forcing this unwelcome realization upon the neonatal and pediatric surgery communities. In the course of this manuscript we will review the history and the physical findings of the disparate etiologies of acquired neonatal intestinal diseases (ANIDs), some which do lead to the common final pathology of NEC and some which do not. New guidelines for distinguishing between ANIDs will also be suggested.

New insights into spontaneous intestinal perforation using a national data set (3): antenatal steroids have no adverse association with spontaneous intestinal perforation

OBJECTIVE

To examine whether antenatal steroids (ANS), alone or with early indomethacin, are associated with spontaneous intestinal perforation (SIP). SIP is a known complication of concurrent post-natal administration of glucocorticoid and indomethacin in extremely low birth weight (ELBW) infants.

STUDY DESIGN

A large de-identified national data set was retrospectively examined for infants with SIP without any report of other malformation or necrotizing entrocolitis. A control group was then derived matching for gender and birth weight (+/- 20 g). Pre- and post-natal variables were tested by both univariate and multivariate analysis to identify associations with SIP.

RESULTS

From January 1996 to June 2004, there were 2 27 711 discharges from Pediatrix neonatal intensive care unit sites. From this population 388 infants with SIP associated with ELBW were compared to matched controls. Infants with SIP were more likely to have received early indomethacin and to have received a combination of early indomethacin with post-natal glucocorticoids (P < 0.05 for both). When used alone (without subsequent indomethacin), ANS showed no association with SIP. When used in conjunction with indomethacin, ANS did not increase the rate of SIP beyond indomethacin alone.

CONCLUSION

ELBW Infants that acquire SIP were more likely to have been exposed to early indomethacin and post-natal glucocorticoids. However, no association was found between SIP and ANS within a well-powered cohort.

New insights into spontaneous intestinal perforation using a national data set: (2) two populations of patients with perforations

BACKGROUND

Spontaneous intestinal perforation (SIP) is increasingly common in the premature infant and has been demonstrated to be associated with early postnatal administration of glucocorticoids and indomethacin. Before survival of the extremely low birth weight (ELBW) infant, SIP was thought to be a rare, congenitally acquired disease sporadically affecting the muscularis of the distal intestine. These disparate views of etiology have not been previously reconciled in the literature.

OBJECTIVES

(1) To establish a cohort of SIP patients in a national data set. (2) To use timing of diagnosis as a unique data element and thereby differentiate between SIP cases which are susceptible to postnatal risk factors versus those occurring at or immediately after birth (and therefore not exposed to postnatal risk factors).

METHODS

A large identified national data set was used to retrospectively look at timing of diagnosis and then the cohort was divided into postnatal treatment groups for further subanalyses. This analysis resulted in the division of the cohort into early and late diagnosis SIP subcohorts. These were then queried retrospectively by univariate analysis to look for differences in demographics between the two (using a P-value < 0.05).

RESULTS

There were 633 patients with SIP evaluated in this data set. The early SIP cohort (0-3 days) was made up of 116 infants with a median BW of 1.401 kg, whereas the late cohort (4-14 days) held 386 infants with a median BW of 775 g. Infants with early SIP were significantly more likely to: be male, have higher Apgar scores, have not received antenatal steroids, surfactant or required mechanical ventilation.

CONCLUSIONS

Two populations of infants acquire SIP: ELBW infants acquire SIP on average between 7 and 10 days of life after exposure to indomethacin and glucocorticoids (either endogenous or exogenous), and infants with early SIP (0-3 days) who are significantly less likely to have been exposed to postnatal risk factors and are less premature.

New insights into spontaneous intestinal perforation using a national data set: (1) SIP is associated with early indomethacin exposure

BACKGROUND

Spontaneous intestinal perforation (SIP) is increasingly common in the premature infant and is associated with significant morbidity. Indomethacin use has been implicated as a co-risk factor for SIP when combined with glucocorticoids, but previous evidence argued against indomethacin being an independent risk factor when used prophylactically.

OBJECTIVES

(1) To establish a homogeneous cohort of SIP patients in a national data set and to contrast them to patients with surgical necrotizing enterocolitis (NEC). (2) To test the hypothesis that early post-natal indomethacin is independently associated with SIP.

METHODS

A large de-identified data set was retrospectively queried by diagnosis, and then multiple antenatal and post-natal variables were tested by both univariate and multivariate analysis to identify associations with SIP. Sub-analyses were also performed to look at the timing of drug administration.

RESULTS

There were 2105 patients evaluated in the data set. Patients were divided into matched controls (n = 581), those with SIP without report of NEC (n = 633) and those with NEC requiring surgery (n = 891). Infants with SIP were more likely to have a patent ductus arteriosus and more likely to be treated with vasopressors than either control or NEC patients. Compared to infants with NEC, patients with SIP were smaller, less mature and required more support. SIP was also diagnosed earlier than NEC (median of 7 vs 15 days). Patients with SIP were more likely to be treated with indomethacin, hydrocortisone or both on days of life 0-3 than controls.

CONCLUSIONS

(1) Surgical NEC and SIP have significant differences in presentation, demographics and morbidity. (2) A detailed look at drug timing revealed that early post-natal indomethacin is independently associated with SIP.

Discharge outcomes of extremely low birth weight infants with spontaneous intestinal perforations

OBJECTIVE

To examine discharge outcomes of extremely low birth weight infants (ELBW) with spontaneous intestinal perforation (SIP).

STUDY DESIGN

A single-center retrospective cohort study of all ELBW infants admitted to the University of Virginia neonatal intensive care unit between July 1996 and June 2004.

RESULTS

We found 35 patients with SIP (incidence 8.4%). The median gestational age was 25 weeks, median birth weight was 722 g, and 71% of the infants were male. Most infants (n=28) with SIP were diagnosed secondary to pneumoperitoneum; however, one-third (7) of infants<25 weeks had occult presentations without pneumoperitoneum. When controlled for gestational age, gender, multiple gestation, indomethacin, and glucocorticoid exposure, infants with SIP have a higher risk of PVL and death than infants without perforation.

SUMMARY

Periventricular leukomalacia and death are significantly associated with SIP in ELBW after adjusting for gestational age, multiple gestation, indomethacin, and glucocorticoid exposure.

The cellular repressor of E1A-stimulated genes mediates glucocorticoid-induced loss of the type-2 IGF receptor in ileal epithelial cells

Glucocorticoids induce hypertrophy of the neonatal ileal mucosa but the molecular mechanisms behind this growth induction remain poorly understood. Ileal epithelial cells (IECs) are dependent upon IGF-II for proliferation both in vivo and in culture. The type-2 IGF receptor (IGFR-2) is a lysosomal transport protein that attenuates IGF-II-driven growth and is highly abundant in the ileum. The cellular repressor of E1A-stimulated genes (CREG) is a secreted phosphoglycoprotein that affects cell fate via ligand binding with IGFR-2, although the mechanism by which it does so is unknown. We hypothesized that glucocorticoids might facilitate IGF-mediated hypertrophy through CREG-mediated degradation of IGFR-2. To test this hypothesis, confluent rat IECs (IEC-18) were cultured for 72 h with or without dexamethasone (DEX) and harvested for Western blot, immunocytochemistry, gene array and CREG immunoneutralization experiments. IGFR-2 and CREG immunohistochemistry were also performed in archived ileal specimens from control and DEX-exposed newborn mice and extremely premature infants to investigate in vivo and clinical relevance. DEX exposure was found to diminish IGFR-2 immunolocalization in cultured rat IECs, newborn mouse ileal mucosa and human neonatal ileal mucosa. Gene array data indicated that IGFR-2 expression was unchanged with DEX treatment, suggesting a mechanism of protein degradation. CREG immunolocalization and abundance was found to be increased by DEX and immunoneutralization of CREG resulted in the abolition of IGFR-2 degradation. We have concluded that CREG is a secreted mediator by which DEX induces degradation of IGFR-2 and speculate that this is a fundamental mechanism of mucosal growth induction.

Early postnatal dexamethasone diminishes transforming growth factor alpha localization within the ileal muscularis propria of newborn mice and extremely low-birth-weight infants

Focal small bowel perforation (FSBP) occurs most commonly in the ileum of extremely low-birth-weight (ELBW) infants. Early postnatal dexamethasone (EPD) administration results in an increased risk for FSBP in this patient population, but the mechanism by which this occurs is unknown. Infants with FSBP have healthy mucosa but thinned smooth muscle, suggesting a mechanism involving the muscularis propria for these perforations. One explanation for these findings would be that dexamethasone alters the tissue availability of pertinent growth factors to the smooth muscle. To explore this possibility, we administered dexamethasone or saline by intraperitoneal injection to newborn mice for 3 days (dosed at 1 microg/g of body weight/day) to simulate EPD protocols. The animals were sacrificed after 72 h of treatment and their ileums harvested and prepared for microscopy. Immunolocalization was performed for three related growth factors (epidermal growth factor [EGF], heparin-binding EGF [h-EGF], and transforming growth factor alpha [TGF-alpha]) and their common receptor. We found TGF-alpha to be abundant and discretely localized in the muscularis propria in control animals but to be diminished in dexamethasone-treated animals. EGF-receptor immunostaining was also decreased with dexamethasone but there was minimal to no detection of EGF or h-EGF in either treatment condition. Surgical and autopsy specimens of the ileum were obtained from seven ELBW infants who either received EPD or not. These tissues were used for immunolocalization of the same growth factors and similar distributions for TGF-alpha were observed in several of these cases. These findings are consistent with an autocrine role for TGF-alpha in ileal smooth muscle proliferation and suggest a mechanism by which EPD might mediate smooth muscle thinning.

Focal small bowel perforation: an adverse effect of early postnatal dexamethasone therapy in extremely low birth weight infants

OBJECTIVE

We tested the hypothesis that early postnatal dexamethasone (EPD) increases the risk of focal small bowel perforation (FSBP) in extremely low birth weight (ELBW) infants.

STUDY DESIGN

The techniques of meta-analysis were applied to studies evaluating EPD, which we identified through a systematic literature search. Studies were included if they were randomized, placebo-controlled trials of EPD, enrolled infants with birth weights < or =1000 g, and reported FSBP as an outcome variable. The Breslow-Day test was used to assess for homogeneity and a summary odds ratio was calculated using the Mantel-Haenszel exact method.

RESULTS

Four studies, with a pooled sample of 1383 infants, were included in the primary analysis. The Breslow-Day test showed a p-value of 0.61, indicating homogeneity among the studies. FSBP was significantly higher in EPD treated infants [odds ratio 1.91, 95% confidence interval (CI) 1.21, 3.07; p=0.004].

CONCLUSION

EPD increases the risk of FSBP in ELBW infants.

The clinical, morphologic, and molecular changes in the ileum associated with early postnatal dexamethasone administration: from the baby's bowel to the researcher's bench

Focal small bowel perforation (FSBP) is a life-threatening event that predominantly affects extremely low birth weight (ELBW) infants. Histopathology from surgical specimens of ileum with FSBP shows a healthy mucosa overlying a thinned muscularis with segmental degeneration. Clinical data strongly support an association between early postnatal administration of dexamethasone (EPD) and FSBP. Additional risk factors, including gestational age, administration of prophylactic indomethacin, and severity of illness, may be synergistic with EPD for the pathogenesis of perforations. Animal models of dexamethasone administration show morphologic changes in the ileum, similar to those seen in ELBW infants, including increased mucosal maturation and thinning of the muscularis. These tissue-specific differences may be mediated by a perturbation in growth factor expression or accumulation. In support of this hypothesis, dexamethasone has been associated with increased IGF-I immunolocalization in the mucosa and decreased immunolocalization in the muscularis. The known growth-promoting functions of IGF-I are consistent with the observed dexamethasone-associated changes within both the mucosa and the muscularis. Ongoing studies in this animal model are exploring the potential mechanisms by which dexamethasone might affect IGF-I availability.

Dexamethasone administration to newborn mice alters mucosal and muscular morphology in the ileum and modulates IGF-I localization

Glucocorticoid exposure accelerates the maturation of small bowel mucosa. We hypothesized that IGF-I, a mitogen and differentiating peptide expressed in small bowel, mediates steroid-induced change within the developing ileum. To investigate this possibility, we intraperitoneally administered 1 microg/gm/d of dexamethasone (DEX) or equal volumes of saline to litter-mate newborn mice. The animals were killed on d 1-3 of life and their ilea were harvested and prepared for microscopy. Tissue sections of ileum were examined for morphologic analyses, mucin staining, immunolocalization of IGF-I and -II, proliferating cell nuclear antigen (PCNA), terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL), and in situ hybridization for IGF-I transcripts. Morphologic comparisons showed increases in goblet cell number, total cell number, and TUNEL-positive cells within the mucosa of DEX-treated animals. In contrast, the number of smooth muscle nuclei per cross-section was unchanged with DEX treatment despite a reduction in the number of PCNA-positive nuclei and an increased bowel circumference. These findings suggest the muscularis stretches to accommodate increasing bowel diameter. IGF-I peptide was localized to the mesenchyme of all control animals. After 48 h of DEX treatment, IGF-I was detected in the epithelia whereas mesenchymal IGF-I localization appeared diminished. In situ hybridization analyses for IGF-I transcripts showed no differences in localization between the groups. We conclude that DEX administration differentially affects adjacent tissues in the newborn ileum and that the associated changes in IGF-I localization are consistent with its participation in this process.

Functional coupling to brush border creatine kinase imparts a selective energetic advantage to contractile ring myosin in intestinal epithelial cells

The B-CK isozyme of cytoplasmic creatine kinase is localized distinctly in the terminal web region of the intestinal epithelial cell brush border (Keller and Gordon: Cell Motil. Cytoskeleton 19:169-179, 1991). Experiments were performed to determine whether this CK is energetically coupled to the myosin II that is present in the circumferential ring and interrootlet structural domains of the brush border terminal web. In isolated brush borders, ATP-dependent circumferential ring contraction and interrootlet myosin solubilization were supported either by an exogenous PEP-pyruvate kinase-based ATP-regeneration system (PEP-PK) or by the addition of phosphocreatine to the endogenous B-CK-based ATP-regeneration system (PCr-B-CK). Addition of an exogenous hexokinase-glucose ATP-hydrolysis system (HK-G) effectively blocked both contraction and myosin solubilization in the PEP-PK assay. In contrast, HK-G had no significant effect on PCr-B-CK-supported brush border contraction, although it did inhibit interrootlet myosin solubilization. Thus, when high-energy phosphate is supplied as phosphocreatine, brush border B-CK imparts to the circumferential ring myosin a selective energetic advantage over other ATPases. These results suggest that myosin and B-CK are functionally coupled in the brush border circumferential ring, where they might comprise one end of an energy circuit that supplies energy for contraction, but that colocalization of CK with myosin in the brush border interrootlet domain is insufficient to establish functional coupling.

Discrete subcellular localization of a cytoplasmic and a mitochondrial isozyme of creatine kinase in intestinal epithelial cells

Two isozymes of creatine kinase have been purified differentially from mitochondrial and cytoplasmic subfractions of intestinal epithelial cells. These intestinal epithelial cell creatine kinases were indistinguishable from the cytoplasmic (B-CK) and mitochondrial (Mi-CK) creatine kinase isozymes of brain when compared by SDS-PAGE, cellulose polyacetate electrophoresis, and peptide mapping. In intestinal epithelial cells, immunolocalization of the Mi-CK isozyme indicates that it is associated with long, thin mitochondria, which are excluded from the brush border at the apical end of each cell. In contrast, immunolocalization of the B-CK isozyme indicates that it is concentrated distinctly in the brush border terminal web domain. Although absent from the microvilli, B-CK also is distributed diffusely throughout the cytoplasm. Terminal web localization of B-CK was maintained in glycerol-permeabilized cells and in isolated brush borders, indicating that B-CK binds to the brush border structure. The abundance and localization of the mitochondrial and cytoplasmic creatine kinase isozymes suggest that they are part of a system that temporally and/or spatially buffers dynamic energy requirements of intestinal epithelial cells.