Caudal dysgenesis in staged human embryos: Carnegie stages 16-23

The severity of expression of malformations of the median axis in the caudal region of human embryos is highly variable and ranges from caudal dysgenesis and sirenomelia to simple sacral hypoplasia. Several forms of sacral dysgenesis may be discovered later in life. This shows that caudal malformations of relatively lesser severity should occur at a greater frequency than actually reported. In the present study we looked at the morphology and histology of some human embryos with caudal dysgenesis. Several developmental alterations of the median axis were observed. These included significant reduction in the craniofacial mesenchyme characterized by hypoplasia of the pharyngeal arches, palatal shelves, and agenesis or hypoplasia of the auricular hillocks at the rostral end, absence of the caudal trunk from midsacral to all coccygeal segments, vertebral fusion or agenesis, defective development of the primary and secondary neural tubes, rectal and urinary tract dysgenesis, and deficiency, malrotation, and deficiency of the limbs at the caudal end. Hindlimb malformations included bilateral agenesis (one case), meromelia, and various forms of abnormal rotation, but no instances of sirenomelia were present. Radial dysgenesis has been reported to be associated with caudal dyplasia in the literature, however, we observed agenesis of the ulna in one and of the fibula in another embryo. There was an impressive association between limb malformations and body wall defects. The histological studies demonstrated caudal vascular deficiency and hemorrhagic lesions in the limbs of the dysplastic embryos. The data suggest that these polytopic field defects arise very early in development possibly as result of disturbances to fundamental developmental events that share common molecular and cellular mechanisms.

Chlorambucil-induced postclosure exencephaly and axial skeletal abnormalities in rat fetuses

Open neural tube defects (NTD) are reported to arise from failure of the embryonic neural folds to close or rupture of a previously closed neural tube. The critical period for dysmorphogenesis of the neural tube by either mechanism is of clinical importance. We had previously reported that single doses of cyclophosphamide (CPA), administered to pregnant rats resulted in reopening of the closed neural tube. The objective of the present study was to determine if other antineoplastic drugs also had similar ability to cause rupture of the closed neural tube. Therefore, single doses of chlorambucil dissolved in bicarbonate buffer were administered to Wistar rats on one of gestation days (GD) 11 through 14 (i.e., well after closure of the neural tube) and fetuses were examined on GD 20. It was observed that the window of susceptibility extended from GD 11 through 14 and that a single dose of 10 mg/kg of the drug was most effective. The affected fetuses had the prosencephalon and mesencephalon parts of the brain protruding through a wide opening of the cranial roof. The exposed brain tissue was hemorrhagic and covered by a thin, transparent and often porous membrane. The cranial vault was missing. Numerous malformations of the axial skeleton and of several nonneural organs were found to accompany the brain defect. Electron microscopic and light microscopic studies revealed extensive cell death, fragmentation, and phagocytosis of dead cells and vacuolization of the neuroepithelium (NE) within 12 h of the drug treatment. Cell death per se was not pronounced in the cranial mesenchyme (ME). The vacuoles in the NE coalesced into small cavities. The ME failed to proliferate adequately and organize itself into the cranial vault primordium. Hemorrhage and disorganization of the NE was progressive. The fluid that escaped into the interstitium seemed to extend into the cysts that developed in the ME externally and into the ventricular system internally. The edema of the ME thus might have contributed to disruption of precursor mesenchymal tissue and consequently to malformation of the cranial bones. Large hematomas and cysts also developed on the basal aspect of the crumbling brain vesicles and appeared to lift the neural tissue out of the shallow cranial cavity. As previously reported for CPA, these data provide evidence for the ability of chlorambucil to cause postclosure exencephaly in rat fetuses for a susceptible period that is considerably long.

An assessment of the developmental toxicity of inorganic arsenic

A critical analysis of the literature base regarding the reproductive and developmental toxicity of arsenic compounds, with emphasis on inorganic arsenicals, was conducted. The analysis was stimulated by the great number of papers that have purported to have shown an association between exposure of pregnant laboratory animals to arsenic compounds and the occurrence of offspring with cranial neural tube defects, particularly exencephaly. For the most part, the literature reports of arsenic developmental toxicity in experimental animals are inadequate for human risk assessment purposes. Despite the shortcomings of the experimental database, several conclusions are readily apparent when the animal studies are viewed collectively. First, cranial neural tube defects are induced in rodents only when arsenic exposure has occurred early in gestation (on Days 7 [hamster, mouse], 8 [mouse], or 9 [rat]). Second, arsenic exposures that cause cranial neural tube defects are single doses that are so high as to be lethal (or nearly so) to the pregnant animal. Third, the effective routes of exposure are by injection directly into the venous system or the peritoneal cavity; even massive oral exposures do not cause increases in the incidence of total gross malformations. Fourth, repetition of similar study designs employing exaggerated parenteral doses is the source of the large number of papers reporting neural tube defects associated with prenatal arsenic exposure. Fifth, in five repeated dose studies carried out following EPA Guidelines for assessing developmental toxicity, arsenic was not teratogenic in rats (AsIII, 101 micromol/kg/d, oral gavage; 101 micromol/m3, inhalation), mice (AsV, 338 micromol/kg/d, oral gavage; est. 402 micromol/kg/d, diet), or rabbits (AsV, 21 micromol/kg/d, oral gavage). Data regarding arsenic exposure and adverse outcomes of pregnancy in humans are limited to several ecologic epidemiology studies of drinking water, airborne dusts, and smelter environs. These studies failed to (1) obtain accurate measurements of maternal exposure during the critical period of organogenesis and (2) control for recognized confounders. The lone study that examined maternal arsenic exposure during pregnancy and the presence of neural tube defects in progeny failed to confirm a relationship between the two. It is concluded that under environmentally relevant exposure scenarios (e.g., 100 ppm in soil), inorganic arsenic is unlikely to pose a risk to pregnant women and their offspring.

Retinoic acid-induced caudal regression syndrome in the mouse fetus

Caudal regression syndrome (CRS) comprises developmental anomalies of the caudal vertebrae, neural tube, urogenital and digestive organs, and hind limbs, the precursors of all of which are derived from the caudal eminence. Although the syndrome is well recognized, the etiology and pathogenetic mechanisms are poorly understood. Genetic and experimental models may provide some important clues to the early events that precede the dysmorphogenesis in CRS. The objectives of this study were to determine the susceptible stages for induction of CRS and to ascertain the early events that precede the development of this syndrome in a mouse model. Single oral doses of 100, 150, or 200 mg/kg retinoic acid (RA) were administered to TO mice on one of Gestation Days (GD) 8 to 12, and fetuses were observed on GD 18. All doses administered on GD 8 or 9 resulted in CRS in a large number of survivors. Agenesis of the tail, caudal vertebral defects, spina bifida occulta/aperta, imperforate anus, rectovesicle or rectourethral fistula, renal malformations, cryptorchidism, gastroschisis, and limb malformations, including the classical mermaid syndrome (sirenomelia), were characteristic features of this animal model. Several craniofacial malformations accompanied CRS in the GD 8 treatment group. Chronologic examination of treated embryos at early stages revealed pronounced cell death in the caudal median axis, hindgut, and neural tube and consequently, failure of development of the tail bud in the high-dose groups. In the 100 mg/kg RA group, patches of hemorrhage occurred initially that subsequently coalesced into large hematomas and the tail progressively regressed. Histologic examination revealed the onset and progression of hemorrhage, edema, and cell death in these embryos. Transillumination and histologic preparations also revealed dilation of the caudal neural tube in the prospective CRS embryos. Thus, a combination of cell death, vascular disruption, and tissue deficiency appears to be the highlight of caudal regression in this model. Symmelia appeared to be due to failure of fission or due to the merger of limb fields rather than a result of fusion of two limb buds. The data are also indicative of caudal agenesis in the high-dose RA groups and caudal regression due to a combination of vascular disruption, edema, and cell death in the lower dose groups of TO mouse embryos.

Sodium valproate augments spontaneous neural tube defects and axial skeletal malformations in TO mouse fetuses [corrected]

The TO mouse exhibits a low incidence (3.65%) of spontaneous exencephaly at birth. The objectives of this study were to determine if sodium valproate (VPA) would augment this background frequency of exencephaly and to characterize its gross and histologic bases. Single doses of 200, 400, or 600 mg/kg of VPA were administered on one of gestation days (GD) 7 to 10 and fetuses were collected on GD 18. Significant augmentation of the background incidence of exencephaly was observed in the GD 7 and 8 treatment groups. Absence of the skull vault, hemorrhage, and degeneration of the exposed brain, polyhydramnios, and a female excess characterized the abnormality. Exencephalic embryos were markedly growth retarded. In addition to craniofacial and urogenital anomalies, severe axial skeletal malformations were found to be consistently associated with exencephaly. Morphometric evaluation of the alizarin red-stained skeleton confirmed significant skeletal growth inhibition. Histologic sections of GD 10 embryos revealed early onset of treatment-related growth retardation. Arrest of closure appeared to affect intermittent segments of the neural tube. The closure defect sometimes only involved the surface ectoderm of the dorsal midline. The unclosed neural tube was at times covered by a continuous layer of surface ectoderm. Cell death per se was not pronounced in the neuroepithelium. The mesenchyme was generally sparse and edema was obvious in embryos with partial closure. Growth inhibition of the optic and otic primordia was marked by pronounced cell death in these structures as well as in the otic and trigeminal ganglia and in the pharyngeal arch mesenchyme. Evidence for neural crest cell migration was also recorded. These data indicate that VPA interacts with genetic susceptibility, augments the frequency of exencephaly, and also induces other malformations in the TO mouse. The widespread malformations of the cranifacial structures are suggestive of the preferential effect of VPA on the neural crest or its derivatives.

Effect of pre-treatment with aspirin on alcohol-induced neural tube defects in the TO mouse fetuses

This study was planned to investigate the effects of ethanol on the genetic susceptibility of the TO mouse to neural tube malformations and to determine the ameliorative effects, if any, of aspirin a potent prostaglandin inhibitor. The TO mouse exhibits a spontaneous incidence of 3.6% exencephaly. The mice were exposed to single (i.p.) dose of 0.03 ml/gm body weight of a solution of (25%v/v) of absolute alcohol in physiological saline on day 7 or 8 of gestation. Subteratogenic doses (150 or 200 mg/kg) of aspirin were administered (i.p.) an hour before ethanol exposure. Fetuses were collected on day 18 and compared with those of the untreated, and saline treated pair-fed pair-watered controls as well as with those of the aspirin alone treatment group. A total of 175 litters were studied. Alcohol caused a three-fold increase against the background incidence of exencephaly. Several craniofacial anomalies and growth retardation were also observed. Alizarin red-S stained skeletal preparations revealed extensive malformations of the craniofacial skeleton in the exencephalic fetuses. Both doses of aspirin administered prior to alcohol treatment significantly accentuated the alcohol-induced prenatal mortality. The rescue effect of aspirin on alcohol-induced intrauterine growth retardation was also significant although fetal weight was not restored to levels comparable to those of the controls. Pre-treatment with aspirin (both 150 and 200 mg/kg) on day 8 of gestation resulted in a numerical, though not statistically significant increase in alcohol-induced exencephaly. On the other hand pre-administration of the lower dose on day 7 exencephaly. On the other hand pre-administration of the lower dose on day 7 of gestation caused a significant reduction while the higher dose gave rise to a significant increase in the incidence of this malformation. Aspirin also reduced the frequency of alcohol-induced arched palate and the baseline exencephaly. These data provide evidence for the possible interaction of alcohol with the genetic susceptibility to exencephaly in this strain of mice. The lack of a clear dose-dependent antagonistic effect of aspirin on alcohol-induced exencephaly suggests that the production of this malformation is probably not mediated by prostaglandin as it was shown for limb and renal abnormalities (Randall, C.L., Anton, R.F. and Becker, H.C. (1991). Aspirin dose dependently reduces alcohol induced birth defects and prostaglandin E levels in mice. Teratology 44, 521-529).