Chlorambucil-induced cell death in embryonic mouse limb buds

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.

Cell death/apoptosis: normal, chemically induced, and teratogenic effect

Cell death is an integral part of a variety of biological processes including cell proliferation, differentiation, and morphogenesis. We review here the morphological and biochemical nature as well as the genetic basis for cell death during normal and abnormal development. Most often referred to in normal development as programmed cell death, this controlled process determines the size, patterning, and function of many tissues. The importance of its proper genetic regulation is demonstrated by the discovery of cell death-specific genes and the several disorders including cancer and teratogenesis that result from repression or enhancement of cell death. In our studies we employed the developing mouse limb, which provides a defined window of active cell death, to elucidate mechanisms of cell death. We have developed markers that reveal in the developing normal limb an apoptotic morphology with phagocytosis and DNA fragmentation. In the limb deformity mutant Hammertoe there is a defective (restricted) cell death pattern, but the morphology remains apoptotic. By the use of these markers, we were able to observe that the teratogen retinoic acid produced enhanced apoptotic cell death. Most interestingly, retinoic acid-induced cell death in the Hammertoe mutant resulted in correction of the mutant phenotype. Future studies will determine the relationship between exogenous agents and endogenous signaling pathways as well as indicate how these interactions can alter the fate of a given cell and potentially ameliorate a genetic abnormality.

Contribution of mesenchymal cell death and mitotic alteration to asymmetric limb malformations induced by MNNG

N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) induces fetal asymmetric limb malformations with exposure of pregnant mice to 50 mg/kg on day 11 of gestation. Hindlimbs were more frequently malformed than forelimbs, and a fourfold greater incidence of postaxial ectrodactyly was found in left forelimbs than in right forelimbs, and a two fold excess in left hindlimbs compared to right hindlimbs. The level of cell death and mitotic index were measured in forelimbs and hindlimbs from treated and control embryos at 1, 4, 18, 24, 48, and 72 hr after exposure to ascertain if these parameters could be correlated with the differential teratogenic susceptibility of the limbs. An increase in necrotic index was first detected in treated limbs at 4 hr, increased at 18 hr, peaked at 24 hr, and began declining at 48 hr to reach the control baseline at 72 hr. At 24 hr, the correlation between the level of cell death and susceptibility of malformation was the strongest, with the left hindlimb having a necrotic index of 58%, the right hindlimb 47%, the left forelimb 30% and the right forelimb 12%. In both forelimbs and hindlimbs, MNNG treatment initially depressed mitotic activity followed by an elevation at 48 hr relative to controls. The magnitude of the depression, extent of the elevation, and overall pattern of mitotic activity could not be uniformly related to limb defects. These results indicate that the amount of cell death in limb buds at 24 hr after MNNG exposure may predict target organ susceptibility. Depressions in mitotic activity and alterations in the pattern of mitosis were also observed which were not as clearly correlated with the incidence of malformations as was the amount of cell death.

Experimental limb dysmorphogenesis as a model of chemical injury response in undifferentiated embryonic tissues: a light and electron microscopical study

A histopathological and ultrastructural analysis of the early stage of limb dysmorphogenesis from 1 hr to 48 hr, following citral administration is presented as a model for studying the reaction to injury of undifferentiated embryonic tissues. Early lesions such as cloudy swelling, hydropic degeneration, enlargement of the extracellular space and retraction of protoplasmic extensions were observed 1 hr after treatment, affecting both epiderm and mesenchyme of the limb. Few pyknotic cells were encountered during the early stages. Cell necrosis and autophagocytosis became more conspicuous at 12 hr post-injection. About the same time the first signs of healing were observed, charcterised by heterophagocytosis and return of mitotic activity. The epiderm being more differentiated, regenerates ad integrum and more rapidly than the mesenchyme. The evolution of these pathological events, which proceed in a spatio-temporal cascade, seem to reflect the pharmacokinetic gradient of the cell-teratogen interactions. The type of limb malformation will depend mainly on the extent of injury and/or healing of the mesenchymal mass, rather than on the injury and/or healing of the surface ectoderm. It is concluded that the reaction to chemical injury of undifferentiated tissues partially mimics the destructive and repair process in adult tissues, but differs in lacking some basic constituents such as oedema, monocyte exudation and granulation tissue formation.

Morphological alterations in the developing fetal rat limb due to maternal injection of chlorambucil

On day 14 of pregnancy, rats were injected i.p. with 9.5 mg/kg of chlorambucil. At term, all the fetuses were stunted and had obvious skeletal malformations. From days 15-20 of gestation there was a steady progression of cell death and histologic derangement in long bone formation. This began on day 15, the day following exposure to chlorambucil, with extensive cell death of chondroblasts in the long bone primordia. By day 16, dead or dying cells were located in the cartilage model and became associated with a higher than normal matrix to cell ratio. In day 17 limbs, many cells of the cartilage model were irregular in size and shape so that the normally precise cellular arrangement was lacking. On day 18, periosteal and enchondrial calcification was delayed and scanty. The fibular hemimelia observed in all drug-treated term fetuses stained with alizarin appeared to result from lack of development of the proximal half of the cartilage model. The observed alterations, particularly bowing of the ulna, in the teratogen-treated fetuses appeared to result primarily from specific cell death and formation of aberrant cell types but abnormal cell products may also play a role.