Morphometric analysis of the developing rat brain

Integration of toxicodynamic and toxicokinetic new approach methods into a weight-of-evidence analysis for pesticide developmental neurotoxicity assessment: A case-study with DL- and L-glufosinate

DL-glufosinate ammonium (DL-GLF) is a registered herbicide for which a guideline Developmental Neurotoxicity (DNT) study has been conducted. Offspring effects included altered brain morphometrics, decreased body weight, and increased motor activity. Guideline DNT studies are not available for its enriched isomers L-GLF acid and L-GLF ammonium; conducting one would be time consuming, resource-intensive, and possibly redundant given the existing DL-GLF DNT. To support deciding whether to request a guideline DNT study for the L-GLF isomers, DL-GLF and the L-GLF isomers were screened using in vitro assays for network formation and neurite outgrowth. DL-GLF and L-GLF isomers were without effects in both assays. DL-GLF and L-GLF (1-100 μM) isomers increased mean firing rate of mature networks to 120-140% of baseline. In vitro toxicokinetic assessments were used to derive administered equivalent doses (AEDs) for the in vitro testing concentrations. The AED for L-GLF was ∼3X higher than the NOAEL from the DL-GLF DNT indicating that the available guideline study would be protective of potential DNT due to L-GLF exposure. Based in part on the results of these in vitro studies, EPA is not requiring L-GLF isomer guideline DNT studies, thereby providing a case study for a useful application of DNT screening assays.

Proliferative cells in the rat developing neocortical grey matter: new insights into gliogenesis

The postnatal brain development is characterized by a substantial gain in weight and size, ascribed to increasing neuronal size and branching, and to massive addition of glial cells. This occurs concomitantly to the shrinkage of VZ and SVZ, considered to be the main germinal zones, thus suggesting the existence of other germinative niches. The aim of this study is to characterize the cortical grey matter proliferating cells during postnatal development, providing their stereological quantification and identifying the nature of their cell lineage. We performed double immunolabeling for the proliferation marker Ki67 and three proteins which identify either astrocytes (S100β) or oligodendrocytes (Olig2 and NG2), in addition to a wider panel of markers apt to validate the former markers or to investigate other cell lineages. We found that proliferating cells increase in number during the first postnatal week until P10 and subsequently decreased until P21. Cell lineage characterization revealed that grey matter proliferating cells are prevalently oligodendrocytes and astrocytes along with endothelial and microglial cells, while no neurons have been detected. Our data showed that astrogliogenesis occurs prevalently during the first 10 days of postnatal development, whereas contrary to the expected peak of oligodendrogenesis at the second postnatal week, we found a permanent pool of proliferating oligodendrocytes enduring from birth until P21. These data support the relevance of glial proliferation within the grey matter and could be a point of departure for further investigations of this complex process.

Clinical and Histological Changes of Intrathecally Administered Gadopentate Dimeglumine (Gd-DTPA) in Normal Rats

Objectives:

This study is carried out to explore clinical and histological changes induced in rats by intrathecal administration of Gd-DTPA via suboccipital spinal injection. 2.5, 5, 10 μmol/g-brain of Gd-DTPA were injected intrathecally to 43 adult male rats and sucrose as control solution with same volume and osmolarity were injected to 18 rats. Animals were sacrificed on day 4 and 14. Sections from the cortex, brain stem, cerebellum and medulla spinalis were obtained to examine for cell loss and apoptosis. In this study, no clinical abnormalities were observed in 69.8 % of rats of Gd-DTPA group and in 83.3 % of rats of sucrose group. Transient neurological signs such as ataxia and paresis were seen in 11.6 % of rats in the Gd-DTPA group and in 5.5 % of rats in the sucrose group. They were seen more frequently in the Gd-DTPA group especially in the highest dose and volume. Histological examination did not revealed necrosis or apoptosis in both groups. This study suggests that intrathecally administered Gd-DTPA may be safe in humans when lower doses per gram of brain are used than rats.

A ‘Best Practices’ Approach to Neuropathologic Assessment in Developmental Neurotoxicity Testing—for Today

A key trait of developmental neurotoxicants is their ability to cause structural lesions in the immature nervous system. Thus, neuropathologic assessment is an essential element of developmental neurotoxicity (DNT) studies that are designed to evaluate chemically-induced risk to neural substrates in young humans. The guidelines for conventional DNT assays have been established by regulatory agencies to provide a flexible scaffold for conducting such studies; recent experience has launched new efforts to update these recommendations. The present document was produced by an ad hoc subcommittee of the Society of Toxicologic Pathology (STP) tasked with examining conventional methods used in DNT neuropathology in order to define the ‘best practices’ for dealing with the diverse requirements of both national (EPA) and international (OECD) regulatory bodies. Recommendations (including citations for relevant neurobiological and technical references) address all aspects of the DNT neuropathology examination: study design; tissue fixation, collection, processing, and staining; qualitative and quantitative evaluation; statistical analysis; proper control materials; study documentation; and personnel training. If followed, these proposals will allow pathologists to meet the need for a sound risk assessment (balanced to address both regulatory issues and scientific considerations) in this field today while providing direction for the research needed to further refine DNT neuropathology ‘best practices’ in the future.

Toxicologic Pathology Analysis for Translational Neuroscience

A half-day American College of Toxicology continuing education course presented key issues often confronted by translational neuroscientists when predicting human risk from animal-derived toxicologic pathology data. Two talks correlated discrete structures with major functions in brains of rodents and nonrodents. The third lecture provided practical advice to obtain highly homologous rodent brain sections for quantitative morphometry in developmental neurotoxicity testing. The last presentation discussed demographic influences (eg, species, strain, sex, age), physiological attributes (eg, body composition, brain vascularity, pharmacokinetic/pharmacodynamic patterns, etc), and husbandry parameters (eg, group housing) recognized to impact the actions of neuroactive chemicals. Speakers described common cases of real-world challenges to animal data interpretation encountered when designing studies or extrapolating biological responses across species. The efficiency of translational neuroscience efforts will likely be enhanced as new methods (eg, high-resolution non-invasive imaging) improve our capability to cross-connect subtle anatomic and/or biochemical lesions with functional changes over time.

Recommended Methods for Brain Processing and Quantitative Analysis in Rodent Developmental Neurotoxicity Studies

Neuropathology methods in rodent developmental neurotoxicity (DNT) studies have evolved with experience and changing regulatory guidance. This article emphasizes principles and methods to promote more standardized DNT neuropathology evaluation, particularly procurement of highly homologous brain sections and collection of the most reproducible morphometric measurements. To minimize bias, brains from all animals at all dose levels should be processed from brain weighing through paraffin embedding at one time using a counterbalanced design. Morphometric measurements should be anchored by distinct neuroanatomic landmarks that can be identified reliably on the faced block or in unstained sections and which address the region-specific circuitry of the measured area. Common test article-related qualitative changes in the developing brain include abnormal cell numbers (yielding altered regional size), displaced cells (ectopia and heterotopia), and/or aberrant differentiation (indicated by defective myelination or synaptogenesis), but rarely glial or inflammatory reactions. Inclusion of digital images in the DNT pathology raw data provides confidence that the quantitative analysis was done on anatomically matched (i.e., highly homologous) sections. Interpreting DNT neuropathology data and their presumptive correlation with neurobehavioral data requires an integrative weight-of-evidence approach including consideration of maternal toxicity, body weight, brain weight, and the pattern of findings across brain regions, doses, sexes, and ages.

Visualizing the engram: learning stabilizes odor representations in the olfactory network

The nature of memory is a central issue in neuroscience. How does our representation of the world change with learning and experience? Here we use the transcription of Arc mRNA, which permits probing the neural representations of temporally separated events, to address this in a well characterized odor learning model. Rat pups readily associate odor with maternal care. In pups, the lateralized olfactory networks are independent, permitting separate training and within-subject control. We use multiday training to create an enduring memory of peppermint odor. Training stabilized rewarded, but not nonrewarded, odor representations in both mitral cells and associated granule cells of the olfactory bulb and in the pyramidal cells of the anterior piriform cortex. An enlarged core of stable, likely highly active neurons represent rewarded odor at both stages of the olfactory network. Odor representations in anterior piriform cortex were sparser than typical in adult rat and did not enlarge with learning. This sparser representation of odor is congruent with the maturation of lateral olfactory tract input in rat pups. Cortical representations elsewhere have been shown to be highly variable in electrophysiological experiments, suggesting brains operate normally using dynamic and network-modulated representations. The olfactory cortical representations here are consistent with the generalized associative model of sparse variable cortical representation, as normal responses to repeated odors were highly variable (∼70% of the cells change as indexed by Arc). Learning and memory modified rewarded odor ensembles to increase stability in a core representational component.

Learning modulation of odor representations: new findings from Arc-indexed networks

We first review our understanding of odor representations in rodent olfactory bulb (OB) and anterior piriform cortex (APC). We then consider learning-induced representation changes. Finally we describe the perspective on network representations gained from examining Arc-indexed odor networks of awake rats. Arc-indexed networks are sparse and distributed, consistent with current views. However Arc provides representations of repeated odors. Arc-indexed repeated odor representations are quite variable. Sparse representations are assumed to be compact and reliable memory codes. Arc suggests this is not necessarily the case. The variability seen is consistent with electrophysiology in awake animals and may reflect top-down cortical modulation of context. Arc-indexing shows that distinct odors share larger than predicted neuron pools. These may be low-threshold neuronal subsets. Learning’s effect on Arc-indexed representations is to increase the stable or overlapping component of rewarded odor representations. This component can decrease for similar odors when their discrimination is rewarded. The learning effects seen are supported by electrophysiology, but mechanisms remain to be elucidated.

Effects of environmentally relevant mixtures of persistent organic pollutants on the developmental neurobiology in rats

We report the developmental neuropathology for rat pups at postnatal day (PND) 37 and PND 77 and the molecular biomarkers for PND 35, 75, and 350 after perinatal exposure to a reconstituted mixture of persistent organochlorine pollutants (POPs) based on the blood profiles of people living in the Great Lake Basin. The developmental neuropathology included routine histopathology evaluation, quantification of cell proliferation and death in the subventricular zone, linear morphometric measurements, and transcriptional analysis. No histopathological, structural, or stereological changes were observed in animals treated with the POPs or Aroclor 1254, on PND 37 or PND 77. While no transcriptional changes were found in Arcolor-treated animals, significant transcriptional changes were observed on PND 350 in female offspring perinatally exposed to 0.13 mg/kg of the POP mixture. Markers of the cholinergic system including acetylcholinesterase and the muscarinic receptors (subtypes M1-M5) were downregulated 2- to 6-fold. In addition, structural genes including neurofilaments (NFLs) and microtubule-associated protein (MAP-2) were downregulated at least 2-fold or greater. Our results support that in utero and lactational exposure to the chemical mixture of POPs lead to developmental changes in adult rat brains.

Computational models of ethanol-induced neurodevelopmental toxicity across species: Implications for risk assessment

Computational, systems-based approaches can provide a quantitative construct for evaluating risk in the context of mechanistic data. Previously, we developed computational models for the rat, mouse, rhesus monkey, and human, describing the acquisition of adult neuron number in the neocortex during the key neurodevelopmental processes of neurogenesis and synaptogenesis. Here we apply mechanistic data from the rat describing ethanol-induced toxicity in the developing neocortex to evaluate the utility of these models for analyzing neurodevelopmental toxicity across species. Our model can explain long-term neocortical neuronal loss in the rodent model after in utero exposure to ethanol based on inhibition of proliferation during neurogenesis. Our human model predicts a significant neuronal deficit after daily peak BECs reaching 10-20 mg/dl, which is the approximate BEC reached after drinking one standard drink within one hour. In contrast, peak daily BECs of 100 mg/dl are necessary to predict similar deficits in the rat. Our model prediction of increased sensitivity of primate species to ethanol-induced inhibition of proliferation is based on application of in vivo experimental data from primates showing a prolonged rapid growth period in the primate versus rodent neuronal progenitor population. To place our predictions into a broader context, we evaluate the evidence for functional low-dose effects across rats, monkeys, and humans. Results from this critical evaluation suggest subtle effects are evident at doses causing peak BECs of approximately 20 mg/dl daily, corroborating our model predictions. Our example highlights the utility of a systems-based modeling approach in risk assessment.

Unbiased cell quantification reveals a continued increase in the number of neocortical neurones during early post-natal development in mice

The post-natal growth spurt of the mammalian neocortex has been attributed to maturation of dendritic arborizations, growth and myelination of axons, and addition of glia. It is unclear whether this growth may also involve recruitment of additional neurones. Using stereological methods, we analysed the number of neurones and glia in the neocortex during post-natal development in two separate strains of mice. Cell counting by the optical fractionator revealed that the number of neurones increased 80-100% from the time of birth to post-natal day (P)16, followed by a reduction by approximately 25% in the young adult mouse at P50-55. Unexpectedly, at the time of birth less than half of the neurones and at P8 only 65% of the neurones expressed neuronal nuclear antigen (NeuN), a marker of mature post-migratory neurones. In accordance with these observations, NeuN acquisition by neurones in layer VIa was delayed until P16. The number of glia reached its maximum at P16, whereas the number of oligodendroglia, identified using a transgenic marker, increased until P55, the latest time of observation. Neurones continued to accumulate in the developing neocortex during the first 2 weeks of post-natal development, underscoring fundamental differences in brain development in the mouse compared with human and non-human primates. Further, delayed acquisition of NeuN by neurones in the deepest neocortical layers and continued addition of oligodendroglia to the neocortex suggested that neocortical maturation should be regarded as an ongoing process continuing into the young adult mouse.

Undertaking positive control studies as part of developmental neurotoxicity testing: a report from the ILSI Research Foundation/Risk Science Institute expert working group on neurodevelopmental endpoints

Developmental neurotoxicity testing involves functional and neurohistological assessments in offspring during and following maternal and/or neonatal exposure. Data from positive control studies are an integral component in developmental neurotoxicity risk assessments. Positive control data are crucial for evaluating a laboratory's capability to detect chemical-induced changes in measured endpoints. Positive control data are also valuable in a weight-of-evidence approach to help determine the biological significance of results and provide confidence in negative results from developmental neurotoxicity (DNT) studies. This review is a practical guide for the selection and use of positive control agents in developmental neurotoxicology. The advantages and disadvantages of various positive control agents are discussed for the endpoints in developmental neurotoxicity studies. Design issues specific to positive control studies in developmental neurotoxicity are considered and recommendations on how to interpret and report positive control data are made. Positive control studies should be conducted as an integral component of the incorporation and use of developmental neurotoxicity testing methods in laboratories that generate data used in risk decisions.

Developmental neuropathology in DNT-studies—A sensitive tool for the detection and characterization of developmental neurotoxicants

Developmental neurotoxicity (DNT-) studies are the first reproduction toxicity studies for which an extended histopathological examination of developing structures is required by the current EPA and OECD guidelines. The morphological screening includes a macroscopic evaluation of the brain and nervous tissue, brain weight parameters, gross morphometry of the brain, neurohistological examinations and a quantitative analysis of major brain areas. This review is intended to give an overview about the needs according to guideline requirements, practical approaches for a successful developmental neuropathology and its preconditions and does include examples of background data on the value and functional meaning of morphological data. A selection of experimental data from literature is also presented in the light of their contribution for the understanding of important, neurodevelopmental disorders in humans.

Cellular and spatial changes in the anuran superior olive across metamorphosis

In many vertebrate species, the superior olive in the auditory brainstem plays an essential role in sound source localization. Little is known, however, about the structural and functional changes in this nucleus during development when alterations in head size and shape as well as in inner ear projections are expected to affect the perception of binaural cues. Using stereological techniques, we investigated the changes in several cellular and spatial features of the bullfrog superior olive across metamorphosis, the time period during which the animal transforms from a totally aquatic larva to a semiterrestrial adult. The total number of cells shows a strongly linear increase from hatchling through late larval stages. The number of neurons decreases during metamorphic climax stages, and recovers to pre-metamorphic climax levels in the early post-metamorphic froglet stage. The number of glial cells increases during the early larval period, and remains relatively stable, with no systematic variation, from late larval to froglet stages. The volume of the superior olive increases rapidly in early larval stages, followed by a much-attenuated rate of growth between late larval and froglet stages. These morphological changes may provide a substrate for the functional restructuring of the bullfrog superior olive, shortly before the switch from aquatic to mostly atmospheric hearing.

A Systems-Based Computational Model for Dose-Response Comparisons of Two Mode of Action Hypotheses for Ethanol-Induced Neurodevelopmental Toxicity

Investigations into the potential mechanisms for ethanol-induced developmental toxicity have been ongoing for over 30 years since Fetal Alcohol Syndrome (FAS) was first described. Neurodevelopmental endpoints are particularly sensitive to in utero exposure to alcohol as suggested by the more prevalent alcohol-related neurodevelopmental disorder (ARND). The inhibition of proliferation during neurogenesis and the induction of apoptosis during the period of synaptogenesis have been identified as potentially important mechanisms for ARND. However, it is unclear how these two mechanisms quantitatively relate to the dose and timing of exposure. We have extended our model of neocortical neurogenesis to evaluate apoptosis during synaptogenesis. This model construct allows quantitative evaluation of the relative impacts on neuronal proliferation versus apoptosis during neocortical development. Ethanol-induced lengthening of the cell cycle of neural progenitor cells during rat neocortical neurogenesis (G13-G19) is used to compute the number of neurons lost after exposure during neurogenesis. Ethanol-induced dose-dependent increases in cell death rates are applied to our apoptosis model during rat synaptogenesis (P0-P14), when programmed cell death plays a major role in shaping the future neocortex. At a human blood ethanol concentration that occurs after 3-5 drinks ( approximately 150 mg/dl), our model predicts a 20-30% neuronal deficit due to inhibition of proliferation during neurogenesis, while a similar exposure during synaptogenesis suggests a 7-9% neuronal loss through induction of cell death. Experimental in vitro and in vivo dose-response research and stereological research on long-term neuronal loss after developmental exposure to ethanol is compared to our model predictions. Our computational model allows for quantitative, systems-level comparisons of mechanistic hypotheses for perturbations during specific neurodevelopmental periods.

Modeling developmental processes in animals: applications in neurodevelopmental toxicology

Biologically based dose-response models can provide a framework for incorporating mechanistic information into our assessments of neurotoxicity considering both kinetic and dynamic processes. We have constructed models for normal midbrain and neocortex development and we have extended these models to evaluate the neurodevelopmental toxicity of ethanol and methyl mercury. Using such modeling approaches, we have been able to test hypothesized modes of action for these neurodevelopmental toxicants. Specifically, we have compared ethanol's effects on neocortical neurogenesis and exacerbation of apoptosis during the synaptogenesis period. We have used methylmercury as an example of how one can link toxicokinetic and toxicodynamic models and also as an example of how mechanistic data on gene expression can support model development. In summary, using examples from our research group, this paper illustrates the need for models that evaluate both qualitative and quantitative kinetic and dynamic factors in order to understand the potential impacts of neurodevelopmental toxicants.

Regulatory developmental neurotoxicity testing: a model study focussing on conventional neuropathology endpoints and other perspectives

Our aim was to investigate a model of the morphologic approach proposed in guidelines for developmental neurotoxicity testing (DNT). Hereto, a limited DNT study [EPA Health Effects Test Guidelines OPPTS 870.6300, 1996a. Developmental Neurotoxicity Study "Public Draft", United States Environmental Protection Agency; Prevention, Pesticides and Toxic Substances (7101); EPA 712-C-96-239, June 1996. ] was carried out with different doses of methylazoxy methanol acetate (MAM), known to affect brain morphology and neuron numbers in the developing brain. After gross examination, the brains of F1-animals were further dissected along neuro-anatomical landmarks to ensure homology between tissues of different individuals. The (relative) weight of the brain (parts) was determined. One brain half (alternating left/right to avoid lateralization) was further used for microscopic slide reading and measurement of brain layer width (linear morphometry); the other was set aside for stereologic investigation in a later phase of the study. In the offspring, a clear reduction in brain size (gross macroscopy) and weight (MAM high- and top-dose groups) was observed on postnatal days (PN) 22 and 62, but this reduction was hard to pinpoint in the microscope as the changes primarily appeared quantitative in nature, rather than qualitative. Linear measurements of brain layer width appeared very sensitive and efficient. This first step of a project is presented and the perspectives of a further stereologic investigation are discussed.

The role of cell death during neocortical neurogenesis and synaptogenesis: implications from a computational model for the rat and mouse

We are quantitatively evaluating the acquisition of neocortical neurons through key stages of development including neurogenesis, migration, and synaptogenesis. Here we expand upon a previous computational model describing neocortical neurogenesis in the rat and mouse [Dev. Neurosci. 24 (2002) 467], to include the period of synaptogenesis (P0-P14) when programmed cell death (PCD) is known to play a major role in shaping the neocortex. We also quantitatively evaluate differing hypotheses on the role of cell death during neurogenesis. This new model construct allows prediction of acquisition of adult neuronal number in the rat and mouse neocortex from the beginning of neurogenesis through synaptogenesis. The mathematical model output is validated by independently derived stereologically determined neuron number estimates in the adult rat and mouse. Simulations suggest cell death during synaptogenesis reduces the neocortical neuronal population by 20-30%, while cell death of progenitor cells and newly formed neurons during neurogenesis may reduce output by as much as 24%. However, higher death rates during neurogenesis as suggested by some research would deplete the progenitor population, not allowing for the vast expansion that is the hallmark of the mammalian neocortex. Furthermore, our simulations suggest the clearance time of dying neurons labeled by TUNEL or pyknosis is relatively short, between 1 and 4 h, corroborating experimental research. This novel mathematical model for adult neocortical neuronal acquisition allows for in silico analysis of normal and perturbed states of neocortical development as well as interspecies and evolutionary analyses of neocortical development.

Quantitative morphometry of hippocampal pyramidal cells: Differences between anatomical classes and reconstructing laboratories

The dendritic trees of hippocampal pyramidal cells play important roles in the establishment and regulation of network connectivity, synaptic plasticity, and firing dynamics. Several laboratories routinely reconstruct CA3 and CA1 dendrites to correlate their three-dimensional structure with biophysical, electrophysiological, and anatomical observables. To integrate and assess the consistency of the quantitative data available to the scientific community, we exhaustively analyzed 143 completely reconstructed neurons intracellularly filled and digitized in five different laboratories from 10 experimental conditions. Thirty morphometric parameters, including the most common neuroanatomical measurements, were extracted from all neurons. A consistent fraction of parameters (11 of 30) was significantly different between CA3 and CA1 cells. A considerably large number of parameters was also found that discriminated among neurons within the same morphological class, but reconstructed in different laboratories. These interlaboratory differences (8 of 30 parameters) far outweighed the differences between experimental conditions within a single lab, such as aging or preparation method (at most two significant parameters). The set of morphometrics separating anatomical regions and that separating reconstructing laboratories were almost entirely nonoverlapping. CA3 and CA1 neurons could be distinguished by global quantities such as branch order and Sholl distance. Differences among laboratories were largely due to local variables such as branch diameter and local bifurcation angles. Only one parameter (a ratio of branch diameters) separated both morphological classes and reconstructing laboratories. Compartmental simulations of electrophysiological activity showed that both differences between anatomical classes and reconstructing laboratories could dramatically affect the firing rate of these neurons under different experimental conditions.

Stereological tools in biomedical research

Stereological studies are more and more frequent in literature, particularly in the development/evolution, pathology, and neurosciences areas. The stereology challenge is to understand the structural inner three-dimensional arrangement based on the analysis of the structure slices only showing two-dimensional information. Cavalieri and Scherle's methods to estimate volume, and Buffon's needle problem, are commented in the stereological context. A group of actions is needed to appropriately quantify morphological structures (unbiased and reproducibly), e.g. sampling, isotropic and uniform randomly sections (Delesse's principle), and updated stereological tools (disector, fractionator, nucleator, etc). Through the correct stereology use, a quantitative study with little effort could be performed: efficiency in stereology means a minimum slices sample counting (little work), low cost (slices preparation), but good accuracy. In the present text, a short review of the main stereological tools is done as a background basis to non-expert scientists.

Changes in cell cycle parameters and cell number in the rat midbrain during organogenesis

We employed 5-bromo-2'-deoxyuridine (BrdU) labeling to identify in vivo changes in the cell cycle patterns of the rat midbrain during the major period of midbrain organogenesis, gestational days (gd) 11 to 16. We also used quantitative stereology to determine changes in absolute cell numbers during these gestational time points. Between gd 12 and 16, the length of S-phase did not change significantly while the fraction of cycling cells decreased from 73 to 11%. The average cell cycle length was determined to be 15 h on gd 12 and 17 h on gd 16, the difference not being statistically significant. The cell number in the midbrain increased from 1.3E5 cells on gd 11 to 1.7E7 cells on gd 16. On gd 12 and gd 13, there was a significant negative correlation between litter position and midbrain cell number, the effect diminishing on later days of gestation. The combined use of quantitative stereology and flow cytometry to study brain development represents a novel application that allows for simultaneous evaluation of changes in cell proliferation kinetics and the resulting effect of those kinetic changes on embryonic midbrain development.