Ovarian estrogen acts to feminize the female rat's corpus callosum

White matter microstructural integrity continues to develop from adolescence to young adulthood in mice and humans: Same phenotype, different mechanism

As direct evaluation of a mouse model of human neurodevelopment, adolescent and young adult mice and humans underwent MR diffusion tensor imaging to quantify age-related differences in microstructural integrity of brain white matter fibers. Fractional anisotropy (FA) was greater in older than younger mice and humans. Despite the cross-species commonality, the underlying developmental mechanism differed: whereas evidence for greater axonal extension contributed to higher FA in older mice, evidence for continuing myelination contributed to higher FA in human adolescent development. These differences occurred in the context of species distinctions in overall brain growth: whereas the continued growth of the brain and skull in the murine model can accommodate volume expansion into adulthood, human white matter volume and myelination continue growth into adulthood within a fixed intracranial volume. Appreciation of the similarities and differences in developmental mechanism can enhance the utility of animal models of brain white matter structure, function, and response to exogenous manipulation.

The effects of puberty and sex on adolescent white matter development: A systematic review

Adolescence, the transition between childhood and adulthood, is characterized by rapid brain development in white matter (WM) that is attributed in part to rising levels in adrenal and gonadal hormones. The extent to which pubertal hormones and related neuroendocrine processes explain sex differences in WM during this period is unclear. In this systematic review, we sought to examine whether there are consistent associations between hormonal changes and morphological and microstructural properties of WM across species and whether these effects are sex-specific. We identified 90 (75 human, 15 non-human) studies that met inclusion criteria for our analyses. While studies in human adolescents show notable heterogeneity, results broadly demonstrate that increases in gonadal hormones across pubertal development are associated with macro- and microstructural changes in WM tracts that are consistent with the sex differences found in non-human animals, particularly in the corpus callosum. We discuss limitations of the current state of the science and recommend important future directions for investigators in the field to consider in order to advance our understanding of the neuroscience of puberty and to promote forward and backward translation across model organisms.

Active feminization of the preoptic area occurs independently of the gonads in Amphiprion ocellaris

Sex differences in the anatomy and physiology of the vertebrate preoptic area (POA) arise during development, and influence sex-specific reproductive functions later in life. Relative to masculinization, mechanisms for feminization of the POA are not well understood. The purpose of this study was to induce sex change from male to female in the anemonefish Amphiprion ocellaris, and track the timing of changes in POA cytoarchitecture, composition of the gonads and circulating sex steroid levels. Reproductive males were paired together and then sampled after 3 weeks, 6 months, 1 year and 3 years. Results show that as males change sex into females, number of medium cells in the anterior POA (parvocellular region) approximately double to female levels over the course of several months to 1 year. Feminization of gonads, and plasma sex steroids occur independently, on a variable timescale, up to years after POA sex change has completed. Findings suggest the process of POA feminization is orchestrated by factors originating from within the brain as opposed to being cued from the gonads, consistent with the dominant hypothesis in mammals. Anemonefish provide an opportunity to explore active mechanisms responsible for female brain development in an individual with male gonads and circulating sex steroid levels.

Developmental changes in visual line bisection in women throughout adulthood

It has been suggested that some hemispheric asymmetries change in a systematic way from young adulthood to older age. However, little is known whether these changes are due to differential aging of a single hemisphere or based on age-related alterations of interhemispheric interactions. A sample of 281 right-handed neurologically healthy participants (151 women), ranging from age 20 to 79, was investigated with a line-bisection task. Previous studies indicate the midpoint estimation shows a consistent leftward bias from the veridical center, which is accentuated when the left hand is used to bisect lines. These findings support the view of a right hemispheric superiority in spatial attention. This study revealed this pattern to be stable in men throughout adulthood. However, women from 50 to 69 years of age showed a reduced leftward bias and a reduced hand effect compared to men and younger women. The results suggest that developmental changes in hemispheric asymmetry of spatial attention are more pronounced in women and support the view that neuromorphological changes during adulthood differ between sexes.

Hormones and temporal components of speech: sex differences and effects of menstrual cyclicity on speech

Voice onset time (VOT) is a salient acoustic parameter of speech which signals the 'voiced' and 'voiceless' status of plosives in English (e.g. the initial sound in 'bat' versus the initial sound in 'pat'). As a micro-temporal acoustic parameter, VOT may be sensitive to changes in hormones which may affect the neuromuscular systems involved in speech production. This study adopted a novel approach by investigating the effects of menstrual cycle phase and sex on VOT. VOT data representing the six plosives of English (/p b t d k g/) were examined for seven women (age 20-23 years) at two phases of the menstrual cycle (day 18-25: high estrogen and progesterone; day 2-5: low estrogen and progesterone). Results indicated that menstrual cycle phase had a significant interaction with the identity of the plosive (F (5,30) = 5.869, P < 0.002). Menstrual cycle phase also had significant effects on the contrasts between cognate voiced and voiceless plosives (F (1,6) = 11.444, P < 0.02); samples from the high hormone phase displayed an enhanced voiced/voiceless contrast. Subsequently, VOT data samples from the two phases of the menstrual cycle were compared with those from five men in order to explore sex differences at different phases of the menstrual cycle. Low hormone phase samples displayed no significant sex differences for either VOT values (F (1,10) = 2.085, P > 0.05), or the contrast between voiced and voiceless cognates (F (1,10) = 0.407, P > 0.05). In contrast, the high hormone phase VOT samples displayed significant plosive by sex interactions (F (5,50) = 4.442, P < 0.005). In addition, significant sex differences were found for the contrasts between cognate voiced and voiceless plosives (F (1,10) = 5.019, P < 0.05); the women displayed a more marked voiced/voiceless contrast. The findings suggest that ovarian hormones play some role in shaping some temporal components of speech.

The aromatase knockout (ArKO) mouse provides new evidence that estrogens are required for the development of the female brain

The classic view of sexual differentiation is that the male brain develops under the influence of testicular secretions, whereas the female brain develops in the absence of any hormonal stimulation. However, several studies have suggested a possible role of estradiol in female neural development, although they did not provide unequivocal evidence that estradiol is indispensable for the development of the female brain and behavior. As a result, the hypothesis subsequently languished because of the lack of a suitable animal model to test estrogen's possible contribution to female differentiation. The recent introduction of the aromatase knockout (ArKO) mouse, which is deficient in aromatase activity because of a targeted mutation in the CYP19 gene and therefore cannot aromatize androgen to estrogen, has provided a new opportunity to reopen the debate of whether estradiol contributes to the development of the female brain. Female ArKO mice showed reduced levels of lordosis behavior after adult treatment with estradiol and progesterone, suggesting that estradiol is required for the development of the neural mechanisms controlling this behavior in female mice. The neural systems affected may include the olfactory systems in that ArKO females also showed impairments in olfactory investigation of odors from conspecifics. Thus, the classic view of sexual differentiation, that is, the female brain develops in the absence of any hormonal secretion, needs to be re-examined.

The aromatase knock-out mouse provides new evidence that estradiol is required during development in the female for the expression of sociosexual behaviors in adulthood

We used estrogen-deficient aromatase knock-out (ArKO) mice to determine whether estrogens contribute to the development of the brain and behavior in females. Female mice of three different genotypes [i.e., wild type (WT), heterozygous (HET), and homozygous (ArKO)] were ovariectomized in adulthood and subsequently tested for odor preferences (choice: intact male vs estrous female) in a Y-maze. When treated with testosterone, ArKO females spent significantly less time sniffing odors (both volatile and nonvolatile) from either male or female stimuli compared with WT and HET females. When given direct access to anesthetized stimulus animals or when given a choice between odor and visual cues from both stimulus animals, ArKO females continued to spend less time investigating the stimuli compared with WT and HET females. These defects in olfactory investigation of ArKO females were partially corrected with estradiol treatment in adulthood. Estradiol-treated ArKO females no longer differed from WT and HET females in the time spent investigating either nonvolatile odors or the anogenital region of anesthetized animals. However, ArKO females still investigated volatile odors and/or visual cues less than WT and HET females. Sexual receptivity was severely impaired in ArKO females after treatments with estradiol and progesterone that successfully induced receptivity in WT and HET females. Furthermore, ArKO females showed diminished levels of male sexual behaviors, whereas WT and HET females readily mounted an estrous female. Together, these findings demonstrate that estrogen is required for normal female development. The concept that the female brain develops in the absence of any hormonal stimulation should therefore be reconsidered.

Neonatal estrogen blockade prevents normal callosal responsiveness to estradiol in adulthood

The rat corpus callosum (CC) is larger in males than females, and is responsive to hormone manipulations during development. We previously demonstrated that P25 ovariectomy (Ovx) enlarged (defeminized) adult CC, while P70 ovary transfer (OvT) counteracted this enlarging effect, resulting in smaller (feminized) CC. Since OvT females were not Ovx'd until P25, they received some neonatal estrogen (E) exposure. Behavioral data suggest that adult responsiveness to ovarian hormones depends upon prior organization by neonatal E. It has not been determined whether a similar phenomenon occurs for the feminization of brain morphology. The current experiment examined whether our previous finding of adult CC responsiveness to ovarian hormones depended upon neonatal E exposure. We investigated this by assessing the effects of P70 ovarian hormone replacement (via ovary transfer or E pellet) in females that received either (1) normal ovarian hormone exposure until P25 Ovx, or (2) the E receptor blocker tamoxifen from birth to P25 Ovx. Females receiving normal neonatal hormone exposure responded to P70 E in the female-typical manner: E reduced CC size. In contrast, females receiving neonatal E blockade responded to adult E in the opposite manner: E increased CC size. As far as we are aware, this is the first report suggesting that neonatal E exposure organizes the female brain so that it responds normally to the organizing actions of E when later exposure occurs. These findings further challenge the traditional model of female brain development, which asserts that normal female brain organization occurs by default, in the absence of gonadal hormone exposure.

Adult ovary transfer counteracts the callosal enlargement resulting from prepubertal ovariectomy

The rat corpus callosum (CC) is larger in males than females, and is sensitive to hormone manipulations during development. Previous research found that, in rats, CC sensitivity to testosterone ended by postnatal day 8 (P8). In contrast, more recent findings demonstrated that CC responsivity to ovarian hormones continued at least through P70. The current experiment extends these findings by showing that the female callosum is still sensitive to ovarian hormones as late as P130, well into adulthood.

Ovarian hormones can organize the rat corpus callosum in adulthood

The rat corpus callosum (CC) is larger in males than females, and is responsive to hormone manipulations during development. Previous data suggest that CC sensitivity to testosterone ends by postnatal day 8 (P8). In contrast, responsivity to ovarian hormones extends as late as P25. The current series of experiments investigates whether ovarian hormone effects on the callosum are permanent and whether CC sensitivity to ovarian hormones extends beyond P25. We found that P70 ovariectomy (Ovx) did not affect callosal size, suggesting that ovarian hormone exposure sometime prior to P70 is sufficient to feminize the CC, and that once the callosum is feminized, the effects can not be reversed. We also found that P25 ovariectomy enlarged, or defeminized, adult female CC, whereas ovary transfer starting on P55 or P70 counteracted this enlarging effect, resulting in feminized adult CC. Thus, although a previously feminized callosum is not affected by P70 ovarian hormone removal, a not-yet feminized callosum can still be feminized after P70. These findings indicate that there is flexibility in the developmental window within which the female brain is responsive to the active feminization process initiated by ovarian hormones.

Sex and species differences in mouse and rat forebrain commissures depend on the method of adjusting for brain size

Sex differences in the forebrain commissures (corpus callosum, hippocampal commissure, and anterior commissure) were examined in B6D2F2 hybrid mice and Sprague-Dawley rats. Twenty-four male-female littermate pairs of mice were perfused at each of 21, 42 and 63 days of age and the midsagittal area of the commissures was measured from en bloc stained tissue. Twenty-two male-female littermate pairs of rats were examined at 110 days of age using the same methods. Male mice had larger bodies than females but no sex differences were found for mouse brain weight or commissure areas. In contrast, a significant sex difference was found for rat body, brain, corpus callosum and hippocampal commissure sizes. Four methods were used to adjust for differences in brain size (ratio, geometric, linear regression, and allometric). When the two species were analysed separately, neither mice nor rats showed significant sex differences in commissure areas relative to brain size if regression or allometric adjustments were made. Even when data from mice and rats were combined into one large group with a wide range of values, no species or sex differences were apparent after adjustments were made for brain size with either the regression or allometric methods. The use of ratios to adjust for differences in overall size is not recommended, especially because this method does not effectively remove the influence of brain size from commissure size; a substantial correlation is often present between the ratio and brain size.

Sex differences in the human corpus callosum: myth or reality?

It has been claimed that the human corpus callosum shows sex differences, and in particular that the splenium (the posterior portion) is larger in women than in men. Data collected before 1910 from cadavers indicate that, on average, males have larger brains than females and that the average size of their corpus callosum is larger. A meta-analysis of 49 studies published since 1980 reveals no significant sex difference in the size or shape of the splenium of the corpus callosum, whether or not an appropriate adjustment is made for brain size using analysis of covariance or linear regression. It is argued that a simple ratio of corpus callosum size to whole brain size is not an appropriate way to analyse the data and can create a false impression of a sex difference in the corpus callosum. The recent studies, most of which used magnetic resonance imaging (MRI), confirm the earlier findings of larger average brain size and overall corpus callosum size for males. The widespread belief that women have a larger splenium than men and consequently think differently is untenable. Causes of and means to avoid such a false impression in future research are discussed.

Sex differences in the development of axon number in the splenium of the rat corpus callosum from postnatal day 15 through 60

Axon number in the splenium was examined at 15, 25 and 60 days of age in male and female rats. The splenium (posterior fifth) of the corpus callosum was found to contain the axons from the visual cortex at all three ages and was extensively sampled with electron microscopy. Overall, there was a 15% decrease in the total number of axons between postnatal day 15 and day 60 in both sexes. The observed decrease in axon number between day 15 and 25 in both males and females is consistent with Elberger's (A.J. Elberger, Transitory corpus callosum axons projecting throughout developing rat visual cortex revealed by DiI, Cereb. Cortex 4 (1994) 279-299) data which suggest that the pattern of visual callosal projections in the rat visual cortex is not restricted to the adult form until the fourth postnatal week. There was a further decrease in axon number between day 25 and day 60 in females only such that by 60 days of age, the total number of axons was equivalent between the sexes. Thus in the rat splenium, males appear to attain the adult number of axons earlier than females. These results also indicate that there is a sex difference in the timing of axon withdrawal in the rat splenium, with axon withdrawal continuing in females after it has ceased in males.

Gender effect on Purkinje cell loss in the cerebellum of the heterozygous reeler mouse

Homozygous mutant mice such as staggerer (sg/sg) or reeler (rl/rl) exhibit a marked ataxia associated with an atrophic cerebellum during the first postnatal weeks and a reduced number of Purkinje cells, the deficit reaching about 75% in sg/sg and 50% in rl/rl as compared to age- and sex-matched mice from the same strain background. These two mutations are classically viewed as recessive, but we have recently shown that heterozygous staggerer (+/sg) mice exhibit a progressive and age-related loss of Purkinje cells between 3 and 12 months of age, despite their apparent clinical normality (Shojaeian-Zanjani et al., 1992). In the present study, we have investigated whether a similar cell loss exists in the cerebellum of heterozygous +/rl mice. The number of Purkinje cells was counted in serial parasagittal sections of the cerebellum of +/rl and their +/+ littermates at 3, 16 and 26 months of age. Our results reveal a 16% deficit in the number of Purkinje cells in 3-month-old +/rl and a 24% one in 16-month-old animals: surprisingly this deficit is only present in the +/rl males, while the females are spared. These results suggest that the reeler gene (D'Arcangelo et al., 1995) exerts its effect on Purkinje cell number in a gender-specific fashion in heterozygous mutant mice.

Absence of postnatal testosterone fails to demasculinize the male rat's corpus callosum

We had previously shown that elimination of testosterone from embryonic day 17 through adulthood reduced the midsagittal area of the male rat corpus callosum (CC). However, day 1 castration, performed after the 2-h post-birth testosterone surge, was without effect. To elucidate the contribution of this surge on the CC, male rats were delivered by cesarean section and castrated within 20 min. This procedure eliminated the 2-h postnatal rise in testosterone levels. The prenatal surge in testosterone, which occurs on embryonic day 18, remained intact. In adulthood, callosal area was examined in castrate males, sham males, and intact females. Castrate males and sham males had significantly larger CCs as compared to females. The two male groups did not differ from each other. Body weight was significantly higher in sham versus castrate males, establishing the effectiveness of the castration. These results show that hormonal organization of the CC in the male is the result of the independent action of prenatal testicular androgens, and suggest that the end of this period marks the end of callosal sensitivity to testicular hormone influence. In addition, this report documents sexual dimorphism of the CC in a third rat strain.

Lack of activational influence of ovarian hormones on the size of the female rat's corpus callosum

The sex difference in the midsagittal area of the adult rat corpus callosum (CC) has been shown to be mediated, in part, by gonadal steroids in early development, with the sensitive period of hormone action in the female extending at least up to postnatal day 25. Given this prolonged sensitivity, the current study attempted to delineate organizational vs. activational influences of gonadal hormones on the female rat CC. In Experiment 1, callosal size was examined across the estrous cycle at 52 and 90 days of age. In Experiment 2, females were ovariectomized at 78 days and CC parameters assessed at 110 days. Last, in Experiment 3, females were ovariectomized at 78 days and sacrificed at 110 days; in addition, sham females were sacrificed during proestrus or estrus. Neither stage of estrous cycle nor adult ovariectomy affected midsagittal CC size. These results provide evidence for organizational effects of ovarian steroids on the female callosum, with the sensitive period of hormone action ending sometime between days 25 and 78.

Sex differences in the distribution of axon types within the genu of the rat corpus callosum

Neuroanatomical sex differences have been documented in the rat neocortex, including dimorphism of its predominant commissure, the corpus callosum (CC). In particular, CC sex differences have been reported in the ultrastructure of the posterior callosal region, the splenium. Since the CC is a heterogeneous fiber tract with its axons arising from distinct cortical areas and passing through restricted regions along its length, it became of interest to ascertain whether cellular sexual dimorphisms may also be present in another division of the CC. To test this hypothesis, electron microscopy was used to examine axon composition in adult male and female rats in the anterior portion, the genu. The number and size of axons, the thickness of the myelin sheath, and the area within the genu occupied by these constituents, were quantified. Results showed a significant sex difference in the ratio of unmyelinated to myelinated axons, with females having a larger proportion of unmyelinated fibers. This effect was present for both (1) the number of axons, and (2) the area taken up by axonal fibers. No differences were found in the size of either axon type, or for myelin thickness. Comparison of these results with those from the splenium and possible mechanisms underlying this dimorphism are discussed.

Anatomical-behavioral relationships: corpus callosum morphometry and hemispheric specialization

We obtained midsagittal measures of the corpus callosum in 60 healthy young adults (right-handed and left-handed males and females), and examined whether individual differences in anatomical measures of callosal connectivity are related to behavioral laterality measures in the same subjects. In an attempt to tap functionally-distinct callosal "channels", four behavioral laterality tasks were used that differed in sensory modality (visual, auditory, tactile) and/or level of cognitive processing (sensory versus semantic). In addition, the tasks had both intrahemispheric and interhemispheric conditions. Sex differences were found for measures of the posterior body (i.e. isthmus) of the corpus callosum, which, in turn, interacted with handedness. In contrast, only handedness effects were found for the behavioral laterality measures. Anatomical-behavioral correlations did not disclose relationships between callosal size and performance on task conditions requiring sensory interhemispheric integration or transfer. Instead, the correlational findings are consistent with the view that the corpus callosum participates in such higher order "control" functions as the support of bilateral representation of language, functional interhemispheric inhibition, and the maintenance of hemispheric differences in arousal. This is consistent with the finding that regional callosal size is related to the number of small diameter fibers, which are presumed to interconnect homologous association cortices in the two hemispheres.

Sex differences in anxiety behavior in rats: role of gonadal hormones

These experiments examined the role of gonadal hormones at both the organizational and activational time periods on sex differences in plus-maze behavior. In the first experiment, adult female Long-Evans rats were found to spend more time on the open arms of the plus maze than adult males, indicating less anxious behavior. In the second experiment, male and female subjects received a neonatal treatment (chemical castration with flutamide or tamoxifen, vehicle injection, or no injection) and a prepubertal treatment (gonadectomy, sham surgery, or no surgery). Adult females receiving either neonatal tamoxifen or prepubertal ovariectomy spent less time on the open arms than control females, but females who received both treatments were the most defeminized subjects. Males were not affected by the absence of gonadal hormones at either time period. These experiment indicate that female gonadal hormones play an important role both organizationally and activationally in plus-maze behavior. The role of the GABA receptor complex in mediating this effect is discussed. Knowledge of sex differences in plus-maze behavior may help to make this maze a more useful tool in investigating anxiety behavior in rats.