White Shemales

As of yet, only two studies have investigated white matter microstructure in transsexuals (Rametti et al., 2011a,b). These investigations report increased FA values in male controls (MCs) compared with female controls (FCs) in several regions in the right hemisphere. Interestingly, female-to-male (FtM) and male-to-female (MtF) transsexuals exhibited FA values between those of MCs and FCs. Furthermore, these values were often more similar to those of their desired rather than their genetic sex. The authors interpreted their findings as increased masculinization in FtM transsexuals and incomplete masculinization in MtF transsexuals. Although these data are very promising, knowledge on additional determinants of observed differences is missing.

white shemales

Calculation of the FA, MD, AD, and RD maps was performed with the FMRIB (Functional MRI of the Brain) software library (FMRIB Software Library version 5.0.5; ) using tract-based spatial statistics (TBSS) (Smith et al., 2006) with default parameters unless specified otherwise. This included adjustment for eddy currents and head movement, as well as removal of the skull and non-brain tissue with the brain extraction tool. After fitting the tensor model (weighted least-squares approach), the TBSS pipeline applies nonlinear registration to match individual FA maps to MNI standard space (target = FMRIB58_FA standard-space image, study-specific mean FA and skeleton, FA threshold = 0.2). A skeleton of white matter tracts is created from the mean FA image, and individual maps are finally mapped to this skeleton. Non-FA images (i.e., AD, RD, and MD) are processed in the same manner by using the transformations obtained from the FA processing procedure. Because men and women differ in overall brain size (Allen et al., 2002; Gong et al., 2009; Giedd et al., 2012), the total intracranial volume (TIV) was extracted from T1-weighted images for inclusion in the statistical analyses. Using the VBM8 toolbox ( -jena.de/vbm/) for SPM8 ( ), structural scans were segmented into gray and white matter, as well as CSF. The TIV was calculated as the sum of these three parameters.

T in the womb and early neonatal life plays a decisive role in sexual brain differentiation, which is believed to underlie a subject's gender identity (Swaab and Garcia-Falgueras, 2009). This organizational effect produces permanent changes in the cellular organization of brain tissue that occurs during critical periods when differences in serum T are highest between sexes (Auyeung et al., 2013). Functional and structural MRI studies in 8- to 11-year-old children found that fetal T predicted neural response to valenced facial cues in reward-related regions, such as nucleus accumbens, caudate, and putamen (Lombardo et al., 2012b). Furthermore, fetal T was shown to predict GMV in the temporoparietal junction, superior temporal sulcus, orbitofrontal cortex, and planum temporale (Lombardo et al., 2012a), as well as white matter asymmetry in corpus callosum size (Chura et al., 2010). T thereby acts either directly on developing neurons or via local conversion to estrogen by the enzyme aromatase, and estrogen then masculinizes certain brain areas, regulates synapse formation, and acts as a neurotrophic factor (Li and Shen, 2005; McCarthy, 2008; Savic et al., 2010).

However, studies indicate that sexual differentiation of the brain also occurs before the onset of hormone production (Lentini et al., 2013). Furthermore, organizational effects of fetal hormones may depend on the chromosomal sex (Carrer and Cambiasso, 2002). Several studies on growth and differentiation of neurons in vitro indicated sexual differences in response to E2: E2 treatment induced earlier axon differentiation in neurons from females but not from males (Díaz et al., 1992), whereas axon growth during E2 treatment was increased in neurons from males but not from females (Cambiasso et al., 2000). In light of these findings, we can speculate on a sex-dependent differential effect of fetal T on white matter microstructure, which explains the group differences found in the present study. Unfortunately, however, this could not be tested directly in the current study because information on fetal T levels was not available.

Our results indicate that organizational effects early in brain development seem to oppose the effects of T later in life. Whereas group differences showed the transition FC > FtM > MtF > MC, indicating an inverse relationship between fetal T and MD, adult plasma T levels showed a strong positive correlation with MD. Indeed, cell culture studies indicate that gonadal steroid hormone effects on neuritogenesis are age dependent and region specific (Carrer and Cambiasso, 2002; McCarthy, 2008). Furthermore, DTI studies corroborate relatively higher FA and AD in males and higher MD and RD in females in many white matter tracts (Hsu et al., 2008; Inano et al., 2011; Menzler et al., 2011), which seems to contradict recent data indicating strong positive correlations between RD and adult T levels in males (Peper et al., 2013). Given the highly dynamic nature of the effects of sex hormones on cellular growth and differentiation, generalizations about their influence should thus be treated with great caution. 041b061a72