Sexually dimorphic nucleus
| Sexually dimorphic nucleus | |
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| NeuroLex ID | Sexually dimorphic nucleus of the preoptic area |
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Anatomical terms of neuroanatomy
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The sexually dimorphic nucleus (SDN) is an ovoid, densely packed cluster of large cells located in the medial preoptic area (POA) of the hypothalamus which is believed to be related to sexual behavior in animals.
Thus far, for all species of mammals investigated, the SDN has been repeatedly found to be considerably larger in males than in females. In humans, the volume of the SDN has been found to be 2.2 times as large in males than in females and to contain 2.1 times as many cells. The human SDN is elongated in females and more spherical in males. No sex differences have been observed in the human SDN in either cell density or mean diameter of the cell nuclei. The volume and cell number of the human SDN considerably decreases with age; although, the decrease in cell number is both sex and age-specific. In males, a substantial decrease in the cell number of the human SDN was observed between the age of 50–60 years. Cell death was more common in females than males, especially among those older than 70 years of age. The SDN cell number in females can drop to 10-15% that found in early childhood.
SDN and its homologues exist widely in human, mammal, and some other animal brains, including:
The volume of SDN in medial preoptic area is modified by hormones, among which testosterone is proved to be of much importance. The larger volume of male SDN is correlated to the higher concentration of fetal testosterone level in males than in females. Also, there is evidence that testosterone acts during specific prenatal period to organize the development of aromatase-expressing neurons into the male-typical SDN (testosterone is transformed to estrogen by aromatase). The effect of testosterone is also reflected by the influence of fetal intrauterine position on the morphology of SDN-POA. Studies demonstrated that male rats which were gestated between two male fetuses (2M) have 2-fold larger SDN-POA volumes than those gestated between two female fetuses (2F). At the same time, the testosterone levels, as well as the 17β-estradiol (product of testosterone) levels, were found to be significantly larger in 2M males than in 2F males on gestation day 21 (testosterone can be transferred from adjacent male fetuses to the target rats). However, evidence fails to show any relationship between SDN volume and female fetal position.
According to some studies, the volume difference of SDN between males and females is related to apoptosis during early development after birth. In rats, central division of the medial preoptic nucleus (MPNc) is an important component of SDN-POA and evidence showed that the number of apoptotic cells within MPNc is greater in females than in males between postnatal day (PD) 7 and PD10. In MPNc, the levels of some proteins, which are related to apoptosis, were shown to be of significant difference between males and females. Such proteins include Bcl-2 and Bax. Bcl-2 is an antiapoptotic protein. The level of Bcl-2 in PD8 male rats is much higher than that in female rats of the same age, hence the number of apoptotic cells of MPNc in PD8 male rats is much lower than PD8 female rats. On the other hand, Bax, a proapoptotic protein, shows lower level in PD8 males than in PD8 females. Also, the number of active caspase-3-ir cells was observed to be greater in females than in males, indicating higher level of apoptosis in female MPNc.
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