biology

Sexually dimorphic nucleus

Sexually dimorphic nucleus is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Sexually dimorphic nucleus rather than just read about it. In short: 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.

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Reference excerpt

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 as 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% of that found in early childhood. SDN and its homologues exist widely in human, mammal, and some other animal brains, including:

the third interstitial nucleus of the anterior hypothalamus (INAH3) in humans; ovine sexually dimorphic nucleus (oSDN) in the medial preoptic area/anterior hypothalamus (MPOA/AH) in sheep; sexually dimorphic nucleus in the preoptic area (SDN-POA) in rats; anterior hypothalamic nucleus (AHdc) in macaques; specific area in medial preoptic nucleus (POM) in quails; etc.

Sexually dimorphic nucleus in medial preoptic area

Formation and organization of SDN in medial preoptic area

Sex differences in SDN caused by exposure to testosterone (prenatal) 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 shown 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.

Sex differences in the volume of SDN caused by apoptosis (postnatal) 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. Apoptosis also occurs in the anteroventral periventricular nucleus (AVPV), which is also a sexually dimorphic area and is located in the periventricular gray area at the rostral extreme of the third ventricle. In contrast with SDN-POA, AVPV has larger size in females than in males. It is proved that Bcl-2 level in AVPV is higher whereas Bax level is lower in females than in males, just as being opposite of those in MPNc. As indicated in these two cases, apoptotic cell death plays a critical role in the formation of sexually dimorphic nucleus, and the apoptotic cell number within SDN negatively correlates with the volume of SDN between different sexes.

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Worked examples

Example 1 — a first encounter with Sexually dimorphic nucleus

Start with the simplest possible case. Write down what Sexually dimorphic nucleus claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Sexually dimorphic nucleus before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Sexually dimorphic nucleus ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Sexually dimorphic nucleus

In research: Sexually dimorphic nucleus appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Sexually dimorphic nucleus in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Sexually dimorphic nucleus is common in secondary-school and first-year university syllabi. It links to neighbouring topics Animal sexuality, Hypothalamus, Limbic system, so understanding it makes those chapters shorter.
In everyday life: Look for Sexually dimorphic nucleus outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Sexually dimorphic nucleus in 20 minutes

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Frequently asked questions

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Why does Sexually dimorphic nucleus matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Sexually dimorphic nucleus?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

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It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Sexually dimorphic nucleus.