Research report
Sexual dimorphism in number and proportion of neurons in the human median raphe nucleus

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Abstract

The number and proportion of neurons in the median raphe nuclei stained by the Golgy–Cox and Nissl methods was compared in males and females infants. When subjects are matched by age and cause of death the number and proportion of fusiform, ovoid and multipolar cells differs significantly between sexes at different ages.

Introduction

Morphological sexual differences have been found in several areas of the human brain, i.e. preoptic area [17], Onuf nucleus [13], amygdala [8], corpus callosum and anterior commisure [1], [3], planum temporale [7] and posterior temporal cortex [19]. Men’s brain seems to be functionally and neuroanatomically more lateralized than women’s brain [2]. The median raphe nucleus (mnr) is the largest of all the raphe nuclei in human brainstem and is part of the serotonergic system [10]. The serotonergic nuclei are involved in neuronal firing, human emotions control, reproduction and sexual behavior [15]. This wide structural sexual dimorphism of the nervous system makes it premature to assign the dimorphic sexual behavior to only one nucleus of the brain [11].

In a preliminary study we found sexual dimorphism in the number and proportion of the different types of neurons in the mnr [5]. Now we extend this study to older infants confirming that finding and studying its ontogeny. Since the Golgy–Cox method (GCm) stains between 1 and 20% of neurons we added the Nissl method (Nm) to assess most of stainable cells.

Section snippets

Material and methods

The autopsies of four males and four females were performed at the Roberto del Rı́o Hospital (Santiago, Chile), according to the ethical norms accepted at that time in Chile. Infant data are summarized in Table 1. They were born after a normal pregnancy. Brains were obtained and fixed in 10% neutral formalin within 12 h postmortem [4]. Blocks 3 mm thick cut nearby plane 19 were processed [12] by the GCm [16]. Blocks were fixed, placed in collodion and cut 120 μm thick. Five histological

Results

Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8 show the photomicrographs and their drawings of the 9/3, 23/22 and 150/210 days female/male pairs respectively. Greater cellularity of the female mnr at the three ages is evident, mainly for multipolar (M) and ovoid (O) neurons. The dendritic network was denser in females. M neurons and glial cells seemed larger in males. Drawings with the Nm are shown on Fig. 9. A remarkable finding is the greater cellularity found in the female sample.

Table 2

Discussion

Of our four hypotheses the clearest refutations were those related to the equality of number and proportion of neurons between males and females in the mnr. Females had more neurons than males. To our knowledge there are no other studies on sexual dimorphism of the reticular formation performed by the GCm and Nm. The differences in the number and proportion of neurons among ages within sexes were significant, but not consistently so due to a few extreme values. A systematic higher number of

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