Research reportSexual dimorphism in number and proportion of neurons in the human median raphe nucleus
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
References (19)
- et al.
Individual differences in brain asymmetries and fiber composition in the human corpus callosum
Brain Res.
(1992) - et al.
Sexual differentiation of monoaminergic neurons – genetic or epigenetic?
TINS
(1991) - et al.
Sexual differentation of the human hypothalamus:ontogeny of the sexually dimorphic nucleus of the preoptic area
Brain Res. Dev. Brain Res.
(1988) - et al.
Age-related changes in fibre composition of the human corpus callosum: sex differences
NeuroReport
(1996) - et al.
Sexual dimorphism of the anterior commisure and massa intermedia of the human brain
J. Comp. Neurol.
(1991) - (1939)
- et al.
Dimorfismo sexual en el núcleo mediano del rafe humano. Estudio preliminar con el método de Golgi Cox
Rev. Med. de Chile
(1999) The Human Cerebral Cortex: Gender Differences in Structure and Function
J. Neuropathol. Exp. Neuro.
(1999)- et al.
Human brain. Cytoarchitectonic left–right asymmetries in temporal speech region
Arch. Neurol.
(1978)