Morphological sex differences and laterality in the prepubertal medial amygdala

Area of Science:

• Neuroscience
• Behavioral Neuroscience
• Neuroanatomy

Background:

• The medial amygdala (MeA) is vital for sex-specific social behaviors.
• The posterodorsal subnucleus of the MeA (MeApd) exhibits significant sex differences in volume, being larger in males.
• Previous research indicated a larger left MeApd in prepubertal males without a difference in neuron number.

Purpose of the Study:

• To investigate the cellular and structural basis of the sexually dimorphic volume of the MeApd.
• To determine if sex differences in MeApd volume are due to neuronal number, dendritic arborization, axonal, synaptic, or glial components.
• To examine laterality in MeApd structural sex differences.

Main Methods:

• Quantification of the volume occupied by dendrites, axons, synapses, and glia in the MeApd of male and female rats (26-29 days old).
• Measurement of MeApd dendritic morphology, including dendritic length and branching patterns.
• Analysis of sex differences in both the left and right MeApd hemispheres.

Main Results:

• In the left MeApd, the sex difference in regional volume was fully explained by increased volume of dendritic shafts and glia in males.
• Males exhibited greater overall dendritic length in the left MeApd, attributed to increased branching rather than longer dendritic segments.
• In the right MeApd, males had more neurons than females, with no significant sex differences in individual neuron dendritic morphology.

Conclusions:

• Sex differences in MeApd volume are primarily driven by dendritic and glial components, not neuronal number, particularly in the left hemisphere.
• The left and right MeApd display distinct patterns of sexual dimorphism, suggesting lateralized functions.
• These findings underscore the importance of considering brain laterality in understanding neuroendocrine regulation and sexually dimorphic behaviors.