Development of the human medullary arcuate nucleus from mid-gestation to the perinatal period: A morphometric study

Development of the human medullary arcuate nucleus (AN) has not been sufficiently investigated. The present study provides morphometric data by examining the brains from preterm and perinatal infants.

Nine brains were obtained from infants aged 21-43 postmenstrual weeks (PW). Serial celloidin sections were cut and stained using the Klüver-Barrera method. After microscopic observations, morphometric parameters [AN volume, numerical density (Nv) and total number (Nt) of neurons, and neuronal profile area (PA)] were analyzed.

The AN was found as a pair of neuronal masses on the ventral medullary surface at 21 PW. Caudally, it was ventrolateral to the pyramidal tract (PT), and rostrally, medial to the PT. In the middle, it was diminished in size or interrupted. The AN neurons were gradually enlarged with age, showing multiplicity in size and shape. The following findings had a marked asymmetry and individual variability: (1) complete or partial inclusion of the AN in the PT; (2) connection between the rostral AN and the pontine nuclei; (3) coexistence of pyknotic neurons. The AN volume increased exponentially with age, while the Nv decreased exponentially. The Nt changed along two phases (decrease-increase) after mid-gestation. The mean PA increased linearly with age. Asymmetry and/or individual variability were demonstrated in the AN volume, Nt, and mean PA.

Asymmetry and individual variability in the AN morphology are present in fetal period. The AN may undergo neuron death and neuroblasts production in tandem after mid-gestation.