Layer-specific neuroanatomical architecture of the quadratus lumborum: a quantitative study using 3D micro-CT and...

The quadratus lumborum (QL) is increasingly recognized as an anatomically important component of the posterior abdominal wall, yet its internal organization remains incompletely defined. This study aimed to characterize the layer-specific architecture of the QL and to quantitatively analyze the distribution of immunolabeled axonal components across its layers.

Ten embalmed adult cadavers (mean age, 82.1 ± 9.9 years) were examined. Gross dissection and morphometric analysis were performed in eight cadavers (16 sides) to identify layer-specific organization and measure width and thickness at standardized anatomical landmarks; measurement reliability was assessed using intraclass correlation coefficients. Non-destructive three-dimensional reconstruction was performed in four phosphotungstic acid-enhanced block specimens using micro-computed tomography (micro-CT; voxel size, 72 μm). Histological and immunofluorescence analyses were performed in six sides using Masson's trichrome staining and double labeling for NF200 combined with choline acetyltransferase (ChAT), tyrosine hydroxylase (TH), or calcitonin gene-related peptide (CGRP). Marker-positive axons were quantified relative to NF200-positive axons.

Gross dissection and micro-CT consistently demonstrated a three-layer organization of the QL, comprising the lumbocostal (LC), iliocostal (IC), and iliolumbar (IL) layers. Morphometrically, the LC was the smallest layer, the IC was the thickest, and the IL was the widest. Neural branches arose mainly from the ventral rami of T12-L4 and entered the muscle along its anterior and lateral aspects as neurovascular bundles. Intramuscular nerves were multifascicular, and mean axonal diameter was greatest in the IC (572.9 ± 41.8 μm) and IL (556.1 ± 38.4 μm) and smallest in the LC (276.9 ± 65.2 μm). Immunofluorescence analysis demonstrated layer-dependent differences in marker distribution: the IC showed the highest proportions of ChAT-positive (26.4%) and TH-positive (32.4%) axons, whereas the LC showed the highest proportion of CGRP-positive axons (24.5%). The IL showed lower proportions of ChAT-, TH-, and CGRP-positive axons and the highest residual NF200-positive fraction (39.7%).

The quadratus lumborum is a reproducible multilayered structure with layer-specific morphometric and immunolabeled neural heterogeneity. These findings provide quantitative anatomical evidence of layer-specific structural and neuroanatomical heterogeneity within the quadratus lumborum.