A Novel Thin Adipose Compartment at the Colonic Mesentery–Perirenal Fat Interface: Histological and Three-Dimensional Morphological Studies

Objective: To elucidate the anatomical characteristics and three-dimensional continuity of a previously unrecognized thin adipose compartment between the colonic mesentery and retroperitoneum, using correlative microscopy and block-face imaging (CoMBI). Summary Background Data: The interface between the colonic mesentery and retroperitoneum has traditionally been considered a fusion fascia (e.g., Toldt’s fascia). However, emerging evidence suggests more complex fascial remodeling, with previous studies lacking comprehensive spatial context. Methods: Seven adult cadavers were examined. Histological analysis was conducted on six specimens using paraffin sections stained with Elastica van Gieson and Masson’s trichrome. One cadaver underwent three-dimensional morphological analysis using CoMBI. Serial block-face images of the perirenal region were captured at 100 μm intervals, and three-dimensional reconstruction segmentation was performed. Results: A distinct thin adipose compartment (0.3–2.0 mm thick) was consistently observed between the colonic mesentery and perirenal fat, enclosed by dense connective tissue and containing small vessels. Similar compartments were also found between the perirenal fat and pararenal fat, and beneath the peritoneum along the abdominal wall. These compartments extended in three directions from the peritoneal reflection and demonstrated craniocaudal continuity, laterally, these form a triad-like junction. Conclusions: The thin adipose compartment represents a structurally organized anatomical unit rather than amorphous filler. Its consistent continuity and integration with adjacent structures support a compartment-based framework of intra-abdominal anatomy, with implications for surgical navigation.