Why is the Umbilicus Concave? A Histological and Three-Dimensional Anatomical Study Revealing the “umbilical sheath”

Background The umbilicus is a distinct anatomical feature of the anterior abdominal wall, characterised by a stable depression that persists after birth. Despite its clinical and surgical relevance, the detailed histological and spatial structure underlying the umbilical concavity remains unclear. This study investigated the fibrous connective tissue structures underlying the umbilical concavity. Methods Five formalin-fixed cadaveric specimens (mean age: 77.4 years) were analysed. Two were used for macroscopic dissection, two for histological examination, and one for quasi-continuous serial sectioning with three-dimensional reconstruction. Results Macroscopic dissection revealed a vertically oriented cylindrical fibrous structure extending from the umbilical fossa to the anterior rectus sheath. Histological analysis confirmed that this structure enclosed a small fat compartment. It was distinct from subcutaneous fat, representing a forward protrusion of extraperitoneal fat, directly contacting the underside of the umbilical skin. Serial sections and 3D reconstruction demonstrated a tunnel-like fibrous structure—termed the “umbilical sheath”—enveloping this protruded fat and bridging the umbilical ring and dermis of the umbilical fossa. This sheath connected the invaginated skin to the deep fascial layers of the linea alba and rectus sheath. Conclusion We identified a previously undescribed fibrous structure, the umbilical sheath, which encloses a localised protrusion of extraperitoneal fat. This structure likely supports and maintains the concavity of the umbilicus. Recognition of this structure may contribute to safer surgical approaches in laparoscopic and robotic procedures by guiding optimal port placement and promoting abdominal wall closure techniques that preserve native anatomical support, potentially reducing the risk of incisional hernia.