The belly button, often overlooked, is both well-researched and under-researched. A recent study may have uncovered the reason most individuals have “innie” belly buttons, potentially linked to a previously unknown structure that pulls the belly button inward.

Researchers made this discovery while investigating optimal techniques for abdominal and hernia repair surgery.

The team’s goal was to minimize occurrences of what is known as an “uncised hernia of the umbilical,” a condition where muscle weakness from prior surgeries allows abdominal tissue to push outward, causing bulging around the belly button.

Satori Muro from the Japanese Institute of Science mentions that this issue often arises, despite the fact that the anatomical details of the umbilical area have not been extensively reported. The researchers conducted a meticulous microscopic analysis of the ventral buttons from five donated human bodies and subsequently created a 3D model of the structure.

The belly button is essentially the remnant of the fetal umbilical cord, which connects to the placenta until birth. Approximately 90% of people show a concave depression where the umbilical scar has healed. However, Muro’s team identified a “fibrous tunnel-like structure” just beneath the skin of the umbilical tissue, seamlessly extending into the deeper layers of tissue. This underlying tissue, known as the abdominal fascia, plays a critical role in stabilizing the abdomen and securing the organs.

This structure is composed of densely packed collagen fibers oriented circumferentially, resembling the umbilical cord. “This sheath seems to anchor the inward curl of the belly button to the deep fascia in multiple directions,” Muro explains, helping to maintain the unique inward shape of the belly button.

However, the team was unable to include individuals with “outie” belly buttons in their study, indicating that further research is necessary to explore the variations in umbilical cord structures among individuals.

“Differences in umbilical cord development or strength may contribute to whether someone’s belly button protrudes outward,” Muro adds.

Kat Sanders from the University of Sydney emphasizes that belly buttons are indeed vital anatomical features, but more extensive research is needed, as five subjects cannot provide a comprehensive understanding for all variations of belly buttons.

“This study reveals aspects of anatomy that are infrequently examined. Given that this area is frequently accessed during laparoscopic procedures, there’s substantial surgical interest,” Sanders states.

Michelle Moscova from the University of New South Wales believes this research will enhance understanding of the structural integrity of the abdomen.

“Umbilical hernias are a common complication after abdominal surgery, where contents such as intestines protrude through a weaker area in the abdomen, leading to inflating around the belly button,” Moscova notes.