We now know why a belly button becomes an ‘innie’

The belly button, often regarded as a simple remnant of our embryonic past, has now emerged as a focal point of scientific inquiry. Recent discoveries have provided insights into why the majority of individuals possess an “innie” or concave navel, a phenomenon now attributed to the presence of the newly identified umbilical sheath.

This landmark discovery originated from research aimed at improving surgical approaches in abdominal and hernia repairs that frequently involve incisions around the navel, medically termed the umbilicus.

The researchers, driven to minimize occurrences of umbilical incisional hernias—in which weakened abdominal muscles allow tissue to bulge—conducted detailed analyses of human anatomical structures. Satoru Muro from the Institute of Science Tokyo highlights that despite its omnipresence, the anatomy surrounding the umbilicus remains largely unexplored.

The research team undertook an extensive microscopic examination of the umbilical region from five human cadavers, ultimately constructing a 3D model of the area. They discovered a fibrous, tunnel-like entity beneath the outer skin of the umbilicus that seamlessly integrates with underlying deep tissues, specifically the abdominal fascia, which is fundamental in stabilizing abdominal structures and organ placement.

“We have identified a fibrous structure composed of densely-arranged collagen fibers, resembling a sleeve, which we have named the ‘umbilical sheath’,” explains Muro. “This sheath appears to anchor the umbilical depression to deep fascia in multiple directions, preserving the inward curvature of belly buttons.”

However, Muro notes the current study did not include any individuals with “outie” belly buttons, indicating that further research is necessary to explore how the umbilical sheath’s characteristics might differ among various individuals.

“Variations in the development or strength of the umbilical sheath may influence the appearance of protruding umbilici,” suggests Muro, prompting further investigation into this area.

Kat Sanders, an anatomy lecturer at the University of Sydney, acknowledges that the belly button has long been overlooked in anatomical studies. She emphasizes the need for broader research as five individuals do not provide a comprehensive enough sample size for generalizations about the whole population’s belly buttons.

According to Michelle Moscova from the University of New South Wales, understanding the structural underpinnings of the belly button could enhance surgical techniques, particularly in laparoscopic procedures. “Abdominal surgeries frequently lead to complications such as umbilical hernias, where abdominal contents, including intestines, can protrude through the weak point around the belly button,” Moscova elucidates.