Re: Embryological explanation for the arcuate line of the rectus sheath?

Dear Andy,

Thank you for your question. I’ll provide a bit of background for other readers.

The rectus abdominus muscles are the muscles one sees as “abs” in a thin person with strong abdominal musculature. They run vertically from the ribs and breastbone to the pubic bone. Like most muscles, they are wrapped in a strong layer of fibrous tissue called fascia. They have the unusual property of a changing fascial thickness along their length. An abrupt change in that thickness on their inner surface can be seen with the naked eye. The level of that abrupt change runs in a curved horizontal line across the interior of the front of the abdomen, usually below the level of the navel. This is called the arcuate line. (This is something one could only see during surgery or in a cadaver; it is not visible from the outside.)

The abdominal muscles form from embryologic mesodermal flaps that fuse at the anterior midline in the 7th or 8th week of embryonic growth. The rectus muscles form separately from the other abdominal muscles in the leading edges of the mesodermal flaps. Because the fascia wrapping the rectus muscles connect with the fascia of the lateral abdominal muscles (the external obliques, internal obliques and the innermost transversus muscle), the fascia of the lateral muscles appear to split and encompass the rectus muscles. Above the arcuate line, this appears to happen within the internal oblique fascia. Below the arcuate line, the split appears to happen behind the transversus muscle. In reality, a thin layer of fascia exists behind the muscles below the arcuate line. It is the innermost fascia of the transversus muscle, called the transversalis fascia. Despite whichever layer of fascia appears to split and contain the muscles, all the fascia fuse to the fascia containing the rectus muscles. There is no contiguous space from the rectus muscle to any of the lateral abdominal muscles. In essence, any fascia covering the rectus muscles is rectus fascia and not the fascia of the lateral abdominal muscles.

There is no clear embryologic explanation for the presence of the abrupt change in thickness of the posterior rectus fascia at this time. Although we explain segmental anatomical changes such as ribs and spinal nerves with the human versions of the Drosophila fruit fly Hox genes, I could not find genetic or embryologic studies to explain the segmental change (abrupt or otherwise) in mammalian posterior rectus sheath thickness.

Although the explanation we receive in medical school about difference in fascial splitting at the arcuate line is elegant and useful for teaching purposes, recent studies reveal the arcuate line does not exist in all people. The thickness of the posterior fascia sometimes diminishes in thickness slowly, without a defined arcuate line (see the article by Rizk below).

It appears that the arcuate line may only be a visible transition from thick to thin fascia. Whether that transition is abrupt and visible or gradual and invisible varies by individual. It is most commonly abrupt, and we teach the landmark in anatomy classes, primarily because of its significance during surgery involving the anterior abdominal wall.

A recent study by plastic surgeons (who would like to locate the arcuate line before rectus flap surgeries) of 18 cadavers demonstrated that the arcuate line varied in its distance from the pubic symphysis. They also noted the arcuate line lay/lie more superior in obese-habitus people and more inferior in thin-habitus people. Other studies have shown complete posterior rectus fascia (extending to the pubic symphysis), duplicate arcuate lines, and other fascial abnormalities that do not appear rare (one doctor identified 40 people without single arcuate lines in a single study). I included references to the studies I mentioned below. The variability in the location and presence of an arcuate line may suggest it does not represent a consistent embryologic process.

Tim Nicholls, MD
Berkeley, California

Rizk NN. The arcuate line of the rectus sheath—does it exist? Journal of 
Anatomy. 1991; volume 175: pp 1–6.

Monkhouse WS, Khalique A. Variations in the composition of the human 
rectus sheath: a study of the anterior abdominal wall. Journal of Anatomy. 
1986; volume 145: pp 61–66.

Cunningham SC, Rosson GD. Localization of the Arcuate Line From Surface 
Anatomic Landmarks: A Cadaveric Study.  Annals of Plastic Surgery, August 
2004; volume 53(2): pp 129-131

Krantz, KE.  Chapter 2.  Anatomy of the Female Reproductive System – 
Abdominal Wall. From: Current Obstetric & Gynecologic Diagnosis and 
Treatment.  9th ed.  DeCherney AH, Nathan L, eds.

Bell RL, Seymour NE.  Chapter 34. Abdominal Wall, Omentum, Mesentery and 
Retroperitoneum – Abdominal Wall. From: Schwartz’s Principles of Surgery.  
8th ed.  Brunicardi FD, Andersen DK, et al, eds.