Timing and sequence between a thick fascia covering muscles and a cutaneous nerve perforating it: a study using histological sections of human fetuses.

A deep fascia is believed to develop after a cutaneous nerve (CN) reaches the subcutaneous tissue. The present study aimed to examine a developmental sequence of the CN perforation of the deep fascia. We examined the occipital and posterior neck region as well as the anterior abdominal wall using histological sections from 12 human early fetuses at approximately 7-9 weeks of gestation age (GA) and 10 midterm fetuses at approximately 12-18 weeks GA. When the anterior abdominal CN reached the anterior aspect of the rectus abdominis muscle at 7 weeks, the rectus sheath was still loose. However, the aponeurosis of the obliquus externus abdominis was relatively tight. Later, depending on the rectus sheath became evident, a "window" for the CN also became clear. After the anterior CN took a wavy course along the aponeurosis, it grew to the skin by 12 weeks. The occipital CNs reached the superficial surface of the semispinalis and ran superiorly along the fascia by 8 weeks. Later, depending on the superior extension of the splenius and trapezius muscles, the occipital CNs issued multiple twigs to perforate the superior margin of the upward extending fascia or muscle. Both a deep fascia and the window for CNs develop simultaneously. However, during several weeks, the CN likely stopped its further growth to the skin and remained on and along the fascia. Likewise, after perforation of the deep fascia, another fascia or muscle likely interferes with the straight course of CNs to the skin.