Fetal anatomy of peripheral lymphatic vessels: a D2-40 immunohistochemical study using an 18-week human fetus (CRL 155 mm).

We demonstrated fetal peripheral lymphatic vessels (LVs) using D2-40 immunohistochemistry in a whole female fetus (18 weeks of gestation, CRL 155 mm) except for the head. There were abundant LVs in the thyroid gland, lung, stomach, small intestine, rectum and pancreas, whereas no LVs were seen in the parathyroid gland, spleen and adrenal cortex. In the liver, except for the gallbladder bed, LVs were still restricted to around hilar thick portal veins and around the hepatic vein terminals. Subcutaneous LVs were well developed throughout the body even in areas where no or few perforating LVs connected with the deep LVs. The diaphragm contained abundant, dilated LVs in the pleural half of its thickness. LVs were also seen not only along supplying arteries of muscles and cartilage but also along the epimysium and perichondrium. LVs ran in a space between the obliquus internus and transversus abdominis but not between the obliquus internus and obliquus externus. Some tight connective tissues such as the sacrotuberous ligament contained abundant LVs. The intervertebral foramen contained a lymphatic plexus. The present observations provide a better understanding of peripheral lymphatic development. The fetal lymphatic morphology seems not to represent a mini-version of the adult morphology.