Mesoderm — one of the three germ layers in an embryo — is the source of embryonic connective tissue. It is classified into two types — mesenchyme and mucoid connective tissue. Mesenchyme, the first connective tissue formed in the embryo, consists of star-shaped stem cells and an extracellular matrix with thin and sparse reticular fibers in a fluid ground substance. This mesenchymal stem cell pool differentiates into all mature connective tissues found in adults. However, a few of these mesenchymal stem cell clusters remain scattered even in the mature tissues for regeneration and repair at later stages of life. Additionally, during fetal development, the mesenchyme in certain parts of the embryo differentiates into the mucoid tissue — the second type of embryonic connective tissue. The mucoid tissue is mainly composed of the ground substance suspended with a few thin collagen fibers and some fibroblast-like cells. Along with the embryo, mucoid tissue is also present in the umbilical cord, where it is called Wharton's jelly. Here, it supports the blood vessels and protects them from physical trauma.