8.15:
Embryonic Connective Tissues
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.
8.15:
Embryonic Connective Tissues
During early development, the embryo forms two types of connective tissues— the mesenchyme and mucoid connective tissue.
The mesenchyme is the first connective tissue that emerges in the developing embryo. It consists of loosely arranged multipotent mesenchymal cells and reticular fibers in the extracellular matrix. This loose arrangement allows easy migration of cells, which is essential for germ layer positioning, patterning, and organ morphogenesis during embryonic development. Mesenchyme is a transient tissue that can differentiate to form all connective tissue types in the body, and is absent in adults. However, scattered mesenchymal stem cells are found in adult tissues, such as the bone marrow and the adipose tissue.
In the course of development, mesenchyme differentiates into the mucoid connective tissue. The mucoid connective tissue is present in the umbilical cord, where it is called Wharton's jelly. It has a gelatinous, jelly-like ground substance rich in hyaluronic acid and chondroitin. This tissue has high turgidity that resists compression and protects structures like blood vessels in the umbilical cord. Mucoid connective tissues, too, are absent postnatally. However, its remnant, termed mucoid-like tissue, is found in the dental pulp of the teeth, the vitreous body between the lens and the retina of the eye, and the nucleus pulposus—the inner core of the intervertebral disc.