The umbilical cord connects the developing fetus and the placenta. It consists mostly of mesenchyme (embryonic connective tissue). Its bluish-pink color is from the abundant ground substance (blue) and sparse collagen fibers (pink).
Fibroblasts produced this extracellular matrix and are dispersed in the connective tissue. Only their nuclei can be identified. Some appear round with heterochromatin, while others are flattened with mostly euchromatin. This suggests some of the fibroblasts are active, while others are inactive.