What are tissues made of?

Tissues are made of cells and extracellular matrix. The cells provide the functional component, while the extracellular matrix—composed of proteins, water, and other molecules—provides structural support and facilitates cellular communication.

The cellular component varies by tissue type. Epithelial tissue consists primarily of tightly packed cells with minimal extracellular matrix, creating effective barriers. Connective tissue, conversely, contains relatively fewer cells dispersed within abundant extracellular matrix—in bone tissue, for example, cells called osteocytes are embedded in a mineralized matrix of calcium and phosphate that provides rigidity. The extracellular matrix isn't just inert scaffolding; it contains proteins like collagen (providing strength), elastin (allowing flexibility), and various glycoproteins that facilitate cell adhesion and signaling. Water constitutes a significant portion of most tissues, enabling nutrient transport and waste removal. In specialized tissues like blood, the liquid component (plasma) serves as the extracellular matrix, carrying cells throughout the body. Understanding tissue composition has practical implications for medicine—collagen supplements aim to support connective tissue health, while tissue engineering attempts to create replacement tissues by combining appropriate cells with biocompatible matrices. The specific proportions and types of cells and matrix materials determine each tissue's unique properties and functions.