Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: Extracellular Matrix of Animal Cells Most animal cells release materials into the extracellular space. The primary components of these materials are proteins, and the most abundant protein is collagen. Collagen fibers are interwoven with carbohydrate-containing protein molecules called proteoglycans. Collectively, these materials are called the extracellular matrix (Figure 4.27). Not only does the extracellular matrix hold the cells together to form a tissue, but it also allows the cells within the tissue to communicate with each other. How can this happen?. Answer: collagen
A:
The extracellular matrix of most animal cells contains abundant amounts of what protein, which helps hold things together?