What type of substance, secreted by fibroblasts, fills the space between connective tissue fibers and cells?

The correct answer is ground substance. Ground substance is an essential component of connective tissue that serves to fill the spaces between cells and fibers. It is a gel-like material that provides a medium through which nutrients and other substances can diffuse between blood vessels and cells. Ground substance is composed of a variety of molecules, including glycosaminoglycans, proteoglycans, and glycoproteins, which help to maintain the structure and function of connective tissue.

In contrast, collagen is a type of protein that provides tensile strength to connective tissues but does not fill the spaces in the way that ground substance does. Fluid matrix is not a specific term recognized in the context of connective tissue components; instead, the components of connective tissue are better categorized as either fibers or ground substance. Elastic tissue, on the other hand, refers to a specific type of connective tissue that contains elastin fibers, which allow for stretch and flexibility but does not serve the role of filling spaces within the tissue. This understanding of ground substance's role highlights its importance in providing the necessary support and environment for cellular functions within connective tissue.