Answer the following question given this paragraph: 

Tight junctions are common at epithelia, which are sheets of cells that form a boundary between a mass of cells and a cavity or space (a lumen ). The membranes of these cells join together, forming a virtually impermeable barrier to fluid. Tight junctions essentially seal adjacent epithelial cells in a narrow layer just beneath their apical surface, which is the portion of the cell exposed to the lumen. The rest of the cell surface is known as the basolateral surface. Tight junctions prevent integral membrane proteins from moving between the apical and basolateral surface, maintaining the properties of those distinct surfaces. For example, receptor-mediated endocytosis occurs at the apical surface and exocytosis at the basolateral surface.


Q: What is the term for sheets of cells that form a boundary between a mass of cells and a cavity or space?


A:
epithelia