These cells aggregate and function together to form tissues. The four basic tissue types are epithelial tissue, connective tissues, nervous tissue, and muscle tissue. Epithelium is lining tissue.
Keratin hardens squamous epithelium.
Connective tissues consist of cells within a matrix. Fibroblasts secrete collagen and elastin. Loose connective tissue has loose collagen strands, whereas dense connective tissue has tightly packed collagen fibrils. Adipose tissue consists of adipocytes hugely swelled with lipid. The matrix of blood is plasma, which carries red blood cells, white blood cells, and platelets.