Mesenchyme, an embryonic connective tissue, gives rise to all mature connective tissues in humans. Mature connective tissues are classified into proper, supporting, and fluid based on their physical forms and functions. The loose connective tissue proper is composed of different cells with loosely arranged protein fibers in the extracellular matrix or ECM. In contrast, the dense connective tissue proper has a relatively acellular ECM with tightly packed thick bundles of collagen fibers. Loose connective tissue is further divided into areolar, adipose, and reticular, while the dense connective tissue is categorized into regular, irregular, and elastic types. The supporting connective tissues—bones and cartilage—comprise the human skeletal system. Bones have calcified ECM, and based on the organization of cells in the ECM, they are compact or spongy. Cartilage has properties intermediate to dense connective tissues and bones and can withstand stress without deformation. It has a gel-like ECM enriched with fibrous proteins and is subdivided into three types — hyaline, fibrocartilage, or elastic. Lastly, blood and lymph are fluid connective tissues with cells floating in a fluid ECM.