Organs are made up of cells and tissues. Types of connective tissue differ primarily in the consistency of the matrix, which may be soft and fluid (adipose tissue), firm or hard (cartilage and bone) or liquid (blood and lymph). Striated muscle cells contain a highly organised array of actin and myosin filaments. Myofilaments in smooth muscle cells are less regularly arranged. Epithelium- barrier/borders, protection, secretion, absorption interfaces between biological compartments and with the environment, and in secretion. Endothelia= epithelial linings of some internal structures, such as blood vessels. Wherever there is muscle nervous tissue is there. Nerve cells are specialised to conduct signals rapidly and precisely throughout the body. They do not divide and are supported by glial cells. Specialised nerve cells contain receptors for light (photoreceptors), heat (thermoreceptors), mechanical pressure or stretch (mechanoreceptors) and chemicals (chemoreceptors). A tendency to maintain a constant internal environment. Many physiological regulatory mechanisms maintain homeostasis- negative feedback.