<P> Animal tissues are grouped into four basic types: connective, muscle, nervous, and epithelial . Collections of tissues joined in structural units to serve a common function compose organs . While all animals can generally be considered to contain the four tissue types, the manifestation of these tissues can differ depending on the type of organism . For example, the origin of the cells comprising a particular tissue type may differ developmentally for different classifications of animals . </P> <P> The epithelium in all birds and animals is derived from the ectoderm and endoderm with a small contribution from the mesoderm, forming the endothelium, a specialized type of epithelium that composes the vasculature . By contrast, a true epithelial tissue is present only in a single layer of cells held together via occluding junctions called tight junctions, to create a selectively permeable barrier . This tissue covers all organismal surfaces that come in contact with the external environment such as the skin, the airways, and the digestive tract . It serves functions of protection, secretion, and absorption, and is separated from other tissues below by a basal lamina . </P> <P> Connective tissues are fibrous tissues . They are made up of cells separated by non-living material, which is called an extracellular matrix . This matrix can be liquid or rigid . For example, blood contains plasma as its matrix and bone's matrix is rigid . Connective tissue gives shape to organs and holds them in place . Blood, bone, tendon, ligament, adipose and areolar tissues are examples of connective tissues . One method of classifying connective tissues is to divide them into three types: fibrous connective tissue, skeletal connective tissue, and fluid connective tissue . </P> <P> Muscle cells form the active contractile tissue of the body known as muscle tissue or muscular tissue . Muscle tissue functions to produce force and cause motion, either locomotion or movement within internal organs . Muscle tissue is separated into three distinct categories: visceral or smooth muscle, found in the inner linings of organs; skeletal muscle, typically attached to bones, which generate gross movement; and cardiac muscle, found in the heart where it contracts to pump blood throughout an organism . </P>

What do plant and animal tissues have in common