Biological<unk> is a collection of interconnected cells that perform a similar function within an organism.
The study of<unk> is known as histology, or, in connection with disease, histopathology.
The classical tools for studying the<unk>  are the wax block, the<unk> stain, and the optical microscope, though developments in electron microscopy, immunofluorescence, and frozen sections have all added to the sum of knowledge in the last couple of decades.With these tools, the classical appearances of the<unk>  can be examined in health and disease, enabling considerable refinement of clinical diagnosis and prognosis.
There are four basic types of<unk> in the body of all animals, including the human body and lower multicellular organisms such as insects. These compose all the organs, structures and other contents.
Epithelium -<unk>  composed of layers of cells that cover organ surfaces such as surface of the skin and inner lining of digestive tract: the<unk>  that serve for protection, secretion, and absorption.
Connective<unk> - As the name suggests, connective<unk> holds everything together. Connective<unk> is characterized by the separation of the cells by an inorganic material, which is called extracellular matrix. Bone and blood are connective<unk> .
Muscle<unk> - Muscle cells contain contractile filaments that move past each other and change the size of the cell. Muscle<unk> also is separated into three distinct categories: visceral or smooth muscle, which is found in the inner linings of organs; skeletal muscle, which is found attached to bone in order for mobility to take place; and cardiac muscle which is found in the heart.
Nervous<unk> - Cells forming the brain, spinal cord and peripheral nervous system.
Tissue 
Biomedical tissue
Wound
Blood
Hair
Exudate
Cell 
Skin appendage