Epithelial tissue This type of tissue is formed of one or several layers of cells, which are closely packed together. Unlike muscular tissue, epithelial cells are not highly dif ferentiated . There are two types: covering and glandular. Covering epithelia. This tissue covers external and internal body surfaces. Its main purpose is to protect and regulate the exchange of substances. Glandular epithelia. This tissue is made up of cells specialised in the secretion of substances. These cells are grouped together, forming glands. Nervous tissue Ner vous tissue transmits and receives ner ve signals. It is made up of neurons and glial cells. A single layer of cells For example, endothelia, which forms the lining of blood vessels and pulmonar y alveoli . Exocrine glands. These glands secrete substances outside the body or into a body cavity through ducts. For example, saliva glands. Mucous membrane (mucosa) tissue covers and protects internal cavities, such as the digestive and respirator y tracts. Several layers of cells For example, the epidermis. This forms the outer, protective layer of human skin . The outer layers of the epidermis are made up of f lat, dead cells that are continuously shedding. In the inner layers, there are live cells undergoing continual cell division . Endocrine glands. These glands have no ducts. This is why they secrete substances called hormones directly into the blood . For example, the thyroid gland . Neuron A highly dif ferentiated cell that has lost the capacity to divide. Astrocyte This cell provides the neurons with nutrients through the blood capillaries. Microglia This cell protects against infections and eliminates damaged cells. Oligodendrocyte This cell forms a protective cover around the neuron extensions. Capillary 26 4. Tissues of the human body
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