Write a detailed note on Epithelial tissue,giving the types of animal tissue.

(A) $\Rightarrow$ Based on the function of cells,there are four types of tissues found in animals: $(i)$ Epithelial,$(ii)$ Connective,$(iii)$ Muscular,and $(iv)$ Neural.
$\Rightarrow$ Epithelial tissue is commonly referred to as epithelium. This tissue has a free surface,which faces either a body fluid or the outside environment,thus providing a covering or a lining for some part of the body. The cells are compactly packed with little intercellular matrix.
$\Rightarrow$ Epithelial tissues are of two types: $(1)$ Simple epithelium,$(2)$ Compound epithelium.
$\Rightarrow$ Simple epithelium is composed of a single layer of cells and functions as a lining for body cavities,ducts,and tubes.
$(1)$ Simple epithelium: Based on the structural modification of the cells,simple epithelium is further divided into three types: $(a)$ Squamous,$(b)$ Cuboidal,$(c)$ Columnar.
$(a)$ Squamous epithelium: It is made up of a single thin layer of flattened cells with irregular boundaries. They are found in the walls of blood vessels and air sacs of lungs and are involved in functions like forming a diffusion boundary.
$(b)$ Cuboidal epithelium: This tissue is made up of a single layer of cube-like cells. It is commonly found in ducts of glands and tubular parts of nephrons in kidneys and its main functions are secretion and absorption.
$(c)$ Columnar epithelium: It is composed of a single layer of tall and slender cells. Their nuclei are located at the base. They are found in the lining of the stomach and intestine and help in absorption and secretion.
$\Rightarrow$ Ciliated epithelium: If the columnar or cuboidal cells bear cilia on their free surface,they are called ciliated epithelium. Their function is to move particles or mucus in a specific direction over the epithelium. They are mainly present in the inner surface of hollow organs like bronchioles and fallopian tubes.
$\Rightarrow$ Glandular epithelium: Some of the columnar or cuboidal cells get specialized for secretion and are called Glandular Epithelium. They are mainly of two types: $(i)$ Unicellular (e.g.,goblet cells of the alimentary canal) and $(ii)$ Multicellular (e.g.,salivary gland). Based on the mode of pouring their secretions,glands are divided into $(i)$ Exocrine (with ducts) and $(ii)$ Endocrine (ductless) glands.
$(2)$ Compound epithelium: It is made up of more than one layer (multi-layered) of cells and thus has a limited role in secretion and absorption. Its main function is to provide protection against chemical and mechanical stresses. It covers the dry surface of the skin,the moist surface of the buccal cavity,pharynx,inner lining of ducts of salivary glands,and of pancreatic ducts.
$\Rightarrow$ Cell Junctions: All cells in epithelium are held together with little intercellular material. Specialized junctions provide both structural and functional links between its individual cells: $(i)$ Tight junctions (help to stop substances from leaking across a tissue),$(ii)$ Adhering junctions (perform cementing to keep neighboring cells together),and $(iii)$ Gap junctions (facilitate the cells to communicate with each other by connecting the cytoplasm of adjoining cells for rapid transfer of ions,small molecules,and sometimes big molecules).