● In `color{violet}("unicellular organisms")` , all functions like digestion, respiration and reproduction are performed by a `color{brown}("single cell.")`

● In the complex body of `color{violet}("multicellular animals")` the same basic functions are carried out by `color{violet}("different groups of cells")` in a well `color{violet}("organised manner.")`

● The body of a `color{violet}("simple organism")` like `color{violet}("𝘏𝘺𝘥𝘳𝘢")` is made of different types of cells and the number of cells in each type can be in `color{violet}("thousands.")`

● The human body is `color{violet}("composed of billions")` of cells to perform various functions.

● In `color{violet}("multicellular animals")`, a group of similar cells along with `color{violet}("intercellular substances")` perform a specific function. Such an `color{violet}("organization")` is called `color{brown}("tissue.")`

● Interestingly, all `color{violet}("complex animals")` consist of only `color{brown}("four basic types")` of tissues.

● These tissues are organised in specific proportion and pattern to form an `color{violet}("organ")` like `color{violet}("stomach, lung, heart and kidney.")`

● When two or more `color{violet}("organs")` perform a common function by their `color{violet}("physical and/or chemical ")` interaction, they together form `color{brown}("organ system")`, e.g., digestive system, respiratory system, etc.

● `color{violet}("Cells, tissues, organs and organ")` systems split up the work in a way that `color{violet}("exhibits division")` of labour and contribute to the survival of the body as a whole.

● The `color{violet}("structure"` of the cells vary according to their `color{violet}"function"`.

● Therefore, the `color{violet}"tissues"` are different and are broadly classified into `color{violet}"four types"`:

(i) `color{violet}"Epithelial"`
(ii) `color{violet}"Connective"`
(iii) `color{violet}"Muscular"`
(iv) `color{violet}"Neural"`

● We commonly refer to an `color{Brown}"epithelial tissue"` as `color{Brown}"epithelium"` (pl.: epithelia).

● This tissue has a `color{violet}"free surface"`, which faces either a `color{violet}"body fluid"` or the `color{violet}"outside environment"` and thus provides a covering or a lining for some part of the body.

● The cells are `color{violet}"compactly packed"` with little intercellular matrix.

● There are `color{violet}"two types"` of epithelial tissues namely `color{Brown}"simple epithelium"` and `color{Brown}"compound epithelium"`.

● `color{Brown}"Simple epithelium"` is composed of a `color{Violet}"single layer"` of cells and functions as a `color{Violet}"lining"` for body cavities, ducts, and tubes.

● The `color{Brown}"compound epithelium"` consists of `color{Violet}"two or more cell layers"` and has `color{Violet}"protective function"` as it does in our skin.

● On the basis of `color{Violet}"structural modification"` of the cells, simple epithelium is further divided into `color{Violet}"three types"`.

● These are:

(i) `color{Brown}"Squamous"`
(ii) `color{Brown}"Cuboidal"`
(iii) `color{Brown}"Columnar"`