Histology is the study of tissues. All animals are made up of tissues, which are a group of cells that work together to perform a common function. During embryonic development, the body is composed of three different layers of tissues; ectoderm, mesoderm, and endoderm. During the developmental process these three layers of tissues differentiate to form the different tissues in the body. The four basic types of tissues are epithelial, connective, muscle, and nervous. Each of these tissues differ in their composition and function.

EPITHELIAL TISSUE

The first of the tissue types is epithelial tissue. An important characteristic of epithelial tissue is that there are many cells that are usually closely packed together. Epithelial tissues cover all exposed surfaces of the body, form glands, and line the inside of all of the passages. The functions of epithelial tissues include protection, secretion and sensation. Epithelial tissue is classified by two methods, the number of layers of cells and the shape of the cells. If an epithelial tissue has one layer of cells, it is called simple. If it has more than one layer of cells, it is called stratified. Epithelial tissue can also be classified by the shape. Irregularly shaped thin flat cells are called squamous. Square cells in which the nucleus is found centered in the middle of the cell are called cuboidal cells, and rectangular cells in which the nucleus is found at the bottom or basolateral surface of the cell are called columnar.

Squamous epithelium can be simple or stratified. If it is simple squamous epithelia, it would be one layer of irregularly shaped cells. This is found in the alveoli of the lung. If it is more than one layer of irregularly shaped cells, that is stratified squamous epithelia. An example of stratified squamous is the skin.

Cuboidal epithelia can be found in many areas of the body and include simple cuboidal, stratified cuboidal, and specialized cuboidal epithelia. Simple cuboidal epithelia is one layer of square cells. Examples are the kidney tubules. Stratified cuboidal is rarely used or found in the body. The specialized cuboidal is called transitional epithelia and is only found in the urinary system, particularly the urinary bladder.

Columnar epithelia can be simple, stratified, and specialized. Simple columnar is found in the intestinal tract. Stratified columnar is a rare type of epithelial tissue, and the specialized columnar, called pseudostratified ciliated columnar epithelia, is only found in the respiratory system.

CONNECTIVE TISSUE

In connective tissue there are many different cells. The cells are highly separated by intracellular substances which consist of protein fibers and a ground substance. The ground substance can be solid, gelatinous, or liquid. Connective tissues differ from epithelial tissue in that there are many different cells and that the cells are loose arranged. The functions of connective tissues are very diverse and include protection, energy storage, defense, and as a general packing material. Connective tissue is classified into three groups: connective tissue proper, fluid connective tissue and supporting connective tissue. Connective tissue proper is the most common type of connective tissue. Within connective tissue proper, there is loose connective tissue, or areolar tissue, dense connective tissue and adipose tissue. Loose connective tissue contains the greatest variety of cells of all the connective tissue. You can find up to 12 different cell types within loose connective tissues. Loose connective is used as a generalized packing material for the body. Dense connective tissue has a very dense arrange of protein fibers and loosely arranged cells. Dense connective tissues will make up tendons and ligaments. Adipose tissue is a specialized connective tissue. It is composed of only one type of cell, the adipocyte. This connective tissue is responsible for storing fat. The supporting connective tissues include bone and cartilage. The ground substance in these two connective tissues ranges from gelatinous to a calcified matrix. The fluid connective tissue includes blood and lymph.

MUSCLE TISSUE

Muscle tissue is a specialized tissue for contraction. There is voluntary and involuntary muscle. Skeletal muscle is a voluntary muscle. Cardiac and smooth muscles are involuntary. Muscle tissue can also be differentiated by the presence of striations. Skeletal and cardiac muscles are striated, and smooth muscle is nonstriated. Skeletal muscle is found on any skeletal element. It is voluntary and striated. Cardiac muscle is only found in the heart, is involuntary and striated. Smooth muscle is found in a variety of places in the body. Any opening or closings are protected by smooth muscle. It is involuntary and nonstriated.

NERVOUS TISSUE

The final type of tissue is nervous tissue. Nervous tissue is also specialized for conducting impulses. Nervous tissue is made up of two different types of cells. Neurons are the functional cells of the nervous system. A neuron is composed of three parts: the dendrites, axon, and cell body. Neuroglia cells are the supporting cells of the nervous system. Neuroglia cells retain the ability to divide mitotically. They function to increase the speed of nerve conduction, in phagocytosis as well as other roles.