ANIMAL TISSUES

The development of a fertilized egg into a newborn child requires an average of 41 rounds of mitosis (241 = 2.2 x 1012). During this period, the cells produced by mitosis enter different pathways of differentiation; some becoming blood cells, some muscle cells, and so on.

There are more than 100 visibly-distinguishable kinds of differentiated cells in the vertebrate animal. These are organized into tissues; the tissues into organs. Groups of organs make up the various systems - digestive, excretory, etc. - of the body.
The actual number of differentiated cell types is surely much larger than 100.

All lymphocytes, for example, look alike but actually represent a variety of different functional types, e.g., B cells, T cells of various subsets.
The neurons of the central nervous system must exist in a thousand or more different functional types, each representing the result of a particular pathway of differentiation.
This page will give a brief introduction to the major types of animal tissues. The links along the left side of the figure will take you directly to the individual paragraphs indicated.

Epithelial
Muscle
Connective
Nerve
Blood

1. Epithelial
Epithelial tissue is made of closely-packed cells arranged in flat sheets. Epithelia form the surface of the skin and line the various cavities and tubes of the body. The epithelia that form the inner lining of blood and lymph vessels are called endothelia. The apical surface of epithelial cells is exposed to the "external environment", the lumen of the organ or the air.

The basolateral surface is exposed to the internal environment (ECF). The entire sheet of epithelial cells is attached to a layer of extracellular matrix that is called the basement membrane or, better (because it is not a membrane in the biological sense), the basal lamina. The function of epithelia always reflects the fact that they are boundaries between masses of cells and a cavity or space. Some examples:

The epithelium of the skin protects the underlying tissues from
mechanical damage
ultraviolet light
dehydration
invasion by bacteria.

The columnar epithelium of the intestine
secretes digestive enzymes into the intestine;
absorbs the products of digestion from it.
An epithelium also lines our air passages and the alveoli of the lungs. It secretes mucus which keeps it from drying out and traps inhaled dust particles. Most of its cells have cilia on their apical surface that propel the mucus with its load of foreign matter back up to the throat.

2.Muscle. Three kinds of muscle are found in vertebrates:
Skeletal muscle is made of long fibers whose contraction provides the force of locomotion and other voluntary body movements.

Smooth muscle lines the walls of the hollow structures of the body, such as the intestine, urinary bladder, uterus, and blood vessels. Its contraction, which is involuntary, reduces the size of these hollow organs.
The heart is made of cardiac muscle.

3.Connective. The cells of connective tissue are embedded in a great amount of extracellular material. This matrix is secreted by the cells. It consists of protein fibers embedded in an amorphous mixture of protein-polysaccharide ("proteoglycan") molecules.
Supporting connective tissueGives strength, support, and protection to the soft parts of the body cartilage.
Example: the outer ear bone.

The matrix of bone contains collagen fibers and mineral deposits. The most abundant mineral is calcium phosphate, although magnesium, carbonate, and fluoride ions are also present.
Binding connective tissueIt binds body parts together.
Tendons connect muscle to bone. The matrix is principally collagen, and the fibers are all oriented parallel to each other. Tendons are strong but not elastic.

Ligaments attach one bone to another. They contain both collagen and also the protein elastin. Elastin permits ligaments to be stretched.
Fibrous connective tissue
It is distributed throughout the body. It serves as a packing and binding material for most of our organs. Collagen, elastin, and other proteins are found in the matrix.
Fascia is fibrous connective tissue that binds muscle together and binds the skin to the underlying structures.
Adipose tissue is fibrous connective tissue in which the cells have become almost filled with oil. The oil is confined within membrane-bound droplets. The cells of adipose tissue, called adipocytes, secrete several hormones, including leptin and adiponectin.
All forms of connective tissue are derived from cells called fibroblasts , which secrete the extracellular matrix.

4. Nerve. Nerve tissue is composed of nerve cells called neurons and glial cells.

NeuronsNeurons are specialized for the conduction of nerve impulses. A typical neuron consists of
a cell body which contains the nucleus;
a number of short fibers — dendrites — extending from the cell body
a single long fiber, the axon.
The nerve impulse is conducted along the axon.
The tips of axons meet:
other neurons at junctions called synapses.
muscles (called neuromuscular junctions)
glands
GliaGlial cells surround neurons. Once thought to be simply support for neurons (glia = glue), they turn out to serve several important functions.

There are three types:
Schwann cells. These produce the myelin sheath that surrounds many axons in the peripheral nervous system.
Oligodendrocytes. These produce the myelin sheath that surrounds many axons in the central nervous system (brain and spinal cord).
Astrocytes. These — often star-shaped — cells are clustered around synapses and the nodes of Ranvier where they perform a variety of functions:
stimulating the formation of new synapses;
modulating the activity of neurons;
repairing damage;
supplying neurons with materials secured from the blood.

(It is primarily the metabolic activity of astrocytes that is being measured in brain imaging by positron-emission tomography (PET) and functional magnetic resonance imaging (fMRI).
In addition, the central nervous system contains many microglia — mobile cells that respond to damage (e.g., from an infection) by
engulfing cell debris
secreting inflammatory cytokines like tumor necrosis factor (TNF-α) and interleukin-1 (IL-1)

5. Blood. The bone marrow is the source of all the cells of the blood.

These include:
red blood cells (RBCs or erythrocytes)
five kinds of white blood cells (WBCs or leukocytes)
platelets (or thrombocytes).

Here there are an example:

HUMAN TISSUES

Biological tissue is a collection of interconnected cells that perform a similar function within an organism.

The study of tissue is known as histology, or, in connection with disease, histopathology.

The classical tools for studying the tissues are the wax block, the tissue 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 tissues can be examined in health and disease, enabling considerable refinement of clinical diagnosis and prognosis. There are four basic types of tissue 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.

1. Epithelium - Tissues composed of layers of cells that cover organ surfaces such as surface of the skin and inner lining of digestive tract: the tissues that serve for protection, secretion, and absorption.
2. Connective tissue - As the name suggests, connective tissue holds everything together. Connective tissue is characterized by the separation of the cells by an inorganic material, which is called extracellular matrix. Bone and blood are connective tissues.
3. Muscle tissue - Muscle cells contain contractile filaments that move past each other and change the size of the cell. Muscle tissue 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.
4. Nervous tissue - Cells forming the brain, spinal cord and peripheral nervous system.

Here there are a fotograph of human tissues: