The Human body consists of at least 200 distinct types of cells. These cells contain essentially the same internal structures yet they vary enormously in shape, size, and functions.
The different types of cells are not randomly distributed throughout the body; rather they occur in organized layers, a level of an organization referred to as tissue or we can say that tissue is a group of cells with their extracellular matrix.
The two separate branches of anatomy basically deal with the study of tissues i.e. Histology (the study of normal tissues) and the histopathology which is the branch of science which deals with the study of abnormal or diseased tissues.
The word “tissue” comes from the French word “,tissu” which is actually meant as “woven”.
French anatomist and pathologist Marie François Xavier Bichat introduced the term in 1801, stating that body functions could be understood better if they were studied at the level of tissues rather than organs.
Types of tissues
The human tissue is classified into four basic categories: epithelial tissue, connective tissue, muscle tissue, and nervous tissue.
These all types of tissues derived from embryonic tissue (ectoderm, mesoderm, endoderm) and sometimes they vary according to species.
- Cells of epithelial tissue form sheets that cover the body and organ surfaces.
- In all animals, most epithelium derives from the ectoderm and endoderm, except the epithelium, which derives from the mesoderm.
- Examples of epithelial tissue include the skin surface and the linings of the airways, reproductive tract, and gastrointestinal tract.
- There are several kinds of the epithelium, including simple squamous epithelium, simple cuboidal epithelium, and columnar epithelium.
- Functions include protecting organs, eliminating waste, absorbing water and nutrients, and secreting hormones and enzymes.
- Connective tissue consists of cells and non-living material, called the extracellular matrix.
- The extracellular matrix may be either fluid or solid. Examples of connective tissue include blood, bone, adipose, tendons, and ligaments.
- In humans, cranial bones derive from the ectoderm, but the other connective tissues come from the mesoderm.
- Functions of connective tissue include shaping and supporting organs and the body, allowing body movement, and providing oxygen diffusion.
- The three types of muscle tissue are skeletal muscle, cardiac muscle, and smooth (visceral) muscle.
- In humans, muscles develop from the mesoderm. Muscles contract and relax to allow body parts to move and blood to pump.
- Nervous tissue is divided into the central nervous system and peripheral nervous system.
- It includes the brain, spinal cord, and nerves. The nervous system derives from the ectoderm.
- The nervous system controls the body and communicates between its parts.
The Embryonic Origin of Tissues
- The fertilized egg or zygote is also known as single-celled. It is formed by the fusion of an egg and sperm.
- The zygote undergoes too rapid mitotic cycles, and forms the embryo by generating many cells.
- The first embryonic cells generated have the ability to differentiate into any type of cell in the body and, as such, are called totipotent, meaning each has the capacity to divide, differentiate, and develop into a new organism.
- As cell proliferation progresses, three major cell lineages are established within the embryo.
- Each of these lineages of embryonic cells forms the distinct germ layers from which all the tissues and organs of the human body eventually form.
- Each germ layer is identified by its relative position: ectoderm (Ecto- = “outer”), mesoderm (Meso- = “middle”), and endoderm (endo- = “inner”). shows the types of tissues and organs associated with each of the three germ layers.
- Note that epithelial tissue originates in all three layers, whereas nervous tissue derives primarily from the ectoderm and muscle tissue from mesoderm.
Different types of tissue membrane
The two broad categories of tissue membranes in the body are
- Connective tissue membranes, which include synovial membranes, and
- Epithelial membranes, which include mucous membranes, serous membranes, and the cutaneous membrane which means the skin.
- When tissue regeneration occurs it is essential that some of the original cells are available to replicate by mitosis.
- The extent to which regeneration is possible depends on the normal rate of physiological turnover of particular types of cells.
- Those with a rapid turnover regenerate most effectively. There are three types.
1.) Labile cells
Labile cells are those in which replication is normally a continuous process. They include cells in:
- epithelium of e.g. skin, mucous membrane, secretory glands, ducts, the uterus lining
- bone marrow
- spleen and lymphoid tissue.
2.) Stable cells
Stable cells have retained the ability to replicate but do so infrequently. This includes:
- liver, kidney and pancreatic cells
- Smooth muscle cells
- osteoblasts and osteoclasts in bone.
3.) Permanent cells
Permanent cells are unable to replicate after normal growth is complete. This includes:
- nerve cells (neurons)
- skeletal and cardiac muscle.