Derivation of Tissues

Cells that are similar in structure tend to group themselves together and form tissues. A tissue, then, is composed of a group of cells that are similar in structure and perform one or more common functions. Some tissues contain intercellular material which is very important in the performance of a particular function belonging to that tissue.

The body tissues and organs develop from the three primary germ layers that form during the growth process of the human embryo.

The tissues derived from the ectoderm are: some epithelial tissue (epidermis or outer layer of the skin, the lining for all hollow organs which have cavities open to a surface covered by epidermis), modified epidermal tissue (fingernails and toenails, hair, glands of the skin), all nerve tissue, salivary glands, and mucous glands of the nose and mouth.

In fact, epithelial tissue can be derived from either the ectoderm or endoderm. The epithelial tissue derived from the endoderm includes the epithelial lining of the digestive tract, except at the open ends, and the epithelial lining of all hollow structures formed as outpockets in the digestive tract. This includes:

• The parenchyma of the liver including communicating or connecting ducts
• The lining of the pharynx and respiratory tract (except the nose). This includes the lungs and the passageways leading from the pharynx to the lungs
• The epithelium of the bladder and urethra
• Glands that form secretions in the digestive tract

Epithelial tissue derived from ectoderm is generally squamous epithelium; epithelial tissue derived from endoderm is essentially glandular epithelium.

There are a variety of body tissues derived from the third or middle primary germ layer known as the mesoderm. These body tissues include:

• Muscles
• Fibrous tissue
• Bone and cartilage
• Fat or adipose tissue
• Blood and lymph vessels
• Blood cells

In the early embryo the first cavity that develops is the coelomic cavity; this is derived from mesoderm. Parts of the urinary and genital systems are derived as outpouchings of the coelomic cavity. Later this coelomic cavity divides into the pleural cavity and the pericardial cavity. The linings of these cavities are composed of a single layer of cells called mesothelium. A few epithelial cells are of mesodermal origin, e.g. endometrium of the uterus, vaginal epithelium, and mucosa of the bladder.

Endothelium derived from mesoderm lines the blood and lymphatic vessels and the walls of the heart. In the capillaries where the endothelium is covered only by a basement membrane, diffusion takes place. It is surrounded elsewhere by supportive layers of connective tissue and smooth muscle. This is necessary because the endothelium is so thin that diffusion would occur otherwise. Many authorities classify this endothelium as connective tissue.