Basic Tissue Types
All of the various tissues of the human body can be categorized into four basic tissue types. All organs are built of these four tissues, which have consistent characteristics and arrangements from organ to organ. Thus an appreciation of the major features of these four basic tissue types can greatly simplify your understanding of the cellular composition of the many organ systems.
Overview of the Four Basic Tissue Types
Epithelial Tissue covers body surfaces (epi, on + thelium, surface). Epithelial tissue consists of cells attached to one another to form an uninterrupted layer of cells that separates the underlying tissues from the outside world. The body's epithelium not only covers its obvious surfaces (such as the epidermis of the skin and the linings of respiratory, urinary, and digestive tracts) but also extends into all of the complex invaginations which form lungs, kidneys, sweat glands, digestive glands, liver, etc. Epithelial tissue provides the essential functions of protection; containment of body fluids; and transport in and out across body surfaces (absorption and secretion). Embryonically, most epithelial tissues are derived either from ectoderm (e.g., epidermis) or endoderm (e.g., epithelium of trachea and lung). [More] [Examples]
Connective Tissue supports other tissues. Connective tissue consists of several cell types and extracellular products which, together, provide essential functions of mechanical reinforcement, immune surveillance, transport/diffusion of nutrients and wastes, and energy storage (fat). Embryonically, connective tissues derive from mesoderm or mesenchyme. [More] [Examples]
Nervous Tissue is responsible for rapid long-distance signalling, coordination, and "thinking". Nervous tissue consists of highly specialized nerve cells and support cells which are derived from embryonic neuroectoderm and neural crest. [More]
Muscle Tissue is specialized for gross movement by means of cellular contraction. Embryonically, muscle derives from mesoderm or mesenchyme. [More]
A note on pathology nomenclature: The names of neoplasms reflect the fundamental nature of their source tissues. Thus a carcinoma is a cancer of epithelial origin, while a sarcoma is a cancer of mesenchymal (connective tissue or muscle) origin.
Parenchyma / Stroma: The parenchyma of an organ consists of that tissue which conducts the specific function of the organ and which usually comprises the bulk of the organ. Stroma is everything else -- connective tissue, blood vessels, nerves, ducts. The parenchyma / stroma distinction provides a convenient way to circumvent the listing of tissue types when discussing an organ.
- The parenchyma of the kidney is epithelial tissue (renal tubules and corpuscles). The blood vessels, nerves, and supporting connective tissue of the kidney comprise the stroma.
- The parenchyma of the spleen is connective tissue (mostly lymphocytes and other blood cells). The supporting fibrous connective tissue of the spleen comprises the stroma.
- The parenchyma of the heart is muscle tissue (cardiac muscle cells). The nerves, intrinsic blood vessels, and connective tissue of the heart comprise the stroma.
- The parenchyma of the brain is nervous tissue (nerve cells and glia). The blood vessels within the brain and the connective tissue associated with these blood vessels are stroma.
- The parenchyma of the malignant neoplasm is cancer cells. Other tissues, including blood vessels, which grow to support the tumor are stroma.
Parenchyma is interesting. Because organ-specific function usually centers on parenchymal cells, histological (and physiological) accounts often emphasize parenchyma. Unfortunately, stroma is commonly ignored as just boring background tissue.
Pay attention to the stroma. No organ can function without the mechanical and nutritional support provided by the stroma. If an organ is inflamed, the signs of inflammation appear first in the stroma. (For an example of inflammation from liver, see WebPath.)
Historical note: Ignoring inconspicuous tissue features can have consequences. Stromal capillaries are seldom evident in tissue specimens. Nothing calls them to one's attention, so they are often ignored and forgotten. Unfortunately, just such inattention may have delayed for decades the realization that interfering with tumor vasculature might powerfully inhibit tumor growth.
Philosophical note: The concept of "four basic tissue types" provides a simple (and powerful) framework for organizing and learning a great wealth of detail. This concept is more than just a convenient intellectual construct. There is a real boundary layer, the basal lamina (visible microscopically with appropriate stain), which separates tissues of different types.
Nevertheless, nature does remind us every so often that her organization is not bound by our simplifying concepts. Although most tissues do correspond closely to one of the four basic tissue types, the usual caveat ("all rules have exceptions") does apply.
Exceptions include germ cells, which don't fit the scheme at all, and several specialized varieties of connective tissue which masquerade as epithelium, such as synovial membranes of joint capsules and stria vascularis of the inner ear.
Endothelium and mesothelium are special cases, usually classified as epithelial even though they derive from mesenchyme.
The best way to appreciate the basic tissue organization of the body is through examples in which this organization is displayed in relatively simple form.
Each of the following examples illustrates the distinct difference between epithelial tissue and connective tissue. Click on a thumbnail image for an annotated enlargement.
Skin Cornea Trachea
Comments and questions: email@example.com
SIUC / School
of Medicine / Anatomy / David
Last updated: 11 December 2007 / dgk