See Also [Histology] and [The Virtual Microscope]
|A tissue is a group of cells working together to perform one or
more specific functions. There
are four tissue types in the human body:
Epithelial tissue - found as the lining and covering of organs and body cavities, the secretory parts of organs and glands, the transport membranes of capillaries and alveolar sacs, and membranes which lubricate organs.
Connective tissue - supports as bone, cartilage, tendons and ligaments, protects as the bony cavities and as protective immune cells in the blood, and stores nutrients.
Nervous tissue - the tissue which carries information in the form of impulses throughout the body.
Muscle tissue - contracts to perform movements such as skeletal muscle movements, propulsion in the GI tract, and pumping blood in the heart.
See Figure 4.13 for Derivation of the above tissues from the primary germ layer. Epithelial tissue is derived from all three germ layers: ectoderm, mesoderm, and endoderm. Connective tissue and muscle are mostly derived from the mesoderm. An exception is the connective glial cells, supportive cells of the nervous system which, together with nerve tissue and the skin, is derived from the ectoderm.
|Epithelial tissue is composed of closely packed cells, anchored
basement membrane consisting of a basal lamina made of glycoprotein
(similar to the glycocalyx on the apical surface) and a reticular lamina
made of collagen. Epithelium is named according to structure, arrangement,
See [Epithelial Cell]
squamous - thin, flat cells
cuboidal - cube shaped cells
columnar - column shaped cells
Structure and arrangement:
simple - one layer thick
stratified - multi-layered
pseudostratified - appears stratified but isn't
transitional - stretchable
ciliated - has hair-like cilia
|Selected epithelial tissues common in the human body:
Stratified squamous epithelium: the epidermis of the body's skin.
|Connective tissue consists of a variety of
widely spaced cells within an intercellular matrix.
The term extracellular matrix (ECM) can also be used, but usually is
applied to the attachments of cells, including other cell types, not just
those in connective tissues. These cells are all derived from mesenchyme stem cells. The matrix may be loose, dense, or
have other specialized characteristics, it may have one or more types of fiber, and has a
substance with semi-solid to fluid gel or other materials. All of these features
structure and function of the variety of these tissues found throughout the body.
fibrocyte (also fibroblast) - the generic or characteristic cell for each type of connective tissue. In the loose and dense tissues the name fibrocyte or fibroblast is used depending on the predominant action and stage of development of the cells. A fibroblast is actively secreting matrix, usually in growing tissue, while a fibrocyte is a mature cell, no longer active in building tissue, but still important in maintenance and managing homeostasis. The same designation is used for cells in other connective tissues, e.g. osteocytes and osteoblasts in bone, chondrocytes and chondroblasts in cartilage. A cell designated as a -clast is dissolving the matrix. For instance osteoclasts are important in bone remodeling by breaking down old matrix before it is replaced. These cells come from a different cell line than the -blasts and -cytes.
Other cells which occur in certain connective tissues, especially the loose varieties, are:
macrophages - phagocytic cells derived from monocytes which are part of the body's first line of defense against invading microorganisms. These cells have a variety of names depending on the tissue such as histiocytes (lungs), Kupffer cells (liver), Langerhan's cells (skin), microglia (nervous tissue).
plasma cells - a type of lymphocyte which secretes antibodies; these plus other white blood cells wander in from the blood.
A loose matrix such as in areolar tissue has much fluid-filled space in between its structural components. A dense matrix does not.
Three types of fibers may be found singly
or in combination:
collagen fibers- high tensile strength with some flexibility. Found in inelastic types of tissues. Collagen is actually a glycoprotein formed into a triple helix (called a fibril), and is found as many as 19 different varieties in various tissues in the body.
elastic fibers - provide organs and tissues with the ability to stretch and recoil. These fibers are thin and interwoven with collagen fibers to prevent tearing. Elastic fibers are made of the protein elastin (randomly coiled and covalently bound to form an elastic matrix), and glycoprotein microfibils, formed into a cross-linked network within the tissue.
reticular fibers - made of the same molecules as collagen but thinner, they form an internal mesh-like network within organs. This produces an endoskeleton or stroma for soft organs such as the spleen, liver, etc.
The ground substance: a viscous, clear substance with a high water content which occupies the space between the cells and fibers. It is composed of proteoglycans (glycoproteins) which are made of a protein core with attached glycosaminoglycans (def) or GAGs. Important GAGs are hyaluronic acid, chondroitin sulfate, heparan sulfate (produces the anticoagulant heparin), keratan sulfate (produces keratin). See [The Structure of a Proteoglycan Complex]. The proteins fibronectin and laminin may also be part of this matrix. These are cell adhesion molecules (CAMS) which help to attach cells, connective as well as other types of cells, to the intercellular or extracellular matrix. They also have proven important in the movement of cells during embryonic development as well as in metastasis.
A semisolid gel makes cartilage firm yet flexible.
Connective tissue proper - composed of two types:
Loose connective tissue is found as part of the dermis of skin, in the underlying supportive layer (called the lamina propria) of the GI, respiratory and genitourinary tracts, also found in serous membranes, and as the body's interstitial tissue. This is the most abundant connective tissue and its matrix contains most of the body's interstitial fluid. Areolar has all three types of fibers.
adipose - Sometimes considered a specialized connective tissue rather than connective tissue proper, adipose is an exception to the general characteristics because its cells are closely packed and it has little matrix. The adipose cells store lipid in a large vacuole which fills each cell. Adipose is important for shock absorption and insulation and is found around many organs such as the heart, eyes, kidneys, spleen etc. as well as under the skin and in the medullary canal of long bones. Subcutaneous fat is a major stored fuel for aerobic activities. Adipose is also found associated with the serous membranes of the body. [See Adipose l.p. and Adipose h.p.]
Dense irregular - Has few cells, mostly fibroblasts, and many fibers, principally collagen, arranged in an irregular pattern to provide strength and withstand stresses to which the organ may be subjected. It makes up the deep layer of the skin's dermis, and it produces the supporting submucosa of the hollow organs (e.g. GI tract), and the capsules of synovial joints.
Dense regular connective tissue - Also known as fibrous or inelastic connective tissue it forms the structure of tendons, ligaments, aponeuroses, fascia, and fibrous joints. It has almost entirely collagen fibers (certain ligaments, called elastic ligaments, have more elastic fibers) in densely packed arrays, with rows of cells between the fiber bundles.
reticular tissue - found as the internal support (stroma) of the kidneys, spleen, liver and many other soft organs. Has reticular fibers only.
elastic connective tissue - found in the walls of the large arteries and in the stroma of the lungs and sparingly in certain elastic ligaments (e.g. those of the spinal column). It makes the arteries flexible to absorb the pulse pressure, and gives the lungs their recoil.
Cartilage: considered a specialized connective tissue and not connective tissue proper. It has semisolid gel made principally of the glycosaminoglycans chondroitin sulfate and hyaluronic acid. The gel gives the cartilage a distinct shape, lots of water turgor, and flexibility. Cells in cartilage are found within spaces called lacunae. The lacuna allows the cell to be bathed in fluid from which it receives nutrients and gets rid of wastes by diffusion. Substances diffuse very slowly through the gel and cartilage itself is avascular.
hyaline cartilage - has organic collagen fibers but they are very finely divided and cannot be seen in the light microscope. Staining gives the matrix a texture which is referred to as the ground substance. Hyaline is the most common type of cartilage, found in the nose, attached to the ribs, as articular cartilage, and as the cartilage model for bone development.
elastic cartilage - has dense bundles of elastic fibers and is rare, found only in the epiglottis and ear.
fibrocartilage - has dense bundles of collagen fibers, it is the major component of the intervertebral disks and the symphysis pubis.
|Vascularization of tissues and Tissue Repair. See [Diagram]
The ability of tissues to repair themselves is related to their blood supply. Tissues well supplied with blood capillaries can usually exhibit functional and rapid repair compared with poorly supplied tissues.
There are two types of repair: functional or parenchymal repair in epithelial tissue, in which the function of the replaced cells continues, and stromal repair or scar tissue in which fibrous tissue knits the damaged parts together but doesn't perform the tissues original function.
Epithelial tissue generally exhibits functional repair. Most epithelial tissues exhibit rapid mitosis and the original function is normally retained. Although the tissue itself has no blood vessels, vessels are a short distance away in the supporting connective tissue which is usually areolar. Epithelial tissue in the skin, in the linings of organs in the GI and respiratory tracts, in the liver, in many glands, and in blood vessels can all replace and repair themselves, the limiting factor generally being the degree of damage and other nutritive and health factors.
Connective tissues, with notable exceptions, have poor vascularization and therefore slow repair and replacement. And the repair of connective tissues, except for bone, is stromal repair, scar tissue which binds the organ but is not the same as the original tissue. The exceptions are areolar, which is the route for blood supply in the skin and in many internal organs, bone, which is richly supplied with blood vessels and adipose tissue. Bone repairs itself much more rapidly and effectively than, for instance, cartilage or fibrous connective tissue (tendons and ligaments). Adipose is highly vacularized and will continue to repair and grow if you let it.
(organ membranes) - these are combinations of
epithelial and connective tissues which have specific functions.
Serous membranes - combined of simple squamous epithelium and areolar connective tissue. Secretes serous fluid as a lubricant for sliding of the tissues. Found as the pericardial sack which prevents friction and abrasion when the heart beats; as the pleural membranes around the lungs; as mesenteries attaching the intestines; as peritoneum lining the abdominal cavity and covering its organs.
Mucous membranes - combined of columnar (may be ciliated or p.c.c.e.) epithelium with areolar connective and smooth muscle. Forms the structure of the GI and respiratory passageways. Specialized glands, or cells called goblet cells, secrete mucus to protect the lining, lubricate the propulsion of food, and remove particulates form the respiratory tract. (See simple columnar epithelium description, above) [Note: the noun mucus has a different spelling than the adjective mucous]
Synovial membranes are not epithelial membranes but are included here because they are important membrane organs which form joint capsules which lubricate joints, and bursae which lubricate the movements of tendons and ligaments. They are composed entirely of connective tissue.
Revised: January 28, 2006
Next: The Cutaneous Membrane a.k.a. The Skin