Loose connective tissue, sometimes called areolar tissue, is a cellular
connective tissue
Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tissue ...
with thin and relatively sparse
collagen fibers. Its
ground substance occupies more volume than the fibers do. It has a viscous to gel-like consistency and plays an important role in the diffusion of oxygen and nutrients from the capillaries that course through this connective tissue as well as in the diffusion of carbon dioxide and
metabolic wastes
Metabolic wastes or excrements are substances left over from metabolic processes (such as cellular respiration) which cannot be used by the organism (they are surplus or toxic), and must therefore be excreted. This includes nitrogen compounds, wa ...
back to the vessels. Moreover, loose connective tissue is primarily located beneath the
epithelia that cover the body surfaces and line the internal surfaces of the body. It is also associated with the epithelium of glands and surrounds the smallest blood vessels. This tissue is thus the initial site where pathogenic agents, such as bacteria that have breached an epithelial surface, are challenged and destroyed by cells of the immune system.
In the past, the designations areolar tissue,
adipose tissue
Adipose tissue, body fat, or simply fat is a loose connective tissue composed mostly of adipocytes. In addition to adipocytes, adipose tissue contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, vascular ...
, and
reticular tissue have been listed as subsets of loose connective tissue. However, they are no longer considered subsets of loose connective tissue. Loose connective tissue is a subset of
connective tissue proper. Furthermore, areolar tissue is the same as loose connective tissue, adipose tissue is a subset of
specialized connective tissue, and reticular tissue is the presence of reticular fibers and reticular cells together forming the stroma of hemopoietic tissue (specifically the red bone marrow) and lymphatic tissue organs (lymph nodes and spleen but not the thymus).
Most cell types in loose connective tissue are transient wandering cells that migrate from local blood vessels in response to specific stimuli. Loose connective tissue, therefore, is a site of inflammatory and immune reactions. In areas of the body where foreign substances are continually present, large populations of immune cells are maintained. For example, the lamina propria, the loose connective tissue of mucous membranes, such as those of the respiratory and alimentary systems, contains large numbers of these cells.
Structure
Composition
It is a pliable, mesh-like
tissue with a fluid
matrix
Matrix most commonly refers to:
* ''The Matrix'' (franchise), an American media franchise
** '' The Matrix'', a 1999 science-fiction action film
** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...
and functions to cushion and protect body
organs.
Fibroblast
A fibroblast is a type of biological cell that synthesizes the extracellular matrix and collagen, produces the structural framework ( stroma) for animal tissues, and plays a critical role in wound healing. Fibroblasts are the most common cells of ...
s are widely dispersed in this tissue; they are irregular branching cells that secrete strong fibrous
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respon ...
s and
proteoglycans as an extracellular matrix.
The cells of this type of tissue are generally connected by a gelatinous substance known as
ground substance primarily made up of collagenous and elastic fibers.
Location
It may be found in tissue sections from almost every part of the body. It surrounds blood vessels and nerves and penetrates with them even into the small spaces of muscles, tendons, and other tissues. It may likewise be present in the mediastinal extremities. Nearly every epithelium rests on a layer of areolar tissue, whose blood vessels provide the epithelium with nutrition, waste removal, and a ready supply of infection-fighting leukocytes when needed. Because of the abundance of open, fluid-filled space, leukocytes can move about freely in areolar tissue and can easily find and destroy pathogens.
The areolar tissue is found beneath the
epidermis
The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the ...
layer and is also underneath the
epithelial tissue of all the
body systems that have external openings.
it makes the skin elastic and helps it to withstand pulling pain. It is also a component of the
lamina propria of the
digestive and
respiratory tracts, the
mucous membranes of
reproductive and
urinary systems, the
stroma of glands, and the
hypodermis
The subcutaneous tissue (), also called the hypodermis, hypoderm (), subcutis, superficial fascia, is the lowermost layer of the integumentary system in vertebrates. The types of cells found in the layer are fibroblasts, adipose cells, and macro ...
of the skin.
It is also found in the
mesentery which is surrounding the intestine.
Fibers
Loose connective tissue is named based on the "weave" and type of its constituent fibers. There are three main types:
*
Collagenous fibers: collagenous fibers are made of collagen and consist of bundles of fibrils that are coils of collagen molecules.
*
Elastic fibers: elastic fibers are made of
elastin and are "stretchable."
*
Reticular fibers: reticular fibers consist of one or more types of very thin collagen fibers. They join connective tissues to other tissues.
Areolar tissue
Areolar tissue ( or ) is a common type of loose connective tissue (and the most widely distributed type of
connective tissue
Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tissue ...
in vertebrates). It is so-named because its fibers are far enough apart to leave ample open space for
interstitial fluid
In cell biology, extracellular fluid (ECF) denotes all body fluid outside the cells of any multicellular organism. Total body water in healthy adults is about 60% (range 45 to 75%) of total body weight; women and the obese typically have a l ...
in between (''
areolar'' is Latin for a "little open space"). It is strong enough to bind different tissue types together, yet soft enough to provide flexibility and cushioning. It exhibits interlacing,
loosely organized fibers, abundant blood vessels, and significant empty space filled with interstitial fluid. Many adjacent
epithelial tissues (which are avascular) get their nutrients from the interstitial fluid of areolar tissue; the
lamina propria is areolar in many body locations. Its fibers run in random directions and are mostly collagenous, but elastic and reticular fibers are also present. Areolar tissue is highly variable in appearance. In many
serous membranes, it appears as a loose arrangement of collagenous and elastic fibers, scattered cells of various types; abundant
ground substance; numerous blood vessels. In the skin and mucous membranes, it is more compact and sometimes difficult to distinguish from
dense irregular connective tissue.
Function
Areolar connective tissue holds
organs in place and attaches
epithelial tissue to other underlying tissues. It also serves as a reservoir of water and salts for surrounding tissues. Almost all cells obtain their nutrients from and release their wastes into areolar connective tissue.
Clinical significance
Organs that are rich in loose connective tissue (such as the eyelids) are usually sites that undergo oedema, indicating
kidney failure or nephrotic syndrome. Therefore, periorbital swelling is one characteristic finding in severe kidney disease.
See also
*
Dense connective tissue
References
{{DEFAULTSORT:Loose Connective Tissue
Connective tissue