8.19:
Loose Connective Tissue
Loose connective tissue, a sub-type of connective tissue proper, is the most abundant type of connective tissue, composed of loosely arranged fibers and a few cells suspended in a dense ground substance.
In an embryo, it is present under the skin as mesenchymal tissue and in the umbilical cord as the mucoid connective tissue.
Adults have areolar, adipose, and reticular loose connective tissues.
Areolar tissues have a loose meshwork of collagen, elastic and reticular fibers combined with interstitial fluid and scattered cells. Areolar tissues connect epithelial cells to the underlying tissue, such as connecting the epidermis to the muscle underneath the skin. These tissues also form a protective covering around the lymphatic and blood vessels.
Adipose tissues primarily comprise adipocytes or fat cells, which have lipid droplets for long-term energy storage. These are found below the subcutaneous layer or around organs and act as shock-absorbing cushions.
Reticular tissues comprise fine, branched reticular fibers and fibroblasts. These tissues provide a supporting framework in lymphatic organs such as the lymph nodes, spleen, tonsils, and bone marrow.
8.19:
Loose Connective Tissue
Loose connective tissue is found between many organs. Its main function is to absorb shock and bind tissues together. It also allows water, salts, and various nutrients to diffuse into cells that are embedded in it or present in adjacent tissues.
Adipose Tissue
Adipose tissue consists primarily of fat storage cells called adipocytes and little extracellular matrix. A large number of capillaries present within adipose tissue allow rapid mobilization of lipid molecules. White adipose tissue is the most abundant type of adipose tissue found in adults. It can appear yellow, due to the presence of carotene and related pigments derived from ingested plant food. White fat contributes mainly to lipid storage and can serve as an insulator from cold temperatures and mechanical injuries. White adipose tissue is especially involved in protecting the kidneys and cushioning the back of the eye. Brown adipose tissue is more common in infants, hence the term "baby fat." In adults, there is a reduced amount of brown fat, found mainly in the neck and clavicular regions of the body. The many mitochondria in the cytoplasm of brown adipose tissue help explain its efficiency at metabolizing stored fat. Brown adipose tissue is thermogenic, meaning that as it breaks down fats, it releases metabolic heat rather than producing adenosine triphosphate (ATP), a key molecule used in metabolism.
Areolar Tissue
Areolar tissue shows little specialization. It contains all the cell types (fibroblast, adipocytes, macrophage, mast cells, etc) and fibers (collagen, elastic fiber and reticular fiber) that are distributed in a random, web-like fashion. It fills the spaces between muscle fibers, surrounds blood and lymph vessels, and supports organs in the abdominal cavity. Areolar tissue also underlies most epithelia and represents the connective tissue component of epithelial membranes.
Reticular Tissue
Reticular tissue is a mesh-like, supportive framework for soft organs such as lymphatic tissue, the spleen, and the liver. Reticular cells produce the reticular fibers that form the network to which other cells attach. It derives its name from the Latin reticulus, which means "little net."
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 4.3: Connective tissue supports and protects