Learn About the Body's Connective Tissue

Dense Fibrous Connective Tissue
Dense Fibrous Connective Tissue. Ed Reschke/Photolibrary/Getty Images

As the name implies, connective tissue serves a connecting function. It supports and binds other tissues in the body. Unlike epithelial tissue, which has cells that are closely packed together, connective tissue typically has cells scattered throughout an extracellular matrix of fibrous proteins and glycoproteins attached to a basement membrane. The primary elements of connective tissue include a ground substance, fibers, and cells.

The ground substance acts as a fluid matrix that suspends the cells and fibers within the particular connective tissue type. Connective tissue fibers and matrix are synthesized by specialized cells called fibroblasts. There are three main groups of connective tissues: loose connective tissue, dense connective tissue, and specialized connective tissue.

Loose Connective Tissue

Loose Connective Tissue
This image of loose connective tissue shows collagenous fibers (red), elastic fibers (black), matrix, and fibroblasts (cells that produce the fibers). Ed Reschke/Photolibrary/Getty Images

In vertebrates, the most common type of connective tissue is loose connective tissue. It holds organs in place and attaches epithelial tissue to other underlying tissues. Loose connective tissue is named so because of the "weave" and type of its constituent fibers. These fibers form an irregular network with spaces between the fibers. The spaces are filled with ground substance. The three main types of loose connective fibers include collagenous, elastic, and reticular fibers.

  • Collagenous fibers are made of collagen and consist of bundles of fibrils that are coils of collagen molecules. These fibers help to strengthen connective tissue.
  • Elastic fibers are made of the protein elastin and are stretchable. They help to give connective tissue elasticity.
  • Reticular fibers join connective tissues to other tissues.

Loose connective tissues provide support, flexibility, and strength required to support internal organs and structures such as blood vessels, lymph vessels, and nerves.

Dense Connective Tissue

Dense Fibrous Connective Tissue
This image of the dermis of the skin shows dense fibrous connective tissue. Irregular collagenous fibers (pink) and fibroblast nuclei (purple) can be seen. Ed Reschke/Photolibrary/Getty Images

Another type of connective tissue is dense or fibrous connective tissue, which can be found in tendons and ligaments. These structures help attach muscles to bones and link bones together at joints. Dense connective tissue is composed of large amounts of closely packed collagenous fibers. In comparison to loose connective tissue, dense tissue has a higher proportion of collagenous fibers to ground substance. It is thicker and stronger than loose connective tissue and forms a protective capsule layer around organs such as the liver and kidneys.

Dense connective tissue can be categorized into dense regular, dense irregular, and elastic connective tissues.

  • Dense regular: Tendons and ligaments are examples of dense regular connective tissue.
  • Dense irregular: Much of the dermis layer of the skin is composed of dense irregular connective tissue. The membrane capsule surrounding several organs is also dense irregular tissue.
  • Elastic: These tissues enable stretching in structures such as arteries, vocal cords, the trachea, and bronchial tubes in the lungs.

Specialized Connective Tissues

Adipose (Fat) Tissue
This image shows a sample of fat tissue with fat cells (adipocytes, blue) surrounded by fine strands of supportive connective tissue. Adipose tissue forms an insulating layer under the skin, storing energy in the form of fat. Steve Gschmeissner/Science Photo Library/Getty Images

Specialized connective tissues include a number of different tissues with specialized cells and unique ground substances. Some of these tissues are solid and strong, while others are fluid and flexible. Examples include adipose, cartilage, bone, blood, and lymph.

Adipose Tissue

Adipose tissue is a form of loose connective tissue that stores fat. Adipose lines organs and body cavities to protect organs and insulate the body against heat loss. Adipose tissue also produces endocrine hormones that influence activities such as blood clotting, insulin sensitivity, and fat storage.

The primary cells of adipose are adipocytes. These cells store fat in the form of triglycerides. Adipocytes appear round and swollen when fat is being stored and shrink as fat is used. Most adipose tissue is described as white adipose which functions in the storage of energy. Both brown and beige adipose burn fat and produce heat.

Cartilage

Hyaline Cartilage
This micrograph shows hyaline cartilage, a semi-rigid connective tissue from a human trachea (windpipe). Steve Gschmeissner/Science Photo Library/Getty Images

Cartilage is a form of fibrous connective tissue that is composed of closely packed collagenous fibers in a rubbery gelatinous substance called chondrin. The skeletons of sharks and human embryos are composed of cartilage. Cartilage also provides flexible support for certain structures in adult humans including the nose, trachea, and ears.

There are three different types of cartilage, each with different characteristics. Hyaline cartilage is the most common type and is found in areas such as the trachea, ribs, and nose. Hyaline cartilage is flexible, elastic, and surrounded by a dense membrane called perichondrium.

Fibrocartilage is the strongest type of cartilage and composed of hyaline and dense collagen fibers. It is inflexible, tough, and located in areas such as between vertebrae, in some joints, and in heart valves. Fibrocartilage does not have perichondrium.

Elastic cartilage contains elastic fibers and is the most flexible type of cartilage. It is found in locations such as the ear and larynx (voice box).

Bone Tissue

Bone Tissue
This micrograph shows cancellous (spongy) bone from a vertebra. Cancellous bone is characterized by a honeycomb arrangement, comprising a network of trabeculae (rod-shaped tissue). These structures provide support and strength to the bone. Susumu Nishinaga/Science Photo Library/Getty Images

Bone is a type of mineralized connective tissue that contains collagen and calcium phosphate, a mineral crystal. Calcium phosphate gives bone its firmness. There are two types of bone tissue: spongy and compact.

Spongy bone, also called cancellous bone, gets its name because of its spongy appearance. The large spaces, or vascular cavities, in this type of bone tissue contain blood vessels and bone marrow. Spongy bone is the first bone type formed during bone formation and is surrounded by compact bone.

Compact bone, or cortical bone, is strong, dense, and forms the hard outer bone surface. Small canals within the tissue allow for the passage of blood vessels and nerves. Mature bone cells, or osteocytes, are found in compact bone.

Blood and Lymph

Red Blood Cells
This is a micrograph of a group of red blood cells (erythrocytes) traveling through an arteriole (small branch of an artery). P.M. Motta & S. Correr/Science Photo Library/Getty Images

Interestingly enough, blood is considered to be a type of connective tissue. Like other connective tissue types, blood is derived from mesoderm, the middle germ layer of developing embryos. Blood also serves to connect other organ systems together by supplying them with nutrients and transporting signal molecules between cells. Plasma is the extracellular matrix of blood with red blood cells, white blood cells, and platelets suspended in the plasma.

Lymph is another type of fluid connective tissue. This clear fluid originates from blood plasma that exits blood vessels at capillary beds. A component of the lymphatic system, lymph contains immune system cells that protect the body against pathogens. Lymph is delivered back to blood circulation via lymphatic vessels.

Animal Tissue Types

In addition to connective tissue, other tissue types of the body include:

  • Epithelial Tissue: This tissue type covers body surfaces and lines body cavities providing protection and allowing for the absorption and secretion of substances.
  • Muscle Tissue: Excitable cells capable of contraction allow muscle tissue to generate body movement.
  • Nervous Tissue: This primary tissue of the nervous system allows for communication between various organs and tissues. It is composed of neurons and glial cells.

Sources

  • "Animal Tissues - Bone." Atlas of Plant and Animal Histology, mmegias.webs.uvigo.es/02-english/guiada_a_oseo.php.
  • "Animal Tissues - Cartilage." Atlas of Plant and Animal Histology, mmegias.webs.uvigo.es/02-english/guiada_a_cartilaginoso.php.
  • Stephens, Jacqueline M. "The Fat Controller: Adipocyte Development." PLoS Biology, vol. 10, no. 11, 2012, doi:10.1371/journal.pbio.1001436.