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Areolar CT

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Areolar connective tissue is like the packing material of the body.  It fills in open spaces.  It is commonly used as the prime example of connective tissue. This is because it can have all three protein fibers.

Photomicrograph of areolar connective tissue, thin skin, at high magnification. The primary color is red, with variations in tone and saturation. Dark purple, branching, non-linear fibers are dispersed irregularly across the frame. The fibers create a complex network against the lighter background tissue. Some white space fills parts of the background along the fibers.
Figure 1: Areolar connective tissue, thin skin. Darkly stained elastic fibers are thinner than the pink-stained collagen fibers in the background

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Cells

Areolar tissue contains several types of cells.
Fibroblasts are the most abundant cell type and are responsible for producing the protein fibers found in areolar tissues.  Areolar tissue contains all three types of protein fibers: collagen, elastin, and reticular.  Because of this, areolar tissue is usually used as the first example of connective tissue presented to students.  Fibrocytes maintain these protein fibers and other portions of the matrix.

Mast cells are immune-associated cells that reside in tissues.  These cells contain vesicles of histamine and respond to allergens and other pathogens.  Antihistamine drugs stop the effects of the histamine, reducing allergic responses.  You can see a mast cell releasing granules of histamine in the bottom right corner of this picture.

Macrophages are also immune-associated cells. Macrophages were once cells circulating in the blood called monocytes, a type of white blood cell.  These cells decide to take residence in a particular tissue.  They roam the tissue and consume via phagocytosis (cell eating) anything they don’t recognize.   

Fibers

Collagen fibers are the most abundant fibers in areolar tissue. These appear as thick, pinkly-stained bundles of proteins.  These collagen bundles are abundant in the dermis of your skin and are the cause of some visible skin conditions.  The tearing of these collagen bundles results in stretch marks.  The sagging of these collagen bundles results in wrinkles. 

Elastic fibers are interspersed among collagen fibers in areolar tissue.  These fibers usually look squiggly, but can be stained different colors.  Elastin fibers take up the pink of a H&E stain. They can also be stained a black color. This makes them more visible. 

Reticular fibers are a specialized type of collagen fiber that forms a delicate mesh-like network within the tissue. Reticular fibers can be hard to locate in areolar tissue.  Also squiggly, they are thinner and usually stain with a blue-black color.  While less abundant than collagen and elastic fibers, reticular fibers are vital for providing structural support. They form a scaffolding or framework within the tissue. Reticular fibers are primarily composed of type III collagen.

Matrix

The extracellular matrix of areolar tissue consists of a gel-like ground substance containing water, glycosaminoglycans (GAGs), proteoglycans, and glycoproteins. Pink-stained collagen fibers are dispersed throughout the matrix. Dark blue-purple stained elastic fibers are also dispersed. Reticular fibers require a different stain to be visible. These fibers provide structural support, elasticity, and tensile strength.  The gel-like ground substance of areolar tissue cushions and protects delicate structures from mechanical stress and trauma.  

Functions

Areolar tissue provides structural support to organs and tissues throughout the body. Its loose, mesh-like structure allows for movement and flexibility. It also provides a framework for the attachment of blood vessels, nerves, and other structures. Areolar tissue acts as a cushioning layer beneath the skin, providing protection against mechanical injury and trauma. It also surrounds and protects blood vessels, nerves, and organs from external forces. Areolar tissue is highly vascularized. It contains a network of blood vessels and capillaries. These facilitate the exchange of nutrients, gases, and waste products between blood vessels and surrounding tissues. Areolar tissue plays a role in the initial stages of wound healing. It provides a scaffold for the migration of fibroblasts and other cells involved in tissue repair and regeneration.

Locations

Areolar tissue is found beneath the epidermis. This is the outer layer of the skin. It is also found beneath the dermis, which is the deeper layer of the skin. It provides support and cushioning for the skin and allows for movement and flexibility. Areolar tissue surrounds blood vessels, lymphatic vessels, and nerves. It provides them with support and protection. It also allows for movement and flexibility. Areolar tissue fills the spaces between muscles, providing support and allowing for movement and coordination between muscles. Areolar tissue forms a layer of connective tissue called the lamina propria. This layer is located beneath the epithelial lining of mucous membranes in various organs. These include the digestive tract, respiratory tract, and urinary tract. It provides support and flexibility to these organs.

Easily Confused With

Dense Irregular, Elastic Cartilage

High-magnification photomicrograph of stained areolar connective tissue, originating from the scalp. Visual elements include polygonal shaped epithelial cells at the top; interwoven collagen fibers appear as wavy, pinkish-red strands; elastic fibers are thinner and darker. Cells with dark stained nuclei are interspersed throughout. The overall composition has a pinkish-red/white hue, indicative of Hematoxylin and Eosin (H&E) staining. The image is tightly cropped to showcase the cellular structure.
Figure 2: Areolar connective tissue, scalp sample. Microscopic view showing cells and fibers in a pinkish-red and white matrix.
Histological slide showing areolar connective tissue (top) stained pink, with interwoven collagen fibers and scattered cells with dark nuclei in a white matrix. Beneath, a section of stained pink plantar tendon cells and collagen fibers. Brightfield microscopy, high magnification.
Figure 3: Areolar connective tissue and plantar tendon stained pink and white, as seen under a microscope.

Figure 1: Skin, homo, Verhoeff stain, 20X, Slide 033
Figure 2: Scalp, hair, H&E, 20X and 40X Slide 107
Figure 3: Thick skin, sole of foot, H&E, 40X Slide 106
All by University of Michigan Histology, licensed under CC BY-NC-SA 3.0.

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General Information
Adipose CT
Areolar CT
Reticular CT
Dense Regular CT
Dense Irregular CT
Elastic CT
Bone CT
Hyaline Cartilage CT
Elastic Cartilage CT
Fibrocartilage CT

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