Connective Tissue Proper
Connective tissue proper varies in its density and arrangement of collagen bundles and generally can be described as either “loose” or “dense.” Again, these are relative terms, and the lines separating these classifications are not distinct but graded.

Loose Connective Tissue Proper
Relative to dense CT, loose CT is typically highly cellular and contains sparse, thin, and loosely arranged fibers and abundant ground substance. Loose CT can further be described as areolar or reticular.

Loose (areolar) connective tissue
Loose (areolar) CT is characterized by relatively loosely arranged collagen and elastic fibers. It is highly cellular, containing fibroblasts, immune cells (e.g., mast cells, macrophages, T cells), and endothelial cells (capillaries). In some cases, you’ll find adipocytes and greater amounts of unstained regions due to extraction of ground substance during tissue processing. It is often found directly underlying epithelia that cover body surfaces or line internal surfaces. In mucosal epithelium (respiratory system, alimentary canal, & genitourinary tract) it is called lamina propria. You can also find it associated with glandular epithelium and surrounding small blood vessels.

The papillary dermis (connective tissue right underneath skin epithelium; see image to the right) has small collagen bundles that are pretty tightly packed but is still considered loose or areolar connective tissue (L). As you examine this slide of human thin skin, note that the distinction between loose CT (L) and dense irregular connective tissue (DICT) is clearer when they are viewed in juxtaposition.

Examine the connective tissue, both loose connective tissue (also known as lamina propria in the colon) and dense irregular connective tissue, and compare it to the appearance of smooth muscle on this slide of the colon . Examine the CT in dermis of the skin in the following examples: thin skin 1, thin skin 2, thick skin 1, and thick skin 2, which have different stains, noting again the different densities of CT.

Clinical note: Following injury to any tissue or organ, connective tissue proper is the usual site of the inflammatory response, a process that involves all the cells of the tissue (cells of CT are discussed later). The events of wound healing mainly involve cells and fibers of the connective tissue and the formation of scars involves excessive or otherwise abnormal production of new fibroblasts and collagen during this process.

Reticular Tissue
Loose reticular CT contains a network of reticular fibers of type III collagen (also called “reticulin”). These fibers are produced by specialized fibroblasts (a.k.a reticular cells) and form an elaborate network through which interstitial fluid or lymph and wandering blood cells pass continuously.

Reticulin fibers stain black with silver stains, as seen in this oval lymph node . Also note the appearance of the collagen bundles in the CT surrounding the lymph node, comparing it with that of the stains examined earlier.
 

Now let's examine dense CT proper, both dense irregular and dense regular CT.