Loose Connective Tissue Proper
Connective tissue proper can be divided into two groups according to how tightly packed their fibers are. We group members of connective tissue proper into:
- Loose connective tissue proper:
- Areolar connective tissue proper
- Adipose tissue
- Reticular connective tissue
- Dense connective tissue proper:
- Dense regular connective tissue
- Dense irregular connective tissue
This chapter will focus on the types of loose connective tissue proper. The following chapter, Dense Connective Tissue Proper, will discuss the types of dense connective tissue proper.
Areolar Connective tissue
Areolar connective tissue is the classic, most-common type of loose connective tissue. Areolar connective tissue is found in many places in our body. It anchors epithelial tissues, as part of the basement membrane. Areolar connective tissue is also found in the dermis of our skin. Areolar connective tissue has a lot of ground substance and blood vessels (highly vascularized). You will see thick bands of collagen (stain pink), that may look wispy like cotton candy. Collagen is a long, rope-like fibrous protein that provides tremendous tensile strength (ability to be pulled without breaking). You will also see thin strands of elastic fibers (stain dark purple/black) that look like dark, thin threads. Elastic fibers provide elasticity to the tissue (ability to be pulled and recoil like a rubber band). There are many cell types that can be found in areolar connective tissue, like immune cells (mast cells, macrophages, lymphocytes). However, the cell type responsible for producing the extracellular matrix fibers are fibroblasts. The suffix -blast means an immature cell.
Table 1: Areolar connective tissue
| Tissue Type | Areolar Connective Tissue |
|---|---|
| General Description | Collagen (pink-staining fibers) and elastic fibers (dark-staining, thread-like fibers) are loosely arranged, contains fibroblasts (secrete materials for extracellular matrix) and other cell types scattered throughout the tissue |
| Location | Widely distributed, located in the papillary layer of the skin, lamina propria of mucous membranes, around organs and capillaries |
| Function | Anchors epithelial tissues to other tissue types, provides a level of support and protection to structures it surrounds (packaging material) |
| Helpful Hints | Go to areas where the tissue is less dense (usually the edges of the section) |
Figure 1: Areolar connective tissue with and without illustration overlay
Check out our YouTube video to help you understand areolar connective tissue:
YouTube Video – Areolar Connective Tissue
Adipose tissue
Adipose tissue is fat tissue, which our body uses for insulation (temperature regulation), protection/cushioning (physical trauma), storage (lipids for fuel). Because it is used for lipid storage, adipose is heavily vascularized. Adipose tissue is made primarily of cells called adipocytes (-cyte means a mature cell). Adipocytes have very little cytoplasm and are entirely filled with a large storage sac of lipid (fat). Due to the staining process, the lipid does not stain. Therefore, adipocytes appear as if they are empty, because the lipid they do not stain. The lipid-filled sac is so large that it pushes the nucleus (blue) against the inside of the plasma membrane (pink).
Table 2: Adipose tissue
| Tissue Type | Adipose tissue |
|---|---|
| General Description | Loose connective tissue with very little extracellular matrix, cells (adipocytes) appear empty with a nuclei pushed to the side |
| Location | Located in the greater omentum, subcutaneous layer associated with the skin, around the kidneys and eyeballs |
| Function | Stores lipids for fuel, insulation, and protection/cushioning |
| Helpful Hints | Students often confuse simple squamous epithelium of the lungs with adipose tissue, view these specimens side by side to compare the differences |
Figure 2: Adipose tissue with and without illustration overlay
Check out our YouTube video to help you understand adipose connective tissue:
YouTube Video – Adipose Connective Tissue
Reticular connective tissue
Reticular connective tissue is found in our lymphoid organs, including lymph nodes, spleen, and bone marrow. Reticular connective tissue is a specialized type of tissue found in locations that act as filters to trap cellular debris or pathogens (lymph nodes and spleen). The main fibers in reticular connective tissue are reticular fibers, which are a specialized type of collagen that forms branching networks (branches on a tree) instead of long, linear strands like we see in other types of connective tissue. These fibers, produced by reticulocytes (special type of fibroblasts), don’t stain very well with typical microscope stains (H&E, which stains pink and purple). Instead, silver stain is typically used, which gives reticular fibers a dark brown color. Reticular connective tissue contains two other types of immune cells: macrophages are found in loosely arranged areas, and lymphocytes are found in tightly packed areas. The cells in reticular connective tissue stain yellow/light orange.
Figure 3 shows reticular connective tissue with and without an illustration overlay. In the microscope image, reticular fibers appear brown and lymphocytes appear yellow/orange. In the illustration overlay, reticular fibers are drawn in purple and lymphocytes are drawn in pink.
Table 3: Reticular connective tissue
| Tissue Type | Reticular Connective Tissue |
|---|---|
| General Description | Reticular fibers loosely arranged around a large quantity of leukocytes (white blood cells) |
| Location | Lymphoid organs including lymph nodes, spleen, and bone marrow |
| Function | Serves as a supporting scaffold for immune cells |
| Helpful Hints | Look for the darkly stained, highly-branched reticular fibers |
Figure 3: Reticular connective tissue with and without illustration overlay
Check out our YouTube video to help you understand reticular connective tissue:
YouTube Video – Reticular Connective Tissue
Chapter Illustrations By:
Juan Manuel Ramiro-Diaz, Ph.D.
Soma Mukhopadhyay, Ph.D.
a thin layer of connective tissue that anchors the cells of epithelial tissues to the deeper structures of our body
Collagen is a fibrous protein that has many structural types
provide elasticity to tissues - the ability to be pulled and recoil like a rubber band
cell type found in connective tissue that actively produces the proteins in the extracellular matrix
containing blood vessels
mature cells found in adipose (fat) tissue - filled with a large storage sac of lipid. The lipid in these cells does not stain well with H&E staining, so the cells appear empty.
a specialized type of collagen that forms branching networks (branches on a tree) instead of long, linear strands
mature cells found in reticular connective tissue, responsible for making reticular fibers
phagocytic cells that engulf (phagocytose) cellular debris or pathogens
a type of lymphocyte (white blood cell) that is part of our adaptive immune system (i.e., B-cells and T-cells)
