This is the most widespread and abundant type of tissue in the human body. Its function is primarily to support, anchor and connect various parts of the body. Although connective tissue exists in a number of forms, all types have three basic structural elements -- cells, fibres and intercellular substance (ground substance).
The most common cell types are fibroblasts, which produce fibres and other intercellular materials. The two most common types of fibres are: collagen (collagenous) and elastic. Collagen fibres are for strength while the elastic ones are for elasticity of the tissue. Both the cells and the fibres are embedded in the intercellular substance. The consistency of this substance is highly variable from gelatin-like to a much more rigid material.
The proportions of the cells, fibres, and intercellular substance vary, depending on a particular nature and function of the connective tissue. For example, a strong connective tissue needs a greater proportion of the collagen fibres and fewer cells. An example would be a dense regular connective tissue, which is found in tendons and ligaments. On the other hand, a connective tissue composed of mostly cells would not be very strong. An example would be an adipose (fat) connective tissue.
Classification of Connective Tissue
I. Connective Tissue Proper -- encompasses all organs and body cavities connecting one part with another and, equally important, separating one group of cells from another. This is a very large and diverse group of tissues and includes adipose tissue (fat), areolar (loose) tissue, and dense regular tissue, among others.
II. Specialized Connective Tissues -- this group includes cartilage, bone, and blood. Cartilage and bone form the skeletal framework of the body while blood is the vascular (transport) tissue of animals.
I. Connective tissue proper
a) Areolar (Loose) Connective Tissue (Slide #31)
Areolar connective tissue is the most widespread connective tissue of the body. It is used to attach the skin to the underlying tissue. It also fills the spaces between various organs and thus holds them in place as well as cushions and protects them. It also surrounds and supports the blood vessels.
The fibres of areolar connective tissue are arranged in no particular pattern but run in all directions and form a loose network in the intercellular material. Collagen (collagenous) fibres are predominant. They usually appear as broad pink bands. Some elastic fibres, which appear as thin, dark fibres are also present. Examine slide #31 and locate these two types of fibres (Figure 10).
The cellular elements, such as fibroblasts, are difficult to distinguish in the areolar connective tissue. But, one type of cells - the mast cells are usually visible. They have course, dark-staining granules in their cytoplasm. Since the cell membrane is very delicate it frequently ruptures in slide preparation, resulting in a number of granules free in the tissue surrounding the mast cells. The nucleus in these cells is small, oval and light-staining, and may be obscured by the dark granules.
Figure 10A: Schematic representation of the areolar connective tissue.
Figure 10B: Microscopic view of areolar connective tissue.
Loose or areolar connective tissue. Thick pink bands are the protein collogen, while the thin dark threads are the protein elastin.
b) Adipose Connective Tissue (Slide #32)
The cells of adipose (fat) tissue are characterized by a large internal fat droplet, which distends the cell so that the cytoplasm is reduced to a thin layer and the nucleus is displaced to the edge of the cell. These cells may appear singly but are more often present in groups (Figure 11). When they accumulate in large numbers, they become the predominant cell type and form adipose (fat) tissue.
Adipose tissue, in addition to serving as a storage site for fats (lipids), also pads and protects certain organs and regions of the body. As well, it forms an insulating layer under the skin which helps regulate body temperature.
Examine slide #32 and note that on your microscopic slides (and in Figure 11), the lipids have actually been removed and the cells appear empty.
Figure 11A: Schematic representation of the adipose connective tissue.
Figure 11B: Microscopic view of adipose connective tissue.
More pictures of adipose tissue.
Why is the nucleus of adipose cells located at the outer edge of the cells?
b) Dense (Fibrous) Regular Connective Tissue (Slide #33)
Dense connective tissue is characterized by an abundance of fibres with fewer cells, as compared to the loose connective tissue. It is also called fibrous or collagenous connective tissue because of the abundance of collagen (collagenous) fibres. Little intercellular substance is present. Furthermore, in this tissue type, the fibres are organized in a regular, parallel pattern (Figure 12). Hence, the name – dense regular (fibrous or collagenous) connective tissue.
Examine slide #33 of the tendon. In addition to the tendons, this type of tissue is also found in ligaments. Hence, the function of this tissue is to anchor skeletal muscle to bone, to attach bone to bone as well as to stabilize the bones within a joint. On your slide, note that the collagen fibres are parallel to one another. Fibroblasts are the only cells visible, and are arranged in rows between the fibres. These fibroblasts function to lay down or create the fibres of the tissue (Figure 12).
Figure 12A: Schematic representation of dense regular connective tissue.
Figure 12B: Microscopic view of tendon
Dense (Fibrous) Regular Connective Tissue
II. Specialized Connective Tissues
a) Cartilage (Slide #16)
Cartilage is a somewhat elastic, pliable, compact type of connective tissue. It is characterized by three traits: lacunae, chondrocytes, and a rigid matrix. The matrix is a firm gel material that contains fibres and other substances. There are three basic types of cartilage in the human body: hyaline cartilage, elastic cartilage and fibrocartilage. In this laboratory, you will examine the most common type of cartilage, the hyaline cartilage. Most of the skeleton of the mammalian fetus is composed of hyaline cartilage. As the fetus ages, the cartilage is gradually replaced by more supportive bone. In the mammalian adult, hyaline cartilage is mainly restricted to the nose, trachea, bronchi, ends of the ribs, and the articulating surfaces of most joints. The function of the hyaline cartilage is to provide slightly flexible support and reduce friction within joints. It also provides structural reinforcement.
Re-examine slide #16 of a cross section of the trachea. Use Figure 13 to locate the band of hyaline cartilage on your slide. The matrix appears as a smooth, solid, blue or pink-coloured substance. Fine protein fibres, are embedded in the matrix, but they are not visible with the light microscope since they do not stain well. Locate the large cartilage cells called chondrocytes, which are trapped within the matrix in spaces called lacunae (singular, lacuna).
Cartilage is a non-vascular tissue. As such, the chondrocytes rely on blood vessels in the tissue surrounding the cartilage for nutrient supply and waste removal. Considering this structural feature, can you make a general comment as to the potential "thickness" of cartilage?
Figure 13A: Schematic representation of hyaline cartilage.
Figure 13B: Microscopic view of hyaline cartilage.