Objectives
- Identify and distinguish different types of connective tissue (CT)
- Identify types of cells found in CT
Connective Tissue Overview
Connective tissue, as it’s name implies, joins different tissues and structures together. For example, tendons are a type of CT which join muscles to bone. CT has 3 components: cells, fibers, and amorphous ground substance.
The fibers and amorphous ground substance collectively form the extracellular matrix (ECM). The extreme differences in properties of different CTs are due to different combinations of properties of constituents. For example, blood is an unusual liquid CT with many red and white cells and a liquid ECM. The fibers are potential fibers in solution (fibrinogen) that only become actual fibers (fibrin) when blood clots into a gelatinous semisolid. On the other end of the spectrum, we have concrete-like CT in bone. Here, the ECM is filled with crystallized calcium salts. Tendons have many collagen fibers arranged parallel to one another like strands in a rope, for tensile strength. One commonly used schema for classification of adult CTs is shown below:
Connective tissues proper
- General types
- Loose (areolar)
- Examples: hypodermis, mesenteries, omentum, lamina propria
- Dense Irregular
- Examples: dermis, periosteum, organ capsules
- Dense Regular
- Examples: tendons, ligaments
- Loose (areolar)
- Embryonic types
- Mesenchyme (middle embryonic germ layer)- gives rise to almost all types of CT
- Mucous CT- found in the umbilical cord
- Special types
- Adipose tissue- found throughout the body
- Hemopoietic tissue (produces blood cells)- covered in a future lesson
- Cartilage & Bone- covered in a future lesson
- Blood- covered in a future lesson
- Lymphatic tissue- covered in a future lesson
CT Fibers:
- Collagen fibers- most abundant type, flexible, tensile strength, stain well with eosin
- Reticular fibers– provide supportive framework, can be seen with periodic acid-Schiff (PAS) stain or silver stains
- Elastic fibers– allow for stretch and distension of tissue, don’t stain well with eosin, stain better with dyes such as orcein or resorcin-fuchsin
CT Cells:
- Fibroblasts– principal cell of CT, synthesis of CT fibers and ECM components
- in typical H&E stains, the cytoplasm may not be visible so look for nuclei that are elongated, disc-like structures and you may see a nucleolus
- Myofibroblasts are similar cells that also display properties of smooth muscle cells but not readily visible in typical H&E stains
- Macrophages (aka histiocytes)- easily visible if displaying phagocytic activity, kidney-shaped nucleus
- Adipocytes– store fat and produce hormones; appear with a washed-out nucleus found at the edge of the cell. Large numbers of these cells accumulate as adipose tissue.
- Mast cells– better seen with fixation and basic dyes which stain the granule-filled cytoplasm
- Adult stem cells– reside in niches in various tissues and organs, difficult to distinguish from other cells
Wandering/transient cell population:
- Lymphocytes
- Plasma cells
- Neutrophils
- Eosinophils
- Basophils
- Monocytes
>CT Slides
This is an example of loose connective tissue (CT), (also known as areolar CT). This was tissue surrounding an organ (mesentery) and spread on a slide. This type of tissue can be found throughout the body. This specimen was stained with the typical H&E stain but also a special stain that highlights elastic fibers which will appear as thin, dark lines throughout the slide. You will also notice some thicker, pink lines, these are collagen fibers. You will also see nuclei throughout the specimen stained purple by the hematoxylin. Notice that the arrangement of fibers is random and relatively sparse, the tissue is not packed with fibers giving it the “loose” nature.
This specimen was taken at the junction of the stomach and the duodenum of the small intestine. For now, focus on the connective tissue found in this specimen, you will review this slide again in the gastrointestinal (GI) lesson. To figure out where the CT is located in this specimen, you should have a general idea of the layers of the (GI) tube structures. The 4 basic layers starting from the lumen and moving outward include:
- Mucosa (M)= surface epithelium + lamina propria (loose CT) + muscularis mucosae
- Submucosa (SM)= layer of dense irregular CT
- Muscularis externa (ME)= a thick layer consisting of multiple layers of muscle
- Serosa (S)= mesothelium (simple squamous epithelium) covering a thin layer of CT (some organs do not have a serosa or a complete layer of serosa and it would be called adventitia which is the CT without the mesothelial cells.
In this slide, you will want to start at the lumen, identify the epithelium and then look for the lamina propria layer. Once you’ve examined that, move a little deeper in the specimen and look for the submucosa. Also view the outer edge of the organ to see if you can identify the CT in the serosa/adventitia layer. The specific stain used in this slide stains the collagen fibers green.
This neurovascular bundle was previously used to identify endothelium. Now you should turn your focus on examining the connective tissue. Neurovascular bundles are found throughout the body and contain loose CT and adipose tissue. You should be able to identify adipocytes in this specimen, look for a thin cytoplasm and the nucleus pushed to the edge of the cell. The center of the cell will appear white since solvents in the processing of tissue removes fat. Arteries and veins also contain thin layers of CT just deep to the endothelium in the subendothelial layer and in the most outer layer. The thickness and visibility of this layer depends on the size of the vessel (easier to identify in larger vessels). The CT of the outer layer of vessels blends into the surrounding CT. There are also numerous capillaries in both cross-section and longitudinal section found throughout the CT holding the neurovasculature bundle together. More details on the different layers of CT in the peripheral nerve will be covered in the nerve tissue lesson.
Skin contains multiple structures (vessels, nerves) and therefore, multiple types of connective tissue. The dermis is filled with dense irregular CT and regions of adipose tissue (loose CT). Dense irregular CT is “dense” because there are more fibers per unit volume compared to loose CT. The “irregular” description comes from the arrangement of fibers creating an irregular pattern. While examining the dense irregular CT, look for the resident fibroblasts. Skin also contains elastic fibers along with collagen fibers but the elastic fibers don’t stain well with H&E.
Tendons connect muscle to bone and are a great example of dense regular CT. Tendons have a large volume of collagen fibers arranged in a regular pattern and you will see the nuclei of fibroblasts scattered throughout the tissue. The regular arrangement of fibers gives tendons more tensile strength. Compare this CT to the dense irregular CT of the skin in the specimen above.
>More Practice
The jejunum is 1 of 3 parts of the small intestine. The tissue projects into the lumen as finger-like projections known as villi. The small intestines contain loose CT in a layer known as the lamina propria. To identify this layer, start with the simple columnar epithelium located in the lumen and just deep to this layer is the lamina propria. When examining the CT, look for the long, thin nuclei of fibroblasts. You may also see some wandering cells such as eosinophils, plasma cells, or lymphocytes. To distinguish the wandering cells from fibroblasts, look at the shape of the nuclei. Eosinophils have a 2-3 lobed nucleus and red granules in the cytoplasm. Plasma cells have a single round nucleus with euchromatin and heterochromatin arranged in a cartwheel appearance at the edge. Lymphocytes have very small, round nuclei compared to other cells and very small amount of cytoplasm.
The lung provides a good example of immune cells known as pulmonary macrophages. These cells pass from the blood through the pulmonary capillaries and across the epithelium of the alveoli (air sacs). Macrophages are typically very large cells so they stand out from the rest. The specific name of the macrophage cell changes depending on the organ it is found. You will learn more specifics as you move through different organs systems.
The lymph node has a CT capsule and this node has been stained with a special stain to highlight the reticular fibers which gives the organ more structure. The carbohydrates found along the reticular fibers reduce silver ions in the stain causing a deposition of silver metal which stains the fibers black.
>More Practice Answers
Slide 52 Jejunum
2. Identify:
2a) Simple columnar epithelium with microvilli
2b) Loose CT
3. Identify:
- Green arrows identify the fibroblast nuclei (long/flat). Other nuclei in the specimen are a mixture of wandering cells (eosinophils, plasma cells, lymphocytes). The quality of the image makes it tricky to identify these but you would look at the shape of the nucleus to determine the type of cell.
4. Identify:
- Dense irregular CT- notice there isn’t a regular pattern, lots of swirls of the collagen represented by the dark pink
5. Identify:
- Loose CT
Slide 38 Lung
- Macrophages
- Wandering cells
- Macrophages
Slide 25 Lymph Node and Liver (focus on lymph node for this lesson)
1. Identify:
- Dense CT
- Reticular fibers are formed by Type III collagen
