(C16 - 19) Slide 29: Trachea and Esophagus, PAS

Using this section, we are not only going to study the appearance and composition of hyaline cartilage, but we will also try to solidify the concepts of appositional and interstitial growth. Because hyaline cartilage and elastic cartilage are so similar, much of what you learn from this slide of hyaline cartilage will apply to the elastic cartilage in the next histological section.

The hyaline cartilage rings in the trachea hold the trachea open and support the soft tissue associated with the organ (C16, T). When ever you look at a slide remember to start at low magnification, do the cruise, get hip and bop, scan the whole slide, and find a good area of tissue to study. Get into the habit of scanning the entire slide, so that you don’t get hung up in some insignificant corner of a slide and miss the big picture. Now find some great hyaline cartilage. Notice how blah the matrix appears (C17, M). You can’t make out anything other than the chondrocytes (C17, c) in the matrix. It appears very homogeneous because proteoglycan aggregates and type II collagen fibrils are not recognizable in most light microscopic preparations.

Now, move over to the edge of the cartilage and appreciate the perichondrium and the progression of cells as they differentiate from perichondrial fibroblasts to chondrocytes (C18). The perichondrium is composed of dense irregular CT and has flattened perichondrial fibroblasts (C18, p). These fibroblasts differentiate into chondroblasts, when they start secreting the components of the cartilage matrix. When chondroblasts are completely surrounded by matrix, they are called chondrocytes. This growth process is appositional growth = growth from the outside (addition of cells by recruitment and matrix production). During their transformation, perichondrial fibroblasts stop secreting type I collagen, fibronectin and predominantly hyaluronic acid and start secreting type II collagen, chondronectin and predominantly chondroitin sulfate. Don’t spend a lot of time trying to distinguish fibroblasts from chondroblasts. You will never be asked to identify a chondroblast. However, perichondrial fibroblasts and chondrocytes are fair game.

Hint: Remember chondrocytes reside in lacunae.

Morphologically, chondrocytes have an eccentric nucleus, a large lipid vacuole and abundance of glycogen. In this preparation, the nucleus is purple, the glycogen is magenta and the lipid vacuole is colorless. The lipid was lost when the tissue was prepared and therefore, there is nothing in the vacuole to stain. Just outside the chondrocyte is a territorial matrix. It usually stains more intensely than the interterritorial matrix. Here, you will see the territorial or capsular matrix as a faint bluish shadow around the chondrocytes. You will not be able to make out the lacunae on this slide because the tissue was fixed too well. Lacunae are the potential spaces that are occupied by the chondrocytes. In poorly fixed tissue, the chondrocytes shrink more than the matrix and spaces are observable.

The clusters of chondrocytes are called isogenous groups (C19, I). All of the cells in an isogenous group had the same precursor cell. Isogenous groups are a result of cell division and along with the production of matrix, they achieve interstitial growth. As more territorial matrix is produced by the chondrocytes, the distances between them increases. The matrix closest to a chondrocyte or isogenous group is the territorial matrix and that between chondrocytes or isogenous groups is called the interterritorial matrix. The interterritorial matrix stains less intensely than the territorial matrix.