The human body is comprised of 4 types of tissues. The four categories are as follows: epithelial, connective (including bone and blood cells), muscle, and nerve.
- Epithelial tissues are protective and cover the outside of organs and/or line the inside of organs. Epithelial tissues can have a single layer of cells or multiple layers of cells. Epithelial tissues are found above the basement membrane.
- Connective tissue has multiple functions, depending on location. It connects different layers of tissues, like we see in the dermis of skin.
- Muscle tissues contract
- Nerve tissues transmit impulses.
Of these tissues, epithelial and connective tissue can divide using mitosis; these cells can become cancerous when they lose the ability to regulate mitosis. If you are familiar with the anatomy of the human brain, you may be wondering where brain tumors come from? They come from the connective tissue surrounding the nerve cells. On the other hand, nerve cells can’t divide and don’t become cancerous. For the purposes of this article, we will focus on the epithelial cells, and explore why it may be more common to see invasive vs in situ cancers in various organs.
- The main role of epithelial tissues is to form a barrier, especially lining hollow organs such as the digestive tract, urinary system, and lungs.
- In some organs there are multiple layers of epithelial cells, and in other organs there is only a single layer of cells.
- Some examples of where multiple layers of epithelial cells can be found are:
- Esophagus, which is lined with stratified squamous epithelium.
- Bladder/ureter, which are lined with transitional epithelium.
- A good example of where you will find just one thin, single layer of cells is the lung. These cells are known as simple squamous cells. Although there is still a basement membrane between the epithelial tissue and the underlying connective tissue.
Let’s take a look at some slides of the bladder, esophagus, lung, and colon.
This is a slide of the bladder; see A-C.
- Transitional tissue
- Submucosa
- Muscularis.
Notice the transitional tissue, which consists of multiple layers of cells.
- In situ cancer is present when the cancerous cells are found only at the transitional tissue level.
- In order for cancer to be invasive it must invade the lamina propria, which is under the epithelial tissue. In situ cancer is when the cancerous cells are found only in the transitional tissue.
This is a slide of the esophagus; see A-C
- Epithelium, which consists of multiple layers of cells. “A” is pointing the the Basal layer of the stratified squamous cells.
- Submucosa
- Muscularis.
- When a pathology report states cancer to be in situ, it means the cancerous cells are only in the epithelial layer and have not crossed the basement membrane.
- In order for cancer to become invasive, it must cross the basement membrane into the lamina propria, muscularis mucosae, or submucosa.
This is the lung; see A, D, E.
A. Lung tissue
D. Bronchus and the
E. Blood vessel
- There is just one single layer of cells, and there aren’t various layers of other tissues in the lung.
- Most cancers in the lung are invasive.
- In situ cancer in the lung is rare.
This is colon.
A. Bottom of the epithelial layer.
B. Submucosa.
C. Muscularis propria.
- While it looks like the epithelial layer is stratified, these are actually folds of epithelial tissue forming ducts.
- The tissue is actually 1 cell thick.
- For AJCC, cancers that involve the lamina propria with no extension through the muscularis mucosae are in situ.
- For SEER, cancers that stay within the epithelium (intraepithelial) are in situ.
In summary, in situ cancer can be seen in tissues with multiple layers such as stratified squamous in the esophagus, and transitional epithelium in the bladder and ureters. In these instances, there are multiple layers of cells above the basement membrane and therefore the cancer can be detected before it is invasive. Tissues that have only a single layer of epithelial cells are more likely to have invasive cancer since the cells have direct contact to the basement membrane.
