author/s: Dr Francesc Tresserra i Casas
What layers form the wall of the colon?
- Epithelium: Constituted by cylindrical or cubic cells arranged in one single layer with presence of interwoven mucosecreting cells. Cylindrical cells show villi that increase the area of the cell to facilitate the ion and liquid absorption function.
- Basal membrane: It gives support to epithelial cells.
- Lamina Propia: It is constituted by laxus connective tissue housing the cells responsible for immunity (lymphocytes, plasmatic cells and macrophages), small vascular structures, mainly capillaries and lymph nodes and fine nervous structures.
- Muscularis mucosa: It is a thin layer of muscular fibres that separates the mucosa (constituted by the epithelium and the lamina propia) from the submucosa.
- Submucosa: It is constituted by connective tissue and fibroblasts. It contains the nerve plexuses located immediately after the muscularis mucosa (Meiner´s plexus) and vascular plexuses constituted by arterioles, venules and lymph nodes.
- Muscular propia: It is formed by an internal layer of smooth muscular fibres arranged in a circular fashion and an external layer arranged longitudinally. Between both layers there is a nervous plexus called Auerbach´s plexus. The muscular layers are crossed by blood and lymph vessels that run to the serosa.
- Subserosa: A layer of connective tissue that separates the muscular propia from the serosa.
- Serosa: It corresponds to the mesothelial layer that lines a large part of the colon, excepting its distal segment and the rectum.
What is a polyp?
A polyp is a benign proliferative lesion of the colorectal mucosa.
Polyps are classified according to their morphology into (see animation):
- Pediculated polyps: when they show a pedicle that unites the head of the polyp with the colorectal mucosa.
- Sessil polyps: when they are flat and are in direct contact with the rest of the colorectal mucosa.
According to their histological characteristics, they can be classified into:
- Hyperplastic o metaplastic polyps (see animation): : It is a non-neoplastic, benign lesion, constituted by elongated glands with a tortuous trajectory.
- Hamartomatous polyps: It is a benign lesion constituted by glands and smooth muscle.
- Inflammatory polyps: It is a benign lesion resulting from an inflammatory process.
- Adenomatous polyps: These are benign intraepithelial lesions constituted by dysplastic cells that can be:
- Tubular adenoma (see animation): When it is conformed by glands.
- Vellous adenoma (see animation): When it is formed by digitiform projections or villi.
- Tubular-vellous adenoma (see animation): When it is formed by glands and villi.
- Other types: Such as serrated adenomas (hyperplastic-adenomatous mixed, combined (coexistence of hyperplastic polyp and adenomatous polyp).
What polyps have the highest risk for malignisation?
The types of polyps that most frequently metastise are vellous and tubular-vellous adenomas, mainly in patients with syndromes of colorectal polyposis.
It is important to point out that the malignant transformation of a colorectal polyp is low, as the incidence of polyps is relatively high in the population. Polyps can remain stable over a long period of time. Hyperplastic polyps rarely become malignant (see animation).
What histological types of colorectal cancer exist?
The most frequent histological type is adenocarcinoma, that is tumours constituted by glands. Depending on the quantity of tumour that forms the glands, they are classified into well differentiated (when over 90% of the tumour forms glands), and moderately or poorly differentiated (when there is little or no formation of glands). The degree of differentiation is an important prognostic factor whereby well differentiated tumours have a more favourable behaviour. Other less frequent histological types are mucinous carcinoma, carcinoma of cells in a sealing ring, clear cell carcinoma and adenoesquamous carcinoma.
How does colorectal cancer spread?
The neoplastic lesion starts at the epithelium, be it in the context of an adenomatous polyp or in the original epithelium in the large intestine. In this period, the lesion is intraepithelial or in situ. In a subsequent phase, it will involve the lamina propia, transforming into an intramucosal lesion. Once it progresses beyond the muscular propia, it transforms into an infiltrating lesion that can spread to the rest of layers of the intestinal wall such as to the submucosal, muscular propia, subserosa, serosa, and from here, it can spread to the pericolonic fat and to adjoining organs. When the lesion is infiltrating it is capable of invading the wall of blood or lymph vessels, penetrating inside. Malignant cells can then move through these vessels, colonising lymph nodes or other organs at a distance, giving rise to metastases.