The oral mucosa is one of the most critical early responding tissues in the body. Dysfunction of this mucosa can lead to reduced oral intake and severe pain, limiting the ability of the patient to continue with treatment.
Microscopic Structure of the Oral Mucosa
The oral mucosa is a mix of keratinising and non-keratinising epithelium. The epithelium of the tongue, the attached gingivae, the hard palate and other masticatory surfaces is usually keratinising; the remainder of the oral cavity (and oropharynx) is non-keratinising. Beneath the muscoal layer are numerous minor savliary glands which vary in concentration. The neurovascular supply to the mucosa lies in the submucosa, together with connective tissue.
Unlike the keratinising epithelium of the skin, the oral mucosa epithelium has a much short life span of about 5 days between generation and loss from the surface layer.
The first reaction is similar to the skin, with erythema developing of the affected mucosa. This takes 5 - 10 days to develop with conventional fractionation, with doses over 20 Gy. Loss of the epithelium is known as mucousitis, and becomes apparent between 10 - 15 days. This is initially patchy but gradually becomes confluent with increasing dose (usually 11 - 20 days to develop).
The latency in these effects is due to the gradual loss of dividing cells in the basal layer of the epithelium. The differentiating cells are relatively resistant to radiation.
An important feature of the oral mucosa is the potential for repopulation. In response to loss of a significant part of the stem cell population, the surviving cells decrease their cell cycle time and tend to create more stem cells rather than differentiating cells. This change in kinetics of the stem cell population prevents further worsening of mucousitis beyond grade III unless dose is given rapidly or other factors intervene. Repopulation begins after 1 week, and is fast enough to counter normal fractionation after two weeks.