The exact pathogenesis is dependent upon the cause. Ulcers and erosions can be the result of a spectrum of conditions including those causing auto-immune epithelial damage, damage because of an immune defect (e.g., HIV, leukemia, infections e.g. herpes viruses) or nutritional disorders (e.g., vitamin deficiencies). Simple mechanisms which predispose the<unk>  to trauma and ulceration are xerostomia (dry<unk>  – as saliva usually lubricates the mucous membrane and controls bacterial levels) and epithelial atrophy (thinning, e.g., after radiotherapy), making the lining more fragile and easily breached. Stomatitis is a general term meaning inflammation within the<unk> , and often may be associated with ulceration. Pathologically, the<unk>  represents a transition between the gastrointestinal tract and the skin, meaning that many gastrointestinal and cutaneous conditions can involve the<unk> . Some conditions usually associated with the whole gastrointestinal tract may present only in the<unk> , e.g., orofacial granulomatosis/oral Crohn's disease. Similarly, cutaneous (skin) conditions can also involve the<unk>  and sometimes only the<unk> , sparing the skin. The different environmental conditions (saliva, thinner mucosa, trauma from teeth and food), mean that some cutaneous disorders which produce characteristic lesions on the skin produce only non specific lesions in the<unk> . The vesicles and bullae of blistering mucocutaneous disorders progress quickly to ulceration in the<unk> , because of moisture and trauma from food and teeth. The high bacterial load in the<unk>  means that ulcers may become secondarily infected. Cytotoxic drugs administered during chemotherapy target cells with fast turnovers such as malignant cells. However, the epithelia of the<unk>  also has a high turnover rate and makes oral ulceration (mucositis) a common side effect of chemotherapy. Erosions, which involve the epithelial layer, are red in appearance since the underlying lamina propria shows through. When the full thickness of the epithelium is penetrated (ulceration), the lesion becomes covered with a fibrinous exudate and takes on a yellow-grey color. Because an ulcer is a breach of the normal lining, when seen in cross section, the lesion is a crater. A "halo" may be present, which is a reddening of the surrounding mucosa and is caused by inflammation. There may also be edema (swelling) around the ulcer. Chronic trauma may produce an ulcer with a keratotic (white, thickened mucosa) margin. Malignant lesions may ulcerate either because the tumor infiltrates the mucosa from adjacent tissues, or because the lesion originates within the mucosa itself, and the disorganized growth leads to a break in the normal architecture of the lining tissues. Repeat episodes of<unk> s can be indicative of an immunodeficiency, signaling low levels of immunoglobulin in the oral mucous membranes. Chemotherapy, HIV, and mononucleosis are all causes of immunodeficiency/immunosuppression with which oral ulcers may become a common manifestation. Autoimmunity is also a cause of oral ulceration. Mucous membrane pemphigoid, an autoimmune reaction to the epithelial basement membrane, causes desquamation/ulceration of the oral mucosa. Numerous aphthous ulcers could be indicative of an inflammatory autoimmune disease called Behçet's disease. This can later involve skin lesions and uveitis in the eyes. Vitamin C deficiency may lead to scurvy which impairs wound healing, which can contribute to ulcer formation. For a detailed discussion of the pathophysiology of aphthous stomatitis, see Aphthous stomatitis#Causes.
Mouth ulcer
Geographic tongue
Stercoral ulcer
Dental abscess
Callous ulcer
Pouchitis
Skin fissure
Tropical ulcer