Actinic keratoses ("AKs") most commonly present as a white, scaly plaque of variable thickness with surrounding redness; they are most notable for having a sandpaper-like texture when felt with a gloved hand. Skin nearby the lesion often shows evidence of solar damage characterized by notable pigmentary alterations, being yellow or pale in color with areas of hyperpigmentation; deep wrinkles, coarse texture, purpura and ecchymoses, dry skin, and scattered telangiectasias are also characteristic. Photoaging leads to an accumulation of oncogenic changes, resulting in a proliferation of mutated keratinocytes that can manifest as AKs or other neoplastic growths. With years of sun damage, it is possible to develop multiple AKs in a single area on the skin.

The lesions are usually asymptomatic, but can be tender, itch, bleed, or produce a stinging or burning sensation. AKs are typically graded in accordance with their clinical presentation: Grade I (easily visible, slightly palpable), Grade II (easily visible, palpable), and Grade III (frankly visible and hyperkeratotic).

Actinic keratoses can have various clinical presentations, often characterized as follows:

- Classic (or common): Classic AKs present as white, scaly macules, papules or plaques of various thickness, often with surrounding erythema. They are usually 2-6mm in diameter but can sometimes reach several centimeters in diameter.

- Hypertrophic (or hyperkeratotic): Hypertrophic AKs (HAKs) appear as a thicker scale or rough papule or plaque, often adherent to an erythematous base. Classic AKs can progress to become HAKs, and HAKs themselves can be difficult to distinguish from malignant lesions.

- Atrophic: Atrophic AKs lack an overlying scale, and therefore appear as a nonpalpable change in color (or macule). They are often smooth and red, and are less than 10mm in diameter.

- AK with cutaneous horn: A cutaneous horn is a keratinic projection with its height at least one-half of its diameter, often conical in shape. They can be seen in the setting of actinic keratosis as a progression of an HAK, but are also present in other skin conditions. 38–40% of cutaneous horns represent AKs.

- Pigmented AK: Pigmented AKs are rare variants that often present as macules or plaques that are tan to brown in color. They can be difficult to distinguish from a solar lentigo or lentigo maligna.

- Actinic cheilitis: When an AK forms on the lip, it is called actinic cheilitis. This usually presents as a rough, scaly patch on the lip, often accompanied by the sensation of dry mouth and symptomatic splitting of the lips.

- Bowenoid AK: Usually presents as a solitary, erythematous, scaly patch or plaque with well-defined borders. Bowenoid AKs are differentiated from Bowen's disease by degree of epithelial involvement as seen on histology.

The presence of ulceration, nodularity, or bleeding should raise concern for malignancy. Specifically, clinical findings suggesting an increased risk of progression to squamous cell carcinoma can be recognized as "IDRBEU": I (induration /inflammation), D (diameter > 1 cm), R (rapid enlargement), B (bleeding), E (erythema) and U (ulceration). AKs are usually diagnosed clinically, but because they are difficult to clinically differentiate from squamous cell carcinoma, any concerning features warrant biopsy for diagnostic confirmation.

Seborrheic keratoses may be divided into the following types:

- Common seborrheic keratosis (basal cell papilloma, solid seborrheic keratosis)

- Reticulated seborrheic keratosis (adenoid seborrheic keratosis)

- Stucco keratosis (digitate seborrheic keratosis, hyperkeratotic seborrheic keratosis, serrated seborrheic keratosis, verrucous seborrheic keratosis) -- Often are light brown to off-white. Pinpoint to a few millimeters in size. Often found on the distal tibia, ankle, and foot.

- Clonal seborrheic keratosis

- Irritated seborrheic keratosis (inflamed seborrheic keratosis)

- Seborrheic keratosis with squamous atypia

- Melanoacanthoma (pigmented seborrheic keratosis)

- Dermatosis papulosa nigra—Commonly found among adult dark-skinned individuals, presents on the face as small benign papules from a pinpoint to a few millimeters in size.

- Inverted follicular keratosis

Also see:

- The sign of Leser-Trélat

A seborrheic keratosis, also known as seborrheic verruca, basal cell papilloma, or a senile wart, is a non-cancerous (benign) skin tumour that originates from cells in the outer layer of the skin (keratinocytes). Like liver spots, seborrheic keratoses are seen more often as people age.

The tumours (also called lesions) appear in various colours, from light tan to black. They are round or oval, feel flat or slightly elevated, like the scab from a healing wound, and range in size from very small to more than across. They can often come in association with other skin conditions, including basal cell carcinoma. Sometimes seborrheic keratosis and basal cell carcinoma occur at the same location, and sometimes seborrheic keratosis progresses to basal cell carcinoma. At clinical examination the differential diagnosis include warts and melanoma. Because only the top layers of the epidermis are involved, seborrheic keratoses are often described as having a "pasted on" appearance. Some dermatologists refer to seborrheic keratoses as "seborrheic warts", because they resemble warts, but strictly speaking the term "warts" refers to lesions that are caused by human papillomavirus.

Most cases of leukoplakia cause no symptoms, but infrequently there may be discomfort or pain. The exact appearance of the lesion is variable. Leukoplakia may be white, whitish yellow or grey. The size can range from a small area to much larger lesions. The most common sites affected are the buccal mucosa, the labial mucosa and the alveolar mucosa, although any mucosal surface in the mouth may be involved. The clinical appearance, including the surface texture and color, may be homogenous or non-homogenous (see: classification). Some signs are generally associated with a higher risk of cancerous changes (see: prognosis).

Leukoplakia may rarely be associated with esophageal carcinoma.

In the context of lesions of the mucous membrane lining of the bladder, leukoplakia is a historic term used to describe a visualized white patch which histologically represents keratinization in an area of squamous metaplasia. The symptoms may include frequency, suprapubic pain (pain felt above the pubis), hematuria (blood in the urine), dysuria (difficult urination or pain during urination), urgency, and urge incontinence. The white lesion may be seen during cystoscopy, where it appears as a whitish-gray or yellow lesion, on a background of inflamed urothelium and there may be floating debris in the bladder. Leukoplakia of the bladder may undergo cancerous changes, so biopsy and long term follow up are usually indicated.

Keratosis (from "keratinocyte", the prominent cell type in the epidermis, and , abnormal) is a growth of keratin on the skin or on mucous membranes. More specifically, it can refer to:

- actinic keratosis (also known as solar keratosis)

- hydrocarbon keratosis

- keratosis pilaris (KP, also known as follicular keratosis)

- seborrheic keratosis

Actinic keratoses are pre-malignant growths. Seborrheic keratoses are not pre-malignant.

Characteristics include a blue/black stain of skin initially. Skin is thin, about 4-5 cell layers thick, which is often related to aging. Histological features include epidermal atrophy and increased number of melanocytes.

AC almost always affects the lower lip and only rarely the upper lip, probably because the lower lip is more exposed to the sun. In the unusual cases reported where it affects the upper lip, this may be due to upper lip prominence. The commissures (corners of the mouth) are not usually involved.

Affected individuals may experience symptoms such as a dry sensation and cracking of the lips.

It is usually painless and persistent.

The appearance is variable. White lesions indicate hyperkeratosis. Red, erosiive or ulcerative lesions indicate atrophy, loss of epithelium and inflammation. Early, acute lesions may be erythematous (red) and edematous (swollen). With months and years of sun exposure, the lesion becomes chronic and may be grey-white in color and appear dry, scaly and wrinkled.

There is thickening whitish discoloration of the lip at the border of the lip and skin. There is also a loss of the usually sharp border between the red of the lip and the normal skin, known as the vermillion border. The lip may become scaly and indurated as AC progresses.

When palpated, the lip may have a texture similar to rubbing the gloved finger along sandpaper.

AC may occur with skin lesions of actinic keratosis or skin cancer elsewhere, particularly on the head and neck since these are the most sun exposed areas. Rarely it may represent a genetic susceptibility to light damage (e.g. xeroderma pigmentosum or actinic prurigo).

Oral lichen planus (also termed "oral mucosal lichen planus"), is a form of mucosal lichen planus, where lichen planus involves the oral mucosa, the lining of the mouth. This may occur in combination with other variants of lichen planus. Six clinical forms of oral lichen planus are recognized:

- "Reticular", the most common presentation of oral lichen planus, is characterised by the net-like or spider web-like appearance of lacy white lines, oral variants of Wickham's straiae. This is usually asymptomatic.

- "Erosive/ulcerative", the second most common form of oral lichen planus, is characterised by oral ulcers presenting with persistent, irregular areas of redness, ulcerations and erosions covered with a yellow slough. This can occur in one or more areas of the mouth. In 25% of people with erosive oral lichen planus, the gums are involved, described as desquamative gingivitis (a condition not unique to lichen planus). This may be the initial or only sign of the condition.

- "Papular," with white papules.

- "Plaque-like" appearing as a white patch which may resemble leukoplakia.

- "Atrophic," appearing as areas. Atrophic oral lichen planus may also manifest as desquamative gingivitis.

- "Bullous," appearing as fluid-filled vesicles which project from the surface.

These types often coexist in the same individual. Oral lichen planus tends to present bilaterally as mostly white lesions on the inner cheek, although any mucosal site in the mouth may be involved. Other sites, in decreasing order of frequency, may include the tongue, lips, gingivae, floor of the mouth, and very rarely, the palate.

Generally, oral lichen planus tends not to cause any discomfort or pain, although some people may experience soreness when eating or drinking acidic or spicy foodstuffs or beverages. When symptoms arise, they are most commonly associated with the atrophic and ulcerative subtypes. These symptoms can include a burning sensation to severe pain. Lichen planus, particularly when concomitant oral or genital lesions occur, significantly affects patients’ quality of life.

Lichen planus affecting mucosal surfaces may have one lesion or be multifocal. Examples of lichen planus affecting mucosal surfaces include:

- "Esophageal lichen planus", affecting the esophageal mucosa. This can present with difficulty or pain when swallowing due to oesophageal inflammation, or as the development of an esophageal stricture. It has also been hypothesized that it is a precursor to squamous cell carcinoma of the esophagus.

- "Genital lichen planus," which may cause lesions on the glans penis or skin of the scrotum in males, and the vulva or vagina in females. Symptoms may include lower urinary tract symptoms associated with stenosis of the urethra, painful sexual intercourse, and itching. In females, "Vulvovaginal-gingival syndrome," is severe and distinct variant affecting the vulva, vagina, and gums, with complications including scarring, vaginal stricture formation, or vulva destruction. The corresponding syndrome in males, affecting the glans penis and gums, is the "peno-gingival syndrome". It is associated with HLA-DQB1.

An invasive tumor arising from a classical lentigo maligna. Usually a darkly pigmented raised papule or nodule, arising from a patch of irregularly pigmented flat brown to dark brown lesion of sun exposed skin of the face or arms in an elderly patient.

Microscopically, inverted follicular keratosis is characterized as a well-circumscribed inverted acanthotic squamous proliferation containing squamous eddies and without significant atypia.

Inverted follicular keratosis is a skin condition characterized by asymptomatic, firm, white–tan to pink papules and considered as a subtype of seborrheic keratosis.

A stucco keratosis is a common benign skin condition characterized by a lesion with a dull or lackluster surface, and with church-spire-like projections of epidermal cells around collagen seen histologically. Stucco keratoses are often light brown to off-white, and are no larger than a few millimeters in diameter. They are often found on the distal tibia, ankle, and foot.

Other names for a stucco keratosis are "digitate seborrheic keratosis," "dyperkeratotic seborrheic keratosis," "deratosis alba," "serrated seborrheic keratosis," and "verrucous seborrheic keratosis".

Lentigo maligna (also known as "lentiginous melanoma on sun-damaged skin") is a melanoma "in situ" that consists of malignant cells but does not show invasive growth. Lentigo maligna is not the same as lentigo maligna melanoma, and should be discussed separately. It typically progresses very slowly and can remain in a non-invasive form for years. The transition to true melanoma is marked by the appearance of a bumpy surface (itself a marker of vertical growth and invasion), at which point it is called lentigo maligna melanoma. It is normally found in the elderly (peak incidence in the 9th decade), on skin areas with high levels of sun exposure like the face and forearms. Some authors do not consider lentigo maligna to be a melanoma. It is commonly thought of as a melanoma precursor. Incidence of evolution to lentigo maligna melanoma is very low, about 2.2% to 5% in elderly patients.

It is also known as "Hutchinson's melanotic freckle". This is named for Jonathan Hutchinson.

Lentigo maligna melanoma is a melanoma that has evolved from a lentigo maligna. They are usually found on chronically sun damaged skin such as the face and the forearms of the elderly. The nomenclature is very confusing to both patients and physicians alike.

Lentigo maligna is the non-invasive skin growth that some pathologists consider to be a melanoma-in-situ. A few pathologists do not consider lentigo maligna to be a melanoma at all, but a precursor to melanomas. Once a lentigo maligna becomes a lentigo maligna melanoma, it is treated as if it were an invasive melanoma.

Actinic cheilitis (abbreviated to AC, also termed actinic cheilosis, actinic keratosis of lip, solar cheilosis, sailor's lip, farmer's lip), is cheilitis (lip inflammation) caused by long term sunlight exposure. Essentially it is a burn, and a variant of actinic keratosis which occurs on the lip. It is a premalignant condition, as it can develop into squamous cell carcinoma (a type of mouth cancer).

Porokeratosis is a specific disorder of keratinization that is characterized histologically by the presence of a cornoid lamella, a thin column of closely stacked, parakeratotic cells extending through the stratum corneum with a thin or absent granular layer.

SCC of the skin begins as a small nodule and as it enlarges the center becomes necrotic and sloughs and the nodule turns into an ulcer.

- The lesion caused by SCC is often asymptomatic

- Ulcer or reddish skin plaque that is slow growing

- Intermittent bleeding from the tumor, especially on the lip

- The clinical appearance is highly variable

- Usually the tumor presents as an ulcerated lesion with hard, raised edges

- The tumor may be in the form of a hard plaque or a papule, often with an opalescent quality, with tiny blood vessels

- The tumor can lie below the level of the surrounding skin, and eventually ulcerates and invades the underlying tissue

- The tumor commonly presents on sun-exposed areas (e.g. back of the hand, scalp, lip, and superior surface of pinna)

- On the lip, the tumor forms a small ulcer, which fails to heal and bleeds intermittently

- Evidence of chronic skin photodamage, such as multiple actinic keratoses (solar keratoses)

- The tumor grows relatively slowly

Porokeratosis may be divided into the following clinical types:

- "Plaque-type porokeratosis" (also known as "Classic porokeratosis" and "Porokeratosis of Mibelli") is characterized by skin lesions that start as small, brownish papules that slowly enlarge to form irregular, annular, hyperkeratotic or verrucous plaques. Sometimes they may show gross overgrowth and even horn-like structures may develop. Skin malignancy, although rare, is reported from all types of porokeratosis. Squamous cell carcinomata have been reported to develop in Mibelli's type porokeratosis over partianal areas involving anal mucosa. This was the first report mentioning mucosal malignancy in any form of porokeratosis.

- "Disseminated superficial porokeratosis" is a more generalized processes and involves mainly the extremities in a bilateral, symmetric fashion. In about 50% of cases, skin lesions only develop in sun-exposed areas, and this is referred to as "disseminated superficial actinic porokeratosis"

- "Porokeratosis palmaris et plantaris disseminata" is characterized by skin lesions that are superficial, small, relatively uniform, and demarcated by a distinct peripheral ridge of no more than 1mm in height.

- "Linear porokeratosis" is characterized clinically skin lesions are identical to those of classic porokeratosis, including lichenoid papules, annular lesions, hyperkeratotic plaques with central atrophy, and the characteristic peripheral ridge.

- "Punctate porokeratosis" is a skin condition associated with either classic porokeratosis or linear porokeratosis types of porokeratosis, and is characterized by multiple, minute, and discrete punctate, hyperkeratotic, seed-like skin lesions surrounded by a thin, raised margin on the palms and soles.

- "Porokeratosis plantaris discreta" is a skin condition that occurs in adults, with a 4:1 female preponderance, characterized by a sharply marginated, rubbery, wide-based papules. It is also known as "Steinberg's lesion". It was characterized in 1970.

Melanoacanthoma (also known as "Pigmented seborrheic keratosis") is a common, benign, darkly pigmented cutaneous condition characterized by a skin lesion with a dull or lackluster surface.

It involves a proliferation of keratinocytes and melanocytes.

Keratosis pilaris results in small bumps on the skin that feel like rough sandpaper. They are skin-colored bumps the size of a grain of sand, many of which are surrounded by a slight pink color in light skinned people and dark spots in dark skinned people. Most people with keratosis pilaris do not have symptoms but the bumps in the skin can occasionally be itchy.

Though people with keratosis pilaris experience the condition year-round, the problem can become exacerbated and the bumps are likely to look and feel more pronounced in color and texture during the colder months when moisture levels in the air are lower. The symptoms may also worsen during pregnancy or after childbirth.

Irritated seborrheic keratosis (also known as "Basosquamous cell acanthoma," and "Inflamed seborrheic keratosis") is a common benign cutaneous condition characterized by a skin lesion with a dull or lackluster surface.

Disseminated superficial actinic porokeratosis (DSAP) is a non-contagious skin condition with apparent genetic origin in the SART3 gene. It most often presents in sun-exposed areas of the body. Some DSAP cases have been reported in patients with acute immune compromised situations, particularly in the elderly. For those with sun damaged skin, the lesions usually begin to appear in the patient's 20s and increase in number and visibility in the 40s or 50s. Commonly, though not always, the number and visibility of lesions is in direct proportion to the amount of sun damage to the affected area.

Lesions generally are characterized by an irregularly shaped thread-like ring that is usually the size of a pencil eraser, though lesions vary and may be half or double that size. The thread-like ring is very thin, much like fabric thread for sewing, and raised such that it is both palpable and visible. The interior of the ring may be rough like sandpaper, or smooth. The interior is often discolored, though colors vary from patient to patient. Lesions, due to their vascular nature, can also vary according to body temperature, environmental temperatures, and other external stimuli. The internal ring color is most often reddish, purplish, pink, or brown.

Some patients report itching and irritation associated with the condition, and many report no notable sensation. Although no known hormonal link has been found, DSAP occurs more commonly in women.

A study in 2000 was done on a Chinese family, in which a locus for a gene was located.