Lips

The lips (Latin: labia) are soft, mobile and reddish paired anatomical structures that encircle the anterior opening leading to the oral cavity. They are part of the digestive system located in the lower aspect of the face, right below the nasal cavity. Together with the cheeks, teeth and gums, the lips form the anterior portion of the oral cavity - the oral vestibule. They form the oral fissure through which the external environment connects with the oral cavity. The lips are the very first structures of the gastrointestinal tract.

Lip anatomy

The lips are composed of two parts - upper lip and lower lip. The main mass of the lip is formed by the vermilion body. The superior lip contains the upper vermillion body, and the inferior lip the lower vermillion body. Lateral endings of both lips join together and form two oral or lateral commissures known as the corners of the mouth. Each corner is connected with the ala of the nose by an indentation line called the nasolabial fold or the smile line.

The superior border of the upper lip goes along the base of the nasal septum, but the inferior border of the lower lip goes along the mentolabial groove. The borders of the lips are defined by the boundary line called the vermillion border that separates both lip bodies from the facial skin. The upper lip contains the white roll (or the lip roll) - a prominent white line at the site where the skin part of the lip meets the facial skin.

Near the middle of the upper lip, the vermilion border of the upper lip goes slightly upwards and on both sides forms two distinct peaks called the Glogau-Klein points. From the central aspects of both summits, the vermillion border further goes down, creating a double curve called the Cupid’s Bow. The depth of the Cupid’s Bow varies among people. Some lips appear uniform in shape, while others have very pronounced depression.

From the Cupid’s Bow to the alar base and columella of the nose goes philtrum - a vertical concavity that connects the upper lip with the nose. It is composed of two philtral columns and a philtral dimple or groove. The philtral dimple is the central concavity, while philtral columns are ridges that stretch along the lateral sides of the philtral groove from the Glogau-Klein points to the nose.

The central aspect of the upper lip below the Cupid's bow has fleshy protuberances called the labial or lip tubercles. Overall, the upper lip usually contains three labial tubercles - one central (upper central labial tubercle) and two lateral (upper left and right lateral labial tubercles) on each side of the central tubercle. The lower lip has only two - lower right and lower left labial tubercles. Internally, the mucous membrane of the lips creates two labial frenula that extend from the midline of each lip to the gums. The upper lip has a superior labial frenulum, while the lower lip contains the inferior labial frenulum.

 

Lip golden ratio

People have defined the ideal appearance of the lips, and the perfect ratio of the vertical height between the upper and lower lips is called the golden ratio. The upper lip against the lower ideally should be 1 : 1.6. But this ratio significantly varies not only in different ethnic groups but even within the same ethnic group.

 

Muscles associated with lip movements

There are several muscles around the lips that provide lip movements. Therefore, these muscles also control the opening and closing of the oral fissure. The primary muscle associated with lip movements is the orbicularis oris. Its marginal part closes the mouth while the labial part pulls the lips forward. Elevators of lips such as the levator labii superioris alaeque nasi and levator labii superioris muscles elevate the upper lip.

In contrast, depressors (depressor anguli oris and depressor labii inferioris) pull the angle of the mouth and lower lip down. Besides that, they also move the lower lip forward. The zygomaticus major and zygomaticus minor muscles elevate the corners of the mouth upward and laterally. And finally, the risorius and buccinator muscles also pull the corners of the mouth laterally.

 

Lip histology

The vermilion bodies are mainly composed of connective tissue and fibers of skeletal muscles. The surface of the lips can be grouped into three zones - the outer surface, the transition or vermilion zone and the inner surface. The outer surface is covered by the skin, while the inner - by the mucous membrane. The site where both surfaces connect is called the transition zone.

The outer surface covered by the skin contains sweat and sebaceous glands, and hair follicles. It is lined with thin keratinized stratified squamous epithelium. Below the epithelium is the submucosa that contains irregular dense connective tissue, lymphatic and blood vessels, nerves, fat cells and minor labial salivary glands. In contrast, the inner surface is lined with a thick layer of stratified squamous non-keratinized epithelium, and it is supported by lamina propria made of loose connective tissue.

The transition zone is modified skin lined with parakeratinized (partially keratinized) stratified squamous epithelium. It does not contain glands or hair follicles. But instead, it has many sensory nerve endings. Because of the lack of glands, the transition zone does not have proper protection, and in this zone, the lips dry out faster, especially during temperature changes. The transition zone appears red as it contains many capillaries and is highly vascular. As previously described, this zone connects the skin with the oral mucosa.

 

Lip functions

Lips are a part of the digestive system, and they have several essential functions associated with digestion, including food intake. Besides digestion functions, lips also provide erogenous function, articulation and tactile senses, and they participate in creating different facial expressions.

The lips have many muscles around them, controlling the size of the oral fissure. One of the main functions of the lips is to help the food and drinks get into the oral cavity. After food intake, the lips close the oral cavity airtight to hold the drinks and food inside. Also, with the help of the lips, babies form a narrow funnel that allows them to suck the milk during breastfeeding. Adults use this suction motion during drinking through the straw. Lips take part in the mastication process by holding the food between lower and upper teeth during chewing. And finally, the lips help to push the food from the oral vestibule to the oral cavity proper.

Lips contain a high number of sensory nerve endings, and they provide tactile senses. For example, lips are sensitive to temperature changes and touches. Also, they are considered an erogenous zone because of many nerve endings. Lips have a crucial role in kissing, and they represent sexuality and sensuality. They also help in the articulation by creating many different sounds. The lips are helpful during whistling, and they are useful while playing various musical instruments such as the saxophone. And finally, they participate in creating various facial expressions. With lips, people can express emotions, for example, when a person is smiling.

 

Lip neurovascular supply

The lips are among the highly vascular anatomical structures. They contain a substantial vascular network providing the arterial supply and venous drainage. Also, the lips have a massive amount of sensory nerve endings.

 

Arterial supply

The upper and lower lips receive arterial blood supply from the superior and inferior labial arteries, respectively. Each lip is supplied by either two superior (upper lip) or two inferior (lower lip) labial arteries that form anastomoses. These arterial vessels arise from the facial artery - a branch of the external carotid artery.

 

Venous drainage

The lips are drained by the superior and inferior labial veins. From the labial veins, blood is carried next to the facial vein that further drains into the internal jugular vein.

 

Lymphatic drainage

The lymphatic drainage from the lateral parts of the upper and lower lips is provided by the vessels that carry lymph to the submandibular lymph nodes, while the medial aspect of the lower lip is drained to the submental lymph nodes.

 

Innervation

The upper lip is innervated by the superior labial nerve arising from the infraorbital nerve. It is a branch of the maxillary nerve (CN V2) - the second division of the trigeminal nerve (CN V). The lower lip receives nerve supply from the mental nerve that originates from the inferior alveolar nerve. The inferior alveolar nerve is a branch of the mandibular nerve (CN V3) - the third division of the trigeminal nerve (CN V).

 

Lip disorders

Many conditions can impact the visual appearance and functions of the lips, affecting the mental health of a person and causing difficulties to smile or eat. Lip disorders can present with mild symptoms such as changes in lip color or with more severe symptoms such as bleeding. Most common conditions include lip cyanosis, cheilitis, oral candidiasis, cold sores, canker sores, mucous cysts and lip cancers. The most common signs of various disorders affecting the lips include:

  • Changes in lip color
  • Dryness
  • Sores or ulcers
  • Rashes
  • Swelling
  • Bleeding
  • Cracked lips with fissures
  • Various bumps and formations within the lips

 

Lip cyanosis

Lip cyanosis appears as a bluish discoloration of the lips, and usually, it is a sign of conditions that affect the heart, vascular system or respiratory system. Lip cyanosis occurs when there is poor circulation or not enough oxygen in the bloodstream, during freezing temperatures and in the case of high levels of abnormal hemoglobin forms (sickle cell anemia).

 

Cheilitis

Cheilitis is inflammation of the lips, presenting with fissures and cracked, scaly lips. The lips can appear dry and irritated, and bleeding might be present. Cheilitis can be an isolated disorder, or it can signify a systemic condition. Many infections, such as HIV, can cause it. Besides that, regular lip licking and chewing, cold and dry weather can result in cheilitis. It can also be caused by prolonged sun exposure, and then it is called actinic cheilitis. Cheilitis can be a sign of an allergic and contact reaction to certain substances or drugs, and it can present in nutritional deficiencies such as vitamin B12 or iron deficiency. Usually, when a person has a deficiency, lip inflammation is mainly present only at the corners of the lips, and it is called angular cheilitis. Some systemic diseases such as lupus and diabetes can present with cheilitis. Diabetes is one of the disorders that cause lip dryness, as high blood sugar levels can result in dry mouth and lips.

 

Oral candidiasis

Oral candidiasis, also known as thrush, is a yeast infection caused by Candida albicans. It presents with white or pale yellow patches on the lips, most commonly on their mucosal part. Patches can cause a burning or itching sensation. Some risk factors for developing thrush include diabetes or other endocrine system disorders, antibiotic or drug usage, poor nutrition and immunosuppression (chemotherapy, HIV infection).

 

Cold sores

Cold sores or herpes labialis, or fever blisters are viral infections caused by the Herpes simplex type 1 virus. Cold sores usually appear as a group of fluid-filled blisters on the outer surface or transition zone of the lips. Blisters become yellow, dry and scabbed over time. Later the scabs fall off. Cold sores can also present as only one lesion, although it rarely happens. They are very painful, contagious and periodically are recurrent. Some of the triggers causing recurrent infections include stress, hormonal changes, traumas and certain diseases.

 

Canker sores

Canker sores, also known as aphthae or aphthous ulcers, are the most common ulcers found on the lips. They appear as small oval or round and painful isolated lesions located on the inner surfaces of the lips. Usually, canker sores have a white center and red border. They are caused by stress, traumas and injuries, certain food types (acidic fruits and spicy food), certain drugs (anti-inflammatory drugs) and allergies (usually, allergy to particular substances within the toothpaste). Several conditions can impact canker sores and worsen their severity, such as nutritional deficiencies and immune system suppressions. Usually, canker sores are self-limiting, and they resolve within one to two weeks.

 

Mucous cysts

Mucous cysts, also known as oral mucocele, can present as small single fluid-containing bumps or swelling within the lips. Most commonly, mucous cysts are found on the surface of the inferior lip. Usually, these cysts are painless, mobile, smooth, round and soft. Most often, fluid-filled cysts develop in minor salivary glands when they become plugged with mucus or in the case of a trauma. Most mucous cysts are self-limiting conditions that resolve within a few weeks.

 

Lip cancers

Lip cancers can affect both lips, but most commonly, they are found within the lower lip. As lips form the oral cavity, lip cancers are classified as oral cancers. And lips are one of the most common sites for oral cancers. Some risk factors for developing lip cancer include excessive tobacco and alcohol consumption, ultraviolet radiation and sun exposure, immune-suppressive conditions and human papillomavirus infection. Squamous cell carcinoma is the most common form of lip cancer.