The Lips: I decided to do my blog entry on the lips! The lips are an interesting tissue which we use for communication, intimacy, and food consumption on a daily basis. Because of their many functions, and histological complexity, I thought they would make a great addition to the tissue blogs:)
The Lips are soft, pliable anatomic structures that form the mouth margin of most vertebrates. They are located beneath the nose on the face, and consist of an upper lip and a lower lip. The upper lip is called the labium superius, and the lower is termed the labium inferius. -The upper lip borders onto the nose, maintaining separation from the cheek by a groove of varying depth, called the nasolabial groove. This groove begins at the wings of the nostrils and runs in a downward lateral direction until reaching the corners of the mouth.
-The lower lip is distinguished from the chin proper by a sharp and deep groove, that is superiorly convex. This groove is known as the labiomental groove (supramental crease), and its depth can vary between individuals based on an individual's lower lip fullness, the prominence of their bony and soft chin, and the individual's age.
- The lower lip is also visibly separated from the cheeks, in some older individuals, by a posteriorly running convex furrow coming from the corner of the mouth, known as the labiomarginal sulcus(prejowl depression).
- The upper and lower lips are connected at the corner of the mouth. These labial commissures are easily seen when the mouth is open, as thin connecting folds, and are vulnerable and sensitive areas.
-The oral cavity is divided into two distinct regions, the oral cavity proper and the oral vestibule. The oral cavity proper is surrounded by inside arches of the teeth and contains a tounge that is movably attached to the mouth floor. Lips create the periphery of the oral vestibule, along with the cheeks, teeth, and alveolar processes. (2)
Lip composition:
The lips are composed of muscles and glands, and are anatomically divided into three regions:
1)the skin
2)the vermillion border
3)the muscous membrane.
- The skin is located in the external environment, and follows a very similar make-up to the common integument found on other body surfaces where abrasive forces do not pose an issue. The skin's outer surface (the epidermis) consists of a thin layer of keratinized stratified squamous epithelium. The epidermis is thin due to the minimal mechanical stresses it encounters, and as a result, some of the layers tend to become less distinct, or fail to exist completely. Two definite constant layers do remain. These are the stratum malpighii (innermost) and the stratum corneum (outermost). Two distinguishing features of the skin's epidermis are the presence of gland ducts and shafts of hairs. No blood vessels or lymphatics are located within the epithelium.
-The connective tissue layer within the external lip is known as the dermis and is composed of a basement membrane and papillary and retucular layers.
-The papillary layer is made of a few short, broad folds, known as the papillary ridges (papillae*) that project into the epidermal ridges. The papillae contain tactile sensory nerve endings and capillary branches, and the folding works to increase the the surface area of the epithelial-connective tissue interface, providing anchorage sites and a better relationship between the blood supply and the epithelium. The connective tissue of this papillary layer is diffuse, and is composed of a network of thin collagen and elastic fibers.
-The reticular layer is much more compact, with collagen collecting to form bundles, creating a mesh-work where hair follicles and secretory portions of sweat* and sebaceous glands* are located. Deep within this layer there is a more diffuse connective tissue, in which larger blood vessels, lymphatic vessels, and nerve trunks are located. The deep layer also forms sheaths for the muscle fibers of the lip.(1,2,4)
-The vermillion border connects the exterior part of the lip(skin) to the interior muscous membrane. This region is also called the transitional zone or the red zone of the lip. The stratum lucidum of the epithelium is highly developed, while the stratum corneum is thinner, forming a thin epithelium. This zone is very lightly keratinized and cells in the middle and superficial layers contain eleidin*, enhancing their translucency.
The dermis of the vermillion border is found underlying the epithelium, and is thrown into deep folds. Papillae are thus numerous, densely arranged, and slender, reaching far into the epithelium (tips covered by thin epithelium). The papillae contain wide and rich capillaries that are seen through the epithelium producing the red color of the lips.
-In the dermis, hair follicles and sweat glands are lacking, and sebaceous glands consist of a few isolated glands located at the mouth corners. (1,2)
-The Mucous Membrane is also known as the oral mucousa. The epithelium of the oral mucousa consists of non-keratinized stratified squamous epithelium. This membrane is continuous between the cheeks and internal lips. The most superficial layer contains cells at the terminal stages of degeneration, which are loose enough to come dislodged by simple forces of the tounge. This layer lacks a stratum lucidum layer and has a plainly present stratum malpighii, composed of 1)stratum basale(most basal) and 2)stratum spinosum.
-The lamina propria is the next region of the oral mucosa and is found below a thin and delicate basement membrane. The papillae of the internal lip's connective tissue are irregular in shape and for the most part are relatively shallow, extending into epithelial ridges only a short distance. The rich vascular supply of this connective tissue (lamina propria) creates intense pink color of the teeth sockets (alveolar mucosa).
-The deeper layer of the connective tissue is composed of heavy bundles of collagen fibers. The submucosa (fibrous content) is heavier but looser because of the presence of fat cell islands.
-There are more arteries and veins in this region and these are larger than those found in the more superficial layers. Secretory acini* of the salivary glands are located in the more shallow connective tissue regions, near the lamina propria. These salivary glands are classified as mixed, however most are of the mucous variety. Sebaceous glands may also be found within the shallow connective tissue of the oral mucosa. (1, 2, 4)
Muscles of the Lips:
-
In the centralmost region within the lip, bundles of striated muscle (the orbicularis oris*) are found. Insertions of some of these muscle fibers occur in the cutaneous part of the lip.
-Most substances of the lips are supplied by this orbicularis oris. Both the skin and the mucous membrane are tightly fixed to the connective tissue that covers the lip's muscular substance, allowing the lips to follow muscle movement without much folding.
-Orbicularis oris muscle combined with the actions of muscles radiating from the cheeks, make lip shape and expression variation possible. (1, 2, 3)
The Arterial Blood Supply to the oral mucosa:
-
The upper lip is supplied by the superior labial artery (which joins the buccal artery).
-The lower lip is supplied by the inferior labial artery (which joins the buccal artery), by the mental artery, and by a branch of the alveolar artery. (3)
The Principle Sensory Nerve Fibers Supplying to the Oral Mucosa:
-The upper lip is supplied by the twigs from the infraorbital branch of the maxillary nerve
-The lower lip is supplied by the mental branch of the alveolar nerve; buccal branch of the mandibular nerve
*The vermillion border is a region with one of the most abundant supplies of sensitive nerve endings. Why else do you think we kiss with our lips and not our fingers ;) (3)
Origin of the Lips:
- The early face development is dominated by the proliferation and migration of the ectomesenchyme, that is involved in the formation of the primitive nasal cavities.
-At 28 days, a localized thickening develops within the ectoderm of the frontal prominence, forming olfactory placodes.
-Underlying mesenchyme around the placodes bulges the frontal rise forward, producing a horse-shoe shaped ridge, and converting the olfactory placode into nasal pit. (The lateral arms are termed lateral nasal process, and the medial arm is called the medial nasal process). Between two processes here, is a depressed frontal process.
-The medial nasal process of both sides and the frontonasal process give rise to:
1) middle portion of the nose
2)middle portion of the upper lip
3)anterior portion of maxilla
4)Primary palate
-
The maxillary processes grow medially and approaches both lateral and medial nasal processes (remains separate from them by distinct grooves). This growth pushes medial nasal process toward midline, causing it to merge with its anatomical partner from the other side (eliminating the frontonasal process).
-Upper lip is formed from the maxillary processes of each side and medial nasal process, with fusion occurring between the forward extent of the maxillary process and the lateral face of the medial nasal process.
-Lower lip is formed from fusion of two mandibular processes. Merging of the two medial nasal processes results in the formation of that part of the maxilla carrying the incisor teeth, the primary palate, as well as part of the lip. (3)
Some Interesting Facts:)
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Because mens facial hair is thicker, especially that of the dermis(connective tissue layer), men show less upper teeth and gums when speaking or laughing, than do woman(3)
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The little bumps you feel when you run your tounge along the inside of your lips (oral mucosa) are often the largest of your glands in that membrane.(1)
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The lips have been shown to be an indicator of a woman's fertility, on the basis that fuller and deeper red lips are correlated to a woman's estrogen levels.(7)
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In newborn infants, the moist mucous membrane (inner surface) is much thicker (higher concentration of papillae and sebaceous glands), and is believed to aid in the suckling behavior. (2)
Functions of the Lips:
- Food intake- keeps food/drink in mouth during mastication, keeps things out, enables suckling, etc.(8)
- Erogenous zone*- due to high concentration of nerve endings.(7)
- Tactile organ- nerve endings enable high levels of sensory reception of touch, warmth, and cold.(8)
- Speech- their articulating ability enable the creation of different sounds, thus making lips and essential part of the speech apparatus.(7,8)
- Facial Expression-lips enable a visual representation of how we feel to be portrayed. (Ex. Smile when we are happy)(7)
***Like all other tissues of the body, disorders and pathologies exist for the Lips.
Two commonly seen pathologies of the Lips are:
Cleft Lip- a relatively common congenital deformality caused by failure of the central to medial lip to fuse properly.
- -This fusion failure results in a fissure in the lip beneath the nostril. This fissure can range from a small pit to a fissure the entire length of the lip, and may occur alone or in conjunction with cleft palate (fissure in the roof of the mouth). (6)
Oral Herpes - is caused by Herpes Simplex virus. Type 1 is responsible for ~80% of cases, and type 2 for the others.
- results in the formation of painful sores on the lips, tounge, roof of mouth, and cheeks. Prior to sore formation, experience burning, itching, and tingling. Later develop blisters, which break down rapidly and appear as tiny grey ulcers on a red base. Later these blisters scap and crust, and look yellow and dry.
(2)
-is contracted by touching infected saliva, mucous membrane, or skin.- - can be accompanied by fever, muscle ache, shallow ulcers in throat, greyish coated tonsils, lymphnode swelling. It can also spread down the chin and neck.
-Three stages exist for the virus:
- Primary infection: virus enters skin/mucous membrane and reproduces; get oral sores.
2. Latency: virus moves to spinal dorsal root ganglia, reproduces and becomes inactive
3. Recurrence: certain stresses (emotional/physical) which cause the virus to react, creating new sores.
Some helpful terms* : (in order they appear)
- Papillae: (ridge) connective tissue thrown into folds/ridges, which contain capillary loops.
- Sweat Glands: any of the glands of the skin that secrete perspiration.
- Sebaceous Glands:microscopic glands in the skin, which secrete oily/waxy matter called sebum to lubricate skin and hair.
- eleidin: a clear protein substance that looks like keratin, found in the strata lucidum layer of the epithelium
- acini: clusters of cells that resemble a many -lobed berry
- orbicularis oris: sphincter muscle around the mouth
- Erogenous zone: area of heightened sensitivity, where stimulation may produce erotic sensations/sexual excitment.
References:
1)DuBrul, L. (1988). Oral Anatomy, 8th Ed. Ishiyaku EuroAmerica, Inc: St.Louis, Missouri.
2)Provenza, V.D. (1986). Oral Histology: Inheritance and Development, 2nd Ed. Lea and Febiger: Philadelphia, PA.
3)Ten Cate, A.R.(1994). Oral Histology: Development, Structure, and Function, 4th Ed. Mosby-Year Book, Inc: St. Louis, Missouri.
4)Lips. (2009). In encyclopaedia Britannica. Retrieved Oct. 20th, 2009, from Encylopedia Britannica Online: http://www.britannica.com/EBchecked/topic/342761/lips.
5)Human Digestive System. (2009). In Encyclopaedia Britannica. Retrieved Oct.20th, 2009, from Encyclopaedia Britannica Online: http://www.britannica/EBchecked/topic/1081754/human_digestive_system.
6)Cleft lip.(2009). In Encyclopaedia Britannica. Retrieved Oct 24th, 2009, from Encyclopaedia Britannica Online: http://www.britannica.com/EBchecked/topic/255229/cleft_lip.
7)Vasliner, Jaan. (2000). Culture and Human Development. Sage Publications, ltd. pp134-136.
8)Romer, A.S., and Parsons, T.S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp 297.
