Oral Mucosa - Comprehensive Notes

Overview of Oral Mucosa

• The oral mucosa is the moist lining inside your mouth.

• It is made up of a top layer called stratified squamous epithelium, which sits on a lower layer called the lamina propria.

• Small tubes from your salivary glands pass through this lining to release saliva.

• A basement membrane acts as a separator between the top epithelial layer and the connective tissue below it.

• This basement membrane includes the basal lamina, made by the epithelial cells, and the reticular lamina from the connective tissue.

• Important parts that help the layers stick together are tonofilaments, hemidesmosomes with attachment plaques, lamina lucida, lamina densa, and anchoring collagen fibers.

• We classify the oral mucosa into three main types based on how they look under a microscope: lining, masticatory, and specialized mucosa.

Basal Membrane and Tissue Attachments

• The basal membrane is a boundary that keeps the top epithelial layer separate from the connective tissue underneath.

• It consists of the basal lamina, which has two parts (lamina lucida and lamina densa), and the reticular lamina, which is part of the connective tissue.

• Special cell connectors called hemidesmosomes use attachment plaques to firmly hold the epithelial cells to this basement membrane.

• This setup helps keep the epithelial layer strong and healthy, while also allowing some substances to pass through and new cells to replace old ones.

Classifications of Oral Mucosa

• There are three primary types of oral mucosa.

• These are lining mucosa, masticatory mucosa, and specialized mucosa.

Lining Mucosa

• Lining mucosa feels soft, stays moist, and can stretch and compress a lot, acting like a cushion for the tissues beneath it.

• You can find lining mucosa on the inside of your cheeks (buccal), lips (labial), the gum area around your teeth (alveolar mucosa), the floor of your mouth, the underside of your tongue, and your soft palate.

• Under a microscope, it looks like non-keratinized stratified squamous epithelium, meaning it does not have a hard, protective outer layer.

• It has fewer and less noticeable ridges and bumps where the epithelium meets the connective tissue, which are called rete ridges and connective tissue papillae.

• This type of mucosa also has a submucosa layer, which makes it easily movable.

• Sometimes, small, yellowish spots called Fordyce's spots can be seen on the surface; these are caused by sebaceous glands trapped deeper in the tissue.

Masticatory Mucosa

• Masticatory mucosa has a firm, rubbery, and resilient surface, designed to withstand the forces of chewing.

• It is found on your attached gums (gingiva), the roof of your mouth (hard palate), and the top surface of your tongue.

• Under a microscope, it shows keratinized stratified squamous epithelium, which means it has a tough, protective outer layer.

• This type of mucosa has many deep and pronounced rete ridges and connective tissue papillae, which helps it stay firmly attached.

• It has little to no submucosa, making it a very stable and firm base.

Specialized Mucosa

• Specialized mucosa is located on the top and sides of your tongue, forming unique structures called lingual papillae.

• These papillae are distinct bumps made up of epithelium and lamina propria.

Epithelium of the Oral Mucosa: 3 Types of Stratified Squamous Epithelium

• The top layer of the oral mucosa, called the epithelium, comes in three main types of stratified squamous epithelium.

• These are non-keratinized stratified squamous epithelium, orthokeratinized stratified squamous epithelium (which is the least common type in the mouth), and parakeratinized stratified squamous epithelium, which has four layers and keeps cell nuclei in its top keratin layer.

• All these types of epithelium serve to protect the oral cavity.

Non-keratinized Stratified Squamous Epithelium

• This type of epithelium has three distinct layers.

• The deepest layer is the basal layer (stratum basale), which is a single line of cube-shaped cells resting on the basement membrane and is responsible for making the basal lamina.

• Above it is the intermediate layer (stratum intermedium), which consists of larger, stacked, many-sided cells and makes up most of this epithelium.

• The most outer layer is the superficial layer (stratum superficiale), where cells are also stacked and many-sided, but the outermost cells flatten out into scale-like shapes called squames.

• These three layers (basal, intermediate, and superficial) sit on top of the lamina propria and, in some areas, a submucosa layer.

Ortho-keratinized Stratified Squamous Epithelium (Orthokeratinized)

• This type of epithelium is characterized by a process called keratinization, which makes it tough and protective.

• You can find it in the masticatory mucosa of your hard palate and attached gums, as well as in the specialized mucosa of some papillae on the top surface of your tongue.

• It has four layers, starting from the deepest:

  • The basal layer (stratum basale) is a single row of cube-shaped cells on the basement membrane, producing the basal lamina.

  • Above this is the prickle layer (stratum spinosum), which forms the main part of orthokeratinized epithelium.

  • Then comes the granular layer (stratum granulosum), which is $3$ to $5$ cells thick, made of flat, stacked cells containing special keratohyaline granules.

  • The most superficial layer is the keratin layer (stratum corneum), made of flattened cells called squames that are filled with keratin, and importantly, these cells do not have nuclei.

• The key features are the presence of keratin granules in the granular layer and the complete absence of cell nuclei in the outermost keratin layer.

Para-keratinized Stratified Squamous Epithelium

• This type of epithelium is found in the masticatory mucosa of your attached gums, often in greater amounts than orthokeratinization, and also on the top surface of your tongue and its specialized papillae.

• It has layers similar to orthokeratinized epithelium, but the main difference is that its outermost keratin layer still contains cell nuclei.

• This keratin layer also has keratohyaline granules, just like the orthokeratinized type, but with the unique feature of nuclei remaining visible in the keratinized cells.

Lamina Propria of the Oral Mucosa

• Every type of oral mucosa has a lamina propria layer directly beneath its basement membrane.

• This layer is divided into two parts.

  • The superficial papillary layer is made of loose connective tissue found within the small bumps called connective tissue papillae, and it contains many blood vessels and nerve tissues.

  • The deeper dense layer is made of strong, dense connective tissue with a lot of fibers.

• Under a microscope, you can see collagen fibers, fibroblasts (cells that make fibers), nerve fibers, a network of tiny blood vessels (capillary plexus), and papillae.

Anatomical and Clinical Correlates: Gingiva and Alveolar Mucosa

• Let's look at some important parts related to your gums and the tissue around them.

• For the attached gums (gingiva), "stippling" refers to the small indentations that make the surface look like an orange peel.

• The mucogingival junction (MGJ) is a clear line that separates the pink attached gums from the redder, looser alveolar mucosa.

• Mucoperiosteum is a type of loose connective tissue that acts like the covering of the bone underneath.

• The alveolar mucosa is a type of lining mucosa, meaning it's nonkeratinized (soft), and it sits below the mucogingival junction.

• Other important boundaries and surfaces include the labial frenum (the fold of tissue connecting your lip to your gum), the free gingival groove (a shallow groove separating the free gingiva from the attached gingiva), the marginal gingiva (the very edge of the gum surrounding the tooth), and the interdental papilla (the gum tissue that fills the space between teeth).

Tongue and Lingual Papillae

• Your tongue is primarily a thick mass of striated muscle covered by oral mucosa.

• On the top (dorsal) and sides (lateral) of your tongue, you'll find different types of bumps called lingual papillae.

• These include filiform papillae, fungiform papillae, foliate papillae, and circumvallate papillae.

• The top surface of the tongue has both masticatory mucosa (which is tough due to orthokeratinization) and specialized mucosa, which forms these lingual papillae.

• Each lingual papilla is a distinct structure made of keratinized epithelium and lamina propria.

Filiform Lingual Papillae

• Filiform papillae are the most common type of papillae found on the main part of the top surface of your tongue.

• They are shaped like fine-pointed cones, about $2$ to $3 ext{ mm}$ tall, and their tips point towards the back of your throat, giving the tongue a velvety feel.

• Importantly, these papillae do not contain any taste buds.

• Under a microscope, they show a thick layer of either orthokeratinized or parakeratinized epithelium covering a core of lamina propria.

• The amount of keratin in these papillae is what makes them appear whiter.

Fungiform Lingual Papillae

• Fungiform papillae are fewer in number compared to filiform papillae and look like reddish dots, typically about $1 ext{ mm}$ across.

• They are slightly raised and have a mushroom-like shape.

• Their epithelium is a thin layer of either orthokeratinized or parakeratinized cells, sitting over a lamina propria core that contains many blood vessels.

• These papillae contain taste buds, which are found in their upper part, and their primary job is to help you sense taste.

Foliate Lingual Papillae

• Foliate papillae appear as $4$ to $11$ vertical, side-by-side ridges on the sides of the back part of your tongue.

• They are leaf-shaped and consist of a layer of orthokeratinized or parakeratinized epithelium covering a lamina propria core.

• Taste buds are found within the epithelium on the side parts of these leaf-shaped structures, and their function is to help with taste sensation.

Circumvallate Lingual Papillae

• Circumvallate papillae are large, raised, mushroom-shaped structures, usually numbering $7$ to $15$, which are arranged in a line just in front of the sulcus terminalis when your tongue is extended.

• They measure about $3$ to $5 ext{ mm}$ in diameter.

• Their epithelium is either orthokeratinized or parakeratinized, covering a core of lamina propria.

• Hundreds of taste buds are found in the epithelium around the base of each of these papillae, and their main role is taste sensation.

• Additionally, von Ebner's glands, which are small, watery salivary glands, are located deep in the submucosa beneath the lamina propria.

• Their ducts release fluid that flushes the area around the taste pores, helping to refresh your taste sensations.

Taste Buds: Structure and Function

• Taste buds are barrel-shaped structures responsible for taste, found on specific lingual papillae (fungiform, foliate, and circumvallate).

• Each taste bud contains $30$ to $80$ spindle-shaped cells that stretch from the basement membrane to the surface of the epithelium.

• These cells include taste receptor cells, which have taste receptors on their surface, and supporting cells.

• Sensory nerve fibers connect to these taste cells and send signals to your brain (CNS) through nerves.

• A small opening called a taste pore is present on the surface.

• Taste buds are constantly renewing themselves, with old cells being replaced approximately every $10$ days.

Taste Cells and Nerve Connection

• Taste cells are located in the middle of a taste bud and have taste receptors on their outer surface.

• The taste pore is the opening at the very top of the taste bud.

• These receptors send signals through nerve fibers to your central nervous system (CNS) so your brain can identify different tastes.

• The path for taste sensation is: taste receptor $<br>ightarrow$ sensory neuron $<br>ightarrow$ CNS.

Taste Perception: Process and Limitations

• The process of tasting involves taste receptors sending messages through nerves to your brain (CNS), which then identifies the specific taste.

• Your ability to taste can be affected by aging, and your sense of smell often becomes weaker as you get older.

• Many things can influence how you perceive taste, such as aging, problems with color or vision, hormonal changes, genetic differences, the temperature inside your mouth, certain drugs and chemicals, tumors in your brain (especially in the temporal lobe), and a blocked nose.

• The old idea of "tongue maps," which suggested that different parts of the tongue taste only certain things, is now considered incorrect; all areas of the tongue that have taste buds can detect multiple tastes.

• While there are four main tastes (sweet, sour, salty, bitter) recognized by slightly different taste cells, other types of sensations can also be felt by any region of the tongue with taste buds.

Pigmentation in the Oral Mucosa

• Melanin pigmentation appears as localized flat spots, known as macules, in the oral mucosa.

• Their color can range from brown to brownish-black.

• In the mouth, this pigmentation is most commonly seen and is usually darkest at the base of the gum tissue between your teeth.

Quick reference: Key numeric and structural details

• Filiform papillae are about $2{-}3 ext{ mm}$ tall, do not have taste buds, and have keratinized epithelium.

• Circumvallate papillae are $3{-}5 ext{ mm}$ wide, arranged in a row of $7{-}15$ structures, and have many taste buds at their bases.

• Each taste bud contains about $30$ to $80$ cells.

• Taste buds renew themselves approximately every $10$ days.

• Non-keratinized epithelium has three layers: basal, intermediate, and superficial.

• Ortho-keratinized epithelium has four layers: basal, prickle, granular, and keratin (also called corneum).

• Parakeratinized epithelium has the same layers as ortho-keratinized, but its keratin layer still contains cell nuclei.

• The lamina propria has two layers: the papillary (superficial) layer and the dense (deep) layer.

Connections to Practice and Pathology

• Knowing the differences between the types of oral mucosa helps us understand how diseases show up in specific areas and how quickly they heal.

  • Lining mucosa is softer, changes shape easily, and heals faster; problems related to dentures often affect this type of mucosa.

  • Masticatory mucosa is tough because of its keratinization and firm base, which helps it resist the forces of chewing; injuries or strong pulls from a frenum can impact the attached gums.

  • Specialized mucosa, found in the lingual papillae, contains taste buds and other sensory parts; changes in this area can affect how you taste and feel things.

• The mucogingival junction (MGJ) is a very important marker for gum health; how well it is maintained affects how easily plaque can be controlled and the risk of gums pulling away from teeth.

• Fordyce's spots are a normal variation, but they can sometimes be confused with a health problem if you don't know what they are.

• Von Ebner's glands play a role in washing away substances from taste pores, which helps to refresh your taste sensations and shows how saliva is connected to your ability to taste.