(Finals) HISTOLOGY / Digestive System I

OVERVIEW OF DIGESTIVE SYSTEM

Digestive System – consists of

Alimentary canal

Associated organs

o Tongue

o Teeth

o Salivary glands

o Pancreas

o Liver

o Gallbladder

ALIMENTARY CANAL

Various segments are especially for digestion

and absorption.

Bolus

Formation of food after maceration and

moistening by the structures of oral cavity and

salivary glands.

Passes rapidly through pharynx to esophagus

keeping the pharynx clear for passage of air.

Passes slowly Gastrointestinal tract, and during

transit through the stomach and small intestine;

the organs for major alterations associated with

digestion, solubilization, and absorption.

Absorption

Occurs through the wall of the small intestine.

Undigested food - Excreted as feces.

Alimentary mucosa

Surface across which most substances enter the

body.

4 Functions

1. Secretion - secretes digestive enzymes, HCL,

mucin and antibodies.

2. Absorption - absorps metabolic substrates,

vitamins, H2O, electrolytes, recyclable materials

(bile components and cholesterol)

3. Barrier – prevention of entry of noxious

substances, antigens and pathogenic organisms.

4. Immunologic protection – Lymphatic tissues

present for body’s 1st line of immune defense.

ORAL CAVITY

Consists of:

1. Mouth

2. Tongue

3. Teeth

4. Supporting structures(periodontium)

5. Major and Minor salivary glands

6. Tonsils

Divided into 2 parts:

1. Vestibule – space between lips, check and teeth.

2. Oral cavity proper – lies behind teeth and

bounded by hard and soft palates, tongue, floor

of mouth and entrance to oropharynx.

3 Major Salivary Glands and its paired structures:

1. Parotid gland – largest, located in infratemporal

region of head.

Excretory duct: Parotid/Stensen’s duct – opens

at parotid papilla, a small elevation on mucosal

surface of the cheek opposite the 2nd upper

molar teeth.

2. Submandibular Gland – located in the

submandibular triangle of the neck.

Excretory duct: submandibular/Wharton’s duct –

opens at a small fleshy prominence (sublingual

caruncle) at each side of lingual frenulum on

floor of oral cavity.

3. Sublingual gland – lying inferior to the tongue

witnin the sublingual folds at the floor of oral

cavity.

*”1 and 2” have long excretory ducts that extend from

secretory portion of the gland to the oral cavity while

“3” has short ducts.

Minor Salivary Glands

Located at the submucosa of the oral cavity.

Empty directly into the cavity via short ducts and

are named for their locations (buccal, labial,

lingual, palatine).

TONSIL

Consists of aggregations of lymphatic nodules

clustered around the posterior opening of oral

and nasal cavities.

Lymphatic tissue is organized into Tonsillar

(Waldeyer’s) ring

(Finals) HISTOLOGY / Digestive System I

4 Types of Tonsils

1. Palatine Tonsil (tonsil) – located at either side

of entrance of oropharynx.

2. Tubal Tonsils – located in the lateral walls of

nasopharynx.

3. Pharyngeal Tonsil (adenoid) – roof of the

nasopharynx.

4. Lingual Tonsil – base of the tongue on its

superior surface.

Oral mucosa – lines oral cavity.

Consists of:

1. Masticatory mucosa - found ion gums and hard

palate.

Has keratinized and parakeratinized

stratified squamous epithelium

Keratinized epithelium lacks Stratum

lucidum.

Nuclei of parakeratinized cells are

pyknotic(highly condensed) .

Midline of hard palate, palatine raphe,

mucosa adheres firmly to underlying

bone.

2. Lining mucosa – found in the lips, cheek,

alveolar mucosal surface, floor of mouth, inferior

surface of tongue and soft palate.

Covers the striated muscles, bone and

glands.

Non-keratinized epithelium except

vermilion border of the lip (keratinized).

Has 3 layers:

o Stratum basale – single layer

of cells rest at basal lamina.

o Stratum spinosum – thick

o Stratum superficiale- most

superficial layer.

Submucosa – underlies lining mucosa except on

the inferior surface of tongue.

Has many collagen and elastic fibers.

Sebaceous glands are present and are

called Fordyce spots.

3. Specialized mucosa – associated with

sensation of taste.

Presence of papillae and taste buds for

taste.

TONGUE

- Muscular organ projecting into the oral cavity.

- Lingual muscles (muscles of tongue) are

o Extrinsic - one attachment outside the

tongue

o Intrinsic – without external attachment

Divisions of dorsal surface of tongue

1. Anterior 2/3

2. Posterior 1/3 via V-shaped; sulcus terminalis

Foramen cecum – the apex of V

Lingual papillae - covers the dorsal surface of the tongue.

4 types of papillae

1. Filiform papillae – smalles and most numerous

Covered by keratinized stratified

squamous epithelium

Doesn’t contain taste buds.

2. Fungiform papillae – mushroom- shaped

Taste buds in stratified squamous

epithelium.

3. Circumvallate papillae – large, dome-shaped

8 – 12 papillae

Ducts of Lingual salivary (von Ebner’s)

glands empty its serous secretion in the

moats.

4. Foliate papillae – has parallel low ridges

separated by deep mucosal clefts.

Taste Buds

Appear as oval, pale-staining bodies extend

through epithelium.

Taste pores- small opening at apex of taste buds.

3 Cell types in taste buds:

1. Neuroepithelial cells – most numerous

Elongated cells extend from basal

lamina to the taste pore.

Supported by tight junctions

Turnover time is 10 days.

At its base, it forms synapses with the

following afferent sensory neurons:

o Facial (cranial nerve VII)

o Glossopharyngeal( CN IX)

o Vagus ( Cranial Nerve X)

2. Supporting Cells – less numerous

Do not synapse, just help.

Turnover time = 10 days

(Finals) HISTOLOGY / Digestive System I

3. Basal Cells - small cells at basal portion.

Stem cell for 2 other types.

Taste

- Chemical sensation in which various tastants

(taste-stimulating substances) in food or

beverages interact with taste receptors.

Reacts with 5 basic stimuli: Sweet, Salty, Bitter, Sour,

Umami

Bitter, Sweet, Umami receptors – activates G protein-

coupled taste receptors belong to T1R and T2R

chemosensory receptor families.

1. Bitter taste – taste buds of circumvallate papillae

- Detected by 30 diff types of T2R chemosensory

receptors. Each is coupled to its own G-protein.

- G-protein stimulates phopholipase C, increasing

the production of Inositol 1,4,5-triphosphate.

- IP3 activates taste-specific Na+ channels causing

influx of Na+ ions for depolarization.

- Depolarization causes voltage-gated Ca2+

channels to open causing influx of Ca2+ which

generate nerve impulse

2. Sweet taste - tip of tongue

- Has 2 protein subunits, T1R2 and T1R3.

3. Umami taste–taste buds of circumvallate

papillae

- Linked to certain amino acids ( L-glutamate and

aspartate) and common to asparagus, tomatoes,

cheese and meat.

- 2 protein subunits, T1R3 and T1R1.

Sour and Salty – Sodium ions and hydrogen protons act

directly on ion channels.

1. Sour taste – more posterolateral to the tip

- Generated by H+ protons formed by hydrolysis of

acidic compounds.

- H+ blocks K+ channels responsible for

depolarization.

- H+ enters cells through amiloride-senstive Na+

channels and PKD1L3 and PKDL1L1.

- Entry of H+ activates voltage-sensitive Ca2+

channels.

2. Salty taste – posterolateral to the tip

- Stimulated by table salt (NaCl) is derived from

the taste of sodium ions.

- Na+ enters amiloride-sensitive Na+ channels and

activates voltage-sensitive Ca2+ channels.

Nerve supply of tongue is provided by cranial nerves and

autonomic nervous system.

1. General sensation

- anterior 2/3 of tongue is carried in manidbular

division of the trigeminal nerve (cranial nerve V)

- posterior 1/3 (sulcus terminalis) is carried by

glossopharyngeal nerve (Cranial nerve IX) and

Vagus nerve (Cranial Nerve X).

2. Taste sensation

- Carried by chorda tympani, branch of facial nerve

(Cranial Nerve VII), glossopharyngeal nerve (CN

IX) and Vagus nerve (CN X).

3. Motor innervations

- Musculature of tongue supplied by hypoglossal

nerve (Cranial nerve XII).

4. Vascular and glandular innervations by the

sympathetic and parasympathetic nerves.

TEETH

- Major component of oral cavity and are essential

for the start of digestive process.

- Children hace 10 deciduous (primary, milk) teeth

in each jaw, on each side:

o Medial(central) incisors

First tooth to erupt at mandible

at approx 6 months old.

o Lateral incisor

Erupts at approx 8 months old.

o Caninte tooth

Erupts at approx 15 months old

o 2 Molar teeth

First – 10 to 19 months old

Second – 20 to 31 months old

*Deciduous teeth are gradually replaced by 16 permanent

(secondary) teeth that usually begins at about the age of 6

until 12 or 13.

Consists of the following:

o Medial (central) incisor – erupts at age of 7-8

o Lateral incisor – age of 8 – 9

o Canine tooth – age of 10 – 12

o 2 Premolar teeth – between ages of 10 -12

o Three Molar teeth

First – age of 6

Second - early teens

Third (wisdom teeth) – late teens or

early twenties.

(Finals) HISTOLOGY / Digestive System I

*Incisors, canines, and premolars have 1 root each except

for the first premolar which has 2 roots.

*Molars have either 2 roots (lower jaw) or 3 roots (upper

jaw) and sometimes 4 roots.

Layers of Teeth

1. Enamel

- Hardest, thin, translucent layer of acellular

mineralized tissue that covers crown of tooth.

- Consists of 96 – 98% Calcium hydroxyapatite.

2. Dentin

- Most abundant dental tissue

- Lies deep to enamel in the crown and cementum

of root.

3. Cementum

- Thin, pale-yellowish layer layer of bone like

calcified tissue covering the dentin of the root of

the teeth.

- Softer and more permeable than dentin and

easily removed by abrasion.

SALIVARY GLANDS

- Major salivary glands are paired glands with

ducts.

Major glands:

1. Parotid gland

- Located subcutaneously, below and in front of

the ear between ramus of mandible and styloid

process of temporal bone.

2. Submandibular gland

- Located under the floor of the mouth.

3. Sublingual gland

- Located in the floor of the mouth anterior to the

submundibular gland.

Minor glands – located at submucosa of the different parts

of oral cavity.

Consists of - lingual, labial, buccal, molar and palatine

glands.

*cords become ducts, bulbous ends become secretory

acini.

Secretory Gland Acini – organized into lobules.

Major Salivary glands – surrounded by a capsule of

dense connective tissue from which septa divides the

secretory portions of gland into lobes and lobules.

Minor Salivary glands – don’t have capsule.

Salivon – basic secretory unit of salivary glands.

Consists of:

1. Acinus - blind sac composed of secretory cells.

2. Intercalated duct

3. Excretory duct

Cells of Acini:

1. Serous cells (protein secreting)

- Pyramidal shape, wide basal surface facing basal

lamina and small apical surface at lumen.

- Have many RER, free ribosome, golgi apparatus

and spherical secretory granules called Zymogen

granules.

2. Mucous cells (mucin secreting)

- Undergo cyclic activity

- Mucus is synthesized and stored in mucinogen

granules.

- After discharge of granules, cell is difficult to

distinguish from serous cells.

3. Both

Myoepithelial cells are contractile cells that embrace basal

aspect of acinar secretory cells.

3 type of Acini:

1. Serous acini – only serous cells and spherical

2. Mucous acini – only mucin and are tubular

3. Mixed – contains both; mucuous acini with a cap

of serous cells.

Serous demilunes

- serous caps

- Artifacts of the traditional fixation method

SALIVARY DUCTS

3 sequential segments:

1. Intercalated duct – leads from the acinus.

- Located between striated and excretory duct

- Lined by low cuboidal epithelial cells

In serous – secreting glands and mixed glands

Secrete HCO3- - into acinar product

Absorb Cl- - from acinar product

2. Striated duct – striations present.

- Located in the parenchyma of the glands

(intralobular ducts)

- Lined by simple cuboidal epithelium

(Finals) HISTOLOGY / Digestive System I

Reabsorption of Na+ from primary

secretion.

Secretion of K+ and HCO3- into the

secretion.

More Na+ are reabsorbed than K+ secreted = hypotonic

3. Excretory duct - larger ducts empty to oral

cavity.

- Constitutes the principal ducts of each of the

major glands.

- Simple cuboidal epithelium

Saliva

- Contains chiefly H2O, CHON, glycoproteins and

electrolytes.

o K concentration = 7x than blood

o Na concentration = 1/10 that of blood

o Bicarbonate = 3x than blood

- Includes combined secretions of all major and

minor salivary glands.

- 1,200 mL of saliva/ day is produced every day.

- Source of calcium and phosphate ions

- Proteins in saliva cover the teeth with protective

coat called acquired pellicle.

Functions of Saliva:

1. Moistening oral mucosa

2. Moistening dry foods to aid swallowing

3. Providing a medium for dissolved and suspended

food materials that chemically stimulate taste

buds

4. Buffering the contents of the oral cavity

5. Digesting carbohydrates with digestive enzyme

alpha-amylase

6. Controlling bacterial flora of the oral cavity by the

lysozyme for staphylococci.

Saliva performs Immunologic functions

-production of Immunoglobulin A (IgA)

*Alpha amylase and Lysozyme – principal enzymes of

saliva.