L04-Histology of digestive glands Flashcards
Major digestive glands
1) Salivary glands
2) Pancreas
3) Liver
Salivary glands overview
- Compound tubuloacinar that produces saliva.
- Basic secretory unit is an acinus; can be serous, mucous or mixed serous and mucous
- Divided into 3 pairs of major salivary glands and numerous minor salivary glands
Major salivary glands
Paired glands with long ducts emptying into the oral cavity; 3 pairs in total:
1) Parotid (serous, below and in front of the ear)
2) Submandibular (mixed, under the floor of the mouth)
3) Sublingual (predominantly mucous with serous elements, floor of the mouth anterior to the submandibular gland inferior to the tongue)
[Note: Major salivary glands (except sublingual glands) are surrounded by a capsule of moderately dense connective tissue with septa dividing the glands into lobes and lobules
The larger blood vessels and excretory ducts are present in the connective tissue septa.]
Minor salivary glands
located in the submucosa of different parts of the oral cavity:
Lingual, labial, buccal, molar, epiglottic and palatine glands
Note: Minor salivary glands are unencapsulated
Secretory gland acini
- Definition: a blind sac composed of secretory cells
- organized into lobules
- Three types:
- *1) Serous**
- protein, enzyme secreting
- watery secretion
- generally spherical
- darkly stained in H&E (basophilic)
- *2) Mucous**
- mucin secreting
- thick and slimy secretion
- generally more tubular
- palely stained in H&E (acidophilic)
- *3) Mixed (serous and mucous)**
- presence of both serous and mucous secretory acini and cells
- presence of serous demilunes in traditional fixation
serous demilunes
- Artifacts of traditional fixation method
- Seen in mixed (serous & mucous) multicellular secretory acini
- under H&E preparation, mucous acini are capped by serous cells
- Serous demilune not observed in rapidly freezed specimens
Myoepithelial cells
Contractile cells present between the gland cells and basal lamina of the epithelium
Innervations of salivary glands
Sympathetically and sympathetically:
1) Sympathetic (cervical sympathetic chain) → slow rate of secretion, high consistency, rich in protein
2) Parasympathetic → copious secretion, low consistency, more watery
Generic scheme of secretory duct hierarchy
Acinus -> Intralobular duct -> interlobular duct -> Interlobar/ lobar duct -> Main duct
Salivary ducts
Three groups: intercalated, striated and excretory:
1) Intercalated duct
- leads from the acinus
- intralobular duct (within parenchyma of glands)
- prominent in serous secretory salivary glands
- short and difficult to identify in mucous secretory salivary glands
- Cl-HCO3 pump for absorption (Cl-) and secretion (HCO3-); thus modify the serous secretiom; hence more commonly associated with serous secretory acini and less with mucous secretory acini
- simple cuboidal
2) Striated duct:
- intralobular duct (within parenchyma of glands)
- presence of “striation” due to extensive infolding of the basal plasma membrane of the ductal cells with longitudinally arranged mitochondria
- diameter of striated duct often exceeds that of secretory acinus
- Na-K pump and the Cl-HCO3 pump to absorption (Na+) and secretion (K+ and HCO3-); thus modify the serous secretiom; hence more commonly associated with serous secretory acini and less with mucous secretory acini
- simple cuboidal or simple columnar
3) Excretory ducts:
- Interlobular and interlobar ducts (travel in interlobular and interlobar connective tissues)
- eventually empty into the oral cavity
- small excretory ducts (simple cuboidal) → gradually change to pseudostratified columnar or stratified cuboidal/squamous as the sizes increase
- Stensen’s duct (parotid duct) and Wharton’s duct (submandibular duct) travel in the connective tissues of the face and neck respectively
Parotid glands
- Completely serous (a lot of basophilic serous acini under H&E) *
- Largest of major salivary gland
- Paired
- Large amount of adipose tissue *
- Below and in front of the ear
- Encapsulated by moderately dense connective tissue with septa dividing the glands into lobes and lobules
- More prominent intercalated and striated ducts
- Parotid/ Stenson’s ducts travel in the connective tissues of the face
Submandibular glands
- Mixed (high amount of serous demilune *, actually predominantly serous)
- Large major salivary gland
- Paired
- under the floor of the mouth
- Encapsulated by moderately dense connective tissue with septa dividing the glands into lobes and lobules
- Relatively prominent intercalated and striated ducts
- Submandibular/ Wharton’s ducts travel in the connective tissues of the neck
Sublingual Glands
- Predominantly mucous * (actually mixed, with few serous acini and serous demilune)
- Smallest of the major salivary gland
- Paired
- Floor of mouth anterior to submandibular glands inferior to the tongue
- Unencapsulated *
- Not prominent intercalated and striated ducts
Saliva (features)
Combined secretions of all the major and minor salivary glands
- 1,200 ml produced a day
- Contains water, various types of proteins and electrolytes
- Principal enzymes: lysozymes and α-amylase
Saliva (functions)
Both protective and digestive functions:
1) Moistening of oral mucosa
2) Aid swallowing
3) Dissolve food to be chemically tasted
4) pH Buffering (high concentration of bicarbonate ions)
5) Digest carbohydrates (α-amylase)
6) Source of calcium and phosphate ions essential for normal tooth development and maintenance (particularly relevant to patients undergoing radiotherapy e.g. nasopharyngeal carcinoma patients in Hong Kong)
7) Saliva performs immunologic functions
i) Lysozyme controls bacterial flora in the oral cavity
ii) Secretion of secretory IgA:
- IgA (is synthesized by plasma cells in the connective tissue surrounding the secretory acini)
- A secretory glycoprotein (synthesized by the salivary gland) is inserted on the basal plasma membrane which function as dimeric receptor for IgA
- Binding of IgA to the receptor from a secretory IgA complex which is internalized by a receptor-mediated endocytosis into the acini cells and transported to the apical membrane and secreted (similar to the secretory IgA transported across the absorptive columnar epithelial cells of intestines).
Panceas anatomy
- An elongate gland with a head, body and tail
- Connected to duodennum via the Main pancreatic duct and accessory pancreatic duct
Endocrine pancreas
aka Islets of Langerhans (1-2% of the pancreas)
- paler stained than acinar cells in pancreas by H&E
- Usually proximal to blood vessels
Function: regulate blood glucose levels
3 principal islet cell types:
A cells (alpha; 15-20%; glucagon)
B cells (beta; 70%; insulin)
D cells (delta; 5-10%; somatostatin)
Minor cell types:
PP cells (pancreatic polypeptide)
D-1 cells (VIP Vasoactive intestinal peptide)
EC cells (enteroendocrine cells; serotinin aka 5HT, secretin, motilin, substance P)
Exocrine pancreatic acinar cells
- Serous glands
- Secretory units: acinar or tubuloacinar
- Distinct basophilia in the basal cytoplasm
- Extensive array of rER and free ribosomes
- Correlates to high level of protein synthesis
- Acidophilic zymogen granules in the apical cytoplasm
- Proenzymes of protease (e.g. trypsinogen, chymotrypsinogen)
- Pancreatic proteases are activated only in small intestine by enterokinases at the microvilli (typsinogen to trypsin); trypsin then converts other inactive enzymes into active enzymes as well as digestion of protein
- lipase, amylase are released in active form
- Well-developed Golgi apparatus
- Opens to intercalated duct (simple cuboidal)
Centroacinar cell
- Centroacinar cells are spindle-shaped cells only found in the exocrine pancreas
- smaller and more palely stained than acinar cells
- Centroacinar cells are an extension of the intercalated duct cells into each pancreatic acinus
- mucin, HCO3- secreting
- Stimulated by secretin to add bicarbonate and water to the exocrine secretion
Liver blood supply
• entering at the porta hepatic together with the bile duct and lymphatic):
- 75%: Hepatic portal vein (venous), from intestines
- 25%: Hepatic artery (arterial); from celiac trunk; oxygenated
• Blood from hepatic portal vein and hepatic artery flows into the sinusoids between hepatocytes leading eventually to the central vein, then to sublobular vein and leaving the liver through the hepatic veins and empty into inferior vena cava.
Sinusoids
- Venous channels between cords of hepatocytes
- Carrying blood from portal areas (from hepatic portal vein and hepatic artery) to central vein
- Lined by incomplete walls formed by endothelium and Küpffer cells — phagocytic (mononuclear phagocytotic system)
- Facilitate exchange of materials between sinusoid and hepatocytes via sinusoidal lumens
Küpffer cells
Location: Liver (e.g. endothelial lining of sinusoids)
Function:
- antigen presnetation
- phagocytosis (mononuclear phagocytotic system)
Histology:
- essentially macrophages
- difficult to distinguish
- Can be visualized with Indian ink/ Carbon particle injection (Küpffer cells will perform phagocytosis on foreign materials, as a result the phagocytic vesicles will be stained and marked, labelling Küpffer cells) - see img below
Ito cells
- aka Hepatic stellate cells (HSC) or perisinoidal cells
- located in perisinoidal space
Functions:
- Store vitamin A
- Antigen presenting
- Involved in liver fibrosis: Differentiate into myofibroblasts in pathological conditions and synthesize collagen type I and III
Space of Disse
- aka Perisinoidal space
- between a hepatocyte and a sinusoid
- It contains the blood plasma, Ito cells and microvilli of hepatocytes
- Site of exchange of materials between blood and liver cells
Bile canaliculi
- a thin tube that collects bile secreted by hepatocytes
- formed by grooves on some of the lateral faces of hepatocytes - surrounded by zonula occludens
- bile canaliculi merge and form bile ductules, which eventually become common hepatic duct
Liver unit classification
Can be classified into three types:
1) Classical lobule (Structural)
2) Portal lobule (Functional - bile drainage)
3) Liver acinus (Functional - blood supply)
Portal area
Contains the portal triad (hepatic artery, hepatic portal vein, bile duct) and lymphatic vessels embedded in connective tissues (that surrounds a classical liver lobule)
[note: hepatic triad refers to types of vessels found in portal area, not number. So there might be more than one of each kind of vessels in a portal area, still considered as triad]
Classic lobule
- Structural in nature
- Theoretical hexagonal (or pentagonal) structures consists of stacks of anastomosing plates of hepatocytes
- More easily recognized in pig’s liver (delineated by connective tissues) but less prominent in human liver
- central vein located at the center of the classic lobule
- anastomosing plates of hepatocytes (one cell thick) radiate out from central vein and are separated by sinusoids
- 6 portal areas at the angles of the hexagon of the classic lobule
- Blood flows from periphery towards central; Opposite for bile
Portal lobule
- Based on the exocrine function of liver (bile secretion)
- Portal area at the center of the lobule (bile drainage)
- Border (imaginary lines between two central veins forming triangular structure)
Liver acinus
- Provides the best correlation between blood perfusion, metabolic activity and liver pathology
- Diamond-shape
- Smallest functional unit of liver
- Two portal areas and two central vein reside on the angles of liver acinus
- Can be devide into zone 1, zone 2, zone 3
- Perfusion, nutrients and metabolism (from greatest to least): zone 1> 2> 3
- pathophysiological events like poisoning, (from most affected to least): zone 1> 2> 3
- Under ischaemic condition, most affected (i.e. most prone to shock): zone 3> 2> 1
