Histology π¬ Flashcards
1
Q
What is the general structure of GIT wall?
A
2
Q
What is the definition of the esophagus?
A
- Muscular tube connecting the throat (pharynx) with the stomach, prevent retrograde flow of stomach content
3
Q
What is the Histological structure of the esophagus?
A
- Mucosa
- Submucosa
- Musculosa
- Adventitia
4
Q
What is the histological structure of the mucosa of esophagus?
A
Epithelium: Stratified squamous non keratinized epithelium
Corium: Loose CT containing; blood vessels, nerves & lymphatics
Muscularis Mucosa: Inner circular & outer longitudinal smooth muscle fibers
5
Q
What is the histological structure of the submucosa of the esophegus?
A
Loose C.T contains esophageal glands (Mucous glands); protect mucosa
6
Q
What is the structure of the musclosa of the esophagus?
A
Upper 1/3: Striated βinvoluntaryβ
Middle 1/3: Mixed
Lower 1/3: IC & OL smooth muscles
7
Q
What is the structure of the adventitia of the esophagus?
A
- Loose C.T
N.B: Serosa in the lower 1-2 cm
8
Q
What are the parts of the stomach?
A
- Cardiac region
- Fundic region: The upper convex part
- Body of stomach: The main part, forming the middle 2/3 of stomach
- pyloric region: Antrum, pyloric canal and pyloric sphincter
9
Q
What is the histological structure of the fundus and body of the stomach?
A
1- Mucosa
2- Submucosa
3- Musclosa
4- AdventetiaοΏΌ
10
Q
What is the Histological structure of the mucosa of the fundus and body of the stomach?
A
Epithelium
- Simple columnar secretory with oval basal nuclei
Corium
- Loose C.T. containing blood Vessels, nerves and lymphatics
- Fundic gland
Muscularis mucosa
- Inner circular & outer longitudinal smooth muscle fibers
11
Q
What is the histological structure of the submucosa of the Fundus and body of the stomach?
A
Loose C.T. containing blood Vessels, nerves & lymphatics
12
Q
Musclosa of the Fundus & body of the stomach
A
Inner: Oblique
Middle: Circular
Outer: Longitudinal
βMore layers to suit functionβ
13
Q
Serosa of the fundus & body of the stomach
A
- Mesothelial layer
- Sub-mesothelial C.T.
14
Q
What is the type of fundic gland?
A
simple branched tubular glands.
15
Q
What is the number of fundic glands?
A
numerous
16
Q
What is the site of fundic gland?
A
occupy most of the corium.
17
Q
What is the shape and arrangement of fundic gland?
βNSPPβ
A
narrow, straight, parallel to each other and perpendicular to the surface.
βIn order to save spaceβ
18
Q
What are the parts of fundic gland?
A
19
Q
What are the cells of fundic gland?
A
- Surface columnar cells
- Mucous neck cells
- EE cells
- Stem cells
- Peptic cells
- Parietal cells
- caveolated cells
20
Q
Compare between surface epithelium and mucus Neck cells
A
21
Q
Compare between peptic cells and parietal cells
A
22
Q
what is the definition of entero-endocrine cells?
A
These are modified cells, which secrete hormones.
23
Q
How many types of enteroendocrine cells does fundic gland have?
A
- The fundic glands have one type (enterochromaffin or E C cell)
- Some secrete serotonin (vasoconstrictor)
- Others secrete endorphin (morphine-like substance).
24
Q
What is the shape of entero-endocrine cells under LM?
A
Shape: Pyramidal or columnar with narrow apex
Nuclei: Rounded, near the base, vesicular
Cytoplasm: Pale basophilic Contains basal granules stained with silver (Argentaffin granules)
25
What is the function of undifferentiated columnar cells?
stem cells for renewal of the cells of the fundic glands
26
What is the function of caveolated cells?
Unknown function
27
What are the characteristics of pyloric glands?
a) They are widely separated from each other
b) They are shorter than the fundic glands.
c) Their secretory parts are coiled, so they are cut in cross and oblique sections.
d) The mucous cells contain flat basal nuclei and mucinogen granules in their apical cytoplasm.
28
What are the cells of pyloric Glands?
1. Surface columnar cells.
2. Mucous neck cells.
3. EE cells.
4. Stem cells.
5. Caveolate cells
(No Peptic cells, No Oxyntic cells)
29
What are the types of Entero-endocrine cells in pylorus?
EC cells, G cells and D cells
30
What is the function of EC cells?
Secrete serotonin and endorphins
31
What is the function of G cells?
Secrete gastric hormone ----> stimulates oxyntic cells to secrete HCI.
32
What is the function of D cells?
Secrete somatostatin hormone, (which inhibit growth) insulin, gastrin and glucagon hormones.
33
Compare between the Fundus and the pylorus
34
What is the function of the stomach?
1. Digestion through pepsin, rennin enzymes.
2. Secretion of
a) HCl (antibacterial; help fat digestion).
b) Mucous to form gastric barrier.
c) Intrinsic factor.
d) some hormones.
3. Limited absorptive function for water, drugs, alcohol, and salts.
35
What is the length of the small intestine?
It is a hollow tubular structure, about 20 feet (6 meters) in length.
36
What are the parts of the small intestine?
1. Duodenum.
2. Jejunum.
3. Ileum.
37
What is the wall of the small intestine formed of?
ο Mucosa.
ο Submucosa.
ο Musculosa.
ο Serosa.
38
What is the gross appearance of the mucosa of small intestine?
The mucosa of the small intestine shows:
οΌ Circular permanent folds called (plica circularis).
οΌ Velvet appearance of the surface due to presence of intestinal villi.
οΌ Minute holes which represent openings of the intestinal crypts.
39
What is the microscopic appearance of the mucosa of small intestine?
A) Epithelium
B) Corium
C) Muscularis Mucosa
40
What is the type of epithelium of the mucosa of small intestine? And what does it cover?
It is of simple columnar type which is:
ο· Covers Intestinal villi.
ο· Lines intestinal crypts.
41
What is the definition of intestinal villi?
they are finger like projections of the mucosa extending from the wall into the lumen of the intestine.
42
What is the length of intestinal villi?
0.5 - 1.5 mm
43
What is the structure of intestinal villi?
Villous epithelium: formed of:
1. Simple columnar absorbing cells. (90%)
2. Goblet cells. (9.5%) "not found in structures above small intestine"
3. Entero-endocrine cells. (0.5%)
Villous core: formed of CT containing blood vessels, nerves and large lymph vessels (central lacteals).
44
What is the site of simple columnar absorbing cells **(Enterocytes)** of small intestine?
they cover the villi and upper part of the intestinal crypts.
45
What is the shape of simple columnar absorbing cells of small intestine under LM?
- Shape: they are tall columnar cells.
- Nuclei: they have basal, oval nuclei.
- Cytoplasm: basophilic.
- The free border: is acidophilic striated (brush border) due to microvilli.
46
What is the shape of simple columnar absorbing cells of small intestine under EM?
a) The free border:
- The microvilli are about 3000 microvilli/cell.
- The microvilli are covered with thick cell coat rich in alkaline phosphatase enzyme needed for the process of absorption.
b) Lateral border:
- It shows irregular interdigitations and belt desmosomes forming complete seal between the absorbing cells.
c) Basal border:
- hemidesmosomes attached with underlying C.T. of lamina propria.
d) Cytoplasm:
- numerous rER, prominent Golgi apparatus and mitochondria.
47
What is the function of columnar absorbing cells?
1) Secretion of lactase, sucrase and isomaltase enzymes.
2) The cells are essential for absorption of useful substances.
48
What is the site of goblet cells in small intestine?
they cover intestinal villi and upper part of intestinal crypts.
49
What is the shape of goblet cells of the small intestine under the light microscope?
- Shape: They are goblet like cells.
- The apical part: expanded and filled with mucin granules.
- The basal part: constricted and contains nucleus and cell organelles.
- Nucleus: it is flat, basal and deeply stained.
- By Hx & E: the apical part of the goblet cells appears as clear, vacuolated or foamy due to dissolved mucin granules.
50
What is the function of goblet cells of the small intestine?
**They secrete:**
1) mucous to lubricate the passage of intestinal content.
2) acid glycoprotein to prevent bacterial invasion of Intestinal mucosa.
51
What is the site of entero-endocrine cells of the small intestine?
cover intestinal villi (about 0.5%) and line intestinal crypts ( 1%).
52
What is the shape of entero-endocrine cells under the LM and the EM?
Similar to entero-endocrine cells described in the stomach.
53
What are the types of entero-endocrine cells lining the small intestine?
54
What what is another name for intestinal crypts of leiberkuhn?
(Intestinal glands = crypts of Lieberkuhn)
55
What is the definition of intestinal crypts of lieberkuhn?
They are simple tubular glands extending from the base of the intestinal villi to the muscularis mucosa.
56
What is the length of intestinal crypts of leiberkuhn?
about 100-200ΞΌm.
57
What is the lining epithelium of Intestinal crypts of leiberkuhn?
intestinal crypts are lined with the following cells:
1. Columnar absorbing cells
2. Goblet cells
3. Entero-endocrine cells
4. Caveolate cells
5. Stem cells
6. Paneth cells.
7. M-cells.
58
What is the site of stem cells in the small intestine?
undifferentiated cells present at base of the crypts between Paneth cells.
59
What is the shape stem cells in the small intestine under LM and EM?
Similar to stem cells described in the stomach.
60
What is the function of stem cells of the small intestine?
differentiate into other types of epithelial cells (cell renewal).
61
What is the site of a paneth cells of the small intestine?
ο· They are grouped in the base of the crypts of the small intestine.
ο· They are absent in crypts of large intestine.
62
What is the shape of paneth cells of the intestine under LM?
Shape: they are columnar or triangular cells with narrow apex.
Nucleus: it is basal, rounded and pale.
Cytoplasm: it shows
- basal basophilia: due to numerous rER.
- Apical acidophilia: due to presence of zymogen granules rich in zinc.
63
what is The shape of paneth cells of the intestine under EM?
ο· These cells are rich in rER, and well developed Golgi apparatus.
ο· The apical part shows large electron dense zymogen granules.
64
What is the function of paneth cells of the intestine?
They secrete lysozyme enzyme which has antibacterial effect as the enzymes digest the cell coat of the bacteria.
65
What are other names for M β cells of the intestine?
(Membrane like cells - Microfold cells)
66
What is the shape of M-cells of the intestine?
ο· They are dome shaped cells with basal cavity.
ο· Basal cavity is packed with intraepithelial leucocytes and macrophages.
67
What is the shape of M-cells of the intestine under EM?
ο· They have short microvilli.
ο· They are attached to the neighboring cells by junctional complex.
ο· The basement membrane is discontinuous to facilitate transport between M-cells and lamina propria.
68
What is the function of M-cells of the intestine?
Transport of intraluminal antigen by endocytosis across the cytoplasm to the closely associated lymphocytes and macrophages in the lamina propria i.e. antigen presenting cells.
69
Corium of the mucosa of the small intestine
ο· It is loose C.T containing blood vessels, nerves and lymphocytes.
ο· The corium is rich in lymphocytes and plasma cells.
ο· Lymphocytes are present in diffuse or aggregated into lymph follicles.
ο· In the ileum, the lymph follicles are collected into confluent mass at the antimesentric border to form (Peyerβs patches) Over it the villi are short or absent and the covering epithelium is rich in M-cells.
70
Muscularis mucosal of the small intestine
It is formed of 2 thin layers of smooth muscle fibres : inner circular and outer longitudinal layers
71
What is the structure of submucosa of the small intestine and what is characteristic for both duodenum and ileum?
It is loose C.T containing
-blood vessels
-nerves
-lymphatics.
* In proximal part of duodenum:
There are (Brunnerβs glands) which are compound tubuloalveolar mucous secreting glands. "absence may cause duedenal ulcer"
* In the ileum: (Peyerβs patches) are present in the corium of the mucosa, pierce muscularis mucosa and appear in submucosa.
72
Musclosa of the small intestine
It is formed of:
-Inner circular layer of S.M.F.
-Outer longitudinal layer of S.M.F.
73
What is the serosa of the small intestine and what is it formed from?
It is a covering layer derived from the peritoneum, It is formed of :
1.Thin layer of simple squamous mesothelial cells.
2.Submesothelial layer of loose C.T containing blood vessels, nerves & lymphatics.
74
What are the differences between the Duodenum, Jejunum and Ileum?
75
# HDAM
What are the functions of the small intestine?
1) Complete the process of food **d**igestion.
2) **A**bsorption of useful products of digestion by blood and lymph capillaries.
3) Secretion of **m**ucous which lubricate food and stool.
4) Secretion of gastrointestinal **h**ormones.
76
What are the factors increasing absorption from the intestinal lumen?
1- Plica circularis ---> β surface area about 3 folds.
2- Intestinal villi ---> β surface area about 10 folds.
3- Microvilli ---> β surface area about 20 folds.
So, these factors increase the surface area by about 600 folds.
77
what is the large intestine consisted of?
consists of caecum, appendix, ascending, transverse and descending colons, rectum and anal canal
78
compare between the small intestine and the large intestine
79
compare between the small intestine and the large intestine
80
compare between the parts of large intestine
81
what is the recto-anal canal?
It is the communication between the rectum and anus.
82
what is the gross appearence of the recto-anal canal?
- The mucosa shows permanent longitudinal folds called (columns of Morgagni).
- Columns of Morgagni are connected distally by anal valves which are transverse folds.
83
what is the microscopic appearence of the epithelium of Recto-Anal canal?
**At the rectum cells**: simple columnar absorptive with goblet cells
**At the columns of Morgagni**: stratified columnar epithelium.
**At the anal valves**: stratified squamous epithelium.
**At the anal orifice**: stratified squamous keratinized epithelium.
84
what is the microscopic appearence of the corium of the recto-anal canal?
**At the rectum & columns of Morgagni**: still present.
**At the anal valves**: fuse with the submucosa.
- At this area,corium shows a group of convoluted veins (internal haemorrhoids).
**At anal orifice**: the fused corium and submucosa become dermis of the skin.
- Contains hair follicles, sebaceous glands and circumanal glands (simple coiled tubular glands)
- dermis shows another group of convoluted veins (external Haemorrhoids).
85
what is the microscopic appearence of the muscularis mucosa of the recto-anal canal?
**At the rectum**: present.
**At the columns of Morgagni**: it fans out and disappears.
86
what is the microscopic appearence of the musclosa of the recto-anal canal?
**At the rectum:** there are inner and outer longitudinal layer of smooth muscle.
**At columns of Morgagni and anal valves:**
- The inner circular layer is thickened to form (Internal anal sphincter).
- The outer longitudinal layer extends as it is.
**At anal orifice**:
- The outer longitudinal layer extend from rectum & lost in the dermis.
- Appearance of striated voluntary muscle to form (external anal sphincter).
| Damage of internal anal sphincter may cause uncontrolled defecation
87
what is the type of glands of the pancreas?
Compound branched tubulo-alveolar gland
| **"like brunner glands and major salivary gland"**
88
what is the histological structure of the pancreas?
89
what is the shape of pancreatic acini under LM?
- They are irregular in shape and size.
**Shape:** pyramidal cells
**Nucleus:** rounded near the base.
**Cytoplasm:** basal basophilic striations, apical acidophilic zymogen granules.
| **"like paneth"**
90
what is the shape of pancreatic acini under EM?
- Ribosomes
- Golgi apparatus.
- **basal part:** mitochondria, rER
- **Apical part:** electron dense secretory granules
91
duct system of pancreatic acini
92
what is the definition of salivary glands?
Salivary glands are glands that secrete saliva into the mouth cavity.
93
what are the types of salivary glands?
major and minor
94
what are the major salivary glands?
- 3 pairs of compound branched tubulo-alveolar glands connected to the oral cavity by main ducts
- Includes:
1) Parotid glands
2) Submandibular glands
3) Sublingual glands
95
what are the minor salivary glands?
- Simple branched tubulo-alveolar glands in the mucous membrane of the mouth cavity.
- They include **b**uccal, **l**abial, **p**alatine and **l**ingual salivary glands
| **"Minor salivary glands: LBLP"**
96
what is the histological structure of salivary glands?
**Stroma:**
- Capsule
- CT trabeculae
- Reticular network
**Parenchyma:**
- Acini
- Duct system
97
what are the charachters of the capsule of the salivary glands?
dense fibrous connective tissue rich in collagen fibers.
98
what are the charachters and types of CT trabeculae (septa) of the salivary glands?
99
what are the characters of the reticular network of the salivary glands?
A network of reticular fibers supports the parenchyma inside the lobules.
100
what does the parencyma of the salivary glands consist of?
**1) Salivary acini:** Consists of group of cells around central lumen. Basket cells are found around the acini to squeezes the secretions into ducts
**2) Duct system**
101
Salivary acini
- formed of secretory cells surrounding a central lumen leading to the duct system
- basket cells squeeze the secretions into ducts
102
compare between the types of salivary acini
103
duct system of the salivary glands
104
what is the **Striated duct (intralobular/secretory/acidophilic)** and what is its shape under LM?
- It is the continuation of intercalated duct inside lobules
**L/M:**
Lined with **Cubical cells**
**Nucleus:** central rounded
**Cytoplasm:** granular acidophilic
**Apical border:** irregular
**Basal part:** striation
**Lateral border:** indistinct
105
what is the shape of striated duct under EM?
**Cytoplasm:** scattered mitochondria
**Apical border:** Microvilli
**Basal part:** infoldings with mitochondria arranged in
rows between the infoldings
**Lateral border:** Interdigitations
106
what is the function of the striated duct of salivary glands?
striated duct is ion transporting absorb Na+ from the saliva and secrete K+ into it. So, the saliva is hypotonic.
107
what are the main differneces between major salivary glands?
108
what is the second largest organ in the human body?
the liver
109
what type of gland is the liver? and what does it secrete?
110
what is the histological structure of the liver?
stroma and parenchyma
111
what is the capsule of glisson? and what charachterizes it?
- connective tissue covered from outside by peritoneum
- It is thickened at the porta hepatis giving off septa dividing the liver into lobes and lobules
112
Hepatic stroma
- Capsule of Glisson
- Septa
- The portal canals or portal tracts
- Reticular fibres CT
113
what are the types of hepatic septa?
- Interlobar septa
- Interlobular septa
114
where is the porta hepatis?
At the under surface of the liver there is a depression named (Porta hepatis)
115
what is the blood supply of the liver?
116
what are the types of liver lobules?
Types:
**1. Classical lobule:** Hexagonal in histologic section.
**2. Portal lobule:** Triangular in histologic section.
**3. Liver acinus of Rappaport:** Diamond shape in histological section.
117
what is the definition of the classic hepatic lobule?
A mass of liver cells which drains its blood into a central vein running in its longitudinal axis.
118
what is the shape of the classic hepatic lobule?
- Hexagonal or pentagonal.
- Portal triad at the angles of the lobule.
119
Parenchyma of the Classic Hepatic lobule
1. **Hepatocytes** forms **plates radiating from the central vein and extends towards the periphery.**
2. **Blood sinusoids** are present **between** the **hepatocyte plates**
3. **Bile canaliculi** within the plates **between neighboring hepatocytes**
120
what is the difference between the liver of human and pig?
**Liver human:** indistinct lobulation
**Liver pig:** distinct lobulation by C.T septa
121
what is the portal tract?
A triangular mass of (C.T) At **some but not all angles** of the hepatic lobules
122
what are the contents of the portal tract?
1. Portal vein
2. Hepatic artery
3. Bile ducts
4. Lymphatics
123
what are hepatic blood sinusoids? and what lines them?
They are **irregular blood channels** between plates of hepatocytes Are lined by:
1. Endothelial cells
2. Von Kupffer cells
124
what separates the sinusoids from hepatocytes?
space of Disse.
125
what is the structure of the endothelial cells of sinusoids?
- Simple squamous cells
- Forms a non contiuous wall to the blood sinusoids.
- The peripheral portions of the cells have fenestrations not covered with diaphragms
126
what is the function of the endothelial cells of sinusoids?
Allow free passage of chylomicrons and plasma from the blood sinusoids to the space of Disse
127
what is the shape of von kupffer cells of sinusoids?
stellate shape.
128
what is the site of von kupffer cells of sinusoids?
situated on the surface of the endothelial cells
129
what is the function of von kupffer cells of sinusoids?
Phagocytose debris, bacteria, foreign bodies, organisms and old erythrocytes.
130
what is the definition of space of disse?
a **narrow space** between the **sinusoids** and the **parenchymal cells** of the liver
131
what is the structure of space of disse?
β’ Long microvilli from the hepatocytes
β’ Plasma
β’ Adipocytes (Ito cell)
β’ PIT cells (natural killer cells)
β’ Reticular fibers
β’ Unmyelinated nerve fibers
132
what are the functions of space of disse?
1. Facilitates **exchange** between blood plasma and liver cells
2. **Prevent collapse of the blood sinusoids** via its contents plasma,microvilli of the hepatocytes and Ito cells and equal hydrostatic pressure with that of the blood sinusoids
3. lipocytes or adipocytes (hepatic stellate cell) **store fat** and fat soluble vitamin A
133
what percent do hepatocytes represent out of liver cell population?
About 80%
134
charachters of hepatocytes
- Branching and anastomosing cords or plates separated by blood sinasiods
- Each plate is formed of two or more rows of cells in width and one cell thick enclosing bile canaliculus
135
what is the LM of Hepatocytes?
**Shape:** Polygonal outline
**Cytoplasm:**
- Pale acidophilic
- vacuolated cytoplasm (dissolved lipid and glycogen).
**Nucleus:**
- Large rounded or oval nucleus
- 25% are binucleated
136
what are the surfaces of hepatocytes?
137
what is the EM of cell membrane of surfaces of hepatocytes?
1. **Cell surface facing the space of Disse:** possesses numerous long microvilli.
2. **Cell surface facing bile canaliculi:** possesses few scarce microvilli.
3. **Cell surface facing adjacent cell:** possesses extensive interdegitations and cell junctions.
138
branches of the portal tract
(portal triad)
**At the angles of the classic hepatic lobule**
- Portal vein and hepatic artery branch at the portal tract + Bile duct
139
what are the organelles found in the cytoplasm of hepatocytes?
**Show all cell organelles and inclusions:**
1. mitochondria.
2. Golgi complex.
3. Free ribosomes
4. RER.
5. SER.
6. peroxisomes.
7. cytoskeleton (microtubules, microfilaments and intermediate filaments)
8. Cell inclusions (lipid and glycogen)
140
what are the types of liver lobules classified according to?
141
charachters of portal lobule
- Triangular in shape (parts from 3 adjacent classic lobules)
- Central veins at the angles and a portal tract at the axis
- Based on bile flow into a common portal tract
142
charachters of liver acinus of rappaport
- A diamond mass of liver parenchyma from 2 adjacent classic hepatic lobules which surrounds a central vascular core.
- Formed of branches from the hepatic artery & portal vein which arise at right angles from the vessels in a portal canal towards the angles which lack a portal canal.
143
what are the zones of liver acinus of rappaport?
zone I
zone II
zone III
144
charachters of zone I of liver acinus of rappaport
- Spindle-shaped
- Close to the vascular core
- Best blood
- High in Oxygen & nutrition
- Minimal metabolites
145
charachters of zone 2 of liver acinus of rappaport
Have poorer blood supply than zone 1
146
charachters of zone 3 of liver acinus of rappaport
- Close to the central vein
- Poorest blood
- Low in oxygen & nutrition
- High wastes and metabolites
147
what is the concept of liver acinus?
1. The differences in the selective damage of hepatocytes caused by various noxious agents or disease conditions
2. in hypoxemia the microscopic evidence of hepatocyte necrosis is more encountered in zone 3 (Centrilobular).
3. In toxemia, zone 1 is the most affected zone
148
where does bile synthesis and secretion take place? and what is it secreted through?
Bile synthesis takes place inside hepatocytes and secreted by the surface facing the bile canaliculi which have short microvilli
β Intrahepatic biliary passages
β Extrahepatic biliary passages
149
what are the intrahepatic biliary passages? and what are their lining?
150
what are the extrahepatic biliary passages? and what are their lining?
151
what is the definition of the gall bladder?
Is a hollow pear shaped organ attached to the lower surface of the liver
152
what is the histological structure of the gall bladder?
Mucosa
Muscle coat (musclosa)
perimuscular coat (outer coat)
153
mucosa of the gall bladder
- Highly folded particularly in empty bladder.
- Consists of:
β Simple columnar secretory Absorptive epithelium
β Corium
β No muscularis mucosa
154
musclosa **(muscle coat)** of the gall bladder
- Thin muscle coat
- Smooth muscle fibers:
β longitudinal
β transverse
β oblique
155
premuscular coat of the gall bladder **(outer coat)**
β’ Thick fibrous connective tissue
β’ Partially covered with peritoneum.
156
what is the function of gall bladder?
β’ Storage of bile.
β’ Concentration of bile (absorption H2O via the epithelial cells).
