HISTOLOGY OF THE DIGESTIVE SYSTEM I ... - Faculty.rmc.edu

HISTOLOGY OF THE DIGESTIVE SYSTEM
I. Introduction
A. Functions of the digestive system
1. Digestion of ingested foodstuffs into molecules which can be absorbed involves two
processes.
a. Mechanical breakdown of food into smaller particles
b. Enzymatic breakdown of macromolecules into their component monomers,
2. Absorption of the resultant products into blood and lymph is enhanced by several
features of the digestive tract.
a. The lining epithelium of the small intestine has active transport enzymes for some
solutes and contains structures which increase uptake of other solutes by simple
diffusion.
b. The surface area of the small intestine lining epithelium is increased due to folds,
villi, and microvilli. This allows for maximum absorption of molecules released by
enzymatic breakdown of macromolecules in food.
c. Highly developed muscle layers (usually smooth muscle) stir food during absorption.
B. Structure of the digestive system (Figure 17.1)
1. The digestive system is a long continuous tube extending from the mouth to the anus
and subdivided into the oral cavity, pharynx, esophagus, stomach, small and large
intestines.
2. Located within the wall of the tubular portion of the digestive tract are numerous glands
(both exocrine and endocrine), referred to as intrinsic glands.
3. Larger exocrine glands referred to as extrinsic or extra-mural glands appear to be
separate from the tubular portion of the digestive tract, but these glands have ducts
which pass through the wall of the digestive tube and deliver the secretory products of
the gland to the location in which they function. Extrinsic glands include the salivary
glands, pancreas, and liver (including the gall bladder).
1

II.
Microanatomy of the Tubular Portion of the Digestive Tract
A. Typical Pattern of Layers in Digestive Tract Walls (Figure 17.1)
1. Mucosa (lining of lumen)
a. Epithelium = varies with location: stratified squamous in the mouth, esophagus,
and anus; simple columnar in the stomach and intestines
b. Lamina propria = loose FECT
c. +/- Muscularis mucosae (thin) - smooth muscle
2. Submucosa
Loose to moderately dense FECT
3. Muscularis externa
Smooth muscle along most of the gut
Skeletal muscle near both ends of the gut
Usually consists of inner circular and outer longitudinal layers
4. Adventitia or Serosa
Adventitia = loose FECT (on organ surfaces embedded in connective tissue)
Serosa = loose FECT + mesothelium (on organ surfaces exposed to body cavities)
B. Oral Cavity and Pharynx (excluding nasopharynx)
1. Mucosa (Plate 44)
a. Epithelium = stratified squamous epithelium (Figure 16.2, 16.4)
The epithelium is heavily keratinized on the upper (dorsal) surface of the tongue, is
moderately keratinized on the hard palate and on parts of the gums, and is nonkeratinized
elsewhere.
b. Lamina propria + Submucosa (Figure 16.4)
The lamina propria and submucosa are not clearly separated in most of the oral
cavity and pharynx. A very thin layer of loose FECT occurs at the base of the
epithelium in most areas. Most of the deeper connective tissue is moderately dense
FECT or dense irregular FECT in most locations. Minor salivary glands
(compound sero-mucous, tubulo-acinar glands) lie in the connective tissue on parts
of the tongue and lateral walls of the oral cavity, and the connective tissue around
those glands is loose FECT. Nodular dense lymphoid tissue lies below the
epithelium lining the crypts of the tonsils.
2

2. Teeth (Figure 16.6, 16.7, 16.10 - 16.20)
a. Enamel (outer layer of crown of tooth)
Enamel is formed by ameloblasts prior to eruption of the tooth. Since the
ameloblasts are destroyed in the process of tooth eruption, enamel cannot be
repaired or replaced on an existing tooth in humans. Enamel contains 96% calcium
salts (hydroxyapatite), 1% organic matrix, and 3% water, making it the hardest
material in the human body.
b. Dentin
Dentin is formed by odontoblasts throughout life. Dentin contains 70% calcium salts
(hydroxyapatite) and 20% organic matrix (90% of which is collagen), and 10%
water.
c. Dental Pulp
(1) Odontoblast layer
Odontoblasts form a simple columnar epithelium between the dentin and the
core of the dental pulp. The odontoblasts extend long apical processes through
the dentin to the dentin- enamel junction. Associations of odontoblast
processes and cell bodies with sensory nerve fibers suggest that odontoblasts
may be involved in pain reception, at least at the dentin-enamel interface.
(2) Core of loose FECT
d. Cementum
The core connective tissue contains fibroblasts and fine collagen fibers similar
to those in mucous CT, but also contains macrophages, mast cells, leucocytes,
and numerous capillaries and nerves. Both sensory and autonomic "motor"
fibers have been reported to be present in dental pulp.
Cementum is a bone matrix-like matrix material which is acellular near the crown of
the tooth, but contains osteocyte-like cementocytes near the root of the tooth.
Unlike bone, cementum is avascular.
e. Periodontal Ligament
The periodontal ligament (or membrane) is a layer of dense collagenous connective
tissue interspersed with loose FECT containing nerve endings and blood vessels.
This tissue attaches the cementum on the root of the tooth to the surrounding
alveolar bone in a strong but flexible manner.
3

f. Alveolar Bone
Alveolar bone consists of a surface layer of compact bone (with some areas of
woven bone) supported by a core of spongy bone. The alveolar bone surrounds the
root of the tooth.
g. Gingiva
Gingiva is covered by a moderately keratinized stratified squamous epithelium in
which dermal papillae are more numerous than in most areas of oral mucosa. The
epithelium at the gingival "cuff" is attached to the tooth surface by basal lamina- like
material, sealing the periodontal ligament from the oral cavity.
3. Tongue (Figure 16.4, 16.5, Plate 45, 46)
a. The tongue is covered by oral mucosa. The epithelium on the upper surface is
heavily keratinized in places, particularly on the tips of filiform papillae.
b. The tongue contains a core of skeletal muscle arranged in bundles. The bundles
run in 3 planes at right angles to each other (i.e. in X, Y, and Z geometric axes)
allowing movement of the tongue in almost any direction and allowing delicate
controlled changes in the shape of the tongue.
c. The upper surface of the tongue (the epithelium plus the adjacent lamina propria)
forms numerous finger- like papillae which are classified into 3 categories.
(1) Filiform Papillae
(a) Pointed tip
(b) Heavily keratinized epithelium
(c) No taste buds
(d) Most numerous type of papilla
(2) Fungiform Papillae
(a) Flattened apex
(b) Moderately keratinized epithelium
(c) Apical taste buds
(d) Second most numerous type
4

(3) Circumvallate Papillae
(a) Flattened apex
(b) Moderately keratinized epithelium
(c) Lateral taste buds in the papilla and in the surrounding structures
(d) Surrounded by a groove with compound serous glands (von Ebner's
glands) secreting into its base
(e) Least numerous type of papilla
C. Esophagus (Figure 17.2 - 17.6, Plate 50, 51)
1. Mucosa
a. Epithelium - Stratified squamous epithelium
b. Lamina propria - loose FECT, may contain branched tubular cardiac glands near the
upper and lower ends of the esophagus
c. Muscularis mucosae - usually longitudinally arranged smooth muscle
2. Submucosa
a. Loose to moderately dense FECT
b. May contain mucous-secreting branched (occasionally compound) tubulo-acinar
glands
3. Muscularis externa
a. Two layers
Inner layer = (more or less) circular
Outer layer = (more or less) longitudinal
b. Skeletal muscle is gradually replaced by smooth muscle
upper 1/3 of esophagus - mostly skeletal muscle
middle 1/3 of esophagus - mixed
lower 1/3 of esophagus - mostly smooth muscle
4. Adventitia/Serosa
Loose FECT -/+ outer mesothelium
5

D. Stomach (Figure 17.5 - 17.16, Plate 51, 52, 54)
1. Mucosa
a. Epithelium
(1) Mainly simple columnar; may be pseudostratified columnar in regions
containing some types of enterochromaffin (APUD) cells (unicellular endocrine
glandular epithelium)
(2) Most cells of the lumen lining are mucous- secreting so the lining epithelium
may be called a sheet gland
(3) Enterochromaffin cells (or APUD cells) are small with basally located light
cytoplasm and dark nuclei. At least some of these cells extend a sensory
process to the luminal surface. Some may have acidophilic granules at the
basal end.
b. Lamina propria - loose FECT
c. Muscularis mucosae - inner circular and outer longitudinal smooth muscle layers,
frequently in bundles
d. Other features
(1) The mucosa forms longitudinal folds or rugae.
(2) The mucosa contains gastric glands (simple tubular glands) which are referred
to as cardiac glands, fundic glands, or pyloric glands based on the gland
structure and the region of the stomach in which the gland is normally found.
Pure cardiac and pyloric glands would contain only mucus-secreting cells plus
occasional enterochromaffin cells while fundic glands contain multiple exocrine
cell types.
(a) Cardiac glands are branched tubular glands which occur in a narrow zone
near the esophageal-stomach junction. They are usually shorter and less
coiled than pyloric glands.
(b) Pyloric glands are located near the pyloric region of the stomach. These
branched tubular glands are usually longer and more highly coiled than
cardiac glands.
(c) Fundic glands occur throughout most of the stomach. These branched
tubular glands contain three exocrine cell types plus numerous solitary
endocrine cells.
[1] Exocrine cell types
[a] Chief cells (or zymogenic or peptic cells) (Figure 17.8, 17.9)
- relatively columnar cells with basal nuclei, basophilic basal
cytoplasm and acidophilic apical cytoplasm containing
acidophilic secretory granules
- serous secretory cells which secrete pepsinogen
- located in the deeper 1/3 of the glands
6

[b] Parietal cells (or oxyntic cells) (Figure 17.8, 17.10, 17.11)
- spherical or triangular cells with central nuclei and canaliculi
and with acidophilic cytoplasm
- secrete HCl and intrinsic factor
- occur singly or in small groups throughout the deeper 2/3 of
the glands
[c] Mucous cells (Figure 17.8)
- columnar cells with pale staining cytoplasm
- mucous secretory cells
- located throughout the glands but predominate in the upper
1/3 of the glands
[2] Enteroendocrine cells (enterochromaffin cells)(Figure 17.12, 17.13)
- different cells secrete a variety of locally acting hormones (mostly
peptides or proteins such as secretin, cholecystokinin, secretin,
and gastric inhibitory peptide) which coordinate gut functions.
Some of the secretory granules may be acidophilic while others
may not stain well.
- some of these cells have the properties of enterochromaffin cells
also called APUD (amine precursor uptake and decarboxylation)
cells
- most of these cells are small, basally located cells with light
cytoplasm and moderately heterochromatic nuclei. A thin
cytoplasmic process may extend to the apical surface of the
epithelium.
2. Submucosa
loose to moderately dense FECT
3. Muscularis externa
two or three (most common) layers of smooth muscle
a. Where two layers are present they are organized into inner circular and outer
longitudinal layers.
b. Where three layers are present they are organized into inner oblique, middle
circular, and outer longitudinal layers.
c. As is the case with other "pear-shaped" or oval organs, the muscle tends to run in
bundles rather than in distinct layers.
4. Serosa
loose FECT and outer mesothelium
7

E. Small intestine (Figure 17.17 - 17.31, Plate 54, 55, 56, 57)
1. Mucosal features occurring throughout the small intestine
a. The mucosa and submucosa of the small intestine are involved in permanent
transverse folds called plicae circulares.
b. Numerous villi (small finger-like projections) extend from the surface into the lumen.
These occur in all regions of small intestine, but do not occur in large intestine
mucosa.
c. Numerous simple tubular glands (Crypts of Lieberkuhn) extend from the surface
into the lamina propria. These occur in both small and large intestine mucosa.
d. The epithelium lining both villi and crypts varies from simple columnar to
pseudostratified columnar (where endocrine cells occur). The following cell types
occur in the epithelia of both crypts and villi (except as noted)
(1) Enterocytes - intestinal absorptive cells (Figure 17.21, 17.26, 17.29 – 17.31)
(a) Enterocytes are tall columnar cells with elongated ovoid somewhat basally
located nuclei and apical microvilli.
(b) Various enzymes (disaccharidases, peptidases, enteropeptidase [also
called enterokinase]) are integral glycoproteins of the plasma membrane
and glycocalyx covering the microvilli.
(c) Functions
[1] The enteropeptidase on enterocytes can activate trypsinogen
(becomes trypsin) which can then activate the other enzymes secreted
by the pabcreas.
[2] The cell surface enzymes on enterocytes digest disaccharides and
peptides (final steps in polysaccharide and protein digestion).
[3] Enterocytes absorb monosaccharides, amino acids, and lipids and
release them into the sub-epithelial connective tissue (lamina propria)
in the villi.
(2) Goblet cells (Figure 17.22) are wine glass-shaped columnar cells with apically
located mucus secretory granules and basally located heterochromatic nucleii.
They are mucus-secreting unicellular exocrine glands.
(3) Enteroendocrine cells (enterochromaffin cells, solitary endocrine cells)(Figure
17.25)
(a) These include many different cells which secrete a variety of locally acting
hormones (mostly peptides or proteins such as secretin, cholecystokinin,
secretin, and gastric inhibitory peptide) which coordinate gut functions.
Some of the secretory granules may be acidophilic while others may not
stain well.
(b) Some of these cells have the properties of enterochromaffin cells also
called APUD (amine precursor uptake and decarboxylation) cells.
8

(c) Most of these cells are small, basally located cells with light cytoplasm and
moderately heterochromatic nuclei. A thin cytoplasmic process may
extend to the apical surface of the epithelium. Some of these cells contain
basally located acidophilic secretory granules.
(4) Paneth cells (Figure 17.24)
(a) Paneth cells are pyramidal shaped cells with basally located spherical
nuclei and apical eosinophilic secretory granules, and are found only at the
base of Crypts of Lieberkuhn (primarily those in the small intestine).
(c) Paneth cells secrete peptidase and lysozyme and can take up some types
of bacteria and protozoa by phagocytosis, so they appear to function to
reduce bacterial invasion into the crypts of Lieberkuhn.
(5) M (microfold) cells (Figure 17.38)
(a) M cells are nearly squamous-shaped epithelial cells which overlie Peyer's
patches and other well-developed mucosal lymphoid nodules in the
intestine. M cells have folded apical surfaces instead of microvilli.
(c) M cells take up molecules from the intestinal lumen by endocytosis and
transfer the molecules to the lymphocytes and antigen-presenting cells in
the lamina propria.
e. The lamina propria (connective tissue under the lining epithelium) is very loose
FECT containing large populations of lymphocytes, plasma cells, granulocytes, and
macrophages. The lamina propria can be viewed as diffuse lymphoid tissue.
f. A muscularis mucosae containing one or two layers of smooth muscle usually
separates the lamina propria from the submucosa. The muscularis mucosae may
be interrupted or absent where glands or lymphoid nodules entend from the
submucosa into the lamina propria.
2. Duodenum (Figure 17.27, Plate 55)
a. The mucosa of the duodenum is typical of the rest of the small intestine (see
above).
b. Submucosa - Loose to moderately dense FECT (which contains Brunner's glands
in the upper duodenum). Brunner's glands are "compound" tubular glands which
secrete sodium bicarbonate and mucus.
c. Muscularis externa - Two layers of smooth muscle (inner circular and outer
longitudinal)
d. Adventitia/Serosa - loose FECT -/+ outer mesothelium
3. Jejunum (Plate 56)
a. The mucosa of the jejunum is typical of the rest of the small intestine (see above).
9

. Submucosa - Loose to moderately dense FECT (which lacks Brunner's glands and
Peyer's patches)
c. Muscularis externa - Two layers of smooth muscle (inner circular and outer
longitudinal)
d. Adventitia/Serosa - loose FECT -/+ outer mesothelium
4. Ileum (Plate 57)
a. The mucosa of the ileum is typical of the rest of the small intestine (see above).
b. Submucosa - Loose to moderately dense FECT. In some areas large lymphoid
nodules (Peyer's patches) extend from the luminal epithelium into the submucosa.
c. Muscularis externa - Two layers of smooth muscle (inner circular and outer
longitudinal)
d. Adventitia/Serosa - loose FECT -/+ outer mesothelium
E. Large Intestine
1. Colon (Figure 17.32, 17.33, Plate 58)
a. The mucosa of the colon resembles the mucose of the small intestine except for the
following features.
(1) No villi are present.
(2) Goblet cells are typically more numerous than in the small intestine.
b. Submucosa - loose to moderately dense FECT
c. Muscularis externa - two layers of smooth muscle (inner circular and outer
longitudinal layers). The outer longitudinal layer is organized into three longitudinal
bands (tenia coli) with only a thin layer around the rest of the surface.
d. Serosa - loose FECT and outer mesothelium
2. Appendix (Figure 17.35, Plate 59)
a. The mucosa of the appendix resembles the colon except that small lymphocytes
are frequently observed crossing the epithelium near lymphoid nodules.
b. The submucosa is loose to moderately dense FECT containing large lymphoid
nodules which frequently extend into the lamina propria, interrupting the muscularis
mucosa.
10

c. The muscularis externa typically contains two layers of smooth muscle (inner
circular and outer longitudinal layers) and is much thinner than in the colon.
d. The serosa consists of loose FECT and an outer mesothelium.
3. Rectum and anal canal (Figure 17.36, 17.37, Plate 60)
a. Mucosa
(1) The epithelium is simple columnar in the rectum and changes over a distance
of a few cells into stratified squamous in anal canal. The stratified squamous
epithelium is initially nonkeratinized, but rapidly changes to keratinized near the
exposed part of the anus.
(2) The lamina propria is loose FECT. It merges with the papillary layer of the
dermis of the skin around the anus.
(3) A muscularis mucosae consisting of smooth muscle is present under the simple
columnar epithelium, but usually disappears under the stratified squamous
epithelium.
b. The submucosa is loose to moderately dense FECT and is highly vascularized.
The submucosa merges with the reticular layer of the dermis of the skin around the
anus.
c. Muscularis externa
(1) Two layers of smooth muscle occur along most of the rectum and anus. The
inner circular layer forms the internal anal sphincter in the anus while the
outer longitudinal layer is fairly consistant throughout the rectum and anus.
(2) Skeletal muscle (outside the smooth muscle layers) forms the external anal
sphincter around the anus.
(3) The muscularis externa disappears at the junction between the anus and the
skin covering the outer body surface.
d. The adventitia along most of the rectum and anus consists of loose FECT which
merges with the hypodermis layer under the skin around the anus. Part of the
rectum is exposed to the abdominal cavity and has an outer mesothelium, making
the outer layer a serosa.
11

III. Microanatomy of Extrinsic Exocrine Glands Associated with the Digestive Tract
A. Terms used to describe ducts of compound exocrine glands
1. Terms based on morphology of the lining epithelium
a. Intercalated ducts
- lined by simple squamous to simple cuboidal epithelia, may have myoepithelial
cells
- smallest ducts, connected to secretory units
b. Striated ducts
- lined by simple columnar epithelium with basal infoldings in the cells
- function in water reabsorption from the secretions of the gland
c. Excretory ducts
- lined with simple columnar to stratified columnar epithelium
2. Terms based on position of the duct
a. Intralobular ducts
- located within a lobule in a gland
- therefore not located in a connective tissue septum
b. Interlobular ducts
- located between lobules (smaller connective tissue-wrapped components) of a
gland
- therefore located within a connective tissue septum which separates lobules of
a gland
c. Interlobar ducts
- located between lobes (larger connective tissue-wrapped components) of a
gland
- therefore located within a connective tissue septum which separates lobes of a
gland
Intercalated ducts and striated ducts are always intralobular, but excretory ducts
may be interlobular or interlobar. Interlobar ducts would be larger in diameter, have
a thicker lining epithelium, and be running in larger connective tissue septa than
interlobular ducts.
12

B. Salivary glands (Figure 16.22 - 16.28, Plate 47, 48, 49)
1. Three pairs of major salivary glands occur in humans
a. Parotid glands
b. Submandibular (submaxillary) glands
c. Sublingual glands
2. Minor salivary glands are widespread within the wall of the oral cavity and usually are
seromucous (tubuloacinar).
3. Organization of the major salivary glands
a. Major salivary glands are compound acinar (serous) or compound tubulo-acinar
(seromucous, also called mixed) glands.
b. Secretory units are usually surrounded by myoepithelial cells.
c. Ducts
(1) Intercalated - simple cuboidal epithelium
(2) Striated - simple columnar epithelium with basal infoldings which concentrates
secretory product by removing sodium ions and water
(3) Excretory - simple columnar, pseudostratified columnar, stratified cuboidal, or
stratified columnar epithelium
d. Capsule - moderately dense FECT
e. Loose to moderately dense FECT septae divide the glands into lobes and lobules
and are continuous with the loose FECT stroma within the lobules.
4. The major salivary glands in humans differ in type of secretion and in the types of ducts
present. Other mammals have different patterns.
a. Parotid = totally pure serous acinar units with intercalated, striated, and excretory
ducts present
b. Submandibular = pure serous acinar units predominate but mucous units capped
with serous demilunes (mixed units) occur in many areas of the gland; intercalated,
striated, and excretory ducts are present.
c. Sublingual = mucous acini and short tubules with serous demilunes (mixed units)
are the most frequent unit. A few serous acini may occur but no large areas of
serous units occur. Only intercalated and excretory ducts occur.
NOTE – the demilune arrangement of the serous cells in seen in routine preparations of the
submandibular and sublingual salivary glands appears to be a fixation artifact, since
rapid freezing shows intermingled serous and mucous cells (see Figure 16.23).
13

5. Salivary gland secretions prepare food for transport in the esophagus
a. Serous secretions - contain fluid and alpha amylase (which fragments and
solubilizes starchy materials) plus some immunoglobulin A (function is unclear)
b. Mucous secretions - provide lubrication for food as it passes down the esophagus
C. Pancreas (Figure 18.19 - 18.22, Plate 64)
1. Composed of exocrine acini and Islets of Langerhans (endocrine) on a stroma of loose
FECT
2. Organization
a. Exocrine portion contains only serous acinar units
b. Ducts
(1) Intercalated ducts = squamous to low cuboidal epithelium; extend into acinus as
centroacinar cells
(2) Intralobular ducts = simple cuboidal to simple columnar, no striated ducts are
present
(3) Excretory ducts are lined by simple columnar epithelium which they are located
within the gland.
c. Capsule is loose FECT which may be covered in some places by mesothelium
3. Composition and function of exocrine secretions of the pancreas
a. Amylase - breaks down starch
b. Trypsinogen, chymotrypsinogen, carboxypeptidase - break down proteins/peptides
c. Ribonuclease - breaks down RNA
d. Lipase - breaks down lipids
14

D. Liver (Figure 18.1 – 18.14, Plate 61, 62)
1. The liver is composed of an epithelial parenchyma (hepatocytes) on a stroma of FECT
(primarily just reticular fibers around hepatocytes). The capsule of the liver is
moderately dense FECT and is surrounded in some regions by mesothelium.
2. Organization
a. A classical lobule has a central "vein" (actually a giant postcapillary venule)
located in the center each lobule.
b. Liver cells (hepatocytes) within a classical lobule are arranged in flat plates (appear
as cords in two dimensions) which radiate out from central vein.
c. Portal triads (portal canals, portal areas) are located at the perimeter of each
lobule. Each portal triad contains
(1) a branch of the hepatic artery (usually an arteriole)
(2) a branch of the hepatic portal vein (usually a venule)
(3) a branch of the bile duct (simple cuboidal epithelium)
A lymphatic vessel is also present in many portal triads.
e. Sinusoidal capillaries located between plates of hepatocytes are lined by
endothelial cells and Kupffer cells (phagocytic). The sinusoids receive blood from
the hepatic portal vein and from the hepatic artery and empty into the central "veins"
which drain into the hepatic vein.
f. Bile canaliculi (narrow channels) located between adjacent hepatocytes collect bile
secreted by the hepatocytes and empty into branches of the bile duct running in the
portal triads.
3. Characteristics of Hepatocytes
a. Hepatocytes are large polyhedral cells with heavily euchromatic nuclei, distinct
nucleoli, and finely granular eosinophilic cytoplasm.
b. Functions include
(1) Glycogen synthesis and subsequent breakdown
(2) Synthesis and secretion of bile
(3) Synthesis and secretion of lipids, lipoproteins, and blood plasma proteins
(albumin, prothrombin, fibrinogen)
(4) Detoxification of materials present in blood
(5) Interconversion of sugars, amino acids, and fatty acids
15

4. Gall bladder (Figure 18.15 - 18.18)
a. Mucosa
(1) Epithelium = simple columnar epithelium with apical microvilli, numerous
mitochondria, and lateral extracellular spaces at the basal ends
(2) Lamina propria = loose FECT, contains branched tubuloalveolar mucous glands
in the region near the neck of the gallbladder
b. Muscularis externa = bundled smooth muscle surrounded by loose FECT
c. Adventitia/serosa = loose FECT containing numerous lymphatic vessels, covered
by mesothelium on the exposed face of the gallbladder
16