866
UNIT 4
Maintenance of the Body
23
Te cells forming the walls of the gastric pits are primarily
mucous cells, but those composing the gastric glands vary in
different stomach regions. For example, the cells in the glands
of the cardia and pylorus primarily secrete mucus, whereas cells
of the pyloric antrum produce mucus and several hormones
including most of the stimulatory hormone called gastrin.
Types of Gland Cells
Glands of the stomach fundus and body, where most digestion
occurs, are substantially larger and produce the majority of the
stomach secretions. Te glands in these regions contain a va-
riety of secretory cells, including mucous neck, parietal, chief,
and enteroendocrine cells.
Mucous Neck Cells
Mucous neck cells
, scattered in the “neck”
and more basal regions of the glands, produce a thin, soluble
mucus quite different from that secreted by the mucous cells of
the surface epithelium (Figure 23.15b). It is not yet understood
what special function this
acidic
mucus performs.
Parietal Cells
Parietal cells
, found mainly in the more apical
region of the glands scattered among the chief cells (described
next), simultaneously secrete
hydrochloric acid
(
HCl
) and
in-
trinsic factor
(Figure 23.15b, c). Although parietal cells appear
oval when viewed with a light microscope, they actually have
three prongs that bear dense microvilli (they look like fuzzy
pitchforks!). Tis structure provides a huge surface area for se-
creting H
1
and Cl
2
into the stomach lumen.
HCl makes the stomach contents extremely acidic (pH 1.5–
3.5), a condition necessary for activation and optimal activity of
protein-digesting enzyme
pepsin
. Te acidity also helps digest
food by denaturing proteins and breaking down cell walls of
plant foods, and is harsh enough to kill many of the bacteria
ingested with foods. Intrinsic factor is a glycoprotein required
for vitamin B
12
absorption in the small intestine.
Chief Cells
Chief cells
occur mainly in the basal regions of
the gastric glands. Te cuboidal chief cells produce
pepsinogen
(pep-sin
9
o-jen), the inactive form of the pepsin. When these
cells are stimulated, the first pepsinogen molecules they release
are activated by HCl encountered in the apical region of the
gland (Figure 23.15c). But once pepsin is present, it also cata-
lyzes the conversion of pepsinogen to pepsin. Te activation
process involves removing a small peptide fragment from pep-
sinogen, causing it to change shape and expose its active site.
Tis positive feedback process is limited only by the amount of
pepsinogen present.
Chief cells also secrete lipases (fat-digesting enzymes). For-
merly thought to be insignificant in fat digestion, these gas-
tric lipases now appear to account for about 15% of overall GI
lipolysis.
Enteroendocrine Cells
Enteroendocrine cells
(en
0
ter-o-en
9
do-
krin; “gut endocrine”), typically located deep in the gastric glands
(Figure 23.15b, c), release a variety of chemical messengers directly
into the interstitial fluid of the lamina propria. Some of these, for
example
histamine
and
serotonin
, act locally as paracrines. Oth-
ers, such as
somatostatin
, act both as paracrines locally and as
the cardia. Te
body
, or the midportion of the stomach, is con-
tinuous inferiorly with the funnel-shaped
pyloric part
. Te
wider and more superior area of the pyloric part, the
pyloric
antrum
(
antrum
5
cave) narrows to form the
pyloric canal
,
which terminates at the
pylorus
. Te pylorus is continuous with
the duodenum through the
pyloric sphincter
or
valve
, which
controls stomach emptying (
pylorus
5
gatekeeper).
Te convex lateral surface of the stomach is its
greater cur-
vature
, and its concave medial surface is the
lesser curvature
.
Extending from these curvatures are two mesenteries, called
omenta
(o-men
9
tah), that help tether the stomach to other
digestive organs and the body wall (see Figure 23.30, p. 889).
Te
lesser omentum
runs from the liver to the lesser curva-
ture of the stomach, where it becomes continuous with the vis-
ceral peritoneum covering the stomach. Te
greater omentum
drapes inferiorly from the greater curvature of the stomach to
cover the coils of the small intestine. It then runs dorsally and
superiorly, wrapping the spleen and the transverse portion of
the large intestine before blending with the
mesocolon
, a dorsal
mesentery that secures the large intestine to the parietal perito-
neum of the posterior abdominal wall.
Te greater omentum is riddled with fat deposits (
oment
5
fatty skin) that give it the appearance of a lacy apron. It also con-
tains large collections of lymph nodes. Te immune cells and
macrophages in these nodes “police” the peritoneal cavity and
intraperitoneal organs.
Te stomach is served by the autonomic nervous system.
Sympathetic fibers from thoracic splanchnic nerves are relayed
through the celiac plexus. Parasympathetic fibers are supplied
by the vagus nerve. Te arterial supply of the stomach is pro-
vided by branches (gastric and splenic) of the celiac trunk (see
Figure 19.24). Te corresponding veins are part of the hepatic
portal system and ultimately drain into the hepatic portal vein
(see Figure 19.29c).
Microscopic Anatomy
Te stomach wall contains the four tunics typical of most of the
alimentary canal, but its muscularis and mucosa are modified for
the special roles of the stomach. Besides the usual circular and
longitudinal layers of smooth muscle, the muscularis externa has
an incomplete innermost layer of smooth muscle fibrils that runs
obliquely
(Figure 23.14a and
Figure 23.15a
). Tis arrangement al-
lows the stomach not only to mix, churn, and move food along the
tract (the job of the circular and longitudinal muscle layers), but
also to pummel the food, physically breaking it down into smaller
fragments and ramming it into the small intestine. (Te oblique
fibers accomplish the ramming by jackknifing the stomach into a
V shape, which provides a propulsive action in the pyloric part.)
Te lining epithelium of the stomach mucosa is a simple
columnar epithelium composed entirely of mucous cells. Tey
produce a cloudy, protective two-layer coat of alkaline mucus in
which the surface layer consists of viscous, insoluble mucus that
traps a layer of bicarbonate-rich fluid beneath it. Tis otherwise
smooth lining is dotted with millions of deep
gastric pits
, which
lead into tubular
gastric glands
that produce the stomach secre-
tion called
gastric juice
(Figure 23.15).
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