Proteomics - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 396

organs like the esophagus, stomach, small intestine, large intestine, and accessory
organs like salivary glands, liver, pancreas, gall bladder, and others. The stomach
part is mainly responsible for absorption of acidic drugs due to its acidic microenvi-
ronment. The presence of proteolytic enzymes is also a major barrier in the absorp-
tion of peptide and proteins. The stomach is followed by the small intestine, which
is divided into three major portions: (1) the duodenum, (2) the jejunum, and (3) the
ileum—one of the most important sites of drug absorption. The large intestine, or
colon, is divided into the cecum, ascending colon, transverse colon, descending
colon, sigmoid colon, rectum, and anus.
Apart from these, accessory glands like the salivary glands, gall bladder, and pan-
creas also play an important role in digestion and absorption as well as assimilation
of proteins and peptides.
8.4.1.1 Stomach
The stomach consists of well-developed mucosal layers like the muscularis mucosa,
the submucosa, and the serosa. Except for the acidic microenvironment and proteo-
lytic enzymes like pepsin found in the stomach, the overall absorptive role of the
stomach is limited due to the absence of microvilli that are responsible for drug or
food absorption.
8.4.1.2 Small Intestine
Anatomically, the small intestine is divisible into three distinct regions: the initial
length of 25 cm is the duodenum, followed by the 2-m-long jejunum and the 2.75-m-
long ileum. The intestinal membrane wall is composed of four basic layers: (1) the
mucosa, (2) submucosa, (3) muscularis mucosa, and (4) serosa.
The mucosal layer that mainly forms the luminal portion of the small intestine is
predominantly important for absorping and digesting food. The mucosal surface is
composed of fingerlike projections called villi and depressed portions called crypts.
These villi and crypts are lined by epithelial cells that project into the intestinal
lumen.
8.4.1.3 Mucosal Surface
The mucosal layer consists of three major layers: (1) an absorptive layer which is a
continuous sheet of columnar epithelial layer, (2) lamina propria and (3) muscularis
mucosa. The lamina propria also contains a large number of lymphoid cells that are
immunologically important. Lymphoid cells, nodules, and Peyer's patches of lamina
propria act as antigen-stimulating organs, and hence their immunological role should
not be overlooked.
8.4.1.4 Absorptive Layers
These cells are also known as enterocytes or brush border cells. They are 20-30 m
high and 8-10 m thick. The presence of microvilli at the epithelial surface renders a
brush-like appearance, hence these cells are called brush border cells. The microvillus