FIGURE 17.24 • Diagram of immunoglobulin A (IgA) secretion and transport. A monomeric form of immunoglobulin A (IgA) is synthesized by the plasma cell. IgAs are secreted into the lamina propria in a dimeric form as dIgA. Dimeric dIgA is composed of two monomeric IgA subunits and a polypeptide J chain also produced by the plasma cell. In the lamina propria dIgA binds to the polymeric immunoglobulin receptor (pIgR) on the basal cell membrane of the enterocyte. The pIgR-IgA complex enters the cell by endocytosis and is carried out within the endocytotic vesicles to the eary endosomal compartment and then to the apical surface (a process called transcytosis). Endocytic vesicles fuse with the apical plasma membrane, the pIgR is proteolytically cleaved and dIgA is released with the extracellular portion of the pIgR receptor. This portion of the pIgR remains with the IgA dimer and become the secretory component (SC) of the secretory IgA (sIgA).
FIGURE 17.25 • Photomicrograph of Brunner’s glands in the duodenum. This photomicrograph shows part of the duodenal wall in an H&E preparation. A distinctive feature of the duodenum is the presence of Brunner’s glands. The dashed line marks the boundary between the villi and the typical intestinal glands (crypts of Lieberkn). The latter extend to the muscularis mucosae. Under the mucosa is the submucosa, which contains Brunner’s glands. These are branched tubular glands whose secretory component consists of columnar cells. The duct of the Brunner’s gland opens into the lumen of the intestinal gland (arrow). 120.
The muscularis externa consists of an inner layer of circularly arranged smooth muscle cells and an outer layer of longitudinally arranged smooth muscle cells. The main components of the myenteric plexus (Auerbach’s plexus) are located between these two muscle layers (Fig. 17.26). Two kinds of muscular contraction occur in the small intestine. Local contractions displace intestinal contents both proximally and distally; this type of contraction is called segmentation. These contractions primarily involve the circular muscle layer. They serve to circulate the chyme locally, mixing it with digestive juices and moving it into contact with the mucosa for absorption. Peristalsis, the second type of contraction, involves coordinated action of both circular and longitudinal muscle layers and moves the intestinal contents distally.
The serosa of the parts of the small intestine that are located intraperitoneally in the abdominal cavity corresponds to the general description at the beginning of the chapter.
Epithelial Cell Renewal in the Small Intestine
All of the mature cells of the intestinal epithelium are derived from a single stem cell population.
Stem cells are located in the base of the intestinal gland. This intestinal stem cell niche (zone of cell replication) is restricted to the lower one half of the gland and contains highly proliferative intermediate cells (as previously explained) and cells at various stages of differentiation. A cell destined to become a goblet cell or absorptive cell usually undergoes several additional divisions after it leaves the pool of stem cells. The epithelial cells migrate upward in the intestinal gland onto the villus where they undergo apoptosis and slough off into the lumen. Autoradiographic studies have shown that the renewal time for absorptive and goblet cells in the human small intestine is 4 to 6 days.