The lamina propria (LP ) makes up the core of the villus. It contains large numbers of round cells whose individual identity cannot be ascertained at this magnification. Note, however, that these are mostly lymphocytes (and other cells of the immune system), which accounts for the designation of the lamina propria as diffuse lymphatic tissue. The lamina propria surrounding the intestinal glands (IGl ) similarly consists largely of lymphocytes and related cells. The lamina propria also contains components of loose connective tissue and isolated smooth muscle cells.
The intestinal glands (IGl ) are relatively straight and tend to be dilated at their base. The bases of the intestinal crypts contain the stem cells from which all of the other cells of the intestinal epithelium arise. They also contain Paneth cells. These cells possess eosinophilic granules in their apical cytoplasm. The granules contain lysozyme, a bacteriolytic enzyme thought to play a role in regulating intestinal microbial flora. The main cell type in the intestinal crypt is a relatively undifferentiated columnar cell. These cells are shorter than the enterocytes of the villus surface; they usually undergo two mitoses before they differentiate into absorptive cells or goblet cells. Also present in the intestinal crypts are some mature goblet cells and enteroendocrine cells.
KEY BGl, Brunner’s glands C, circular (inner) layer of muscularis externa D, duct of Brunner’s gland GC, goblet cells IGl, intestinal glands (crypts) L, longitudinal (outer) layer of muscularis externa LP, lamina propria ME, muscularis externa MM, muscularis mucosae Muc, mucosa S, serosa SubM, submucosa V, villi arrow, duct of Brunner’s gland asterisk, leaflike villus dashed line (top figure), boundary between base of villi and intestinal glands
The jejunum is the principal site of absorption of nutrients in the small intestine. The villi are more finger like than leaf like and are covered largely with absorptive columnar epithelial cells (enterocytes), although goblet cells and enteroendocrine cells are also present. The stem cells for all of these cells and the Paneth cells that secrete the antibacterial enzyme lysozyme are found deep in the intestinal gland. Replicat-ing cells line the lower half of the gland.
Jejunum, monkey, H&E ×22.
This is a longitudinal section of the jejunum, showing the permanent circular folds of the small intestine, the plicae circulares (PC ). These folds or ridges are mostly arranged with their long axis at roughly right angles to the longitudinal axis of the intestine; therefore, the plicae circulares shown here are cut in cross section. The plicae circulares consist of mucosa (Muc) as well as sub-mucosa (SubM ). The broad band of tissue external to the submucosa is the
Plica circularis, jejunum, monkey, H&E ×60.
Part of the plica circularis marked by the bracket in figure above is shown at higher magnification. Note the muscularis mucosae (MM ), the intestinal glands (GI ), and the villi (V ). The boundary between the glands and villi is marked by the dashed line. Some of the glands are cut longitudinally; some are cut in cross section; most of the villi have been cut longitudinally. In conceptualizing the mucosal structure of the small intestine, it is important to recognize that the glands are  epithelial depressions that project into the wall
Intestinal villi, jejunum, monkey, H&E ×500.