chapter 5 Epithelial Tissue G LAN D S TABLE Classification of Multicellular Glands5.6 Classification Typical Location Features Simple tubular Large intestine: intestinal Secretory portion of the gland glands of the colon is a straight tube formed by the secretory cells (goblet cells) Simple coiled Skin: eccrine sweat gland Coiled tubular structure is tubular composed of the secretory portion located deep in the dermis Simple branched Stomach: mucus-secreting Branched tubular glands with tubular glands of the pylorus wide secretory portion are formed by the secretory cells and produce a viscous mucous secretion Simple acinar Urethra: paraurethral Simple acinar glands develop and periurethral glands as an outpouching of the transitional epithelium and are formed by a single layer of secretory cells Branched acinar Stomach: mucus-secreting Branched acinar glands with glands of cardia secretory portions are formed by mucus-secreting cells; the short, single-duct portion opens directly into the lumen Compound Duodenum: submucosal Compound tubular glands with tubular glands of Brunner coiled secretory portions are located deep in the submucosa of the duodenum Compound Pancreas: excretory portion Compound acinar glands with acinar alveolar-shaped secretory units are formed by pyramid-shaped serous-secreting cells Compound Submandibular salivary Compound tubuloacinar glands tubuloacinar gland, mammary gland, can have both mucous branched  lacrimal gland tubular and serous branched acinar secretory units; they have serous end-caps (demilunes) Simple GlandsCompound Glands hh
FIGURE 5.41 • Serous-secreting compound gland. Photomicro-graph of pancreatic acinus (A; outlined by the dotted line) with its duct (D). The small round objects within the acinar cells represent the zymogen granules, the stored secretory precursor material. 320.
removed from the lumen of the acinus and are shaped as crescents or demilunes (half-moons) at the periphery of the mucus acinus.
Most epithelial cells have a finite life span less than that of the whole organism.
Surface epithelia and epithelia of many simple glands belong to the category of continuously renewing cell populations. The rate of cell turnover (i.e., the replacement rate) is characteristic of a specific epithelium. For example, the cells lining the small intestine are renewed every 4 to 6 days in humans. The replacement cells are produced by mitotic activity of self-maintaining adult stem cells. They are located in sites called niches. In the small intestine, niches of adult stem cells are located in the lower portion of the intestinal glands (crypts; Fig. 5.42). They then migrate and differentiate into four principal cell types. Enterocytes (columnar absorptive cells), goblet cells (mucus-secreting), and enteroendocrine cells (regulatory and hormone-secreting) continue to differentiate and mature while they migrate up along the villi to the surface of the intestinal lumen. The migration of these new cells continues until they reach the tips of the villi, where they undergo apoptosis and slough off into the lumen. The fourth cell type, Paneth cells, migrate downward and reside at the bottom of the crypt. The transcription factor Math1 expressed in the intestinal epithelium determines the fate of the cell. The cells committed to the secretory lineage (i.e., they will differentiate into goblet, enteroendocrine, and Paneth cells) have increased expression of Math1. Inhibition of Math1 expression characterizes the default developmental pathway into absorptive intestinal cells (enterocytes).