Objectives
- Name the parts of the digestive tract and the primary function of each.
- Describe the structure of the tongue, teeth, and gingiva.
- Describe the development of the teeth.
- Compare the digestive tract organs in terms of the four layers comprising their walls and relate any structural variations to differences in organ function.
- Know the distinguishing regional structure of each digestive tract component.
- Name the secretory product(s), the distinguishing structural features, and (where appropriate) the staining properties of each secretory cell type in digestive tract mucosa.
- List the features of the small intestine that promote nutrient absorption and trace the steps in this process.
- Identify the organ, region, cell types present, and type of section (i.e., transverse or longitudinal) in a micrograph of any part of the digestive tract.
MAX-Yield™ Study Questions
1. List the digestive tract organs in the order food passes them (I.A). What happens to the food in each (I.C.1–3)?
2. Sketch a cross-section of a generalized tubular organ of the digestive tract that shows the layered structure of its walls (Fig. 15–1; I.B.1–4) and indicate the location of the following:
Lumen
Mucosa
Submucosa
Muscularis externa
Serosa
Epithelium
Lamina propria
Muscularis mucosae
Submucosal (Meissner’s) plexus
Myenteric (Auerbach’s) plexus
Mesothelium
Attachment of the mesentery
3. Describe the oral cavity in terms of its epithelial lining, the muscle type in its walls, and the structural difference between the hard and soft palates (II.A).
5. Name the four types of lingual papillae and compare them in terms of their characteristic shape, taste bud distribution, and relative abundance (II.C.1–4).
8. Sketch a tooth and its surrounding structures in sagittal (midline longitudinal) section (Fig. 15–2) and label the following:
Gingiva
Alveolar bone
Crown
Neck
Root
Apical foramen
Enamel
Cementum
Dentin
Pulp
Periodontal ligament
Epithelial attachment (of Gottlieb; III.D.3)
10. Describe tooth pulp in terms of its predominant tissue, major cell types, blood supply, and innervation (III.C.4 and 5.b; Fig. 15–2).
11. Describe the periodontal ligament in terms of its composition, location, functions, attachments, and the effects of dietary vitamin C and protein deficiency (III.D.1; Fig. 15–2).
12. Compare ameloblasts, odontoblasts, and cementoblasts in terms of their embryonic origin (III.E), the layer of tooth structure formed by each (III.E), and their survival into adulthood (III.C.5.c, 6.c, and 7).
13. Beginning with the dental laminae, name, in order, the stages of crown development (Fig. 15–3; III.E.1).
Ameloblasts
Odontoblasts
Enamel organ
Inner enamel epithelium
Outer enamel epithelium
Stellate reticulum
Dental papilla
Cervical loop
15. By the time a deciduous tooth is shed, prior to permanent tooth eruption, only the crown remains. What happens to the root (III.E.4)?
16. Describe the oral pharynx (IV) in terms of its epithelial lining and the muscle type in its walls.
Epithelium
Location of the mucus-secreting glands
Tissue type of the muscularis externa
Outer covering (serosa versus adventitia)
18. Sketch the stomach’s outline, showing the boundaries of the cardia, fundus, body, and pylorus (Fig. 15–4).
19. Name the epithelium covering the stomach’s luminal surface and lining the gastric pits (VI.B.1).
20. Compare the mucosal glands in the stomach’s four regions in terms of their secretory products and the depth of their gastric pits (VI.C.1–3).
21. Name four secretory cell types in the gastric glands (VI.B.3–6) and compare them in terms of their secretory product(s), staining properties, distribution, and organelles.
22. How does the stomach’s muscularis externa differ from that of the other digestive tract organs (I.B.3; Table 15–1)?
23. Name, in the order that food passes them, the three segments of the small intestine and the type of epithelium lining each (VII).
24. List three features of the small intestine wall that increase the surface area and thus promote nutrient absorption (VII.A, B.1 and 3).
25. Name five key cell types in the epithelium lining the intestinal lumen and the crypts of Lieberkühn (VII.B.3–8). Compare them in terms of their primary (including secretory) function, distribution, and distinguishing structural or staining properties.
26. Compare the duodenum, jejunum, and ileum in terms of the presence of submucosal glands, amount of lymphoid tissue, and number of goblet cells (Table 15–1; VII.C.1–3).
27. Compare the absorption of lipids, amino acids, and carbohydrates from the small intestine lumen (VII.B.3.a–d) in terms of the mechanism of entry into the absorptive cells, modification (if any) in the absorptive cell (e.g., packaging), and their uptake by blood versus lymph.
29. Indicate how the large and small intestines differ in terms of the presence of plicae circulares and villi (VIII.A), goblet cell number (VIII.A), amount of lymphoid tissue (VIII.A), and the outer layer of their muscularis externa (VIII.C).
30. Describe how the appendix (IX) differs from the colon in terms of overall diameter, depth of crypts, abundance of lymphoid follicles, and muscularis externa.
31. Compare the stomach’s rugae (VI.A) and the small intestines’ circular folds (plicae circulares) (VII.A) in terms of their permanence.
33. Describe the trends in wall structure from the duodenum through the colon (VII.C; VIII.A–D) in terms of the increase or decrease in:
Synopsis
The digestive tract is a series of organs forming a long muscular tube whose continuous lumen opens at both ends to the exterior. The organs include the oral cavity, oral pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (cecum; appendix; ascending, transverse, descending, and sigmoid colon; rectum), and anal canal.
Each organ’s wall has four concentric layers (Fig. 15–1): the mucosa, submucosa, muscularis externa, and serosa or adventitia. (To master digestive tract histology, first learn the general composition and location of each layer and then focus on distinguishing features of each organ; Table 15–1.) Distinguishing structural features make more sense when considered in relation to organ functions (I.C).
Mucosa. This layer borders the lumen and has three parts. The epithelium derives from endoderm throughout the tract, except in the oral cavity and anal canal, where it derives from invaginating ectoderm. The epithelium is stratified squamous in the oral cavity, oral pharynx, esophagus, and anal canal; it is simple columnar in the stomach, intestines, and rectum. The lamina propria is the loose connective tissue layer containing blood and lymphatic vessels beneath the epithelium. The muscularis mucosae is a thin, smooth muscle layer bordering the submucosa.
Submucosa. This dense, irregular connective tissue layer contains blood and lymphatic vessels and the submucosal (Meissner’s) nerve plexus. Some organs are characterized by glands and lymphoid nodules in this layer.
Muscularis externa. This comprises two smooth muscle layers (inner circular and outer longitudinal) through most of the tract. Between them lies the myenteric (Auerbach’s) plexus. The muscle around the oral cavity is skeletal; where it is absent (e.g., hard palate, gingiva), the submucosa binds tightly to the bone. In the upper esophagus, this layer contains skeletal muscle, which is replaced by smooth muscle in the lower portion. The stomach’s muscularis externa has three layers: outer longitudinal, middle circular, and inner oblique. The colon’s outer longitudinal layer is gathered into three bands called the teniae coli. Smooth and skeletal muscles encircling the anal canal form involuntary and voluntary sphincters, respectively.
Serosa and adventitia. The tract’s outer covering differs according to location. The esophagus and rectum are surrounded and held in place by a connective tissue adventitia similar to that around blood vessels. Intraperitoneal organs (stomach, jejunum, ileum, transverse and sigmoid colon) are suspended by mesenteries and covered by a serosa (i.e., a thin layer of loose connective tissue covered by simple squamous epithelium, or mesothelium). Retroperitoneal organs (duodenum, ascending and descending colon) are bound to the posterior abdominal wall by adventitia and are covered on their anterior surfaces by serosa.
The primary functions of the digestive tract include the absorption of nutrients and water and the excretion of wastes and toxins.
