Cui Dongmei. Atlas of Histology: with functional and clinical correlations. 1st ed

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Name of

Tissue

Productive

Cells

Inorganic

(Mineral) %

Organic % Water % Sequence

of Hard

Tissue

Formation

Sequence

Productive

Cell

Formation

Activity Embryological

Origin

Dentin Odontoblasts 70 20 10 First Odonto-

blasts second

Lifetime Dental papilla

(mesoderm)

Enamel Ameloblasts 96 3 1 Second Amelo-

blasts ﬁ rst

Before

tooth

eruption

Enamel organ

(ectoderm)

Cementum Cementoblasts 65 23 12 Third Cemento-

blasts third

Lifetime Dental sac/

follicle

(mesoderm)

Alveolar

bone

Osteoblasts 60 25 15 — — Lifetime;

respond

to stresses

and

tensions

Mesoderm

TABLE 14-3 Dental Hard Tissue

CLINICAL CORRELATIONS

Figure 14-16A.

itamin D–Resistant Rickets. H&E, 476

Vitamin D–resistant rickets, commonly known as X-linked

hypophosphatemic rickets, is characterized by resistance to con-

ventional vitamin D treatment, decreased reabsorption of phos-

phate by the renal tubules, abnormalities in bones and teeth,

osteomalacia, and hypocalcemia. It is an X-linked autosomal

dominant disorder. Patients with these rickets disorders do not

respond to high-dose vitamin D treatment. Signs include rickets,

short stature, bowing of lower limbs, seizures, congestive cardiac

failure, and tooth defects including calciﬁ cation defects in dentin,

enlarged pulp chambers, pulpitis, and pulp necrosis. Treatment

with 1, 25-vitamin D (which is not dependent on normal hor-

monal mechanisms or organ systems to activate) plus controlled

phosphate therapy may improve the condition.

Figure 14-16B.

Dentin Dysplasia. H&E, 35; inset 7

Dentin dysplasia is an autosomal-dominant tooth disorder

. Teeth

in this disorder are sometimes called rootless teeth, because they

often have very short and conical roots. Dentin dysplasia can be

classiﬁ ed into types I and II. Type I, also called the radicular type,

is characterized by short roots and pulp obliteration. Type II, also

called the coronal type, is characterized by a “thistle tube” pulp.

Radiographically, extension of the pulp chamber into the root is

usually observed in dentin dysplasia. Pulp stones and sudden con-

striction of the pulp chamber are common. Dentin dysplasia type

II shares some similarities with dentinogenesis imperfecta type II,

but in dentin dysplasia the permanent dentition has normal color

or only slight discoloration. Histologically, globules of dentin and

disorganized dentinal tubules are characteristic of this condition.

The left image shows globules of dentin; the right image shows a

low-power photograph of a dentin dysplasia.

Calcification

defect in dentin

Dentin

Broad

root

Globules

of dentin

Dentin

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Odontoblasts

Predentin

Dentin

Cell-free

zone

Cell-free

zone

Cell-rich

zone

Pulp core

Blood

vessel

Apical foramen

Radicular

pulp

Radicular

pulp

Coronal

pulp

Coronal

pulp

Blood vessels

and nerves

Blood vessels

and nerves

Figure 14-17A. Dental pulp. H&E, 136

Dental pulp is a specialized loose, cellular mucous connective

tissue that ﬁ lls the pulp chamber in the central core and

root canals of the tooth. Fibroblasts are the most numer-

ous cells in the pulp; they produce connective tissue ﬁ bers

(mainly type I and III collagen, ﬁ bronectin, and elastin)

and ground substance. They maintain the pulp matrix. The

second most numerous cells are odontoblasts ( producing

dentin). Other defense cells, such as macrophages and

lymphocytes, may be found in the pulp. No mast cells or

adipocytes (fat cells) are found in the pulp. The pulp con-

tains many blood vessels and nerve ﬁ bers, which enter and

leave at the apical foramen. Most of the nerve ﬁ bers are

afferent ﬁ bers; they are either C ﬁ bers or Ad ﬁ bers, and

both carry pain information through the trigeminal system

to the brain. The efferent ﬁ bers are autonomic nerve ﬁ bers

and innervate the smooth muscle of the blood vessels.

Dental pulp plays an important role in producing dentin

and providing nutrients and sensory input for the dentin.

Dental pulp

Predentin

3. Cell-rich zone

1. Odontoblast

layer

2. Cell-free zone

(zone of Weil)

Odontoblasts

Cell-free

zone

Dentin

Dentinal

tubules

Figure 14-17B. Dental pulp. H&E, 136; inset 190

Dental pulp is derived from mesenchymal tissue that forms

the dental papilla during tooth development. The papilla

becomes pulp in the mature tooth. The pulp can be divided

into four zones from the periphery to the center. (1) The

odontoblast layer forms a single cell layer along the periph-

eral edge of the pulp. These cells have processes extending

into the dentin. (2) The cell-free zone, also called the zone

of Weil, is directly under the odontoblast layer. This layer

has ﬁ bers, cellular processes, axons, and capillaries running

through it but contains no cell nuclei. (3) The cell-rich zone

is beneath the cell-free zone, and has many cells and nuclei

of cells densely packed in rows. This layer has ﬁ broblasts,

undifferentiated mesenchymal cells, neural plexuses, and

capillaries. If some odontoblasts die, the undifferentiated

mesenchymal cells in this layer will differentiate into new

odontoblasts. (4) The pulp proper (pulp core) is the cen-

tral part of the pulp and contains blood vessels and nerves

within the loose, mucous connective tissue. The layers of

both cell-free and cell-rich zones are more visible in the

coronal than the radicular pulp region (Fig. 14-17A).

CLINICAL CORRELATION

Figure 14-17C.

Pulp Abscess. H&E 23; inset 168

Pulp abscess refers to an abscess involving the pulp tissue of a tooth.

Pulp abscesses are usually sequelae of dental caries, but they can also

develop in teeth showing no detectable lesions. Pulp abscesses can also

occur after restoration work has been performed. They are characterized

by severe, intermittent pain that may intensify when a patient reclines.

Sharp pain may also be triggered by cold liquids, tapering off to a dull,

pulsating pain. Periapical tissues may not be involved, and the affected

tooth may not show any difference from other healthy teeth in percussion

or pressure tests. T

reatment includes using antibiotics and undergoing

root canals and even tooth extraction.

Pulp cavity

Abscess with

neutrophils

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Figure 14-18C. PDL and alveolar bone. H&E, 34; inset 72

The PDL is a dense ﬁ brous connective tissue with a direct nerve

and blood supply. It is located between the cementum and

the alveolar bone, which surrounds the tooth root. Fibroblasts are

the main cells responsible for the formation of the PDL. The PDL

supports the tooth root by forming a strong attachment between

the cementum and alveolar bone by Sharpey ﬁ bers (Fig. 14-19A).

The functions of the PDL are to provide sensory information

regarding pain and pressure, to provide signals to the alveolar

bone regarding bone remodeling, and to maintain tooth position

in the dental arch. The alveolar bone (also called the alveolar pro-

cess) surrounds and supports the tooth root and is part of the

maxilla and mandible. It is constantly remodeled and is built up

by osteoblasts and absorbed by osteoclasts. Alveolar bone has the

general characteristics of bones. It consists of both compact and

cancellous bones (see Chapter 5, “Cartilage and Bone”).

Enamel

space

Enamel

space

DCJ

CEJ

Acellular

cementum

Acellular

cementum

Dentin

DCJ

Cementoblasts

Predentin

Dental

pulp

Dental

pulp

Odontoblasts

Figure 14-18A. Acellular cementum, cervical region of tooth

root. H&E, 181; inset 158

The periodontium includes the cementum, the PDL, and the

alveolar bone. These structures surround and support the

tooth in the tooth socket. Cementum is a thin layer of hard tis-

sue ( calciﬁ ed matrix) that does not have a direct blood supply.

It covers the dentin and seals the dentinal tubules at the root

region. Cementum is thicker at the root apex than at the cervix

region and can be divided into acellular and cellular cementum.

Acellular cementum has no cementocytes embedded within the

matrix and is often found at the cervical two thirds of the root,

attached to the dentin. The junction between the dentin and

cementum is called the dentinocemental junction (DCJ), and the

junction between the cementum and enamel is the CEJ. There are

three forms of the CEJ: (1) the cementum overlaps the enamel

(60%); (2) the cementum meets the enamel end-end (30%); and

(3) there is a gap between the cementum and the enamel (10%).

Periodontium

Blood

vessels

Blood

vessels

Cellular

cementum

Cellular

cementum

Cementocyte

Periodontal ligament (PDL)

Dentin

Predentin

Odotoblast

Odontoblast

Dental

pulp

Dental

pulp

Cementoblast

Sharpey fibers

in cementum

Sharpey fibers

in cementum

Figure 14-18B. Cellular cementum, apical region of tooth

root. H&E, 181; inset 408

Cellular cementum is often found at the apical third of the tooth

root and is similar to bone with a calciﬁ ed intercellular matrix.

During the process of formation of cementum some cemento-

blasts are trapped in the matrix, and each one is surrounded by

a lacuna. These cementoblasts then become cementocytes. The

matrix of cellular cementum is deposited more rapidly than that

of acellular cementum. Acellular cementum has a slower forma-

tion rate and formation continues throughout life. The slower

growing cementum allows ﬁ bers (Sharpey ﬁ bers, left inset) from

the PDL to become trapped in the matrix of the cementum to

form the tooth attachment. Cementum is much more resistant

to reabsorption than bones. This characteristic helps maintain

root integrity and prevents exposure of the root dentin, which

is important as teeth are moved around during orthodontic

procedures.

Alveolar

bone

Alveolar

bone

Dentin

PDL

Cementum

Dental pulp

Alveolar bone

Bone

marrow

Bone

marrow

PDL

Cementum

PDL

Alveolar bone

Dentin

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Horizontal fibers

Alveolar crest fibers

Gingival fibers

Oblique fibers

Apical fibers

Alveolar

bone proper

Supporting

bone

Fig. 14-18A

Fig. 14-18B

Fig. 14-19A,B

PDL

Sharpey fibers

in alveolar bone

Sharpey fibers

in alveolar bone

Dentin

Alveolar bone

Cementum

PDL

Figure 14-19A. PDL and alveolar bone, tooth root. H&E, 68; inset 408

The cementum and PDL are derived from the dental sac. During root formation, some ﬁ bers from the PDL are trapped within the

cementum and the alveolar bone. These Sharpey ﬁ bers bridge the space between the cementum and the alveolar bone and serve to

suspend the tooth within the alveolar socket. The principal ﬁ bers of the PDL are organized in several groups based on their orientation

and location: apical, oblique, horizontal, alveolar crest, interradicular, and gingival ﬁ bers. The interradicular ﬁ bers are only present

between multirooted teeth. The gingival ﬁ bers attach the gingiva to the hard tissue of the tooth. The alveolar bone provides support

and protection for the tooth root. The alveolar bone includes the alveolar bone proper and supporting bone. The alveolar bone proper

is a thin layer of compact bone which lines the tooth socket and has Sharpey ﬁ bers embedded in it; it is remodeled constantly to adapt

to stresses and tensions. The supporting bone is composed of compact bone and cancellous bone. The compact bone forms the cortical

plate, which provides surface strength. The cancellous bone makes up the central core of the alveolar bone and contains bone marrow.

CLINICAL CORRELATION

Figure 14-19B.

ooth Ankylosis. H&E, 68

Tooth ankylosis is the fusion of a mineralized root surface to its surround-

ing alveolar bone. It is characterized by a typical metallic percussion sound

and loss of the PDL space in radiographs. Periapical inﬂ ammation and

subsequent tissue repair are believed to be the cause of ankylosis. Ankylo-

sis is more common in deciduous teeth and usually results in impaction of

a subjacent permanent tooth. Impaction of incisors is much less common

than it is in the mandibular third molars. This slide shows impaction of a

central incisor with focal loss of the PDL the incisor.

SYNOPSIS 14-1 Pathological and Clinical Terms for the Oral Cavity

Microdontia ■ : disproportionately small teeth (disturbances occur at bud stage).

Macrodontia

■ : disproportionately large teeth (disturbances occur at bud stage).

Dens invaginatus (dens in dente)

■ : developmental anomaly in tooth formation at cap stage in which the epithelium invaginates

into the pulp space to form enamel and dentin, creating a “tooth within a tooth” as shown in the radiographic imaging.

Misalignment

■ : a condition in which teeth are too crowded and incorrectly positioned.

Tooth decay (caries)

■ : bacterial destruction of teeth, including erosion of the enamel and damage to the pulp tissue.

Hypersensitivity

■ : a painful condition, especially to touch, sweetness, and cold drinks caused by exposed dentin.

Periodontal disease

■ : a chronic inﬂ ammatory condition of gingival and surrounding periodontal tissues. Calculus and

plaque are the major causes of the disease.

Gingivitis

■ : infection and inﬂ ammation of the gingival tissue.

Periodontitis

■ : infection and inﬂ ammation affecting the periodontium, characterized by gingivitis, destruction of the alveolar

bone and periodontal ligament, and the formation of periodontal pockets.

Pulpitis

■ : an inﬂ ammation of the pulpal tissue of a tooth.

White lesions

■ : lesions on the mucosa of the oral cavity that have a white coating such as leukoplakia.

Red lesions

■ : chronic red oral mucosal lesions in which underlying vascular structures become more visible (as in

erythroplakia).

Alveolar

bone

Alveolar

bone

Dentin

Junction between

alveolar bone

and dentin

(absence of PDL)

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279

Digestive Tract

Introduction and Key Concepts for the Digestive Tract

Figure 15-1 Overview of the Digestive Tract

Figure 15-2 Orientation of Detailed Digestive Tract Illustrations

Figure 15-3 General Structure of the Wall of the Digestive Tract

Esophagus

Figure 15-4A Overview of the Esophagus

Figure 15-4B Upper Esophagus

Figure 15-4C Middle Esophagus

Figure 15-5A Lower Esophagus, Esophagogastric Junction

Figure 15-5B Clinical Correlation: Barrett Esophagus

Figure 15-5C Clinical Correlation: Esophageal Carcinoma

Stomach

Figure 15-6 Overview of the Stomach

Figure 15-7A Cardiac Region, Esophagogastric Junction

Figure 15-7B Fundic Region of the Stomach

Figure 15-7C Body Region of the Stomach

Figure 15-8A Pyloric Region of the Stomach

Figure 15-8B Clinical Correlation: Gastric Ulcer (Peptic Ulcer)

Figure 15-8C Clinical Correlation: Gastrinoma (Zollinger-Ellison Syndrome)

Small Intestine

Figure 15-9 Overview of the Small Intestine

Figure 15-10A,B Duodenum, Small Intestine

Figure 15-10C Clinical Correlation: Peptic Duodenitis

Figure 15-11A Plicae Circulares, Villi, and Microvilli

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Figure 15-11B Villi of the Small Intestine

Figure 15-12A Columnar Absorptive and Goblet Cells of the Small Intestine

Figure 15-12B Goblet, Columnar Absorptive Cells, and Microvilli

Figure 15-13A Paneth Cells, Small Intestine

Figure 15-13B Enteroendocrine Cells, Small Intestine

Figure 15-14A Submucosal/Meissner Plexus, Small Intestine

Figure 15-14B Muscularis Externa, Small Intestine

Figure 15-14C Myenteric/Auerbach Plexus, Muscularis Externa of the Small Intestine

Figure 15-15A Jejunum, Small Intestine

Figure 15-15B Ileum with Peyer Patches, Small Intestine

Figure 15-15C Mucosa of the Ileum, Small Intestine

Large Intestine

Figure 15-16 Overview of the Large Intestine

Figure 15-17A,B Colon, Large Intestine

Figure 15-17C Mucosa of the Colon, Large Intestine

Figure 15-18A Clinical Correlation: Colon Polyps

Figure 15-18B Clinical Correlation: Colorectal Cancer

Figure 15-18C Clinical Correlation: Meckel Diverticulum

Figure 15-19A Appendix and Cecum

Figure 15-19B Anorectal Junction

Figure 15-19C Clinical Correlation: Hemorrhoids

Figure 15-20A Clinical Correlation: Ulcerative Colitis

Figure 15-20B Clinical Correlation: Crohn Disease

Synopsis 15-1 Pathological and Clinical Terms for the Digestive Tract

Table 15-1 Digestive Tract

Introduction and Key Concepts

for the Digestive Tract

The digestive system is composed of the oral cavity, digestive

tract, and digestive glands with associated organs. The “Oral

Cavity” is discussed in Chapter 14; the “Digestive Glands and

Associated Organs” are discussed in Chapter 16. The diges-

tive tract is discussed in this chapter. It includes the esopha-

gus, stomach, small intestine, and large intestine. The digestive

tract is a continuation of the oral cavity, and its main functions

are to ingest food and to digest the food as it passes along the

tract. In this process, nutrients and water are absorbed, and

waste materials are prepared for elimination from the body.

Each section of the digestive tract has its unique histological

features, which are closely associated with the function of that

part of the tract, although there are some common character-

istics: (1) Organs of the digestive tract are all hollow; (2) they

are composed of four general tunic layers: mucosa, submu-

cosa, muscularis externa, and adventitia or serosa; (3) they

are innervated by the enteric portion of the autonomic ner-

vous system, also known as the enteric nervous system (or the

“second brain”); (4) they include epithelium, connective tissue,

muscle, blood and lymphatic vessels, lymphatic nodules, and

nerve ﬁ bers; and (5) they contain glands in the lamina propria

or submucosa.

General Structure of the

Digestive Tract

Based on its histological organization, the wall of the digestive

tract can be divided into four tunics (Fig. 15-3).

1. Mucosa is the innermost layer of the digestive wall. It

includes epithelium, lamina propria, and muscularis muco-

sae. The epithelium consists of simple columnar epithelium

lining most of the tract and stratiﬁ ed squamous epithelium

lining the two ends, the esophagus and anal canal. The

lamina propria is a loose connective tissue that contains

abundant ground substance, many ﬁ bers, and numerous

connective tissue cells such as ﬁ broblasts, macrophages,

mast cells, plasma cells, and leukocytes (see Chapter 4,

“Connective Tissue”). Various types of glands are found in

the lamina propria depending on the region of the digestive

tract. The muscularis mucosae is a very thin layer of smooth

muscle, which is the boundary between the mucosa and the

submucosa. It is usually arranged in an inner circular and

outer longitudinal layer. However, the muscularis mucosae

varies in different regions, and it is often difﬁ cult to distin-

guish between the muscle layers.

2. Submucosa is a thick layer of dense irregular connective

tissue. This layer contains blood vessels, lymphatic vessels,

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281

and submucosal (Meissner) plexuses, which contain nerve

ﬁ bers and neurons of the enteric nervous system. In some

regions of the digestive tract, this layer is characterized by

mucous glands or lymphatic nodules.

3. Muscularis externa is composed of two or three oblique,

circular, and longitudinal muscle layers, which vary from

region to region. Most of the muscularis externa consists

of smooth muscle ﬁ bers, but the upper and middle esophagi

contain some skeletal muscle. The myenteric (Auerbach)

plexuses (nerve ﬁ bers and neurons of the enteric nervous sys-

tem) are located between the muscle layers. They innervate

and control contraction of the muscularis externa.

4. Serosa and adventitia are coverings of the outermost wall

of the digestive tract. Most parts of the digestive tract are

covered by serosa, a thin layer of loose connective tissue

lined by mesothelium. The mesothelium produces a lubri-

cating ﬂ uid that reduces friction during movement of the

organs against each other (see Chapter 3, “Epithelium and

Glands,” Fig. 3-2A,B). The serosa is the visceral layer of the

peritoneum and covers the wall of the digestive tract where

it connects to the mesentery in the peritoneal cavity (intrap-

eritoneal organs). The adventitia is a layer of loose connec-

tive tissue without mesothelium that covers the upper region

of the esophagus, part of the duodenum, and the lower part

of the digestive tract, such as the rectum and anal canal.

Adventitia covers regions of the digestive tract where it is

connected to other organs or to the body wall (e.g., retro-

peritoneal organs).

Esophagus

The esophagus is the upper part of the digestive tract, con-

necting the oral cavity to the stomach. The major function of

the esophagus is to provide passage for food from the mouth

to the stomach. The luminal surface of the esophagus is lined

by nonkeratinized stratiﬁ ed squamous epithelium. Mucous

glands called esophageal glands are located in the submucosa

of the esophagus. The muscularis externa consists of two lay-

ers of muscle: inner circular and outer longitudinal layers. Both

skeletal and smooth muscle ﬁ bers are found in the muscularis

externa of the esophagus. The proportions of skeletal and

smooth muscle ﬁ bers are different in different regions of the

esophagus. The esophagus can be divided into three regions:

the upper esophagus, middle esophagus, and lower esophagus

(Figs. 15-4A to 15-5A).

1. The upper esophagus connects the oropharynx to the mid-

dle esophagus. This segment contains numerous esophageal

glands in the submucosa. These glands secrete mucus to

lubricate the esophageal wall so that food will pass through

easily. The upper esophagus contains only skeletal muscle

ﬁ bers in the muscularis externa. These are voluntary muscle

ﬁ bers and are innervated by the glossopharyngeal nerve (cra-

nial nerve [CN] IX) (see Fig. 15-4B).

2. The middle esophagus has mucosa similar to that of the

upper esophagus. The esophageal glands in the submucosa

are less numerous than in the upper esophagus. The mus-

cularis externa contains both skeletal and smooth muscles

(Fig. 15-4C).

3. The lower esophagus connects the esophagus to the car-

dia of the stomach. This region contains large numbers of

mucous glands in the lamina propria and submucosa. These

are called esophageal cardiac glands and produce mucous

secretions to protect the lower esophagus from being dam-

aged by reﬂ ux of acidic gastric juices from the stomach.

The lower esophagus contains only smooth muscle ﬁ bers in

the muscularis externa. These are controlled by the enteric

branches of the vagus nerve (CN X) (see Fig. 15-5A).

Stomach

The stomach is a “J”-shaped sac (hollow) organ. It temporarily

stores food, mixes food with gastric juice, and initiates the pro-

cessing of food by breaking it down into simpler substances that

are easier to digest. The stomach can be divided into the cardia,

fundus, body, and pylorus. The inner surface of the stomach

is lined by simple columnar epithelium composed mainly of

surface mucous cells. The surface epithelium of the stomach is

invaginated into the lamina propria to form gastric pits. These

pits serve as ducts for the glands in the lamina propria, which

vary from region to region in the stomach.

1. The cardiac region connects to the lower esophagus at the

esophagogastric junction, which is characterized by a change

from the nonkeratinized stratiﬁ ed squamous epithelium of

the esophagus to the simple columnar epithelium of the

stomach. A thickened smooth muscle ring called the gastroe-

sophageal sphincter (lower esophageal sphincter) or cardiac

sphincter surrounds the opening at the junction of the lower

esophagus and cardiac region of the stomach. This smooth

muscle contracts to prevent the acidic stomach contents from

entering the esophagus. The glands in the lamina propria of

the cardia are called cardiac glands and are branched tubu-

lar glands with coiled secretory portions. The cardiac gland

contains mainly mucus-secreting cells and some stem cells,

enteroendocrine cells, and, occasionally, parietal cells. The

mucus-secreting cells mainly produce mucus and lysozymes.

The mucus protects the stomach wall from acidic gastric

juices; lysozymes destroy bacterial membranes, preventing

bacterial infections (Fig. 15-7A).

2. The fundic and body regions form the largest portions of the

stomach. Their mucosa has similar histological characteris-

tics, including short gastric pits and long branched tubular

glands in the lamina propria. The glands are called fundic or

gastric glands in both the fundus and the body regions. The

gastric glands contain mainly parietal cells and chief cells,

along with some stem cells, mucous neck cells, and enteroen-

docrine cells. Parietal cells are more numerous in the superior

regions of the glands; these cells produce large quantities of

hydrochloric acid (HCl), creating an acidic environment to

help digestion. Parietal cells also secrete intrinsic factor (IF),

which is required for the absorption of vitamin B12. Chief

cells are located in the more inferior regions of the glands;

they secrete precursor enzymes such as pepsinogen, which is

activated by HCl and becomes pepsin. Pepsin helps to break

down proteins (particularly protein collagen) into simpler,

more absorbable compounds. Chief cells also secrete precur-

sors of lipases, which help in lipid digestion (Fig. 15-7B,C).

3. The pyloric region is the lower end of the stomach, which

connects with the duodenum. Its mucosa is similar to that of

the cardia, with long gastric pits and short, coiled secretory

portions. A

circular smooth muscle ring called the pylorus

sphincter (pyloric valve) surrounds the end of the pylorus

region. This valve controls the entry of stomach contents

into the duodenum. The glands in the lamina propria of

the pylorus are called pyloric glands and contain primarily

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Organ Systems

mucus-secreting cells and two special types of enteroendo-

crine cells: gastrin-secreting cells (G cells) and somatostatin-

secreting cells (D cells), as shown in Figure 15-8A. These

enteroendocrine cells regulate gastric (HCl) secretion.

Small Intestine

The small intestine (Figs. 15-9 to 15-15C) is a hollow organ of

small diameter that is typically 6 to 7 m long. It is the major

site for the absorption of nutrients. Important features of the

small intestine are villi and microvilli, which increase surface

area for absorption. Intestinal glands called glands (crypts)

of Lieberkühn are located in the lamina propria of the small

intestine. Villi project into the lumen of the intestine; the glands

of Lieberkühn open into the mucosa at the base of the villi

(Fig. 15-11A,B). The small intestine can be divided into three

parts: the duodenum, jejunum, and ileum.

1. The duodenum is the shortest segment of the small intestine,

about 20 to 25 cm long. It has small openings called duodenal

papillae (minor and major [Figs. 16-9A and 16-15A]), which

allow pancreatic juice and bile to enter the digestive tract.

It has a similar general structure to other parts of the small

intestine (Fig. 15-9). However, the Brunner glands (mucus-

secreting gland) in the submucosa are a unique feature of the

duodenum (Fig. 15-10A,B).

2. The jejunum is much longer than the duodenum, about

2.5 m long (two ﬁ fths of the rest of the small intestine). It has

long villi and a somewhat increased number of goblet cells. It

has neither Brunner glands nor Peyer patches (Fig. 15-15A).

3. The ileum is the longest segment, about 4 m long (three ﬁ fths

of the rest of the small intestine). It has short villi with sig-

niﬁ cantly increased numbers of goblet cells on the surface

of the mucosa. There are clusters of lymphatic nodules in

the lamina propria of the ileum; sometimes these lymphatic

nodules extend into the submucosal layer. These clusters of

lymphatic nodules are called Peyer patches and are unique to

the ileum (Fig. 15-15B,C).

Large Intestine

The large intestine (Figs. 15-16 to 15-19B) is a hollow organ

with a relatively large diameter compared to the small intestine

and is about 1.5 m long. It is the last region of the digestive tract

and is the major site for absorption of water and salts. It also

forms, stores, and eliminates feces. Most of the regions of the

large intestine have tunics that are similar to those of the small

intestine, but there are no villi in the mucosa. There are large

numbers of goblet cells in the large intestine. These cells produce

mucus, which helps in the formation of the feces and protects and

lubricates the surface of the intestinal wall. The large intestine

includes the cecum, appendix, colon, rectum, and anal canal.

1. The cecum is the most proximal region of the large intestine.

It is a small, blind pouch of the large intestine where the

ileum connects to the ascending colon. A sphincter muscle, a

thickening of the muscularis mucosae, is called the ileocecal

valve and is located at the junction of the ileum and cecum.

It prevents the contents of the large intestine from backing

up into the small intestine (Fig. 15-16).

2. The appendix is a small, blind tube that attaches to the

posterior-medial wall of the cecum. It has the general tunic

structure of the intestine and a small irregular lumen. There

are many lymphatic nodules in the lamina propria (Fig.

15-19A).

3. The colon is the longest segment of the large intestine. It

includes the ascending colon, transverse colon, descend-

ing colon, and sigmoid colon. The proximal half of the

colon is responsible for the majority of the absorption

of water and salt; the distal half of the colon has only a

small absorptive function and is predominantly for pro-

cessing and storing feces. The colon does not have villi; it

has a smoother surface than the small intestine. Columnar

absorptive cells and goblet cells line the mucosa. The large

intestinal glands, the glands (crypts) of Lieberkühn, con-

tain primarily goblet cells, columnar cells, enteroendocrine

cells, and stem cells. Lymphatic nodules may also be found

in the lamina propria. The muscularis externa consists

of inner circular layers of muscle; the outer longitudinal

smooth muscle layer becomes three teniae coli (Figs. 15-16

and 15-17A–C).

4. The rectum and anal canal are the last segments of the large

intestine. The junction between the rectum and the anal

canal is called the “anorectal junction.” The mucosa of the

rectum is similar to that of the colon but has fewer glands

of Lieberkühn. The main function of the rectum is the tem-

porary storage of feces. The sensory receptors in the rectum

send signals to the brain when feces need to be evacuated.

The anal canal is the distal end of the large intestine. Most

of the anal canal is lined by stratiﬁ ed squamous epithelium,

although simple cuboidal epithelium may be present at the

anorectal junction. Sebaceous glands and hair follicles may

be found at or near the anal opening. There are many veins

in the lamina propria and submucosa of the anal canal. The

term hemorrhoids refers to the condition in which these

veins become chronically swollen and inﬂ amed in the rectal

and anal regions (Fig. 15-19B).

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CHAPTER 15

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Digestive Tract

283

Esophagus

Small intestine

Small

intestine

Small

intestine

Stomach

Large intestine

Rectum

Sigmoid

Ascending

colon

Descending

colon

Transverse

colon

Cecum

Appendix

Figure 15-1. Overview of the digestive tract. H&E, 5 to 6

The digestive tract, also called the alimentary canal, includes the esophagus, stomach, small intestine, and large intestine. (1) The

esophagus transports food to the stomach. Its luminal surface is lined by stratiﬁ ed squamous nonkeratinized epithelium and contains

mucous glands in the lamina propria. It has both skeletal and smooth muscles in the muscularis externa. (2) The stomach temporarily

stores and digests food. Its luminal surface is lined by simple columnar epithelium and contains gastric glands in the lamina propria.

It has three layers of smooth muscle in the muscularis externa. (3) The small intestine digests and absorbs carbohydrates, proteins,

and lipids. The small intestine includes the duodenum, jejunum, and ileum. The luminal surface is lined by simple columnar epithe-

lium and contains the glands (crypts) of Lieberkühn in the lamina propria. There are two layers of smooth muscles (inner circular

and outer longitudinal muscles) in the muscularis externa. (4) The large intestine includes the cecum, appendix, colon, rectum, and

anal canal. The main functions of the large intestine are the absorption both of a large volume of the water that enters it (90%)

and of electrolytes (e.g., Na

and Cl

−

) and the formation of the feces. Most parts of the large intestine are lined by simple columnar

epithelium, but stratiﬁ ed squamous epithelium lines the anal canal. The glands of Lieberkühn are located in the lamina propria. The

muscularis externa contains an inner layer of circular smooth muscle and an outer layer of longitudinal smooth muscle, which forms

three teniae coli (see Fig. 15-16). In general, the submucosa of the digestive tract is a thick layer of connective tissue containing blood

vessels, lymphatic vessels, and submucosal (Meissner) plexuses consisting of nerve ﬁ bers and neuron cell bodies in the enteric nervous

system. The myenteric (Auerbach) plexuses located in the muscularis externa are also components of the enteric nervous system.

Digestive Tract

I. Esophagus

A. Upper esophagus (skeletal muscle)

B. Middle esophagus (mixed skeletal and smooth muscles)

C. Lower esophagus (smooth muscle)

II. Stomach

A. Cardia

B. Fundus

C. Body

D. Pylorus

III. Small Intestine

A. Duodenum

B. Jejunum

C. Ileum

IV. Large Intestine

A. Cecum

B. Appendix

C. Colon (ascending, transverse, descending, and sigmoid)

D. Rectum

E. Anal canal

CUI_Chap15.indd 283 6/2/2010 3:23:33 PM

284

UNIT 3

■

Organ Systems

Digestive Tract with Figure Numbers

Esophagus

Figure 15-4A

Figure 15-4B

Figure 15-4C

Figure 15-5A

Figure 15-5B

Figure 15-5C

Stomach

Figure 15-6

Figure 15-7A

Figure 15-7B

Figure 15-7C

Figure 15-8A

Figure 15-8B

Figure 15-8C

Small intestine

Figure 15-9

Figure 15-10A

Figure 15-10B

Figure 15-10C

Figure 15-11A

Figure 15-11B

Figure 15-12A

Figure 15-12B

Figure 15-13A

Figure 15-13B

Figure 15-14A

Figure 15-14B

Figure 15-14C

Figure 15-15A

Figure 15-15B

Figure 15-15C

Large intestine

Figure 15-16

Figure 15-17A

Figure 15-17B

Figure 17-17C

Figure 15-18A

Figure 15-18B

Figure 15-18C

Figure 15-19A

Figure 15-19B

Figure 15-19C

Figure 15-20A

Figure 15-20B

Fig. 15-15A

Fig. 15-15B,C

Fig. 15-4A

Fig. 15-4B

Fig. 15-5A

Fig. 15-7A

Fig. 15-7B

Fig. 15-7C

Fig. 15-8A

Fig. 15-17A,B,C

Fig. 15-9B

Fig. 15-19B

Fig. 15-4C

Fig. 15-10A,B

Fig. 15-19A

Figure 15-2. Orientation of detailed digestive tract illustrations.

CUI_Chap15.indd 284 6/2/2010 3:23:37 PM