Essentials of Anatomy and Physiology

bladder fills, these cells become flattened. Transitional
epithelium enables the bladder to fill and stretch with-
out tearing the lining.

SIMPLE CUBOIDAL EPITHELIUM

Simple cuboidal epitheliumis a single layer of cube-
shaped cells (Fig. 4–3). This type of tissue makes up
the functional units of the thyroid gland and salivary
glands. These are examples of glandular epithelium;
their function is secretion. In these glands the
cuboidal cells are arranged in small spheres and secrete
into the cavity formed by the sphere. In the thyroid
gland, the cuboidal epithelium secretes the thyroid
hormones; thyroxine is an example. In the salivary
glands the cuboidal cells secrete saliva. Cuboidal

epithelium also makes up portions of the kidney

tubules. Here the cells have microvilli (see Fig. 1–1),

and their function is the reabsorption of useful materi-

als back to the blood.

SIMPLE COLUMNAR EPITHELIUM

Columnar cells are taller than they are wide and are

specialized for secretion and absorption. The stom-

ach lining is made of columnar epithelium that

secretes gastric juice for digestion. The lining of the

small intestine (see Fig. 4–3) secretes digestive

enzymes, but these cells also absorb the end products

of digestion from the cavity of the intestine into the

blood and lymph. To absorb efficiently, the columnar

cells of the small intestine have microvilli, which you

72 Tissues and Membranes

Thyroid secretions (hormones)

Simple cuboidal

Simple columnar

Goblet cells

Cilia

Connective tissue

Ciliated

Example: Small intestine

(approximately 430X)

B

Example: Trachea (approximately 430X)

A C

Example: Thyroid gland (approximately 430X)

Figure 4–3. Epithelial tissues. (A) Simple cuboidal. (B) Simple columnar. (C) Ciliated.

QUESTION:What is the function of the cilia that line the trachea?