Porth's Essentials of Pathophysiology, 4e

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Cell Structure and Function

C h a p t e r 1

Neural crest (cranial and sensory ganglia and nerves, adrenal medulla, melanocytes and Schwann cells)

Neural tube (central nervous system, retina, posterior pituitary gland)

Somites (skeletal and trunk muscles and connective tissue)

Dorsal aorta

Visceral serous membranes (peritoneum, pleura, pericardium)

Urogenital ridge (genitalia, gonads, urinary structures)

Somatic serous membrane (peritoneum)

Dorsal mesentery

Surface ectoderm (epidermis, hair, nails, cutaneous and mammary glands)

Lateral and ventral abdominal walls (abdominal muscles and connective tissue)

Peritoneal cavity

Surface

Epithelium of gastrointestinal and respiratory tracts, thyroid, and parathyroid glands, tonsils and pharyngotympanic tube

Endoderm

Mesoderm

FIGURE 1-13. Cross-section of human embryo illustrating the development of the somatic and visceral structures.

Ectoderm

to the underlying basement membrane (Fig. 1-14). The characteristics and geometric arrangement of the cells in the epithelium contribute to their function. In an epi- thelium formed from a single layer of epithelial cells, the free or apical surface is directed toward the exterior sur- face or lumen of an enclosed cavity or tube, the lateral surface communicates with adjacent cells and is charac- terized by specialized attachment areas, and the basal surface rests on the basement membrane anchoring the cell to the surrounding connective tissue. Epithelial tissues are classified according to the shape of the cells and the number of layers that are present: simple, stratified, and pseudostratified. Glandular epi- thelial tissue is formed by cells specialized to produce a fluid secretion. The terms squamous (thin and flat), cuboidal (cube shaped), and columnar (resembling a column) refer to the cells’ shape (Fig. 1-15). Simple Epithelium Simple epithelium contains a single layer of cells, all of which rest on the basement membrane. Simple squamous epithelium is adapted for filtration; it is found lining the blood vessels, lymph nodes, and alveoli of the lungs. The single layer of squamous epithelium lining the heart and blood vessels is known as the endothelium . A similar type of layer, called the mesothelium , forms the serous

membranes that line the pleural, pericardial, and perito- neal cavities and covers the organs of these cavities. A simple cuboidal epithelium is found on the surface of the ovary and in the thyroid. Simple columnar epithelium lines the intestine. One form of a simple columnar epi- thelium has hair-like projections called cilia , often with specialized mucus-secreting cells called goblet cells . This form of simple columnar epithelium lines the airways of the respiratory tract. Stratified and Pseudostratified Epithelium Stratified epithelium contains more than one layer of cells, with only the deepest layer resting on the basement membrane. It is designed to protect the body surface. Stratified squamous keratinized epithelium makes up the epidermis of the skin. Keratin is a tough, fibrous protein that polymerizes to form intermediate filaments that are abundant in the outer cells of skin. A stratified squamous keratinized epithelium is made up of many layers. The layers closest to the underlying tissues are cuboidal or columnar. The cells become more irregular and thinner as they move closer to the surface. Surface cells become totally filled with keratin and die, are sloughed off, and then replaced by the deeper cells. A stratified squamous nonkeratinized epithelium is found on moist surfaces such as the mouth and tongue. Stratified cuboidal and