Chapter Summary

I. SKIN

A. Epidermis

The epidermis constitutes the superficial, epithelially derived region of skin. It is composed of four cell types: keratinocytes, melanocytes, Langerhans cells, and

Merkel cells. The keratinocytes are arranged in five layers, and the remaining three cell types are interspersed among them. The five layers of the epidermis are

1. Stratum Basale

A single layer of cuboidal to columnar cells that stand on the basement membrane. This is a region of cell division. It also contains melanocytes and Merkel cells.

2. Stratum Spinosum

Composed of many layers of polyhedral prickle cells bearing intercellular bridges. Mitotic activity is also present. It also contains Langerhans cells and processes of melanocytes.

3. Stratum Granulosum

Cells that are somewhat flattened and contain keratohyalin granules. It is absent as a distinct layer in thin skin.

4. Stratum Lucidum

A thin, translucent layer that is also absent in thin skin.

5. Stratum Corneum

Composed of squames packed with keratin. Superficial squames are desquamated.

B. Dermis

The dermis is a dense, irregular, collagenous connective tissue subdivided into two layers: papillary and reticular.

1. Papillary Layer

The dermal ridges (dermal papillae) and secondary dermal ridges interdigitate with the epidermal ridges (and interpapillary pegs) of the epidermis. Collagen fibers are slender in comparison with those of deeper layers of the dermis. Dermal ridges house capillary loops and

Meissner’s corpuscles.

2. Reticular Layer

The reticular layer of skin is composed of coarse bundles of collagen fibers. It supports a vascular plexus

and interdigitates with the underlying hypodermis. Frequently, it houses hair follicles, sebaceous glands, and sweat glands. Krause’s end bulbs and pacinian corpuscles may also be present.

II. APPENDAGES

A. Hair

Hair is an epidermal downgrowth embedded into dermis or hypodermis. It has a free shaft surrounded by several layers of cylindrical sheaths of cells. The terminal end of the hair follicle is expanded as the hair bulb, composed of connective tissue papilla and the hair root. The concentric layers of the follicle are

1.Connective Tissue Sheath

2.Glassy Membrane

A modified basement membrane.

3. External Root Sheath

Composed of a few layers of polyhedral cells and a single layer of columnar cells.

4. Internal Root Sheath

Composed of three layers: Henle’s layer, Huxley’s layer, and the cuticle.The internal root sheath stops at the neck of the follicle, where sebaceous gland ducts open into the hair follicle, forming a lumen into which the sebum is delivered.

5. Cuticle of the Hair

Composed of highly keratinized cells that overlap each other.

6. Cortex

The bulk of the hair, composed of highly keratinized cells.

7. Medulla

A thin core of the hair whose cells contain soft keratin.

B. Sebaceous Glands

Sebaceous glands are in the forms of saccules associated with hair follicles. They are branched alveolar holocrine glands that produce an oily sebum. Secretions are delivered into the neck of the hair follicle via short, wide ducts. Basal cells are regenerative cells of sebaceous glands, located at the periphery of the saccule.

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C. Arrector Pili Muscle

Arrector pili muscles are bundles of smooth muscle cells extending from the hair follicle to the papillary layer of the dermis. They cradle the sebaceous gland. Contractions of these muscle fibers elevate the hair, forming “goose bumps,” release heat, and assist in the delivery of sebum from the gland into its duct.

D. Sweat Glands

1. Sweat Glands

Simple, coiled, tubular glands whose secretory portion is composed of a simple cuboidal epithelium. Dark cells and light cells are present with intercellular canaliculi between cells. Myoepithelial cells surround the secretory portion.

2. Ducts

Composed of a stratified cuboidal (two-cell-thick) epithelium. Cells of the duct are darker and smaller than those of the secretory portions. Ducts pierce the base of the epidermal ridges to deliver sweat to the outside.

E. Nail

The horny nail plate sits on the nail bed. It is bordered laterally by the nail wall, the base of which forms the lateral nail groove. The eponychium (cuticle) is above the nail plate. The hyponychium is located below the free end of the nail plate. The posterior aspect of the nail plate is the nail root, which lies above the matrix, the area responsible for the growth of the nail.

CHAPTER OUTLINE

Graphics

Graphic 12-1 Conducting Portion of

Respiratory System p. 284

Graphic 12-2 Respiratory Portion of

Respiratory System p. 285

Tables

Table 12-1 Summary Table of Respiratory System Table 12-2 Components of the Blood-Air Barrier

Plates

Plate 12-1 Olfactory Mucosa, Larynx p. 286