Stacey Mills_Histology for Pathologists_9781496398949

1

Skin Andrew Kanik

EMBRYOLOGY  3 Epidermis  3 Dermis  4

HISTOLOGIC VARIATIONS ACCORDING TO ANATOMIC SITES  18

PATHOLOGIC CHANGES FOUND IN BIOPSIES AND INTERPRETED AS “NORMAL SKIN”  20

Epithelial Skin Appendages  4

SPECIMEN HANDLING  21

HISTOMORPHOLOGY  5 Epidermis  5 Dermis  15

ARTIFACTS  21

Subcutaneous Tissue  16 Blood Vessels, Lymphatics, Nerves, and Muscle  16 HISTOLOGIC DIFFERENCES OF SKIN WITH AGE  18 Newborns and Children  18 Elderly  18

STAINING METHODS  22

Histochemical Stains  22 Immunofluorescence  23 Immunohistochemical Stains/Molecular Studies  23 REFERENCES  27

The skin accounts for about 15% of the total body weight and is the largest organ of the body. It is composed of three layers: (a) epidermis, (b) dermis, and (c) the subcutaneous adipose tissue. Each component has its unique and com- plex structure and function (1–3), with variations accord- ing to age, gender, race, and anatomic location. Functions of the skin are extremely diverse. It serves as a mechanical barrier against external physical, chemical, and biologic noxious substances and as an immunologic organ. It par- ticipates in body temperature and electrolyte regulation. It is an important organ of sensuality and psychological well-being. In addition, it is a vehicle that expresses not only primary diseases of the skin, but also diseases of the internal organs. An understanding of the skin’s normal histology is essential to the understanding of pathologic conditions.

The ectoderm gives rise to epidermis and its append- ages. The mesoderm provides the mesenchymal elements of the dermis and subcutaneous fat (4). Developmental abnormalities in the ectoderm produce among others a variety of syndromes grouped under the umbrella term of ectodermal dysplasias (5). Initially, the embryo is covered by a single layer of ectodermal cells which by the 6th to 8th week of develop- ment differentiates into two layers, the basal layer and an overlying second layer called periderm. Because of mitotic activity, the basal layer becomes the germinative layer and additional rows of cells develop from this proliferating layer, forming a multilayer of cells between the ectoderm and periderm (4). By the 23rd week, keratinization has taken place in the upper stratum, and the cells of the periderm have already been shed (4,6,7). Of interest is that the CD30 antigen, considered to be restricted to tumor cells of Hodgkin disease and anaplastic large cell lymphoma, par- ticipates in the terminal differentiation of many fetal tissues including the skin (8). Cell junction proteins are expressed in the early two- layered embryonic epidermis and as early as the 8th week of estimated gestational age (9). By the end of the first

EMBRYOLOGY

Epidermis Basic knowledge of the embryology of the skin is important because it helps to understand some postnatal pathology.

Copyright © 2020 Wolters Kluwer Health, Inc. Unauthorized reproduction of this content is prohibited.

3

LWBK1701-C01_p001-030.indd 3

13/11/18 12:50 PM