Lectures: Intro to the Integument Block, Language of Dermatology, Common Infections of the Skin, Pre-Lab Orientation, Skin Histology Lab, Physiology of the Skin

Skin! One of my favorite organs. It protects us from mechanical and chemical insults, micro-organisms and UV light. It is filled with receptors for touch, pressure, pain and temperature. It keeps us insulated and warm, but it also acts to cool us down through evaporation of sweat. And, not to forget about MSK, it synthesizes Vitamin D (the MSK Block is where we learned all about muscles and bones).

This Daily Doodle shows a cross section of a hair follicle and the various layers of skin, including the epidermis, dermis and hypodermis. The epidermis is the top layer that you can see and touch. The dermis layer is underneath and contains blood vessels, nerves, sebaceous glands, the “root” of the hair follicle and arrector pili muscles, which are tiny muscle that give us “goose bumps.” The hypodermis contains mainly adipose tissue and is the deepest layer.

Keratinocytes are derived from ectoderm and make up 90% of the epidermis. The main function of keratinocytes is to store melanin. The epidermis can be divided into 4 main layers composed mostly of keratinocytes arranged in four morphologic layers (from deepest to most superficial).

1. Stratum basale, also called basal layer (Blue layer of cells in Daily Doodle on left-side): Consists of a single layer of small cuboidal cells that are attached to the basement membrane by hemidesmosomes. This layer contains mitotically active (stem) cells that provides a constant supply of new keratinocytes. It is usually pigmented as a result of pigment transfer from neighboring melanocytes.
2. Stratum spinosum, also called spinous or “prickle” layer (dark purple): Contains large, polyhedral cells with visible intercellular connections (desmosomes) which give the cells a “prickly” appearance. Most cellular maturation occurs at this layer.
3. Stratum granulosum, also called granular layer (purplish pink): Contains 1-3 layers of flattened cells, which contain keratohyaline granules which contribute to the process of keratinisation. Lysosomes are also prominent in this layer.
4. Stratum corneum, also called cornified layer (pink): Consists of fused, flattened, keratinized cells in multiple layers (also called corneocytes). The keratinocytes in this region has lost their nucleus (anucleate) and cytoplasmic organelles. They are composed almost entirely of keratin filaments (proteins). It forms the principal diffusion barrier of the skin.

A closer view of the epidermis is shown on the left with a melanocyte (orange) and a Langerhans cell (green). Melanocytes produce melanin which is a major determinant of skin pigmentation and protects us against UV radiation. Langerhans cells are members of the immune surveillance system, keeping watch for any invaders.

References:

1. H. Lui, “Cutaneous Gross Anatomy and Language of Dermatology”. UBC Year 2 Lecture Notes. January 3, 2012
2. J. Dutz, “Post-Lab Session: Physiology of the Skin”. UBC Year 2 Lecture Notes. January 3, 2012
3. B. Young and J. W. Heath, “Wheater’s Functional Histology”. Harcourt Publishers Limited. 2000