The Skin: An Introduction

Supported by the Savannah and Barry French Poodle Memorial Fund
University of Pennsylvania School of Veterinary Medicine
Last Modified: August 21, 2005

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Michael H. Goldschmidt, MSc, BVMS, MRCVS, Diplomate ACVP Professor and Head, Laboratory of Pathology and Toxicology Chief, Surgical Pathology Department of Pathobiology
Frances S. Shofer, PhD, Adjunct Associate Professor of Epidemiology and Biostatistics

The skin, or integument, is the largest organ of the body.

It consists of 3 layers:

  • the epidermis and it’s associated adnexal structures
    • apocrine glands
    • sebaceous glands
    • eccrine glands
    • hair
    • nails
  • the dermis
  • the panniculus adiposus

The skin protects the body from the external environment. It is a dynamic organ that responds rapidly to changes.


The epidermis is the outer skin layer of the skin and consists of two types of cells:

  • keratinocytes
  • non-keratinocytes

Keratinocytes are the major component of the epidermis and are so called because they produce the intracellular protein keratin (keratinization).

The epidermic is divided into 4 layers:

  • stratum basale or basal cell layer where all cell division occurs. As these cell mature (differentiate) they move up in the epidermis to form the:
  • stratum spinosum or spinous cell layer, so named because on microscopic examination the cells appear to be attached to each other by small spines (desmosomes).
  • stratum granulosum or granular cell layer, so named because on microscopy there are numerous intracellular basophilic (blue) granules.
  • stratum corneum or cornified layer which consists of multiple layers of flattened cells without a nucleus that form the outer protective layer of the epidermis.

The movement of the epidermal cells from the stratum basale to the stratum corneum is a dynamic process involving:

  • cell proliferation
  • cell differentiation
  • cell death

Non-keratinocytes include

  • Melanocytes (the pigment producing cells).
  • Merkel cells, which are cells that function as slowly adapting mechanoreceptors for touch.
  • Langerhan cells that function as immune surveillance cells

Melanocytes and Merkel cells reside in the stratum basale, Langerhans cells are in the stratum spinosum.


The dermis is made up of:

    • dense irregular connective tissue which contains primarily fibrocytes
    • blood vessels and lymphatics
    • nerves
    • smooth muscle fibers (arrector pili)
    • Mast cells, lymphocytes and macrophages.
    • fat or adipose tissue

The adnexa (see above) are found in the dermis with an intimate association with the overlying epidermis.

Panniculus adiposus

Panniculus adiposus is made up of:

    • fat
    • blood vessels
    • nerves


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