The structure of the human vaginal stratum corneum and its role in immune defense

Abstract

The superficial layers of the human vaginal epithelium, which form an interface between host and environment, are comprised of dead flattened cells that have undergone a terminal cell differentiation program called cornification. This entails extrusion of nuclei and intercellular organelles, and the depletion of functional DNA and RNA precluding the synthesis of new proteins. As a consequence, the terminally differentiated cells do not maintain robust intercellular junctions and have a diminished capacity to actively respond to microbial exposure, yet the vaginal stratum corneum (SC) mounts an effective defense against invasive microbial infections. The vaginal SC in reproductive-aged women is comprised of loosely connected glycogen-filled cells, which are permeable to bacterial and viral microbes as well as molecular and cellular mediators of immune defense. We propose here that the vaginal SC provides a unique microenvironment that maintains vaginal health by fostering endogenous lactobacilli and retaining critical mediators of acquired and innate immunity. A better understanding of the molecular and physicochemical properties of the vaginal SC could promote the design of more effective topical drugs and microbicides.

Keywords: Female genital tract; HIV; immunity; microbicides; microflora; stratum corneum; vagina.

Figure 1. Comparison of morphology of epidermal vs. vaginal epithelia

A) Structure of normal human vulvar epidermis. SB = stratum basalis; SS = stratum spinosum, SG = stratum granulosa, SC = stratum corneum. B) Structure of normal human vaginal epithelium. (SB= stratum basalis, SuB = suprabasal layer; SC= stratum corneum). Both specimens were collected fresh after surgery, fixed in formalin, mounted in paraffin, sectioned at 6 um and stained with hematoxylin and eosin.

Figure 2. Characteristics of Human Vaginal Stratum Corneum

A) Transmission electron micrograph showing vaginal epithelial cells transitioning into the stratum corneum. Note pyknotic nucleus, corneodesmosomes and glycogen granules. B) Transmission electron micrograph showing fully differentiated epithelial cells at the apical surface of the vaginal straum corneum. These cells are devoid of nuclei and cytoplasmic organelles, and are filled with glycogen. The cells are loosely attached with remnants of intracellular junctions. C) Scanning electron micrograph of the surface of the vaginal epithelium showing exfoliation of flattened corneocytes. D) Interferon-β is concentrated in the stratum corneum of the human vaginal epithelium, demonstrated by immunohistology.