Human reconstructed tissue models have become increasingly important during the last few years. Reconstructed models already replace animal testing for skin corrosion and irritation. They provide more physiological test systems as compared to cell monolayers and allow for topical testing of undiluted and water insoluble materials. They can be used for substance testing in the regulatory context of the cosmetics directive and under the REACH legislation.

Skin corrosion

3-D human epidermal models consisting of a basement membrane, proliferating keratinocytes and stratum corneum with an intact barrier function are available.

Historically, dermal corrosion has been determined using the Draize Rabbit Skin Test. The European Centre for the Validation of Alternative Methods (ECVAM) has validated and accepted these reconstructed human models for skin corrosion testing, according to OECD (Organisation for Economic Cooperation and Development) TG 431 “In Vitro Skin Corrosion: Human Skin Model Test” as an alternative to animal testing.

The test consists of topical application of the test material to the model for (a) 3 minutes and (b) 60 minutes, followed by immediate determination of the cytotoxic effect. Cytotoxicity is expressed using the widely accepted MTT ET-50 (ET-50: exposure time needed for a chemical to reduce viability to 50% of controls) tissue viability assay. A substance is classified as corrosive if the cell viability is reduced below 50 % after 3 min of exposure or 15 % after 1 hour.

Skin irritation

The assessment of dermal irritation has typically involved the use of laboratory animals, such as the Draize skin irritation test. 3-D human epidermal models are available and have been accepted by ECVAM as alternatives to animal testing, for skin irritation according to OECD TG 439 ‘’In vitro skin irritation: Reconstructed human epidermis test method’’.

The test consists of topical application of the test material to the model for a specified time, followed by a 42 hour incubation period and immediate determination of the cytotoxic effect. Cytotoxicity is expressed using the widely accepted MTT ET-50 tissue viability assay. Relative cell viability is calculated for each tissue as a percentage of the mean of the negative control tissues. Skin irritation potential of the test materials is predicted if the remaining relative cell viability is below 50%.

Eye irritation

3-D human corneal models are available to be used as alternatives to animal testing (such as the Draize Rabbit Eye Test) to assess ocular irritation where chemicals can be classified as non-irritating, mild, moderate or severely irritating. In addition Sub-Draize" testing is possible with these models, allowing quantifiable discrimination among mild, milder, and mildest product formulations. Cytotoxicity is expressed using the widely accepted MTT ET-50 (ET-50: exposure time needed for a chemical to reduce viability to 50% of controls) tissue viability assay. Based on the ET-50, an approximation of the Draize Rabbit Eye Score can be determined and the chemical classified accordingly.