Effect of thyroid hormone and epidermal growth factor on tactile hair development and craniofacial morphogenesis in the postnatal rat.

The facial region of the developing rat is surrounded by seven highly innervated hair follicle groups, of which the lateral tactile hairs have an invariant position between the palpebral fissures and the developing external ears. In this study, we tested the ability of triiodothyronine (T3) or epidermal growth factor (EGF) to alter the direction of lateral tactile hair growth in neonatal rats. To quantitate this effect, we measured the angular displacement of the lateral hairs from the sagittal plane in photomicrographs of 36 six-day-old littermate Sprague-Dawley rats treated on postnatal days 0-5 with saline, T3 (100 ng/g body weight), or EGF 500 ng/g body weight. Midline angular displacements were as follows: control, 45.8 degrees +/- 1.7; T3, 40.9 degrees +/- 1.7; EGF, 59.8 degrees = 2.1; mean +/- SEM, P less than .05 versus controls for T3 and EGF. In addition, the timing of specific topological events of craniofacial development, ie, incisor eruption, ear canal opening, and eyelid unfusion, was compared for the various treatment groups. Thus, at the dosages tested, 1) both hormones accelerate incisor eruption (T3 greater than EGF), 2) EGF is a much more potent accelerator of palpebral morphogenesis than T3, 3) EGF retards while T3 accelerates external ear development, and 4) both hormones elicit a comparable degree of somatic growth retardation. These data provide quantitative evidence in the neonatal rat of differences as well as similarities between two powerful craniofacial morphogens.