Prenatal superficial musculoaponeurotic system anlage

Introduction

The superficial musculoaponeurotic system (SMAS) is an acronym describing a facial subcutaneous anchored three-dimensional (3D) fibro-adipose-muscular tissue network connected to mimic muscles. SMAS transfers, distributes and reinforces mimic facial muscles contractions to the skin determining mimic expression and facial fold formation. The aim of this study was the histomorphological analysis of prenatal SMAS (pre-SMAS) development in analogy to the adult SMAS typology.

Method

Histological serial sections (n = 7300 of 31 embryos and fetuses) of different staining obtained from the historic Carnegie Collection, Washington D.C. and from the Historical Histological Collection, University Witten/Herdecke were analyzed. All specimens head and neck area were investigated microscopically regarding facial mimic muscles genesis, pre-SMAS development and chronological changes.

Results

Three pre-SMAS Types were described. Pre-SMAS Type I consisting of mesenchymal SMAS with low vascularity and minimal structural changes covered Regio Frontalis and Regio Occipitalis. Pre-SMAS Type II consisting of mesenchymal SMAS with low vascularity and minimal structural changes covered Regio Periocularis, Regio Preparotidea and Regio Nasalis. Pre-SMAS Type III consisting of SMAS with muscular integration and connective tissue remodeling covered Regio Parotidea, Regio Perioralis, and Regio Cervicalis. There were no morphological similarities between adult SMAS architecture compared to pre-SMAS anlage in Regio Preparotidea, Regio Frontalis, Regio Occipitalis, Regio Periocularis, Regio Cervical and Regio Nasalis. Regio parotidea pre-SMAS showed analogies in connective tissue architecture to adult type I SMAS. Regio Perioralis pre-SMAS showed isolated muscle fibers, which were aligned perpendicular to the skin level. Subcutaneous facial adipose tissue was undetectable up to the 22nd week of gestation.

Conclusions

Pre-SMAS anlage metamorphosis seems not to emerge synchronously with mimic muscle development, but its changes and tissue differentiation are closely related to mimic muscle activity. Therefore following ontogenetic hypothesis was proposed: the development of pre-SMAS anlage follows the law “form follows functional activity”.