Fetal anatomy of the lower cervical and upper thoracic fasciae with special reference to the prevertebral fascial structures including the suprapleural membrane

The aim of this study was to find basic rules governing the fetal anatomy of the deep cervical fasciae and their connections to the mediastinal fasciae. We examined the histology of paraffin-embedded preparations of 18 mid-term fetuses (5 between 9 and 12 weeks of gestation, 3 between 15 and 18 weeks, and 10 between 20 and 25 weeks). The prevertebral lamina of the deep cervical fasciae (PLDCF) developed as an intermediate aponeurosis for the bilateral bellies of the longus colli muscles. In contrast, the alar fascia developed as a connecting band between the bilateral adventitiae of the common carotid artery. The retropharyngeal fascia became evident much later than the latter two fasciae. The fascia covering the thymus was thicker than the fascia for the strap muscles (the pretracheal lamina of the cervical fascia). The primitive suprapleural membrane, or Sibson's fascia, contained veins and fatty tissues, and was composed of the alar fascia rather than the PLDCF, tranversalis fascia, or endothoracic fascia. The prevertebral two-laminar configuration was rather evident in the early stages of development because, in the later stages, the fasciae together provided a multilaminar structure, especially in the lateral area in front of the longus colli, which suspended the cupula pleurae. To consider a continuation from the base of the neck to the upper mediastinum, the alar fascia seems to be a key structure for connecting the vascular sheath to the parietal pleura.