Development of the stapedius muscle and unilateral agenesia of the tendon of the stapedius muscle in a human fetus.

The objective was to analyze the development of the stapedius muscle to understand an isolated unilateral absence of the tendon of the stapedius muscle in a human fetus. The study was made on 50 human embryos and fetuses aged 38 days to 17 weeks post-conception. The stapedius muscle was formed by two anlagen, one for the tendon, which derives from the internal segment of the interhyale and another for the belly, located in the second pharyngeal arch, medially to the facial nerve and near the interhyale. In the interhyale, two segments were observed forming an angle and delimited by the attachment of the belly of the stapedius muscle. The internal segment will form the tendon. The lateral segment of the interhyale was attached to the cranial end of the Reichert's cartilage (laterohyale), and normally it disappears at the beginning of the fetal period. The right unilateral agenesia of the tendon of the stapedius muscle, observed for the first time in a human fetus of 14 weeks post-conception development (PCd), was brought about by the lack of formation or the regression of the internal segment of the interhyale. It presented a belly of the stapedius muscle with an anomalous arrangement, and with a pseudo tendon originated by the persistence of the external segment of the interhyale.

Embryonic anastomosis between hypoglossal nerves.

This article presents two cases of anastomosis of hypoglossal nerves in the suprahyoid region in human embryos of CR length 10.75 and 17.5 mm. This variation was studied in two human specimens at this stage of development and compared with the normal arrangement of the hypoglossal nerves in embryos at the same stage. The anastomotic branches were of similar caliber to the main trunks. In both cases the anastomosis was located dorsal to the origin of the geniohyoid muscles and caudal to the genioglossus muscles, lying transversally over the cranial face of the body of the hyoid bone anlage. The anastomosis formed a suprahyoid nerve chiasm on the midline in the embryo of 10.75 mm CR length.

Anatomical relationships of the cleidoatlanticus muscle. Interpretation about its origin.

An unusual muscular variation, the cleidoatlanticus muscle, was observed on the right-hand side of the lateral cervical region. The upper third of the muscle was concealed by the sternocleidomastoid muscle. There was a loop of nerves surrounding the muscle, formed by an anastomosis between the transverse cervical nerve and the greater auricular nerve. A fine vascular-nervous pedicle (formed by a small branch from the transverse cervical artery and by a branch from the medial supraclavicular nerve) entered the deep surface of the muscle at the junction of its middle and lower thirds. Taking into account the relationships that presented with the superficial branches of the cervical plexus, we consider that the cleidoatlanticus muscle is derived from the sternocleidomastoid muscle.

Morphogenesis of the second pharyngeal arch cartilage (Reichert's cartilage) in human embryos.

This study was performed on 50 human embryos and fetuses between 7 and 17 weeks of development. Reichert's cartilage is formed in the second pharyngeal arch in two segments. The longer cranial or styloid segment is continuous with the otic capsule; its inferior end is angulated and is situated very close to the oropharynx. The smaller caudal segment is in contact with the body and greater horn of the hyoid cartilaginous structure. No cartilage forms between these segments. The persistent angulation of the inferior end of the cranial or styloid segment of Reichert's cartilage and its important neurovascular relationships may help explain the symptomatology of Eagle's syndrome.