Development of the stapedius muscle and pyramidal eminence in humans

Abstract

The aim of the study was to systematize the key developmental phases of the stapedius muscle and the pyramidal eminence to clarify their formation, as well as to understand the variations and anomalies that can affect these structures. Sixty human embryos and fetuses between 38 days and 17 weeks of development were studied. The stapedius muscle is 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 medial to the facial nerve and near the interhyale but forming a completely independent anlage. In the interhyale, two segments were differentiated, these forming an angle; at the vertex, the belly of the stapedius muscle is attached. The internal segment is located from the attachment of the belly of the stapedius muscle to the anlage of the stapes, forming the anlage of the tendon of the stapedius muscle. The external segment completely disappears at the beginning of the fetal period. The pyramidal eminence is formed by an anlage independent of Reichert's cartilage, from the mesenchymal tissue of the tympanic cavity, which condenses around the belly of the stapedius muscle from 12 weeks of postconception development. The length of the tendon of the stapedius muscle in adults varies, depending on the attachment site of the belly of the stapedius muscle in the interhyale, which would determine the length of the internal segment (anlage of the tendon) and consequently the tendon length. This variation depends on the greater or lesser persistence of the angulation observed during development, between the tendon and the belly of the stapedius muscle.

Development of the stapedius muscle canal and its possible clinical consequences

“…Some authors simply mention how the laterohyale process intervenes in its formation [1], which is discussed by Louryan [6] and RodriguezVazquez [5], describing one the origin in the interhyale process and the other as being based on the mesenchymal tissue of the tympanic cavity, whilst for Hough [2] the mesenchymal cells of the second pharyngeal arch intervene. The close relationship with the facial nerve is also described [4] and how the stapedius muscle is housed on its interior [3,7].…”

“…Spector and Ge [4] observe how, between weeks 16 and 19, the pyramidal eminence presents a cartilaginous structure and its ossification process ends between development weeks 25 and 27, whilst for Rodriguez-Vazquez [5] the pyramidal eminence is formed by an anlage independent of Reichert's cartilage, from the mesenchymal tissue of the tympanic cavity which condenses around the belly of the stapedius muscle after 12 weeks.…”

Ear and Further Anatomic Anomalies in Children Undergoing Stapedotomy

“…Another middle ear anomaly worth mentioning, which was intraoperatively identified, was the absence of the stapedius muscle tendon in 1 ear. Based on embryologic knowledge, the stapedius muscle is formed from 2 rudiments; 1 for the belly which is located in the second pharyngeal arch and 1 for the tendon, derived from the interhyale [18] . According to the majority of researches, stapes and the rest of the ossicles are also formed from the second pharyngeal arch [18] .…”

“…Based on embryologic knowledge, the stapedius muscle is formed from 2 rudiments; 1 for the belly which is located in the second pharyngeal arch and 1 for the tendon, derived from the interhyale [18] . According to the majority of researches, stapes and the rest of the ossicles are also formed from the second pharyngeal arch [18] . These facts provide an evidence-based explanation for the combination of middle ear malformations with stapes fixation.…”

“…Anomalies during the embryologic development can affect not only stapes but also the rest of the middle ear structures. Closer embryologic analysis enlightens the coexisting external ear canal and temporal bone dysplasias as well [18] .…”