Reichert cartilage

**Definition:** Reichert cartilage is the cartilaginous skeletal rod of the 2nd pharyngeal (branchial/hyoid) arch, derived from neural crest mesenchyme. Mnemonic: "Reichert = Second arch = Stapes, Styloid, Stylohyoid ligament, Hyoid (lesser horn and upper body)" (the "S's").

**Derivatives of Reichert cartilage:** - Stapes (except the footplate's vestibular surface/annular ligament region, which has otic capsule contribution) - Styloid process of the temporal bone - Stylohyoid ligament - Lesser horn of the hyoid - Superior (upper) part of the body of the hyoid

**Other 2nd arch derivatives:** - Muscles: muscles of facial expression, stapedius, stylohyoid, posterior belly of the digastric, platysma, auricular muscles - Nerve: facial nerve (CN VII) - Artery: stapedial artery and hyoid artery (mostly regress)

**Comparison with the 1st and 3rd arches:** - 1st arch (Meckel cartilage): malleus, incus, sphenomandibular ligament, anterior ligament of malleus (CN V2/V3). The mandible and maxilla form by membranous ossification, not from Meckel cartilage directly. - 3rd arch: greater horn and lower body of the hyoid (CN IX, stylopharyngeus, common carotid and proximal ICA). - 4th–6th arches: laryngeal cartilages (thyroid, cricoid, arytenoid, etc.), CN X (superior laryngeal from the 4th, recurrent laryngeal from the 6th).

**Clinical significance:** - **Treacher Collins syndrome** (1st and 2nd arch neural crest deficiency; TCOF1 mutation, AD): mandibular and malar hypoplasia, downslanting palpebral fissures, conductive hearing loss, and ear malformations. Ossicle abnormalities can involve the stapes. - **Eagle syndrome:** an elongated styloid process or calcified stylohyoid ligament (Reichert derivatives) causes pharyngeal or facial pain, odynophagia, and a foreign-body sensation. Pain can be triggered by head turning, and rarely carotid compression occurs. - **Stapes anomalies:** congenital stapes fixation or malformation causes conductive hearing loss. Otosclerosis is an acquired condition (stapes footplate fixation), not a developmental Reichert defect. - **Hyperacusis:** from stapedius paralysis in Bell palsy, because the stapedius is a 2nd arch muscle innervated by CN VII. - **Cervical sinus/cyst anomalies:** the 2nd arch overgrows the 3rd and 4th arches and normally obliterates the cervical sinus. Persistence causes branchial cleft cysts, usually along the anterior border of the sternocleidomastoid. - **Ossification:** formed by endochondral ossification from cartilage.

**Boards pearl:** If a question gives a structure and asks for the arch, "stapes, styloid process, lesser horn of the hyoid" points to the 2nd arch (Reichert) with CN VII. "Malleus and incus" points to the 1st arch (Meckel).