buccopharyngeal membrane

**Structure and formation** - During gastrulation (week 3), the prechordal plate region at the cranial end of the embryonic disc is where ectoderm and endoderm stay fused. Mesoderm does not migrate between them here. - This fused region is the buccopharyngeal (oropharyngeal) membrane. The cloacal membrane is the matching structure at the caudal end. - Cranial folding (about week 4) brings the membrane ventrally. It then lies at the floor of the stomodeum, a depression lined by ectoderm. The stomodeum is bounded by the frontonasal prominence, the maxillary prominences, and the mandibular prominences (1st pharyngeal arch).

**Rupture** - The membrane breaks down at about day 24 to week 4. This creates continuity between the amniotic cavity and the foregut (pharynx). - The result is the oral cavity opening into the pharynx. Ectoderm lines the anterior mouth, and endoderm lines the pharynx. Rathke pouch comes from stomodeal ectoderm and forms the anterior pituitary (adenohypophysis). - The boundary between ectoderm and endoderm is roughly at the oropharyngeal junction. The anterior two-thirds of the tongue and the oral mucosa come from ectoderm and 1st arch tissue, while pharyngeal structures are endoderm-derived.

**Clinical correlations** - Persistent buccopharyngeal membrane (oropharyngeal membrane atresia) is extremely rare. It is associated with a membranous obstruction of the oral opening and can occur with other craniofacial anomalies. - Rathke pouch remnants can give rise to a craniopharyngioma, a suprasellar tumor in children with calcifications and bitemporal hemianopia. Pharyngeal hypophysis remnants are rare. - A caudal analog is the cloacal membrane. Failure of mesoderm to migrate there causes bladder or cloacal exstrophy. Persistence leads to imperforate anus.

**High-yield pearls** - It contains no mesoderm, so it is a bilaminar ectoderm-endoderm membrane. - The prechordal plate is located just cranial to the notochord and induces forebrain development. - Other places with ectoderm-endoderm contact and no mesoderm are the cloacal membrane and, in some descriptions, the pharyngeal membranes between arches (the pharyngeal clefts and pouches). - Cranial: buccopharyngeal membrane, which becomes the mouth. Caudal: cloacal membrane, which becomes the anus and urogenital openings.