stomodeum

**Formation:** Around week 4, cephalic folding and growth of the brain and facial prominences create an ectoderm-lined depression beneath the developing forebrain. This depression is the stomodeum. Its floor is the buccopharyngeal membrane, a bilayer of ectoderm (outside) and endoderm (inside) with no intervening mesoderm. The membrane ruptures at about day 26 (week 4), opening the foregut to the amniotic cavity.

**Boundaries (5 facial prominences surround it):** - Frontonasal prominence (superior) - Paired maxillary prominences (lateral), from pharyngeal arch 1 - Paired mandibular prominences (inferior), from pharyngeal arch 1

**Derivatives (ectodermal origin):** - Epithelium of the anterior two-thirds of the oral cavity, including the lips and cheeks - Enamel of the teeth (ameloblasts are oral ectoderm) - Salivary glands (parotid is ectoderm; submandibular and sublingual are endoderm) - Rathke's pouch, an upward diverticulum of stomodeal ectoderm that becomes the anterior pituitary (adenohypophysis: pars distalis, pars tuberalis, pars intermedia) - The posterior pituitary (neurohypophysis) comes from neuroectoderm of the diencephalon (infundibulum), not the stomodeum.

**Clinical correlations:** - Craniopharyngioma arises from remnants of Rathke's pouch (the craniopharyngeal duct). It is a suprasellar tumor in children. It causes bitemporal hemianopia and has calcifications and cystic "motor-oil" fluid. It is distinct from Rathke's cleft cyst. - Persistent buccopharyngeal membrane is a very rare cause of oral atresia. - Failure of fusion of the facial prominences causes cleft lip (maxillary and medial nasal prominences fail to fuse, which is the intermaxillary segment/philtrum defect) and cleft palate (palatine shelves of the maxillary prominences fail to fuse). - Holoprosencephaly and midline defects can alter stomodeal and facial development.

**Related embryology:** - Proctodeum is the caudal counterpart: an ectodermal pit that meets the hindgut at the cloacal membrane. The pectinate (dentate) line marks this junction. - Three membranes where ectoderm and endoderm directly contact: buccopharyngeal membrane, cloacal membrane, and the oropharyngeal/pharyngeal membranes of the pharyngeal clefts and pouches (closing plates).

**High-yield mnemonic:** Stomodeum = "Stoma" (mouth) + "deum" (to go in). Its ectoderm gives the anterior pituitary and enamel; the neuroectoderm gives the posterior pituitary.