BRS Embryo Head Neck Eye Ear

Taste sensation from the mucosa for the oral part of the tongue is carried by the chorda tympani branch of the facial nerve (CN VII). This part of the tongue forms from pharyngeal arch 1, so the trigeminal nerve (CN V) will carry sensory innervation from the mucosa.

The stapes is innervated by CN VII.

First arch syndrome results from abnormal development of pharyngeal arch 1 due to a lack of migration of neural crest cells. Treacher Collins syndrome is associated with underdevelopment of the zygomatic bone, down-slanting palpebral fissures, and deformed lower eyelids and external ears.

The middle ear cavity develops from pharyngeal pouch 1 as it evaginates to form the tubotympanic
recess.

Pharyngeal groove 1 gives rise to the external auditory meatus.

A pharyngeal fistula forms when pharyngeal pouch 2 and pharyngeal groove 2 persist. Therefore, these fistulas are found on the lateral aspect of the neck, usually along the anterior border of the sternocleidomastoid muscle.

Aqueous humor enters the venous circulation via the canal of Schlemm. Blockage of this canal results in increased intraocular pressure (glaucoma).

The thyroid gland forms from a diverticulum in the midline of the floor of the pharynx.
The thyroid migrates caudally and passes ventral to the hyoid bone. During this migration, the thyroid remains connected to the tongue by the thyroglossal duct. If a part of the thyroglossal duct persists, a cyst develops, usually near the hyoid bone.

Unilateral cleft lip is the most common congenital malformation of the head and neck. Cleft lip occurs when the maxillary prominences fail to fuse with the medial nasal prominences and when the underlying somitomeric mesoderm and neural crest fail to proliferate, resulting in a persistent labial groove. Cleft lip occurs in 1 of 900 births and may be unilateral or bilateral

The optic cup and its derivatives, the retina and optic nerve, develop from the diencephalon.

The lens forms from the lens placode that is induced by the optic cup.

The saccular region of the otic vesicle gives rise to the cochlear duct, which houses the spiral
organ of Corti.

The intermaxillary segment, which plays a critical role in the formation of the definitive adult palate, forms when the two medial nasal prominences fuse in the midline.

The tubotympanic recess is derived from pharyngeal pouch 1. It gives rise to the tympanic
cavity and the auditory (eustachian) tube; the auditory tube interconnects the tympanic cavity
with the nasopharynx.

The parathyroid and thymus migrate in a caudal and medial direction during development; therefore, ectopic glandular tissue may be found anywhere along the migratory path.

Failure of the choroid (optic) fissure to close results in a cleft of the iris, a coloboma iridis. This defect may extend into the ciliary body, choroid, optic nerve, or retina. Congenital aphakia, absence of the lens, may result from defective development of the lens placode.

Aqueous humor is produced by the ciliary processes of the ciliary body. It flows from the posterior chamber, through the pupil, into the anterior chamber, and finally to the canal of Schlemm, which empties into the extraocular veins.

The perilymph enters the subarachnoid space of the posterior cranial fossa via the cochlear
aqueduct, which contains the perilymphatic duct.

The site of retinal detachment is between the pigment epithelial layer and the layer of outer segments M rods and cones; this corresponds to the intraretinal space between the inner and outer layers of the optic cup. Retinal detachment occurs when fluid from the vitreous compartment passes through a retinal hole and separates the pigment epithelial layer from the layer of outer segments of rods and cones.

The utricular region of the otic vesicle gives rise to the endolymphatic sac and duct, and
semicircular ducts.

The auricle (pinna) of the external ear is innervated by cranial nerves V3 (mandibular division),
VII, IX, and X; cervical nerves C2 and C3 also innervate the auricle.

The cochlear duct is derived from a thickening of the surface ectoderm called the otic placode.

The hyaloid canal (Cloquet's canal) is found in the vitreous body. In early development, a hyaloid artery passes through the vitreous body to perfuse the developing lens; in the late fetal period, this artery obliterates to form the hyaloid canal.

The otic vesicle arises from an invagination of the surface ectoderm called the otic placode.
The otic vesicle is found adjacent to the rhombencephalon.

Retrolental fibroplasia results from hyperoxygenation of premature infants. In premature infants, high oxygen concentration results in vaso-obliteration of the terminal arterioles, leading to hemorrhage and infarction of the retina. This phenomenon is peculiar to the incompletely vascularized peripheral retina

The axons of the optic nerve are not completely myelinated until 3 months after birth. Myelinated axons are normally not found in the retina. The optic nerve is not a true peripheral nerve but a tract of the diencephalon; when severed, the optic nerve does not regenerate.
Myelination in the CNS is accomplished by oligodendrocytes; oligodendrocytes are not found in the retina.

The ciliary body is derived from the anterior two layers of the optic cup (neuroectoderm), which form the epithelium and form an anterior extension of the choroid (mesoderm).