Face and Neck Development Notes

Occurs during the fourth week of prenatal development.
Involves ectoderm, mesoderm, and endoderm.
Key processes: primitive mouth, mandibular arch, maxillary process, frontonasal process, and nose formation.
Relies on fusion of tissue swellings. Furrows are eliminated by mesenchyme migration.

Stomodeum: Originates from the first branchial arch and ectodermal depression. Forms the oral cavity proper.
Maxillary process: Arises from the mandibular arch and neural crest cells. Develops into the midface, upper lip sides, cheeks, secondary palate, posterior maxilla, zygomatic bones, and part of temporal bones.
Frontonasal process: Composed of ectodermal tissue and neural crest cells. Contributes to the medial and lateral nasal processes.
Nasal pits: Formed from nasal placodes. Develop into nasal cavities.
Medial nasal processes: Derived from the frontonasal process. Fuse to form the intermaxillary segment, contributing to the middle of the nose, philtrum, anterior maxilla, primary palate, and nasal septum.
Lateral nasal processes: Originate from the frontonasal process. Form the nasal alae.

The stomodeum, or primitive mouth, appears in the fourth week.
It's initially separated from the primitive pharynx by the oropharyngeal membrane.
Disintegration of the oropharyngeal membrane increases the depth of the primitive mouth.

Two mandibular processes appear inferior to the primitive mouth during the fourth week.
These processes fuse to form the mandibular arch, which becomes the mandible.
Meckel cartilage forms within the mandibular arch.
The mandibular arch gives rise to the lower face, lower lip, mandible, mandibular teeth, and associated tissue.

The frontonasal process forms as a tissue bulge at the cephalic end of the embryo.
Placodes (lens, otic, and nasal) develop on the outer surface of the embryo.
Nasal placodes form in the anterior part of the frontonasal process.

Tissue around the nasal placodes grows, initiating nose development.
Nasal placodes submerge, forming nasal pits.
Nasal pits deepen, creating nasal sacs separated from the stomodeum by the oronasal membrane.
Medial nasal processes fuse internally to form the intermaxillary segment.
Lateral nasal processes form on the outer part of the nasal pits.

The upper lip forms during the sixth week via fusion of the maxillary process and medial nasal process.
Maxillary processes form the sides of the upper lip, while medial nasal processes contribute to the philtrum.

Failure of the maxillary process to fuse with the medial nasal process results in cleft lip.

Neck development parallels facial development, beginning in the fourth week and completing during the fetal period.
It originates from the primitive pharynx and branchial apparatus.

The anterior part of the foregut forms the primitive pharynx (oropharynx).
The endoderm of the pharynx lines the branchial arches and pharyngeal pouches, but ectoderm lines the oral and nasal cavities.

The branchial apparatus includes branchial arches, grooves, membranes, and pharyngeal pouches.
Branchial arches appear as stacked bilateral swellings inferior to the stomodeum in the fourth week.
Each arch has its own cartilage, nerve, vascular, and muscular components.
The first arch is the mandibular arch (Meckel cartilage), and the second is the hyoid arch (Reichert cartilage).
Branchial grooves are external grooves between neighboring arches.
Pharyngeal pouches develop as endodermal evaginations from the pharynx walls.

The second branchial grooves may not obliterate, leading to branchial cleft cysts.