Content Reviewers:Yifan Xiao, MD
Early in embryonic development, during the 3rd week post-fertilization, the embryo is a flat, disc-shaped organism made up of three layers of pluripotent cells called germ layers, which give rise to all the organs and tissues in the body: an inner layer, called endoderm, a central layer, called mesoderm, and an outer layer, called ectoderm.
By week 4 of development, as a result of the folding of the embryo along the rostrocaudal axis and the lateral axis, it takes on a more recognizably “human” form—but to be honest, it still looks more like a shrimp than a baby.
Lateral to the neural tube is the paraxial mesoderm, which partially segments rostrally to form somitomeres and fully segments caudally to form somites, the first in the series being the occipital somites.
At the back of the stomodeum, there’s a two-layered membrane, called the buccopharyngeal membrane, made up of ectoderm and endoderm.
The buccopharyngeal membrane initially separates the stomodeum from the foregut, but soon disintegrates, allowing free access between the stomodeum and the foregut.
At the same time, six little bulges or thickenings of the mesoderm, sprout from the primitive pharynx to become the branchial, or pharyngeal, arches.
These arches are paired, symmetrical bumps that form on each side on the lateral aspect of the embryo, in a craniocaudal fashion, going from head to tail.
At the same time, neural crest cells from the midbrain and the first two rhombomeres migrate bilaterally to the region and infiltrate the mesoderm bumps where they support the development of embryonic connective tissue needed for craniofacial development, called ectomesenchyme.
The first pharyngeal arch splits up into two processes — the maxillary process, which lies lateral to the stomodeum and extends slightly above; and the mandibular process, which lies near the lower border of the stomodeum.
The mandibular processes on either side grow towards each other and merge into a single structure very early on.
Now, development of the face begins in week 4 when two patches of ectoderm on the frontal prominence start to proliferate, forming two thickenings known as the nasal placodes.
During week 5, the mesodermal cells surrounding each nasal placode proliferate rapidly to form a horseshoe shaped swelling.
The inner half is called the medial nasal process and the outer half, the lateral nasal process.
The region of the frontal prominence where these changes take place and the nose will develop is called the frontonasal process.
As the mesoderm proliferates, the nasal placodes appear to sink downwards to form nasal pits.
The base of the nasal pits are lined by the oronasal membrane, which separates it from the primitive oral cavity.
At the same time, the maxillary processes start to proliferate towards the center while remaining separated from the lateral nasal process by the naso-optic groove, and the medial nasal process by the bucconasal groove.