Week 6: ectoderm at center of each pit invaginates to form nasal pit, dividing the rims into lateral and medial nasal processes.
Mesenchymal proliferation around the nasal placodes allows the horseshoe-shaped medial and lateral nasal prominences to develop
The invagination into mesenchyme at the point of fusion between the maxillary process and the lateral nasal process forms the nasolacrimal duct
Fusion of the two medial nasal processes form the nasal bridge and septum. The inferior tips of the medial nasal process fuse to form the intermaxillary process – becomes philtrum
nasal pits grow backward and evolve into the early nasal fossae.
Invagination of these fossae is halted by the nasobuccal membrane
Week 7: nasal fin grows from the floor of the nasal sac to separate oral from nasal cavity
This then thins down to form the oronasal membrane. This ruptures to form an opening – primitive choana
The floor of the nasal cavity forms from the posterior extension of the intermaxillary process – this is theprimary palate
At the same time, the cells lining the walls of the nostrils proliferate to form epithelial plugs, which obliterate the nostrils.
Week 8-9: Medial walls of maxillary process form palatine shelves.
These grow downward and at the end of week 9, rotate rapidly upward (right first then left – thought to be due to rapid synthesis and hydration of hyaluronic acid) to fuse (ventral to dorsal) – forming secondary palate.
At the same time, downgrowth from the frontonasal process forms the nasal septum, with an opening posteriorly (definitive choana).
nostrils reopen by resorption of the epithelial plug
medial nasal process ultimately gives rise to one half of the nasal septum and the medial crus of the lower lateral alar cartilage.
The lateral nasal process develops into the external wall of the nose, nasal bones, upper lateral cartilage, alae, and lateral crus of the lower lateral cartilage.
The apex and dorsum of the nose come from the frontonasal process