The facial features of the human embryo develop rapidly very early on in pregnancy, beginning around the fourth week after conception. Many of the structures of the face originate from a group of cells called cranial neural crest cells. These cells move in a distinct pattern from the neural tube located in the back of the embryo to create the various structures of the face. Aberrations in the formation or behaviour of these cells are the main causes of abnormalities in the head and face.
During the first three days* of development, the fertilized ovum, or egg, is located in the fallopian tube. As it travels down the tube it undergoes rapid divisions to form a cluster of cells called the morula. These cells then organize themselves to form the blastocyst and by the end of week five the fully formed blastocyst comes into contact with the uterine wall for implantation.
During the second week of development, the inside of the blastocyst, known as the embryoblast, becomes two layers, the hypoblast and the epiblast layers. Together, these layers form an oval-shaped disc-like structure.
In the third week of development a streak is created on the surface of the epiblast, during which time the cells of the epiblast detach and migrate. These migrating cells create three layers, the ectoderm, mesoderm and endoderm layers, which go on to contribute cells to form all the tissues and organs in the human body.
In the third to fourth weeks of development, a cord, called the notochord, is formed. This induces the cells in the overlying ectoderm to thicken forming a region called the neural plate along the back of the embryo. The edges of the neural plate elevate to form the neural folds that eventually fuse to form the neural tube. During this process, neural crest cells are formed along the entire length of the tube at the tip of the folding neural folds. Neural crest cells formed from the head or cranial region are called cranial neural crest cells, or CNCC’s. These cells multiply and soon start migrating over long distances in distinct paths from the back of the embryo towards the front of the embryo. Once they arrive at their target destinations in the facial regions, CNCCs further develop and mature and ultimately contribute to a substantial amount of the structures in the head and neck region, such as bones, cartilage, and nerves. Growth, migration and subsequent differentiation of CNCCs are critically important for proper development of the normal structures of the facial region.
Soon after the CNCC’s reach their final destination, the facial structures begin to take shape externally. By the fourth week of development, the embryo is characterized by five facial swellings: the frontonasal prominence, and the paired maxillary and mandibular prominences. These structures are formed in part from the migration and proliferation of the CNCC’s from different regions of the neural tube.
In a five week old embryo, the nasal placodes, which will go on to become the olfactory system, and the optic placodes, which will become the lenses of the eyes, will form. In addition, the maxillary and mandibular prominences enlarge and grow forward and towards the middle, eventually giving rise to the upper and lower jaws, respectively.
In a six week old embryo, the two mandibular prominences fuse to form the lower jaw. At this point the outline of the mouth is visible.
In the seven week old embryo the maxillary prominences grow and fuse to form the border of the nostril and the upper lip. Merging of the maxillary and mandibular prominences forms the cheeks and the corners of the mouth. The philtrum of the upper lip and palate are also created at this time.
In this flurry of activity in the first 7 weeks after fertilization, the five facial prominences give rise to the formation of the forehead and sides of the face, middle and sides of the nose, the philtrum, the upper lip, the palate, and the lower jaw.
By the 7th week of human embryonic development most of the facial structures can be observed. In the next few months of development, the initial cartilaginous skeleton of the face is replaced by bone and there is an overall increase in shape and size of the different structures of the face. From childhood to adulthood, the face continues to develop through further growth and remodelling.
*Note: age of embryo is based on "developmental age" - time period after fertilization.