EMBRYOLOGY TIMELINE

Development of the Face and Neck

Week 1

◦ A single sperm penetrates an ovum, resulting in fertilization and forms a zygote

◦ The zygote undergoes mitosis and develops from a morula into a blastocyst. The ovum and sperm’s chromosomes also join and cross ove

◦ The blastocyst implants into the endometrium

Week 2

◦ The implanted blastocyst continues to grow in size and develops embryonic cell layers

◦ The bilaminar embryonic disc develops with a superior (epiblast) and inferior (hypoblast) layer and is suspended between the amniotic cavity and the yolk sac

◦ The placenta forms from the trophoblast layer (the outside layer of cells on the blastocyst) and the endometrial lining

Week 3

◦ -The primitive streak forms in the bilaminardisc, creating bilateral symmetry through a furrowed thickening in the middle of the disc

◦ -Cells from the epiblast layer migrate towards the middle and create the mesenchyme (embryonic connective tissue) and mesoderm (middle layer of cells).

◦ - The formation of the mesoderm turns the bilaminar disc into the trilaminar embryonic disc with three layers called the ectoderm (epiblast), mesoderm, and endoderm (hypoblast).

Week 3 continued…

◦ - The cephalic (head) end and caudal (tail) end become apparent

◦ - At the cephalic end, the oropharyngeal membrane begins to develop which only has endo and ectoderm and turns into the stomodeum or primitive mouth. At the caudal end, there is a cloacal membrane (endo and ectoderm only).

◦ - The Central Nervous System begins to develop: neuroectoderm cells differentiate from ectoderm and these cells make the neural plate. The neural plate goes through invagination and forms a neural groove. It begins to further invaginate and produces neural fold at the crest of the neural plate.

◦ - Neural crest cells develop and move into the mesenchyme. They will later become various neck and face structures and oral tissues

◦ -The mesoderm begins to form 38 paired cuboidal segments on either side of the developing neural groove, called somites. These will form the skeletal system, muscles, and dermis.

Week 4

◦ The trilaminar disc goes through embryonic folding which creates a more recognizable embryo. This helps embryonic development continue in the correct position and also forms the future digestive tract with three regions; the foregut, midgut, and hindgut. The foregut forms the primitive pharynx and the midgut and hindgut form from the rest of the pharynx and digestive tract.

◦ - The neural tube fuses, will become the spinal cord

Week 4 continued…

◦ - The stomodeum enlarges due to the disintegration of the oropharyngeal membrane

◦ - Arm and leg buds begin to develop

◦ -Neck development begins

◦ - Tissue around the nasal placodes on the frontonasal process grows and begins to develop into the nose and nasal region

◦ - Stacked bilateral swellings of tissue appear inferior to the stomodeum. These are the branchialarches, six pairs of U shaped bars with a core of mesenchyme formed by neural crest cells

◦ - Four pairs of pharyngeal pouches form from evanigations on the lateral walls lining the pharynx

◦ - The tongue begins to develop as a triangular median swelling (the tuberculum impar).

Week 4 continued…

◦ - Mandibular processes develop from neural crest cells and mesenchyme and later in the week, fuse to form mandibular arch

◦ - Maxillary process forms from mandibular arch on either side of the stomodeum

◦ - Maxillary process begins to fuse with each medial nasal process to form upper lip and with the mandibular arch to form the labial commissures

◦ - Frontonasal process forms which will produce the nasal placodes, nasal pits, medial and lateral nasal processes, and intermaxillary segment to form the nose and palate

Week 5

◦ -Intermaxillary segment forms through fusion of the two medial nasal processes

and becomes the anterior ⅓ of the hard palate

◦ - Placodes, areas of ectoderm tissue with special sense organs, develop. Lens, otic, nasal.

◦ - Primitive muscle cells in the mandibular arch begin to differentiate and orient themselves to the sites where they will form the masticatory muscles

Week 5 continued…

Two lateral lingual

swellings (distal tongue

buds) develop on each

side of the tuberculum

impar from the growth

of mesenchyme of the

mandibular arch. These

grow and merge

together to form the

body of the tongue.

Week 6

◦ - The maxillary processes give rise to 2 lateral palatine processes/palatal shelves. These grow medially towards each other and result in the median palatine raphe.

These form the posterior ⅔ of the hard palate, soft palate, and uvula.

◦ - Formation of the upper lip through the joining of the maxillary process and the 2 medial nasal processes is complete

◦ - Lateral lingual swellings continue to grow to form the body of the tongue

Week 7

◦ - Mandibular muscle mass has enlarged, muscle cells continue to migrate where they will differentiate into the four different masticatory muscles

◦ - Copula, overgrowth of the mesenchyme from the 3rd and 4th branchial arches, forms the base of the tongue along with epiglottic swelling

◦ - The palatal shelves continue to fuse and grow together

Week 8

◦ -The body and base of the tongue fuse, completing the development of the tongue, the sulcus terminalis marks the fusion of the anterior and posterior parts of the tongue

◦ - After the fusion of the tongue, it contracts and moves anterior and inferior to keep out of the way of the developing palatal shelves

◦ - Lingual papillae begin to appear, first circumvallate and foliate

Week 9

The vertical nasal

septum begins to fuse

with the horizontally

orientated palate

- Fungiform papillae

appear

Week 10

◦ - Mandibular muscle masses have become organized into the four muscles of mastication and nerve branches innervate these muscles

◦ - The nasal septum continues to grow

◦ - Filiform papillae appear

Week 11

- Taste buds develop

- The nasal septum

continues to grow

Week 12- The vertical nasal septum is completely fused with the palate, separating the nasal and oral cavity

- All three processes (maxillary and 2 medial nasal) fuse and the formation of the palate is complete. Mesenchyme from the 1st and 2nd branchial arches migrate to the soft palate to make the palatal muscles.