Disc composed of 2 germ layers: ectoderm and endoderm

Week 3: Primitive streak- development of mesoderm (the third germ layer)Primitive streak starts at the tail end of the embryo. Ectodermal cell migrates to the region of primitive streak, invaginate beneath the ectodermal layer and form the sandwiched third layer. The third layer and the primitive streak will spread from the tail end toward the head end, later become the future mesoderm.

*note: the spreading third layer never reaches the head end and far tail end*

Early development: fertilized cell cell division cell differentiation (As cell number increased, different region undergoes differentiation. One of the differentiations includes the development of the neural and blood system which provides means of communication between the different cells and groups of cell as well as to the external environment)

Plate tube (for primitive gut) c-shape curve at head and tail

Development of the Third Layer: Mesoderm and Ectomensenchyme

Ectomesenchyme Development: Neural Crest Cell

Neural plate: Thickening of the ectoderm at the head end of the embryo

Neural groove: margins of the neural plate thickens and form a groove

Neural crest cell: As the groove gets deeper, bulges formed which give rise to future neural crest. After the neural fold meet and becomes the neural tube, the bulge (neural crest) is separated from the tube and become neural crest which is the later ectomesenchyme

Therefore, mesoderm and ectomesenchyme has different origin- one is from primitive streak and the other is from neural plate.

24th to 28th day (week 4): Facial Development

Mouth developmentWeek 4: presence of a depression gives rise to stomodeum (primitive oral cavity)

Buccopharyngeal membrane break down leading to direct communication between stomodeum and the digestive track, only then mouth is developed

neck development : week 4

It arises from the branchial arches and the primitive pharynx, which is part of the digestive tube.

The caudal part of the primitive pharynx forms the oesophagus, and a ventral outgrowth leads to formation of the larynx, and trachea which end at the developing lung

Late week 2 and early week 35 Process developed as 5 tissue masses produced by the multiplication of ectodermal cell. This is: 2 x mandibular, 2 x maxillary and 1 frontonasal processes.

Branchial arches developes.

Later development:Changes for form primitive nasal cavities24 days: 1st branchial arch starts to bud off and maxillary process formedStomatodeum borders now are:

- Rostral: developing brain- Caudal: 1st branchial arch- Lateral: maxillary process

Frontonasal process forms 2 nasal pits and lateral and medial nasal process forms around itMaxillary process grows and pushes lateral and medial nasal process towards midline process fuse to form:

- Middle part of nose- Middle part of upper lip- Anterior part of maxilla- Primary palate (part that holds 1, 2)

o Nerve: incisive branch of nasopalatine branch of maxillary division of trigeminal nerve (CNV)

- Fusion of shelves from maxillary processo Nasal and oral cavity are separated

- Nerve: greater palatine branch of maxillary division of trigeminal nerve (CNV)

7th to 8th week: formation of secondary palate

Formation of tongue

- Tuberculum impar: mesenchymal swelling in midline of mandibular process of 1st branchial arch

- 2 other swellings appear (1 on either side of tuberculum impar) and merge to form large mass

- Anterior 2/3 of tongueo Mucus membrane: formed by the large masso Nerve for sensory supply: mandibular division of trigeminal nerve (CNV) (via

lingual nerve) Derived from 1st branchial arch

o Nerve for taste: chorda tympani from facial nerve (CNVII) Derived from 2nd branchial arch

- Posterior 1/3 of tongueo Formed from: hypobranchial eminence

Large midline swelling from 3rd branchial archo Nerve: Glossopharyngeal nerve (CNIX)

~10 weeks: Formation of mandible

- Develops from: mandibular process from the 1st branchial archo Mandibular process fuse so mandibular arch formed o Bone of mandible form from mesenchymal tissue and Meckel’s cartilage

disappears- Trough formed to contain inferior dental nerve- Bone forms compartments for developing teeth and closes over the tooth germs- 10 weeks: rudimentary mandible formed from intramembranous ossification- Role of Meckel’s cartilage: formation of mental foramen

Formation of maxilla

- Develops from: condensation of mesenchyme from 1st branchial arch- No branchial arch cartilage in maxillary process formed by intramembranous

ossification- 16th week: maxillary sinus is formed