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• Prenatal growth and development occurs in three phases: embryonic, fetal, and embryonic stage.
• Fertilization is the fusion of male (spermatozoa) and female (ova) germ cells to form a zygote.
• Blastocyst development starts approximately five to six days after fertilization and involves the blastocyst emerging from its protective covering and beginning implantation in the uterus.
• The morula stage consists of 16 cells.

Two-Layered Embryo (Bilaminar Disc)

• The outer cell mass forms the trophoblast, while the inner cell mass forms the embryo.
• This stage is referred to as a two-layered embryo or bilaminar disc.
• The bilaminar disc consists of two main layers.
  • Epiblast (ectoderm)
  • Hypoblast

Three-Layered Embryo (Trilaminar Disc)

• During gastrulation in the third week, a third germ layer, the mesoderm, forms.
• The trilaminar disc is also called a three-layer embryo.
• Gastrulation is the process where the bilaminar embryonic disc reorganizes into a trilaminar disc.
• The migration of epiblast cells to the primitive streak and the subsequent development of the mesoderm is called gastrulation.

Neurulation

• Neurulation initiates the formation of the central nervous system and involves the neural plate forming the neural tube.
• Neurulation starts with the formation of the neural plate, which expands cranially to caudally.
• The cranial end of the neural plate signifies the future brain.
• The narrow caudal end signifies the region of the spinal cord.
• The folds created at the neural plate become the neural groove.
• Fusion of the neural tube usually begins in the middle of the embryo.
• The process of neurulation is complete once the neural tube has completely fused.

Neural Crest Cells

• Neural crest cells are multipotent stem cells found on the side of the neural tube.
• NCCs migrate throughout the embryo and differentiate into various cell types.
• They receive inductive signals to undergo epithelial-mesenchymal transformation.
• Neural crest cells aid in the development of the peripheral nervous system.
• The folding embryo transforms from a flat disc into a three-dimensional structure.
• NCCs give rise to a diverse range of cell types and travel towards the face and ears.

Defects Due to Deficient CNCC Migration

• Mandibulofacial Dysostosis (Treacher Collins syndrome) results from deficient CNCC migration.
• Other conditions: Pierre Robin sequence, cleft palate, and craniosynostosis.

NCC and Head Formation

• Rhombomeres are eight bulges critical for development in the midbrain and rhombomeres 1 and 2.
• First stream contributes to face structures.
• Second stream contributes to jaw structures.
• Rhombomeres 3 and beyond contribute to pharyngeal structures.
• Somite formation begins as paraxial mesoderm cells organize into somitomeres.
• Somitomeres compact, bind with an epithelium, and separate from the presomitic paraxial mesoderm to form somites.
• Homeobox transcription factor genes: Otx2, Msx, Dlx, Barx are invovled

Branchial Arches

• Stomatodeum is the primitive mouth.
• Boundaries of primitve mouth
  • Frontal prominance
  • Cardiac bulge
• The buccopharyngeal membrane is a structure that ruptures to separate the foregut.
• Pharyngeal arches have
  • Vertical grooves
  • Pharyngeal pouches
  • Pharyngeal clefts
• The mandibular arch arises from the first bronchial arch.
• Hyoid arch arises from the second bronchial arch.
• The first pharyngeal groove forms the external auditory canal.
• The first pharyngeal pouch forms the middle ear and eustachian tube.
• The 2nd and 5th arches grow over the other arches.
• The second pharyngeal pouch forms palatine tonsils.
• The third pharyngeal pouch forms the inferior parathyroids and thymus.
• The fourth pharyngeal pouch - superior parathyroids
• The fifth pharyngeal pouch - ultimobranchial body
• Branchial arches are separated by branchial grooves but internally by pharyngeal pouches.

Vascular Development

• Aortic arch vessels pass through pharyngeal arch tissue and then disappear after 4 weeks
• Third pharyngeal arch vessels become the right and left common carotid that supplies the face
• via the internal carotid and stapedial arteries at 5 weeks.

Muscle Development

• Muscle cells in the first arch appear at the fifth week.
• They spread within the first arch into each muscle site of origin at 6th -7th week
• The muscle mass of the first arch forms muscles of mastication. -Muscles of the second arch grow upward forming the muscles of the face and muscles of facial expression.
• The muscle of the mandibular arch remains in the first arch and the muscles of the second arch will move upward to become the muscles of the face.

Neural Development

• Nerves develop in conjunction with developing muscle fibers.
• Nerve V grows into the 1st Arch
• Nerve VII grows into the 2nd Arch
• Nerves V, IX, X contribute to tongue muscles.
• 1 arch Cartilage is Meckel's cartilage.

Innervation and Vascularization of Pharyngeal Arches

• The nerve of the first brachial arch is the 5th cranial trigeminal nerve involved in the innervation of muscles for mastication.
• Facial nerve is for facial muscle
• The nerve of the 3rd brachial arch is for glossopharyngeal nerve

Reichert Cartilage

• Reichert cartilage (2nd arch) - stapes, styloid process, lesser horn, upper body of hyoid
• 3rd arch cartilage - greater horn, lower part of hyoid body
• 4th arch cartilage - hyoid cartilage
• 5th arch cartilage - no derivatives
• 6th arch cartilage - laryngeal cartilage

Development of CF Skeleton (cartilages)

• Cartilages Include: nasal capsule (ethmoid) - Earliest cartilage, audtiry capusle, sphenoid, auditory capsule, basioccipital cartilage
• Cartilages form single cartilaginous unit initially, then separate later to form individual bones Transformed via endochondral bone formation
• Sphenoid cartilage arises posterior to the ethmoid, later forming the wings of the bone laterally. Occipital cartilage is behind.

Craniofacial Development

• Meckel's cartilage (1st arch) becomes the malleus and incus.
• Meckel's cartilage will not become the mandible and it becomes part of a middle ear.

Development of CF Skeleton (bones)

• Cranial bones: membrane(flat)bones, frontal, parietal, squamous portion of temporal, intraoccipital.
• Bone formation: intramembranous bone formation
• Facial bones: premaxilla, maxilla, zygomatic bone, petrous portion of temporal Appear as ossification centers and increase in size and diameter. They spreads anteriorly, posteriorly and upward

Maxilla

• Maxillary bones grow medially into the palate

Mandible

• The body grows laterally to Meckel's cartilage then joins with cartilaginous condyle The body of mandible and cartilaginous condyle replace the Meckel's cartilage Develops as sevral unit Condylar Body Alveolar Process Angular process Coronoid process

Development of CF Skeleton (types of Sutures)

• Sutures are fibrous joints
• Close united sufaces
• includes Facial sutures: zygomaticomaxillary, frontomaxillary, zygomaticotemporal, Cranians sutures
• Syndesmosis – fibrous joint where 2 adjacent bones are linked by strong membrane or ligaments
• Types of sutures simple - uncomplicated band

Suture Classification

• Serrated - interdigitating type Squamosal - beveled or overlapping
• Synchondrosis: cartilaginous joint with hyaline cartilage, grows by forming new cartilage,
• synchondrosis articulations grow by forming new cartilage in the center of the suture
• Ethmosphenoid _ Sphenoccipital
• Synostosis is a type of suture in which there is bone-to-bone union e.g. midpalatal suture
• Clinical Implications* Craniosynostosis is a condition in which the sutures close too early, causing problems with normal brain and skull growth arch expansion and midpalatal suture

Development of the Face

• Development of face occurs at weeks 4-7
• Olfactory (nasal) placode thickens.
• Nasal pit is formed.
• Lateral nasal process, Medial, process
• The nostrils deepen as the tissues around them continue to grow anteriorly.
• At 6 weeks Broadening of face occurs by lateral growth of brain
• eyes and maxillary processes at the front occurs and mouth slit widens
• nasal pit, LNP, MNP

Medial Nasal Process (MNP)

• The frontal area is now known as the frontonasal process Broadens face
• Philtrum = MNP, upper lip= MNP+2MaxSeg

Palatal Shelf

• Lateral nasal process has broadening of face
• Eye=broad side
• During the 5th week are the nasal placodes ( thickened areas of epithelium in upper border of the lip).
• The eyes now approach the front of the face.
• Third of face occurs and Broadens the face

Palate Formation

• A lack of contact or fusion of the medial nasal process and the maxillary processes will result in either a unilateral or a bilateral cleftlip
• Border of the lip:* lack of fusion MNP + MaxP =CL Epithelium covering, of the Medial and Nasal process Contact zone of fusion known as the nasal fin
• Lack of contact and Epithelium = pull apart =orbicularis oris for support
• 6 weeks –Pits continue to the roof of mouth

Development of Development

Initialy only a common space occurs Two palatine shelves have fused horizontal the horizontal platal shelves, contact in the midline It starts from the primary palate and goes posteriorly, fusion (The initial contact occurs behin) then merges

1. 7Weeks- platatine shelfs are vetical
2. 8 weeks, elevate
3. 9 week final fussion
4. 10 weeks –Palatal shelves fused with nasal spetum

Development of Tongue

• Tuberculum impar arise in he middles/midline
• -lateral lingual swellings Quickly merge togther from large mass of mucous membrane
• The postiror one- third develops from (Large swelling) from mesenchyme of the second , third, and fourth archs
• The swelling consists of the copula and the hypobrachial eminence Over and devlopes Posterior part of the fourth arch markds the developemnt for the epigottis (the Swelling now covers copua

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