Embryology- head, face, in oral cavity

• Sessions covering embryology of the head, face, oral cavity, oral tissues, crown, root, eruption and exfoliation are self-directed workbook activities
• A live oral embryology consolidation session includes group work and a quiz
• Formative assessments include workbooks, discussions and a quiz
• Summative assessment is an E-assessment for the Biomedical Sciences module

Learning Outcomes

• Key phases of embryogenesis and development of head, face, and oral cavity structures should be outlined
• Embryonic origins (cellular layers) of head, face, and oral cavity structures should be defined
• Developmental process interruptions' consequences on face and oral cavity structures should be explained

Definitions

• Embryology explores tissue development
• Oral Embryology explores oral tissue development
• Embryogenesis is the process of embryological development and the formation of tissues and organs
• Embryonic origins are the ectoderm, mesoderm, and endoderm tissue layers of the tri-laminar embryonic disc
• Derivatives are the structures formed from origins/tissue layers

Embryogenesis Phases

• Three key phases of embryogenesis and development: Fertilisation, cleavage, proliferation, migration and differentiation

3 Key Phases of Embryogenesis

• Early Embryogenesis (Weeks 1-4): Blastocyst, embryonic origins, neural crest cells, and pharyngeal arches development
• Embryogenesis (Weeks 5-8): All tissue development including the face, palate, tongue, jaws and skull
• Fetal Development (Week 9 - term): Growth and maturation of the fetus

Early Embryogenesis: Week 1

• The embryoblast is a cluster of cells within the cavity
• The trophoblast lines the cavity with cells
• The blastocoel is the primary yolk sac and fluid filled space
• The embryo develops from zygote to blastocyst phases via fertilization, cleavage, proliferation, migration and differentiation

Early Embryogenesis: Weeks 2-3

• The embryoblast becomes a bilaminar embryonic disc comprised of the ectoderm and endoderm
• The bilaminar disc becomes a trilaminar disc by forming the mesoderm
• The three layers: ectoderm, endoderm and mesoderm are the embryonic origins

Early Embryogenesis: Weeks 3-4

• During weeks 3-4 the embryo goes though key stages of development
• The neuroectodermal tissues differentiate from the ectoderm and form the neural plate
• Later the neural plate bends with the ends joining at the neural plate border
• The closure of the neural tube disconnects the neural crest from the epidermis and the component cells differentiate

Early Embryogenesis: Neural Crest Cells derivatives

• The neural crest cells are key in the development of the head, face, and oral cavity and are highly specialised
• Neural crest cells migrate throughout the embryo giving rise to cranial nerves, peripheral nervous system and teeth

Early Embryogenesis: Week 4

• The embryo begins to take shape through tissue folding
• The stomodeum (primitive mouth), bucco-pharyngeal membrane, and primitive gut are formed
• The membrane ruptures to allow communication between the stomodeum and gut

Early Embryogenesis: Week 4 Continued

• The embryo develops pharyngeal arches (or branchial arches)
• The arches create building blocks for the face and oral cavity
• Each arch has its own nerve, blood and cartilage supply
• Different structures formed in different arches will have different nerve supplies

Pharyngeal Arches and derivatives

• 1st Mandibular Arch: Trigeminal nerve (cranial nerve V), 1st aortic arch blood vessel, Muscles of mastication, Merkel's Cartilage, resulting in Mandible/Maxilla/zygomatic bones
• 2nd Hyoid Arch: Facial Nerve (VII cranial nerve), 2nd aortic arch blood vessel, Muscles of facial expression, Reichert's Cartilage, resulting in part of the hyoid bone, styloid process and the stapes
• 3rd Arch: Glossopharyngeal Nerve (cranial nerve IX), 3rd aortic arch blood vessel, Stylopharyngeal muscle, None, resulting in part of the hyoid bone, connective tissue of the thymus and inferior parathyroid.
• 4th Arch: Superior laryngeal branch of Vagus nerve (cranial nerve X), 4th aortic arch blood vessel, Laryngeal muscles, Cartilage of the larynx, resulting in thyroid, corniculate and cuneiform

Embryogenesis: Weeks 5-8

• The head, face and oral cavity which includes the jaws, tongue, palate and skull develop

Development of the face: Week 4-6

• One frontonasal, two maxillary, and two mandibular prominences fuse together to form the face
• The face continues to develop in detail throughout this phase

Development of the primary palate: Week 6-7

• The palate forms in two parts
• The primary palate forms at the same time as the face
• The frontonasal prominence and the medial nasal prominences fuse together
• The tongue fills the space and hence the primary palate forms first

Development of the secondary palate: Week 7-8

• Secondary palate formation occurs after the primary palate
• The palatine shelves grow around the developing tongue
• The palatine shelves fuse along the midline with the primary palate as the tongue retracts
• Incisive foramen forms at the midline between the secondary and primary palates

Development of the tongue: Week 4-7

• The tongue develops from the 1st, 2nd, and 4th pharyngeal arches
• The anterior 2/3 of the tongue forms from the lateral lingual swellings (1st arch) and the tuberculum impar (2nd arch)
• The copula/hypobranchial eminence form the posterior 1/3 of the tongue
• Cellular apoptosis allows the tongue to separate from the floor of the mouth

Development of the alveolar bone: Week 7

• The maxilla and mandible jaws undergo ossification like normal bone
• A neural and alveolar portion develops to support the tooth
• Secondary cartilages develop to support bone growth

Development of the skull

• Skull development takes place in the cranial vault (brain case), cranial base and the facial bones
• The skull bones derive from neural crest cells and mesoderm tissue
• The cranial vault bones do not fully fuse until 6-7 years old to allow for brain growth

Fetal Development

• The fetal stage (week 9 - birth) focuses on growth of the fetus
• The structures continue to develop

Clinical Significance

• Embryogenesis involves complex processes
• Complex developmental anomalies and defects can occur

Clinical significance of Embryology

• Embryogenesis is a complex set of processes subject to malfunctions or interruptions
• These issues may result in developmental anomalies
• Aetiology of anomalies can be from genetic or environmental factors

Cleft lip and palate

• Facial clefts are congenital anomalies that affect the face and oral cavity
• They can be complete/incomplete and unilateral/bilateral
• Cleft lip and palate affects approximately 1 in 700 live births in the UK
• If not diagnosed or treated, can impact feeding, speech, hearing, and development

Types of facial clefts

• Cleft lip (CL) from failure of fusion of the two medial nasal processes and/or the maxillary and mesial nasal processes
• Cleft palate (CP) from failure of fusion of the palatine shelves together and/or the nasal septum
• Cleft lip and palate (CLP) results from a combination of each of the above

Management of cleft lip and palate

• Diagnosed early, treatment is affective and can reduce impact
• Multi-disciplinary teams start treatment early
• Patients may remain at a higher risk for oral disease due to impact on tooth development