Pharyngeal Arches, Clefts And Pouches Flashcards
Describe the formation of the neural tube in the 3 layer embryo (Ectoderm, Mesoderm and Endoderm)
- Thickening within the ectoderm layer at the rostal end of embryo
- Forms the neural plate
- Neural plate develops raised margins called neural folds
- The deepening midline depression is known as the neural groove
- Neural folds eventually fuse
- And separate from the ectoderm forming the neutral tube
Anterior portion of neutral tube expands to form
• Forebrain
• Midbrain
• Hindbrain
Posterior portion of neural tube expands to form
•Spinal cord
Describe the origin of the neural crest cells (NCCs) and explain their role in embryological development of the oro-facial complex
ORIGIN
•As the neural tube forms
•NCC separate from the neuroectoderm (neural tube epithelium) on the lateral aspect
•Migrate to the mesoderm
ROLE OF NCCs IN EMBRYOLOGICAL DEVELOPMENT OF ORO-FACIAL COMPLEX
• In the head region have a role in assisting with the formation of the cranial sensory ganglion
• Differentiate to form most of the connective tissue of the head
• Neuroectoderm cells reinforce the mesoderm (mesenchyme) to the head region referred to as ectomesenchyme
• Cells have the capacity to migrate and differentiate within developing embryo
•Proper migration of NCC is essential for development of the face and the teeth
• All tissues of the tooth except enamel and it’s supporting structures are derived from NCCs
Spinal sensory ganglia
Sympathetic neurons
Schwann cells
Pigment cells
Meninges
Results of abnormal migration or absence of NCCs in the developing oro-facial complex
Underdevelopment results in failure of NCCs to migrate to facial region
Prevents proper dental development
E. G Trencher Collins syndrome
Derivatives of ectoderm
Neuroectoderm
-Retina
-CNS
-Pineal body
-Posterior pituitary
Surface ectoderm
-Epidermis
-Hair, Nails
-Enamel
-Parenchyma of salivary glands
Derivatives of endoderm
Epithelial components of trachea, bronchi, lungs, pharynx, thyroid, tympanic membrane, tonsils, parathyroid, GIT, liver, pancreas, Urinary bladder
Derivatives of mesoderm
- Paraxial
Muscles of trunk (myotome)
Skeleton except skull (Sclerotome)
Dermis of skin
Connective tissue (Dermatome) - Intermediate
Urogenital - Connective tissue
Muscles of viscera
Serous membrane of pleura
Pericardium and peritoneum
Blood and lymph cells
Tooth pulp
Dentin
Cementum
Periodontal ligament
Alveolar bone
Cartilage of 1st arch
- Mandible and maxilla
- Meckel’s cartilage
Incus and malleus
Sphenomandibular and sphenomalleolar ligament
Nerves of the 4 branchial arches
- Trigeminal nerve
- Facial nerve
- Glossopharyngeal nerve
- Vagus nerve
Difference between genetic development and congenital abnormalities with examples
Genetic development abnormalities E. g Down Syndrome and Cystic fibrosis
• Caused by: one or more abnormalities in genome
Autosomal dominant
- One mutated copy needed
- Affected person has affected parent
- 50% chance of inheritance
Autosomal recessive
-Two copies of genes needed
-Affected person usually has unaffected parents
Congenital abnormalities E. G Congenital heart defect, microencephaly
• Structural or functional defect
•Due to
Environmental E. G Infectious agent, X-ray radiation, Drugs, Hormones and Nutritional deficiencies
Genetics
Multifactorial
Unknown cause
Innervation of anatomical structures in the oro-facial complex with reference to the embryological development of these structures
Oral cavity- Branches of trigeminal
Floor of the mouth- Lingual nerve of mandibular
Hard palate- Greater palatine and nasopalatine of maxillary nerve
Soft palate- Lesser palatine of maxillary
Tongue- Chorda Tympani the sensory of facial nerve
Cheeks- Buccal nerve from mandibular nerve
Define teratogens and give examples
Teratogens is an agent that disturbs the development of an embryo or fetus
°It can cause a birth defect, halt pregnancy and cause a miscarriage
° Greatest risk is between 3-8 weeks of gestation
Exposure before 3 weeks results in miscarriage
Exposure after 8 weeks organs are more developed and less sensitive
Examples
Radiation
Chemicals
Maternal infection
Drugs
Rubella (Contagious viral infection)
Hormones
Nutritional deficiencies
Why the origin of the mucosal lining of the oral cavity differs from that of the pharynx and the rest of the GIT
✓The oral cavity is lined by the ectoderm while the pharynx and the rest of the GIT are lined by the endoderm
✓ Buccopharyngeal membrane separates the stomodeum (mouth pit) from the foregut
✓ Head folding establishes the primitive stomatodeum or oral cavity bounded by the developing brain and cardiac plate
Derivatives of the clefts and pouches
Ectoderm- Clefts
Endoderm-pouches
POUCHES
1st- Eustachian tube and tympanic canal
2nd- Fossa of palatine tonsils
3rd- Thymus and Inferior parathyroid
4th to 6th- Superior parathyroid and ultimobrachial body
CLEFTS
1st- External Auditory Meatus
2nd to 4th- Normally obliterated by over growth of the second arch but sometimes a cervical sinus may persist
NB The arch grows faster than the other pharyngeal arches
