Embryology of palate and craniofacial/oral structures

• Embryology is a subfield of biology that deals with embryos and embryo development, which is important for understanding and treating developmental/congenital defects
• Epigenesis: idea that complex features arise from simpler features through interactive processes
• Preformation: all structures of an individual are formed upon conception and developmental processes are simply the growth of these structures in size; however this would mean a complete human is inside a human inside a human and so on, and was disproved by the invention of the microscope and development of cell theory
• Modern developmental biology mixes genetics and traditional embryology
• The early blastula is a hollow structure with the trophoblast outside and the embryoblast inside; the embryoblast will eventually form everything in the body while the trophoblast will form the placenta
• Theoretically, the embryoblast an be cultured to form the whole individual and can differentiate into every cell type of the body
• Pluripotent are cells that can differentiate into any cell but before 2016 they could only be obtained from human embryos (very difficult); after 2016 they can be induced from most cell types by introducing Yamanaka factors
• Once the blastocyst stage is reached, implantation begins where the embryo travels from the ovary to the uterus via the Fallopian tube
• Ectopic pregnancies (implantation outside of the womb) can be life-threatening
• During gastrulation, the inner cell mass begins to form structures, firstly the bilaminar embryonic disc containing the epiblast and hypoblast
• From the bilaminar disc, some epiblast cells migrate to the space between the epiblast and hypoblast, eventually forming the trilaminar disc with the ectoderm, mesoderm and endoderm
• The ectoderm gives rise to organs and tissues such as the epidermis, dentine and enamel of teeth, and the neural crest derivatives
• Mesoderm gives rise to bone, cartilage, tendons, muscles, circulatory system, kidneys, etc.
• Endoderm gives rise to the digestive system, respiratory system, liver, bladder, etc.
• During neurulation, the ectoderm bends inwards to form the neural tube, which will form the central nervous system while the new surface ectoderm forms the epidermis; neural crest cells migrate out to form other tissues, including craniofacial tissues
• Neural tube defects are often attributed to deficiences in folic acid, thus supplements are recommended for pregnant women
• Neurocristopathy: diseases caused by defects in the neural crest
• After neural crest migration, the pharyngeal arches are created, which contributes to most of the craniofacial bones
• The closer the neural tube defect is to the head region, the more catastrophic it is
• The medial nasal process/intermaxillary segment gives rise to a triangular region of the palate (primary palate) that bears 4 teeth (the incisors); maxillary process gives rise to the rest of the canines, premolars and molars
• The primary palate develops at the end of the 5th week from the intermaxillary segment, while the secondary palate (includes soft and hard palate) develops from the maxillary process
• The palate arises from the fusion between the intermaxillary segment and the two maxillary processes; they fuse to form one functional piece
• The two lateral palatine processes project from the maxillary prominences and gr ow downwards on either side of the tongue; the primary plate fuses with the lateral palatine processes as well as the nasal septum
• Fusion of the palate completes by the 9th week and extends to the uvula by the 12th week
• The fusion of the palate is a complex procedure so many things can go wrong, thus a cleft palate is one of the most common birth defects
• Cleft palates are frequently associated with a unilateral or bilateral cleft lip, 50% are a cleft palate only
• Anterior cleft malformation: includes cleft lip, results from deficiency of mesenchyme in the maxillary prominences and intermaxillary segment
• Posterior cleft malformation: includes clefts of the posterior palate
• Cause of cleft palate: improper growth of the primordia involved in palate development
• Cause of cleft palate: palatal shelf did not elevate early enough or there was an issue in the lower jaw, e.g. a smaller jaw leads to the tongue falling backwards and blocking secondary palate closure
• Cleft palates may be genetic (syndromic or non-syndromic, i.e. is it a symptom of a wider disease), due to environmental interactions (smoking, alcohol consumption), or excessive vitamin A intake
• Hemifacial microsomia: congenital condition where the patient has unbalanced development of the face, believed to be caused by insufficient blood supply during early development
• Treacher Collins syndrome: underdeveloped facial bones, including cheek bones and lower jaw, and cleft palates are common; caused by early apoptosis of neural crest cells