GROUP AND INDIVIDUAL LEARNING BDS 2 EMBRYOLOGY COMMONWEALTH

GROUP AND INDIVIDUAL LEARNING: BDS 2 EMBRYOLOGY

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EMBRYOLOGY • Basic Terminology • Embryonic Stage • Stage 1: Fertilisation • Stage 2: Morphogenesis • Foetal Stage • Cephalogenesis • Developmental disturbances • Questions/Answers are infused into the slides

TERMINOLOGY • Growth = process by which a single fertilised ovum is transformed into a mature organism of complex functions and morphology • Development = complexity of structure and function • Purpose? • Basic functions of cellular changes • Induction • Competence • Differentiation • Proliferation • Morphogenesis • Maturation

TERMINOLOGY Extensive and diverse effects of a few GFs is achieved by the expression of combinations of cell-surface receptors that require simultaneous capture of different inducers (GFs => initiate homeobox genes)

EMBRYONIC: FERTILISATION • Timing = 0 -4 weeks • Defined by cellular proliferation and migration, with some differentiation (Diploid) Zygote; formed via the process of fertilisation + mitosis => Morula => Blastocyst Structure • Primary yolk sac • Trophoblast cells • Embryoblast cells

EMBRYONIC: FERTILISATION Zygote => Morula => Blastocyst Developmental Disturbances What’s occurring at a molecular, cellular and tissue level? Meiosis in production of gametes; issues? Ectopic pregnancy

EMBRYONIC: FERTILISATION • Bilaminar germ disc • Embryoblasts => ectodermal + endodermal sac • Prochordal plate • Primitive streak => • Invagination => • Ectoderm => Mesoderm • Trilaminar germ disc • Ectoderm = • Mesoderm = • Endoderm =

Neuroectoderm (NCCs + Neural tube) formed from ectoderm => undergoes folding to form neural groove => neural fold => neural tube Mesoderm => paraxial mesoderm => somites, move cephalorostrally Somites = sclerotome + myotome + dermatome Gives rise to the vertebrae, rib cage, skeletal Bones, cartilage, tendons and skin

Spina bifida Ectodermal dysplasia

EMBRYONIC: MORPHOGENESIS • Time = 4 -8 weeks • Differentiation of all major internal and external structures NCCs => ectomesenchyme => branchial arches => control regions of CT, cartilage, bone, pulp, dentine, cementum and PDL (not E) growth Pharyngeal pouches are composed of all three germ layers NCCs => rhombomeres form the fore/mid /hindbrain

EMBRYONIC: MORPHOGENESIS • Embryonic folding: rostrocaudally and on lateral axes • Migration of tissues to form pharyngeal pouches by different Homeobox genes

FOETAL STAGE • Time = > 8 weeks • Embryo becomes foetus; defined by maturation and proliferation, differentiation and morphogenesis (refinement) = organs, bones, BVs, musculature, brain, etc • Related to Dentistry = cephalogenesis (4 weeks +) + tooth development (6/7 weeks +) • Developmental disturbances Aminocentesis (amniotic fluid test) - Amniotic fluid is sampled at wk 14 -16 to check for chromosomal abnormalities, neural tube defects, parents with inborn errors or X-linked disorders (hemophilia, colour blindness, Rickets) Systemic tetracycline - AB therapy that is not advised during pregnancy because of intrinsic staining of primary/permanent dentition (depending when drug is given), also has other side effects on children - Yellow-brown stains bound to dentine (visible because of transparency of E)

CEPHALOGENESIS • Begins 4 th week • Stomodeum = primitive mouth • 5 facial processes • Recap: notochord/neural tube (NS), segmented blocks (somites, support) and branchial arches (each arch forms the basis of a region of your body, i. e branchial arch 1 + 2 form the face) Branchial arch • Innner aspect is endoderm with ectoderm covering • Central core is mesenchyme derived from mesoderm • NCCs => ectomesenchyme => arch cartilage

MD ARCH + LOWER FACE • • Two bulges under stomodeum (processes) Bulges fuse together = Md arch (= lower lip, Md, teeth) During growth, 1 st branchial arch lays down Meckel’s cartilage Bony Md forms by intramembranous ossification, starts at the split b/w incisive and mental nerve (IAN)

MX PROCESSES + MIDFACE • Forms = upper lip, cheeks, secondary palate and posterior portion of Mx (Mx canines – posterior teeth), labial commissures

FRONTONASAL PROCESS • Forms = upper face, forehead, bridge of nose, primary palate and nasal septum • Placodes = pits of ectoderm that form special sense organs - submerge • Frontonasal process = medial and lateral nasal processes • Medial NPs fuse = middle of nose, philtrum, intermaxillary segment (Mx incisors, primary palate, nasal septum) • Lateral NPs = form alae Fusion of the lateral/medial/Mx processes form the nostrils and most clefts

CLEFTS

SECONDARY PALATE • Mx processes => 2 Pa shelves/lateral palatine processes • Signaling for fusion given by depression of tongue • Fusion forms mid-palatine raphe • Forms hard palate, posterior to incisive foramen; and forms soft palate/uvula

REFERENCES • Nanci A, 2012. Ten Cate’s Oral Histology, 8 th Edn, Missouri • Mosby Elsevier • Sadler, Thomas W. y 2009. Langman's Medical Embryology. Lippincott Williams & Wilkins • Netter, Frank H & Cochard, Larry R, 2002. Netter's Atlas of human embryology. Teterboro