Embryology 2 Lecture Notes PDF

School of Medicine Embryology 2 Dr. Umesh Bindal Copyright © Griffith University. Griffith University claims copyright ownership of all material on this video unless expressly stated otherwise. No part of the video may be copied (except as allowed by the copyright law of your country) or further dis...

School of Medicine Embryology 2 Dr. Umesh Bindal Copyright © Griffith University. Griffith University claims copyright ownership of all material on this video unless expressly stated otherwise. No part of the video may be copied (except as allowed by the copyright law of your country) or further disseminated without the express and written permission of Griffith University. To seek copyright permission, email: [email protected]. 3rd week of embryonic development Embryo at the end of week 2 (Recap) 2 layers of the germ disc Complete implantation in the uterine wall Two cavities formed ( Amniotic and chorionic) Two layers of extraembryonic mesoderm formed Establishment of uteroplacental circulation Week 3: Days 14-21 Two-layer germ disc Primitive streak forms Gastrulation forms tri-laminar embryo Neural induction Left-right asymmetry 0.4mm - 2.0mm Gastrulation- animated video will play https://youtu.be/3AOoikTEfeo Development of Notochord Forward migrating cells in the midline forms a cord of cells - “Notochordal Process”. Longitudinal section Later the canal disappears and a solid cord of cells results – The definitive notochord Coronal /transverse section Functions of Notochord The proteins expressed in the notochordal cells – Further fascilitate transformation of epiblast cells to primary germ layers – Have an inductive effect on neurulation Notochord acts as a primary axis along which the tissues organize during early development. – Spine, muscles, GIT Mechanical support during flexion of the embryo Gives rise to Nucleus pulposus of the intervertebral discs Persistence of remnants of Primitive streak - Sacro-coccygeal Teratoma Persistence of remnants of primitive streak proliferation and differentiation embryonic tumours containing a variety of tissue types including glands, cartilage, muscle, hair, and teeth, etc. Tumour can be Cystic/solid or mixed type. SCT can be highly vascular. SCT has an incidence of 1 in 35,000-40,000 live births End of Week 3 – Neurulation animated video will playhttps://youtu.be/lGLexQR9xGs The fusion of neural fold starts at the cervical region. Then extend cranially and caudally. Non-fused areas are directly in contact with amniotic fluid. They are called anterior & posterior neuropores. Anterior close two days early than posterior Anterior Neuropore Neural tube (blue) Ectoderm Cervical region Cut edge of amnion Posterior Neuropore 22 days 24 days Neuropores are fused by 28 days. 28 days Neural Tube Defects Defective development/Failure of fusion of neural tube leads spectrum of anomalies which are closely associated with defective development of vertebral column, skull, and overlying body wall. Defective development/fusion of Anterior neuroporeAnencephaly – a fatal defect Defective development/fusion of Posterior neuroporeSpina Bifida – Commonly occur in the lumbosacral region. – Neurological dysfunction varies depending on the type of spina bifida. Spina Bifida Spina Bifida Occulta – Vertebral arches unfused and underdeveloped, vertebral canal is open but meninges and spinal cord are in canal. – Site is covered with skin. Some cases (2025%) symptomless, others involve peripheral nerve and lower limb abnormalities. https://radiopaedia.org/cases/spina-bifida-occulta-4?lang=gb Spina Bifida Cystica – Meningocele Cyst containing only meninges with intact neural tube with possible functional neurological abnormalities. – Myelomeningocele Cyst contains Neural and Meningeal tissues. Spina Bifida Oculta Spina Bifida Cystica Meningocoele 1. Vertebral arches unfused 1 + Cyst contain Meninges Myelo meningocoele 1 + Cyst can contain neural, Meninges or an open lesion Folic Acid and NTD The rate of NTD was markedly reduced worldwide following Folic acid supplementation programs during pregnancy. There are several hypotheses in place to explain this association. – Folic acid is vital for synthesis of nucleic acids which is required in abundance during early fetal development due to very high cellular proliferation taking place. – Folic acid is implicated in reducing Homocystein levels which promote closing of neural tube (increased homocysteine levels are associated with NTD). – Some people are genetically predisposed (carrying certain mutations) and optimal folic acid levels probably reduce this risk (Gene environment interaction). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3799525/ Fate of Neural crest cells Neural crest cells migrate toward different regions/tissues of the body. Sympathetic Neural tissues derived from NCC Neural crest cells ganglia Dorsal root ganglia Melanocytes DRG Autonomic ganglia Preaortic ganglia (Coeliac, superior mesenteric, inferior mesenteric) Enteric NS & Adrenal medulla Non-neural tissues derived from NCC Melanocytes Facial and Cranial bones Meninges Preaortic ganglia Enteric ganglia Adrenal medulla Flexion/ Folding of the embryo Embryo Flexion – Starts at the end of Week 3 2 types – Lateral (Transverse) – Cephalocaudal (Sagittal plane) Outcomes – 3 flat discs transformed into cylindrical body shape 3 tubes of primitive germ layers are arranged within each other; the endodermal tube being in the center. – Amniotic cavity completely covers the embryo. – Positioning of certain organs (Eg. Heart and umbilical cord) A – Cephalocaudal C – Lateral Embryonic folding/ flexion video will playhttps://youtu.be/yXUv4MPuNTA Lateral folding of embryo (Coronal sections) Amnion AC 3 flat discs of germ layers CC YS Chorion Amnion Chorion with chorionic villi Chorion Amnion Ectodermal flexion pulls with it the amnion expands AC Picture-Langman Embryology Part of yolk sac cavity absorbed into Endodermal tube Amnion After flexion 4 Weeks – Differentiation of mesoderm While flexion is taking place, the mesoderm undergoes expansionspreads between ectoderm and endoderm. There are 3 components – Paraxial mesoderm ( Close to the Neural tube) – Intermediate mesoderm (Smaller component) – Lateral plate mesoderm (Most distal) Paraxial Intermediate Ectoderm Endoderm Lateral plate 4 Weeks – Differentiation of mesoderm Coronal section Paraxial mesoderm Somites – Dermatome – Myotome – Sclerotome Ectoderm Superior view Intermediate mesoderm Lateral plate mesoderm Splanchnic mesoderm gives rise to visceral peritoneum/pleura, connective tissue and smooth muscles of the gut. Gives rise to GU system Somatic mesoderm gives rise Parietal peritoneum/pleura, connective tissue around the body wall. Bones of the limbs Dermis anterolateral body wall and limbs. Somites Sclerotome migrate around notochord & Neural tube. form vertebrae and facial skeleton Dermatome dermis of skin – migrate under the ectoderm (Epidermis) Myotome skeletal muscles – migrate into the somatic mesoderm. Example of contributing structures to the development of of body wall (e. T4 segment) T4 Sclerotome  T4 Dermatome  T4 Myotome T4 segment Erector spinae T4 vertebra + Ribs NCC  Dermis of 4th ICS (Posterior part) Epidermis Sensory nerves Somatic mesoderm Dermis of 4th ICS (anterolateral part) T4 segment Neural tube T4 Myotome  - intercostal - abdominal Limb muscles T4 Body wall muscles Both myotome and dermatome of each body segment receives innervation from the spinal nerves. Dermatome is innervated by the sensory neurons developed from the neural crest cells. These neurons grow into the dorsal horn of the spinal cord. Myotome is innervated by the motor nerves developed from the neural tube (Anterior horn). Defective flexion and migration of mesoderm The body folds centrally fuse. Mesoderm migrate underneath ectoderm. Defective mesodermal migration and defective fusion of body folds (commonly occur around the umbilical cord) leads to anterior abdominal wall defects. Omphalocoele – failure of midgut to return Gastrochisis – A larger defect on anterior to abdomen due to a defect around the abdomen - viscera outside. umbilical ring Derivatives of Germ layers Paraxial mesoderm Somites Neuro-ectoderm Gives rise to CNS and PNS Face and Limb development 26 day 4mm Wk 4 - 28 day 5mm Face development starts middle of 4th week with appearance of Pharyngeal arches (4). Each arch receives Ectoderm, Mesoderm, Endoderm and Neural crest cells contributing to different tissues types. Limbs develop as extensions from the trunk. Each limb bud receives ectodermal covering and mesoderm (connective tissue). 32 day 6mm Fetus & Ultra Sound Scan Safe , non invasive method. Used to – Diagnose pregnancy – demonstrate gestational sac. (after 4-5 wks) – Assess the Fetal growth and Age Using Crown Rump length (5-10wks) Biparietal Diameter Femur length Abdominal diameters – Detect the amount of amniotic fluid Polyhydroamnios Oligohydroamnios – Detect congenital anomalies (Usually done around 18-20 wks) – Nuchal Translucency test Nuchal Translucency test Sonographic appearance of a collection of fluid under the skin behind the fetal neck late in first-trimester (11-14wks) of pregnancy. In conditions like Down’s syndrome (and other congenital disorders) there is increased amount of fluid in this area increasing nuchal translucency. Chorionic villous sampling Usually done 10-12 weeks Chorionic tissue is used for – Chromosomal analysis – DNA studies Common Indications: Maternal age 35 years or older at estimated date of delivery Family Hx/Previous child with a chromosome abnormality or genetic disorder Positive antenatal screening tests Risk of miscarriage and congenital anomalies?? Diagnostic Amniocentesis Usually done after 14 weeks – Sufficient amount of amniotic fluid present. Amniotic fluid is used for – Biochemical analysis – Eg. Alfa fetoprotein – Chromosomal analysis – Takes longer (about 1-2 weeks to grow cells isolated from amniocentesis) – DNA studies Common Indications: Maternal age 35 years or older at estimated date of delivery Family Hx/Previous child with a chromosome abnormality or genetic disorder Positive antenatal screening tests Carries a risk of miscarriage.