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Embryology

Development from conception to birth

QuestionAnswer
Embryology - study of origin and development of single individual - embryonic period: first 8 weeks - fetal week: 9 weeks to birth
Week 1 - fertilization in lateral third of uterine tube - zygote (fertilized oocyte) moves toward uterus - cleavage: daughter cells form zygote
Stages of week 1 development - zygote - 4 cell - morula: solid cluster of 12-16 cells - early blastocyst - late blastocyst (implantation at this stage)
Blastocyst - fluid filled structure of about 60 cells - inner cell mass: forms embryo - trophoblast: helps form placenta
Week 2 - 2 layered embryo - bilaminar embryonic disc: inner cell mass divided into 2 sheets (epiblast and hypoblast)
Primitive streak - raised groove on dorsal surface of epiblast - appears in week 3
Gastrulation - process of embryo growing from 2-layered disc (epiblast and hypoblast) to three layered disc with 3 germ layers (endoderm, mesoderm, ectoderm) which are all derived from epiblast
Week 3: Formation of germ layers - Endoderm: formed from migrating cells that replace the hypoblast - Mesoderm: formed between epiblast and endoderm - Ectoderm: formed from epiblast cells that stay on dorsal surface
Notochord - day 16: made of mostly mesoderm and some endoderm - defines body axis and forms midline that divides left & right sides of body - future site of vertebral column
Neurulation - neural plate: ectoderm in dorsal midline thickens to form neural plate - neural groove: ectoderm of neural plate folds inward until hollow neural tube is pinched off into body
Induction - ability of one group of cells to influence developmental direction of other cells - example: notochord signals that stimulate surrounding ectoderm to form brain & spinal cord
Neural tube - cranial part becomes the brain, rest becomes the spinal cord - closure of neural tube begins in week 3 and usually ends by week 4
Neural crest - cells originate from ectodermal cells on lateral ridges of neural plate - forms sensory nerve cells which are positioned just external to closed neural tube
Somites - masses of mesoderm distributed along 2 sides of neural tube - eventually become dermis (dermatome), skeletal muscle (myotome), and vertebrae (sclerotome)
Paraxial mesoderm - area of mesoderm that forms just lateral to neural tube on both sides - starting cranially and proceeding caudally, divides into series of blocks called somites
Intermediate mesoderm - begins as continuous strip of tissue just lateral to paraxial mesoderm - each segment is attached to a somite - influenced by segmentation of somites
Lateral plate - most lateral part of mesoderm - splits into 2 layers that have a wedge between them (coelom) - somatic mesoderm: next to ectoderm - splanchnic mesoderm: next to endoderm
Coelom - becomes serous body cavities for peritoneal, pericardial, pleural cavities
Week 4 - embryo folds laterally and head and tail - primitive gut emerges: site of future digestive and respiratory structures
Fetal period - week 9 to birth - has been sexual differentiation of genitalia - organ systems established in embryonic period continue to grow and differentiate - period of rapid growth
Created by: peckman12