Development from conception to birth
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| Embryology | - study of origin and development of single individual
- embryonic period: first 8 weeks
- fetal week: 9 weeks to birth
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| Week 1 | - fertilization in lateral third of uterine tube
- zygote (fertilized oocyte) moves toward uterus
- cleavage: daughter cells form zygote
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| Stages of week 1 development | - zygote
- 4 cell
- morula: solid cluster of 12-16 cells
- early blastocyst
- late blastocyst (implantation at this stage)
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| Blastocyst | - fluid filled structure of about 60 cells
- inner cell mass: forms embryo
- trophoblast: helps form placenta
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| Week 2 | - 2 layered embryo
- bilaminar embryonic disc: inner cell mass divided into 2 sheets (epiblast and hypoblast)
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| Primitive streak | - raised groove on dorsal surface of epiblast
- appears in week 3
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| 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
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| 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
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| 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
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| 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
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| 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
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| 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
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| 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
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| Somites | - masses of mesoderm distributed along 2 sides of neural tube
- eventually become dermis (dermatome), skeletal muscle (myotome), and vertebrae (sclerotome)
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| 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
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| 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
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| 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
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| Coelom | - becomes serous body cavities for peritoneal, pericardial, pleural cavities
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| Week 4 | - embryo folds laterally and head and tail
- primitive gut emerges: site of future digestive and respiratory structures
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| 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
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