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EmbryologEmbryolog
yy Review Review
EmbryologyEmbryology
EmbryologyEmbryology – study of origin and – study of origin and development in utero—prenatal development in utero—prenatal formation, growth and formation, growth and differentiationdifferentiation
Prenatal periodPrenatal period Embryonic periodEmbryonic period – first 8 weeks – first 8 weeks Fetal periodFetal period – remaining 30 weeks – remaining 30 weeks
Embryonic PeriodEmbryonic Period
Fetal PeriodFetal Period
The Embryonic PeriodThe Embryonic PeriodWeek 1Week 1 – from zygote to blastocyst – from zygote to blastocyst Fertilization (Conception)Fertilization (Conception) – in lateral – in lateral
third of uterine tubethird of uterine tube ZygoteZygote (fertilized oocyte) moves toward the (fertilized oocyte) moves toward the
uterusuterus BlastomeresBlastomeres – daughter cells formed from – daughter cells formed from
zygote through mitotic division called zygote through mitotic division called cleavagecleavage
MorulaMorula – – solidsolid cluster of 12–16 blastomeres cluster of 12–16 blastomeres BlastocystBlastocyst – fluid-filled embryonic – fluid-filled embryonic
stage– ~ 60 cellsstage– ~ 60 cells
The Embryonic PeriodThe Embryonic Period
Events in first week Events in first week ZygoteZygote4-cell4-cellMorulaMorulaEarly blastocystEarly blastocystLate blastocyst— implantations Late blastocyst— implantations
occur at this stageoccur at this stage
Fertilization and the Events Fertilization and the Events of the of the
First 6 Days of DevelopmentFirst 6 Days of Development
Week 2 – BlastulationWeek 2 – Blastulation
Two-layered embryo formationTwo-layered embryo formation Bilaminar embryonic discBilaminar embryonic disc – inner – inner
cell mass divided into two sheetscell mass divided into two sheets EpiblastEpiblast and the and the hypoblasthypoblastTogether they make up the Together they make up the
bilaminar embryonic disc bilaminar embryonic disc
Amniotic sacAmniotic sac – formed by an – formed by an extension of epiblastextension of epiblastOuter membrane forms the Outer membrane forms the amnionamnion
Inner membrane forms the Inner membrane forms the amniotic sac cavityamniotic sac cavity
The cavity is filled with amniotic The cavity is filled with amniotic fluidfluid
Week 2 – BlastulationWeek 2 – Blastulation
Yolk sacYolk sac – formed by an lateral – formed by an lateral extension of hypoblastextension of hypoblast
Digestive tubeDigestive tube forms from yolk forms from yolk sacsac
NOTNOT a major source of nutrients a major source of nutrients for embryofor embryo
Tissues Tissues aroundaround yolk sac g yolk sac gives ives rise to rise to earliest blood cellsearliest blood cells and and blood vesselsblood vessels
Week 2 – BlastulationWeek 2 – Blastulation
Implantation of the Implantation of the BlastocystBlastocyst
Implantation of the Implantation of the BlastocystBlastocyst
Implantation of the Implantation of the BlastocystBlastocyst
Disorders of implantationDisorders of implantation
Ectopic pregnancyEctopic pregnancy Tubal— 95%Tubal— 95% PeritonealPeritoneal Douglas pouchDouglas pouch
Pracental previaPracental previa-Placenta forms at the inner cervical -Placenta forms at the inner cervical
osos
-Characterized with uterine bleeding-Characterized with uterine bleeding
Week 3 – Trilaminar Week 3 – Trilaminar embryoembryo
Primitive streakPrimitive streak – raised – raised groove on the dorsal surface of groove on the dorsal surface of the epiblastthe epiblast
GastrulationGastrulation – a process of – a process of invagination of epiblast cellsinvagination of epiblast cellsBegins at the primitive streakBegins at the primitive streakForms the three primary Forms the three primary germ layersgerm layers
Week 3 – Tri-laminar Week 3 – Tri-laminar embryoembryo
Three Germ LayersThree Germ Layers EndodermEndoderm – formed from – formed from
migrating cells that replace the migrating cells that replace the hypoblast hypoblast
MesodermMesoderm – formed between – formed between epiblast and endodermepiblast and endoderm
EctodermEctoderm – formed from epiblast – formed from epiblast cells that stay on dorsal surfacecells that stay on dorsal surface
Note that all layers derive from Note that all layers derive from epiblast cellsepiblast cells
Week 3: The Primitive Week 3: The Primitive StreakStreak
Week 3 – Tri-laminar Week 3 – Tri-laminar embryoembryo
Week 3: Notochord Week 3: Notochord formationformation
Primitive nodePrimitive node – a swelling at – a swelling at one end of primitive streakone end of primitive streak
NotochordNotochord forms from forms from primitive node and endodermprimitive node and endoderm
NotochordNotochord – defines body axis – defines body axis Is the site of the future Is the site of the future vertebral columnvertebral column
Appears on day 16Appears on day 16
Week 3: Notochord and Week 3: Notochord and MesodermMesoderm
Week 3: Notochord and Week 3: Notochord and MesodermMesoderm
Week 3: NeurulationWeek 3: NeurulationNeurulationNeurulation – ectoderm – ectoderm
starts forming brain and starts forming brain and spinal cordspinal cord
Neural plateNeural plate – ectoderm in – ectoderm in the dorsal midline thickensthe dorsal midline thickens
Neural grooveNeural groove – ectoderm – ectoderm folds inwardfolds inward
Week 3: NeurulationWeek 3: NeurulationNeurulationNeurulation (continued) (continued)Neural tubeNeural tube – a hollow tube – a hollow tube pinches off into the bodypinches off into the body
CranialCranial part of the neural tube part of the neural tube becomes the brainbecomes the brain
Folic acid deficiency at this Folic acid deficiency at this stage causes neural tube stage causes neural tube defectsdefects
Week 3: NeurulationWeek 3: Neurulation
Neural crestNeural crestCells originate from Cells originate from ectodermal cellsectodermal cells
Forms sensory nerve cellsForms sensory nerve cellsInductionInduction
Ability of one group of cells Ability of one group of cells to influence developmental to influence developmental direction of other cellsdirection of other cells
Week 3: Mesodermal Week 3: Mesodermal differentiationdifferentiation
SomitesSomites – Body – Body segementationsegementationParaxial mesodermParaxial mesodermIntermediate mesodermIntermediate mesoderm – begins as a continuous – begins as a continuous strip of tissue just lateral strip of tissue just lateral to the paraxial mesodermto the paraxial mesoderm
Lateral plateLateral plate – most lateral part of – most lateral part of the mesodermthe mesoderm
CoelomCoelom – becomes serous body – becomes serous body cavitiescavities
Somatic mesodermSomatic mesoderm – – apposed to the ectodermapposed to the ectoderm
Splanchnic mesodermSplanchnic mesoderm – – apposed to the endodermapposed to the endoderm
Week 3: Mesodermal Week 3: Mesodermal differentiationdifferentiation
Parts of the mesoderm
Neurulation and Neurulation and notocordnotocord
Neuralation and Neuralation and notocordnotocord
Week 4 – Embryonic Week 4 – Embryonic foldingfolding
Folding of embryo laterally Folding of embryo laterally and at the head and tailand at the head and tail
Embryonic disc bulges; Embryonic disc bulges; growing faster than yolk sacgrowing faster than yolk sac
Primitive gutPrimitive gut – encloses – encloses tubular part of the yolk sactubular part of the yolk sac
Site of future digestive tube Site of future digestive tube and respiratory structuresand respiratory structures
Week 4 – The Body Takes Week 4 – The Body Takes ShapeShape
Week 4 – FoldingWeek 4 – Folding Derivatives of the germ layersDerivatives of the germ layers
EctodermEctoderm forms forms Brain, spinal cord, and epidermis Brain, spinal cord, and epidermis
EndodermEndoderm forms formsInner epithelial lining of the gut Inner epithelial lining of the gut tube tube
Respiratory tubes, digestive organs, Respiratory tubes, digestive organs, and urinary bladderand urinary bladder
NotochordNotochord – gives rise to nucleus – gives rise to nucleus pulposus within intervertebral discspulposus within intervertebral discs
Week 4 – Folding and Week 4 – Folding and systemic developmentsystemic development
MesodermMesoderm – forms – forms MuscleMuscle BoneBone DermisDermis Connective tissues Connective tissues
Mesoderm differentiates further and Mesoderm differentiates further and is more complex than the other two is more complex than the other two layerslayers
Week 4 – System Week 4 – System developmentdevelopment
MesodermMesoderm (continued) (continued) SomitesSomites divides into divides into
SclerotomeSclerotome DermatomeDermatome MyotomeMyotome
Intermediate mesodermIntermediate mesoderm forms forms Kidneys and gonadsKidneys and gonads
MesodermMesoderm (continued) (continued)Splanchnic mesodermSplanchnic mesoderm
Forms musculature, connective Forms musculature, connective tissues, and serosa of the tissues, and serosa of the digestive digestive and respiratory structuresand respiratory structures
Forms heart and most blood vesselsForms heart and most blood vesselsSomatic mesodermSomatic mesoderm – forms – forms
Dermis of skinDermis of skinBonesBonesLigamentsLigaments
Week 4 – System Week 4 – System developmentdevelopment
Derivatives of Germ Derivatives of Germ LayersLayers
Figure 3.10
The Germ Layers in The Germ Layers in Week FourWeek Four
Week 5-8 – Week 5-8 – OrganogenesisOrganogenesis
Limb buds formLimb buds form Embryo first looks recognizably Embryo first looks recognizably
human (week 8) human (week 8) Head is disproportionately Head is disproportionately
large large All major organs are in place All major organs are in place
Fetal period- summaryFetal period- summary
Fetal period- summaryFetal period- summary
Fetal period- summaryFetal period- summary
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