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