2a embryonic development

Survey of Embryonic Development

Human Anatomy and Physiology, 7eby Elaine Marieb & Katja Hoehn

Copyright © 2007 Pearson Education, Inc.,publishing as Benjamin Cummings.

Diagrams showing the size of a human conceptus from fertilization to the early fetal stage

Fertilization1-week conceptus

2-week conceptus3-week embryo

Embryo

4-week embryo5-week embryo

6-week embryo

7-week embryo

8-week embryo

9-week fetus

12-week fetus



Sperm penetration and the cortical reaction (slow block to polyspermy

Fusion of oocyteand sperm plasmamembranes

Corticalreaction

Sperm nucleus engulfedby oocyte cytoplasm

Oocyte cytoplasmCortical granule

Oocyte plasmamembrane

Extracellularspace

Zona pellucida

Sperm nucleus

Granulosa cellsof corona radiata

Acrosomalreaction

Acrosome

Oocyte insecond meioticdivision

First polar body

Zona pellucidaCorona radiataSperm

(a)

(b)



Events immediately following sperm penetration

Daughtercells

Anaphase offirst cleavagedivision

Sperm and ovumchromosomes

Zygote

Male pronucleusMitotic spindleCentrioleFemale pronucleus

Polarbodies

Ovumnucleus

Sperm pro-nucleus

Detachedsperm tail

Sperm

Extracellular space

Second meiotic divisionof oocyteSecond meiotic divisionof first polar body

Coronaradiata

Zonapellucida

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)



Blastocystcavity

Degeneratingzonapellucida

Inner cell massBlastocyst cavityTrophoblast

Zygote(fertilized egg)

4-cell stage2 days

Morula3 days

Early blastocyst4 days

Implanting blastocyst 6 days

Cavity ofuterus

Uterus

EndometriumOvulation

Ovary

Fertilization(sperm meets egg)

Uterine tube

Oocyte(egg)

Sperm

(a) (b) (c) (d) (e)

(a)

(b)

(c)

(d)

(e)

Cleavage from zygote to blastocyst



Hormonal changes during pregnancy

Ovulationand fertilization

Birth

Human chorionicgonadotropin

Progesterone

Estrogens

Gestation (weeks)

4 80 12 16 20 24 28 32 36

Rel

avti

ve b

loo

d l

evel

s



Implantation of the blastocyst

Endometrial stromawith blood vesselsand glands

SyncytiotrophoblastCytotrophoblastInner cell mass(future embryo)

Lumen of uterus

Uterine endometrialepitheliumInner cell mass

Trophoblast

Endometrium

Blastocyst cavityLumen of uterus

(b) (c)

(a)



Formation of the three primary germ layers

Bilayeredembryonic disc

Amniotic sac

Amniotic sac (amnion)Yolk sac

Cut edgeof amnion

Left

Right

Primitivestreak

HypoblastEpiblast

Yolk sac

Future openingbetween mouth and pharynx

Head

(b) . . . turned 90 (̊c) 3-D view (d) Section view in (e)

(a) Embryo (See Figure 28.7b)

(e) Bilayered embryonic disc, lateral-superior view

Yolk sac

Tail



Formation of the three primary germ layers

Head

Cut edgeof amnion

Yolk sac(cut edge)

RightLeft

Sections

Tail

Primitive streak

Primitivestreak

Endoderm

Mesoderm Definitiveendoderm

EpiblastHypoblast

Ectoderm

(f) Bilayered embryonic disc, superior view

(g) 14–15 days

(h) 16 days



Neurulation

(a) 17 days

(b) 19 days

Amnion

Amniotic cavity

Neural plate

Ectoderm

Mesoderm

Notochord

Endoderm

Yolk sacTail

Head

Cutedge ofamnion

Primitivestreak

Surfaceectoderm

Neuralplate

Futureneural crest

Notochord

Notochord

Cut edgeof amnionNeuralplate

Primitive nodePrimitive streak

Neural grooveNeuralfold

Neuralgroove Neural

crest

Left Right

Tissues derived from the three germinal layers

• Ectoderm: nervous tissues, epidermis and melanocytes.

• Mesoderm: bones, muscles, mesenchyme, reproductive system, urinary system.

• Endoderm: digestive system, respiratory system, endocrine system.



Events of placentation, early embryonic development, and extraembryonic membrane formation

Maternalblood vessels

Lacuna (intervillusspace) containingmaternal blood

EndometriumAmniotic cavity

Primarygerm layers:

• EctodermChorionic villus

Chorion

Amnion

Yolk sac

Extraembryonicmesoderm

Chorionbeing formed

Lumen of uterus

Proliferatingsyncytiotrophoblast

CytotrophoblastAmniotic cavityBilayeredembryonic disc:• Epiblast• HypoblastEndometrialepithelium

• Mesoderm

• Endoderm

Formingbody stalkAllantois

Extra-embryoniccoelom

(a) 71/2-day implanting blastocyst

(b) 12-day implanted blastocyst

(c) 16-day embryo

Embryonic membranes

• Amnion: encases and protects embryo against mechanical and temperature trauma.

• Yolk sac: transfers nutrients to embryo. Replaced by placenta in humans.

• Allantois: repository of embryo wastes. Replaced by placenta in humans.



Decidua basalis

Maternal blood

Chorionic villus

Umbilical bloodvessels inumbilical cord

Amnion

Amniotic cavity

Yolk sac

Extraembryoniccoelom

Chorion

Deciduacapsularis

Lumenof uterus

(d) 41/2-week embryo





Placenta

Decidua basalis

Chorionic villi

Yolk sacAmnionAmnioticcavityUmbilical cord

UterusLumen ofuterus

Deciduacapsularis

(f) 13-week fetus

Extraembryoniccoelom




Myometrium

Maternalveins

Maternalarteries

Umbilical arteriesUmbilical veinConnection toyolk sac

Chorionic villuscontaining fetalcapillaries

Fetal arterioleFetal venuleAmnion

(g)

Umbilical cord

Maternal bloodin lacuna(intervillus space)

Stratum basalis ofendometrium

Maternal portionof placenta(decidua basalis)

Fetal portion ofplacenta (chorion)

2a embryonic development

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