Human Embryonic Development: From Fertilization to Birth, Lecture notes of Medicine

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Human Embryology

Weeks 1 - 4

Week 1

Week 1

Fertilisation to Implantation

Fertilisation to Implantation

For this presentation embryonic age will be expressed as days or weeks

For this presentation embryonic age will be expressed as days or weeks

from fertilisation. On this basis human pregnancy lasts about 266

from fertilisation. On this basis human pregnancy lasts about 266

days or 38 weeks.

days or 38 weeks.

Obstetricians time pregnancy from the last day of the menstrual period

Obstetricians time pregnancy from the last day of the menstrual period

on the assumption that fertilisation takes place 2 weeks later. on the assumption that fertilisation takes place 2 weeks later. ForFor

obstetricians pregnancy lasts 280 days or 40 weeks.

obstetricians pregnancy lasts 280 days or 40 weeks.

2 cells~ 24 hours

58 cells~ 5 days

107 cells~ 6 days

oocyte

fertilised egg

or zygote – 2

nuclei egg and

sperm

2-cell stage 8-cell stage

(3 days)

morula blastocyst

zona

pellucida

trophoblast

or future placenta

inner cell mass or

future embryo

Human embryos in the first week of development

Embryo “hatching”

From the zona

pellucida

Three day old embryos

Three day old embryos

Ectopic

Ectopic

Pregnancies

Pregnancies

Implantation in a non-

Implantation in a non-

uterine site occurs at a uterine site occurs at a

rate of about 0.

rate of about 0.

The

The ampulla

ampulla of the uterine

of the uterine

tube is the most common

tube is the most common

ectopic

ectopic implantation site.

implantation site.

In vitro fertilisation

In vitro fertilisation

The process by which one or more eggs

The process by which one or more eggs

( (ooooccyytteess)) aarree ffeerrttiilliisseedd oouuttssiiddee tthhee

b booddyy.. FFeerrttiilliissaattiioonn iiss eeiitthheerr aacchhiieevveedd

b byy ppllaacciinngg aa ddrroopplleett ooff wwaasshheedd ssppeerrmm

(~ 50 , 000 ) onto each egg or if the

(~ 50 , 000 ) onto each egg or if the

s sppeerrmm ccoouunntt iiss llooww aa ssiinnggllee ssppeerrmm ccaann

b bee iinnjjeecctteedd iinnttoo eeaacchh eegggg ((IInnttrraa--

C Cyyttooppllaassmmiicc SSppeerrmm IInnjjeeccttiioonn – – IICCSSII))..

• • EEmmbbrryyooss ccaann bbee ppllaacceedd iinnttoo tthhee uutteerruuss

at the 6 - 8 cell stage ( 3 - days culture)

at the 6 - 8 cell stage ( 3 - days culture)

b buutt ssoommee cclliinniiccss ccuullttuurree tthhee eemmbbrryyooss

f foorr 55 oorr 66 ddaayyss ttoo eennssuurree hheeaalltthhyy

b bllaassttooccyysstt ssttaaggee eemmbbrryyooss aarree ppllaacceedd

into the uterus.

into the uterus.

oocyte

fertilised egg

(zygote)

blastocyst

4 - cell embryo

Stem Cell Research

• Stem cells are unspecialized cells that can renew

themselves for long periods through cell division.

Under certain experimental conditions, they can be

induced to become cells with special functions

such as the beating cells of the heart muscle or the

insulin-producing cells of the pancreas.

• Human embryonic stem cells are obtained from the

inner cell mass – the early embryo.

• In the USA it is forbidden to destroy a human

embryo to obtain stem cells. So they are not

allowed to remove the inner cell mass or totipotent

cells. There is interest in the possibility that at the

8 - cell stage the cells are no longer totipotent but

are still pluripotent (ie can form the 240 + human

cell types).

Multiple Gestations- TWINS

Multiple Gestations- TWINS

About 1 : 80 human births ( 0. 8 %) is a twin pregnancy.

About 1 : 80 human births ( 0. 8 %) is a twin pregnancy.

• • AAbboouutt 22 // 33 ooff ttwwiinnss aarree ffrraatteerrnnaall ((ddiizzyyggoottiicc)) – – ddeerriivveedd ffrroomm 22 eeggggss..

One third ( 1 : 250 ) are identical twins (monozygotic)

One third ( 1 : 250 ) are identical twins (monozygotic)

• • 3355 %% ooff MMZZ ttwwiinnss ddiivviiddee bbeettwweeeenn 22 - - 88 cceellll ssttaaggee ,, ggeett ttwwoo bbaabbiieess wwiitthh ttwwoo

a ammnniioonnss,, ttwwoo cchhoorriioonnss,, aanndd eeiitthheerr oonnee ffuusseedd oorr ttwwoo sseeppaarraattee ppllaacceennttaass--

c caannnnoott tteellll ddiiffffeerreennccee bbeettwweeeenn tthheessee ttwwiinnss aanndd ffrraatteerrnnaall uunnttiill ggeenneettiicc

testing is done.

testing is done.

6655 %% ooff MMZZ ttwwiinnss ooccccuurr bbyy ddiivviissiioonn ooff tthhee iinnnneerr cceellll mmaassss aafftteerr ffiirrsstt

week

week

. At this point two embryos will develop with two amniotic sacs, one . At this point two embryos will develop with two amniotic sacs, one

chorionic

chorionic sac and a common placenta.

sac and a common placenta.

IVF has greatly increased the number of fraternal (

IVF has greatly increased the number of fraternal ( dizygotic

dizygotic ) twins and in

) twins and in

22000011 tthhee rraattee ooff ttwwiinnss iinn tthhee UUSSAA wwaass 33 %%..

((aa)) SSeeppaarraattiioonn aatt tthhee ttwwoo

b bllaassttoommeerree ssttaaggee ccaann

lead to twinning.

lead to twinning.

( (bb)) SSpplliittttiinngg ooff tthhee iinnnneerr

cell mass is the most

cell mass is the most

common cause of

common cause of

identical twins.

identical twins.

(c)

(c) Incomplete separation

Incomplete separation

of the inner cell mass

of the inner cell mass

can lead to conjoined

can lead to conjoined

t twwiinnss..

MONOZYGOTIC TWINS

MONOZYGOTIC TWINS

Week 2

Week 2

Implantation

Implantation

From The Developing Human

From The Developing Human

MMoooorree aanndd PPeerrssaauudd 66

tthh

EEddiittiioonn

From

From Langman

Langman ’

’ s

s Medical Embryology 9

Medical Embryology 9

th

th

Edition

Edition

Lippincott Williams & WilkinsLippincott Williams & Wilkins

Implanted Human

Implanted Human

Blastocyst

Blastocyst

16 days

16 days

gestation

gestation

21 days

21 days

circulation

circulation

established

established

Week 3

Week 3

Gastrulation

Gastrulation

" "TThhee mmoosstt iimmppoorrttaanntt eevveenntt iinn yyoouurr lliiffee iiss nnoott bbiirrtthh,, mmaarrrriiaaggee,,

oorr ddeeaatthh,, bbuutt ggaassttrruullaattiioonn..""-- LL.. WWoollppeerrtt..

Gastrulation

Gastrulation

The inner cell mass in the 15-16 day human embryo

The inner cell mass in the 15-16 day human embryo

consists of two layers

consists of two layers

an upper

an upper epiblast

epiblast and a lower

and a lower

hypoblast.

hypoblast. Gastrulation

Gastrulation converts this

converts this bilaminar

bilaminar disc

disc

into three layers. An upper

into three layers. An upper ectoderm

ectoderm , a middle

, a middle

mesoderm

mesoderm and a lower

and a lower endoderm

endoderm .

.

14 - day embryo Gastrulation in a 16 - day human embryo 18 - day embryo