Fertilization and Fertilization and

embryogenesisembryogenesis

1.1. Ovulation and fertilizationOvulation and fertilization

2.2. Assisted reproductionAssisted reproduction

3.3. EarlyEarly developmentdevelopment ofof thethe humanhuman embryoembryo

�� Cleavage and blastogenesisCleavage and blastogenesis

�� ImplantationImplantation

�� Bilaminar germ discBilaminar germ disc

�� Trilaminar germ disc (Gastrulation)Trilaminar germ disc (Gastrulation)

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Ovarian cycleOvarian cycle

�� menstrualmenstrual cyclecycle (28 days)

–– phasesphases::

��menstrualmenstrual – 3-5 days

(menstruation)(menstruation)

�� proliferativeproliferative ((follicularfollicular))

phasephase – 5-14 day

�� secretory secretory ((lutealluteal)) phase phase ––till 26-27 day

�� premenstrualpremenstrual (ischemic)

phasephase – 1-2 days

�� fertile windowfertile window – ~7 days

the time from 5 days before

until 1–2 days after ovulation

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�� ovulationovulation::�� the process by which an oocyte is the process by which an oocyte is released from the Graafian folliclereleased from the Graafian follicle

� a cyclic processcyclic process, blocked during pregnancy

� takes place around 1414--15 15 dayday� preovulatory Graafian follicle

� stigmastigma

�� liberation of the ovumliberation of the ovum� corona radiata

OvulationOvulation

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�� fertilisatiofertilisatio ((fecundatiofecundatio):):�� thethe processprocess byby whichwhich thethe male male and female gametes fuseand female gametes fuse

�� occursoccurs inin thethe ampulla ampulla of the uterine tube

�� preceded by insemination preceded by insemination ––the introduction of sperm into the female reproductive tract

�� main results of fertilizationmain results of fertilization::�� restorationrestoration ofof thethe diploiddiploid number number of chromosomes

�� determination of the sex determination of the sex of the new individual

�� initiation of cleavageinitiation of cleavage

� without fertilization the oocyte degenerates 24h after ovulation!

FertilizationFertilization

Designer:Designer:AndyAndyKurovetsKurovets

““““““““Fertilization WatchFertilization WatchFertilization WatchFertilization WatchFertilization WatchFertilization WatchFertilization WatchFertilization Watch””””””””

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FertilizationFertilization

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�� spertamozoa:spertamozoa:�� capacitation capacitation – lasts ~ 7h

� removal of a glycoprotein coat and seminal plasma proteins

�� acrosome reactionacrosome reaction – release of:�� hyaluronidasehyaluronidase � penetration of the corona radiata barrier

�� trypsintrypsin--like substancelike substance �

digestion of the zona pellucida�� acrosinacrosin (zonalysin) � helps the spermatozoon cross the zona pellucida

�� phasesphases:�� penetration of the penetration of the corona radiatacorona radiata

� 300-500 spertamozoa reach the site� only oneone penetratespenetrates

�� penetration ofpenetration of the the zona pellucidazona pellucida� inner acrosomal membrane dissolves�� zona reaction zona reaction – avoids

consequent penetration

�� fusion of oocytefusion of oocyte --sperm cell membranessperm cell membranes�� cortical reaction cortical reaction ––polyspermy is prevented

�� resumption of second meiotic resumption of second meiotic divisiondivision � female pronucleusfemale pronucleus

�� stage of male and female pronucleistage of male and female pronuclei�� metabolic activation of the eggmetabolic activation of the egg

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�� in vitro in vitro fertilization (fertilization (IVFIVF):):� a process by which egg cells are fertilized

by sperm outside the womb, in vitro� provides the opportunity to alleviate

infertility from a variety of causes� to karyotype fertilize ova as a means of

averting chromosomal anomalies � disadvantage of the technique – the low

success rate (20%) of the procedure� four or five ova are collected, fertilized,

and placed in the uterus � multiple births

�� gamete intrafallopian transfer (gamete intrafallopian transfer (GIFTGIFT))::� introduction of oocytes and sperm into

the ampulla of the fallopian tube

�� zygote intrafallopian transfer (zygote intrafallopian transfer (ZIFTZIFT):):� fertilized oocytes are placed in the

ampullary region of the fallopian tube

�� intracytoplasmicintracytoplasmic spermsperm injectioninjection ((ICSIICSI))– in severe male infertility:� a single sperm is injected into the

cytoplasm of the egg to cause fertilization� an alternative to using donor sperm for IVF

Assisted reproductionAssisted reproduction

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�� segmentatiosegmentatio – a series of mitotic divisions:

� begin of the process ~30 h� duration – 3-4 days

� cleavage division:�� total cleavagetotal cleavage

�� unequal cleavageunequal cleavage�microblastomeres

�macroblastomeres

�� asynchronousasynchronous

�� main events:main events:�� increase inincrease in number number of cells in the embryo

�� decrease in size decrease in size of cells (blastomeres)

� formation of a morulamorula(mulberry)� 16 blastomeres� ~3 days after fertilization

CleavageCleavage

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�� Blastocyst formationBlastocyst formation:�� inner cell massinner cell mass ––

embryoblastembryoblast� embryonic pole

� abembryonic pole

�� outer cell massouter cell mass ––

trophoblasttrophoblast� cytotrophoblast

� syncytiotrophoblast

�� disappearancedisappearance ofof the the zona pellucidazona pellucida

� entering the uterine cavity� 4-5 day

BlastogenesisBlastogenesis

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�� mechanismmechanism:

�� adhesion:adhesion:

�cytotrophoblast

�syncytiotrophoblast

�� invasion:invasion:�erosion of the uterine mucosa (endometrium)

�embryoblast –bilaminar germ diskbilaminar germ disk�� amniotic cavityamniotic cavity

�� trophoblasttrophoblast –– primary villiprimary villi�� chorionic cavitychorionic cavity

�� implantacioimplantacio (Lat.(Lat. in, in, within + within + plantare,plantare, to plant)to plant)

nidationnidation, nidacionidacio (Lat.(Lat. nidus, nidus, nestnest))

� begin of the process – 6-7 day

� duration – ~40 h

ImplantationImplantation

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�� implantation sitesimplantation sites::

�� normalnormal sitessites ofof implantationimplantation (("plug""plug"))::

�� uterine cavityuterine cavity� posterior uterine wall

� lateral uterine wall

� superior cervix

�� abnormalabnormal implantationimplantation sites:sites:

�� uterine (Fallopian) tube uterine (Fallopian) tube ((tubaltubal pregnanciespregnancies)) –– 98%

�� infundibular tubalinfundibular tubal

�� ampullar tubalampullar tubal

�� isthmic tubalisthmic tubal

�� cervical canal cervical canal (cervicalervical pregnanciespregnancies) – <1% (placenta previaplacenta previa)

�� ovaries (ovaries (primary ovarian pregnancy)ovarian pregnancy)

�� mesenterymesentery

�� abdominal cavity (abdominalabdominal cavity (abdominal pregnanciespregnancies))

}} extrauterine extrauterine ((ectopicectopic)) pregnancypregnancy

~1% of all pregnancies

ImplantationImplantation

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�� epiblastepiblast ��

amniotic cavityamniotic cavity:�outer amnioblastsamnioblasts

� amnionamnion

� inner amnioblastsamnioblasts� embryoembryo

�� hypoblasthypoblast ��

yolk sacyolk sac::�� primitive yolk sacprimitive yolk sac((exocoelomic cavityexocoelomic cavity))

�� secondarysecondary yolk sac yolk sac ((definitive yolk sacdefinitive yolk sac))

�� 22ndnd week of development:week of development:

�� epiblast epiblast layer

�� hypoblast hypoblast layer

Bilaminar germ diskBilaminar germ disk

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�� extraembryonicextraembryoniccoelomcoelom ��

chorionic cavitychorionic cavity�� somatopleuric somatopleuric mesodermmesoderm

�� splanchnopleuricsplanchnopleuricmesodermmesoderm

�� connecting stalkconnecting stalk�� umbilical cordumbilical cord

�� trophoblasttrophoblast ––lacunar stagelacunar stage

��primary villiprimary villi

��maternal maternal sinusoidssinusoids

��primitiveprimitiveuteroplacentaluteroplacentalcirculationcirculation

Bilaminar germ diskBilaminar germ disk

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�� formationformation ofof primitiveprimitive streakstreak

� primitive (Hensen'sHensen's) ) nodenode

�� primitive pitprimitive pit

�� proliferationproliferation and invaginationand invagination

of the epiblastof the epiblast � formation of

three germ layersthree germ layers::

� definitive ectodermectoderm

� intraembryonic mesodermmesoderm

�� endodermendoderm

�� 33rdrd week of development:week of development:

�� gasrtulation:gasrtulation: formation formation of all three germ layersof all three germ layers

Trilaminar germ diskTrilaminar germ disk

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Trilaminar germ diskTrilaminar germ disk

� formation of notochordnotochord::

�� prenotochordal cellsprenotochordal cells ��

prechordal plateprechordal plate

� bilaminar

notochordal platenotochordal plate

� definitive notochordnotochord �

the basis of the axial skeleton

(vertebral column)

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�� anteroposterior axisanteroposterior axis::

� cells at the anterior (cranial) margin

of the embryonic disc

(anterior visceral endoderm)

� head-forming genes, including OTX2, LIM1, and HESX1 and the secreted factor cerberus

� chordin, noggin, and follistatin

� in more caudal regions

� brachyury (T) gene

�� leftleft--right axis:right axis:

� FGF-8, secreted by cells in the node and streak

� induces Nodal and Lefty-2expression on the left side

� these genes upregulate PITX2, a transcription factor responsible for left sidedness

Establishment of the body axesEstablishment of the body axes

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� expansion of the embryonic disc::� mainly in the cephalic region - by a

continuous migration of cells from the primitive streak region in a cephalic direction

� the region of the primitive streak remains more or less the same size

� invagination of surface cells in the primitive streak and their subsequent migration forward and laterally continues until the end of the fourth week

� the primitive streak shows regressive changes, rapidly shrinks, and soon disappears

� in the cephalic part, germ layers begin their specific differentiation by the middle of the third week

� in the caudal part, differentiation begins by the end of the fourth week

Growth of the embryonic discGrowth of the embryonic disc

�� embryonic disc:embryonic disc:

�� flat and almost round flat and almost round initially

�� elongated elongated -- a broad cephalic and a narrow caudal end

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17 Thank youThank you……