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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)
Prof. Dr. Nikolai LazarovProf. Dr. Nikolai Lazarov
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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
Prof. Dr. Nikolai LazarovProf. Dr. Nikolai Lazarov
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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
Prof. Dr. Nikolai LazarovProf. Dr. Nikolai Lazarov
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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””””””””
Prof. Dr. Nikolai LazarovProf. Dr. Nikolai Lazarov
5
FertilizationFertilization
5
�� 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
Prof. Dr. Nikolai LazarovProf. Dr. Nikolai Lazarov
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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
Prof. Dr. Nikolai LazarovProf. Dr. Nikolai Lazarov
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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
Prof. Dr. Nikolai LazarovProf. Dr. Nikolai Lazarov
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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
Prof. Dr. Nikolai LazarovProf. Dr. Nikolai Lazarov
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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
Prof. Dr. Nikolai LazarovProf. Dr. Nikolai Lazarov
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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
Prof. Dr. Nikolai LazarovProf. Dr. Nikolai Lazarov
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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
Prof. Dr. Nikolai LazarovProf. Dr. Nikolai Lazarov
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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
Prof. Dr. Nikolai LazarovProf. Dr. Nikolai Lazarov
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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
Prof. Dr. Nikolai LazarovProf. Dr. Nikolai Lazarov
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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)
Prof. Dr. Nikolai LazarovProf. Dr. Nikolai Lazarov
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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
Prof. Dr. Nikolai LazarovProf. Dr. Nikolai Lazarov
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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
Prof. Dr. Nikolai LazarovProf. Dr. Nikolai Lazarov
17 Thank youThank you……
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