Fertilization and Fertilization and embryogenesis embryogenesis 1. 1. Ovulation and fertilization Ovulation and fertilization 2. 2. Assisted reproduction Assisted reproduction 3. 3. Early Early development development of of the the human human embryo embryo Cleavage and blastogenesis Cleavage and blastogenesis Implantation Implantation Bilaminar germ disc Bilaminar germ disc Trilaminar germ disc (Gastrulation) Trilaminar germ disc (Gastrulation)
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Fertilization and Fertilization and
embryogenesisembryogenesis
1.1. Ovulation and fertilizationOvulation and fertilization
�� 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!
�� 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