Fertilization G.LUFUKUJA1

Scope 1. Capacitation 2. Acrosome reaction 3. Phases of fertilization G.LUFUKUJA2

Capacitation Functional process & final step in sperm maturation that takes place in female genital duct and requires contact with secretions of female oviduct It involves the destabilization of the acrosomal sperm head membrane allowing greater binding between sperm and oocyte. This change is facilitated by the removal of sterols (e.g. cholesterol) and non-covalently bound glycoproteins. The result is a more fluid membrane with an increased permeability to Ca 2+sterolscholesterolglycoproteins G.LUFUKUJA3

Capacitation - cont… Capacitation represents A change in the sperm membrane A removal of blocking agents An activation of receptor sites and/or Acquisition of competence to respond to other agents A period of conditioning that occurs in the female reproductive duct G.LUFUKUJA4

Acrosome reaction Acrosome cap contains acrosin (proteolytic) & hyaluronidase (mucolytic & zonalysing) enzymes Penetration of the corona radiata Penetration of the zona pellucida Binds with ZP3 in a human-specific interaction Release of acrosin facilitates the process Fusion of sperm-oocyte membrane Induces acrosome to release degradation enzymes G.LUFUKUJA5

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Phases of fertilization G.LUFUKUJA7

Consequences of fusion of gamete cell membranes Cortex of oocyte releases granules to the exterior to prevent entry of other sperm Zona reaction Neuraminidase makes zona pellucida refractory MII resumes & resulting in extrusion of 2 nd polar body G.LUFUKUJA8

Entry of a sperm & formation of pronuclei G.LUFUKUJA9

Formation of pronuclei Once formed M & F pronuclei move close to another Pronuclear envelopes break up M & F pronuclei fuse Zygote results G.LUFUKUJA10

Fusion of pronuclei & formation of zygote G.LUFUKUJA11

Consequences of fertilization 1. Completion of MII of ovum 2. Restoration of diploid number 3. Determination of sex 4. Initiation of cleavage 5. Appearance in maternal blood of early pregnancy factor (EPF) G.LUFUKUJA12

Events of the first week of life Cleavage and morula formation G.LUFUKUJA13

Cleavage and morula formation It is a division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a compact mass called the morula. Cleavage ends with the formation of the blastula.cellsembryozygotescell cycles blastomeresmorulablastula G.LUFUKUJA14

Cleavage ZYGOTE 2 cells 4 cells 8 cells Many cells (solid ball ) morula BLASTULA (hollow ball ) Cross section of blastula Blastocoel Cleavage 32 cells G.LUFUKUJA15

Segregation of blastomeres into embryoblast and trophoblast Cells of the morula will give rise to Embryoblast and Membranes & placenta G.LUFUKUJA16

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Implantation/DECIDUAL REACTION Blastocyst being naked interacts directly with endometrium. In the presence of progesterone secretory cells form called decidual cells i.e. DECIDUAL REACTION decidual rxn is the changes of the endometrium of the uterus preparing it for implantation of the blastocyst Endometrial glands enlarge; the uterine wall highly vascularized and edematous By end of first week embryo will implant Uterine lining maintained by progesterone from corpus luteum; Later trophoblast produce hCG(human chorionic gonadotropin) which supports corpus luteum G.LUFUKUJA18

Attachment of blastocyst to endometrial epithelium - 6d G.LUFUKUJA19

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Syncytiotrophoblast penetrates endometrial epithelium - 7d G.LUFUKUJA21

Second week of life G.LUFUKUJA 1. FORMATION OF THE:- EMBRYO DISC,AMNIOTIC CAVITY, AMNION, & YOLK SAC 2. CHORION FORMATION 22

Embryonic disc The ICM differentiates into two cell types; a layer of columnar cells called the epiblast adjacent to the trophoblast, and a layer of cuboidal cells called the hypoblast adjacent to the Blastocoele (blastocyst cavity).columnar cellstrophoblast cuboidal cellsBlastocoele G.LUFUKUJA 23

Bilaminar disc… The hypoblast lies beneath the epiblast and extraembryonic endoderm (including the Yolk sac) is derived from this layerepiblastYolk sac Cells migrate from the hypoblast to form exocoelomic (Heuser’s) membrane that encloses the blastocyst cavity which will become the primary yolk sac. G.LUFUKUJA 24

AMNIOTIC CAVITY & AMNION, YOLK SAC As Implantation continues a small space appears between the embryoblast & the cytotrophoblast Flat amniogenic cells called amnioblasts delineate from the epiblast and surround a new cavity; the amniotic cavity by organizing to form a membrane, the Amnion. G.LUFUKUJA 25

The extraembryonic mesoderm During subsequent stages of development, there appears a population of cells that lie between the cytotrophoblast and the exocoelomic membrane. These cells give rise to the extraembryonic mesoderm. As cavities develop in the extraembryonic mesoderm they coalesce and a new space is formed called the extraembryonic coelom (chorionic cavity)cavities extraembryonic coelom Formation of extraembryonic coelom and yolk sac result into split of the extraembryonic mesoderm into Extraembryonic somatic mesoderm & the Extraembryonic splanchnic mesoderm surround the yolk sac G.LUFUKUJA 26

G.LUFUKUJA 27 The extraembryonic mesoderm…

G.LUFUKUJA 28 The extraembryonic mesoderm…cont..

PROCHORDAL PLATE Cells of the hypoblast at a localized area cranially change into columnar cells that is referred to as the prochordal plate an important organizer of the H&N and a position of the future mouth. 29G.LUFUKUJA

Chorion The chorion is one of the membranes that exist during pregnancy between the developing fetus and mother (the fetal membranes).membranesfetus fetal membranes The chorion and the amnion together form the amniotic sac.amnionamniotic sac It is formed by extraembryonic mesoderm and the two layers of trophoblast that surround the embryo and other membranes. The chorionic villi emerge from the chorion, invade the endometrium, and allow transfer of nutrients from maternal blood to fetal blood.mesodermtrophoblastchorionic villiendometrium G.LUFUKUJA30

G.LUFUKUJA31 Chorion…

CHORIONIC VILLI The chorion undergoes rapid proliferation and forms numerous processes, the chorionic villi, which invade and destroy the uterine decidua and at the same time absorb from it nutritive materials for the growth of the embryo. processesuterine deciduaembryo The greater part of the chorion is not in contact with the decidua capsularis (decidua facing the uterine cavity). Hence this part of the chorion becomes smooth, and is named the chorion læve; as it takes no part in the formation of the placenta On the other hand, the villi on that part of the chorion which is in contact with the decidua placentalis (basalis) increase greatly in size and complexity, and hence this part is named the chorion frondosum. G.LUFUKUJA 32

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twining Monochorionic twins are twins that share the same placenta.twinsplacenta It occurs in 0.3% of all pregnancies (in caucasians), and in 75% of monozygotic (identical) twins, when the split takes place beyond the third day after fertilization. monozygoticfertilization G.LUFUKUJA 34

Monochorionic twins The remaining 25% of monozygous twins become dichorionic diamniotic. The condition may affect any type of multiple birth, resulting in monochorionic multiples.multiple birth G.LUFUKUJA 35

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