FERTILIZATION TO BLASTOCYST FORMATION By Dr Samina Anjum

PREVIEW FERTILIZATION CLEAVAGE BLASTOCYST FORMATION UTERUS AT THE TIME OF IMPLANTATION MENSTRUAL CYCLE

Fertilization is the process by which fusion of male and female gametes occurs in the ampullary region of the uterine tube.

Only 1% of the sperms deposited in the vagina enter the cervix. Movement of sperm from cervix to uterine tube occurs by muscular contractions of uterus and uterine tube & by their own propulsion.

Cont… 2-7 hrs to reach the uterine tube. On reaching the isthmus they become less motile and cease their migration. At ovulation, sperms become motile again- chemoattractants released by cumulus cells

SPERMATOZOA must undergo: Capacitation Acrosome reaction

CAPACITATION: READYING THE SPERM Period of conditioning in the female reproductive tract (7 hrs in humans). Epithelial interactions b/w mucosal surface of tube and sperm Glycoprotein coat & seminal plasma proteins are removed from the plasma membrane that overlies the acrosomal region of the spermatozoa. Only Capacitated sperm is more active, undergoes chemotaxis & can cross corona cells & under go acrosomal reaction.

ACROSOME REACTION Occurs after zona binding Zona proteins: ZP1, ZP2, ZP3 Sperm specifically binds to ZP3 glycoprotein Initiates acrosome reaction

ACROSOME REACTION 1 3 2 4

PROCESS OF FERTILIZATION Phase I: Penetration of Corona Radiata. Phase II: Penetration of Zona pellucida Phase III: Fusion of Oocyte and sperm cell membranes.

PHASE 1:PENETRATION OF CORONA RADIATA Passage of sperm through the corona radiata depends on: Tubal mucosal enzymes Flagella action Ejaculation 200-300 million sperm, 300 to 500 reach the site of fertilization Only one of these fertilizes the egg, others aid the fertilizing sperm in penetration.

PHASE 2:PENETRATION OF THE ZONA PELLUCIDA Zona is a glycoprotein shell that maintains sperm binding --- mediated by ZP3 that induces the acrosome reaction. Release of acrosomal enzymes Penetration of ZP & contact with oocyte plasma membrane. Permeability of ZP changes on contact - Cortical reaction - Zona reaction

PHASE 3:FUSION OF OOCYTE & SPERM PLASMA MEMBRANES For initial adhesion interaction of integrins on Oocyte & their ligands, disintegrins on sperm. After adhesion, fusion of plasma membranes occur Actual fusion is accomplished between the plasma membranes of the oocyte and the membrane that covers the posterior region of sperm head Both Head and tail of a sperm enters the cytoplasm of the oocyte, but the sperm plasma membrane remains behind.

THE EGG RESPONDS IN THREE WAYS: 1.CORTICAL & ZONA REACTIONS 2. RESUMPTION OF 2ND MEIOTIC DIVISION OF OOCYTE 3.METABOLIC ACTIVATION OF EGG

1.CORTICAL REACTION Refers to a massive exocytosis of cortical granules lining the plasma membrane of oocyte seen shortly after sperm-oocyte fusion.  Cortical granules contain a mixture of enzymes, including several proteases, which diffuse into the zona pellucida following exocytosis from the egg. These proteases alter the structure of the zona pellucida, inducing the zona reaction. Oocyte membrane become impenetrable to other spermatozoa

2.ZONA REACTIONS The zona reaction refers to an alteration in the structure & composition of the zona pellucida catalyzed by lysosomal enzymes from cortical granules. Sperm receptors in the zona pellucida are destroyed to prevent sperm binding and penetration - These reactions prevents polyspermy

2. RESUMPTION OF 2ND MEIOTIC DIVISION OF OOCYTE Definitive oocyte

3.METABOLIC ACTIVATION OF EGG Activating factor is carried by spermatozoon Formation of male pronucleus Tail detaches and degenerates At this stage, the male and female pronuclei are indistinguishable. The two pronuclei fuse eventually, loose nuclear envelops . During growth of male and female pronuclei (both haploid) each replicates its DNA and then undergo first mitotic division.

CONSEQUENCES OF FERTILIZATION Stimulates the secondary oocyte to complete meiosis II. Restores the normal diploid number of chromosomes (46). Results in variation of human species as maternal and paternal chromosomes intermingle. The embryo contains only maternal mitochondria because the sperm mitochondria are dispersed into the egg cytoplasm and discarded. Determines the sex of the embryo. Initiation of cleavage: Without fertilization oocyte degenerates in 24hrs

CLEAVAGE IN HUMANS Cleavage is the rapid succession of mitotic cell divisions , resulting in an increase in the number of cells, blastomeres which become smaller with each division Cleavage is thus a fractionating process rather than a process of growth i.e. no new protoplasm has been formed

Cleavage occurs as the zygote passes along the uterine tube toward the uterus. During cleavage, zygote lies within the thick zona pellucida.

Uncompacted blastomeres cell outlines are distinct Until the 8-cell stage, blastomeres form a loosely arranged clump Uncompacted blastomeres cell outlines are distinct

COMPACTION After the third cleavage, blastomeres maximize their contact with each other, held together by tight junctions leading to deformation of their round shape. This change in shape of the embryo is called compaction. Compacted blastomeres cell outlines are indistinct

MORULA Approximately, 3 days after fertilization, cells of the compacted embryo divide again to form a 16- cell (12-32) morula (L., morus; mulberry) Compaction segregates inner cells, which communicate extensively by gap junctions, from outer cells. Inner cells of the morula constitute the inner cell mass, & surrounding cells compose the outer cell mass.

BLASTOCYST FORMATION About the time the morula enters the uterine cavity, glandular secretion from uterine cavity, begins to penetrate through the zona pellucida into the intercellular spaces of the inner cell mass.

BLASTOCYST FORMATION Gradually, the intercellular spaces become confluent, and finally, a single cavity, the blastocoele forms. At this time ZP disappears (at the end of 4th day) allowing implantation to occur

TIME CONSUMED IN COMPLETING CLEAVAGE

IMPLANTATION Begins on the 6th day after fertilization once zona pellucida disappears. L selectins on trophoblast cells and its CHO receptors on uterine epithelium mediate initial attachment of the Blastocyst to the uterus. Implantation is the result of mutual trophoblastic and endometrial action.

UTERUS AT THE TIME OF IMPLANTATION

CHANGES IN UTERINE MUCOSA CORRESPONDING TO OVARY DURING A REGULAR MONTHLY CYCLE

MENSTRUAL CYCLE Menstruation is the shedding of the lining of the uterus (endometrium) accompanied by bleeding. Menstruation starts during puberty (at menarche) and stops permanently at menopause

MENSTURAL CYCLE

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