PANTOMATH

FIRST WEEK OF DEVELOPMENT

THE OOCYTE

• At the time of ovulation, the 1st meiotic division is complete
• Secondary oocyte is about to undergo the 2nd meiotic division

THE OOCYTE

• At this stage it enters the fallopian tube
• Fertilization occurs in the ampulla of the uterine tube
• If not fertilized it moves to uterus and is resorbed

SPERMATOZOA

• Formed in the seminiferoustubule.
• After spermiogenesis they enter the epididymis where they are stored and become functionally mature.

SPERMATOZOA

• Vas Deferens
• Urethra
• Accessory glands
• Deposited in female genital tract

SPERMATOZOA

• 200- 600 million deposited in the vagina
• Only 1% enter the cervix
• Movement to the fallopian tube takes
• 2-7 hours(Own propulsion & uterine cilia)

SPERMATOZOA

• Once they reach the tube they become less motile.
• Incapable of fertilization until:
  • Capacitation.
  • Acrosome reaction.

CAPACITATION:

• Conditioning in the female genital tract (mainly in uterine tubes)
• Lasts 7 hours
• Glycoprotein coat and seminal proteins are removed from the surface of acrosome
• Only capacitated sperm can penetrate corona radiata and undergo acrosome reaction

VIABILITY OF GAMETES

• Oocytes: 12 hrs in vivo (In vitro… 24 hours).
• Sperms: 48 hours in the female genital tract  (Frozen, for years)

FERTILIZATION

• At ovulation sperm regain motility   (Chemical attractants secreted by cells of corona radiata).
• Sperm Begins its contact with the oocyte.
• Ends with intermingling of maternal and paternal chromosomes.

FERTILIZATION

• Many spermatozoa reach the  site of fertilization.
• Only one fertilizes the oocyte.

PHASES OF FERTILIZATION

1. Penetration of the corona radiata.
2. Penetration of the Zona Pellucida.
3. Fusion of oocyte and sperm cell membranes.

ZONA REACTION

When the head of the sperm contacts the oocyte surface:

• Cortical granules lining the plasma membrane of the oocyte release lysosomal enzymes.
• Permeability of the zona pellucida changes to prevent sperm penetration.

FERTILIZATION COMPLETE 

• Female pronucleus ( 22+X)
• Male pronucleus (22+X or 22+Y)   
• The pronuclei lose their nuclear membranes and their chromosomes get mixed (44+XX or 44+XY)

The Zygote is formed   

…… DAY 1-The Miracle of life beginsRESULTS OF FERTILIZATION

1. Restoration of diploid number of chromosomes
2. Determination of the genetic sex
3. Initiation of cleavage

DAY 2

• A typical mitotic division takes place and an embryo having 2 cells is formed

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• The 2 daughter cells are still surrounded by the zona pellucida

Day 3

• The morula is formed around 3 days after fertilization.
• The cells become smaller with each division and are called blastomeres.

Day 4

• The inner cells clump together and are called the inner cell mass or embryoblast
• The surrounding cells compose the outer cell mass or trophoblast

Day 5 

• The cells of the trophoblast are flattened
• The cavity of the blastocyst is the blastocoele

TROPHOBLAST 

• The trophoblast has the property of being able to stick to uterine tissue
• Its cells can bury into or “eat up” other cells
• They can therefore, invade or burrow into tissues with which they come in contact

ZONA PELLUCIDA

• The zona pellucida prevents the embryo from sticking to abnormal sites as it travels down the uterine tube.
• Initially the embryo derives its nutrition from the substances stored within the ovum (yolk) and from diffused uterine secretions.
• By the time the blastocyst is formed additional sources of nutrition are required.

Day 5

• Thus the zona pellucida disappears and the blastocyst sticks to the uterine endometrium
• Thus the function of the zona pellucida is to prevent implantation at an abnormal site

IMPLANTATION

• The process by which the trophoblast of the blastocyst sticks to and invades the endometrium.
• Implantation begins on Day 6 after fertilization.
• The normal site for implantation is in the superior part of the posterior wall of the uterine cavity

CLINICAL CORRELATESASSISTED REPRODUCTIVE TECHNIQUES PROBLEMS WITH ART

• Expense
• Ethical issues
• Increased rates of multiple pregnancies
• Increased prematurity, low birth weight and infant mortality

TECHNIQUES OF ART

• In vitro fertillization (IVF): fertilized eggs monitered until 8 cell stage and then implanted into the uterus.
• Gamete Intra Fallopian Transfer (GIFT): Ova and sperm introduced into the Fallopian tube.
• Zygote Intra Fallopian Transfer (ZIFT).
• Intra Cytoplasmic Sperm Injection (ICSI): in oligospermia.

SUMMARY OF EVENTS OF WEEK 1

• Day 1: Fertilization.
• Day 2: 2 cell stage.
• Day 3: Morula.
• Day 4: Embryo enters uterine cavity Early blastocyst.
• Day 5: Late blastocyst Zona pellucida disappears.
• Day 6: Implantation begins.