Fertilization

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General Embryology
Fertilization

Embryology

Ø   A  phenomena which occurs before birth

Ø  Three basic events takes palce

Ø   multiplication, differentiation and degeneration.

 

Gametogenesis

ØGametogenesis is maturation of primordial germ cells through meiosis.

Primordial Germ cells,3rd.week:

Spermatogenesis:

Oogenesis

 

ØOogenesis can be divided in three phases:

A. Prenatal- 7 millions maximum by 5th.  month.

B. Postnatal

C. Ovulation

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Ovulation  & menstrual cycle

Ovulation

when ?

What is safe period ?

  Corpus Luteum.

Corpus luteum:

Ø What is it? a glandular structure.

Ø  Function?  It secrets progesterone.

Ø  Fate:

      a)  Corpus luteum of menstruation.

          ( Corpus albicans)

       b) Corpus luteum of pregnancy.

Mens

•Cyclic changes in the endometruim, comprising of  menstrual phase( 3-6) days, proliferative phase of (6-14 days, follicular cells- estrogen), secretory phase ( 15- 28 days- Corpus luteum-progesterone) and ischaemic ( premenstrual phase-27-28days).

Fertilization:

Ø   Definitions: It is the process by which the  

     male and female gametes fuse, to form 

     the zygote.

ØThe fertilization process takes approximately 24 hours.

 

•

Fertilization(Cont.)

Ø  Fertilization is a complex sequence of coordinated molecular events that begins with contact between a sperm and an oocyte

ØWhere ? Occurs normally in the ampulla  

     of the uterine tube

in vitro fertilization:

ØThis fusion can occur within the body (in vivo fertilization) or using the new technologies outside the body (in vitro fertilization).

ØTransgenic and gene knockout studies in animals have shown that carbohydrate binding molecules and gamete-specific proteins on the surface of the sperms are involved in sperm-egg recognition and their union.

Ø  ? Cloning: Development of  somatic cell.

Modification of the spermatozoa [before it can fertilize the ova]

   A. Capacitation:

ØIt is activation – enzymatic process on the acrosomal cap of the sperm to allow it to penetrate the corona radiata.

ØGlyoprotein coat and seminal proteins are removed from the cell membrane.

Modification of the spermatozoa( Cont.)

 B.The Acrosomal Reaction:

ØOccurs after the sperm binds to the zona pellucida.

ØBinding of the sperm to the zona is facilitated by ZP3 protein and receptors in the sperm cell membrane.

ØEnzymes – acrosin and trypsin like substance are released to allow the sperm to pass through the zona pellucida.

Phases of fertilization:

ØPhase 1: penetration of the corona radiata.

    The enzyme hyalurodinase and the movements of the spermatozoa bring this about. Tubal mucosal enzymes also appear to assist the dispersal.

ØPhase 2: penetration of the zona pellucida.

    The enzymes esterases, acrosin, and neuraminidase appear to cause lysis of the zona pellucida,

Ø Phase 3: fusion of the oocyte and spermatozoa cell membranes.

 The plasma or cell membranes of the oocyte and sperm fuse and break down at the area of fusion. The head and tail of the sperm enter the cytoplasm of the oocyte, but the sperm's plasma membrane remains behind

Zonal reaction:

  Zonal reaction:  this is to prevent polyspermy [more   than one sperm fertilizing a ova] after the first   sperm passes through the zona pellucida.

   The zona undergoes physiochemical changes that prevent   entry of other sperms. This change is brought by lysosomal enzymes released from the   secondary oocyte.

•The nucleus of the mature ova is called the FEMALE   PRONUCLUEUS.

•The head of the sperm [nucleus] – MALE PRONUCLEUS.

•Duplication of DNA in the pronuclei.

•Disintegration of nuclear membrane.

•Maternal and paternal chromosomes intermingle  - at metaphase of first mitotic division of the zygote.

 

 

Results of fertilization:

ØCompletion of 2nd meiotic division of the ova.

ØRestoration of diploid number: two haploid  gametes [n] produce a diploid zygote [2n].

ØDetermination of chromosomal sex.

ØInitiation of cleavage – a series of mitotic cell   divisions within the zygote.

ØSpecies variation – the zygote contains a new   combination of chromosomes and genes. Results in variation of the human species  through mingling of maternal and paternal chromosomes.

Ø

Contraceptive methods:

ØA.  Barrier technique:  Condom

ØB.  Pills: Inhibit ovulation permit

               menstruation

ØC.  Depo Provera   ( Depo-provera):

                                     IM Injection.

ØD.  IUD.

ØE.  Vasectomy and tubal ligation.

Task 2.

   Mention different types contraceptive methods that is available in Nepal.

Implantation
First week!

First week of development

1.Zygote

2.Morula

                  3.Loss of Zona pellucida

4.Implantation

            5.Blastocyst formation

Implantation

Ø is the process by which the blastocyst embeds itself into the endometrium of the uterus.

ØThe normal site of implantation is usually in the anterior or posterior wall of the body of the uterus.

ØThe endometrium is already prepared for this process - secretory phase of mens.

5.Blastocyst formation

  A.  Inner cell mass. ( Embryoblast)

  B. Outer cell mass. (Trophoblast. )

  C . Blastocyst cavity.

At the end of first  week of development

Summary of the first week

ØZygote formation

ØCleavage and blastomere

ØMorula which enters the uterine cavity

ØBlastocyst- Embryoblast,trophoblast & blastocele

ØZona pellucida disintegrates

ØEmplantation begins

 

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