embryology : fertilization and cleavage.

human embryology topic fertilization and cleavage described in simpler form with diagrams and flow chart.

1. Fertilization & Cleavage By: Dr. Ritika Dixit Associate professor Dept. of Rachana Sharir

2. It is the process of fusion of two mature germ cells, an ovum and a spermatozoon to form a single cell Zygote. Fertilization is a process of fusion of cell division. Haploid gametes fuse to form a diploid undifferentiated zygote. Fertilization

3. Site of fertilization: -- Ampulla of fallopian tube. Stages of fertilization: Approximation of gametes Contact and fusion of gametes Effects of fertilization.

4. Transport of spermatozoa & ovum in female genital tract towards the ampulla of uterine tube. Transportation of spermatozoa is affected by the following factors: 1. Liquefaction of semen: semen contains fibrinolysin that liquefies semen within 30 mins . After ejaculation 1. Approximation of gametes

5. 2. Contractions of uterine tubes: prostaglandins of semen stimulate peristaltic contraction in the female genital tract. 3. Effects of oxytocin : secretion of oxytocin from neurohypophysis also produces uterine contraction. 4. Aspiration of sperms : repeated uterine contractions generate vacuum that aspirates sperm into uterine cavity and later into uterine tube. 5. Chemotaxis : sperm is attracted towards ovum by chemicals secreted by corona radiata cells that surround the ovum.

6. Only 1% of deposited sperm in vagina enter the uterine cervix and only 300-500 sperms reach the fallopian tube. # transport of Ovum – Ovum enters the fimbriated part of fallopian tube due to cilliary beats and rhythmic contractions of uterine tube musculature. Transcoelomic migration: ovum released by an ovary aspirated into fallopian tube by transcoelomic migration. # ovum takes 25 mins to reach ampulla of uterine tube.

7. Three barriers: Sperm must break three barriers- corona radiata , zona pellucida and vitelline membrane. 2. Contacts and fusion of gametes

8. 4 Phases are involved in the penetration of sperm into ovum as follows: Phase I- acrosome reaction Acrosome cap establishes multiple contacts with plasma membrane of sperm head and releases acrosomal enzymes. Acrosome enzymes include hyaluronidase acrosine

9. Phase II – hyaluronidase disintigrates corona radiata Sperm head binds with Zp2 and Zp3 receptor proteins of zona pellucida Acrosine disintegrates zona pellucida Disintigrin peptide of sperms and integrin of vitelline membrane of ovum help in fusion of sperm plasma membrane with vitelline membrane

10. Phase III – Calcium wave for the depolarization of oocyte Sperm fusion with vitelline memb. Of oocyte induces calcium wave and depolarizes vitelline memb. Calcium wave is responsible for the following changes : Secondary oocyte completes second meiotic division and one set of chromosomes as a female pronucleus and expel another set as a second polar body.

11. ii. Ca wave triggers release of cortical granules to form oocyte that hydrolyses Zp3 receptors on zona pellucida and prevents further binding of sperms. iii. Release of cortical granules alters the vitelline membrane and induces vitelline block that prevents polyspermy .

12. Phase iv– nuclear fusion Only nucleus and tail of sperm enter into the cytoplasm of oocyte leaving behind body and cytoplasm Male pronucleus approaches towards the female pronucleus. Tail of sperm degenerates. Both male and female pronuclei replicate their DNA and lose their nuclear membrane. - Finally cytoplasm divides to form two cells with diploid number of chromosomes. It is called zygote.

13. Completion of second meiotic division of secondary oocyte. Restoration of diploid numbers of chromosomes. Determination of chromosomal sex. Initiation of cleavage. 3. Effects of fertilization

14. It is aproces of repeated mitotic segmentation of zygote within zona pellucida to give rise to small cells called blastomeres . In a cleavage fertilised ovum is divided to form 2 cell 3-cell 4-cell stage and so on. Duration : upto 6-7 days after fertilization. Cleavage

15. 16- cell stage looks similar to mulberry. Morula consists of – -inner cell mass or embryoblast . -Outer trophoblast ( trophoectoderm ) that forms coverings of embryo. - morula is covered by zona pellucida . Morula