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

1. 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.

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

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.
-

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 :
i. 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.

16. i. Completion of second meiotic division of secondary
oocyte.
ii. Restoration of diploid numbers of chromosomes.
iii. Determination of chromosomal sex.
iv. Initiation of cleavage.

19.  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.

21.  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.