Fertilization And Further Development Of The Embryo

Fertilization and further development of the embryo

What is ovum?  The

secondary oocyte about finish its Meiosis II.

A single ejaculation of 3-5 ml contains around 300 to 500 million sperms!! Out of these only ONE is required to fertilize the ovum.

Then why so many???

Mechanics of fertilization  Coronal

block  Zona block  Peri vitelline block  (in addition to the pH differences in vagina and cervix, preventing the entry of multiple sperms).

FERTILIZATION  Defined

as the fusion of male and female gametes (haploid) resulting in a diploid embryo.  Life begins at fertilization.  The secondary oocyte completes its Meiosis II and becomes the Ovum(female pronucleus).  The sperm loses its tail and its head becomes a male pronucleus.

MECHANICS (CONTD)  ZONA

PELLUCIDA is opened by the acrosomal enzymes from the successful sperm.  Zona becomes an impermeable barrier to other sperms to prevent polyspermy.

The male and female pronuclei immediately position themselves for cell division. Hence there is NO actual fusion of gametes as seen in lower animals. Fusion means mixing up of both nucleus and cytoplasm of gametes. There is no cytoplasm in the Sperm!!

FERTILIZATION  FERTILIZATION

USUALY OCCURS IN THE AMPULA OF THE UTERINE TUBE.  More than two weeks after last normal menstrual period (LNMP).

Effects of Fertilization  Diploid

number is restored.  Sex of the embryo(at genetic level) is established.  It’s the father who determines the sex of the embryo (it all depends on whether x bearing or y bearing sperm fertilizes the ovum).  Cleavage is initiated.

Cleavage  Once

fertilization is over the conceptus starts to divide continuously and this process is called “Cleavage”  (strictly speaking there is no zygote in humans. It should be called a fertilized ovum but who cares)?

Cleavage  The

first two cells, if separated, can develop into separate individuals (MZ twins).  As it progresses to more cells this property is lost.  ALL OF US STARTED AS TWINS!!

Morula  When

the cleaving embryo reaches 16-20 cell stage it is called the MORULA.  IT IS THE MORULA THAT ENTERS THE UTERINE CAVITY STILL SURROUNDED BY ZONA PELLUCIDA.  By a week after fertilization Morula enters  the uterine cavity.

blastocyst  Some

cells in the morula die (apoptosis) and give rise to a cavity.  Cells which remain are pushed to one end and this stage is called ‘blastocyst”.  At the same time zona starts to dissolve to “hatch” the blastocyst.

blastocyst  Has

inner cell mass and outer cells called trophoblast.  Trophoblast is highly phagocytic.  To prevent this property zona was essential.

What's happening in the endometrium during this time? Uterine

glands enlarge. Uterine vessels also enlarge and become more tortuous. In effect it is all set to receive the embryo to nourish it.

Changes in outer wall of the blastocyst  First

it is single layered (trophoblast)  Then it divides into outer syncitio trophoblast and inner cytotrophoblast.  Syncitio trophoblast starts to eat its way into endometrium and opens the uterine blood vessels so that the entire blastocyst is bathed in maternal blood.  Gradually entire blastocyst goes into the entire thickness of the endometrium.  This process is called ‘implantation”.

TRILAMINAR BLASTOCYST In

addition to outer syncitiotrophoblast and inner cytotrophoblast, one more layer (inner most) is formed and the blastocyst becomes three layered structure. All these three layers are collectively called the “CHORION” (SYNCITIO+CYTO+ EXTRA EMBRYONIC MESODERM)

Clinical aspects  Tubal

pregnancy.  Ectopic pregnancy.  GIFT  ICSI  in vitro fertilization

Freak incidences of fertilization  One

sperm fertilizes an empty ovum and duplicates. The entire chromosomes are paternal in origin. The placenta is formed in this condition with no traces of embryo.  Sometimes two sperms can fertilize each other.  These abnormal fertilization processes give rise to a condition called “hydatidiform mole”

Placenta in molar pregnancy

Contraception  Condoms,

diaphragms, CuT, TUBECTOMY  VASECTOMY.  SPERMICIDAL VACCINE, CREAMS