د. مها السماك
Lec.4Fertilization:
It is the process by which the female and male gametes fuse. It occurs in the ampullary region of the uterine tube.
The sperms can stay alive for about 24 hours in the female reproductive tract before fertilization, they pass rapidly from the vagina into the uterus and then into the uterine tube. They must undergo some changes to be capable for fertilization, these changes are :
Capacitation : During which some protein of plasma membrane of the sperms head are removed. It lasts about 7 hours in human (inside the female reproductive tract).
Acrosome reaction : The appearance of small openings in the wall of the acrosome, permitting the enzymes to escape from the acrosome to penetrate the corona radiata and zpna pellucida.
Fertilization requires 24 hours to occur, and it takes place as follows:
Penetration of the corona radiata : Of the 200-300 million spermatozoa deposited into female genital tract, only one of these is meeded for fertilization. The sperm will penetrate the corona radiate by the combined action of sperm enzymes and tubal mucosa enzyme.
Penetration of the zona pellucida : The sperm penetrates the zona pellucida by the aid of enzymes released from the acrosome. Once the sperm touches the zona pellucida, it is firmly attached to it and penetrate it rapidly.
Fusion oocyte-sperm cell membranes : When the sperm touch the oocyte cell membrane, the two plasma membranes fuse and both the head and tail of the sperm will enter the cytoplasm of the oocyte, but the cell membrane is left on the oocyte surface.
As soon as the sperm enters the oocyte, the zona pellucida and oocyte membrane become impermeable to other sperms, this is known as cortical and zona reaction. Moreover, the oocyte finishes its second meiotic division immediately after entry of the sperm, one of the daughter cells is the second polar body, and the other is the definitive oocyte, its chromosome (22+X) arranged in a vesicular nucleus known as female pronucleus.
The sperm inside the cytoplasm of the oocyte will pass toward the female pronucleus, then the tail degenerates and the head become swollen forming the male pronucleus.
The two pronucleus approach each other , become in contact, they replicate their DNA, then fuse (now the zygote have 46 double structured chromosome and 4n DNA). After fusion the chromosomes arrange on the spindle in preparation for a normal meiotic division. The 46 structured chromosomes split longitudinally at the centromere and the chromatids move to the opposite poles thus providing each cell of the zygote with the normal number of chromosome and DNA. The zygote in this stage is called two cell stage zygote
The main results of fertilization are :
Restoration of the diploid number of chromosome : (i.e. 23 pairs of chromosome) half of the chromosome from father and the other half from the mother and thus the zygote have a new combination of chromosome.
Determination of the sex of the embryo : if the sperm carries X chromosome the new individual will be female while if the sperm carries Y chromosome, the new individual will be male.
Initiation of cleavage : cleavage is a series of meiotic divisions that the two cell stage zygote undergoes, it leads to increase in the number of the cell, which become smaller in size and known as blastomeres . three days after fertilization and after about 3-4 division, the zygote (embryo) is about to enter the uterine lumen and it looks like mulberry, known as morula . the morula contain 12-16 cell stage of two groups of cells, a group of centrally locaterd cells called the inner cell mass (which gives rise to the tissue of the embryo proper) and a surrounding layer of cells called the outer cell mass (which form the trophoblast that will form part of the placenta).
Blastocyst formation :
When the morula enter the uterine lumen, fluid begins to penetrate through the zona pellucida into the intercellular spaces of the inner cell mass, forming small fluid filled space called blastocele and the embryo in this stage is called blastocyst . in which the inner cell mass grouped at one pole and they are called embryoblast , while the outer cell mass will form the wall of the blastocyst and they are called trophoblast . at this time (during blastocyst stage) the zona pellucida starts to disappear allowing implantation to begin.
Implantation occur at about sixth day (after fertilization). The trophoblastic cells over the embryoblast begin to penetrate and invade the uterine mucosa by the action of proteolytic enzymes produced by the trophoblast.
Abnormal zygote :
Zygote with multinucleated blastomeres.
Non cleaving zygote may be because they are not penetrated properly by sperm, the meiotic mechanism is not functioning.
Most of the abnormal zygotes are lost during the early stages of development, a process called self-cleaning .
Abnormal implantation sites (ectopic pregnancy) :
Usually the blastocyst implants along the posterior or anterior wall of the body of the uterus. Occasionally it implants close to the wall internal os and so at later stages of development ,the placenta will over bridge the os (it is called placenta previa) causing severe bleeding in the last months of pregnancy and during delivery.
Abnormal implantation sites include :
Abdominal cavity : the blastocyst attaches to the peritoneal lining of the rectouterine cavity or it may attaches to the peritoneal covering the intestinal tract.
Ovary : blastocyst implanted in the ovary (primary ovarian pregnancy).
Uterine tube : called tubal pregnancy .
Ectopic pregnancy usually leads to death of the embryo and severe bleeding by the mother during the second month of pregnancy.