Process of fertilization in human beings

Question

Describe the complete process of fertilization in human beings with the help of diagram?

Answer 1

It is amazing that it only takes one sperm to penetrate the egg and complete the fertilization process!  What started out as a mass exodus with 150 million plus sperm cells decreased to a mere 100 to actually reach the Fallopian tubes and from that mere 100 it only takes 1 to begin the process of creating a human being.  The length of time it takes the fertilized egg to reach the uterus after fertilization is between three and five days.  During this time it undergoes many cell divisions in a process called CLEAVAGE which is the division of the cells of the zygote without any subsequent increase in size.  By the time the ovum reaches the uterus it is referred to as a BLASTOCYST which consists of an outer sphere of cells from which the extra embryonic structures develop and an inner cell mass from which the EMBRYO develops.  This blastocyst now attaches itself to the wall of the endometrium and growth begins. This attaching is referred to as IMPLANTATION.  Often eggs can become fertilized and implantation will not take place due to the unfavourable conditions of the uterus – this is another reason for infertility with some couples.  So the GAMETES which each contained 23 chromosomes now become a ZYGOTE with a full 46 chromosomes – ½ from the mother and ½ from the father!   It is at this point of fertilization that every genetic aspect of the child is determined, including the gender! A brief synopsis of gender determination:  Each gamete from a male can have a X or a Y chromosome (XY determines male) and each gamete from a female can have a X or a X chromosome (XX determines female).  So when the gametes join to form a zygote they can be either XX or XX or XY or XY – so you have a 50:50 chance of becoming a male or a female!!  More about this process will be studied in Unit C on genetics. 

Occasionally a second egg may be released by the woman – when this occurs two eggs could be fertilized – this results in FRATERNAL TWINS – conceived at the same time but not genetically identical.  IDENTICAL TWINS are the result of the dividing cells of the zygote not staying together that well during the time before they become a blastocyst and two embryos can develop.  Since both twins come from the same zygote they have identical genetic information.  Even though they have identical genetic information they are not identical in all facets of life as environmental and spiritual factors will contribute to their individuality.  
 
After implantation occurs the TROPHOBLAST cells (the outer layer) of the blastocyst secrete digestive enzymes which eat into the endometrium. The endometrium will then grow over the blastocyst after being stimulated by the hormone progesterone. The trophoblastic cells will then form the amniotic sac, the placenta and the umbilical cord.