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Fertilisation occurs when the male’s sperm and female’s egg combine, and their nuclei fuse. These two cells are considered haploids which is a cell that contains one set of chromosomes (Diffen.com, 2019). The combination of these two reproductive cells creates a diploid cell, which is called a zygote, which is a cell that contains two sets of chromosomes which is enough to make a human; half of which is from the mother and half from the father. (Opentextbc.ca, 2019)
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Upon entry of the vagina, millions of sperm are overwhelmed by the acidity which is approximately pH 3.8; Millions more of the sperm are blocked from the entry of the uterus by the thick cervical mucus. Others are destroyed by phagocytic uterine leukocytes; this reduces the sperm from millions to thousands. The journey through the uterine tubes usually takes 30 minutes to 2 hours and is believed to be facilitated by uterine contraction. however, if they do not encounter the oocyte, they can survive in the uterine tubes for 3-5 days (Opentextbc.ca 2019).
The female reproductive system prepares for the sperm through a process called capacitation or priming, where the fluids in the uterine tubes improve the mobility of the male gamete as well as depleting the cholesterol molecules embedded in the membrane of the head of the sperm, which help the release of lysosomal enzymes which are digestive enzymes needed for the sperm to penetrate the oocytes exterior once contact is made (Opentextbc.ca 2019).
During ovulation fertilisation can occur; ovulation is the release of the female eggs from the ovary during the menstrual cycle (Naturalcycles.com, 2019). Fertilisation needs to occur along the distal uterine tube because the unfertilized female gamete cannot survive a 72-hour journey, the female gamete is surrounded by 2 protective layers; Zona pellucida which is a glycoprotein matrix that acts as a barrier to sperm entry and The corona radiata which is the external layer of follicular cells and it acts as support and provides nourishment to the egg (Ib.bioninja.com.au, 2019).
As it is swept along the distal uterine tube, the egg encounters the surviving sperm, which is due to the chemical attractants released by the cells of the corona radiata, upon contact the sperm needs to penetrate through these two barriers, first burrow through the corona radiata it will then bind to the receptors of the zona pellucida, which initiates acrosomal reaction. The enzymes released during the acrosomal reaction clears a path for the sperm to reach the female gamete. (Opentextbc.ca, 2019) This allows the sperm to reach the egg and their membranes to fuse, therefore fertilizing the egg, this initiates first cell division creating a zygote, other the next seven days the zygote will go through mitosis, becoming a blastocyst. (UCSF Medical Center, 2019)
Approximately 3 days after fertilisation, the 16-cell zygote will reach the uterus, once inside it will float freely for several days; after a week the blastocyst will come in contact with the uterine wall, embedding itself via the trophoblast cells, implantation typically occurs on the posterior wall, however if the endometrium isn’t fully developed the blastocyst will unbind and find a better spot, about 50-75% of blastocysts fail to implant which means that it will shed along with the endometrium during menstruation, when successful the trophoblast cells will fuse with the endometrium forming a syncytiotrophoblast; an epithelial covering of the vascular embryonic placental villi, that creates a nutrient circulation between the embryo and the mother, it grows to approximately 13 cm, in response the uterine mucosa rebuilds itself and envelops the blastocyst. The Trophoblast will then secrete a human chorionic gonadotropin (hCG), which is a hormone that directs corpus luteum to enlarge and continue to produce progesterone and oestrogen (suppresses LH and FSH) to prevent menstruation. The release of these hormones is important to create an environment for the embryo to develop, implantation is completed by the middle of the second week and by the third the trophoblast would have released enough hCG to show positive on a urine pregnancy test. (Opentextbc.ca,b 2019)
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During pregnancy the uterus has functions to support the bearing the embryo, the inner cells mass forms into two-layered disc of embryonic cells, as well as a space called the amniotic cavity, the epiblast will extend the surround the membranous sac to form an amnion which fills with amniotic fluid which helps provide the embryo from trauma, Also It forms a placenta a structure that provides oxygen and nutrients to the embryo as well removes waste from the blood of the embryo. (Mayo Clinic, 2019) during week 3 the yolk sac becomes the allantois which is a primitive excretory duct of the embryo which later becomes the urinary bladder. The stalks of the yolk sacs and allantois establish the outer structure of the umbilical cord, the chorion (outermost membrane) then surrounds the embryo. (Opentextbc.ca.b 2019)
The placenta provides nutrients through simple diffusion, molecules such as oxygen, carbon dioxide and other lipids solubles will be exchanged through simple diffusion. Others will move across the placenta by facilitated diffusion, such as water-soluble glucose. Amino acids and iron are exchanged by active transport (Opentextbc.ca.b, 2019)
Relaxin is released which is a hormone that prepares the mother for childbirth by increasing the elasticity of the symphysis pubis joint as well as the pelvic ligaments, this makes room for the foetus and allows the expansion of the pelvis for childbirth. In addition, relaxin helps dilate the cervix during labour (Opentextbc.ca.b, 2019).
Progesterone levels plateau and drop, this hormone prevents uterine contractions, however oestrogen levels will continue to rise this makes the myometrium more sensitive to stimuli which promote uterine contractions, fetal cortisol also rises which boost oestrogen secretion from the placenta. The posterior pituitary also boosts its secretion of oxytocin which is the hormone that stimulates contractions (Opentextbc.ca.c, 2019)
The processes can be divided into three stages, these are cervical dilation, the expulsion of the child and afterbirth, cervical dilation is the stage where the cervix will dilate fully, which allows the head of the child to pass through, this is the longest stage of childbirth. Expulsion beings when the new-borns head enters the birth canal and ends with the birth of the newborn, during the after-birth stage the placenta and associated membranes are expelled. (Opentextbc.ca.c, 2019)
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