After meiosis process what follows is the fertilization process. The mature sperm binds to the receptor found on the egg surface, and it fuses with the plasma membrane of the egg. This results into the development of diploid cell organism whose genetic composition is derived from the DNAs from both parents (Barsacchi 136). Fertilization induces several changes within the cytoplasm of the egg that are critical to further development. These alterations usually activate the fertilized egg thus initiating the mitotic cell division of the embryo.
Mitosis refers to the process of cell division that occurs strictly in somatic cells. When fertilization occurs, that is the meeting of haploid sperm and haploid egg, a single cell that is diploid is formed. The single cell formed then undergoes mitotic cell division leading to the formation of two diploid cells (Cameron & Barry 120). The two diploid cells formed again divides to form four diploid cells. This process continues geometrically in distinct progressions. These progressions are 1, 2, 4, 16, 64, and on to billions of cells. This means that the most important function of mitosis is growth. The other function, however, is for repair of worn out or damaged cells.
Mitosis is a process that has four distinct stages with every stage having specific cellular activities that occur in them. The first phase is the Interphase. In this stage, chromosome replicates making one copy that is identical to the other one. At the same time, the chromosomes are invisible in the nucleus and have their original lengths and thickness (Cameron & Barry 145). Some microtubules originate from the chromosomes centromeres forming radial arrays that are referred to as asters. The duplicated chromosomes remain in loosely packed forms of chromatin fibers.
The next stage is the Prophase. During this phase, the nucleoli found within the nucleus automatically disappear. The formed chromatin fibers coil tightly and later condense to form chromosomes. The chromosomes then align themselves in pairs that are later joined at the one point called the centromere. Spindle fibers start to form from microtubules within the cytoplasm (Barsacchi 147). The centrosomes are forced to move away from one another due to the propelled lengthening of microtubules/ spindle fibers found between them. Each chromosome is referred to as chromatin.
The next stage is known as Metaphase. In this stage, chromosomes align themselves along the cell center. This imaginary line where the chromosomes align themselves is referred to as the metaphase plate. The centrioles are formed at each cells poles leading to the production of spindle fibers (Cameron & Barry 167). These fibers then attach to centromeres of the chromosome pairs. The next stage is known as the Anaphase. In this stage, separation of paired chromosomes occurs at the centromere. The two halves that are formed migrate to opposite cells poles along the fibers (spindle). The poles of the cell at the same time move further from each other leading to the pinching of the cell center. By the end of this stage, each cell pole possesses complete chromosome set.
The last phase is referred to as Telophase. The microtubules lengthen and daughter nuclei start to form at every pole. Nuclear envelopes form; nucleoli reappear and, the chromatids uncoil. Cell division occur leading to the formation of two diploid daughter cells. The cells then return to the first stage, interphase for a fresh round of division and the process goes on and on leading to growth.
Development of embryo after fertilization to birth
After fertilization, an embryo is formed within amniotic sac under uterus lining. At this stage, most internal organs and some external structures of the body starts building. The formation of the organs mostly begins after three weeks of pregnancy. The embryo first elongates to form a human shape. Formation of the spinal cord and the brain begin developing shortly. However, the blood vessels and the heart are the first to form at around 16th day of pregnancy. The heart starts pumping blood at the 20th day followed by appearing of red blood cells. By week ten, almost all of the body organs have completely formed. The only exceptions are the spinal cord and the brain which form continuously throughout the pregnancy period. Major birth defects occur during the period of organs formations. The embryo here is very vulnerable to viruses, drugs, and radiation effects.
Within the ten weeks of pregnancy, both the fetus and the placenta have been under continuous development for six weeks. The placenta thus forms villi that attach to the uterine wall. The embryos blood vessels develop within the villi and passes to the placenta via the umbilical cord. A gaunt membrane acts as the separation factor between the mothers blood and the embryos blood within the villi (Barsacchi 150). The arrangement promotes the exchange of materials between the embryos and the mothers blood. The embryo generally floats in the amniotic fluid contained in the amniotic sac. This fluid gives space for the embryo to grow smoothly and freely. It also protects the embryo from external shock.
By the 10th week of pregnancy (the 8th week after complete fertilization), the embryo becomes a fetus. At this stage, the already formed structures grow and extensively develop. There are distinct developments that occur at specific times. By the end of the 12th week of the pregnancy, the fetus is developed enough to occupy the whole uterus. By around fourteen weeks of pregnancy, the fetal sex can be clearly identified, and the finger of the fetus can grasp. Between sixteen to twenty weeks, the fetus can make movements, and the placenta is completely formed (Cameron & Barry 177). Also, the hair appears both on the skin and the head, and the eyebrows together with eyelashes are already formed. By the 24th week, the fetus can successfully survive outside the mothers womb. The pregnant woman starts gaining weight rapidly.
The lung is another organ that continues with its development and maturity up to very few days to delivery time. The brain at the same time throughout the pregnancy period accumulates numerous new cells. The placenta is widely branched to form a treelike arrangement. It is this mechanism that increases the contact area between the uterine wall and the placenta that facilitates the efficiency in exchange of nutrients and waste materials from the fetus (Barsacchi 148). By the 25th week, the fetus head positions itself for delivery. Between the 37-42 weeks of pregnancy, the fetus weighs around seven pounds and is about twenty inches in length hence the delivery occurs within this period.
Barsacchi, Giuseppina. "Genes, Evolution and the Development of the Embryo." The Theory of Evolution and Its Impact. Springer Milan, 2012. 131-158.
Cameron, Noel, and Barry Bogin, eds. Human growth and development. Academic Press, 2012.
Moore, Keith L., Trivedi Vidhya Nandan Persaud, and Mark G. Torchia. The developing human. Elsevier Health Sciences, 2011.
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