Zygote

How The Embryo Develops After Fertilisation - Human Development Animation - Zygote Cell Division Vid

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• published: 18 Feb 2016
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During the first 12 hours after conception, the fertilized egg cell remains a single cell. After approximately 30 hours, it divides from 1 cell into 2 and 15 hours later, the 2 cells divide into 4. And at the end of 3 days, the fertilized egg cell has become a berry-like structure made up of 16 cells. This structure is called a morula, which is Latin for mulberry. The cells continue to divide 8 or 9 days following conception into a blastocyst. Although it is only the size of a pinhead, the blastocyst is composed of hundreds of cells. The blastocyst is slowly carried by tiny hair-like projections in the fallopian tube called cilia toward the uterus. During the critically important process of implantation, it must attach itself to the uterine lining where it will be able to get nourishment from the mother’s blood supply. If the blastocyst is unable to attach, the pregnancy will fail to survive. Human embryogenesis is the process of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, human development entails growth from a one celled zygote to an adult human being. Fertilisation occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage of prenatal development commences. Embryogenesis covers the first eight weeks of development and at the beginning of the ninth week the embryo is termed a fetus. Human embryology is the study of this development during the first eight weeks after fertilisation. The normal period of gestation (pregnancy) is nine months or 38 weeks. The germinal stage, refers to the time from fertilization, through the development of the early embryo until implantation is completed in the uterus. The germinal stage takes around 10 days. During this stage, the zygote, which is defined as an embryo because it contains a full complement of genetic material, begins to divide, in a process called cleavage. Cleavage stage Cleavage (embryo) 8-cell embryo, at 3 days This first division marks the beginning of the cleavage process which continues with the division of the first two cells by mitosis to give four cells which then divide to give eight cells and so on. This is quite a slow process taking from 12 to 24 hours for each division. The dividing cells which are termed blastomeres (blastos Greek for sprout) are still enclosed within the strong membrane of glycoproteins (termed the zona pellucida) of the ovum, which the successful spermatozoon managed to penetrate. The zygote (which is large compared to any other cell) undergoes further cleavage, increasing the number of cells without any increase in the size of the initial zygote. This means that the proportion of nuclear genetic material is greater than that of the cytoplasm in each cell. When eight blastomeres have formed they are undifferentiated and aggregated into a sphere. The cells begin to form gap junctions by this time, enabling them to develop in an integrated way and co-ordinate their response to physiological signals and environmental cues. When the cells number about sixteen or thirty-two the solid sphere of cells is termed a morula. At this stage the cells start to bind firmly together in a process called compaction, and cleavage continues as cellular differentiation begins. Baby began to develop at the moment of conception; this is when the mother's egg is fertilized by the father's sperm. Pregnancy is considered to begin at first day of the mother's last menstrual period. Conception typically occurs at what is considered the 2nd week of pregnancy. Conception is also the beginning of the germinal period of prenatal development. This stage occurs during the first 2 weeks of development (or the 2nd - 4th weeks of pregnancy). The union of the sperm and egg cell forms a single cell called a zygote. The zygote moves along the Fallopian tube toward the uterus. At the same time, the zygote is rapidly dividing in a process called cleavage. Once it reaches the uterus, the zygote becomes what is called a blastocyst, and it begins to implant into the wall of the uterus. The germinal period ends when the blastocyst is fully implanted into uterine tissue. By 3 to 4 days after fertilization, the dividing cells of the embryo assume a spherical shape and the embryo is called a morula. By 4 to 5 days, a cavity forms within this ball of cells and the embryo is then called a blastocyst. The cells inside the blastocyst are called the inner cell mass and give rise to the head, body, and other structures vital to the developing human. Now, the blastocyst is ready to make direct contact with the endometrium. When it does, both the endometrium and the blastocyst will exchange hormones, allowing the blastocyst to connect to the uterine wall, a process known as implantation.