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HUMAN EMBRYOLOGY DEVELOPMENT C By Carly and Katie First Trimester: 0-12 weeks Fertilization • Fertilization occurs in the fallopian tube, it involves the joining of a sperm cell with a secondary oocyte. The secondary oocyte develops into a fertilized ovum, or zygote. The zygote is composed of 46 chromosomes, 23 from the sperm cells and 23 from the oocyte. Implantation • The sperm cells travel through the cervix into the uterus, although they do not all reach fallopian tubes. It takes approximately 3-5 days for the fertilized ovum to travel from the fallopian tube to uterus. • Cell division, or cleavage, occurs and the zygote gets smaller. The zygote develops into a fluid filled structure known as the blastocyst. • In the uterus the blastocyst attaches to the wall of the endometrium, this is referred to as implantation. Zygote, Embryo, Fetus • 4 days after fertilization, the zygote becomes an embryo. • The embryo remains an embryo for 8 weeks, then it will be referred to as a fetus. • The ‘yolk sac’ is another key element which acts as the circulatory system for the embryo. 3 Layers of the Fetus • The fetus is composed of 3 layers: • Ectoderm (outer) – skin, hair, finger nails, brain, retina… • Mesoderm (middle) – muscles, blood vessels and blood, reproductive structures… • Endoderm (inner) – liver, pancreas, thyroid, parathyroid, urinary bladder… Development in First Trimester • After four weeks, a complex vascular network has formed inside of the placenta. The placenta is the site in which gas and nutrient exchange occurs between a mother and her baby. The umbilical chord also serves as an exchange site for nutrients. • https://www.inkling.com/read/the-developing-human-moore9th/chapter-7/placenta • By the end of the first month, the 1 cm long embryo is 500x larger than the fertilized egg. Many important organs/systems have begun to develop, such as the heart, brain and tiny fingers and toes. By the 9th week sucking reflex is evident and arms and legs are beginning to move and the mother can feel these kicks. Second Trimester: 14-20 weeks Development in Second Trimester • In the second trimester the fetus, or baby, is about 8 cm long. All of the organs have formed, not all fully developed yet, and the fetus gradually increases size. • The fetus is starting to look more like a human, and kicks even more in the womb. Soft hair covers the body, and six months into the pregnancy the baby develops eyelids with lashes and bone cells. Third Trimester: 28-40 weeks Development in Third Trimester •In the third trimester, the baby is growing rapidly. The body mass is getting bigger, and the organs continue to enlarge. Video • https://www.youtube.com/watch?v=9TxwAkW3h1Y