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Fertilization and Development • Life’s Greatest Miracle • http://video.pbs.org/video/1841157252/ Prenatal Stages of Human Life • Prenatal (fertilization - birth) • Embryo - (fertilization - 8 weeks of gestational phase) – Zygote - the point of conception, fertilization – Blastula (blastocyst) - the period between conception and embryonic stage – Embryo - the embryonic period starts at three weeks and continues until the end of the 8th week of pregnancy • Fetus (8 weeks of gestational phase - birth) Fertilization • In animals, fertilization is the fusion of gametes (sperm and egg). • Mammals rely on internal fertilization through copulation (sexual intercourse) to deliver the sperm to the female. • The male inserts his penis into the opening of the vagina, the passage into the female's other sexual organs. Once the male ejaculates, a large number of sperm cells move to the upper vagina (via contractions from the vagina) through the cervix and across the length of the uterus toward the ovum— a considerable distance compared to the size of the sperm cell • After finding the egg, the sperm binds to the egg, causing an acrosome reaction to release the enzyme hyaluronidase, which digests the outer layer of the egg. • The sperm’s nucleus and flagellum enter the egg, but not the mitochondria. The egg “activates" once it fuses with a single sperm cell (its cell membrane changes to prevent fusion with other sperm). • This protective coat is called a fertilization membrane. • When the sperm and egg unite, it makes a diploid zygote. • In medicine, the beginning of pregnancy is when a sperm enters an ovum and forms a zygote. • Now, in western medicine, pregnancy is defined as beginning when a zygote becomes implanted in a woman's uterus. This occurs when the zygote becomes embedded into the endometrium where it makes a placenta. • * important for sake of medical research (stem cells, etc.) Growth of Organism once fertilization has happened • The zygote undergoes rapid mitotic divisions with no significant growth (a process known as cleavage). • The resulting daughter cells are called blastomeres. • About 5-6 days after fertilization the zygote transforms into a mass of cells called a blastocyst (blastula). • The blastocyst (blastula) becomes attached to the endometrium, a process referred to as implantation (this usually happens 6-10 days after fertilization) • At this point, the blastula is a fluid-filled ball. • The blastula contains an inner cell mass and that contains stem cells. Where do stem cells come from? • Stem cells are the body’s master cells, they are undifferentiated and can become any cell in your body. • Early stem cells are found in the inner cell mass of the blastocyst. Once the inner cell mass is removed from the blastocyst, the stem cells are placed in a culture dish where they grow and replicate over time. In normal reproductive development, the inner cell mass becomes all the tissues of the body. • Once implantation begins, cell division stops temporarily. When cell division starts again, all cells that are produced are not alike. • Differentiation is the process by which cells develop into specialized tissues and organs. • At around 16 days after fertilization, gastrulation happens and this is when the blastula folds in on itself (blastula can be called gastrula) • The inner cell mass inside the blastocyst rearranges itself into 3 distinct layers which are responsible for the development of the organs and accessory structures: • Endoderm (“inside skin”) • Ectoderm (“outside skin”) • Mesoderm (“middle skin”) • Endoderm turns into the inner linings of the digestive tract, as well as the linings of the respiratory passages. It also forms many glands/organs, such as the liver and pancreas. • Ectoderm turns into the skin, nails, the epithelium of the nose, mouth and anal canal; the lens of the eye, the retina and the nervous system. • Mesoderm forms the muscles, circulatory and excretory systems of the body. • At the 3rd week of development four membranes begin to form: 1. chorion: the protective sac around the embryo. It makes up most of the placenta. It contains many villi (finger-like projections), which allow for exchange of nutrients, gases and wastes between mother and embryo. 2. Yolk sac: Transports nutrients to the embryo during the second and third weeks 3. Amnion: a tough, thin, transparent membrane that envelops the embryo. It’s interior space, the amnionic cavity, becomes filled with a watery, amnionic fluid. The amnion functions to: insulate the embryo/fetus, protect it from infection, dehydration, impact and temperature change, and help dilate the mother’s cervix at the start of labour. 4. Allantois: involved in the formation of blood cells and its blood vessels become the vein and arteries of the umbilical cord • Placenta (part of chorion): a thick bed of tissues and blood vessels embedded into the wall of the uterus that provides an indirect connection between mother and fetus • 3 functions of placenta: • It transports material between mother and fetus • It provides nutrients and energy to the fetus • It secretes HCG hormone • *** Sugar, water, oxygen, hormones, infectious agents, toxic substances and drugs can cross the placental barrier. • Umbilical cord: a connecting tube between the embryo and the placenta, carrying carbon dioxide and nitrogen waste from the embryo and oxygen and nutrients to the embryo. • This all happens before 20 days. • At 20 days, the foundations of brain, spinal cord and nervous system are established • Day 21 – heart begins to beat • Day 26 – arm buds appear • Day 28 – backbone and muscles are forming Second month (5-8 weeks) • the face is forming • ears covered by thin membrane and continue developing • brain, liver, kidneys, bloodstream, and digestive system are developing • arms and legs developed • By the end of the second month, the embryo has become a fetus. It is about 1 and 1/8th inches long and weighs 1/30th of an ounce Third month (9-12 weeks) • the fetus' eyelids are fused • facial features are present, the nose and outer ears are formed • nail beds established • movement such as head turning or sucking begins • teeth buds forming • all internal organs formed • swallows amniotic fluid Fourth month (13-16 weeks) • strong heart beat begins • lanugo or fine body hair develops • At the end of the fourth month, the fetus is about 6 inches long and weighs four ounces. Fifth month (17-20 weeks) • heartbeat can be heard with a stethoscope • finger and toe nails formed • sucks thumb • responds to noise • grows hair and eyebrows • movements become increasingly vigorous Sixth month (21-24 weeks) • eyes are open • a creamy substance called vernix covers the skin • skin is wrinkled and the fetus appears very thin • At the end of the sixth month, the fetus is about 11-14 inches long and weighs 1 1/2 pounds. • The placenta and umbilical cord are now fully developed and continue to increase in size. • At 38 weeks, the uterus will have reached the breastbone and you may begin to experience early signs of labour. • During this period, the most dramatic change in the fetus is its continued growth. By the seventh month it weighs about two and a half pounds and is approximately 15 inches in length. • By the eighth month the fetus weighs about 5 pounds and is 16 to 18 inches long. During the last month of pregnancy the fetus weighs seven or eight pounds and may be more than 20 inches long. In addition: • skin wrinkles become less pronounced as body fat accumulates and begins to smooth out the body • eyes open and close • fetus responds to light • lanugo disappears • fetus stores iron and builds skeleton