http___magazin.net.hr_roditeljstvo_treci-tjedan-trudnoce_1.xml
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How far does a sperm have to swim? Sperm swimming to an egg. If fertilization occurs, the egg will have all the "instructions" to grow into a new organism. That one cell will become two, then four, then eight, then sixteen, and on and on and on. And after about 9 months, that one cell will have become a new baby. But it all starts with the sperm swimming to the egg. A sperm cell is about two thousandths of an inch long. And although they are small, they can swim roughly 8 inches in an hour. To reach an egg, they will ultimately they have to swim around 192,000 times there own length. Cleavage and Implantation A day or two after an ovary releases an egg, the egg may unite with a sperm. Sperm are deposited in the vagina during sexual intercourse. They propel themselves through the uterus and enter a fallopian tube . This is where fertilization usually takes place. When a sperm penetrates the egg, it triggers the egg to complete meiosis. The sperm also undergoes changes. Its tail falls off, and its nucleus fuses with the nucleus of the egg. The resulting cell, called a zygote , contains all the chromosomes needed for a new human organism. Half the chromosomes come from the egg and half from the sperm. Morula and Blastocyst Stages The zygote spends the next few days traveling down the fallopian tube toward the uterus, where it will take up residence. As it travels, it divides by mitosis several times to form a ball of cells called a morula . The cell divisions are called cleavage . They increase the number of cells but not the overall size of the new organism. As more cell divisions occur, a fluid-filled cavity forms inside the ball of cells. At this stage, the ball of cells is called a blastocyst . The cells of the blastocyst form an inner cell mass and an outer cell layer, as shown in Figure below . The inner cell mass is called the embryoblast . These cells will soon develop into an embryo. The outer cell layer is called the trophoblast . These cells will develop into other structures needed to support and nourish the embryo. Blastocyst. The blastocyst consists of an outer layer of cells called the trophoblast and an inner cell mass called the embryoblast. The blastocyst fluid-filled cavity is also known as the blastcoel or blastcoele. Implantation The blastocyst continues down the fallopian tube and reaches the uterus about 4 or 5 days after fertilization. When the outer cells of the blastocyst contact cells of the endometrium lining the uterus, the blastocyst embeds in the endometrium. The process of embedding is called implantation . It generally occurs about a week after fertilization. For a detailed look at fetal development, see http://php.med.unsw.edu.au/embryology/index.php?title=Main_Page . Summary - Fertilization is the union of a sperm and egg, resulting in the formation of a zygote. - The zygote undergoes many cell divisions before it implants in the lining of the uterus. Practice Use these resources to answer the questions that follow. - http://www.hippocampus.org/Biology Biology for AP* Search: Embryonic Development (Reproduction & Devel. in Animals) - Distinguish between the animal and vegetal poles. Describe the following: - clevage - blastula - blastocoel - http://www.hippocampus.org/Biology Biology for AP* Search: Human Development - What develops from the blastocyst? - Define placenta. - What are embryonic stem cells? - What is implantation? Review What happens during fertilization? Where does it usually take place? What is implantation? When does it occur?
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Treći tjedan Nakon oplodnje, perioda zigote i morule, nastaje blastocista. Blastocista se implantira, ili usadi, u sluznicu maternice te se nastavi razvijati. Mama Ovulirat ćete ovaj tjedan i uz malo sreće ostati trudni. Međutim, vaša će se trudnoća brojati od prvog dana posljednje menstruacije, a ne od dana oplodnje! Prilikom probijanja sluznice može doći do laganog krvarenja. To je normalno i nije ništa zabrinjavajuće. Vaše tijelo tada ispušta imunosupresivni protein poznat kao "rani trudnički faktor" koji pomaže da vam tijelo ne prepozna vašu tek začetu bebu kao uljeza. Beba Oplodnja će se dogoditi ovaj tjedan. Jajašce i spermij će se sresti u tzv. Fallopijevoj tubi koja povezuje jajnik s maternicom. Oplođeno jajašce dalje putuje i ugrađuje se u zadebljanu sluznicu maternice. To putovanje može potrajati 7 do 10 dana. Tijekom tog putovanja jajašce se multiciplira dok ne dosegne veličinu od oko 0,1 do 0,2 milimetara. Službeno ime vašeg djeteta je blastocista. Naime, u roku od otprilike 24 sata nakon što spermij prodre u jajnu stanicu dolazi do spajanja muških i ženskih kromosoma. U narednih nekoliko dana stanica se dijeli. Za četiri ili pet dana nakon oplodnje od novonastalih stanica formira se šuplja kuglica (sitnija od glavice pribadače) koju čine vanjska ovojnica i unutrašnja stanična masa (embrioblast). Ta se kuglica naziva blastocista. Mnoge stanice koje tvore vanjsku ovojnicu blastociste s vremenom će se razviti u tkivo koje ne tvori dijelove embrija. Od embrioblasta će se razviti dijete. Blastocista se pričvrsti za stijenke maternice i počne stvarati posteljicu, kroz koju će plod iz majčinog krvotoka dobivati kisik i hranu te odbacivati otpadne tvari. Ovu 'šuplju kuglicu', čine trofoblast, to jest vanjska ovojnica blastociste, i malena nakupina od tridesetak stanica, takozvani embrioblast, koja je pričvršćena za unutrašnju stijenku kuglice. S vremenom se iz trofoblasta razvije posteljica, a embrioblast postane ljudski embrij. Tata Jeste li znali da prilikom ejakulacije u prosjeku izbacite oko 200 do 300 milijuna spermija, a samo njih dvjestotinjak stigne do zrelog jajašca? web preporuke cosmopolitan.hr16 najljepših smokey eyesa ikada elle.hrBaš poput majke oglasnik.hrSvakog utorka nova ponuda radnih mjesta!