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Week #1: Fertilization……… MORULA MORULA to Implantation A. Fertilization (1) • The development of a human being begins with the fusion of an ovulated oocyte and a sperm fertilization – The oocyte and sperm are called germ cells – Each is haploid • contains 23 chromosomes following meiosis • this is the amount of DNA needed by somatic (body) cells Mitosis : chromosomes double 1x, the nucleus (cell) divides 1x Meiosis : chromosomes double 1x, the nucleus (cell) divides 2x A. Fertilization (2) • This produces a zygote –this is a single cell –it means “united” –it is diploid • it contains 46 chromosomes • like all somatic cells zygote B. • The embryo now rapidly undergoes a series of mitotic divisions called cleavage • This results in an –increase the number of cells Cleavage C. Cell Differentiation • By day 4, the embryo forms a morula –a ball containing 16cells –the cells have now differentitated into • an inner cell mass – gives rise to the embryo proper (namely YOU) •an outer cell mass – gives rise to the embryonic contribution to the placenta morula (means “mulberry”) D. Cavitationand the Blastocyst • Next a fluid-filled cavity forms between the inner cell mass and the outer cell mass • This is called “cavitation” • This gives rise to the blastocyst inner cell mass outer cell mass cavity “blastocyst” “blastocyst”= “germ-bag” E. Implantation • On about day 7, the blastocyst implants itself into the wall of the mother’s uterus Week #2: BilaminarDisk Formation •The inner cell mass now differentiates into 2 distinct cell layers epiblast –the epiblast –the hypoblast hypoblast • This is called the bilaminar embryonic disk (1) Week #2: BilaminarDisk Formation (2a) • A cavity forms next to the epiblast –It is called the amniotic cavity –it becomes filled with amniotic fluid –this will eventually surround and suspend the embryo and provide • a shock-free environment • a constant temperature –and allow free movement amniotic cavity Week #2: BilaminarDisk Formation (2b) • The cells lining the amniotic cavity are shed on occasion • These cells, along with proteins, carbohydrates, fat, enzymes, hormones and embryonic excretions can be extracted and examined • This procedure is called an amniocentesis amniotic cavity Week #2: BilaminarDisk Formation (3) • A cavity also forms next to the hypoblast and is called the yolk sac –it will contribute to respiratory and digestive system structures amniotic cavity yolk sac A few words about the outer cell mass (1) •The outer cell mass –is the embryo’s contribution to the placenta –becomes intimately associated with the inner lining of the uterus in which the embryo is embedded A few words about the outer cell mass (2) • Where the 2 surfaces join –many blood vessels from the embryo and from the mom come close together –they exchange nutrients and wastes via diffusion –no actual blood flows between the embryonic and maternal circulation Week #3: Gastrulation (1)… • With further cell differentiation, a 3rdcell layer forms between the epiblastand hypoblast • The embryo is now a trilaminardisk • This process is called gastrulation Week #3: Gastrulation(2)… • The epiblastis now called the ectoderm • The hypoblast is now called the endoderm • The middle layer, composed of cells that migrated from the epiblast, becomes the mesoderm • These 3 cell layers give rise to all the cells of your body! Week #3: Gastrulation(3a)… •The ectoderm will give rise to –organs that maintain contact with the outside world • epidermis (epithelial lining of the skin) • nervous system Week #3: Gastrulation(3b)… •The endoderm will give rise to –the epithelial lining of the digestive and respiratory tracts –plus outgrowths of the digestive tract that give rise to the: • liver • pancreas Week #3: Gastrulation(3c)… •The mesoderm will give rise to –connective tissues • • • • connective tissue proper bone cartilage blood –muscle That is, basically everything between the inner and outer linings of your body! Week #3: …and Notochord formation • The formerly round, 3-layer disk elongates • A rod-like structure forms in the mesoderm • It is called the notochord –it extends the length of the embryo (from head to tail) –it will contribute to the axial skeleton notochord cross section Notochord at 5 weeks • This is to give you a better idea of the notochord extending from head to tail • It forms the basis for the axial skeleton This is our next lecture! The Axial Skeleton gives rise to 3 Components: • the skull • the vertebral column • the thoracic cage: –the sternum –the ribs Week #4: Neurulation • Ectoderm overlying the notochord now –invaginates and –differentiates into the neural tube –this becomes the nervous system • brain • spinal cord …. Neural Tube Formation • Here is a view of the neural tube as if forms superficial to the notochord Here are the brain and spinal cord in you! Week #4:and Formation of the Fetus (1) • The shape of the embryo changes drastically – from an elongated disk it rolls up into a – “C”-shaped… • (from head to toe) tube • (from side to side) Week #4:and Formation of the Fetus(2) • The embryo now has –an outside • lined by ectoderm –an inside • lined by endoderm ectoderm –and everything in between • from mesoderm –and is now called a fetus • all the major organ systems have now been established endoderm Week #4:and Formation of the Fetus(3) • And the fetus now bulges into the amniotic cavity Week #4:and Formation of the Fetus(4) • It remains connected to the placenta via the umbilical cord • It contains very important blood vessels! Placental Blood Supply • Oxygenated blood for the fetus is supplied by the Mom (high O2 from mom) Summary • Week #1: – Fertilization →zygote formation – Cleavage →morulaformation – Beginning of cell differentiation and cavitation→blastocyst formation – Implantation • Week #2: – Differentiation continues →bilaminardisk formation – Formation of amniotic cavity and yolk sac • Week #3: – Gastrulation→trilaminardisk – Notochord formation • Week #4: –Neurulation – Major shape changes in embryo – Formation of the fetus Four Weeks • By the end of the 4thweek of gestation the upper and lower limb buds have formed • (The fetus is at very high risk of malformations at this age) Figure 29–7a, b Eight Weeks • By the end of the 8thweek the fingers and toes have formed nowface begins to • The look human-like • (The fetus remains at very high risk of malformations through the 8th week) Figure 29–7c, d Twelve Weeks • This is the period of functional maturation of all organ systems • (By this age, the fetus is at lower risk of malformations) Figure 29–7c, d Fertilization is defined as the A. increase in cell number B. joining of the sperm and oocyte C. formation of an inner and outer cell mass D. development of a cavity E. formation of a trilaminardisk The ectoderm gives rise to A. muscle B. bones C. lining of the digestive system D. liver E. skin and nervous system The neural tube gives rise to A. muscle B. connective tissue C. skin D. liver E. brain and spinal cord At what developmental stage does the embryo implant into the uterus? A. fertilized oocyte B. morula C. blastocyst D. trilaminardisk E. fetus At what developmental stage does the trilaminardisk form? A. fertilized oocyte B. morula C. blastocyst D. gastrulation E. fetus