Biology: Ch. 3 Embryogenesis and Development

• A second oocyte is ovulated from the follicle on approximately day 14 of the menstrual cycle
• The secondary oocyte travels into the fallopian tube, where it can be fertilized up to 24 hours after ovulation
• Fertilization occurs in the widest part of the fallopian tube called ampulla
• When the sperm meets the secondary oocyte in the fallopian tube, it binds to the oocyte and release acrosomal enzymes that enable the head of the sperm to penetrate the corona radiate and zona pellucida,
• The first sperm that comes in contact with the secondary oocyte'e cell membrane forms a tube-like structure known as the acrosomal apparatus and enters to finish the meiosis II
• After penertation of the sperm through the cell membrane, the cortical reaction, a release of calcium ion, occurs. These calciums depolarizes the membrane of the ovum in which it prevents the fertilization of the ovum by multiple sperms, and the increase calcium concentration increase the metabolic rate of the newly formed diploid zygote. The now depolarized and impenetrable membrane is called fertilization membrane
• Twins can be dizygotic ( fraternal) twins form from fertilization of two diff eggs with 2 diff sperms. Each zygote will implant in uterine wall, and each will develop its own placenta, chorion, and amnion.
• In dizygotic twins, if the zygote implant close together the placentas may grow onto each other
• Fraternal twins are no more genetically similar than any other pain of sibilings
• Monozygotic ( identical) twins form when a single zygote splits into two. The genetic material is identical same as the genome
• In identical twins, if the division is incomplete, we would have conjoined twins, where the two offsprings are physically attached together
• If split occurs in 2-3 days we would have dichotionic / diamniotic: they have separate amnion and chorion
• If spli happedn in 3-8 days they have dichorionic/ monoamniotic: have same placenta (chorion), but separate amnion ( sacs)
• If split occurs 8-13 days then we have monoamnionic/ monochorionic, so they have the same placenta and sacs
• As the zygote moves to the uterus for implantation it undergoes rapid mitotic cell division called cleavage as the number of cells increase, but the size stays the same
• Indeterminate cleavage results in cells that are capable of becoming any cell in the organism like monozygotic ( identical) twins
• Determinate cleavage results in cells that are committed to differentiating into specific cell type of differentiating
• In the 16 cell stage, it would be called morula
• Once the morula is formed undergoes a blastulation, which it forms a blastula, a hollow ball of cells with a fluid-filled inner cavity known as blastocoel. The mammlian blastula is known as blastocyst that develops into outer cell mass and inner cell mass. Then, they will develop in order into trophoblast cells surrounding the blastoceol and gives rise to the chorion and later the placenta, ,whereas the inner cell mass gives rise to embryoblast and give rise to blastoceol and the organism itself
• The blastula ( blastocyst) moves through the fallopian. tube to the uterus, where it burrows into the endometrium.
• Trophoblast cells are specialized to create an interface btw the maternal blood supply and the developing embryo.
• The trophoblasts give rise to chorion and later the placenta
• The trophoblasts form chorion villi, which are microscopic fingers. As these chorionic villi develop into placenta they support maternal-fetal gas exchange.
• The embryo is connected to the placenta by umbilical cord
• The umbilical cord consists of 2 arteries and one vein. The one vein carries freshly oxygenated blood rich with nutrient from the placenta to embryo
• The 2 umbilical cord arteries would carry deoxygenated blood and waste to the placenta for exchange
• Until the placenta is functional, the embryo is supported by yolk sac, which is also the site of early blood cell development
• Two extraembryonic membranes are allantois that is involved in early fluid exchange btw the embryo and yolk sac.
• Ultimately, yhr umbilical cord is formed from remnants of the yolk sac and allantois
• The allantois is surrounded by amnion, a thin tough membrane filled with amniotic fluid as shock absorber during pregnancy, lessening the impact of maternal motion on the developing embryo
• Gastrulation is the generatoin of the three distinct cell layers
• The three layers of cells are called primary germ layers
• Selective transcription means the only genes needed for a particular cell type are transcribed
• Induction, is the ability of one group of cells influencing the fate of other nearby cells
• This process is mediated by chemical substances. called inducers which diffuse from the organizing cells to the responsive cells
• Once the three germ layers are form, neurulation, or the development of the nervous system can begin.
• Nervous system is derived from ectoderm
• In the mesoderm cells there is notochord in which it releases some type of growth factors on the ectoderm causing them to slide inward to form neural folds, the curve in neural groove, and tip of the neural folds are called neural crest cells in which it would outward to form the peripheral nervous system.
• the neural plate woulod further go inward to a complete circle called neural tube
• early development is highly sensitive time