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Chapter 4

Chapter 4 - Chapter 4 Prenatal Development and Birth From...

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Chapter 4: Prenatal Development and Birth From conception to birth 1. The period of the zygote (germinal period): - 1 st phase of prenatal development; from conception until organism becomes firmly attached the uterine wall ( implantation ). - As the zygote moves up the fallopian tube and divides by mitosis, the resulting cells form a ball-like structure called a blastocyst . - The inner layer of the blastocyst will become the embryo, and the other layer of cells will develop into tissues that protect/nourish it. - Implantation: blastocyst tendrils burrow into uterine wall, tapping the blood supply; takes ~48 hours, occurs 7-10 days past ovulation. - Nearly ¾ zygotes fail to properly implant, and thus do not develop. - Implantation in fallopian tube= ectopic pregnancy, must abort. Support systems - Once implanted, the outer blastocyst layer forms 4 major support structures: one membrane, the amnion , is a watertight sac that cushions the developing organism, regulates temperature, etc. - 2 nd membrane: yolk sac floating in amniotic fluid; provides blood cells until the embryo can produce its own. - 3 rd membrane: chorion - surrounds amnion, gathers nourishment, and eventually becomes the lining of the placenta . - 4 th membrane: the allantois, which forms the umbilical cord . Purpose of placenta - Placenta is fed by blood vessels from mother and embryo, but it prevents the bloodstreams from mixing (it is a semi-permeable barrier that allows gases (O2 + CO2), salts, and various nutrients). - Maternal blood delivers O2 and nutrients to embryo via umbilical cord, which transports back CO2 and metabolic wastes. - Placenta= site of all metabolic transactions that sustain an embryo.
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2. The period of the embryo (weeks 3-8): - By the 3 rd week, the embryonic disk differentiates into 3 cell layers: o Outer later- ectoderm - will become the nervous system. o Middle layer- mesoderm - circulatory system, muscles, bones. o Inner layer- endoderm - digestive system, lungs, vital organs. - Very rapid speed of development during period of embryo- within the 3 rd week, a portion of the ectoderm folds into a neural tube that soon becomes the brain and spinal cord. - By the 4 th week, the heart has formed and begun to beat. - The eyes, ears, nose and mouth are also beginning to appear. - At this point, embryo is ¼ inch long (10,000 x larger than zygote). - During 2 nd month, embryo grows ~1/30 th inch a day; primitive tail appears (enclosed by tissue), limbs develop, basic skeleton, etc. - 7 th /8 th weeks: sexual development begins w/ appearance of a genital ridge called the indifferent gonad – if it is a male, Y chromosome triggers gonad to produce testes; no direction is given if the embryo is female, and it produces ovaries. -
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Chapter 4 - Chapter 4 Prenatal Development and Birth From...

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