fertilization and development anatomy & physiology mrs. quezada
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Fertilization and development
Anatomy & PhysiologyMrs. Quezada
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Aim: How does fertilization occur?
Key terms review: • Pregnancy: events that occur from time of
fertilization until birth• Gestation period: period of development
from last menstrual period until birth• Embryo: fertilized egg until 8 weeks• Fetus: 9 weeks until birth
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• Fertilization occurs when the sperm’s chromosomes combine with those of an egg
• Fertilized egg= zygote• Sperm deposited in the vagina cannot fertilize
the oocyte immediately, they must be capacitated first
• Capacitated= sperm motility increases and membrane becomes weak to fuse with the egg (occurs in the female)
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Structure of sperm
• Acrosome- contains enzymesto penetrate egg• Midpiece- contains mitochondria• Tail- propels the sperm• Nucleus- contains DNA
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• Sperm use olfactory receptors to locate the egg
• The egg is surrounded by the corona radiata and the zona pellucida- transparent layers of extracellular matrix
• The sperm penetrates this layer using enzymes that digest the corona first, then binding to the zona pellucida
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• The binding of sperm (releases Ca2+) to the zona pellucida causes the acrosomal reaction- breakdown of plasma membrane and acrosomal membrane, releasing enzymes that digest holes in the zona pellucida
• Binding of the sperm to receptors on the egg leads to the fusing of sperm and oocyte membrane and the contents of the sperm entering the oocyte cytoplasm
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Blocks to Polyspermy
• Polyspermy- entry of several sperm into an egg
• This is prevented in human by the release Ca2+- causes the cortical reaction in the egg- zonal inhibiting proteins are released from granules and destroy the sperm receptors on the egg
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Aim: What events occur during embryonic development?
• After fertilization of the egg, cleavage and implantation take place
• Cleavage- series of rapid mitotic divisions• ZygoteMorulaBlastocyst (hollow ball of
cells)
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• Implantation- the outer cells of the blastocyst- trophoblast cells adhere to the endometrium and secrete digestive enzymes and growth factors
• The blastocyst burrows into the thick, velvety lining of the uterus
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• After implantation, the corpus luteum is maintained by secretion of human chrorionic gonadotropin (HCG) by the trophobast cells- keeps progesterone levels high
• The embryo obtains nutrition from the endometrium for the first month, but by the second or third month the placenta is fully formed
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• Placentation- formation of the placenta, a temporary organ that forms during pregnancy
• Cells from the inner blastocyst give rise to the chorionic villi- finger-like projections that absorb nutrients and oxygen from the mother’s blood
• Cells from the endometrium create the rest of the placental tissue
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Functions of the placenta
• Provides nutrients and oxygen for the developing fetus, removes waste products, and maintains levels of progesterone and estrogen (corpus luteum breaks down after third month)
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• Gastrulation- formation of three distinct layer in the embryo, from which all other tissues and organs will develop (third week)
• Ectoderm- nervous system and epidermis• Mesoderm- all other organs and tissues• Endoderm- digestive and respiratory systems
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• Organogenesis- formation of body organs and organ systems- occurs by week 8 of pregnancy
• After week 8, growth and further development of organs take place until the end of pregnancy (week 40)