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Embryological Stages. March 23, 2011. Female Reproductive System. Fertilization: 0 hours. E gg is released from the ovary & moves into the fallopian tube to wait for the arrival of the sperm Fertilization - results in a zygote requires the sperm & egg nucleus to fuse
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Embryological Stages March 23, 2011
Fertilization: 0 hours • Egg is released from the ovary & moves into the fallopian tube to wait for the arrival of the sperm • Fertilization- results in a zygote • requires the sperm & egg nucleus to fuse • Following fusion, egg plasma membrane undergoes changes that prevent entrance of other sperm • Then wave-like action of cilia move the fertilized egg toward the uterus for implantation to take place
Cleavage 2-cell: 24 hours • Cleavage: process by which a zygote divides by mitosis to form two new cells • mitosis and DNA replication occur repeatedly • Cell is no longer just one cell
4-Cell: 36 hours • Cleavage continues as two cells will divide to form 4 cells, and so on • Continue dividing until form a blastocyst
8-Cell: 48 hours 8-CELL STAGE IS KEY DIFFERENCE!!! • Either Protostomes or Deuterostomes depending on species • Protostomes (clams, worms, & insects) undergo spiral cleavage- cells divide and they do not sit on top of each other; they appear to spiral
Deuterostomes • Deuterostomes- (starfish & vertebrates) cleavage results in 8 cells sitting directly on top of each other. This is called radial cleavage
Morula 16-Cell: 60-84 hours • The eight cells now continue to divide until you are simply a large solid ball of cells (16-32 cells). Morula
Blastula: 6 days • At this time, the morula begins to form a hollow ball of cells called the blastula. • The blastula contains a fluid filled space known as the blastocoel. BLASTOCOEL BLASTULA
Gastrula: ~15 days • Blastula begins to fold inward on one side • When the folding occurs, it creates a horse shoe shaped structure that is 2 cell layers thick • Outer layer of the ball is called the ectoderm. • Inner layer of the ball is called the endoderm. • Eventually, another cell layer will begin to form between the endoderm and ectoderm. This layer is called the mesoderm. Mesoderm
3 Tissue Layers In Depth • Ectoderm- forms nervous system & outer layer of skin • Mesoderm- forms muscle, bone, & cardiovascular system • Endoderm- organs of digestive system, lining of respiratory system & bladder
Gastrula (cont.) • The point where the horse shoe almost touches is called the blastopore (small hole) • In protostomes this will eventually form the mouth. • In deuterostomes this will form the anus Mesoderm