describe in post zygote events leading in implantation of placenta formation in human
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(a) During coitus, the semen having sperms is released into the vagina.
i. It passes through the cervix of the uterus and reaches the ampullary-isthmic junction of the fallopian tube.
ii. The ovum is also released into the junction for fertilisation to occur.
iii. The process of fusion of the sperm and the ovum is known as fertilisation.
iv. During fertilisation, the sperm induces changes in the zona pellucida and blocks the entry of other sperms. This ensures that only one sperm fertilises an ovum.
v. The enzymatic secretions of the acrosomes help the sperm enter the cytoplasm of the ovum.
vi. This causes the completion of meiotic division of the secondary oocyte, resulting in the formation of a haploid ovum (ootid) and a secondary polar body.
vi. Then, the haploid sperm nucleus fuses with the haploid nucleus of the ovum to form a diploid zygote.
vii. Mitosis starts as the zygote moves through the isthmus of the oviduct (cleavage) and forms 2, 4, 8 and 16 daughter cells called blastomeres.
viii. The 8−16 cell embryo is called a morula, which continues to divide to form the blastocyst. The morula moves further into the uterus.
ix. The cells in the blastocyst are arranged into an outer trophoblast and an inner cell mass.
x. The trophoblast gets attached to the uterine endometrium and the process is called implantation. This leads to pregnancy.
xi. The inner cell mass gets differentiated to form the embryo.
i. It passes through the cervix of the uterus and reaches the ampullary-isthmic junction of the fallopian tube.
ii. The ovum is also released into the junction for fertilisation to occur.
iii. The process of fusion of the sperm and the ovum is known as fertilisation.
iv. During fertilisation, the sperm induces changes in the zona pellucida and blocks the entry of other sperms. This ensures that only one sperm fertilises an ovum.
v. The enzymatic secretions of the acrosomes help the sperm enter the cytoplasm of the ovum.
vi. This causes the completion of meiotic division of the secondary oocyte, resulting in the formation of a haploid ovum (ootid) and a secondary polar body.
vi. Then, the haploid sperm nucleus fuses with the haploid nucleus of the ovum to form a diploid zygote.
vii. Mitosis starts as the zygote moves through the isthmus of the oviduct (cleavage) and forms 2, 4, 8 and 16 daughter cells called blastomeres.
viii. The 8−16 cell embryo is called a morula, which continues to divide to form the blastocyst. The morula moves further into the uterus.
ix. The cells in the blastocyst are arranged into an outer trophoblast and an inner cell mass.
x. The trophoblast gets attached to the uterine endometrium and the process is called implantation. This leads to pregnancy.
xi. The inner cell mass gets differentiated to form the embryo.
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(i) Implantation: The embedding
of a fertilised mammalian egg (embryo) into the inner thick wall of the uterus (womb) where it will continue its development is called implantation.
(ii) Placenta: It is a complex double-layered spongy vascular tissue in human female formed by the joint activity of maternal and foetal tissues in the wall of uterus that is meant for attachment, nourishment and waste disposal for the foetus.
(b) The average duration of human pregnancy is 40 weeks or 280 days.
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