Fertilisation, embryo development upto blastocyst formation, implantation
- During copulation, semen is released by the penis into the vagina. Fertilization is the penetration of a mature ovum by the spermatozoa and fusion of the respective male and female pro nucleus.
- In humans, Fertilization takes place in ampullary-isthmic junction of Fallopian tube.
- The capacitated sperm undergoes acrosomal reaction and releases various chemicals contained in it. These chemicals are collectively called sperm lysins.
- The egg extends around the entering sperm finger like process, called microvilli, which constitute a fertilization cone called microvilli, which take the entire sperm into the egg.
- The sperm entry stimulates the egg to resume and complete the suspended meiosis-II. This produces a haploid mature ovum.
- At this time, the head of spermatozoa, which consists of nucleus, separates from middle piece and tail and becomes male pronucleus. The 2nd polar body degenerates.
- The nucleus of the ovum is now called the female pronucleus. The male and female pronucleus move towards each other and nuclear membranes disintegrate.
- After karyogamy, the ovum is now a diploid cell having 23 pair's of chromosomes ans is formed zygote.
- A glycoprotein fertilizin composed of monosaccharides and amino acid is secreted by ovum. Penetrating sperm also has a protein substance called antifertilizin on its surface. The interaction of fertilizin of an ovum with anti-fertilizin of a sperm called agglutination is species-specific.
- Embryonic development or embryogenesis is the development of embryo from fertilized ovum and its subsequent development into a oyoung organism.
- Cleavage refers to a series of rapid mitotic divisions of the zygote following fertilization, forming a many called blastula. Cleavage in fertilized egg of human starts in Fallopian tube forming 2, 4, 8, 16 daughter cells called blastomeres.
- Embryo with 8-16 blastomeres is known as Morula (little mulberry). At the next stage of development, which produces an embryo with about 64 cells, a cavity is formed within cell mass. The cavity is called blastocoel and the embryo is termed the blastocyst.
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