Cleavage
As the zygote passes along the uterine tube to the uterus it indergoes cleavage i.e a series of rapid mitotic cell divisions. The new cells are known as Blastomers. After several divisions a mulberry shaped mass of 16 blastomeres is formed which is know as morula. It is surrounded by a protective layer known as the zona pellucida ( secreted by granulosa cells).
Third to Fourth day
A) Blastocyst formation:
Three days after fertilization the morula enters the uterus. Uterine fluid passes through the zona pellucida, and fills the spaces, which appear between the central cells of the morula. these spaces soon coalesce to form a blastocyst cavity and thus developing human is called Blastocyst.
B) Differentiation of blastocyst cells:
As the amount of fluid in the blastocyst cavity increases, the cells becomes separated into two parts.
(i) Trophoblast or outer cell mass: It is flattened and eventually gives rise to embryonic part of the placents.
(ii) Embryoblast or inner cell mass: It is centrally located and is the primordial of the embryo.
Fifth Day
A) Degeneration of Zona pellucida:
About five days after fertilization the zona pellucida disappears and the blastocyst enlarges.
Sixth day
A) Initiation of implantation:
The trophoblast attaches to the endometrial epithlium to initiate the provess known as Implantation.
B) Differentiation of trophoblast:
The attached region of the trophoblast differentiates into two layers cytotrophoblast (the inner cellular layer) and syncytiotrophoblast (an external syncytial layer), which invade the endometrial epithelium and connective tissue by the and of the seventh day.
Seventh day
A) Formation of hypoblast:
By the end of the first week a layer of cuboidal cells known as the hypoblast is formed on the ventral surface of the inner cell mass. It given rise to the primitive endoderm.
As the zygote passes along the uterine tube to the uterus it indergoes cleavage i.e a series of rapid mitotic cell divisions. The new cells are known as Blastomers. After several divisions a mulberry shaped mass of 16 blastomeres is formed which is know as morula. It is surrounded by a protective layer known as the zona pellucida ( secreted by granulosa cells).
Third to Fourth day
A) Blastocyst formation:
Three days after fertilization the morula enters the uterus. Uterine fluid passes through the zona pellucida, and fills the spaces, which appear between the central cells of the morula. these spaces soon coalesce to form a blastocyst cavity and thus developing human is called Blastocyst.
B) Differentiation of blastocyst cells:
As the amount of fluid in the blastocyst cavity increases, the cells becomes separated into two parts.
(i) Trophoblast or outer cell mass: It is flattened and eventually gives rise to embryonic part of the placents.
(ii) Embryoblast or inner cell mass: It is centrally located and is the primordial of the embryo.
Fifth Day
A) Degeneration of Zona pellucida:
About five days after fertilization the zona pellucida disappears and the blastocyst enlarges.
Sixth day
A) Initiation of implantation:
The trophoblast attaches to the endometrial epithlium to initiate the provess known as Implantation.
B) Differentiation of trophoblast:
The attached region of the trophoblast differentiates into two layers cytotrophoblast (the inner cellular layer) and syncytiotrophoblast (an external syncytial layer), which invade the endometrial epithelium and connective tissue by the and of the seventh day.
Seventh day
A) Formation of hypoblast:
By the end of the first week a layer of cuboidal cells known as the hypoblast is formed on the ventral surface of the inner cell mass. It given rise to the primitive endoderm.
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