CBSE Class 12-science Answered

• Cleavage is a unique embryological process which involves a series of rapid mitotic divisions of the zygote that transforms the single-celled zygote into a multicellular structure called blastula.
• Human zygote undergoes holoblastic and equal cleavages. Cleavage occurs in the fallopian tube.
• Cleavage increases cell number but does not result in growth. It restores the cell size and nucleocytoplasmic ratio, which is a characteristic of a species

Morula Formation

• Morula consists of a group of 16-32 centrally located cells from the inner cell mass and the surrounding cell mass.
• The inner cell mass gives rise to the tissues of embryo proper, while the outer cell mass forms the trophoblast, which later develops into the placenta.
• Morula reaches the uterus about 4-6 days after fertilisation with zona pellucida remaining intact.

Blastocyst Formation

• The rearrangement of small blastomeres initiates differentiation of morula into a blastodermic vesicle or blastcoel.
• The embryo is called blastocyst which is composed of an outer envelope of cells, the trophoblast or trophoectoderm and an inner cell mass called embryoblast.
• The side of the blastocyst to which inner cell mass is attached is called the embryonic or animal pole while the opposite side is the abembryonic pole.
• The trophoblast encircles the blastocoel and inner cell mass. The inner cell mass gives rise to the embryo.
• The cells of the trophoblast help to provide nutrition to the embryo. They form the extra embryonic membranes, chorion and amnion and a part of the placenta.
• The cells of the trophoblast that are in contact with the inner cell mass are called cells of Rauber

Implantation   

• The attachment of the blastocyst to the uterine wall is called implantation. It occurs after 7 days of fertilisation.
• The trophoblast develops two layers, syncytiotrophoblast and cytotrophoblast in the region of contact between the blastocyst and endometrium.
• The blastocyst sinks into a pit formed in the endometrium and gets completely surrounded by the endometrium. It forms villi to obtain nourishment.
• The cells of the inner cell mass differentiate into two layers, hypoblast and epiblast.
• The hypoblast and epiblast form a flat disc called the embryonic disc.
• The chorionic cells of placenta secrete a hormone called human chorionic gonadotropin (HCG). It carries out several functions such as:
  • Maintains corpus luteum.
  • Stimulates secretion of progesterone by the corpus luteum.
  • Maintains the endometrial lining of the uterus and helps in its growth during pregnancy.
• By 16^th week of pregnancy, the placenta begins to produce sufficient amount of progesterone and the corpus luteum regresses.

Gastrulation

• Transformation of the blastocyst into gastrula with primary germ layers by the rearrangement of cells is called gastrulation.
• It involves cell movements called morphogenetic movements that help to attain new shape and morphology of the embryo.

• Formation of Embryonic Disc
• Formation of Extra Embryonic Coelom
• Formation of Chorion and Amnion
• Formation of Yolk Sac
• Formation of Primitive Streak
• Formation of Germ/Embryonic Layers