Question

About how many cells make up blastocyst?

Answer 1

Hint: Cleavage is a series of rapid and repeated mitotic divisions of zygote which converts the singles celled zygote into many celled blastula. In humans, zygote, cleavage occurs during its passage through the fallopian tube to the uterus.

Complete answer:
The formation of a blastocyst begins about 5 days after fertilization when a fluid-filled cavity in the morula, the early embryonic stage of a 16-cell ball, opens up. After rapid cleavage, the blastocyst comprises 200–300 cells.
The outer layer of the cells of the morula is called trophoblast or trophectoderm. The cells of trophoblast become flattened. Some fluid passes into the modular from the uterine cavity and partially separates cells of the inner cell mass from those of the trophoblast. The fluid collects in a cavity called the blastocoel or blastocyst cavity. The blastocyst separates the trophoblast from the inner cell mass except on one side. As the quantity of fluid increases, the morula enlarges rapidly and assumes the shape of a cyst. It is then called blastocyst.
The inner cell mass is attached to the side of the blastocyst called an embryonic or animal pole and opposite side is the abembryonic pole.
The trophoblastic does not take part in the formation of the embryo proper. It remains external to the embryo and gives rise to the extraembryonic membrane for the protection and placenta for the nourishment of the embryo. The cells of the trophoblast which are in contact with the inner cell mass are known as cells of Rauber. Zona pellucida becomes thinner and finally disappears after the formation of blastocyst.

Note: Fertilization is followed by implantation. Thereafter there is transformation of the blastocyst into the gastrula with three primary germ layers by rearrangement of the cells through a process called gastrulation.