Arrange the following events in the correct order pertaining to fertilization in the human reproductive system.

(A) The blastocyst becomes embedded in the endometrium.

(B) Finger-like projections appear on the trophoblast called chorionic villi.

(C) The blastomeres are arranged into trophoblast and the inner cell mass.

(D) The zygote divides miotically and transforms into an embryo with 8-16 blastomeres, called a morula.

Choose the correct answer from the options given below.

(D), (C), (A), (B)

The correct answer is Option (1) - (D), (C), (A), (B)

(D) The zygote divides miotically and transforms into an embryo with 8-16 blastomeres, called a morula.

(C) The blastomeres are arranged into trophoblast and the inner cell mass.

(A) The blastocyst becomes embedded in the endometrium.

(B) Finger-like projections appear on the trophoblast called chorionic villi.

During copulation, semen is ejaculated from the penis into the vagina, a process known as insemination. The motile sperm cells rapidly swim through the cervix, enter the uterus, and ultimately reach the ampullary region of the fallopian tube. Simultaneously, the ovum released by the ovary is transported to the same ampullary region, where fertilization occurs.

Fertilization can only happen when both the egg and sperm reach the ampullary region at the same time. This is why not all instances of copulation result in fertilization and subsequent pregnancy.

Fertilization refers to the process of the fusion of a sperm cell with an egg cell, leading to the formation of a zygote.

Following fertilization, the haploid nucleus of the sperm cell combines with that of the egg cell, resulting in the formation of a diploid zygote. The zygote initiates mitotic divisions while traveling through the isthmus of the oviduct, a process known as cleavage. These divisions give rise to 2, 4, 8, and eventually 16 daughter cells called blastomeres.

The embryo with 8 to 16 blastomeres is called a morula . The morula continues to divide and transforms into blastocyst as it moves further into the uterus. Within the blastocyst, the blastomeres are organized into two distinct layers: the outer layer referred to as the trophoblast and an inner group of cells attached to the trophoblast known as the inner cell mass. The trophoblast layer becomes attached to the endometrium, while the inner cell mass differentiates into the embryo.

Following attachment, the cells of the uterine lining undergo rapid division, enveloping and embedding the blastocyst within the endometrium of the uterus. This crucial process is termed implantation, and it marks the onset of pregnancy. After implantation, finger-like projections appear on the trophoblast called chorionic villi which are surrounded by the uterine tissue and maternal blood. The chorionic villi and uterine tissue become interdigitated with each other and jointly form a structural and functional unit between developing embryo (foetus) and maternal body called placenta.