Question

At what stage does the developing embryo get attached to the inner lining of the mother's uterus? Explain the developmental changes the embryo undergoes until the establishment of three germinal layers.

Answer 1

The human embryo goes through several processes and changes and develops into a multicellular and fully formed development. This whole process is known as embryonic development. The embryo differentiates into three germinal layers and the process is called gastrulation.

Complete answer:
After five to six days of fertilization, the developing embryo reaches the blastocyst stage and hatches out of zona pellucida. Then it attaches or implants itself in the inner lining of the mother's uterus and receives its nourishment and oxygen from the cell in the lining for the next week.
When the zygote divides into two cells by mitosis the beginning of cleavage occurs and the two cells divide into four cells, then eight cells, and so on. When the number reaches 16 it is referred to as a morula and at this stage, the cells start to bind together which is known as compaction and cleavage continues as cellular differentiation. In blastulation, at first, the cells divide into an outer cell layer called trophoblast and an inner cell mass. The blastomere and trophoblast become indistinguishable due to compaction. As the fluid quantity in the morula increases, it takes the shape of a cyst. When the cells of the trophoblast get flattened and the inner cell mass gets attached to the trophoblast on one side, the morula is known as a blastocyst. By this time the embryo needs additional sources for nutrition and therefore, it sticks to the uterine endometrium. The zona pellucida disappears. The blastocyst develops further to give rise to new tissues and organs and also a 3 layered disc structure which is known as an embryonic disk. These layers are called germ layers and the process is known as gastrulation. These layers are ectoderm, endoderm, and mesoderm.

Note: In the case of humans, embryonic development lasts for 9 months which is divided into three trimesters. The 1st semester lasts from week 1 to week 12, the 2nd semester from week 13 to week 24, and the third one from the 25th week to the birth of the baby. Gastrulation occurs in the first trimester and it gives rise to three germinal layers in the embryo. In the case of a human embryo, the ectoderm gives rise to the epidermis, the neural system, and the neural crest, endoderm to the epithelium of digestive and respiratory system and organs related to digestive systems like liver and pancreas and the middle layer mesoderm gives rise to cell types such as muscle, bones and connective tissues.