In humans, fertilization of the secondary oocyte (also called ovum) occurs inside the female body, more precisely in the fallopian tube. It is important to remember that the secondary oocyte is parked in the metaphase II of meiosis, and this meiosis will only be complete if the secondary oocyte is fertilized by a sperm.
If there is fertilization of the secondary oocyte by a sperm, meiosis will complete, originating a zygote, which is carried to the uterus by muscle contractions of the fallopian tube. It is important to remember that the zygote's development starts during its journey to the uterus, which can take three to four days.
Segmentation in mammals, and consequently in humans, is holoblastic and equal, that is, the zygote divides completely, forming blastomeres of approximately size. Before reaching the uterus, the zygote undergoes numerous divisions until it becomes a morula with about sixteen blastomeres.
Around the fifth day after fertilization, the embryo is already in the phase of
blastula, which can also be called blastocyst or blastocyst, and is already implanted in the uterus in a process called nidation. In the blastula stage we can see a layer of cells, called the trophoblast, that produces enzymes that digest the tissues of the uterus, opening cavities in the endometrium, in order to get nutrients for the embryo that is implanted in the uterus. In response to this trophoblast action, the uterine wall promotes the proliferation of blood vessels in that region, forming a highly vascularized structure called the uterine decidua.From the moment the embryo nidated in the uterus, the placenta began to form from the uterine decidua and the chorion villi that are embedded in it.
About three weeks after fertilization, the main organs of the nervous, digestive and circulatory systems begin to form and the heart begins to beat. At five weeks, the embryo begins to develop arms and legs and have muscle contractions. By the ninth week after fertilization, the embryo measures about 2.5 centimeters. At this stage, bone cells (osteoblasts) appear in the embryo's cartilage, initiating the ossification process. During this period, the embryo already has a human appearance and is called fetus.
The growth and development of the fetus continues. In the fifth month of pregnancy, the fetus measures about 20 cm and weighs approximately 500g. After 40 weeks after the first day of the last period, the baby is born.
By Paula Louredo
Graduated in Biology
Source: Brazil School - https://brasilescola.uol.com.br/biologia/desenvolvimento-do-embriao.htm