The human development starts in this week, when occurs the fertilization of the egg, which is complete when you mix the maternal and paternal chromosomes. The stimulus of fertilization takes out the oocyte from its state of rest, thereby leading to remove part of your DNA, and activates it, favoring the constitution of the zygote.

Expert: Texts of embryology, human


The response to fertilization is the segmentation of the zygote that consists of mitotic divisions repeated the zygote, giving rise to a rapid increase in the number of cells called blastomeres, which become smaller with each division.
The blastomeres are oval-shaped to rounded and edges are relatively smooth. Not all have the same size, but they denote the same aspect.
First, the zygote divides into 2 cells; then these cells are divided into 4 blastomeres, 8 blastomeres, and so on.
The segmentation occurs during the passage of the zygote along the uterine horn into the uterus and ends between 5 and 6 days later.
The purpose of the segmentation coincides with the structuring of the blastocyst, which consists of around 110 cells.
Initially, the blastomeres are entities with individuality of its own, separate from each other, and retain all the potentiality of the egg, but then change shape and aligned to each other closely to form a ball of cells compact, which is called morula. This phenomenon is known as compaction.
In fact, the blastomeres resulting from the first divisions of segmentation, retain all the totipotencialidad of the zygote, until shortly before the compaction. Therefore, each blastomere, separately, is able to generate an embryo completely.
In this process the areas of cell contact of the blastomeres are groomed, by the loss of their microvilli, and to establish strong unions and of the type opening between the inner cells, especially between the blastomeres more peripherals.
To the extent that the number of blastomeres increases, the size of each one of them gradually decreases.
The compaction occurs at the 4°or 5° day of this week and it marks the start of the formation of the embryo. That is to say, the cells lose their potential individual and initiate a differentiation process that leads them to acquire morphological and functional characteristics special.
The inner cells of the morula are surrounded by a layer of cells that form the cellular layer externally. The inner mass gives rise to the tissues of the embryo, while the mass of external form the placenta.
The morula is composed of 12 blastomeres, or more, to form 3 days after fertilization and enters the uterus. It is so called because it looks like a mora.
The journey of the egg, the uterine horn, occurs during the 5 first days of development, a period in which the embryo is gradually increasing its number of cells, without modifying the initial size of the zygote.
The egg human reaches the stage of 2 blastomeres between 30 and 35 hours of development; the status of 4 blastomeres between 40 and 53 hours; from morula to 80 hours and the state of the blastocyst, between 85 and 96 hours.
From the state of 2 blastomeres up to the state of morula and blastocyst, each cell has microvilli are short and numerous. The blastomeres are held together by junctional complexes, including gap junctions intermediate junctions of opening, strong joints. Until the state of 16 cells, at which time it initiates the compaction of the morula limits cytoplasm are well defined.
The maintenance of the pregnancy during the 1st week after ovulation and fertilization, is dependent on the endocrine activity of the corpus luteum. To synthesize the hormone hCG are sent chemical signals to the ovary that prevents new ovulations.
The hCG can be detected in the blood of the pregnant woman, 20 days after the start of the last menstrual period, which corresponds to the secretion made by an embryo of about 5 to 6 days.
The biological role attributed to this secretion as early for hCG is the to inhibit the immune reaction of the mother and to prevent rejection of the embryo, which is like a graft genetically different from the maternal tissues. From this it follows that the endocrine activity of pregnancy starts in the period of segmentation of the egg.

Stages of segmentation
  • Zygote
  • 2 blastomeres
  • 4 blastomeres
  • Morula


The little time that penetrated the morula in uterus (4 days after fertilization), between the blastomeres displayed a fluid-filled space called the cavity of the blastocyst. As the fluid increases, the blastomeres are separated into 2 parts:
  • A layer of outer cells thin called trophoblasts that forms the embryonic placenta.
  • A group of blastomeres, which is located in the center, known as a mass of inner cells, which give rise to the embryo.
The blastocyst increases in size rapidly as it floats freely in the uterus and is nourished by secretions of the uterine glands.
Towards the end of this week, the zygote human has gone through the stages of morula and blastocyst and has started her nest in the mucosa of the uterus, where it implants to form on the surface of the compact layer of the endometrium and is nourished by maternal tissues.

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