THE DEVELOPMENT OF EMBRYO
Day 1. Fertilization
In the beginning is the egg. Inside the fallopian tube, a mature human egg waits in a state of arrested development. Just released from the ovary, it is the largest cell in the human body. The egg is packed with nutrients, growth factors, enzymes and proteins — nearly everything it needs to jump start the development of a human embryo. Except for one little thing — it needs a sperm. If a sperm doesn’t penetrate the egg’s tough outer membrane to activate it within the next 24 hours, the egg will die.¬¬
The sperm cells contain the male genetic contribution to the new genome that will be produced at the completion of fertilization. ...view middle of the document...
Still dependent on nutrients and genetic instructions contributed by the egg, the embryo divides to form two cells, then four cells, eight cells and then lastly, sixteen cells. The cell division at this stage is called cleavage since furrows appear as the cytoplasm divides. The daughter cells are called blastomeres. An embryo with 16 – 32 cells called a morula.
Day 3 to 5: Blastulation
Cleavage itself is the first stage in blastulation, the process of forming theblastocyst. Cells differentiate into an outer layer of cells (collectively called thetrophoblast) and an inner cell mass. With further compaction the individual outer blastomeres, the trophoblasts, become indistinguishable, and are still enclosed within the zona pellucida. This compaction serves to make the structure watertight since the cells will later secrete fluid. The inner mass of cells differentiate to become embryoblasts and polarise at one end. They close together and form gap junctions in order to facilitate cellular communication. This polarisation leaves a cavity, the blastocoel in which is now termed the blastocyst. (In animals other than mammals, this is called the blastula). The trophoblasts secrete fluid into the blastocoel. By this time the size of the blastocyst has increased which makes it 'hatch' through the zona pellucida which then disintegrates.
The inner cell mass will give rise to the embryo proper, the amnion, yolk sac and allantois, while the fetal part of theplacenta will form from the outer trophoblast layer. The embryo plus its membranes is called the conceptus and by this stage the conceptus is in the uterus. The zona pellucida ultimately disappears completely, and the now exposed cells of the trophoblast allow the blastocyst to attach itself to the endometrium, where it will implant. The formation of thehypoblast and epiblast occurs at the beginning of the second week, which are the two main layers of the bilaminar germ disc. Either the inner cells embryoblast or the outer cells trophoblast will turn into two sub layers each other. The inner cells will turn into the hypoblast layer that will surround the other layer called epiblast layer, and these layers will form the embryonic disc in which the embryo will develop. The place where the embryo develops is called the amniotic cavity, which is the inside the disc. Also the trophoblast will develop two sub-layers; the cytotrophoblast that is front of the syncytiotrophoblast that is inside of the endometrium. Next, another layer called the exocoelomic membrane or Heuser’s membrane will appear and surround the cytotrophoblast, as well as the primitive yolk sac. The syncytiotrophoblast will grow and will enter a phase called lacunar stage, in which some vacuoles will appear and be filled by blood in the following days. The development of the yolk sac starts with the hypoblastic flat cells that form the exocoelomic membrane, which will coat the inner part of the cytotrophoblast to form the primitive yolk sac....