Embryology 2: Second Week of Development PDF

EMBRYOLOGY 2 Dr.Ban EMBRYOLOGY L2 SECOND WEEK OF DEVELOPMENT One of the most important features of the second week is the completion of implantation and establishment of fetomaternal interactions. BLASTULATION  Is the stage in e...

EMBRYOLOGY 2 Dr.Ban EMBRYOLOGY L2 SECOND WEEK OF DEVELOPMENT One of the most important features of the second week is the completion of implantation and establishment of fetomaternal interactions. BLASTULATION  Is the stage in early embryonic development that produces the blastula which develops into the blastocyst which is a cluster of dividing cells (blastomeres)made by a fertilized egg.  The blastocyst comprises of an outer layer of trophoblast cells which will form the placenta and the inner cell mass the embryoblast which will ultimately form the embryo. 1 EMBRYOLOGY 2 Dr.Ban Stages of Blastocyst Embryo Development THE BILAMINAR EMBRYONIC DISC The bilaminar embryonic disc, is the distinct two-layered structure of cells formed in an embryo takes place by day eight in the development of the human embryo.It is formed when the inner cell mass( embryoblast) forms a bilaminar disc of two layers. Once the blastocyst enters the uterus, the zona pellucida degenerates to expose the underlying trophoblast layer, the blastocyst is partially incorporated into the 2  EMBRYOLOGY 2 Dr.Ban endometrium of the uterus (into which it enters due to the proteolytic enzymes of the syncytiotrophoblast cells), and in order to fit in, the blastocyst must partially collapse and change its shape, establishing a connection between the blastocyst and the mother. The second week is often referred to as the week of twos. It’s the week when the embryoblast, extraembryonic mesoderm(which has important functions in maternal-fetal protection and communication, as well as in primitive erythropoiesis),and trophoblast each separate into two distinct layers.Additionally, there are two cavities that develop within the embryonic unit at this time as well. The embryoblast will turn into two sub-layers:  The embryoblast forms embryonic disc, which is a bilaminar disc of two layers: a-an upper layer called the epiblast b-a lower layer called the hypoblast The disc is stretched between fluid-filled cavities the amniotic cavity and the yolk sac.  The trophoblast is a thin layer of cells that helps a developing embryo attach to the wall of the uterus, protects the embryo, and forms a part of the placenta, it 3  EMBRYOLOGY 2 Dr.Ban will develop two sub-layers: a-cytotrophoblast b-syncytiotrophoblast IMPLANTATION  is a complex biochemical and mechanical process that begins in the first week of gestation and extends into the second week.  After ovulation, the endometrial lining becomes thickened, with its secretory glands becoming elongated, and is increasingly vascular in preparation for accepting the embryo.  The placenta develops once the blastocyst is implanted, connecting the embryo to the uterine wall.  Concerning syncytiotrophoblast some vacuoles will appear and will be filled by blood in the following days.  Syncytiotrophoblast expands and erodes maternal capillaries and endometrial glands ,the blood will begin to penetrate and flow through the trophoblast to give rise to the uteroplacental circulation. Also contents the glands will be released inside the lacunae. The syncytiotrophoblast also produces human chorionic gonadotropin, a hormone that stimulates the release of progesterone from the corpus luteum. 4 EMBRYOLOGY 2 Dr.Ban Progesterone enriches the uterus with a thick lining of blood vessels and capillaries so that it can oxygenate and sustain the developing embryo. The embryo is joined to the trophoblastic shell by a narrow connecting stalk that develops into the umbilical cord to attach the placenta to the embryo. 5