Second Week Embryology PDF

 2nd week Implantation  Definition: embedding blastocyst into endometrium  Site upper part of posterior uterine wall near fundus  Time : begin at the end of 1st week ti...

 2nd week Implantation  Definition: embedding blastocyst into endometrium  Site upper part of posterior uterine wall near fundus  Time : begin at the end of 1st week till the end of 2nd week  Mechanism:  Loss of zona pellucida causes trophoblast exposure  Exposed trophoblast has 2 powers:  Adhesive power: to endometrium  Histolytic power: by enzymes that help blastocyst to digest endometrium.  Fibrin plug fill the digest part of endometrium Anatomist “second week”  Anomalies: Placenta previa: Implantation of blastocyst into lower uterine segment. It may be: 1. Placenta previa lateralis: Placenta is away from Internal os 2. Placenta previa marginalis: Placenta partially cover Internal os 3. Placenta previa centralis: Placenta is completely cover Internal os Ectopic pregnancy: Implant ion of blastocyst outside uterine cavity. It may be 1. Tubal "commonest type": Caused by early loss of zona pellucida or delayed transport of zygote 2. Ovarian: it may be Primary: implantation into ovary or Secondary: to tubal pregnancy rupture 3. Peritoneal “abdominal”: Mainly in recto-uterine pouch Anatomist “second week”  Decidua  Definition : endometrium of pregnant uterus  Structure : as endometrium at secretory phase  Hormonal control : under effect of progesterone  Parts : Decidua capsularis: Definition: part of endometrium covering the fetus Fate: Fuse & Degenerate Decidua parietalis Definition: part of endometrium lining uterine cavity Fate: Fuse & Degenerate Decidua basalis: Definition: part of endometrium between fetus & myometrium Fate: Form maternal part of placenta Anatomist “second week”  Blastocyst changes “Chorionic vesicle formation” 1.Trophoblast becomes: 1a.Syncytio-trophoblast: outer layer of trophoblast which: Produce proteolytic enzymes to erode the endometrium Secrets HCG "human chorionic gonadotropin" to keep coprpus luteum secreting progesterone for 4 months 1b.Cytotrophoblast: inner layer of trophoblast 2.Inner cell mass becomes Bilaminar disc formed of 2a.Epiblast: upper columnar layer 2b.Hypoblast: lower cuboidal layer 3.Blastocele becomes 3a.Amniotic cavity: between epiblast "form floor" & amnioblast "form roof" 3b.Primary Yolk sac: between hypoblast "form roof" & Hausser membrane "form floor" 4,Extra-embryonic mesoderm appears and divided by Extra-embryonic coelom into 4a.Somatic mesoderm: lining cytotrophoblast & covering amniotic cavity 4b.Splanchnic mesoderm: covering yolk sac N.B: Somatic & splanchnic mesoderm remain incontact at the caudal end of baby forming connecting stalk “umbilical cord” N.B: Chorion is formed of syncytiotrophoblast , cytotrophoblast & somatic extra-embryonic mesoderm Anatomist “second week”