Second Week Embryology Lecture Notes PDF

Level 1 semester 1 (Embryology Lecture Notes) Dr. Dalia Saleh Embryology Lecture Notes "Second Week" By the end of this lecture, you should be able to: Explain the mechanism of implantation, normal site & abnormal sites...

Level 1 semester 1 (Embryology Lecture Notes) Dr. Dalia Saleh Embryology Lecture Notes "Second Week" By the end of this lecture, you should be able to: Explain the mechanism of implantation, normal site & abnormal sites Demonstrate steps of bilaminar embryonic disc formation Implantation: It is the process of embedding of the blastocyst within the functional layer of the endometrium during the progestational phase of the menstrual cycle. Time: It takes place by the end of the first week and continues through the second week. Site: The implantation usually occurs within the posterior wall of the uterus, near midline, near fundus. Steps: o Degeneration of the zona pellucida must take place first. o The trophoblastic layer differentiates into an inner, actively proliferating layer (the cytotrophoblast) and an outer layer (the syncytiotrophoblast), which erodes maternal tissues to establish a primitive uteroplacental circulation. Abnormal implantation: Inside uterus (placenta previa): Definition: Implantation of a blastocyst in the inferior segment of the uterus nears the internal os of the cervix. What are its types? Placenta praevia lateralis: away from internal os of cervix. Placenta praevia marginalis: partially covers the internal os. Placenta praevia centeralis: completely covers the internal os. 1 Level 1 semester 1 (Embryology Lecture Notes) Dr. Dalia Saleh Outside the uterus (ectopic pregnancy) Tubal Pregnancy: This is the most common form of ectopic pregnancy. It occurs when the blastocyst implants within the tube commonly in the ampulla. Usually occurs because of delayed transport. Ovarian pregnancy: Fertilized ovum is directly implanted on the ovary either primary or secondary to rupture of the tubal pregnancy. Abdominal pregnancy: Most commonly occurs in the rectouterine pouch. Decidua: It is the endometrium of the pregnant uterus at the secretory phase under the influence of progesterone. What are its parts? Decidua basalis: Is the part of the endometrium, which lies between the fetus and the muscle wall of the uterus Decidua capsularis: Is the part of the endometrium which overlies the fetus. Decidua parietalis: Is the part of the endometrium, which lines the rest of the uterine cavity. What is the fate of its different parts? Decidua basalis: forms the maternal part of the placenta. Decidua capsularis and parietalis: degenerate later on. 2 Level 1 semester 1 (Embryology Lecture Notes) Dr. Dalia Saleh Second week changes: Bilaminar embryonic disc The inner cell mass differentiates into the epiblast and hypoblast, forming a bilaminar embryonic disc. Two cavities are formed: The amniotic cavity dorsal to the epiblast The yolk sac ventral to the hypoblast. Trophoblasts differentiation: It differentiates into: o The syncytiotrophoblast grows into the endometrium to contact endometrial blood vessels and glands. It also secretes human chrionic gonadotrophin) (HCG) which stimulates the corpus luteum to keep secreting progesterone. o The cytotrophoblast is mitotically active forming villi. Development of the extra–embryonic mesoderm and coelom A new layer of tissue is formed from the hypoblast and the cytotrophoblast. It lies between the trophoplast externally and the primary yolk sac and amniotic cavity internally Isolated spaces appear within the extra embryonic mesoderm. These spaces rapidly fuse to form a large cavity, which is the extra– embryonic coelom. The extraembryonic mesoderm now is divided into two layers: 1) Somatic layer: lining the trophoplast. 2) Splanchnic layer: covering the primary yolk sac. The two layers connected at the caudal end of the embryo by the connecting stalk (the future umbilical cord) The trophoblast together with somatic layer of the extraembryonic mesoderm form the chorion The blastocyst is then called chorionic vesicle 3