Second Week Of Embryological Development PDF

2 ndWEEK OF DEVELOPMENT IMPLANTATION Lecturer, Ceren ERDEM ALTUN [email protected] ISTANBUL OKAN UNIVERSITY 12.21.2023 Without cell growth in the embryo, division into 2 occurs mitotic divisions take place. Each mitotic division takes about 24 hours. On the 4th day, the compaction stage, that...

2 ndWEEK OF DEVELOPMENT IMPLANTATION Lecturer, Ceren ERDEM ALTUN [email protected] ISTANBUL OKAN UNIVERSITY 12.21.2023 Without cell growth in the embryo, division into 2 occurs mitotic divisions take place. Each mitotic division takes about 24 hours. On the 4th day, the compaction stage, that is, the borders of the blastomeres become invisible with each other and they are connected to each other. On the 5th day, the number of 200-500 cells reaches the blastocyst stage. The blastocyst falls into the uterus. This stage is called early blastocyst. (first cellular differentiation occurs at this stage) EARLY EMBRYO DEVELOPMENT PN’s fuse and dissolve in the cytoplasm (1C stage) The first cleavage starts 24 h after the fertilization, 2 cells produced The cleavage occurs by mitosis The cells are called blastomeres The cells become smaller by each division EARLY EMBRYO DEVELOPMENT Embryo have 4 blastomeres after ~48 h (2 nd day after fertilization) 8 blastomeres after ~72 h (3 rd day after fert.) Blastomeres are loosely attached to each other EARLY EMBRYO DEVELOPMENT Blastomeres starts to come together tightly attaching each other compactly in the 3-4 th day of development This stage is called the compaction EARLY EMBRYO DEVELOPMENT The cells goes on cleaving to form morula on the 4-5 th day of development They are tightly attached to each other Cell boundaries are indistinguishable After the morula stage, a cavitation begins in the early blastocyst stage and the space increases and becomes the blastocoele. blastocoele; It is filled with nutrientcontaining fluid  secreted by the endometrial layer of the uterus. During the divisions, the zona pellucida gradually becomes thinner  because the zona no longer functions It thins out and bursts from a point, the embryo starts to get rid of hatching is called blastocyst, the act of coming out is called hatched. If the embryo cannot come out of the zona, it cannot be implanted. In IVF, especially in elderly female patients and in cases where frozen embryos are thawed and transferred, zona does not burst because it hardens, at that point assisted hatching technique is applied. As fluid increases in the blastocyst cavity, it divides the blastomere into two parts: 1) Outer cell mass (ICM)= Trophoectoderm; forms the embryonic part of the placenta. The trophoblast flattens out to form the epithelial wall of the blastocyst. 2) Inner cell mass(OCM)= Embryoblast; Inner cells compact to give fetus. trophoectoderm At this stage, cells can no longer transform into another cell, their fate is determined.  called pluripotent cells. blastocoel ICM embryoblast OCM IMPLANTATION IMPLANTATION: The attachment of the embryo in the blastocyst stage to the uterine wall (endometrium) is called. Location: posterior wall of the endometrium, close to the fundus. time: 6-7 days after fertilization. implantation takes about 1 week. The first week of embryo development takes place in the fallopian tubes, and the second week in the uterus in the endometrium. UTERUS  Uterine wall consits of three layers: 1) Endometrium: the inner layer of the uterus - Functionalis (functional layer) - Basalis (basal layer) 2) Myometrium: a thick muscle layer 3) Perimetrium: the outer periton membrane Endometrium shows periodic differences in every menstruel cycle (28 days) by hormonal control Uterine receptivity finishes on 20-24 th day of the menstrual cycle UTERUS  During menstruel cycle uterus have 3 stages: 1) Follicular/ proliferative phase: follicular development 2) Secretory/ progestational phase: starts 2-3 days after ovulation. Endometrium is reestablished 3) Menstruel phase: if implantation don’t occur, the layers become degenerated and menses (bleeding) occurs  Implantation occurs in the secretory phase UTERUS In the follicular stage, the surface of the endometrium begins to thicken. Endometrium >11 mm ideal, In IVF, the endometrium is thickened by giving progesterone and estrogen externally. ENDOMETRIAL RECEPTIVITY  The endometrium is ready to accept the embryo. - Includes; morphological(artery lengthens, endometrium thickens) Genetic metabolic actions.  Surface receptors undergo superficial modification to accept the embryo.  Factors affecting implantation : - quality of the embryo - Endometrial receptivity - maternal age - Timing - hormonal status - metabolic and genetic status IMPLANTATION WINDOW Defines a time and a place. there is a short period of time during which the implantation window is open  a certain area of the endometrium is ready to receive the embryo for a short time. This event takes place 6-10 days after ovulation. Endometrial receptivity and implantation window are affected by; - cytokines (IL-1β and IL-1α), - β-HCG - signaling mechanisms produced by the blastocys STAGES OF IMPLANTATION The process of implantation may be separated into a series of developmental phases starting with the blastocyst hatching and attachment to the endometrium  These steps: - Apposition: first contact - Adhesion: adhesion (here may not like and implant the embryo  natural selection) - Penetration: start to sink in - Invasion: full embedded STAGES OF IMPLANTATION STAGES OF IMPLANTATION After the first attachment, signaling pathways are triggered and the entire endometrium undergoes changes. This is called a decidual reaction. After this point, the endometrium transforms into decidua cells. Decidua cells begin to synthesize and store lipid and glycogen (they will feed the embryo), decidua cells are larger, lighter colored and rich in glycogen than fibroblasts. At the penetration stage, the embryo will release enzymes that break down proteins and push itself deeper. STAGES OF IMPLANTATION It is well embedded in invasion and seals itself with a clot (fibrin plug). Nothing is visible on the uterine surface anymore. DURING IMPLANTATION; 1) Cytotrophoblast: 2) syncytiotrophoblast  The protrusions of trophoblast cells that adhere into the endometrium differentiate to become a new type of cells, syncytiotrophoblast.  The rest of the trophoblasts surrounding the inner cell mass will be referred to hereinafter as cytotrophoblasts.  As this process takes place, changes ocur in ICM, producing a bilaminar embryonic disc composed of two layers, the epiblast and hypoblast  At the end of the first week, a layer of cells, called the hypoblast, first appears on the surface of the embryoblast, facing the blastocyst cavity  The embryonic disc gives rise to germ layers that form all the tissues and organs of the embryo. 1: desidua cells 2: hypoblast 3: syncytiotrophoblast 4: Cytotrophoblast 5: Epiblast Multiple pregnancy is the development of more than one fetus in the uterus during the same gestational period. Multiple pregnancy usually occurs when more than one oocyte is ovulated and fertilized in the same cycle. Rarely, a single zygote splits into two in the early stages of division, forming egg different embryos. Since these embryos carry the same genetic material, they are very similar to each other. These embryos often share the same placenta monozigotic dyzigotic Their genetic material is not the same, and they are as similar to each other as siblings born at different times. TEŞEKKÜR EDERİM…