Cleavage, Blastocyte Formation and Implantation PDF

Cleavage, Blastocyst formation and Implantation By Dr. A.U. Agu MB,BS; MSc; PhD  Cleavage Is the process of subdivision of zygote into smaller cells. Once the zygote reached 2 cell stage (30hrs after ferti), it undergoes a series of mitoti...

Cleavage, Blastocyst formation and Implantation By Dr. A.U. Agu MB,BS; MSc; PhD  Cleavage Is the process of subdivision of zygote into smaller cells. Once the zygote reached 2 cell stage (30hrs after ferti), it undergoes a series of mitotic division increasing the number of cells. The cells become smaller with each division & form the blastomere  The blastomeres are seen as loosely arranged clump of cells until eight-cell stage. Thereafter, the blastomeres maximize their contact with each other forming a compact ball of cells held by tight junction. This process of compaction, segregates inner cells from outer cells At about 3 days after fertilization, the compacted cells divide again to form 16-cell morula (mulberry)  Inner cells constitute the inner cell mass (ICM) which gives rise to embryo proper The surrounding cells (outer cells), composed the outer cell mass (OCM) & gives rise to trophoblast which later contribute to placenta. NOTE.  4-cell stage is formed at about 40hrs after ferti  12-16 cell stage - at about 3 days  Late morula – 4 days  Blastocyst formation About the time the morula enters the uterine tube, some fluid enter into the intercellular space of inner cell mass via the zona pellucida. As the fluid increases, the ICM becomes separated from the OCM and appears at one pole. A single cavity or cyst is formed called blastocele. The embryo is thus called blastocyst, ICM and OCM become embryoblast and Trophoblast respectively  A single cavity or cyst is formed (blastocele). The embryo is thus called blastocyst, ICM and OCM become embryoblast and trophoblast respectively The trophoblast flatten & form epithelial wall of the blastocyst Then zona pellucida disappears to allow implantation The side of attachment of embryoblast is called embryonic pole while the opposite side is abembryonic pole  Function of zona pellucida The trophoblast has the property of being able to stick into tissues & its cell can ‘eat up’ other cells Thus they can invade & burrow into tissue they come in contact Zona pellucida prevents the embryo from sticking to the uterine tube  Uterus at time of implantation The mucosa of the uterus is in secretive phase The uterine glands and arteries are coiled & the tissues are succulent Thus 3 distinct layers of are recognized in the endometrium;  Superficial compact layer  Intermediate sponge layer  Thin basal layer  Normally, implantation occur along the anterior of posterior wall of the body of uterus.  Implantation The trophoblastic cells over the embryonic pole begin to penetrate b/w the epithelial cells of the uterine mucosa Attachment and inversion of trophoblast involves;  Integrins – expressed by trophoblast  Laminin - extracellular matrix molecule(EMM)  Fibronectin - EMM  Integrin receptors for laminin promote attachment Those of fibronectin stimulate migration These molecules also regulate trophoblast differentiation.  Day 8 Blastocyst is partially embedded in the endometrial stroma In the area over the embryoblast, the trophoblast differentiated into 2 layers;  Cytotrophoblast – has mitotic figures  Syncytiotrophoblast Thus, cells of cytotrophoblast divide & migrate into syncytiotrophoblast where they fuse & loose their individual cell membranes  The embryoblast also differentiates into 2 layers:  Hypoblast  Epiblast Within the epiblast, a small cavity appears & this enlarges to form the amniotic cavity Epiblast cells line the amniotic cavity. Those adjacent to cytotrophoblast are called amnioblast Endometrial stroma adjacent to implantation site becomes edematous & highly vascular The tortuous glands secrete glycogen & mucus  Blastocyst is more Day 9 deeply embedded The penetration defect in the surface epithelium is closed by fibrin coagulum Trophoblast dev further with vacuoles appearing in the syncytiotrophoblast The vacuoles fuse to form large lacunae & this stage is called lacunar stage  At the abembryonic pole, flattened cells from hypoblast forms exocoelomic (Heuser’s) membrane that lines the inner surface of cytotrophoblast The hypoplast & the membrane form the lining of primitive yolk sac (exocoelomic cavity)  Days 11 and 12 Blastocyst is completely embedded in the endometrium Surface epith almost entirely covers the defect in the uterine wall The blastocyst produces a slight protrusion into the uterine lumen The lacunar spaces form an intercommunicating network  The syncytiotrophoblast penetrate deeper into the stroma & erode the endothelial lining of maternal capillaries (sinusoids) The lacunae become continuous with the sinusoid & maternal blood enters the lacunar system Trophoblast erodes more sinusoids, & maternal blood flows via the trophoblast system, establishing uteroplacental circulation  New cell populations (extraembryonic mesoderm) appear b/w the inner surface of cytotrophoblast & outer surface of primary yolk sac. They are derived from primary yolk sac. Large cavities develop in the extraembryonic mesoderm The cavities form spaces called chorion cavity or extraembryonic coelom  The extraembryonic mesoderm lining the cytotrophoblast & amion is called extraembryonic somatopleuric mesoderm Those covering the primary yolk sac is called extraembryonic splanchnopleuric mesoderm Decidua reaction first occur at site of implantation but soon occur throughout the endometrium  Day 13 The surface defect of the endometrium is healed Sometimes, bleeding occur at the implantation site due to increased blood flow to the lacunar spaces Trophoblast is characterized by villous structures Hypoblast proliferate & form a new cavity within the primary yolk sac & is called secondary or definitive yolk sac  During its formation, a portion of primary yolk sac (exocoelomic cavity) is pinched off & is called exocoelomic cyst Extraembryonic coelom expand & form a large cavity called chorionic cavity With dev of blood vessels, connecting stalk becomes the umbilical cord  Clinical correlates Synsytiotrophoblast produces h C G, thus PT can be positive by end of 2nd week Abnormal implantation – At internal os giving placenta previa – Ectopic gestation Abnormal blastocyst causing abortion of hydatidform mole Thanks for listening