First and Second Week of Development PDF

FERTILIZATION, CLEVAGE AND IMPLANTATION: First week of development Prof. Dr. Aylin YABA UÇAR Department of Histology and Embryology Learning objectives • Explain the phases of fertilization • Define important results of fertilization • Describe the clevage and compaction of embryo • Define the ph...

FERTILIZATION, CLEVAGE AND IMPLANTATION: First week of development Prof. Dr. Aylin YABA UÇAR Department of Histology and Embryology Learning objectives • Explain the phases of fertilization • Define important results of fertilization • Describe the clevage and compaction of embryo • Define the phases of implantation PREPUBERTY FETAL LIFE meiotic arrest LH FSH OVULATION Oocyte maturation meiosis mitosis oogonia REPRODUCTIVE PERIOD Primary oocyte PI PI PI MII Translational control Blastocyst 8C 4C 2C Pre-implantation embryonic development 1C fertilization Main Events During Fertilization • • • • • • • • Capacitation Acrosomal reaction Penetration of corona radiata and zona pellucida Binding and fusion of the oocyte and sperm cell membranes Cortical and zona pellucida reactions Resumption of 2nd meiotic division Metabolic activation of the egg Restoration of diploid number of chromosomes Capacitation A series of maturational changes that ejaculated spermatozoa must undergo in order to fertilize. Capacitation • Changes the surface characteristics of the sperm – seminal plasma factors that coat the surface of the sperm are removed – surface charge is modified – receptor mobility becomes more restricted • Capacitation leads to: – Ability to bind to the zona pellucida – Ability to undergo the acrosome reaction – The acquisition of hypermotility Effect of Capacitation Andrew Raid et al., Asian Journal of Andrology, 2010 Diana S. Chu, PlosOne, 2018 • Increased activity of adenylyl cyclase leading to increasing levels of cAMP • Increased rate of tyrosine phosphorylation • Activation of Ca2+ channels resulting in increased intracellular Ca2+ levels • Release of seminal fluid glycoconjugates from the surface of the head of the spermatozoon • Extensive modification of the plasma membrane by removal of cholesterol, the predominant inhibitor of capacitation, and redistribution of phospholipids and carbohydrate moieties Capacitation Sperm hyperactivity is necessary for breaking physical barriers that protect the secondary oocyte from fertilization. • No change in sperm morphology • Motility change – hyperactivation Acrosome reaction • Breakdown and merging of the plasma membrane and the outer acrosomal membrane allowing the release of acrosomal enzymes. • Is mediated by binding ZP3. Free swimming sperm and those negotiating the cumulus are acrosome intact. • Ca++ influx is a prerequisite. • Results in release of acrosin, esterase, neuraminidase and other enzymes facilitates penetration of ZP. Penetration of corona radiata and zona pellucida Likely Sperm Receptors: b1,4 galactosyltransferase SP 56 P 95 a mannoside a mannosidase Galactosyl receptors Lectin-like fructose binding protein Sperm Passage of a sperm through the corona radiata of the oocyte: • Dispersal of the follicular cells of the corona radiata results mainly from the action of the enzyme hyaluronidase, which is released from the acrosome of the sperm. • Tubal mucosal enzymes also appear to assist hyaluronidase. • Movements of the tail of the sperm are important during penetration of the corona radiata. Binding of Zona Proteins • The zona pellucida, which is 13 µm thick in humans, consists principally of four glycoproteins—ZP1 to ZP4. • ZP2 and ZP3 combine to form basic units that polymerize into long filaments. These filaments are periodically linked by cross-bridges of ZP1 and ZP4 molecules. 2 1 3 Binding of Zona Proteins • Three sulfated glycoproteins: ZP1, ZP2, ZP3 of 200, 120 and 83 kDa respectively. • Basic structural unit: ZP2-ZP3 dimers cross linked by ZP1. • ZP3: polypeptide + N and O linked oligosaccharides. The oligosaccharides mediate initial sperm binding, the polypeptide initiates the acrosome reaction (via G protein mediated Ca++ permeability). • ZP2: secondary sperm receptor, after fertilization, prevents polyspermy. Nearly 20,000 cumulus cells are extruded with the oocyte at ovulation. Although not necessary for fertilization, denuded oocytes have low fertility. 1 PB Ovum ZP Cumulus cells Prof. Dr. Aylin Yaba Uçar Sperm binds oligosaccharide on ZP3 3 2 1 Prof. Dr. Aylin Yaba Uçar Binding to ZP3 leads to influx of Ca++ 3 2 1 Ca++ Prof. Dr. Aylin Yaba Uçar Prof. Dr. Aylin Yaba Uçar The proteolytic enzyme acrosin, esterases, neuraminidase, appears to cause lysis of the zona pellucida, thereby forming a path for the sperm to follow to the oocyte. • Sperm passing the ZP are actively motile. • After crossing the ZP and perivitelline space, the sperm become apposed to the oolemma at the equatorial segment. • Following fusion, the sperm membrane becomes part of the oocyte membrane. Fusion of the oocyte and sperm cell membranes Assoc. Prof. Aylin Yaba Uçar Cortical and zona pellucida reactions Cortical Granules Prof. Dr. Aylin Yaba Uçar Cortical Reaction Induced by IP3-mediated increased intracellular Ca++ Release of trypsin-like protease Zona Reaction Block to Polyspermy Loss of sperm binding due to alterations in ZP2-3 Cortical granules Prof. Dr. Aylin Yaba Uçar • Immaturity or excessive aging may result in an incomplete block to polyspermy. • In the presence of excess spermatozoa such as in IVF (in vitro fertilization), the polyspermy rate can be as high as 5%. Resumption of 2nd meiotic division • Breakdown of Sperm Nuclear Membrane • Nuclear Decondensation • Nuclear Dispersion Prof. Dr. Aylin Yaba Uçar • Formation of pro-nuclear membrane • Formation of enlarged nucleus with 8 nucleoli Prof. Dr. Aylin Yaba Uçar • Formation of pro-nuclear membrane • Formation of enlarged nucleus with 8 nucleoli Prof. Dr. Aylin Yaba Uçar Apposition of pronuclei Prof. Dr. Aylin Yaba Uçar Fusion of male and female pronuclei Restoration of diploid number of chromosomes Zygote The zygote is genetically unique because half of its chromosomes come from the mother and half are derived from the father. This mechanism forms the basis for biparental inheritance and variation of the human species. Prof. Dr. Aylin Yaba Uçar Phases of Fertilization Results of Fertilization • Stimulates the secondary oocyte to complete the second meiotic division, producing the second polar body. • Restores the normal diploid number of chromosomes (46) in the zygote. • Results in variation of the human species through mingling of maternal and paternal chromosomes. Results of Fertilization • Determines the chromosomal sex of the embryo; an X-bearing sperm produces a female embryo and a Y-bearing sperm produces a male embryo. • Causes metabolic activation of the oocyte, which initiates cleavage of the zygote. CLEAVAGE OF ZYGOTE • 30 hrs after fertilization Blastomeres Assoc. Prof. Aylin Yaba Uçar After the eight-cell stage, the blastomeres change their shape and tightly align themselves against each other—compaction. • This phenomenon may be mediated by cell surface adhesion glycoproteins. • Compaction permits greater cell-to-cell interaction and is a prerequisite for segregation of the internal cells that form the inner cell mass . What determines whether a blastomere will form inner cell mass or trophoblast? Development 2014 Development 2016 BLASTULATION Expanded Blastocyst Trophectoderm Blastocoelic cavity Inner Cell Mass Blastocyst Hatching Blastocyst Blastocyst hatches from zona pellucida before implanting Hatching Blastocyst Hatched Blastocyst Zona pellucida An implantation window refers to the period of time in which the uterus is ready to receive the blastocyst for implantation. Nature Medicine 2012 IMPLANTATION 1. Apposition 2. Adhesion – attachment of blastocyst to the surface of endometrium – day 5 3. Invasion – introduction of blastocyst inside the uterine wall resulting from its lytic digestion – days 6-12 4. Implantation Nature Medicine 2012 FORMATION OF BLASTOCYST • The trophoblast, the thin outer cells that give rise to the embryonic part of the placenta. • The embryoblast, a discrete group of blastomeres that is the primordium of the embryo • During this stage of development— blastogenesis—the conceptus is called a blastocyst • Approximately 6 days after fertilization, the blastocyst attaches to the endometrial epithelium • The cytotrophoblast, the inner layer of cells •The syncytiotrophoblast, the outer layer consisting of a multinucleate protoplasmic mass formed by the fusion of cells The cytotrophoblast is a mitotically active inner cell layer producing cells that fuse with the syncytiotrophoblast, the outer erosive layer. The syncytiotrophoblast is not mitotically active and consists of a multinucleate cytoplasmic mass; it actively invades the epithelium and underlying stroma of the endometrium. Scanning electron microscopy (SEM) photomicrographs of luminal surface of human endometrial biopsies were taken from (A) early, (B) mid and (C) late luteal phases of normal menstrual cycle The hypoblast forms the roof of the exocoelomic cavity and is continuous with the cells that migrated from the hypoblast to form the exocoelomic membrane. This membrane surrounds the blastocystic cavity and lines the internal surface of the cytotrophoblast. The exocoelomic membrane and cavity soon become modified to form the primary umbilical vesicle. The embryonic disc then lies between the amniotic cavity and primary umbilical vesicle. The lacunae are soon filled with a mixture of maternal blood from ruptured endometrial capillaries and cellular debris from eroded uterine glands. The fluid in the lacunae—embryotroph—passes to the embryonic disc by diffusion. The communication of the eroded uterine vessels with the lacunae represents the beginning of the primordial uteroplacental circulation. • Uteroplacental circulation – starts day 12 day, when maternal blood from eroded uterine vessels provide nutritional support to the developing embryo. • When maternal blood flows into the lacunae, oxygen and nutritive substances become available to the extraembryonic tissues over the large surface of the syncytiotrophoblast. • Oxygenated blood passes into the lacunae from the spiral endometrial arteries in the endometrium; deoxygenated blood is removed from the lacunae through endometrial veins. As the extraembryonic coelom forms, the primary umbilical vesicle decreases in size and a smaller, secondary umbilical vesicle forms. • During formation of the secondary umbilical vesical, a large part of the primary umbilical vesicle is pinched off. • The umbilical vesicle contains no yolk; however, it may have a role in the selective transfer of nutritive materials to the embryonic disc. Development of Chorionic Sac The end of the second week is characterized by the appearance of primary chorionic villi. • https://www.youtube.com/watch?v=bIdJOiXp p9g After implantation, the endometrium undergoes decidualization. During pregnancy, the portion of the endometrium that undergoes morphologic changes is called the decidua or decidua graviditas. Decidualization • As its name implies, this layer is shed with the placenta at parturition. The decidua includes all but the deepest layer of the endometrium. The stromal cells differentiate into large, rounded decidual cells. • The uterine glands enlarge and become more coiled during the early part of pregnancy and then become thin and flattened as the growing fetus fills the uterine lumen. Cell reports 2016 Three different regions of the decidua are identified by their relationship to the site of implantation: • The decidua basalis is the portion of the endometrium that underlies the implantation site. • The decidua capsularis is a thin portion of endometrium that lies between the implantation site and the uterine lumen. • The decidua parietalis includes the remaining endometrium of the uterus. Ectopic Pregnancy