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EMBRYOLOGY(1) Dr.Ban Lec 1:Embryogenesis: First week, Ovulation, Fertilization and implantation Human embryogenesis is a complicated process by which a fertilized egg develops into an embryo. During the first eight weeks of developm...

EMBRYOLOGY(1) Dr.Ban Lec 1:Embryogenesis: First week, Ovulation, Fertilization and implantation Human embryogenesis is a complicated process by which a fertilized egg develops into an embryo. During the first eight weeks of development, the conceptus shifts from a single-celled zygote into a multi-layered, multi- dimensional fetus with primitively functioning organs. Embryogenesis covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus. In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs. During the first eight weeks of development, the concepts shifts from a single-celled zygote into a multi-leveled, multi-dimensional fetal body plan which utilizes primitively functioning organs. Development of the human embryo is divided into developmental events by week. Week 1 is a major part of the germinal stage of development, a period of time that continues from fertilization through uterine implantation. Important events that occur during the first week of human embryonic development are 1 EMBRYOLOGY(1) Dr.Ban highly dependent on prior events that create the ideal environment for fertilization and implantation to occur, they include: a- gamete approximation b- contact and fusion of gametes c- fertilization d- mitotic cleavage of the blastomere e- morula formation f- blastocyst formation g- implantation of the blastocyst. OVULATION:  At puberty, the female begins to undergo regular monthly cycles controlled by the hypothalamus which secretes gonadotropins. These hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), stimulate and control cyclic changes in the ovary.  Ovulation is a complex interaction between the pituitary gland in the brain, the ovaries and the uterus work to create the perfect environment 2 EMBRYOLOGY(1) Dr.Ban for ovulation (the release of an egg) to occur, for the sperm and egg to meet and for the fertilized egg to implant itself in the uterus.  Every month the pituitary gland, releases a hormone which tells the ovaries to produce a number of fluid-filled cysts called follicles. Under normal conditions, only one of these follicles reaches full maturity, and only one oocyte is discharged; the others degenerate and become atretic.  In the days immediately preceding ovulation, under influence of FSH and LH, the follicle grows rapidly to become a mature vesicular named graafian follicle then right before ovulation, the egg inside the follicle detaches itself the follicle swells, then it ruptures, expelling the oocyte with follicular fluid.  After ovulation the follicle turns into the corpus luteum which serves as a temporary endocrine gland, by releasing female sex hormones namely progesterone and estrogen.  Subsequently, the corpus luteum shrinks and forms a mass of fibrotic scar tissue, the corpus albicans. 3 EMBRYOLOGY(1) Dr.Ban Cumulus oophorus a cluster of cells that surround the oocyte both in the ovarian follicle and after ovulation the innermost layer of these cells is the corona radiate. Corona radiate is the innermost layer of cumulus oophorus ,it is a thick layer that surrounds the zona pellucida it is composed of proteins, carbohydrates and hyaluronic acid. Zona pellucida is the inner protective glycoprotein layer that surrounds the plasma membrane of mammalian oocytes. FERTILIZATION: Fertilization is the process of fusing a mature male gamete and a mature female gamete to form a single cell, the zygote. 4 EMBRYOLOGY(1) Dr.Ban Prior to fertilization, sperm undergo a process of capacitation is a period of conditioning in the female reproductive tract involves epithelial interactions between the sperm and the mucosal surface of the tube,only capacitated sperm can pass through the corona cells and undergo the acrosome reaction. Acrosome reaction: which occurs after binding to the zona pellucida, is induced by zona proteins. This reaction culminates in the release of enzymes needed to penetrate the zona pellucida, including acrosin- and trypsin-like substances. RESULTS OF FERTILIZATION A- Restoration of the diploid number of chromosomes, half from the father and half from the mother. Hence, the zygote contains a new combination of chromosomes different from both parents. B -Determination of the sex of the new individual. An X-carrying sperm produces a female (XX) embryo, and a Y-carrying sperm produces a male (XY) embryo. Hence, the chromosomal sex of the embryo is determined at fertilization. C- Initiation of cleavage MORULA FORMATION:  After fertilization, the resulting one-celled zygote will rapidly undergo multiple mitotic cleaves as it travels around four days to reach the uterine cavity. 5 EMBRYOLOGY(1) Dr.Ban  The process of cleavage results in the production of blastomeres, despite rapid increases in the number of cells during cleavage, the conceptus (The conceptus includes all structures that develop from the zygote, both embryonic and extraembryonic)remains a similar size and form due to the compaction of its cells.  When the cell’s number is around sixteen the solid sphere of cells within the zona pellucida is referred to as a morula  About the time the morula enters the uterine cavity, fluids begin to penetrate through the zona pellucida into the intercellular spaces of the inner cell mass and finally, a single cavity, the blastocoel forms. BLASTULATION(BLASTOCYST FORMATION) The blastocyst possesses an inner cell mass (ICM) which subsequently forms the embryo. The outer layer of the blastocyst consists of cells collectively called the trophoblast. This layer surrounds the inner cell mass and a fluid-filled cavity known as the blastocoel. Blastocoel is the first cavity formed during embryonic development, Blastocyst is a mass of cells that is formed a few days after fertilization. 6 EMBRYOLOGY(1) Dr.Ban Cells differentiate into an outer layer of cells called the trophoblast and an inner cell mass which differentiate to become embryoblasts. The increase in size of the blastocyst causes it to hatch through the zone pellucida , which then disintegrates. IMPLANTATION Implantation is a complex biochemical and mechanical process that begins in the first week of gestation and extends into the second week. The process of implantation can be subdivided into a-Period of apposition where the blastocyst establishes weak interactions with the uterine wall. b-Attachment phase occurs when definitive binding of the blastocyst to the uterine epithelium is more established. c-Invasion occurs when the blastocyst begins to burrow into the endometrium. PHASES:apposition, adhesion/attachment and invasion/penetration. 7