Embryo: Developmental Biology and Zona Pellucida Essay

Embryology notes WEEK ONE
Maturation of sperms * Freshly ejaculated sperm are unable to fertilize an oocyte * Must undergo conditioning period –capacitation * Lasts aprox. 7 hours * Glycoprotein coat and seminal proteins are removed from the surface of the sperm acrosome * Capacitated by secretions in the uterus or fallopian tubes * The acrosome of the capacitated sperm binds with the glycoprotein of the zona pellucida

Fertilization * Fertilized in the ampulla of the fallopian tube * May occur in other parts of the tube but never in the uterus * Chemical signals (attractants) secreted by the oocyte and surrounding follicular cells, guide the capacitated sperm to the oocyte * Begins with contact of sperm and oocyte * Ends with intermingling of maternal and paternal chromosomes at metaphase of the first meiotic division of the zygote * Any defects may cause the zygote to die * Fertilization process takes 24 hrs

Phases of fertilization * Passage of sperm through corona radiate * Penetration of zona pellucida * When the sperm penetrates the zona pellucida a zona reaction occurs that does not allow any other sperm to penetrate it * Fusion of the cell membranes of the oocyte and sperms * Completion of the second meiotic division of oocyte and formation of female nucleus * Formation of the male pronucleus * The male and female pronuclei replicate there DNA * The oocyte that contains 2 haploid pronuclei is called an ootid * The ootid then becomes a zygote and the chromosomes arrange themselves in a cleavage spindle to get ready for the cleavage of the zygote * An XY zygote makes a male and an XX zygote makes a female

Cleavage of the Zygote

* Repeated divisions of the zygote * Results in rapid increase in the number of cells in the fallopian tube * The zygote is in the zona pellucida during cleavage * The cells become smaller with each division * When there are 12-32 blastomeres the developing human is now called a morula
Formation of the Blastocyst * 4 days after fertilization the morula enters the uterus * A fluid filled cavity appears in the morula and is called a blastocystic cavity * As fluid increases in the cavity the blastomeres split into two parts 1. A thin outer layer , the trophoblast, which gives rise to the embryonic part of the placenta 2. A group of centrally located blastomeres, the embryoblast, which gives rise to the embryo * During this stage of development the developing human is called a Blastocyst * The embryoblast now projects into the blastocystic cavity and the trophoblast forms the walls of the Blastocyst * After the free Blastocyst has floated in the uterine secretions for aprox. 2 days the zona pellucida gradually degenerates and disappears * The early embryo derives nourishment from uterine glands * Aprox. 6 days after fertilization the Blastocyst attaches to the endometrial epithelium * As soon as it attaches the trophoblast rapidly differentiates into 2 layers 1. An inner layer of cytotrophoblast 2. An outer layer of syncytiotrophoblast * By the end of the first week the Blastocyst is superficially implanted in the compact layer of the endometrium * It derives its nourishment from eroded maternal tissues

Summary of the First Week * Oocytes are produced by the ovaries (oogenesis) and are expelled from them during ovulation the fimbriae sweep the egg into the fallopian tube for it to be fertilized in the ampulla * Sperms are produced in the testes (spermatogenesis) and are stored in the epididymis ejaculation of semen results in the deposit of millions of sperms in the vagina * When an oocyte is contacted by sperm it completes its second meiotic division as a result the mature oocyte and a second polar body are formed the nucleus of the mature oocyte constitutes the female pronucleus * After the sperm

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Embryo: Developmental Biology and Zona Pellucida Essay

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