Gametogenesis and Fertilization PDF

EMBRIOLOGY department of histology, cytology and embriology  GENERAL EMBRYOLOGY An intricate and miraculous process by which a single cell gives rise to a highly developed multicellular human being. A continuous process that begins when an oocyte (ovum) is fertiliz...

EMBRIOLOGY department of histology, cytology and embriology  GENERAL EMBRYOLOGY An intricate and miraculous process by which a single cell gives rise to a highly developed multicellular human being. A continuous process that begins when an oocyte (ovum) is fertilized by a sperm to form a zygote which differentiates in to definitive organ system and thereafter in to their early functional stage.  Common terms used in embryology Oocyte (Ovum)- a mature secondary oocyte ready for fertilization. Sperm or spermatozoa- male gamete Zygote- diploid, fertilized cell which has the potential to produce an embryo. Cleavage- continuous, rapid mitotic cell divisions occurring in zygote just after fertlization. Morula- as a result of cleavage, zygote appears like a mulberry bush onsisting of small tightly packed cells. Conceptus- the developing embryo or fetus along with its associated membranes. Embryo- first eight weeks of developing human when the primordia of almost all the organs and system have appeared.  EMBRIOGENESIS 1- fertilization and formation of a zygote, 2- fragmentation and formation of blastula, 3- gastrulation - the formation of germ layers and a complex of axial organs, 4- histogenesis (tissue formation), 5- organogenesis, 6- system genesis.  Embryogenesis - closely related to progenesis and early postembryonic period. Progenesis - this is the period of development of germ cells (egg and sperm).  GAMETOGENESIS It is the production of (Gamete Formation) mature male & female gametes (Sperms & Ova).  Spermatogenesis:  It is the series of changes by which the primitive germ cells (spermatogonia) are transformed into mature sperms.  Oogenesis:  Sequence of events by which the primitive germ cells (oogonia) are transformed into mature oocytes.  MEIOSIS  It is the cell division that takes place in the germ cells to produce male & female gametes.  It consists of two cell divisions, meiosis I & meiosis II during which the Diploid number of chromosomes (46) is reduced to Haploid number (23). FIRST MEIOTIC DIVISION At the beginning of meiosis I, (prophase) male & female germ cells replicate their DNA so that  each of the 46 chromosomes is duplicated into sister Chromatids. WHAT IS THE DIFFERENCE BETWEEN MITOSIS & MEIOSIS? HAPLOID  Spermatogenesis Total time taken 64 days. Starts after onset of puberty (13-16 years). Includes all events by which spermatogonia (primordial germ cells ) are transformed into spermatozoa, the mature sperms. Comprises of three phases: 1. Spermatocytosis (mitosis) 2. Meiosis 3.Spermiogenesis SPERMATOGENESIS  AIM:  Formation of sperms with haploid number of chromosomes.  SITE:  Seminiferous tubules of the testis.  TIME:  From puberty till old age.  DURATION:  About two months  N.B. Sperms are stored and become functionally mature in the Epididymis.  It is change in SPERMIOGENESIS shape (metamorphosis) through which Spermatids are transformed into mature Sperms: 1. Nucleus is condensed and forms most of the head. 2. Golgi apparatus forms the Acrosome. 3. Mitochondria forms a spiral sheath. 4. Centriole elongates to form the axial filament.  Male reproductive cells Formed in the convoluted seminiferous tubules of the male reproductive gland. The sperm has - head - tail: - intermediate part - mane, - terminal The sperm head contains the nucleus and the acrosomal cap.  The acrosome has a set of enzymes: hyaluronidase and protease.  The nucleus contains 23 pairs of chromosomes (1n) In the connecting part of the caudal region, there are centrioles: proximal The tail was formerly thought to move symmetrically in a helical shape. The neck or connecting piece contains one typical centriole and one atypical centriole such as the proximal centriole-like.The midpiece has a central filamentous core with many mitochondria spiralled around it, used for ATP production for the journey through the female cervix, uterus, and uterine tubes. During fertilization, the sperm provides three essential parts to the oocyte: (1) a signalling or activating factor, which causes the metabolically dormant oocyte to activate; (2) the haploid paternal genome; (3) the centriole, which is responsible for forming the centrosome and microtubule system Female reproductive cell An ovocyte is a large, immobile cell The number of oocytes is significantly less than sperm Ovocyte has a nucleus (X chromosome) The cytoplasm contains SER, CG, ribosomes, mitochondria, cortical granules and vitelline inclusions  Oogenesis Yolk granules contain proteins, phospholipids and carbohydrates. The ovocyte has a number of membranes: oolemma, The zona pellucida (consists of glycoproteins and glycosaminoglycans). The "corona radiata" is represented by oocyte microvilli and follicular cell processes the granular membrane consists of follicular cells. The egg (and corona radiata) at ovulation Corona radiata Zona pellucida (ZP-1, -2, and -3) Cortical granules  Oogenesis Vs Spermatogenesis Similarities PGC originate from the same source and at the same time. Occurs in sex cells. Both undergo two reduction divisions-meiosis. Cells from columnar epithelium contribute to form supportive cells-sertoli cells in males and follicular cells in females.  Oogenesis Vs Spermatogenesis Differences Oogenesis Spermatogenesis Differentiation starts in IU Starts at puberty. life. Both divisions are Meiosis is completed only completed before if fertilization occurs. release of sperms. Duration-64 days. Cells may remain Equal spermatids. dormant for years. Spermatocytes are of Cytokinesis is not equal- two types-23x & 23y. one main cell and one polar body are formed. Secondary oocytes are alike -23x. liquids from the prostate gland and seminal vesicles are added, which help dilute the concentration of sperm and provide a suitable environment for them. Fluids contributed by the seminal vesicles are approximately 60 percent of the total semen volume; these fluids contain fructose, coagulasis, amino acids, citric acid, phosphorus, potassium, and hormones known as prostaglandins. The prostate gland contributes about 30 percent of the seminal fluid; the constituents of its secretions are mainly citric acid, acid phosphatase, calcium, sodium, zinc, potassium, protein-splitting enzymes, and fibrolysin (an enzyme that reduces blood and tissue fibres). A small amount of fluid is secreted by the bulbourethral and urethral glands; this is a thick, clear, lubricating protein commonly known as mucus. Sperm swimming penetrates the Corona Radiata Sperm binding to the zona pellucida – Fertilization ZP3 is critical Binding induces the Acrosomal reaction Membrane fusion Everts Acrosomal sac - releases hydrolytic enzymes (acrosin is a membrane bound serine protease) Cell membrane fusion Prevention of polyspermy fast block - membrane depolarization slow block - Ca++ mediated cortical granule vesicle fusion with membrane; hydrates perivitelline space, zona pellucida elevates Metabolic activation of egg - Ca++ mediated – increase intracellular pH  This so-called cortical reaction prevents other sperm from fertilizing the egg (aka “polyspermy”) Cortical granule enzymes digest ZP proteins so other sperm can no longer bind. Hyaluronic acid and other proteoglycans are also released that become hydrated and swell, thus pushing the other sperm away. Fertilization Meiosis II complete Formation of male and female pronuclei Decondensation of male chromosomes Fusion of pronuclei Zygote Transport through the oviduct At around the midpoint of the menstrual cycle (~day 14), a single egg is ovulated and swept into the oviduct. Fertilization usually occurs in the ampulla of the oviduct within 24 hrs. of ovulation. Series of cleavage and differentiation events results in the formation of a blastocyst by the 4th embryonic day. Inner cell mass generates embryonic tissues Outer trophectoderm generates placental tissues  Cleavage A variant of mitotic division, in which daughter cells (blastomeres) are formed without further growth to the size of the mother. Blastomeres do not diverge and form morula  Cleavage types: Complete and incomplete Uniform and uneven Synchronous and asynchronous Cleavage depends on the type of egg that has been fertilized. 2 Cell Stage Individual cells = blastomeres Mitotic divisions maintain 2N (diploid) complement Cells become smaller Blastomeres are equivalent (aka totipotent). 4 cell; second cleavage 4 equivalent blastomeres Still in zona pellucida 8 Cell; third cleavage Blastomeres still equivalent Embryo undergoes compaction after 8-cell stage: first differentiation of embryonic lineages Caused by increased cell-cell adhesion Cells that are forced to the outside of the morula are destined to become trophoblast--cells that will form placenta The inner cells will form the embryo proper and are called the inner cell mass (ICM).  Formation of the blastocyst Sodium channels appear on the surface of the outer trophoblast cells; sodium and water are pumped into the forming blastocoele. Note that the embryo is still contained in the zona pellucida. Early blastocyst Later blastocyst Day 3 Day 5 inner cell mass blastocoele  “Hatching” of the blastocyst: preparation for implantation Hatching of the embryo from the zona pellucida occurs just prior to implantation. Occasionally, the inability to hatch results in infertility, and premature hatching can result in abnormal implantation in the uterine tube. Implantation  Implantation Days 6 –12 Adhesion, blastocyst to endometrium Trophoblast proliferation Syncytiotrophoblast Secretion of hydrolytic enzymes Breakdown of endometrium  Implantation and placentation (day 8) Trophoblast further differentiates and invades maternal tissues – Cytotrophoblast: stem cell population – Syncytiotrophoblast: invasive fused cells (syncytium) derived from cytotrophoblast – Breaks maternal capillaries, trophoblastic lacunae fill with maternal blood Inner cell mass divides into epiblast and hypoblast: – Epiblast contributes to forming the overlying amniotic membrane and amniotic cavity – Hypoblast contributes to forming the underlying yolk sac.  Implantation and placentation (day 9) Trophoblast further differentiates and invades maternal tissues – Cytotrophoblast: stem cell population – Syncytiotrophoblast: invasive fused cells (syncytium) derived from cytotrophoblast – Breaks maternal capillaries, trophoblastic lacunae fill with maternal blood Inner cell mass divides into epiblast and hypoblast: – Epiblast contributes to forming the overlying amniotic membrane and amniotic cavity – Hypoblast contributes to forming the underlying yolk sac.  Implantation and placentation (day 12) Trophoblast further differentiates and invades maternal tissues – Cytotrophoblast: stem cell population – Syncytiotrophoblast: invasive fused cells (syncytium) derived from cytotrophoblast – Breaks maternal capillaries, trophoblastic lacunae fill with maternal blood Inner cell mass divides into epiblast and hypoblast: – Epiblast contributes to forming the overlying amniotic membrane and amniotic cavity – Hypoblast contributes to forming the underlying yolk sac. Implantation and placentation (day 13) Trophoblast further differentiates and invades maternal tissues – Cytotrophoblast: stem cell population – Syncytiotrophoblast: invasive fused cells (syncytium) derived from cytotrophoblast – Breaks maternal capillaries, trophoblastic lacunae fill with maternal blood Inner cell mass divides into epiblast and hypoblast: – Epiblast contributes to forming the overlying amniotic membrane and amniotic cavity – Hypoblast contributes to forming the underlying yolk sac.  Week 3: Days 14-21 Two layer germ disc Primitive streak forms Gastrulation forms tri-laminar embryo Neural induction Left-right asymmetry 0.4mm - 2.0mm  Gastrulation At gastrulation the two layered epiblast is converted into the three primary embryonic germ layers: – Ectoderm: outside, surrounds other layers later in development, generates skin and nervous tissue – Mesoderm: middle layer, generates most of the muscle, blood and connective tissues of the body and placenta – Endoderm: eventually most interior of embryo, generates the epithelial lining and associated glands of the gut, lung, and urogenital tracts Thank you