Molecular BASIS OF FERTILIZATION

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1 COLLEGE OF HEALTH SCIENCE DEPARTMENT OF PHYSIOLOGY PRESENTATION ON: Molecular BASIS OF FERTILIZATION By TEKETEL ERISTU Kediso 1

2 Presentation Outline Introduction Fertilization Types of Fertilization Cellular Events Sperm Transport and Capacitation Ovum Reaction to Sperm Penetration References 2

3 Objectives At the end of the presentation you will be able to: Define Fertilization Know the types of Fertilization Describe Cellular Events Explain Sperm Transport and Capacitation Discuss ovum reaction to Sperm Penetration 3

4 INTRODUCTIO Fertilization Father s Sperm Mother s Egg Child s Cell 4

23 46 Early Embryo 46 46 46 46 46 46 46 46 46 46 46 46 46 46 46 46 sperms produced by meiosis 23

5 Fertilization: Union of a Spermatozoal nucleus, of paternal origin, with an egg nucleus, of maternal origin, to form the primary nucleus of an embryo Early Embryo sperms produced by meiosis 23 fertilization zygote 23 sperm mother cell ovum mother cell ova produced by meiosis but only one develops to maturity 5

6 CONT 6

7 Types of Fertilization 1.External Fertilization: is common in aquatic animals Parents release their gametes into the environment 7

8 CONT. Internal Fertilization occurs in nearly all terrestrial animals Sperm are deposited in or close to the female reproductive tract Gametes unite within the tract following mating 8

9 1.3 Cellular Events a) Ovum Maturation Cell Type Ploidy Process Oogoniu diploid Oocytogenesis m (mitosis) Process Completion 3rd trimester (forming Oocytes) Primary diploid Ootidogenesis Halts in prophase 1 Oocyte (Meiosis 1) (Folliculogenesis) Seconda haploid Ootidogenesis ry Ocyte (Meiosis 2) Ovum haploid Halted in metaphase 2 untill fertilization 9

10 CONT 2N (diploid) cell replicates its DNA, resulting in a 4N cell (meiosis I) 2N cells (meiosis II) 1N (haploid) cells 10

11 b) TransportThree Stages: 1. Ovulation Cont... Expulsion of the oocyte Captured the Ciliated fimbriated end of the fallopian tube The Ciliary action and Peristaltic waves of oviduct musculature bring the ovum into the ampulla of the fallopian tube Fertilization Normally occurs in the ampulla ( ampularyisthmic junction) of fallopian tube and Usually within 24 hrs of after ovulation. 11

12 CONT b.transportation (utero-tubal Junction, junction b/n Uterus and Oviduct) and Implantation of the Zygote C.Utetro tubal junction uterine cavity 12

13 Cont. 1. Follicle in Ovary is ready to ovulate 2. New corpus Luteum 3. Ovulated Ovum in the second meiotic division 4. Formation of second polar body after fertilization 5. Fusion of egg and sperm Pronuclear; and 6. Beginning of first mitotic division of zygote. 13

14 1.4 Sperm Transport and Capacitation a. Transport: Velocity = 1-4mm/min (300 x sperms reach the oviduct). Flaggelar movement (to-and-fro) + ATP The sperm are first deposited in the vagina: they then pass up this cavity and through the cervix into the uterus, up the uterus, through the junction between the uterus and oviduct (Uterotubal Junction), and up the isthmus of the oviduct to the usual area of fertilization in the oviduct: the ampullary isthmic junction. 14

15 Cont 1.Vaginal Sperm After about one minute of deposition in the Vagina the semen becomes thicker and less liquid ( Coagulation). After about 20 minutes, the semen again liquefies, Stimulates some sperm to swim more rapidly. 15

16 Cont The environment in the vagina is usually acidic ( about PH 4.2) and this level of acidity inhibit semen motility. The presence of semen in the vagina, however, increases the vaginal PH to a basic 7.2, which in turn increase sperm motility. 16

17 Cont 2. Cervical Sperm The cervical canal is lined by a complicated series of narrow folds and crypts and is blocked by a sticky mass of cervical mucus and tiny cervical fibers. Estrogen cause the mucus to become more liquid and makes gaps wider. 17

18 CONT The cervical fibers vibrate in rhythm with the tail beat frequency of normal sperm, this may allow the normal sperm to move through the cervix; where as sperm with abnormal or absent tail beats are detained. Other sperms enter cervical Crypts, where they may die or lost or they remain as a reservoir of sperm that may enter the uterus. 18

19 Cont 3. Uterine Sperm Upon leaving the cervix, the sperm travel up the uterus to the uterotubal junction. The uterus fluid is watery but sparse The swimming rate of sperm (3mm/min) cannot account to climb up uterine lumen. 19

20 CONT.. The muscle contraction and movement of cilia in the female reproductive tract that facilitate sperm transport. Uterotubal junction allows the gradual entrance of sperm in the isthmus of the oviduct. About half of the sperm enter the wrong oviduct and only a few hundred make it to the general proximity of the waiting egg. 20

21 Cont 21

22 b.capacitation Sperms cannot fertilize oocytes when they are newly ejaculated. Thus Capacitation Is a process of sperm becoming fertile a process of essential changes in the spermatozoa that enables them to fertilize the egg. occurs during transport in female genital tract capacitated sperm have the ability to fertilize the egg The process of capacitation takes 5-7 hours. 22

23 Cont. Major Changes in sperm cell membrane during Capacitation Changes in surface glycoproteins, caused by secretion of FGT Cholesterol is removed- increase Fluidity Glycoproteins are lost- expose zona binding proteins Proteins are phosphorylated Fluctuations occur in the intracellular levels of calcium ionsessential for hyperactivation. 23

24 CONT. Effects of Capacitation on Sperm Increased rate of metabolism Hyperactivation: flagellum beats more rapidly Changes in sperm glycoproteins allow sperm egg binding Pro-Acrosin (inactive) is converted to acrosin (active) 24

1.5 Ovum Reaction to sperm penetration Zona Binding Zona proteins: ZP1, ZP2, ZP3 Initiates acrosome reaction a) Acrosomal Reaction After binding to the corona radiata the sperm reaches the

25 1.5 Ovum Reaction to sperm penetration Zona Binding Zona proteins: ZP1, ZP2, ZP3 Initiates acrosome reaction a) Acrosomal Reaction After binding to the corona radiata the sperm reaches the zona pellucida. Sperm head binds to a ZP2 glycoprotein in the zona pellucida. This binding triggers the acrosome to burst, releasing enzymes that help the sperm get through the zona pellucida. 25

26 Cont The acrosome reaction is associated with the release of acrosome enzymes that facilitate fertilization Acrosomal enzymes: esterases, acrosin,hyaluronidase,and neuraminidase cause lysis of the zona pellucida 26

27 On binding sperm is induced to undergo the acrosomal reaction (ZP3) Influx of Ca2+ into the sperm cytosol Contents of the acrosome are released (+ exocytosis) i. Proteinases/acrosin) + hyaluronidase penetration of ZP. ii. Exposes other proteins on the sperm surface that bind to ZP2 help the sperm maintain the binding to zona iii.exposes a protein in the sperm plasma membrane that mediates the binding and fusion of this membrane with that of the egg. Penetration thru ZP 15-25min. 27

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29 Cont 29

30 Cont 30

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B. Cortical Reaction Once sperm penetrates zona pellucida, the zona reaction occurs: This reaction makes the zona pellucida impermeable to other sperms.

32 B. Cortical Reaction Once sperm penetrates zona pellucida, the zona reaction occurs: This reaction makes the zona pellucida impermeable to other sperms. Egg Activation Cortical Reaction exocytosis of cortical granules Zona block -biochemical changes eliminates sperm binding This prevents fertilization of an egg by more than one sperm. 32

33 Cont Mechanisms: Two mechanisms can operate to ensure that only one sperm fertilizes the egg. 1.Primary block to polyspermy Rapid depolarization of the egg plasma membrane prevents further sperm from fusing and thereby acts as a fast primary block to polyspermy. But the membrane potential returns to normal soon after fertilization, so that a second mechanism is required to ensure a longer-term, secondary block to polyspermy 33

34 ii. Secondary block to polyspermy A local increase in cytosolic Ca2+ (spreads in a wave). CONT prolonged Ca2+ oscillations. Activate the egg to begin development, The cortical granules release their contents by exocytosis Proteolytic cleavage of ZP2 and the hydrolysis of sugar groups on ZP3 The zona becomes hardened, so that sperm no longer bind to it. 34

35 35

36 C. Nuclear Reaction Cortical reaction pulls the sperm nucleus. Loses the membrane that surround the head. Tail incorporated into the egg cytoplasm. Naked nucleus male pronucleus Approaches haploid nucleus of the ovum female pronucleus Male pronucleus + female pronucleus diploid zygote GENOME of a NEW ORGANISM mitotic process. 36

37 2 References 1. Human Reproductive Biology Richard E.JONES, 3rd edition. 2. Advanced Reproductive Physiology Lecture notes, Dr. Getahun 3. Different internet Sources 37

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Internal Fertilization

Internal Fertilization Fertilization which takes place inside the female body is called internal fertilization(the union of the gametes within the female body after insemination) Occurs in the widest part