Bio 415 lecture 2 Fertilization

1. Bio 415 lecture 2Fertilization بسم الله الرحمن الرحيم

2. Fertilization • Fertilization is part of mating or copulation between male and female. • Fertilization is the fusion of the sperm nucleus with the ova nucleus to produce a zygote (fertilized egg). • Fertilization brings the haploid nuclei of sperm and egg together, forming a diploid zygote. • The sperm’s contact with the egg’s surface initiates metabolic reactions in the egg that trigger the onset of embryonic development.

4. Chromosomes in Human Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes. A diploid cell (2n) has two sets of chromosomes, Where the diploid number is 46 (2n = 46).

5. Sex Determination • The sex chromosomes in mammals are called X and Y. • Human females have a homologous pair of X chromosomes (XX) • Human males have one X and one Y chromosome. • A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n). • For humans, the haploid number is 23 (n = 23) • Each set of 23 consists of 22 autosomes and a single sex chromosome • In an unfertilized egg (ovum), the sex chromosome is X • In a sperm cell, the sex chromosome may be either X or Y

6. Fertilization in animal kingdom There are two types of fertilization (external and internal). External fertilization in amphibians and fish

7. Fertilization in animal kingdom

8. Internal fertilization in mammals • Fertilization in mammals occurs in the oviduct. • The ova is viable for approximately 24 hours after ovulation.

9. What are the factors that affect the sperm’s potential for successfully fertilizing the egg? 1- capacitation of sperm. 2- Structures surrounding the egg – barriers.

10. 1- Capacitation of sperm • It is the process that allows the acrosome reaction to occur, so the sperm can penetrate the zona pellucida of the oocyte. • The physiological changes that confer on the sperm the ability to fertilize are called capacitation. • Capacitation includes multiple physiological and biochemical modifications. • occurs in the female’s vagina. Vaginal secretions cause a molecular change in the sperm plasma lemma (removal of decapacitating factor - semen proteins, results in increased membrane fluidity,). • Takes 4-5 hr. in humans, 1 hr. in mice, 6 hr. in rabbits.

11. 1- Capacitation of sperm • The biochemical changes associated with the capacitation process include: • an efflux of cholesterol from the plasma membrane leading to an increase in membrane fluidity and permeability to bicarbonate and calcium ions. • hyperpolarization of the plasma membrane, changes in protein phosphorylation and protein kinase activity • increases in bicarbonate (HCO3) concentration and intracellular pH, Calcium and cyclic adenosine monophosphate (cAMP) levels. • Galactosyltransferase enzyme is a protein in the sperm plasma lemma that binds to the zona pellucida 3 protein (ZP3) receptor around the ova and initiates the acrosome reaction.

12. 1- Capacitation of sperm

13. Effects of Capacitation on Sperm • Increased rate of metabolism. • Flagellum beats more rapidly; Result: Sperm are more motile. • Changes in sperm plasma lemma proteins allow sperm-egg binding and occurrence of the acrosome reaction. • Pro-Acrosin (inactive) is converted to acrosin (active). • Sperm become capable of chemotaxis.

14. 2- Structures surrounding the egg - barriers • Follicle cells. • Zona pellucida (ZP)in mammals. • (vitelline membrane in non-mammals). • Oolemma (plasma membrane of ova).

15. Structure of Ovum (egg) • Ovum is a larger nutritive cell. Like an animal cell it also contains all cell organelles. • Its size varies in different animals. Ovum is generally round/spherical in shape. • It is covered by a thin membrane known as plasma membrane. Beside this, it is also covered by other covering membranes known as egg membranes.

16. Egg membranes These are divided into three-1. Primary egg membrane- Secreted by ovum itself.( viteline .m) 2. Secondary egg membrane- Secreted by ovarian tissue like follicular membrane. 3. Tertiary egg membrane- Secreted or generated by various regions of uterus or oviduct. Importance of the egg membranes:-These membranes provide nutrition and protection for developing embryo.

17. Fertilization Stage The acrosome reaction. The Penetration of the zona pellucida. Fusion of plasma membranes of oocyte and sperm. 2nd meiotic division of oocyte is completed Formation of male and female pronuclei. Membranes of the pronuclei break down, chromosomes condense and arrange themselves for mitotic cell division.

18. Fertilization 1- Acrosome reaction • The acrosome reactionmust be completed before the sperm can fuse with the secondary oocyte. • Occurs when sperms come into contact with the corona radiata of the oocyte • Perforations develop in the acrosome • Point fusions of the sperm plasma membrane and the external acrosomal membrane occur • The acrosome reaction is associated with the release of acrosome enzymes that facilitate fertilization • Passage of sperm through the corona radiata depends on enzyme action: • Hyaluronidase released from sperm acrosome • Tubal mucosal enzymes • Flagella action also aids corona radiata penetration

19. Acrosome Reaction

20. Ovum and Sperms: (in vitro) From this photograph, it should be clear that the heads of human sperm are less than 1/20 the diameter of human eggs.  Arrows point to sperm heads Advanced Fertility Center of Chicagohttp://www.advancedfertility.com/

21. Fertilization2- penetration of the zona pellucida • Penetration of the zona pellucida around the oocyte: • Acrosomal enzymes: esterases, acrosin, and neuraminidase cause lysis of the zona pellucida. • Once sperm penetrates zona pellucida, the zona reaction occurs: fertilization membrane will form. • This reaction makes the zona pellucida impermeable to other sperms. Block to poly spermy. • When more than one sperm manages to enter the ovum (dispermy = 2; triploidy = 3 the called poly spermy), the fetus nearly always aborts.

22. Diagram of fertilization steps in mammals

23. Fertilization3- fusion of plasma membranes of oocyte and sperm • Fusion of plasma membranes of oocyte and sperm. • Head of a sperm enter the cytoplasm of the oocyte, but the sperm plasma membrane remains behind.

24. Fertilization4- second meiotic division of oocyte is completed • 2nd meiotic division of oocyte is completed. • The secondary oocyte was previously arrested in metaphase of the 2nd meiotic division, and now forms the mature ovum and another polar body.

25. Ova cortical granules reaction • Once the sperm cell membrane fused the ova membrane, the ova cortical granules fused with ova membrane and released its content to the out side of the ova in the perivitelline space. • The released materials from the cortical granule will form a hyaline layer between the ova membrane and the vitelline membrane. • This layer will prevent the entry of any other sperm to the ova and considered as the second bloke to polyspermy.

26. Diagram of fertilization steps in frog

27. Fertilization5- formation of male and female pronuclei • Formation of male and female pronuclei: • Chromosomal material of the sperm decondensates and enlarges • Chromosomal material of the ovum decondensates following the completion of meiosis • At this stage, the male and female pronuclei are indistinguishable. • As they grow, the pronuclei replicate their dna  still 1n (haploid)-( 23 chromosomes, each in chromatid pairs in human).

28. Fusion of the pronuclei (in vitro) • The male and female pronuclei are indistinguishable from one another. • The second polar body can be seen (blue arrow). • The plasma membranes of the two pronuclei are dissolving and one diploid nucleus will remain. Advanced Fertility Center of Chicagohttp://www.advancedfertility.com/

29. Fertilization6- membranes of the pronuclei break down • Membranes of the pronuclei break down, chromosomes condense and arrange themselves for mitotic cell division • On membrane dissolution, there is 1 cell with 46 chromosomes = diploid (2N) in human • The first cleavage follows shortly, leaving 2 cells, each with 46 chromosomes in human. • Mitosis in the new zygote uses centrioles derived from the sperm. • The oocyte has no centrioles.

30. Diagram of fertilization steps

31. Fertilization facts • Completed within 24 hours of ovulation in human • Approximately 400 to 600 MILLION sperms are deposited at cervical opening during ejaculation. • Some sperm are held up by the folds of the cervix and are gradually released into the cervical canal; this gradual release increases the chances of fertilization. • Most human sperms do not survive longer than 48 hours in the female genital tract. • Only about 200 sperms reach the fertilization site; most degenerate and are absorbed by the female genital tract.

32. The results of fertilization • Stimulates the secondary oocyte to complete meiosis. • Restores the normal diploid number of chromosomes (46 in human). • Results in variation of human species as maternal and paternal chromosomes intermingle. • The embryo contains only maternal mitochondria because the sperm mitochondria are dispersed into the egg cytoplasm and discarded. • Determines the sex of the embryo. • The sex chromosome (y or x) carried by the successful sperm determines embryonic sex.

33. Summary of biochemical interaction during fertilization

34. After fertilization, embryonic development proceeds through cleavage, gastrulation, and organogenesis • Important events regulating development occur during fertilization and the three stages that build the animal’s body • Cleavage: cell division creates a hollow ball of cells called a blastula • Gastrulation: cells are rearranged into a three-layered gastrula • Organogenesis: the three layers interact and move to give rise to organs

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