fertilization for public health students HO2016.pptx

Fertilization:
 The process of fusion or union of the spermatozoon with the
mature ovum is known as conception /fertilizaiton/
impregnantation.

1. Fertilization B y: Zekarias B. (MSc. Clinical Anatomy)

2. Fertilization Chapter 3 3/1/2024 Fertilization 2

3. FERTILIZATION Definition: The process of fusion of two mature germ cells, an ovum and a spermatozoon to form a single cell zygote . Site of fertilization: ─The ampulla of fallopian tube which is the widest portion. ─ Lateral third of fallopian tube. 3/1/2024 Fertilization 3

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5. Abnormal sites: ─ Peritoneal cavity ─ Ovarian follicle Pecularities : Two highly specialized haploid gametes are fused resulting in the formation of most unspecialized or undifferentiated diploid zygote. 3/1/2024 Fertilization 5

6. Phases of Fertilization Passage of a sperm through the corona radiata . Penetration of the zona pellucida . Fusion of plasma membranes of the oocyte and sperm. Completion of the second meiotic division of oocyte and formation of female pronucleus . Formation of the male pronucleus . As the pronuclei fuse into a single diploid aggregation of chromosomes, the ootid becomes a zygote. 3/1/2024 Fertilization 6

7. Mechanism of fertilization is complex, for the descriptive purpose it can be studied through following stages, - Approximation of the gametes - Contact & fusion of the gametes - Effects of fertilization 3/1/2024 Fertilization 7

8. Approximation of the gametes: It involves the transport of the sperms and ova in the female genital tract to reach the uterine tube. 3/1/2024 Fertilization 8

9. Cont.. During the ascent of spermatozoa through female genital tract the spermatozoa are gradually reduced in number by barriers provided by abrupt constrictions at the cervix & uterine ostium of the tube. As a result remarkably reduced number of sperms(about 300-500 sperms) appears at the site of fertilization. 3/1/2024 Fertilization 9

10. Cont … Most of the sperms die within 24hrs after ejaculation. Transport of the oocyte into the uterine tube is encouraged by the ciliary beats & rhythmical contractions of the musculature of uterine tube. 3/1/2024 Fertilization 10

11. 2.Contact & fusion of the gametes A direct contact between the spermatozoa & zona pellucida of secondary oocyte is necessary before actual fusion of the gametes. Three barriers protect the female gamete before penetration by the fertilizing sperm. The barriers are provided by the following, 1)Corona radiata 2) Zona pellucida 3) Vetilline membrane 3/1/2024 Fertilization 11

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13. Cont .. Prior to penetration the spermatozoa undergo a process of capacitation , followed by acrosome reaction in the female genital tract. Capacitation : Increase the activity of the sperm Acrosome reaction: Enzyme contained in the Acrosome released and react with the zona pellucid 3/1/2024 Fertilization 13

14. Not in mammals 3/1/2024 Fertilization 14

15. Cont.. The cytoplasm of mature ovum contains two pronuclei . The sperm head with its nuclear envelop . It contains 1N haploid chromosomes is somewhat swollen to form male pro nucleus . In the same way the nucleus of mature ovum forms female pronucleus . At this stage both male & female pronuclei also called ootids approach each other for fusion. 3/1/2024 Fertilization 15

16. A. Scanning electron micrograph of sperm binding to the zona pellucida . B. The three phases of oocyte penetration. ► In phase 1, spermatozoa pass through the corona radiate barrier; ► In phase 2, one or more spermatozoa penetrate the zona pellucida ; ► In phase 3, one spermatozoon penetrates the oocyte membrane while losing its own plasma membrane. Inset shows normal spermatocyte with acrosomal head cap. 3/1/2024 Fertilization 16

17. RESULT OF FERTILIZATION 1)Completion of second meiotic division of the female gamete Immediately after the entrance of a spermatozoon into the secondary oocyte, the latter completes the second maturation division. 3/1/2024 Fertilization 17

18. Cont … 2)Restoration of diploid number of chromosomes in the zygote The head of the spermatozoon containing haploid chromosomes swells and form male pronucleus Both male and female pronuclei each containing 1N DNA complement replicate their DNA to form DNA duplex before their actual fusion . 3/1/2024 Fertilization 18

19. Cont.. The male and female pronuclei meet each other near the centre of the ovum and the nuclear membrane disappear. This results in the nuclear fusion with the restoration of the diploid number of chromosome which is constant for the species . 3/1/2024 Fertilization 19

20. 3)Determination of chromosomal sex Out of total sperm population half of them contain x bearing chromosomes and the remaining half y bearing chromosomes. When an X bearing spermatozoon fertilize an ovum the zygote contains 2X chromosomes plus 44 autosomes & a female child is formed . If the ovum is fertilized by Y bearing spermatzoon the sex of zygote will be male containing XY chromosomes plus 44 autosomes . Therefore the father decides the chromosomal sex of the off spring. 3/1/2024 Fertilization 20

21. 4)Initiation of cleavage division Fertilization provides energy for the repeated mitotic division of the single celled zygote. It results in the formation of a mass of cells called the morula . 3/1/2024 Fertilization 21

22. The line of entry of the spermatozoon probably decides the cephalo caudal polarity of the zygote. Also bilateral symmetry of the embryo . 6)Embryo contain only maternal mitochondria because sperm mitochondria are dispersed in cytoplasm of ovum and discarded . RESULT OF FERTILIZTION ON OVARY – the corpus luteum changes to corpus luteum of pregnancy. ON UTERUS – the secretory changes starts in endometrium. ON ZYGOTE – cleavage division starts. 3/1/2024 Fertilization 22 5)Determination of the polarity of the embryo