Physiology of conception (gametogenesis)

1. Physiology of conception(gametogenesis) Dr Pooja Rajbhara Senior Resident OBGY

2. Learning Objectives • Explain the relationship between differentiation and development and specify the various stages of development. • Brief gametogenesis. • Describe the process of fertilization. • List the three prenatal periods and describe the major events associated with each.

3. Differentiation and development • Development involves: 1) Cell division and differentiation • Differentiation is the creation of different types of cells 2)  changes in anatomical structures • Anatomical changes include gradual modification of physical and physiological characteristics • Development begins at fertilization

4. Stages of development Development can be divided into: Prenatal and postnatal development • Prenatal development begins at fertilization and ends with birth • Prenatal development includes: • Embryological development Changes occurring during the first two months after fertilization • Fetal development Begins at the start of the ninth week and continues until birth • Postnatal development • Commences at birth and continues to maturity

5. Gametogenesis • The process involved in the maturation of two highly specialized cells, spermatozoon in male and ovum in female before they unite to form zygote, is called gametogenesis. • It includes: Oogenesis Spermatogenesis

7. Fertilization (conception) • Fertilization is fusion of two haploid gametes (egg and sperm) each with 23 chromosome to produce a zygote that contains 46 chromosomes • Fertilization occurs in the uterine tubes • Within a day of ovulation • Spermatozoa cannot fertilize an ovum until after capacitation

8. Fertilization

10. Fertilization Steps • Step 1 Ovulation • At ovulation the oocyte is in metaphase of meiosis II • Both the occyte and the polar body is surrounded by corona radiate • Step 2 Fertilization • Oocyte is surrounded by the sperms • Acrosomal enzyme from several sperms creates gaps in corona radiata • One sperm makes contact with the oocyte membrane • Sperm and oocyte fused • The process of meiosis will complete

11. Fertilization

13. The Oocyte at Ovulation • Ovulation occurs before the oocyte is completely mature i.e. oocyte is ovulated before the process of meiosis is completed • Ovulated oocyte is in metaphase of meiosis II • If the egg is fertilized the process of meiosis will complete • If fertilization does not occur, the oocyte disintegrate without completing the process of meiosis • Oocyte is surrounded by the corona radiate • Spermatozoa release hyaluronidase and acrosin • Enzymes required to penetrate corona radiate • Single spermatozoan contacts oocyte, fertilization begins • Oocyte activation • Polyspermy prevented by membrane depolarization and cortical reaction

14. The Stages of Prenatal Development Embryonic and Fetal Periods • Induction • During prenatal development differences in cytoplasmic composition trigger changes in genetic activity • Gestation periods • The gestation period is divided into three integrated trimesters

15. The First Trimester • The first trimester is the period of embryological and early fetal development • Four processes occur during the first trimeter • 1) Cleavage • 2) Implantation • 3) Placentation • 4) Embryogenesis

16. The First Trimester Cleavage • Cleavage - cleavage is the first cell division • Zygote becomes a preembryo then a blastocyst • Blastocyte includes: • Trophoblast – outer layer of cells • Inner cell mass – cluster of cells at one end of blastocyst

17. Cleavage and Blastocyst Formation

18. The First Trimester Implantation and Placentation • Implantation • Attachment of blastocyst into the uterine endometrium • Occurs about 7 days after fertilization • Placentation • Blood vessels form around blastocyst and placenta develops • The placenta is a complex organ that permits exchange between the maternal and embryonic circulatory systems

19. Stages in Implantation Animation: Location of the Endometrium (see tutorial)

21. Thank you