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BIOCHEMICAL CHANGES DURING OOGENESIS

GOMATHI.MMSc ZOOLOGY

BHARATHIAR UNIVERSITY

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OBJECTIVES

• INTRODUCTION• OOGENESIS• PROCESS OF OOGENESIS• BIOCHEMICAL CHANGES DURING OOGENESIS

1. Changes during proliferative phase2. Changes during Growth phase3. Changes during Maturation phase

• SIGNIFICANCE OF OOGENESIS• CONCLUSION

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REPRODUCTION

• What type of cell division takes place?

• MEIOSIS + MITOSIS

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GAMETOGENESIS

• GAMETES :Oocytes and Spermatozoa (n)

OOGENESIS SPERMATOGENESIS

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OOGENESIS

• Production and maturing of ovum. • The differentiation of the ovum • “CREATION OF HAPLOID EGG CELLS USING

MEIOSIS”

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BEGINS BEFORE BIRTH

ENDS AT MENOPAUSE

ACCELERATES AT PUBERTY

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PRIMORDIAL GERM CELLS (PGCs)

GAMETES

GAMETOGENESIS: OOGENESISDURING 2ND WEEK OF EMBRYO PGC FORMED IN EPIBLAST 4TH WEEK PGC BEGINS TO MIGRATE FROM THE YOLK SAC 5TH WEEK MIGRATION ENDS IN DEVELOPING GONADS

(OVARY) DIFFERENTIATE INTO OOGONIA - 7 MILLION 6TH WEEK CELL DEATH BEGINS -MAJORITY OF OOGONIA

DEGENERATED

7TH WEEK REMAINING PRIMARY OOCYTES HAVE ENTERED PROPHASE OF MEIOSIS I – DIPLOTENE STAGE (RESTING PHASE)

• Oogonia divide rapidly from the second to the seventh month of gestation to form roughly 7 million germ cells. After the seventh month number of germ cells drops precipitously.

AT BIRTH 2 MILLION PRIMARY OOCYTES BEGINNING OF PUBERTY 400,000 TILL 50 YEARS (MENOPAUSE) 500 ONLY OVULATED

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• Oogonia divide rapidly from the second to the seventh month of gestation to form roughly 7 million germ cells. After the seventh month number of germ cells drops precipitously.

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PROCESS OF OOGENESIS

MULTIPLICATION PHASE

GROWTH PHASE

MATURATION PHASE

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BIOCHEMICAL CHANGES DURING OOGENESIS

• NUCLEUS• CYTOPLASM• RNA• YOLK – PROTEINS• HORMONES- GONADOTROPIN HORMONES ,

OVARIAN HORMONES

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BIOCHEMICAL CHANGES

• Gametes formed by oogenesis contains all the materials needed to initiate and maintain metabolism and development.1. Form haploid nucleus,2. All the organelles involved in fertilization have to be constructed,3. All the mRNAs & proteins have to positioned properly in the

oocyte,4. All the membrane proteins involved in coordinating the

interactions with sperm have to be synthesized and in place,5. The accumulated material in the oocyte cytoplasm includes energy

sources and energy –producing organelles(yolk & mitochondria)6. The enzymes and precursors for DNA, RNA and protein synthesis,7. Stored messenger RNAs,8. Structural proteins,9. Morphogenetic regulatory factors that control early embryogenesis

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CELLULAR COMPONENTS STORED IN THE MATURE OOCYTE OF XENOPUS LAEVIS

COMPONENT APPROXIMATE EXCESS OVER AMOUNT IN LARVAL CELLS

MITOCHONDRIA 100,000

RNA POLYMERASES 60,000 -100,000

DNA POLYMERASES 100,000

RIBOSOMES 200,000

TRNA 10,000

HISTONES 15,000

DEOXYRIBONUCLEOSIDE TRIPHOSPHATES

2,500

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• Oogenesis vary among species to species.• E.g. Sea urchin & Frogs - routinely produces

hundreds or thousands of eggs at a time.• Humans and most mammals - only a few eggs are

produced during the lifetime of an individual.Most of the biochemical changes takes place in the growth

phase

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1) MULTIPLICATION PHASE

MITOSIS MITOSIS• PGCs OOGONIA 1⁰ OOCYTE

• No conspicious changes have been traced upto the formation of primary oocyte.

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2)GROWTH PHASE

1. Oocyte increases in size

• Nutrients & other materials synthesized, then these substances accumulate in the cytoplasm.Young oocyte of frog - 50µ in diameterMature oocyte of frog- 20-40 times larger.Hen’s oocyte- 200 times larger.Mouse oocyte- 43 times larger.

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2. Period of growth: Frog 3 years oogenesis – first 2 years size increase

gradually then 3rd year accumulation of yolk increases its size.

Human – 12 or 13 years

3. Stages of growth phase:i. Previtellogenesis (cytoplasmic & nuclear

materials grow- no yolk synthesis)ii. Vitellogenesis (synthesis of yolk )

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•

YOLK FORMATION

mRNAtRNArRNA

Nucleus increase in sizeNucleoli increase in

numberMitochondria increase in

numberCorticle granules in GB

Follicle Cells/Nurse Cells

PREVITELLOGENESIS VITELLOGENESIS

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CHANGES DURING PREVITELLOGENESIS

• Nuclear sap produced in a large amount – nucleus increases its size = Germinal vesicle.

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• Homologous chromosomes pair together.• Active mRNA synthesis – GENE TRANSCRIPTION IN AMPHIBIAN OOCYTES:• During the diplotene stage, certain

chromosomes stretch out large loops of DNA, causing the chromosome to resemble a lamp brush. These lamp brush chromosomes can be revealed as the sites of RNA synthesis by in situ hybridization.

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• Oocyte chromosomes can be incubated with a radioactive RNA probe, and autoradiography used to visualize the precise location where the gene is being transcribed.

Figure shows diplotene chromosome I of the newt Triturus cristatus after incubation with radioactive histone mRNA.It is obvious that a histone gene (or set of histone genes) is located on one of these loops of the Lampbrush chromosome (Old et al. 1977).

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VITELLOGENESIS

• Liver produce inactive/precursor vitellogenin• Transported through blood to ovary enzymes protein kinase

convert it into active vitellogenin- mitochondria

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• The growing diplotene oocyte is actively transcribing the genes for zona pellucida proteins ZP1, ZP2, and ZP3.

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MATURATION PHASE

• The diploid primary oocyte reduced into haploid & form ovum.

• 1st meiotic division – 1 polar body having small amount of cytoplasm

- larger cytoplasm having cell is known as secondary oocyte.2nd meiotic division – produce 3 polar bodies and a single ovum

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• The time of maturation varies in different species.

• It may occur after / before fertilization or at the time of fertilization.

• Sea urchin – before• Vertebrates – after• Acidian – at time of fertilization

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OOGENESIS IN MAMMALS

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HORMONAL CHANGES DURING OOGENESIS

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FSH promotes the growth & development of oocyteLH triggers ovulation FSH, LH promote meiotic maturation division of oocyte

and stimulate follicle cells to synthesis vitellogenin.

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• During the process of oogenesis, oocytes of many animal species undergo meiotic arrest prior to the completion of chromosomal reduction and it is in this state that they undergo tremendous growth. The length of time that oocytes remain in this arrested state and the nature of the stimulus which reinitiates meiosis are species dependent

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SIGNIFICANCE OF OOGENESIS

• WITHOUT OOGENESIS THERE IS NO FERTILIZATION, REPRODUCTION, LIFE…..

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THANK YOU

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REFERENCES

1. Scott F.Gilbert, 2009, Developmental Biology 8th edition, Sinauer Associates,inc.,Publishers,USA.

2. Berry Mitchell, Tam Sharma, 2005, “Embryology an illustrated color text 2nd edition, Churchill Livingstone Elsevier publishers, Pg: 1-10.

3. Mace Welene Vorlhac, Anne Villeneure, 2010, “Oogenesis, The Universal Process”, Wiley lackwell publishers, Pg: 38-41

4. Edwin F.Bartholonew, William C.Ober, Claire W.Garrison, Karhleen Welch, Talph T Hutchings, frederic H Matini, Judi.L.Nath, 2009, “Fundamentals fo Anatomy & Physiology” 8th edition, pearson, Benfamien cumming publication, Pg: 1061 – 1070.

5. Gary A. Jhibodeau, Kevin T.Patton, 2009, Anthong’s textbooks of Anatomy & Physiology 7th edition, moshy an Inprint & Elsevies publication, Pg: 941 – 945.

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REFERENCES

6.http://www.endotext.org/female/female1/female1.htm7.http://howmed.net/anatomy/embryology/oogenisis-and-ovarian-cycle/8.http://artificialinsemination.wordpress.com/about/reproductive-anatomy/ 9.http://www.womens-healthcare.org/articles/estrogen.html

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REFERENCES

• 8. http://buffonescience9.widispaces.com/UNIT+3-+cell+reproduction.

• 9. http:///pc1.clpccd.cc.ea.us//pc/zingg/anat/alecture/ach27flsldo11.htm.

• 10. http://www.fastbleep.com/biology-notes/32/159/859.

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