Primordial Germ Cells they undergo mitosis and form oogonia → undergo further division by mitosis to form the primary oocyte or become atretic (not mature and break down). At meiosis 1, the primary oocyte starts and is stopped at prophase 1, causing it to enter a resting stage until further stimulated.
1.PRENATAL
In the 4th month, the oogonia are grouped in clusters, surrounded by flat supporting cells in an ovary. Some show mitosis; others have differentiated into primary oocytes and entered prophase of the first meiotic division.
By 7th month, Almost all oogonia are transformed into primary oocytes in the prophase of the first meiotic division.
By the time of birth: there are no oogonia, each primary oocyte is surrounded individually by a single layer of follicular cells, forming the primordial follicle.
2.POSTNATAL
After birth at puberty, some primordial follicles are chosen and mature under hormones. → Resumes meiosis 1 so follicle cells change shape and size → Flat to cuboidal and form a red layer called Zona Pellucida and have granulosa cells.= primary follicle → Multilayer Primary follicle → Vesicular follicle (meiosis 1 is nearly done at this point) → in Mature vesicular ( compleated meiosis 1 )
3.This then develops into a secondary oocyte (starts meiosis 2 but ends in metaphases 2, not complete meiosis 2 unless fertilised) and polar body.
If secondary oocyte DOES become fertilised → forms mature oocyte and 3 polar bodies.. These polar bodies are used as a nutrient source
MALE
Primordial germ cells migrate into the gonads and go to testes.= Spermatogensis
PGC divide by mitosis to form spermatogonia → two types are A and B .
Type A = stem cells
Type B= Differentiate to primary spermatocyte by mitosis
primary spermatocyte then undergo meiosis 1 → form 2 secoundary spermocytes → Undergo meiosis 2 → form 4 spermatids
Sertoli cells aids this process.
Spermatogensis
Process of the sperm getting ready to undergo fertilisation.
Condensation of nucleus
Loss of excess cytoplasm
acrosomal cap is formed
3. Fertilisation
Results in the fusion of the male and female gametes.
Sperm goes thru 3 different stages.
Penetration of corona radiata
Penetration of zona pellucida
Fusion of the oocyte and sperm cell membranes
**mature ovum once ovulated is viable for only 24 hours while the mature sperms can stay alive for about 2-7 days So if an ovum is not fertilized within 24 hours once it has been ovulated there will be no fertilization or zygote formation
STAGE 1: Dispersal and passage through the corona radiata cells
Sperm enters vagina ↓ Sperm enters cervix ↓ Sperm travels to uterine tubes (30 min - 6 days, via muscular contractions and flagellar movement) ↓ Capacitation in uterine tube (~7 hours, removal of glycoprotein coat) ↓ Stage 1: Penetration of corona radiata ↓ Distributing of corona radiata cells via:
Hyaluronidase from sperm acrosome
Tubal mucosal enzymes
Sperm tail movements
STAGE 2: to pass through the zona pellucida
Sperm reaches zona pellucida
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Sperm binds to zona pellucida
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Acrosomal cap releases enzymes (acrosin and trypsin-like substances)
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Enzymes help sperm penetrate zona pellucida
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First sperm reaches close to oocyte membrane
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Two reactions occur:
Oocyte releases cortical granules → triggers Zona pellucida undergoes zona reaction
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Zona pellucida becomes impermeable to other sperm entering in
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Polyspermy (when more than one sperm fertilises the egg) prevention complete
STAGE 3:Fusion of sperm and oocyte membranes ****
Secondary oocyte completes meiosis 2
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acrosome had already been worn off at this stage and only the head and some part of tail enter the oocyte cytoplasm, so practically the sperm only comes with its chromosomes rest of the content like cytoplasm, mitochondria etc are mainly thought to be provided by the oocyte.
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sperm’s nuclear content becomes a male pronucleus
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oocyte chromosomes are arranged into a female pronucleus
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tail detaches and degenerates
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pronuclei will come in close contact and the chromosomes will be arranged on a mitotic spindle to prepare for a normal mitotic division.
Site of fertilisation = Ampulla of the uterine tube
RESULTS OF FERTILISATION:
restore diploid number
sex of embryo is determined
cleavage starts → cell division that occurs in an embryo
4. Cleavage and morulation
As male and female pronuclei come together + replicating their DNA. It forms 23 double stranded chromosome to prepare for normal mitotic division.
First clevage occurs 30 hrs after fertilisation
zygote → blastomeres
Zygote will then undergo series of miotic divisions→ result in 4-cell then 8-cell then 16-cell (morula) and then 32-cell→this causes cell number to increase but with each division the blastomers are becoming smaller in size
5. Blastocyst
After the nine cells stage the newly formed cells will start to form tight intercellular junctions and will form a compact ball of cells. = compaction
THEN →→→
Segregation of cells occurs.
Layer of cells inside are the INNER CELL MASS. → gives arise to tissue of embryo proper
Layer of outer cells are OUTER CELL MASS- forms trophoblast → form placenta
Morula enters the uterine cavity → fluid penetrate through the Zona pellucida
→ inner cells are called embryoblast and the outer cells are called trophoblasts and the whole structure is now called a blastocyst.
Around the same time Zona pellucida is disappearing or splitting and this blastocyst will hatch from the zona pellucida while it is entering the uterine cavity, ready for the implantation.