ERS26 Fertilisation And Post-implantation Development Flashcards
Embryonic origin of germ cells
Extraembryonic tissue (later replaced by Endoderm) (Junction between Embryo and Yolk sac)
—> Primordial germ cells (detected by ALP stain ~24 day)
—> Mitosis, Proliferate
—> migrate through Dorsal mesentery
—> into Embryonic gonads (Gonadal ridges, close proximity to Mesonephros)
—> Spermatogonia / Oogonia
Female:
Primordial germ cells continue to proliferate
—> millions of Oogonia (max no.) (5th month)
Male:
Spermatogonia retain capacity to proliferate throughout life
Meiosis
- occurs only during gamete production
- 2 cell divisions (Meiosis 1 and 2)
- formation of 4 gametes each containing 23 chromosomes and 1N (haploid) amount of DNA
- Normal somatic cell: 2N, 2C (46)
- before Meiosis, after DNA synthesis: 2N, 4C (92)
- after Meiosis 1: 1N, 2C (46)
- after Meiosis 2 (i.e. gametes): 1N, 1C (23)
Function:
1. ↓ no. of chromosomes within gametes —> ensure no. of chromosomes (46) is maintained through generations
- Redistributes maternal + paternal chromosomes, ↑ exchange of small amounts of maternal and paternal DNA via crossover
—> ensure genetic variability
Female gametogenesis (Oogenesis)
Before birth:
Female primordial germ cells (2N, 2C: 2 complement sets of chromosomes) derived from hindgut arrive in ovary in week 4
—> Mitosis
—> **Oogonia (2N, 2C) in ovaries
—> Oogonia undergo Meiosis, DNA replication (8-9 weeks, completed by 6 months)
—> **Primary oocyte (2N, 4C) (within Primordial follicle)
—> Arrest at Diplotene stage of Meiosis 1 until puberty
—> Growth of oocyte and follicle (Primary follicle)
—> Remain in ovarian follicle until puberty
After birth:
—> Follicular maturation (Secondary follicle): Follicle recruited for development in menstrual cycle
—> Meiosis 1 resumed
—> Unequal cell division occurs in Meiosis of oocytes
—> 1 Large cell: **Secondary oocyte (Mature egg: 1N, 2C) + 1 Small cell (1st polar body: residual body destined to degeneration) (Tertiary follicle)
—> Arrest again at Metaphase 2 upon **LH surge until fertilisation
—> Ovulation
—> Sperm entry and fertilisation
—> Resumption of Meiosis 2
—> large ***Pronucleated / Mature oocyte (1N, 1C: Haploid) + 2nd polar body
Male gametogenesis (Spermatogenesis)
Before birth:
Primordial germ cell (2N, 2C)
—> certain degree of proliferation
After birth:
Puberty
—> Spermatogonia (2N, 2C) commit to Meiosis (Spermatogonia produce daughter cells throughout life)
—> Primary Spermatocyte (2N, 4C)
—> Meiosis 1
—> Secondary Spermatocyte (1N, 2C)
—> Meiosis 2
—> Spermatids (1N, 1C) (non-motile, without tail)
—> Morphological changes (***Spermiogenesis)
—> Spermatozoa (1N, 1C)
Spermiogenesis:
1. Nucleus
- DNA and chromosomes repackaged with ***Protamine (instead of Histone)
—> ↓ nucleus size
- Cytoplasm
- **Enzyme, **Golgi apparatus concentrated at tip of head
—> ***Acrosome (collection of amorphous enzymes and protein-rich material) - Mitochondria
- lining up along **Midpiece of sperm
—> **Mitochondrial sheath - Flagellum
- growing out at tail end of sperm
Spermatozoa vs Oocyte
Primordial germ cell: 2N, 2C
Oogonia: 2N, 2C
Primary oocyte: 2N, 4C
Secondary oocyte: 1N, 2C
Pronucleated / Mature oocyte: 1N, 1C
Spermatogonia: 2N, 2C Primary spermatocyte: 2N, 4C Secondary spermatocyte: 1N, 2C Spermatid: 1N, 1C Spermatozoa: 1N, 1C
Events in fertilisation
Sperms deposited in female reproductive tract, Oocyte around infundibulum
Tertiary follicle (出—>入):
- ***Corona radiata (highly cellular layer: Cumulus / Granulosa cells)
- Zona pellucida (non-cellular translucent Glycoprotein materials)
- ***Perivitelline space
- Cortical granules (protein rich mixture of material line up at ***peripheral of oocyte)
- Secondary oocyte (arrested at Metaphase 2)
- 1st Polar body
Corona radiata (Protein + Hyaluronic acid)
Highly cellular layer with an intercellular matrix consisting of
- Proteins
- High [carbohydrates] (esp. Hyaluronic acid)
Penetration:
1. Sperm head
—> collection of enzymes (e.g. protease, proteinase, hyaluronidase)
—> ***Hyaluronidase: major role in penetration of Corona radiata (digesting intercellular matrix in Corona radiata)
- Active swimming movements of spermatozoa
—> propel sperm through Corona radiata
Zona pellucida (Glycoprotein)
Human Zona pellucida (4 glycoproteins)
- ZP1
- ZP2 (alternate with ZP3 —> protein polymers / long filaments —> cross-linked by ZP1)
- ZP3
- ZP4/B (unknown function)
—> Polypeptide chains highly decorated with **Oligosaccharides (*receptor for sperm head)
Mutations of ZP1, 2, 3 —> Female infertility
Penetration:
Spermatozoa binds tightly to Zona pellucida by Plasma membrane of sperm head (display variety of protein) (~ ligand-receptor interaction)
—> Sperm receptor: O-linked Oligosaccharide attached to Polypeptide core of **ZP3 molecule
—> **Galactosyl transferase (or other molecules) on surface of sperm head —> specific binding sites for ZP3
Acrosome reaction (stimulated by ZP3):
Binding of sperm to Zona pellucida
—> **Ca influx through plasma membrane of sperm head
—> trigger Acrosome reaction in spermatozoa
—> fusion of **outer Acrosomal membrane with overlying sperm plasma membrane
—> pinching off of fused parts as small vesicles
—> perforations on sperm head
—> release of enzymes in Acrosome
—> 1. Acrosomal enzyme action (e.g. ***Acrosin: serine proteinase) + 2. Mechanical propulsion of spermatozoa
—> digestion + penetration of Zona
—> sperm plasma membrane touch oocyte membrane
—> fuse
Binding and Fusion of Spermatozoon and Egg
Acrosomal reaction seem to cause some change in membrane properties of sperm
—> Spermatozoon bind to and fuse with plasma membrane of Egg (兩個既plasma membrane fuse埋一齊)
—> **Molecules of sperm plasma membrane (~ ligand) bind to **Integrin molecules (~ receptor) on egg surface
—> fusion
—> content of spermatozoon sink into egg (leaving plasma membrane left outside incorporated into egg’s plasma membrane) (除左件衫)
Prevention of polyspermy
Block of polyspermy begins with propagation of a **wave of Ca from site of sperm-egg fusion
—> Ca wave passes through egg
—> acting on Cortical granules (lined up on peripheral of egg)
—> **Cortical granules fuse with plasma membrane
—> release **Hydrolytic enzymes + **Polysaccharides into **Perivitelline space (Zona再入面嗰層)
—> diffuse into Zona (由入面滲出去)
—> **Hydrolyse sperm receptor molecules (ZP3) in Zona + ***hardening of Zona pellucida
—> “Zona / Cortical reaction”
—> eliminate ability of other spermatozoa to adhere to / penetrate Zona
Metabolic activation of egg
Penetration of sperm
—> ***release of Ca from internal store (initiation event)
—> rapidly intensify respiration + metabolism of egg
Decondensation of sperm nucleus
Chromatin is very tightly packed in mature spermatozoa
—> due to **disulfide linkages among **Protamine molecules
—> after sperm head enter cytoplasm of egg
—> permeability of sperm nuclear membrane ↑
**↓ Glutathione in ooplasm
—> **↓ cross-linking disulfide group (女幫男)
—> Protamines rapidly lost (desembled)
—> Histones begin to associate with male chromosomes (reassembled)
—> Male ***pronucleus formation
Completion of meiosis in egg
Sperm penetration
—> Secondary oocyte complete Meiosis 2
—> **Pronucleated oocyte + 2nd polar body
—> **Male pronucleus also forming
—> **DNA replication occurs in developing haploid pronuclei
—> approximation of pronuclei
—> nuclear membrane break down
—> chromosomes intermingle
—> maternal + paternal chromosomes organised around a **mitotic spindle
—> in preparation for ordinary mitotic division
—> Zygote
Results of fertilisation
- Stimulate Secondary oocyte to ***complete 2nd meiotic division
- ***Restores normal diploid no. of chromosomes in zygote
- ***Variation of human species by mingling of maternal and paternal chromosomes (zygote: genetically unique)
- Determine chromosomal ***sex of embryo
- ***Metabolic activation of oocyte + Initiates cleavage
In-vitro fertilisation
ICSI: Intracellular sperm injection
