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Repro > Fertilisation and Embryonic Development > Flashcards

Fertilisation and Embryonic Development Flashcards

1
Q

sperm migration

A

Majority of sperm do not reach site of fertilization
o destroyed by vaginal acid
o majority drain out of vagina
o cannot penetrate cervical mucus
o get destroyed by leukocytes in uterus
o half go in the fallopian tube not containing an oocyte (fallopian tubes alternate each month which one releases an egg)
- only ~2000-3000 spermatozoa reach oocyte
- takes 5-10min for spermatozoa to reach distal end of fallopian tube
- Spermatozoa have to undergo changes before fertilization can occur –> capacitation & acrosome reaction
** vagina is an opening to exterior: needs protective mechanisms to protect from bacterias etc as a result: destroys sperm as well

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2
Q

sperm activation

A
  1. Capacitation: Acquisition of hypermobile tail
    - Sperm head changes to gain capacity to fertilize
  2. Acrosome reaction
    - Sperm head modified
    - Release of enzymes to penetrate zona pellucida
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3
Q

sperm capacitation

A
  • Chemical changes of sperm after entering female reproductive tract
  • In isthmus of fallopian tube
  • Takes ~10h
  • Fluid in female reproductive tract leaches cholesterol from sperm plasma membrane
    o Sperm head becomes penetrable to Ca2+ ions
    o Hypermotility of sperm
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4
Q

capacitation process

A
  • Ca2+ influx, cholesterol oxidized and removed from sperm head and tail by albumin (blue)
  • Remodeling of protein complexes in sperm head so they can interact with zona pellucida
  • Increase in pH and temperature in fallopian tube (flush out tube to avoid sperm degradation)
  • Spermatozoa acquire hypermobile tail & capacity to fertilize
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5
Q

fertilisation timing

A
  • Egg has to be fertilized within 12-24h after ovulation
  • Takes ovum 72 hours to reach the uterus -> sperm has to fertilize the egg in ampulla of the fallopian tube
  • Sperm viable for up to six days
  • Fertilization involves chemical changes in the Spermatozoa and in the Oocyte
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6
Q

fertilisation process

A

(1) Sperm penetration of cumulus cells
(2) attachment to zona pellucida & acrosome reaction
(3) exocytosis of acrosomal contents - enzymes digest the zona pellucida
(4) penetration of zona pellucida
(5) entry into perivitelline space and binding to exposed docking proteins on the oocyte membrane
(6) membrane fusion between sperm and ovum -> sperm nucleus enters cytoplasm (7) cortical reaction: enzymes released
(8) block to polyspermy

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7
Q

acrosome reaction

A
  • Acrosome = cap-like structure over the anterior half of the sperm’s head
  • Acrosome reaction induced by calcium and progesterone + other factors
  • Membrane surrounding the acrosome fuses with the plasma membrane of the sperm’s head –> exposure of acrosome contents:
    o Digestive enzymes
    o Antigens which can bind oocyte membrane
  • Acrosome vesicle fuses with the zona pellucida and releases digestive enzymes which soften the glycoprotein matrix of the zona pellucida –> sperm can penetrate
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8
Q

sperm and fertilisation

A
  • After digestive enzymes have softened glycoprotein matrix of the zona pellucida sperm penetrates zona pellucida
  • Sperm can enter perivitelline space and bind to proteins on oocyte membrane
  • Membrane fusion of sperm and oocyte –> sperm nucleus (and centriole) enters oocyte
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9
Q

preventing polyspermy (cortical reaction)

A
  • Process initiated after fertilization triggered by Ca2+ release from ER by the release of cortical granules from ovum
  • -> Calcium wave starts at site of sperm penetration
  • Permanent barrier to sperm entry gradually established = slow block of polyspermy in many animals
  • Exocytosis of cortical granules from ovum -> contents of the cortical granules released outside the cell -> modify extracellular matrix (zona pellucida) by serine proteases
  • -> becomes impenetrable to sperm entry = physical barrier and also receptors to bind sperm down-regulated
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10
Q

result of fertilisation

A
  • One cell with haploid DNA from 1 sperm and haploid DNA from 1 ovum = cell is now diploid
  • DNA is initially present in two separate areas within the cell (“head of sperm” & nucleus of oocyte)
  • Pronuclei fuse => zygote with diploid number of chromosomes
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11
Q

formation of a zygote

A

Genome has to be duplicated before the zygote can become divided into 2 daughter cells  before mitosis DNA replication in male and female pronuclei

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12
Q

events occurring directly after fertilisation

A
  • Genome has to be duplicated for first meiotic division

- DNA replication occurs in male and female pronuclei

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13
Q

fertilisation process

A
  1. After the sperm penetrates the secondary oocyte, the oocyte completes meiosis II, forming the ovum and second polar body.
  2. Sperm and ovum nuclei swell, forming pronuclei
  3. The DNA in each pronucleus replicates. The pronuclei approach each other and a mitotic spindle forms between them.
  4. Chromosomes of the pronuclei intermix. Fertilization is accomplished and the cell, now called a zygote, is ready for the first cleavage division.
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14
Q

female and male contributions to the zygote

A

Female Oocyte:
- 1 nucleus with 23 chromosomes
- Large amount of cytoplasm
o Granules for cortical reaction to block polyspermy, mRNA and proteins for fertilization, cleavage, cell fate determination, embryo axis orientation
o Mitochondria, nucleolus, centriole pair, ribosomes
- Surrounded by zona pellucida = extracellular matrix – protection
- Corona radiata – nourishment
Male Sperm:
- 1 nucleus with 23 chromosomes
- Very few mitochondria

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15
Q

events after zygote formation

A
  • Aim: Grow early embryo
  • First Cleavage:
  • Formation of a two cell embryo by mitosis
  • First mitotic division 24h after fertilization
  • Results in two cell embryo – each cell is now called a blastomere
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16
Q

cleavage and cell cycle events

A
  • Zygote undergoes further cleavage = rapid cell division without growth
    o Cell cycle consists mainly of S phase and M phase (DNA synthesis and mitosis)
    o Very little protein synthesis (skips G1 and G2)
    o Oocyte brought lots of cytoplasm with proteins and organelles for these cleavages to occur
  • Cleavage into 4 cell embryo: 4 blastomeres
  • Cleavage into 8 cell embryo = 8 blastomeres
17
Q

early cell fate

A
  • Lineage tracing of embryos developed in vivo after labeling a blastomere by photoconversion at the two-cell stage
  • Labeled embryos developed in vivo to the blastocyst stage
  • Distribution of cells not equally
18
Q

morula

A
  • Morula: ≥16 cells after 72h (still surrounded by zona pellucida)
  • Hatching: Morula sheds zonal pellucida
19
Q

blastocyst

A
  • Blastocyst: consists of ~100 cells
  • Inner cell mass
    o Becomes embryonic disc, which will form embryo and three or four extraembryonic membranes (becomes foetus)
  • Trophoblast
    o Display immunosuppressive factors
    o Participate in placenta formation
  • Blastocoel
    o Fluid filed cavity
  • Blastocyst enters uterine cavity ~ 4-5 days after fertilization
20
Q

implantation

A
  • Blastocyst implantation begins day 6-7
  • Blastocyst nourished by uterine secretions
  • Blastocyst Adhesion: Trophoblast cells of Blastocyst adhere to site with proper receptors and chemical signals
  • Trophoblast cells proliferate and form two distinct layers
    o Cytotrophoblast: inner layer of cells
    o Syncytiotrophoblast:
    • cells in outer layer lose plasma membranes, becoming multinuclear mass
    • Send out long protrusions that invade and digest endometrium
  • Blastocyst fully implanted in endometrium by middle of week 2 after fertilization
21
Q

hormones during implantation of blastocyst

A
  • Implantation is usually completed by day 12 after ovulation (day 26 of menstrual cycle); about same time menstruation would occur
  • human chorionic gonadotropin (hCG) is secreted by trophoblast cells and later chorion
    o Corpus luteum is maintained by hCG to prevent menstruation
    o Corpus luteum continues secretion of progesterone and estrogen
    o hCG levels rise until end of month 2
    o Decline as placenta begins to secrete progesterone and estrogen

Repro (16 decks)

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