early embryonic development

Question 1

describe fertilization

Answer 1

Fertilization occurs in the ampulla of the uterine tube within 12 hours of ovulation
Sperm and Ovum are haploid cells (23)
• Fertilization restores the diploid chromosome number (46)
1. sperm cell penetrated corona radiata cells of oocyte
2. ZP3 protein in zona pellucida allows the sperm head to bind to zona pellucida which stimulates acrosome exocytosis of degradative enzymes
3. enzymes enable sperm to penetrate zona pellucida
4. front end of sperm plasma membrane fuses with plasma membrane of the oocyte so sperm DNA enters same cytoplasm as oocyte DNA
4. oocyte immediately undergoes final step of reduction division (meiosis) to produce 23 chromosomes and polar bodies
5. end result : male pronucleus and female pronucleus in maternal cytoplasm with maternal mitochondria and maternal mRNA

Question 2

how many genes in mitochondria

Answer 2

37 genes - 13 protein-coding ones

Question 3

what is the three-person embryo

Answer 3

when the maternal mitochondria is not healthy so a donor oocyte without nuclear material is used

1. mothers egg fertilized - nuclear material removed and kept
2. donors egg fertilised - nuclear material removed and destroyed
3. parents nuclear material placed into a donor egg

Question 4

when does the first division start

Answer 4

Initiation of cleavage occurs within 36 hours of fertilization

Question 5

describe how the fertilized egg divides to form a ball of cells in the first week

Answer 5

1. DNA replicate and Maternal and paternal chromosomes line up on spindle equator
2. chromatids separate forming and 2 cells formed
3. No cell growth in early cell cycles as zona pellucida is intact (Back to back synthesis and mitosis) so cells become smaller
   - Little expression of parental genes
   - Maternal cytoplasm shared

Question 6

describe compaction

Answer 6

compaction begins at 8 cells in which central cells are linked by gap junctions thus making them each less distinct and more as a unit
(physically looks like each cell is less of a distinct unit and instead more joined and smooth)

• Ant/post axis of embryo established (but not morphologically visible)

Question 7

what is the morula

Answer 7

• Morula (16+ cells) solid ball of cells with inner and outer layers
• The morula is transported along the uterine tube and arrives at the uterus (approx 30-
  40cells) approx 3 - 4 days after fertilization
• Zona pellucida disappears
• Outer cells divide to become trophectoderm (trophoblast) + Inner cells -> Inner cell mass.

Question 8

describe the process leading up to the blastocyst formation

Answer 8

at 4.5 days

• cavity (blastocoel) form in inner cell mass thus named blastocyst
• outer cells named trophoblast (near the inner cell mass) and trophectoderm (around blastocoel) and inner cell mass can be also named embryoblast
• Still free unattached in uterus

Question 9

where and when should cells be removed for genetic testing

Answer 9

5-6 days, remove from trophectoderm

Question 10

why do genetic testing

Answer 10

to detect single-gene disorders or chromosome abnormalities

Question 11

describe the binding process of the blastocyst to the uterus wall

Answer 11

AT 5.5 -6 DAYS:
• Binding at the uterus epithelium occurs (embryonic pole attaches)
• Down regulation of anti-adhesion molecule MUC-1 at epithelium allows binding via selectins (embryo) to glyco-components on epithelial cells (uterus)
• Similar mechanism to white blood cell adhesion to blood vessel walls
• Integrins, laminin and fibronectin involved in initial penetration

Question 12

describe the implantation of the blastocyst

Answer 12

AT DAY 6- 7:
- Trophoblast becomes the “invasive” syncytiotrophoblast (layer that penetrates) and cytotrophoblast (layer closer to inner cell mass)
• Syncytiotrophoblast multinculeate syncitium invasion via metalloproteases to break down tissue AND IMPLANT
• Immunosuppression of host/graft and graft/host reactions

AT DAY 8:
Trophoblast divides and part becomes “invasive” syncytiotrophoblast and cytotrophoblast
• Two layers form from inner cell mass in the embryo – epiblast (above) and hypoblast (below)
• Then amniotic cavity begins to form as a space formed within the epiblast

AT DAY 9:
- Hypoblast cells coat the blastocyst cavity to form: the primary yolk sac
• Spaces develop within the syncytiotrophoblast
• Amniotic cavity has formed as a space formed within the epiblast
• Extraembryonic mesoderm will soon form deep to trophoblast (between trophoblast and yolk sac)

DAY 11 TO 12:
- Embryonic disk has two layers - bilayered (0.1 - 0.2 mm)
• The blastocyst burrows completely into the endometrium
• Syncytiotrophoblast (SCT) cells erode through the walls of large maternal capillaries which
bleed into the spaces establishing the primitive placental circulation (breakthrough bleeding may occur)
- New layer between the yolk sac and cytotrophoblast (extraembryonic mesoderm)
• Spaces form in extraembryonic mesoderm which forms chorionic cavity upon fusion

Question 13

what happens by end of week 2 (time at which woman expects her next period)

Answer 13

2 layers develop in the trophoblast
• Syncytiotrophoblast
• Cytotrophoblast

2 layers in the Inner Cell Mass
• Epiblast - ectoderm
• Hypoblast - endoderm

2 cavities form
• Amniotic cavity
• Chorionic cavity

• No clear body axis (morphologically)