TR056/PG1007 Lecture 2 Fer4lisa4on, Implanta4on and Bilaminar Germ Disc Forma4on (Weeks 1 and 2) Dr. Neil Docherty In the Last Lecture……………… We examined how male and female gametes arose from
the hormonally controlled maturation of primordial germ
cells and covered the details of meiotic cell division and
the generation of genetic diversity
We addressed meiotic non-disjunction as a cause of
chromosomal abnormalities and birth defects and you
received a handout with further information on Down’s
Syndrome (Trisomy 21) as an example of this.
I suggested that you should focus on being able to;
1 Describe the steps in meiotic cell division
2. Correlate the structural and functional features of
male and female gamete maturation
Today’s Objec6ves • To iden4fy the key stages in fer4lisa4on and describe the ini4al phase of cell division of the zygote • To chart the development of the blastocyst and detail the key steps in implanta6on • To link the changes in the endometrium and ovary following ovula4on, fer4lisa4on and implanta4on • To track the changes in the inner and outer cell masses during the second week of development TO REVIEW THE ABOVE BY MEANS OF A SHORT INTERACTIVE EXERCISE AT THE END OF THE LECTURE Germ Cells to Fer6lisa6on Summary Primordial Germ Cells testes Mitosis Primary spermatocyte Meosis I and II ovary Primary oocyte Meosis I Meosis II Spermatazoon secondary oocyte fer4lised oocyte Mitosis fer4lisa4on Two cell stage Fer6lisa6on Where? • The gametes meet and fuse in the ampullary region (X) of the uterine tube Spermatazoa that reach the ampulla (5-­‐7 hours) can remain viable for several days X
At ovula4on, the release of chemotac4ns reac4vates mo4lity of the spermatazoa Fer4lisa4on usually occurs between 12 and 24 hours a]er ovula4on Key Events During Fer6lisa6on The secondary oocyte is protected 1.  Penetra4on of the corona radiata and the zona pellucida by spermatazoon 2. Fusion of gamete membranes and blockade of access for other spermatazoa (cor6cal and zonal reac6ons in oocyte) 3. Comple4on of meiosis II in secondary oocyte Have a look at the handout on intersexuality to see what happens and why when there
Is fertlisation with non-disjunction of sex chromosomes
Changes in the Spermatazoa Allowing for Penetra6on of Oocyte 1.  Capacita6on Interac4ons between spermatazoon and the uterine tube epithelium cause decapping of the head region of the sperm (Allows spermatazoa to pass through corona radiata) 2. Acrosome Reac6on -­‐Occurs upon binding of sperm head to the zona pellucida (ZP3 ligand) -­‐Causes the release of proteases (trypsin, acrosin) required for penetra4on of the zona pellucida Fertilisation
Pro-nuclei approach
(having replicated DNA)
Formation of zygote
and first cleavage
30 hours post ovulation
Two Cell Stage to Late Morula Early divisions give rise to increasingly small cells (blastomeres) Cleavage Divisions By 3 days a 16-­‐cell structure called the morula is formed -­‐Cells on the inside of the ball =Inner Cell Mass (form embryo) -­‐Cells on outside of the ball =Outer Cell Mass (contribute to placenta) The Blastocyst Entry of fluid between intercellular spaces forms a cavity (blastocele) The structure is now given the name blastocyst =INNER CELLS-­‐embryoblast =OUTER CELLS-­‐trophoblast Entry into
uterus
4.5-­‐6 days Summary of the First Week
Normally in anterior or posterior uterine wall What about ectopic pregnancy? B S C
B-Basal S-Spongy C-Compact
(layers of the endometrium)
Note that at implantation, the endometrium is in the progestational phase
under control of progesterone from the corpus luteum. The corpus luteum is
sustained by human chorionic gondaotrophin (hCG) release from the trophoblast
How are these hormones communicating between the tissues?
What do you suspect happens in the absence of fertilisation?
Forma4on of the Bilaminar Germ Disc (Week 2 (The Week of Twos)) By day 8, the implanted blastocyst is undergoing a series of changes OUTER CELL MASS (TROPHOBLAST) Inner aspect outer aspect CYTOTROPHOBLAST
SYNCYTIOTROPHOBLAST divide and populate INNER CELL MASS (EMBRYOBLAST) outer aspect (towards trophoblast) Inner aspect (by cavity) HYPOBLAST
EPIBLAST (A cavity emerges within =AMNIOTIC CAVITY) Blastocyst Development DAY 6
DAY 8
The Blastocyst Is A Source of Embryonic Stem Cells Cells are pluripotent hep://stemcells.nih.gov/info/faqs.asp This site is from the NIH, federal research funding body in the U.S.A. and details informa4on on all types of stem cell therapy Stem Cells are Inducible From Adult Cells This is the focus of the PG1004 tutorial on Thursday with Prof. Campbell https://medicine.tcd.ie/physiology/assets/docs/lecturenotes/ND/HHD/JF/PG1004/PG1004%20Tutorial%207%20Induced
%20Pluripotent%20Adult%20Stem%20Cells.pdf
Development (Day 9) Note the amnio4c cavity and the cytotrophoblast and synci6otrophoblast* The synci4otrophoblast enters the lacunar stage Forma4on of the exocoelemic membrane from the hypoblast generates the primi6ve yolk sac The penetrated blastocyst is sealed in place now by a fibrin clot. *What are syncitia and where
have we seen them before?
Development (Day 12) Establishment of the uteroplacental circula6on Forma4on of the extra-­‐embryonic mesoderm and emergence of the chorionic cavity Surface defect is now healing with uterine epithelium Development (Day 13) The cytotrophoblast starts to project columns ( primary villi) into the syncy4otrophoblast The hypoblast cells proliferate to form a new cavity, the secondary yolk sac Development of the connec4ng stalk from the extraembyronic mesoderm The defini4ve bilaminar disc will now gastrulate to form the three cell layers that give rise to all the organs and 4ssues (next lecture) Your Learning Objec4ves Your learning from today should focus on being able to; 1) Iden,fy the key stages in fer4lisa4on and describe the paeern of cell division of the zygote 2) Chart the development of the blastocyst and detail the key steps in implanta6on 3) Link the changes in the endometrium and ovary following ovula4on, fer4lisa4on and implanta4on. 4) Track and describe the changes in the inner and outer cell masses during the second week of development.