PG1007 Lecture 2 Fertilisation, Implantation and Bilaminar Germ Disc Formation

TR056/PG1007  

Lecture  2  

Fer4lisa4on,  Implanta4on  and  Bilaminar  Germ  Disc  Forma4on  (Weeks  1  and  2)  

Dr.  Neil  Docherty  

In  the  Last  Lecture………………  

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

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.

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    

Primary  spermatocyte     Primary  oocyte  

ovary  testes   Mitosis  

Spermatazoon                  secondary  oocyte          fer4lised  oocyte  

fer4lisa4on  

Mitosis  

Two  cell  stage  

Meosis  I  and  II    Meosis  I  

Meosis  II  

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  

At  ovula4on,  the  release  of    chemotac4ns  reac4vates    mo4lity  of  the  spermatazoa  

Fer4lisa4on  usually  occurs    between  12  and  24  hours  a]er  ovula4on  

X

Key  Events  During  Fer6lisa6on  

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  

The  secondary  oocyte  is  protected    

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      

Pro-nuclei approach Formation of zygote (having replicated DNA) and first cleavage

30 hours post ovulation

Fertilisation

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

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?

B S C B-Basal S-Spongy C-Compact

(layers of the endometrium)

Normally  in    anterior  or    posterior  uterine  wall  What  about  ectopic  pregnancy?  

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)  

CYTOTROPHOBLAST      SYNCYTIOTROPHOBLAST  

Inner  aspect   outer  aspect  

divide              and                    populate  

INNER  CELL  MASS  (EMBRYOBLAST)  

HYPOBLAST            EPIBLAST  

Inner  aspect  (by  cavity)  

outer  aspect  (towards  trophoblast)  

(A  cavity  emerges  within    =AMNIOTIC  CAVITY)  

Blastocyst  Development  

DAY 6

DAY 8

The  Blastocyst  Is  A  Source  of    Embryonic  Stem  Cells  

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    

Cells  are  pluripotent  

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  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.  

Your  Learning  Objec4ves