Development of bilamina germ disc

Dr Sharifa Abdul Aziz2008/2009

Summary of 1st week• Human development begins at fertilization.• Before fertilization, several important events

occurs like gametogenesis in male and oogenesis in female with each haploid nos. of chromosomes to meet each other to form zygote.

• Fertilization is complete when maternal and paternal chromosomes intermingle during metaphase of 1st meiotic division of zygote.

• As it passes along the uterine tube towards uterus , zygote undergoes cleavage( series of mitotic division).-- blastomeres.

Contd.• 3days later- morula is formed (16 cell stage).• Cavity occurs in morula- embryoblast (inner

cell mass) embryo proper+ some extraembryonic tissue.

• A blastocyst cavity- fluid-filled space• The trophoblast- thin outer layer of

cells.embryonic part of placenta+ extraembryonic structures.

• Trophoblast at embryonic pole - differentiate into cytotrophoblast + syncytiotrophoblast.

• Syncytiotrophoblast - invades endometriumspotted bleeding.

Implantation

• Is the penetration of blastocyst into the superficial compact layer of uterine endometrium. On 11th or 12th day after fertilization.

Contd.

• Implantation started at 1st week is completed by 2nd week.

• 2nd week – two layers of germ disc and two layers of trophoblast & two sac occur.

• The erosive syncytiotrophoblast invades- endometrial connective tissue embedded in the endometrium.

• Endometrial cells- apoptosis. Proteolytic enzymes+ COX-2 (prostacyclin) + fas ligand present at the implantation site- involved in this process.

2nd week• Day 8 : the blastocyt partially embedded in

endometrial stroma.• 2 layers : inner cell mass or embryoblast.• Embryoblast ectoderm(epiblast). endoderm(hypoblast).• Amniotic cavity : small cavity appears within the

epiblast.• Amnioblast : cells adjacent to cytotrophoblast.• Trophoblast : inner mononucleated cells :

cytotrophoblast.• Outer layer of multinucleated cells without cell

boundary : syncytiotrophoblast.

• Day 8 : the blastocyt partially embedded in endometrial stroma.

• 2 layers : inner cell mass or embryoblast.• Embryoblast ectoderm(epiblast). endoderm(hypoblast).• Amniotic cavity : small cavity appears within the

epiblast.• Amnioblast : cells adjacent to cytotrophoblast.• Trophoblast : inner mononucleated cells :

cytotrophoblast.• Outer layer of multinucleated cells without cell

boundary : syncytiotrophoblast.

• Day 8 : the blastocyt partially embedded in endometrial stroma.

• 2 layers : inner cell mass or embryoblast.• Embryoblast ectoderm(epiblast). endoderm(hypoblast).• Amniotic cavity : small cavity appears within the

epiblast.• Amnioblast : cells adjacent to cytotrophoblast.• Trophoblast : inner mononucleated cells :

cytotrophoblast.• Outer layer of multinucleated cells without cell

boundary : syncytiotrophoblast.

• Day 8 : the blastocyt partially embedded in endometrial stroma.

• 2 layers : inner cell mass or embryoblast.• Embryoblast ectoderm(epiblast). endoderm(hypoblast).• Amniotic cavity : small cavity appears within the

epiblast.• Amnioblast : cells adjacent to cytotrophoblast.• Trophoblast : inner mononucleated cells :

cytotrophoblast.• Outer layer of multinucleated cells without cell

boundary : syncytiotrophoblast.

• Day 8 : the blastocyt partially embedded in endometrial stroma.

• 2 layers : inner cell mass or embryoblast.• Embryoblast ectoderm(epiblast). endoderm(hypoblast).• Amniotic cavity : small cavity appears within the

epiblast.• Amnioblast : cells adjacent to cytotrophoblast.• Trophoblast : inner mononucleated cells :

cytotrophoblast.• Outer layer of multinucleated cells without cell

boundary : syncytiotrophoblast.

• Day 8 : the blastocyt partially embedded in endometrial stroma.

• 2 layers : inner cell mass or embryoblast.• Embryoblast ectoderm(epiblast). endoderm(hypoblast).• Amniotic cavity : small cavity appears within the

epiblast.• Amnioblast : cells adjacent to cytotrophoblast.• Trophoblast : inner mononucleated cells :

cytotrophoblast.• Outer layer of multinucleated cells without cell

boundary : syncytiotrophoblast.

Contd.• Trophoblast - differentiate into

cytotrophoblast( mononucleated layer of cells , mitotically active form new cells migrate into syncytiotrophoblast and lose their cell membrane.

• Syncytiotrophoblast rapidly expanding multinucleated mass on the outer layer produce hCG (basis of pregnancy test).

• Embryoblast - epiblast (thicker layer, tall columnar cells, hypoblast (small cuboidal cells adjacent to exocoelomic cavity).

• Two sacs – amniotic sac adjacent to epiblast(ectoderm) and yolk sac PYS(adjacent to endoderm(hypoblast)

Bilamina germ disc

Bilamaina germ disc

Bilamina germ disc

Day 9• Blastocyst more deeply embedded in

endometrium.• Lacuna stage : vacuoles appear in

syncytiotrophoblast-- fused- lacuna.• At abembryonic pole ; flattened cells appear

from endoderm and lines the inner surface of cytotrophoblast heuser’s membrane.

• Day 11,12 : at abembryonic pole : only one layer – cytotrophoblast only

• Cells of syncytiotrophoblast penetrate into maternal blood vesels -- uteroplacental circulation starts.

Contd.

• Extraembryonic mesoderm : fine loose connective tissue cells deriv. fr yolk sac in between cytotrophoblast and heuser’ s membrane.

• Cavities appears in it coalesc into extraembryonic coelom or chorionic cavity.

• Mesoderm b/c ----somatopleuric (chorionic plate) and splanchnopleuric mesoder (pleura and peritoneum).

• Decidua reaction : endometrial cells become polyhedral and loaded with glycogen and lipids & is oedematous.

EEM(primary mesoderm)

Day 13

• Villi formation : trophoblast is characterised by villus structure.

• Cytotrophoblast cells proliferate into syncytiotropho surrounded by syncytium--. Primary villi.

• Secondary villi : mesoderm + inside cyto dan syncytiotrophoblast.

• Tertiary villi : blood vessel dev. from mesoderm inside cyto and syncytiotrophoblast.

Villi formation

Contd.• Secondary yol sac or definitive yolk sac : form

inside the primary yolk sac.• Extraembryonic coelom expands and form large

cavity- chorionic cavity.• Chorionic plate : mesoderm lining the inside of

cytotrophoblast.• Connecting stalk: later become the umbilical

cord.• Buccopharyngeal membrane : area where

columnar cells and cuboidal cells stick at the cephalic region- oral cavity.