Development of Embryonic and Extra-Embryonic Structures

Question 1

Fertilisation

Answer 1

Sperm penetrate zona pellucida (glycoprotein shell), fusion of oocyte and sperm cell membranes
Result:
Restore diploid no. chromosomes, determination embryo sex (sperm carry X or Y), initiate cleavage

Question 2

Cleavage

Answer 2

Zygote reach 2 cell stage, series of mitotic divisions increase no. cells, cells (blastomeres) become smaller w/ each division
Before 8 cell stage = loosely arranged clump
After 3rd cleavage - blastomeres max contact w/ each other, form compact balls held by tight junctions, segregates inner (communicate via gap junctions) and outer cells
Approx 3 days after fertilisation = 16 cell morula
Inner cell mass - embryo proper
Outer cell mass (surrounding cells) - trophoblast (placenta)

Question 3

Blastocyst formation

Answer 3

Morula enter uterine cavity, fluid penetrate zona pellucida (ZP) into intracellular spaces of inner cell mass (ICM), blastocoel (cavity) forms
Embryo = blastocyst
ICM = embryoblast cells
Trophoblast flattens to form epithelial wall of blastocyst, ZP disappear to allow implantation
Day 6: Trophoblastic cells penetrate between epithelial cells of uterine mucosa (implants)

Question 4

Embryonic stem cells

Answer 4

Derived from ICM
Before 8 cell stage - totipotent
After - pluripotent
Potential cure diabetes, Alzheimer’s, Parkinson’s, anaemias, spinal cord injuries etc

Question 5

Ectopic pregnancy (clinical)

Answer 5

Implantation occurs outside uterus

Question 6

Bilaminar germ disc

Answer 6

Day 8 - blastocyst partially embedded in endometrial stormer (inner layer of uterus)
Trophoblast differentiate into 2 layers: cytotrophoblast (inner mononulceated cells, actively proliferating) and synctiotrophoblast (outer multinucleate zone w/ out distinct cell boundaries, erodes maternal tissue)
Embryoblast differentiates:
Epiblast (amnioblasts [line amniotic cavity] and embryo) and hypoblast (surround primitive yolk sac)
Epiblast + hypoblast = bilaminar disc
Amniotic and yolk sacs form

Question 7

Implantation

Answer 7

Day 6-9 = embryo fully implanted in endometrium
Syncytiotrophoblast volume increase as cells fetch from proliferating cytotrophoblast and fuse
Apposition: Trophoblast cells come in contact w/ endometrial cells which have formed crypts in uterine wall
Adhesion: Trophoblast cells invade endometrial tissue
Active finger-like processes from syncytiotrophoblast, penetrate between endometrial cells, pull embryo into uterine wall = villi
Expanding syncytiotrophoblast surrounds blastocyst
Uterine blood vessels expand, close proximity to fetal blood (allow exchange via diffusion)

Question 8

Placenta

Answer 8

Uteroplacental circulation = maternal and fetal blood flow through placenta in close proximity, exchange gases and metabolites via diffusion
Day 9 - vacuoles open in syncytiotrophoblast (trophoblastic lacunae) , maternal capillaries expand to form sinusoids
Extension of cytotrophoblast grow into blood-filled lacunae = primary chorionic stem villi
Day 16 - cytotrophoblast penetrates core of primary villi = secondary chorionic stem villi
Blood vessels form and connect w/ vessels in embryo - establish uteroplacental circulation
Villi containing differentiated blood vessels = tertiary chorionic stem villi
Umbilical chord: 1 umbilical vein (high ox), 2 umbilical arteries (low ox) surrounded by wharton’s jelly