Fertilisation to Trilaminar Embryo

Question 1

Fertilisation Steps (4)

Answer 1

1. Sperm binds in a human specific interaction with Zona pellucida glycoprotein (ZP3 – receptor, other ZP glycoproteins 1-4)
2. Acrosomal enzymes released from sperm head (makes way into egg)
3. Egg and sperm plasma membrane fuse and sperm contents enter egg
4. Sperm entry triggers - Meiosis II completion, Release of cortical granules by oocyte

Question 2

Cleavage - First Cel Divisions

Answer 2

Zyogte undergoes cell division
8 cell stage Blastomere - totipotent (any cell)

Question 3

Mosaicism - 2 or more cell lines, non-disjunction, trisomy 21

Answer 3

2 or more cell lines with different chromosome complements
Can occur via mitotic non-disjunction
Some cells with normal complement and some with trisomy - less prominent features

Question 4

Compaction of Morula

Answer 4

Morula = 16+ blastomeres
Within zona pellucida
Begin to form cell junctions
Fluid enters through zona pellucida

Question 5

Blastocyst

Answer 5

Day 4/5
Fluid moves into cavity
Trophoblast - outer layer, foetal part of placenta
Inner cell mass
Hatching - embryo hatch from zona pellucida
Implantation

Question 6

Implantation

Answer 6

Day 6
Move along uterus to posterior or anterior uterus
Invade endometrium lining - trophoblasts

Question 7

Corpus Leuteum

Answer 7

Endocrine organ
Important for pregnancy
Produces progesterone for maintaining lining

Question 8

Day 7.5

Answer 8

Invade endometrial lining and implant properly
Active process by trophoblasts
Trophoblasts divide into Cyto- and Syncytiotrophoblasts (chorionic gonadotropin stimulates corpus luteum)

2 Layers - epiblast (dorsal) and hypoblast (ventral)

Question 9

Day 9

Answer 9

Implantation complete
Vacuoles appear in syncytiotrophoblast and unit to form lacunae
ENlarged blood vessels in endometrial lining - form uteroplacental circulation
Two cavities formed –> Amniotic cavity & Primitive Yolk sac

Question 10

Day 12

Answer 10

Establishment of Uteroplacental circulation
Maternal blood flows into lacunae to diffuse to embryoblast
Extraembryonic mesoderm develops and degenerates and gives rise to chorionic cavity

Question 11

Day 13

Answer 11

Further development of chorionic cavity and presence of connecting stalk
Second wave of hypoblast cells migrate to definitive yolk sac

Question 12

Implanted embryo

Answer 12

Uteroplacental circulation forms as syncytiotrophoblasts extend and contacts maternal blood supply
Amniotic cavity present
Chorionic cavity developed and encircles

Question 13

Week 3 - Formation of Primitive Streak

Answer 13

Appears at midline at caudal end of epiblast –> body axes
Cranial end - pit and node
Know what direction cells are in now

Question 14

Gastrulation

Answer 14

Movement of epiblast cells through primitive streak to form three germ layers
Body axes established
Hypoblast cells replaced by definitive endoderm
Back under themselves to form mesoderm and spread out
Epiblast - ectoderm

Question 15

Fate of Ectoderm

Answer 15

Epidermis of Skin
Nervous system

Question 16

Fate of Mesoderm

Answer 16

Paraxial mesoderm - axial skeleton voluntary muscle, parts of dermis (back)

Intermediate mesoderm - urogenital systems

Lateral plate mesoderm - somatic layer (most of dermis, lining of body wall, parts of limbs), visceral layer (CV system, mesothelial covering organs, smooth muscle)

Question 17

Fate of Endoderm

Answer 17

Lining of gut tube, lining of respiratory tract, lining of bladder and urethra