Fertilization to Gastrulation

Question 1

Pre-fertilization

Answer 1

– capacitation – takes up to 7 hours – sperm maturation
o Female – has all eggs when she is born
 Eggs began cell division but did not finish – arrested in 2nd meiotic division
• Must be stimulated/fertilized to proceed
 Granulosa (follicular) cells (most exterior) – “helper” cells
• Corona radiate – granulosa cells that specifically surround the egg
 Zona pellucida – “egg shell”

Question 2

Fertilization

Answer 2

– takes 24 hours; ends when male nucleus moves in and comes together with female nucleus; takes place in ampulla of fallopian tube
o Acrosomal reaction – head of sperm comes in contact with exterior of egg and releases enzymes
 Acrosin – destroys zona pellucida
 Hyaluronidase – destroy corona/granulosa/follicular cells
o Penetration – inner acrosomal membrane dissolves
o Fusion – initiates: membrane of sperm and egg fuse together
 Zona reaction – internal granulosa cells release contents that causes the membrane to change properties and prevent polyspermy
 Male pronucleus formation
 Division of female pronucleus to give 2nd polar body
 Unmasking of mRNA in egg cytoplasm – mRNA is released so that translation can occur of essential proteins and not rely on DNA

Question 3

Post-Fertilization Cell and Implantation

Answer 3

• multiple cell divisions occur to reach a critical mass – 16 cell morula with zona pellucida around it
• blastocyst – contains an inner cell mass encircled in trophoblast cells; day 4-5
o zona pellucida degenerates
o blastocyst cavity
o trophoblasts – secrete hCG that allows you to confirm a pregnancy
• day 5-6 begins to implant into uterus
• day 9 fully implanted into uterus
• ectopic pregnancy - abnormal implantations
o fallopian tubes – most common
o cervix
o opening of uterus
o abdominal cavity
o ovary

Question 4

Bilaminar Disc

Answer 4

(day 8)
o Trophoblasts differentiate into cytotrophoblasts and syncytiotrophoblasts
 cytotrophoblasts still surround the embryo too
o Syncytiotrophoblasts – multinucleated cells – migrate into uterine wall
 Pick up nutrients from mother via uterine lining
o Inner cell mass divides into epiblast (upper layer) and hypoblast and gives rise to 2 cavities
 Epiblasts – gives rise to all structures of a fetus; encircles amniotic cavity
 Hypoblasts – gives rise to extraembryonic mesoderm; encircles primitive yolk sac
o Epiblast cells migrates between hypoblast cells and cytotrophoblast cells and gives rise to chorionic cavity
o Hypoblast cells divide and gives rise to secondary/true/definitive yolk sac and the primitive yolk sac is excreted
 Hypoblast cells will designate head region and longitudinal axis of embryo
 Splanchnic mesoderm (from epiblast) surround yolk sac; gives rise to germ cells and cardiovascular system

Question 5

Gastrulation

Answer 5

– process of going from bilaminar disc to trilaminar disc

Question 6

Trilaminar Disc

Answer 6

– gives rise to the 3 germ layers (ecto, meso-, and endoderm)
o Primitive streak - bulge forms at caudal portion of trilaminar plate due to epiblast cells proliferating and migrating at midline
 Endoderm layer is formed – inner lining of GI tract and digestive glands; respiratory system
 Mesoderm layer is formed – everything else (connective tissue, bone, muscle, blood vessels, cartilage, etc.)
 Top portion of cells become ectoderm – central nervous system, skin, eyes, ears
o Prechordal plate – made of hypoblst; forms at the cranial/cephalic end
 Gives rise to oropharyngeal membrane
o Cloacal membrane – gives rise to anus at caudal end

Question 7

Epiblast Migration

Answer 7

• Epiblast cells that migrate through primitive node stay at midline and form notochordal process
o Signals ectoderm over it to thicken and form neural plate
o Notochordal process reorganinzes to notochord
o Notochord signals the neural plate to fold and form a tube which gives rise to CNS
o Lateral to notochord are somites forming