Embryology Flashcards
Fusion of sperm and oocyte
Fertilization
What is fertilization?
Fusion of sperm and oocyte
What is the product of fertilization?
Zygote
What is a zygote?
Product of fertilization
When is the zygote formed?
Day 1
During ovulation, the primary oocyte is arrested in which meiotic phase?
Metaphase 2
What is the purpose of the cortical reaction of a fertilized egg?
Blocks further sperm from entry
What process blocks further sperm from entry into a fertilized egg?
Cortical reaction
When does the cortical reaction happen?
After the first sperm makes contact with the ovum
Where is the corpus luteum formed?
In ruptured, ovulated follicle
When does the embryo implant?
Within 5-6 days
Where does the embryo impact?
Uterus
What allows the embryo to implant in the endometrium?
Syncytiotrophoblast cells
How are the syncytiotrophoblast cells formed?
Differentiate from trophoblast cells of the blastocyst
What cells are created from the differentiation of trophoblast cells?
Cytiotrophoblast (on blastocyst) and syncytiotrophoblast (inside endometrial lining)
The 3 definitive germ layers are formed from the ___.
Epiblast
Epithelial mesenchymal transition (EMT) is a feature of ___.
the movement of epiblast out of the primitive streak
During the monthly menstrual cycle, primary oocyte in the ovary mature under the influence of ___ (___) and ___ ensures at the midpoint of the cycle.
pituitary hormones (FSH & LH), ovulation
During the monthly menstrual cycle ___ promotes the thickening of ____ to prepare for ____ of a blastocyst.
progesterone, uterine lining, implantation
Which hormones initiate oocyte maturation & ovulation, corpus luteum formation, and endometrial growth?
Pituitary hormones FSH & LH
What do FSH & LH do?
Initiate: 1) oocyte maturation & ovulation; 2) corpus luteum formation (LH); and 3) endometrial growth.
What happens if fertilization & implantation does not occur?
Corpus luteum degenerates
What does the corpus luteum make?
progesterone
What is the purpose of progesterone?
thickens the uterine lining for implantation
What phase is triggered from the lack of progesterone?
Menstrual phase
What is the LH surge?
Sudden rise in LH
What is the purpose of the LH surge
induces maturation of a large follicle at the surface of the ovary
What happens after follicle maturation following the LH surge?
Follicle ruptures and releases oocyte into the uterine tube (oviduct)
What happens to the ruptured follicle after the LH surge
Granulosa cells of the follicle forms the corpus luteum
What does the corpus luteum secrete?
Progesterone
What happens during fertilization?
1) sperm penetrate zona pellucida of ovum; 2) fuses with oocyte membrane due to acrosome reaction of sperm; 3) cortical reaction of ovum bars entry of other sperm; 4) oocyte and sperm membranes fuse; 5) Nucleopores of oocyte and sperm dissolve so DNA can merge
What is the product of fertilization?
Zygote
What happens after zygote formation?
Cell division
What forms from zygote cell division by day 4?
Morula
When is the morula formed?
Day 4
Where is the morula located?
Uterus
How does the morula evolve in the uterus?
Trophoblast outer lining and embryoblast cell mass on one side creates a blastocoel, forming the blastocyst
When does the blastocyst form?
Day 5
When does implantation happen?
Day 5-6
What implants?
Blastocyst
Where does implantation happen?
Endometrium of uterus
What happens at implantation?
Blastocyst outer wall differentiates into cellular and syncytial trophoblast layers
What does the syncytiotrophoblast layer do?
1) invades uterine lining; 2) triggers inflammation and proliferation of blood vessels; 3) secretes human chorionic gonadotrophin (hCG) that maintains the corpus luteum
What does hCG do?
Maintains corpus luteum
What does the embryoblast do at implantation?
Rearranges into bilaminar disk
What happens when the syncytiotrophoblast invades the uterine endometrium?
Forms cavities (lacunae) that connect endometrial blood vessels and fill with maternal blood
What are lacunae?
syncytiotrophoblast cavities that connect endometrial blood vessels
Cavity that forms within the inner cell mass
Amniotic cavity
Types of cells in bi-layered disk
epiblast (above) and hypoblast (below)
What is the yolk sac?
cavity derived from the original cavity within the blastocyst
At bilayered disc formation, what two cavities are formed?
amniotic and yolk sac
How is the embryo attached to the placenta?
By a connecting stalk
Connecting stalk
attached embryo to placenta
What is the chorion or chorionic cavity?
The new cavity that surrounds the embryo when the bilayered disc forms
What is gastrulation?
the formation of 3 germ layers
What happens during gastrulation?
Cells in epiblast (eventually ectoderm) of bilayered disc move to the primitive streak and migrate between the epiblast and hypoblast to form mesoderm (fill in space) and endoderm (replace hypoblast)
When does gastrulation happen?
Day 14
Where is the primitive streak located?
Caudal on bilayered disc
Oropharyngeal membrane
future location of mouth and heart development
What does primitive streak signal?
start of gastrulation
What happens to epiblast cells during gastrulation?
epithelial to mesenchymal transformation to create mesoderm and endoderm
What are the subdivisions of the mesoderm?
Paraxial (next to neural tube) > intermediate > lateral plate
How is the lateral plate mesoderm further divided?
Somatic/parietal layer & splanchnic/visceral layer
What are the derivatives of the paraxial mesoderm?
head musculature, vertebrae, longitudinal spinal muscles, dermis (center)
What are the derivatives of the intermediate mesoderm?
urogentical system
What are the derivatives of the lateral place mesoderm?
somatic: muscular body wall
splanchnic: smooth muscle of gut
How is the neural tube formed?
formed from midline epiblast cells (ectoderm), which sinks into embryo above the notochord
What are neural crest cells?
Ectodermal cells involved in neural tube formation that migrate from closed neural tube and undergo EMT to form PNS, bones, etc
How are somites formed?
By paraxial mesoderm along both sides of neural tube
What happens when somites undergo EMT?
Redisperse to form dermatome (dermis under skin), myotome (longitudinal spinal muscle), and sclerotome (spinal vertebrae)
What is the importance of EMT in embryogenesis?
1) gastrulation; 2) neural crest cells & PNS; 3) somites
Why are the first 8 weeks of embryogenesis especially sensitive to teratogenic environmental factors?
Lots of growth and differentiation is happening in those first 8 weeks (gastrulation, neurulation, organ formation), so damage here will be more serious and pregnancy failure more likely
Why do multiple cell divisions of oocytes create potential risk for initiation of a successful pregnancy?
Each round of mitosis carries a small risk of chromosome damage or uneven distribution to daughter cells, which increases risk of pregnancy failing early on
Why does the delay at the initiation of meiosis for many years before ovulation create risk for initiation of a successful pregnancy?
Chromosome damage in oocyte due to radiation, environmental chemicals, etc.
Why is inflammation important for implantation?
Inflammation triggers and invasion of blood vessels into the implantation site, which are essential for formation of a placenta for oxygen/nutrition. Without it, pregnancy will likely fail.
What is the importance of the primitive node?
Provides signals for migrating cells to guide them during gastrulation
What is the notochord?
Transient structure that influences how the embryo folds by secreting SHH to guide tissue differentiation and forms the longitudinal axis for the embryo
What are the 3 body axes and when are they determined during embryonic development?
1) dorsal-ventral (blastocyst)
2) cranial-caudal (blastocyst)
3) left-right (bilayered disc)
How is the left-right body axis formed?
FGF8 made in primitive node is accumulated on the left side due to cilia flow > express Nodal > express Lefty2 > express PITX2, which is the most important for left side development
Could also be due to gap junctions allowing electrical or chemical stimulation on one side
What is the impact of dysfunctional or immotile cilia in the primitive node?
Disrupt specification of left side
What can cause immotile cilia?
Inherited mutation of dyne protein that powers ciliary movement
Situs invertus
Internal organ location is flipped
What can cause situs invertus?
primary ciliary dyskinesia, kartagener’s syndrome, or environmental influences in early development
How does closure of neural tube happen?
Started in cervial region (middle) and goes cranial and caudal
What are the transient structures of the neural tube?
Anterior neuropore (cranial) and posterior neuropore (caudal)
When do the neuropores usually close?
Week 4
What is the most common neural tube defects?
Most happen when folds fail to fuse during week 4
Anencephaly
Upper cranium and brain fail to develop due to anterior neuropore failure to open
Spina bifida
General term for neural tube defects
SPina bifida occulta
Failure of spinal vertebrae to fuse over spinal cord
What is the importance of folic acid during pregnancy?
Reduces incidence of neural tube closure deficits by 70%
In what order do hormones exert action in oocyte maturation, ovulation, decimal thickening, and implantation?
FSH - LH - Progesterone - hCG - Progesterone
What do somites produce?
Vertebral bones
EMT is an integral part of ___
Formation of mesoderm/endoderm and neural crest cells
Neural crest gives rise to ___
peripheral nerve ganglia
Embryonic kidneys and gonads are derived from ___
intermediate mesoderm
Why does conception fail about 50% of the time?
DNA replication is error prone so each cell division has a probability of error. Chromosomes in non-dividing cells can be damaged by teratogens. Since ovulation occurs later in life, damage can accumulate over time.
Hormonal regulators of female reproduction
Pituitary FSH (oocyte maturation) & LH (ovulation and meiosis), progesterone (uterine thickening for implantation), and hCG (corpus luteum stimulation)
What changes have occurred by the time the oocyte is ready to implant?
Mitosis, zygote, morula, blastocyst + embryoblast by day 4. Implantation within 1.5 days.
What changes occur in the blastocyst wall as implantation begins?
Differentiates into cytotrophoblast and syncytiotrophoblast.
What changes occur in embryoblast as implantation begins?
New amniotic cavity and differentiation into epiblast and hypoblast layers
How are germ layers formed during gastrulation?
Epiblast cells migrate into primitive streak and emerge into new space opened below the epiblast
What are the first internal structure within the embryo and how do they form?
1) Notochord formed from midline mesoderm ahead of primitive pit as a result of gastrulation
2) Neural tubes from ectodermal folds on either side of midline rise up at cervical region and fuse towards head and tail to detach from ectoderm
How does the notochord and neural tube lead to neural crest formation?
Cells from apex of neural tube undergo EMT and migrate throughout the body
Neural tube defects
1) Spina bifida (general)
2) Occulta - defect covered by skin
3) Meningocoele/meningomyelocoele - exposed cysts containing the intact spinal cord
4) Rachischisis - open and unfolded neural tube (paralysis)
Folic acid can reduce risk
