Exam 1 Embryology Thread Flashcards
Fetal Dating
Fertilization age ⇒ from the time of fertilization.
Menstrual age ⇒ from the start of the mother’s last menstrual period.
Embryonic period ⇒ first 8 weeks following fertilization
Fetal period ⇒ time after 8 weeks following fertilization
Ovulation
LH surge from anterior pituitary on day 14.
Releases secondary oocyte and 1st polar body surrounded by zona pellucida and corona radiata.
Capacitation
- Removal of seminal proteins from acrosome
- Occurs during sperm migration through uterine tube
- Only capacitated sperm can penetrate the corona radiata and undergo acromsome reaction
Week 1
Summary
Fertilization and Cleavage
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Fertilization
- Fusion of male and female gametes to form a zygote
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Cleavage
- Series of mitotic divisions
- Zygote makes week-long journey down oviduct
- Zygote becomes blastocyst by end of week 1
Fertilization
Overview
Usually occurs in the distal 1/3 of oviduct called ampulla
Only capacitated sperm can penetrate corona radiata to reach the zona pellucida.
- Sperm binds to zona pellucida and undergoes acrosome reaction.
- Cortical reaction follows which makes oocyte impermeable to other sperm.
- Fusion of secondary oocyte and sperm allow sperm nucleus and tail to enter oocyte.
- Sperm nucleus swells to form male pronucleus.
- This initiates completion of meiosis II by seocndary oocyte to form mature ovum ⇒ becomes female pronucleus.
- Fusion of male and female pronuclei forms the zygote.
Acrosome Reaction
- Triggered when sperm reaches zona pellucida of oocyte.
- Release of acrosin
- Hydrolytic enzyme that allows penetration of the zona pellucida by sperm cell
Cortical and Zona
Reactions
Triggered when a single sperm penetrates the zona pellucida and contacts the surface of the oocyte.
Prevents polyspermy.
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Cortical reaction
- release of cortical oocyte granules (lysosomes)
- makes oocyte membrane impermeable to other sperm
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Zona reaction
- alters structure of zona pellucida
- no other sperm can bind to or penetrate it
Cleavage
The repeated mitotic division of the zygote that results in an increasing number of daughter cells ⇒ blastomeres.
New proteins translated from maternal mRNA stored in ovum.
Cells double in number with each division during early cleavage ⇒ doubling
Blastomeres still contained within the zona pellucida and become smaller with each division ⇒ compaction
Morula
Zygotes reaches morula stage when it is between 16-32 cells.
Solid ball with an inner cells mass and outer cell mass.
Inner cell mass ⇒ embryo and fetus
Outer cell mass ⇒ placenta and supportive membranes
Blastocyst Formation
32 cell morula begins to accumulate fluid and become hollow.
Fluid filled cavity called the blastocoele.
Morula now known as the blastocyst.
Cells of the blastocyst differentiate into two groups:
Inner cell mass ⇒ embryoblast
Outer cell mass ⇒ trophoblast
Blastocyst escapes through a hole in the zona pellucida to start implantation on day 7.
Totipotent vs Pleuripotent
All daughter cells during early cleavage are totipotent.
At the blastocyst stage cells become pleuripotent.
Embryoblast ⇒ able to form the whole person
Trophoblast ⇒ only able to form supporting structures
Week 2
Summary
“Week of Twos”
- Implantation of the blastocyst
- Division of the two layers of the blastocyst into 4 layers
- Embryoblast ⇒ epiblast and hypoblast
- Trophoblast ⇒ cytotrophoblast and syncytiotrophoblast
- Determination of the dorsal/ventral body axis
- Two cavities develop: amniotic cavity and yolk sac
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Extraembryonic mesoderm appears midweek
- Somatic and splanchnic layers
- Establishment of the placenta
Implantation
Erosion of the endometrial tissues by enzymatic activity of the outer layer of the trophoblast.
Blastocyst Connections
Cells of the trophoblast connected via tight junctions.
Cells of the embryoblast connected via gap junctions.
Protects the embryo from rejection by the mother’s immune system.
Blastocyst
Day 9
- Two cavities:
- Amniotic cavity
- Primary yolk sac
- Embryoblast is two layers:
- Epiblast
- Hypoblast
- Trophoblast is two layers:
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Syncytiotrophoblast
- Produces enzymges to break down endometrial wall and facilitate implantation
- Lacunar spaces here will soon communicate with maternal sinusoids
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Cytotrophoblast
- undergo mitosis and fuse with syncytiotrophoblast forming a syncytium
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Syncytiotrophoblast
Blastocyst
Day 12
- Implantation complete
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Extraembryonic mesoderm appears and breaks down into two layers:
- Somatic layer ⇒ attached to cytotrophoblast
- Splanchnic layer ⇒ attached to yolk sac
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Connecting stalk maintains connection between trophoblast and embryo
- Will become central core of umbilical cord
- Maternal blood from maternal sinusoids fill in lacunar spaces of syncytiotrophoblast ⇒ establishes hemochorial placenta
Blastocyst
Day 13
- Blastocyst demonstrates polarity
- Embryonic pole attached to the connecting stalk
- Opposite side is the abembryonic pole
- Trophoblast now called chorion
- Blastocyst cavity now called chorionic cavity
- Lined by mesoderm so also called extraembryonic coelom
Bubble on a bubble suspended in a balloon.
Bubble Analogy
Bubble on a bubble suspended inside a balloon.
Epiblast & amnion ⇒ bubble on top
Hypoblast & yolk sac ⇒ bubble below
Suspended within the ballon of the chorionic cavity by the connecting stalk of extraembryonic mesoderm.
Dorsal/Ventral
Body Axis
Seperation of inner cell mass (embryoblast) into two distinct layers determines the dorsal/ventral axis.
Ectopic Implantation
Optimum site for implantation is anterior and posterior wall of the body of the uterus.
Ectopic implantations in other locations occur.
Usually rupture causing bleeding or may be absorbed by maternal immune cells.
Placenta Previa
Placenta located inferior to the embryo.
Can support the fetus to term.
May produce heavy vaginal bleeding and placenta delivered first at birth.
Gastrulation
The process of converting the bilaminar embryonic disc into a trilaminar disc.
- Begins early in week 3 with appearance of the primitive streak and primitive node in the epiblast.
- Primitive node is at the cranial end establishing the cranial/caudal axis.
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Epiblast cells disconnect and migrate into the primitive node and primitive streak.
- First they displace hypoblast cells replacing them with endoderm
- Next they form mesoderm
- Remaining epiblast cells become ectoderm.
- Oropharyngeal membrane @ cranial end will eventually form the mouth
- Cloacal membrane @ caudal end will eventually form the anus.
- At the end of gastrulation, the primitive node and primitive streak regress and disappear.
**Epiblast forms all three germ layers of the trilaminar embryonic disc.
Primordial Germ Cells
A very small group of epiblast cells that migrate early into the primitive streak establishes a line of primordial germ cells (PCGs).
PCGs migrate from the primitive streak into the posterior wall of the yolk sac.
During 4th week, PCGs will migrate from yolk sac, through dorsal mesentary, and into developing gonads by week 5.
Fate Mapping
- Epiblast cells just cranial to the primitive node migrate into the primitive node ⇒ becomes endoderm and prechordal plate ( important in development of head region)
- Epiblast cells at edges of the primitive streak pass into the primitive streak ⇒ become mesoderm that then migrate into head region to form cardiogenic area
- Lastly, waves of epiblast cells migrate into primitive node and primitive streak to form notochord and body mesoderm.
- At the end of gastrulation, remaining epiblast cells become ectoderm.
Notochord Formation
After formation of cardiogenic mesoderm, subsequent waves of mesoderm aggregate at the midline. The first of these gives ride to the notochord.
After migrating through the primitive streak, cells that become the notochord spend a short time within the endoderm layer.
They leave the endoderm to form a semi-solid cylindrical midline rod ⇒ notochord.
Notochord forms the central axis of the embryonic disc ⇒ medial/lateral axis.
Mesoderm Differentiation
End of week 3 to week 4.
Mesoderm becomes segregated into 4 functional groups of cells:
Medial to lateral:
- Notochord
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Paraxial mesoderm
- Will form somites that eventually form the vertebrae and deep muscles of the back
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Intermediate mesoderm
- Forms genitourinary structures
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Lateral plate mesoderm
- Splits into two layers
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Somatic layer ⇒ fixed to surface ectoderm
- Forms muscles, bones, and skin of body wall
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Splanchnic layer ⇒ fixed to endoderm
- Forms the wall of many of the hollow viscera
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Somatic layer ⇒ fixed to surface ectoderm
- Splits into two layers
Neurulation
Begins in week 3 and continues through week 4.
Occurs due to inductive influence of Sonic hedge hog (Shh) secreted by the notochord.
- Midline surface ectoderm thickens to form neural ectoderm
- Raised lateral edges of neural ectoderm called neural folds that flank the midline neural groove
- Neural plate lengthens via convergent extension
- Neural folds meet in posterior midline to form the neural tube
- Neural tube closure starts from the middle and proceeds both cranially and caudally.
- Cranial unfused portion of neural tube ⇒ anterior neuropore
- Close on day 25
- Failure ⇒ anencephaly
- Caudal unfused portion of neural tube ⇒ posterior neuroport
- Closes on day 28
- Failure ⇒ spina bifida
- Cells from the lateral edges of the neural folds become detached to form the neural crest
- Cranial unfused portion of neural tube ⇒ anterior neuropore
- Neural tube sicks below the surface ectoderm
Neural Crest Cells
Neural crest tissue develops from cells that bud off the lateral aspect of the neural folds during neurulation.
Undergo epithelial-mesenchymal transformation and migrate throughout the body of the embryo.
Contributes to a wide array of organs and structures.
Critical to development of heart, craniofacial structures, most of the peripheral sensory and motor ganglia.
Embryonic Folding
Folds in the sagittal and transverse planes.
Flat disc ⇒ tubes within a tube
Body tube ⇒ gut tube, vascular tube (aorta), neural tube
Body cavity tube ⇒ coelom
- Cardiogenic area initially cranial moves into the body cavity
- Endoderm incoorporated into the gut tube and umbilical cord
- Splitting of the lateral plate mesoderm to form the lining of the coelom
- Closure of the ventral body wall
Body Plan
Development
Three body axes formed at different times:
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Dorsal/ventral axis
- Week 2 when embryoblast forms epiblast (dorsal) and hypoblast (ventral)
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Cranial (rostral)/caudal axis
- Week 3 by the appearance of the primitive node (cranial) and primitive streak (caudal)
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Medial/lateral axis
- Midweek 3 by formation of the notochord in the midline
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Left/right axis
- Refined in week 3
Right/Left
Asymmetry
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Fibroblast growth factor 8 (FGF8) secreted by cells in the primitive node and primitive streak.
- Cells have nodal cilia that beat clockwise ⇒ causes gradient of FGF 8 to form on the left side
- FGF8 causes cells on the left to activate the NODAL gene that encodes for transforming growth factor (TGF) Nodal
- Nodal (TGF) results in accumulation of serotonin (5-HT) on the left leading to induction of MAD3
- MAD3 inhibits NODAL on the right side
- Cells on the right also express Monamine oxidase (MOA) that breaks down serotonin (5-HT) providing further right/left asymmetry.
