Exam 1 Embryology Thread

Question 1

Fetal Dating

Answer 1

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

Question 2

Ovulation

Answer 2

LH surge from anterior pituitary on day 14.

Releases secondary oocyte and 1st polar body surrounded by zona pellucida and corona radiata.

Question 3

Capacitation

Answer 3

• 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

Question 4

Week 1

Summary

Answer 4

Fertilization and Cleavage

1. Fertilization
   
   • Fusion of male and female gametes to form a zygote
2. Cleavage
   
   • Series of mitotic divisions
   • Zygote makes week-long journey down oviduct
   • Zygote becomes blastocyst by end of week 1

Question 5

Fertilization

Overview

Answer 5

Usually occurs in the distal 1/3 of oviduct called ampulla

Only capacitated sperm can penetrate corona radiata to reach the zona pellucida.

1. Sperm binds to zona pellucida and undergoes acrosome reaction.
2. Cortical reaction follows which makes oocyte impermeable to other sperm.
3. Fusion of secondary oocyte and sperm allow sperm nucleus and tail to enter oocyte.
4. Sperm nucleus swells to form male pronucleus.
5. This initiates completion of meiosis II by seocndary oocyte to form mature ovum ⇒ becomes female pronucleus.
6. Fusion of male and female pronuclei forms the zygote.

Question 6

Acrosome Reaction

Answer 6

• Triggered when sperm reaches zona pellucida of oocyte.
• Release of acrosin
  
  • Hydrolytic enzyme that allows penetration of the zona pellucida by sperm cell

Question 7

Cortical and Zona

Reactions

Answer 7

Triggered when a single sperm penetrates the zona pellucida and contacts the surface of the oocyte.

Prevents polyspermy.

• Cortical reaction
  
  • release of cortical oocyte granules (lysosomes)
  • makes oocyte membrane impermeable to other sperm
• Zona reaction
  
  • alters structure of zona pellucida
  • no other sperm can bind to or penetrate it

Question 8

Cleavage

Answer 8

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

Question 9

Morula

Answer 9

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

Question 10

Blastocyst Formation

Answer 10

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.

Question 11

Totipotent vs Pleuripotent

Answer 11

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

Question 12

Week 2

Summary

Answer 12

“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
• Extraembryonic mesoderm appears midweek
  
  • Somatic and splanchnic layers
• Establishment of the placenta

Question 13

Implantation

Answer 13

Erosion of the endometrial tissues by enzymatic activity of the outer layer of the trophoblast.

Question 14

Blastocyst Connections

Answer 14

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.

Question 15

Blastocyst

Day 9

Answer 15

• Two cavities:
  
  • Amniotic cavity
  • Primary yolk sac
• Embryoblast is two layers:
  
  • Epiblast
  • Hypoblast
• Trophoblast is two layers:
  
  • Syncytiotrophoblast
    
    • Produces enzymges to break down endometrial wall and facilitate implantation
    • Lacunar spaces here will soon communicate with maternal sinusoids
  • Cytotrophoblast
    
    • undergo mitosis and fuse with syncytiotrophoblast forming a syncytium

Question 16

Blastocyst

Day 12

Answer 16

• Implantation complete
• Extraembryonic mesoderm appears and breaks down into two layers:
  
  • Somatic layer ⇒ attached to cytotrophoblast
  • Splanchnic layer ⇒ attached to yolk sac
• 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

Question 17

Blastocyst

Day 13

Answer 17

• 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.

Question 18

Bubble Analogy

Answer 18

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.

Question 19

Dorsal/Ventral

Body Axis

Answer 19

Seperation of inner cell mass (embryoblast) into two distinct layers determines the dorsal/ventral axis.

Question 20

Ectopic Implantation

Answer 20

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.

Question 21

Placenta Previa

Answer 21

Placenta located inferior to the embryo.

Can support the fetus to term.

May produce heavy vaginal bleeding and placenta delivered first at birth.

Question 22

Week 3 and 4

Summary

Answer 22

Begins the embryonic period.

Basic organization of major body organ systems established.

Period of rapid cell division and differentiation.

Very susceptible to teratogens.

1. Gastrulation
2. Notochord formation
3. Mesoderm differentiation
4. Neurulation
5. Embryonic folding
6. Development of a body plan

Question 23

Gastrulation

Answer 23

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.
• 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.

Question 24

Ectoderm Derivatives

Answer 24

1. CNS
2. PNS
3. Retina, lens, iris, cornea of the eye
4. Epidermis
5. Glands in the skin
6. Pituitary gland
7. Enamel of teeth

Question 25

Mesoderm Derivatives

Answer 25

1. Most bones and muscles 2. Lining of the body cavities 3. Heart and blood vessels 4. Dermis 5. Kidneys and ureters 6. Gonads and ducts 7. Dentin and tooth pulp

Question 26

Endoderm Derivatives

Answer 26

1. Lining of the respiratory tract 2. Thyroid and parathyroid glands 3. Thymus and linings of tonsillar crypts 4. Linging of the GI tract and associated gland ducts 5. Parenchyma of liver and pancreas 6. Lining of urinary bladder, urethra, lower 2/3 of vagina 7. Lining of tympanic cavity

Question 27

Epithelial-Mesenchymal Transformation

Answer 27

During gastrulation, epiblast cells detach through **downregulation of cadherin** and become **mesenchymal cells** ⇒ _epithelial-mesenchymal transformation._ Mesenchyme refers to a loose collection of cells that are **undifferentiated** and can form any layer of the embryonic disc.

Question 28

Primordial Germ Cells

Answer 28

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.

Question 29

Fate Mapping

Answer 29

* 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**.

Question 30

Notochord Formation

Answer 30

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.

Question 31

Mesoderm Differentiation

Answer 31

End of week 3 to week 4. Mesoderm becomes segregated into 4 functional groups of cells: Medial to lateral: 1. **Notochord** 2. **Paraxial mesoderm** * Will form _somites_ that eventually form the _vertebrae and deep muscles of the back_ 3. **Intermediate mesoderm** * Forms _genitourinary structures_ 4. **Lateral plate mesoderm** * Splits into two layers 1. **Somatic layer** ⇒ fixed to surface ectoderm * Forms _muscles, bones, and skin of body wall_ 2. **Splanchnic layer** ⇒ fixed to endoderm * Forms the _wall of many of the hollow viscera_

Question 32

Neurulation

Answer 32

Begins in week 3 and continues through week 4. Occurs due to inductive influence of **Sonic hedge hog** (Shh) secreted by the **notochord**. 1. Midline surface ectoderm _thickens_ to form **neural ectoderm** * Raised lateral edges of neural ectoderm called **neural folds** that flank the midline **neural groove** 2. Neural plate lengthens via **convergent extension** 3. Neural folds _meet in posterior midline_ to form the **neural tube** 4. 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** 5. Neural tube sicks below the surface ectoderm

Question 33

Neural Crest Cells

Answer 33

**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.**

Question 34

Embryonic Folding

Answer 34

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

Question 35

Body Plan Development

Answer 35

Three body axes formed at different times: * **Dorsal/ventral axis** * **Week 2** when embryoblast forms epiblast (dorsal) and hypoblast (ventral) * **Cranial (rostral)/caudal axis** * **Week 3** by the appearance of the primitive node (cranial) and primitive streak (caudal) * **Medial/lateral axis** * **Midweek 3** by formation of the notochord in the midline * **Left/right axis** * Refined in **week 3**

Question 36

Right/Left Asymmetry

Answer 36

1. **Fibroblast growth factor 8 (FGF8)** secreted by cells in the _primitive node and primitive streak_. 1. Cells have nodal cilia that beat _clockwise_ ⇒ causes **gradient of FGF 8 to form on the left side** 2. FGF8 causes _cells on the left_ to activate the ***NODAL*** gene that encodes for **transforming growth factor (TGF) Nodal** 3. **Nodal (TGF)** results in accumulation of **serotonin (5-HT)** on the _left_ leading to induction of **MAD3** 4. MAD3 **inhibits NODAL on the right side** 5. _Cells on the right_ also express **Monamine oxidase (MOA)** that **breaks down serotonin (5-HT)** providing further right/left asymmetry.