Embryology Flashcards
What is the conceptus called during weeks 1-2 of development?
Zygote
Morula
Blastocyst
What are the major events that occur during weeks 1-2 of development?
Fertilization
Implantation
What weeks do the embryonic period refer to?
Weeks 3-8
What is the conceptus referred to during weeks 3-8?
Embryo
What is the major event during weeks 3-8?
Organogenesis
What do weeks 9-38 refer to?
Fetal period
What is the conceptus named during weeks 9-38?
Fetus
What major events occur during weeks 9-38?
Growth
Maturation
Describe the control of development:
Teratology:
Study of abnormal development
Birth defect:
Birth defect = Congenital abnormality = Congenital anomaly = abnormality present at birth.
Malformation:
Developmental structural abnormality arising in embryonic period (weeks 3-8).
Deformation:
Developmental structural abnormality 2ndary to extrinsic disruption after embryonic period
Sequence:
Multiple abnormalities all developing in a sequence
Syndrome:
Multiple abnormalities developing in no known sequence
What normally occurs during the pre-fertilization stage of development?
Normal development : Meiosis produces egg and sperm.
Abnormal development : Genetic abnormalities
- Example : Downs syndrome (Trisomy 21)
Describe broad results of normal vs abnormal development during weeks 1 and 2:
Normal development of fertilized egg:
- Mitosis
- Move to uterus
- Begin implantation in uterine wall
Abnormal development :
- Not susceptible to teratogens.
- So, abnormal development is 2ndary to inherited.genetic abnormalites.
- ~40-50% of all conceptuses spontaneously abort (= miscarriage) during these weeks 2ndary to lethal genetic abnormalities.
What is a miscarriage?
loss of an embryo or fetus before the 20th week of pregnancy (before it is viable).
Broadly describe the results of normal vs. abnormal development during the embryonic period (weeks 3-8):
Normal development :
- Form organs (Organogenesis)
Abnormal development :
- Susceptible to all types of birth defect etiologies.
- This is the most vulnerable time for the embryo
- Major anomalies develop since development of a structure is disrupted when it is made of only a few cells. These now-abnormal cells then build an abnormal organ.
Broadly describe the results of normal vs. abnormal development during the fetal period:
Normal development:
- Growth and maturation
Abnormal development:
- Susceptible to all types of birth defect etiologies.
- Minor anomalies develop since development of a structure is disrupted when it is already nearly fully formed.
What direction does development occur in?
Cranial –> caudal
Proximal –> distal
What are the conventional views of a developing conceptus?
The placenta essentially functions as the ___, ___ and ____ for the fetus. Explain.
The placenta functions as the fetal lungs, kidneys and GI tract.
The placenta is a vascular structure formed by the embryo. In it maternal blood releases nutrients and O2 into the fetal blood, and fetal blood releases wastes into the maternal blood.
Does fetal and maternal blood mix via the placenta?
No
Where does fetal urination occur?
Into the amniotic fluid
What does the fetus breathe?
Amniotic fluid
Where does the fetus defecate in utero?
The fetus does not defecate in utero
What occurs during Week 1 of development?
Fertilization
Cell division (mitosis)
Zygote → Morula → Blastocyst
Implantation in uterus
What occurs during week 2 of development?
-The inner cell mass forms 2 layers (bilayer)
- Epiblast –> embryo
- Hypoblast –> disappear
- Amniotic cavity appears.
- A primitive streak is formed
- forms in the epiblast
- Dorsal, caudal midline
Gastrulation:
Process of forming the 3 germ layers from the primitive streak.
Ectoderm, Mesoderm, Endoderm
Trilaminar embryo are formed from _____
the three germ layers
What is somite formation?
The formation of horizontal body segments
What is neurulation?
The formation of the nervous system
What 4 things occur during Week 3 of development?
Gastrulation
Trilaminar embryo
Somite formation
Neurulation
Mitosis of the epiblast occurs at the ____, resulting in the formation of new cells, which migrate under the _____ layer and form two new layers, the ___ and ___.
Mitosis of the epiblast occurs at the primitive streak, resulting in the formation of new cells, which migrate under the epiblast layer and form two new layers, the mesoderm and endoderm.
What structures arise from the ectoderm?
outer layer of skin (epidermis) and teeth (enamel), nervous system
What structures arise from the mesoderm?
muscle (all types), bone, connective tissues, blood and blood vessels, deeper layer of skin (dermis)
What structures arise from the endoderm?
Epithelial lining of GI and respiratory tracts
Mesenchyme:
Aggregates of undifferentiated cells (mostly from mesoderm)
Somites:
horizonal body segments (trunk, limb) composed of skin, muscle, bone, connective tissue
(color schemes in pictures are different)
Horizontal body segments form from _____ during somite formation.
Horizontal body segments form from mesoderm (paraxial) during somite formation.
More somites are added at the caudal end.
In their horizontal territory, each somite divides into (3):
Dermatome → skin dermis
Myotome → muscle
Sclerotome → bone
What occurs during week 4 of development?
Organogenesis
- Folding : Longitudinal, Transverse
- Germ layer differentiation
- Pharyngeal arches
- Heart forms and begins beating
- 4 limbs begin developing
Week 4 begins with a ___ embryo. Folding creates a ___ trunk and moves the heart into the _____.
Week 4 begins with a flat embryo. Folding creates a 3D trunk and moves the heart into the newly formed thorax.
During longitudinal folding, the cranial and caudal ends fold _____, which moves the heart from cranial into the new thorax.
Transforms the conceptus from a flat embryo to a ___-shaped embryo.
During longitudinal folding, the cranial and caudal ends fold ventrally, which moves the heart from cranial into the new thorax.
Transforms the conceptus from a flat embryo to a C-shaped embryo.
During ____ folding, the lateral sides of the embryo fold ventrally until they meed and fuse in the ventral/anterior midline.
During transverse folding, the lateral sides of the embryo fold ventrally until they meed and fuse in the ventral/anterior midline.
Transverse folding creates body cavities (called ___) and the ____.
Transverse folding creates body cavities (called coelom) and the GI tract.
At the beginning of Week ___, most major organ systems are present but rudimentary. During these weeks there is continued development of structures → fairly mature by end of Week 8.
Week 5
_____ folding moves the heart into the newly formed thorax during week 4.
Longitudinal
Longitudinal folding moves the ____ into the newly formed thorax during week 4.
heart
Longitudinal folding moves the heart into the newly formed ____ during week 4.
thorax
______ folding creates a 3-D trunk. This brings the right and left sides together in ventral midline.
Transverse
Transverse folding creates a ____. This brings the right and left sides together in ventral midline.
3-D trunk
Transverse folding creates a 3-D trunk. This brings the right and left sides together in _____ midline.
ventral
Describe the steps of development of blood-carrying vessels:
Mitosis of mesoderm cells (called angioblasts) –> clusters everywhere
Lumens develop
Vessel walls create blood cells. Later, blood cells arise in liver, spleen and bone marrow.
Fusion of neighbor vessels, to form network of interconnected vessels.
By week ___, the embryo is already large enough to need heart and vessels to carry blood everywhere. The heart pumps blood to the ___ and ____ via arteries and veins.
By week 4, the embryo is already large enough to need heart and vessels to carry blood everywhere. The heart pumps blood to the body and placenta via arteries and veins.
The heart starts as a right and left ____, called ____.
The heart starts as a right and left blood vessel, called cords.
The ____ is the most cranial structure of the early embryo
heart
Transverse folding during week ___ brings the 2 cords together in anterior midline, which fuse and create one heart ___. It develops all 3 layers. (____, _____, and _____)
Transverse folding during week 4 brings the 2 cords together in anterior midline, which fuse and create one heart tube. It develops all 3 layers. (myocardium, epicardium, and endocardium)
The heart tube begins to beat on day ____.
22 or 23
What is the order of blood flow through the heart tube?
Sinus venosus –> atrium –> ventricle –> bulbus cordis –> truncus arteriosus
The heart tube is a single, straight tube (no right and left divisions inside yet).
It is subdivided into the regions above.
What are the 5 structures important for atrial subdivision?
2 sequential septa; 3 sequential openings:
Septum primum
Foramen primum
Foramen secundum
Septum secundum
Foramen ovale
(always need a hole in the wall between the atria)
The septum primum grows from the ___ to the ____.
endocardial cushion
The first hole formed during atrial subdivision is called WHAT?
The foramen primum
A second septum, called the ______ grows to the ____ of the first (septum primum), from the roof to the endocardial cushion during atrial subdivision.
A second septum, called the septum secundum grows to the right of the first (septum primum), from the roof to the endocardial cushion during atrial subdivision.
What is the name of the hole formed in the septum secundum?
Foramen ovale
Describe the 6 steps of the formation of 5 structures during atrial subdivision:
- Septum primum grows from roof to endocardial cushion. (1,2,3)
- Always need a hole. 1st is named Foramen primum. It disappears. (2,3,4)
- Holes form in the septum. (3)
- Coalesce to form 1 hole. Named Foramen secundum. (4)
- A second septum grows on the right side of the first, from roof to endocardial cushion. Named Septum secundum. (5,6)
- This septum has a hole, named Foramen ovale. This septum covers the Foramen secundum. (6)
Describe the blood flow through the heart after the completion of atrial subdivision:
Blood flows from the right atrium –> foramen ovale –> foramen secundum –> left atrium.
This occurs due to pressure differences in the atria.
The Septum ___ is pushed to the left and acts as an open valve of the foramen ___.
The Septum primum is pushed to the left and acts as an open valve of the foramen ovale.
(Ventricle subdivision) The interventricular _____ grows from the floor to the endocardial cushion, closing the interventricular ______.
(Ventricle subdivision) The interventricular septum grows from the floor to the endocardial cushion, closing the interventricular foramen.
Most mistakes during ventricle subdivision occur near the ________.
endocardial cushion
The atrioventricular valves arise from the ______.
endocardial cushion
The truncus arteriosus subdivides into the ____ and the _____.
The truncus arteriosus subdivides into the aorta and the pulmonary trunk/artery.
The ____ subdivides into the aorta and the pulmonary trunk/artery.
The truncus arteriosus subdivides into the aorta and the pulmonary trunk/artery.
The division of the truncus arteriosus into the aorta and the pulmonary trunk/artery requires forming a ______ and requires ______ to achieve the final positions.
The division of the truncus arteriosus into the aorta and the pulmonary trunk/artery requires forming a longitudinal septum and requires twisting to achieve the final positions.
What are the 6 structures necessary uniquely for in-utero circulation?
2 arteries: Umbilical arteries (iliac artery → placenta)
1 vein: Umbilical vein (placenta → Inf. Vena cava)
3 shunts:
- Ductus venosus to help umbilical vein bypass liver
- Oval foramen between heart atria to bypass pulmonary circulation. Blood flows right atrium → left atrium (right-to-left shunt)
- Ductus arteriosus between pulmonary artery and aorta to bypass pulmonary circulation. Blood flows pulmonary artery → aorta (right–to left shunt)
What is the purpose of the ductus venosus?
Helps the umbilical vein bypass the liver
What is the purpose of the oval foramen?
Allows blood to flow through the right atrium into the left atrium (right to left shunt) to bypass the pulmonary circulation.
Most IVC blood passes through foramen → LA → LV → aorta → upper body
Most SVC blood passes RA → RV → pulm.artery
What is the function of the ductus arteriosus?
Located between the pulmonary artery and aorta to bypass pulmonary circulation. The blood flows through the pulmonary artery → aorta (right–to left shunt).
What is the remnant of the umbilican vein called?
Ligamentum teres
What are the remnants of the umbilical arteries called?
Medial umbilical ligaments
What is the remnant of the ductus venosus?
Ligamentum venosum
(Closes at birth, premanent by 1 week)
What is the remnant of the oval foramen?
Oval fossa
(Closes at birth by septum primum valve; permanent by ~5 years)
The ductus arteriosus can be clinically kep open with ____.
Prostaglandins E1 and E2
The ductus arteriosus can be clinically closed with _____.
Indomethacin
