Embryology Flashcards
List the events that occur between fertilization and implantation
Fertilization -> Zygote -> Cleavage -> Morula -> Blastocyst -> Blastocyst Hatching -> Implantation
Describe zygote formation
Stage two. After fertilisation a mature oocyte (pronucleus) and polar body form. Male nucleus enlarges to form a pronucleus and the tail drops off. The pronuclei fuse and become a zygote.
Describe cleavage
Stage three. Cells begin to divide at about 30 hours after fertilization. Note that the zona pellucida remains the same size as the cells decrease in size as they divide.
Describe a morula
Stage four. A mass of about 30 cells formed around the 96 hour mark (3-4 days).
Describe a blastocyst
Stage five. On the 4th day after fertilisation the morula becomes a blastocyst as it forms a thin outer layer and an inner cell mass.
Describe a hatched blastocyst
Stage 6. The zona pellucida dissolves and there is a free (or hatched) blastocyst.
Describe implantation
Stage 7. The trophoblasts that surround the free blastocyst form the invasive syncytiotrophoblasts that enable implantation around day 8-9 after fertilisation
What is gastrulation?
the formation of the trilaminar disc or embryo from the bilaminar disc during the third week of development.
Describe the process of gastrulation.
At the end of the second week of development the primitive groove is formed at the caudal end of the bilaminar disc. The primitive streak is formed as the epiblast cells undergo invagination.
As the epiblast cells invaginate they displace the hypoblast cells forming the endoderm. Epiblast cells also move between the endoderm and epiblasts forming the mesoderm and the cells remaining in the epiblast become the ectoderm.
What are the three primary germ layers?
Ectoderm, mesoderm, endoderm.
List two structures/tissues that develop from the ectoderm.
Epidermis and CNS.
List two structures/tissues that develop from the mesoderm.
The urogenital system and the heart.
List two structures/tissues that develop from the endoderm.
Epithelial linings of the respiratory and digestive tracts and the glandular organs of the liver and pancreas.
List the organs that develop from the ectoderm.
Epidermis, CNS, eye and inner ear
List the organs that develop from the paraxial mesoderm.
Somites that then form the axial skeleton and associated musculature and overlying dermis
List the organs that develop from the intermediate mesoderm.
The urogenital system.
List the organs that develop from the lateral mesoderm.
The heart, diaphragm and linings of the cavities.
List the organs that develop from the endoderm.
The linings of the respiratory and digestive tracts as well as the associated glandular organs of the pancreas and liver.
What are the three layers of the mesoderm?
Paraxial, intermediate and lateral.
What are the two layers of the lateral mesoderm?
Splanchnic (visceral) and somatic (parietal)
List three extra embryonic coeloms and their multiple names.
Blastocystic cavity (yolk sac, umbilicle vesicle), Chorionic cavity (gestational sac, extra embryonic coelom), Amniotic cavity (sac)
Describe the progression of the blastocystic cavity
The blastocystic cavity is also called the yolk sac and the umbilical vesicle at different stages. The hypoblast cells migrate around the walls the blastocystic cavity and it becomes the yolk sac. During the folding of the embryo part of the yolk sac is incorporated into the embryo with the endoderm to become a primitive gut tube. Part of it remains as a communication (umbilical vesicle) and during gut development the small intestines herniate into this space until there is room in the abdominal cavity around the 10th week of development.
Describe the progression of the chorionic cavity.
The chorionic cavity is also called the gestational sac and extra embryonic coelom. Once hypoblast cells line the blastocystic cavity to form the yolk sac a cavity forms between the hypoblast cells and the outer layer of cytotrophoblast cells. This is the chorionic cavity (gestational sac) This disappears round the 8-9th week of gestation (7-8th week of development) as the amnion grows larger and takes up more space.
Describe the progression of the amniotic cavity.
The amniotic sac is formed between two layers of epiblast cells. It increases in volume to surround the folding embryo and merges around the umbilical vesicle to for the amniotic sac.
What are dizygotic twins?
Known as non-identical twins. They follow the normal developmental path and are a result of two different spermatozoa fertilising two different secondary oocyte.
What are monozygotic twins?
Result of a misstep in the developmental path. They arise from the same fertilization event and share the same genetic sequencing making them identical. The timing of twinning determines what sort of twinning occurs.
What are the different types of monozygotic twinning?
Monochorionic, monoamniotic, diamniotic.
How does monochorionic diamniotic twinning occur?
It is most common for twinning to occur in the blastocyst stage. In this case twins will share the same chorion (gestational sac) and the same placenta but have their own amnion.
How does monochorionic monoamniotic twinning occur?
It is rare for twinning to occur during the bilaminar stage just before the primitive streak appears. This results in shared amnion, chorion and placenta.
What are some complications of monochorionic monoamniotic twinning?
Birth weight differences, amniotic fluid problems, umbilical cord entanglement or compression, twin to twin transfusion syndrome, trap sequence and increased risk of birth defects.
List structures formed from the ectoderm.
Epidermis, cns, eye, inner ear, neural crest cells.
List structures formed from the mesoderm.
Skeletal muscles, connective tissue (bones, cartilage, blood cells), urogenital system, heart and CVS, visceral smooth muscle and serosa linings of cavities.
List structures formed from the endoderm.
Epithelial linings of respiratory and digestive tracts as well as glandular organs of the liver and pancreas.
What tests can be performed on amniotic fluid during pregnancy?
Amniocentesis and alpha-fetoprotein assay can both test for chromosomal abnormalities in the fetus. A spectrophotometric study may be used to test for hemolytic disease of the fetus.
Explain the formation of the abdominal cavity.
The abdominal cavity is orginally a part of one big thoracic abdominal cavity until the septum transversum is folded under the pericardial sac and the pericardialperitoneal canals are closed by the formation of the lungs.
What are the primary tissues that form the GI tract.
The epithelium formed by the endoderm, the visceral smooth muscle formed by the mesoderm and the serosa lining of the cavities also formed by the mesoderm.
What is an omphalocele and when is it normal?
when the gut is projected into the umbilical vesical due to lack of space in the abdominal cavity and is normal up until 10 weeks of development when it should retreat back into the abdominal cavity.
What is spina bifida?
A splitting of the vertebral arches as a failure of the neural tube to close.
List the different types of spina bifida.
Spina bifida occulta, spina bifida cystica, spina bifida meningocele, spina bifida meningomyelocele, rachischisis.
Describe spina bifida occulta.
Usually asymptomatic. Occurs in about 10% of otherwise normal people and is usually found incidentally on x-ray. Externally it may present as a patch of hair, dimple or discolouration of the lumbosacral area.
Describe Spine bifida cystica
More serious and has two presentations meningocele and meningomyelocele. Both will result in disability. Meningocele usually results in less serious disability as it is only a projection of the meninges and CSF whereas meningomyelocele also involves a herniation of the neural tissue and may be covered or uncovered. The level of disability is dependent on the level of the defect. The higher it occurs the worse the disability as it involves more nerves and muscles.
Describe rachischisis.
Most serious and is incompatible with life often involving anencephaly. The spinal cord is an open flattened mass of tissue and the cranial part of the neural tube has not closed.
When does the embryonic period end?
8 weeks.
The epididymis and ductus deferens develops from the
Mesonephric duct.
What vitamin is associated with a reduction in spina bifida?
Folic acid.
Human chorionic gonadotrophin is secreted by;
The trophoblast.
The axial skeleton derives from the;
Paraxial mesoderm