Embryology short answers Flashcards
Describe the events that occur between implantation and fertilisation
Fertilization -> Zygote -> Cleavage -> Morula -> Blastocyst -> Blastocyst Hatching -> Implantation
Zygote - 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.
Cleavage - 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.
Morula - A mass of about 30 cells formed around the 96 hour mark (3-4 days).
Blastocyst - On the 4th day the morula becomes a blastocyst as it forms a thin outer layer and an inner cell mass.
Hatched Blastocyst - the zona pellucida dissolves and there is a free (or hatched) blastocyst.
Implantation - The trophoblasts that surround the free blastocyst form the invasive syncytiotrophoblasts that enable implantation around day 8-9
Briefly describe the process of gastrulation and include in your answer the name of the 3 germ layers that are formed in this process.
Gastrulation is the formation of the trilaminar disc or embryo from the bilaminar disc during the third week of development.
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.
List 2 structures/tissues that are derived from each of these layers. ( 3 marks)
The ectoderm forms the Epidermis and the CNS.
The mesoderm forms the urogenital system and the heart.
The endoderm forms the epithelial linings of the respiratory and digestive tracts and the glandular organs of the liver and pancreas.
Describe the three primary germ layers and list the organs that develop from each of them
The three primary germ layers are the Endoderm, Mesoderm and Ectoderm.
The Ectoderm forms the Epidermis, CNS, eye and inner ear.
The Mesoderm comprises of three layers in itself. The paraxial, intermediate and lateral mesoderm.
The paraxial mesoderm is found either side of the neural plate and forms somites that then form the axial skeleton and associated musculature and overlying dermis.
The intermediate mesoderm is located either side of the dorsal aorta and forms the urogenital system.
The lateral mesoderm is made up of two layers somatic (parietal) and splancnic (viscera). It forms the heart, diaphragm and linings of the cavities.
The endoderm forms the linings of the respiratory and digestive tracts as well as the associated glandular organs of the pancreas and liver.
List three extra-embryonic coeloms and describe their progression
Blastocystic cavity, Chorionic cavity, Amniotic 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.
The chorionic cavity is also called the gestational sac. 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.
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.
Compare dizygotic to monozygotic twins. Explain the complications of monochorionic monoamniotic twins
Dizygotic twins are also called fraternal or non-identical twins. They are the result of two different fertilization events in which two mature oocytes are fertilized by two different spermatozoa.
Monozygotic twins are a result of a single fertilisation event and are also known as identical twins. They have the same genetic code and are formed due to a misstep in the developmental sequence. The time twinning occurs determines whether they will be monochorionic and monoamniotic or diamniotic.
It is most common for twinning to occur in the blastocyst stage. In this case twins will share the same chorion but have separate amnion.
It is very rare for twinning to occur during the bilaminar stage. If this occurs just before the formation of the primitive streak twins will be share a chorion, amnion and placenta.
Due to the proximity of the twins during development this can result in 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 the structures that are formed from the ectoderm, mesoderm and endoderm respectively
ectoderm - epidermis, cns, eye, inner ear, neural crest cells.
mesoderm - Skeletal muscles, connective tissue (bones, cartilage, blood cells), urogenital system, heart and CVS, visceral smooth muscle and serosa linings of cavities.
Endoderm - epithelial linings of respiratory and digestive tracts as well as glandular organs of the liver and pancreas.
Explain the development of the amniotic cavity and how this impacts on the yolk sac. What tests can be performed on amniotic fluid during pregnancy?
The amnion first forms between two layers of epiblast cells dorsal to the embryo. As the embryo folds the yolk sac is folded into it and the amnion grows to surround the embryo, merging around the remaining part of yolk sac that will be form the umbilical vesicle.
Amniocentesis, Alpha-fetoprotein assays, spectrophotometric studies can be performed during pregnancy to test for a range of chromosomal anomalies
Explain the formation of the abdominal cavity and the primary tissues that form the gastrointestinal tract. What is an omphalocoele -when is it’s occurrence normal or abnormal?
The abdominal cavity is initially a part of one large cavity including the thorax. As the embryo undergoes massive growth and begins folding the septum transversum is tucked underneath the primitive heart tubes to create two cavities, thoracic and abdominal, with two pericardalperitoneal canals communicating between the two. As the lungs develop they will form a membrane over these canals to fully separate the cavities.
The GI tract is primarily comprised of endodermal epithelial lining and glandular organs of the liver and pancreas, mesodermal visceral smooth muscle and the serosa lining of the cavity also formed by the mesoderm.
Omphalocoele is the term used to describe the herniation of the GI tract into the umbilical vesicle during embryonic development and occurs due to massive growth of the intestines and not enough space in the abdominal cavity for them. It is normal up until weeks 10 of development when the omphalocoele should return to the abdominal cavity.
Discuss the cause of spina bifida and comment on the range of abnormality.
Spina bifida is a splitting of the vertebral arches as a failure of the neural tube to close.
The abnormality can range from asymptomatic to incompatible with life.
Spina bifida occulta 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.
Spina bifida cystica is 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.
Rachischisis is the 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.
