Embryology Flashcards
1
Q
List the events that occur between fertilization and implantation
A
Fertilization -> Zygote -> Cleavage -> Morula -> Blastocyst -> Blastocyst Hatching -> Implantation
2
Q
Describe zygote formation
A
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.
3
Q
Describe cleavage
A
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.
4
Q
Describe a morula
A
Stage four. A mass of about 30 cells formed around the 96 hour mark (3-4 days).
5
Q
Describe a blastocyst
A
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.
6
Q
Describe a hatched blastocyst
A
Stage 6. The zona pellucida dissolves and there is a free (or hatched) blastocyst.
7
Q
Describe implantation
A
Stage 7. The trophoblasts that surround the free blastocyst form the invasive syncytiotrophoblasts that enable implantation around day 8-9 after fertilisation
8
Q
What is gastrulation?
A
the formation of the trilaminar disc or embryo from the bilaminar disc during the third week of development.
9
Q
Describe the process of gastrulation.
A
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.
10
Q
What are the three primary germ layers?
A
Ectoderm, mesoderm, endoderm.
11
Q
List two structures/tissues that develop from the ectoderm.
A
Epidermis and CNS.
12
Q
List two structures/tissues that develop from the mesoderm.
A
The urogenital system and the heart.
13
Q
List two structures/tissues that develop from the endoderm.
A
Epithelial linings of the respiratory and digestive tracts and the glandular organs of the liver and pancreas.
14
Q
List the organs that develop from the ectoderm.
A
Epidermis, CNS, eye and inner ear
15
Q
List the organs that develop from the paraxial mesoderm.
A
Somites that then form the axial skeleton and associated musculature and overlying dermis
16
Q
List the organs that develop from the intermediate mesoderm.
A
The urogenital system.
17
Q
List the organs that develop from the lateral mesoderm.
A
The heart, diaphragm and linings of the cavities.
18
Q
List the organs that develop from the endoderm.
A
The linings of the respiratory and digestive tracts as well as the associated glandular organs of the pancreas and liver.
19
Q
What are the three layers of the mesoderm?
A
Paraxial, intermediate and lateral.
20
Q
What are the two layers of the lateral mesoderm?
A
Splanchnic (visceral) and somatic (parietal)
21
Q
List three extra embryonic coeloms and their multiple names.
A
Blastocystic cavity (yolk sac, umbilicle vesicle), Chorionic cavity (gestational sac, extra embryonic coelom), Amniotic cavity (sac)
22
Q
Describe the progression of the blastocystic cavity
A
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.
23
Q
Describe the progression of the chorionic cavity.
A
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.
24
Q
Describe the progression of the amniotic cavity.
A
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.
25
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.
26
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.
27
What are the different types of monozygotic twinning?
Monochorionic, monoamniotic, diamniotic.
28
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.
29
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.
30
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.
31
List structures formed from the ectoderm.
Epidermis, cns, eye, inner ear, neural crest cells.
32
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.
33
List structures formed from the endoderm.
Epithelial linings of respiratory and digestive tracts as well as glandular organs of the liver and pancreas.
34
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.
35
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.
36
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.
37
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.
38
What is spina bifida?
A splitting of the vertebral arches as a failure of the neural tube to close.
39
List the different types of spina bifida.
Spina bifida occulta, spina bifida cystica, spina bifida meningocele, spina bifida meningomyelocele, rachischisis.
40
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.
41
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.
42
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.
43
When does the embryonic period end?
8 weeks.
44
The epididymis and ductus deferens develops from the
Mesonephric duct.
45
What vitamin is associated with a reduction in spina bifida?
Folic acid.
46
Human chorionic gonadotrophin is secreted by;
The trophoblast.
47
The axial skeleton derives from the;
Paraxial mesoderm
48
Which of the following is correct about the intra-embryonic coelom?
a. It is formed from the amniotic cavity
b. It never connects to the extra-embronic coelom
c. It is formed from the yolk sac
d. It is lined by the intermediate mesoderm
e. It is lined by somatic and splanchnic lateral mesoderm
E
49
Why does a horse shoe kidney remain in the pelvis?
Obstruction by the inferior mesenteric artery.
50
Which of the following is incorrect about a diaphragmatic hernia?
Select one:
a. Diaphragmatic hernias are commonly due to failure of formation of the septum transversum
b. Failure of fusion of the pleuroperitoneal membrane is an important factor in the defect
c. The lung may not be able to develop normally because it is compressed
d. The herniation is commonly in the posterolateral region of the diaphragm
e. Intestines may protrude into the thoracic cavity
A
51
What is the most common site for an extra uterine pregnancy?
The ampulla of the fallopian tube.
52
Which of the following is associated with gastrulation?
Select one:
a. Invagination of epiblasts
b. Formation of a bi-laminar disc
c. extravasation of the hypoblast
d. fusion of the neural folds
A
53
Potter's syndrome is associated with;
Select one:
a. kidney ectopia
b. unilateral aplasia of the kidney and polyhydramnios
c. an extra functioning kidney
d. bilateral aplasia of the kidneys and oligohydramios
D
54
What do kidneys develop from?
The intermediate mesoderm.
55
Which of the following is true about somites?
Select one:
a. they are not associated with development of muscles and skin
b. they develop in a cranio-caudal sequence
c. they develop from the lateral mesoderm
d. they start to present in week 5 of development
B
56
Which of the following is correct?
Select one:
a. Neural folding is completed first at the cranial region
b. Neural folding is completed first at the cervical region
c. Neural folding is completed first at the lumbosacral region
d. Neurulation is completed by the end of the third week
B
57
Which of the following is correct about meningomyelocele?
Select one:
a. It involves protrusion of both nerve tissue and meninges
b. It involves the protrusion of meninges only
c. It presents with a dimple in the lumbosacral region
d. It involves failure of fusion of vertebrae only
e. It is unlikely that paralysis of the lower limbs will occur
A
58
The mesonephros are present during which time frame?
Select one:
a. From Week 5 onwards
b. Weeks 4-8
c. Weeks 2-4
d. Weeks 5-12
e. Week 4
B
59
A sacrococcygeal teratoma develops from remnants of;
Select one:
a. the neural plate
b. the primitive streak
c. the notochord
d. the cloacal membrane
e. the yolk sac
B
60
The epithelial lining of the gastrointestinal tract originates from the;
Select one:
a. lateral mesoderm
b. endoderm
c. ectoderm
d. epiderm
B
61
The permanent kidney is the ;
Select one:
a. cloaca
b. metanephros
c. pronephros
d. mesopnephros
B
62
Which of the following is incorrect?
Select one:
a. Gestational age is the number of weeks since the first day of the last normal menstrual period (LNMP)
b. Embryonic age can only be determined in cases where the actual date of conception is known
c. Gestational age may not be able to be determined if a woman has very irregular periods
d. Embryonic age is the number of weeks since conception
e. Embryonic age is normally 2 weeks before the LNMP
E
63
Where are the fimbriae, infundibulum, ampulla, isthmus, interstitium and uterine ostium?
From distal to proximal - fimbrae, infundibulum, ampulla (tube), Isthmus (end of tube), interstitium, ostium (opening)
64
Describe the formation of the bilaminar disc from the inner cell mass.
During the second week of development the inner cell mass of the blastocyst differentiates to form the bilaminar disc. This is made up of epiblasts and hypoblasts.
65
Where can ectopic preganancies occur?
The ampulla of the fallopian tube (80%) however they may also occur in the fimbria, isthmus, on the ovary and in the abdominal cavity.
66
What is a serious consequence of an ectopic pregnancy?
Rupture may lead to an acute life-threatening haemorrhage.
67
How does the intra embryonic coelom form?
develops as the embryo folds laterally. These will allow for the development of abdominal and thoracic cavities. It communicates with the extra embryonic coelom for a time.
68
What are the functions of amniotic fluid?
Reduces infection – Has antibacterial properties.
Reservoir – provides fluid and nutrients
Insures freedom of movement for the embryo, space for development, absorbs blows, and keeps the embryo from sticking to the placenta and protects the umbilical cord by providing a cushion between the foetus and the umbilical cord reducing the risk of compression.
69
What are the relevance of the early cavities in ultrasound imaging?
The gestational sac is used to determine if an intrauterine pregnancy is present and it can be seen using transvaginal ultrasound imaging from about 4-5 weeks. Confirmation of an ongoing pregnancy cannot be absolutely confirmed with ultrasound imaging unless a yolk sac or embryo is seen which usually appears around 5.5 weeks.
70
Describe the formation of the placenta.
After a blastocyst has hatched trophoblast cells near the epiblasts of the inner cell mass become the invasive syncytiotrophoblasts. These implant the embryo into the maternal endometrium. The syncytiotrophoblast cell layer covers the surface of the placenta. It forms as a result of differentiation and fusion of the underlying cytotrophoblast cells, a process that continues throughout placental development.
71
Describe the structure of the placenta.
The basic structural unit of the placenta is the chorionic villus. The villi are vascular projections of foetal tissue surrounded by chorion. The chorion consists of two cellular layers: the outer syncytiotrophoblast which is in direct contact with maternal blood within the intervillous space, and the inner cytotrophoblast. The intervillous space is a large cavernous expanse into which the villi reach.
72
What is the function of the placenta?
The placenta provides oxygen and nutrients to the foetus, removes waste products from its blood, secretes hormones and attaches the foetus to the uterine wall. Gas exchange, metabolic transfer, endocrine function, drug transfer.
73
Where is the notochord?
Some mesenchyme cells migrate cranially on the midline from the primitive node and pit forming the notochordal process which will eventually reach the prechordal plate. The notochord forms cranially from the primitive pit and only extends caudally later.
74
What is the notochords function?
The notochord serves as the axial skeleton of the embryo until other elements, such as the vertebrae, form and controls aspects of LR symmetry and dorsal ventral positioning.
75
Describe neural tube formation.
As the notochord develops the embryonic ectoderm thickens to form the neural plate. This then invaginates along its central axis to form the neural groove. The neural folds on each side move together and fuse to form the neural tube. Once the neural tube is formed neural crest cells migrate to the sides as it separates from the surface ectoderm.
76
What divides the two layers of the lateral mesoderm?
Intra embryonic coelom.
77
What three body cavities does the intra embryonic coelom give rise to?
Pericardial, pleural, peritoneal.
78
What forms the limb elements?
paraxial (somites) mesoderm and lateral plates as well as the covering ectoderm.
79
What are the three parts of the primitive gut tube?
Foregut, midgut, hindgut
80
What does the foregut give rise to?
Pharynx, oesophagus, stomach, liver and most of the small intestines. The foregut is also the site from which the respiratory diverticulum arises and from this the lung buds.
81
What does the midgut form?
From the end of the foregut at bile duct opening to 2/3 of the transverse colon.
82
What does the hind gut form?
From the distal 1/3 of the transverse colon to the rectum.
83
What does a teratoma form from?
The remainder of the primitive streak that normally degenerates or disappears.
84
What do teratomas consist of?
As they are made up of pluripotent cells teratomas contain various types of tissue such as bone, hair, teeth and nerve cells.
85
What are some common locations for teratomas?
Ovarian cystic teratoma or dermoid cyst
Sacrococcygeal (most common at 40%)
Other locations occurring in childhood include testis, ovaries, mediastinum, retroperitoneum and head and neck.
86
What are the two directions of embryonic folding?
Craniocaudally and lateroventrally
87
How does folding occur?
Rapid uneven growth.
88
How is the diaphragm formed?
By the septum transversum. Due to massive growth of the neural tube it becomes folded under the developing heart. Early on there are openings between the pleural cavity and the peritoneal cavity, the pericardialperitoneal canals. As the lungs develop a membrane will close these canals and the diaphragm will be complete.
89
What is gastroschisis?
Similar to a congenital omphalocele except that it involves all layers of the abdominal wall, from the peritoneum to the skin.
90
What is a congenital omphalocele?
The persistence of herniation of the abdominal viscera into the proximal part of the umbilical cord. 1/5000 births and can be associated with other cardiac and urogenital defects.
91
What is meckels diverticulum?
Meckels diverticulum is the most frequent malformation of the GIT. It is located in the distal ileum and is typically 3-5cm long and is a remnant of the omphaloenteric (vitelline) duct. It is mostly asymptomatic.
92
How does oesophageal stenosis or artresia occur?
Disturbed separation of the lung buds from the esophagus.
93
Define neurulation.
The formation of the neural tube.
94
Where does neural tube formation begin?
The region of the future neck and proceeds in cranial and caudal directions. Closure of the cranial or anterior end (neuropore) occurs on the 25th day while the caudal neuropore closures occurs by the end of the fourth week.
95
How does the brain and spinal cord develop?
The cranial two-thirds of the neural plate and tube represent the future brain, and the caudal one-third of the neural plate and tube represent the future spinal cord.
The walls of the neural tube thicken to form the brain and the spinal cord. The neural canal of the neural tube is converted into the ventricular system of the brain and the central canal of the spinal cord.
96
How is spina bifida diagnosed?
Spina bifida can be detected by high levels of alpha-fetoprotein (AFP) in the amniotic fluid. This is because AFP is in the CSF which leaks thru the opening in the spine into the amniotic fluid. (High AFP levels can also indicate other fetal conditions)
Spina bifida can be diagnosed prenatally by ultrasound. The vertebra can be visualised by 12 weeks of gestation, and defects in closure of the vertebral arches may be detected.
97
Where does genitourinary development occur?
The kidneys develop in the sacral region and ascend superiorly into the renal fossa as the embryo matures. wheras the gonads descend into the sacral region.
98
What are the three stages of renal development?
Pronephroi, mesonephroi, metanephroi.
99
When do the pronephroi exist?
Week 4-5. they are not functional. Their ducts are used by the mesonephroi.
100
When are the mesonephroi functional?
Week 4 - 8. it Then begins to gradually degenerate (although parts of its duct system become associated with the male reproductive organs)
101
What is the timeline for metanephric development?
Begins in week 5, functional from week 8.
102
Describe the progression of the ureteric bud.
The ureteric bud is a diverticulum formed from the mesonephric duct (Wolfian duct) close to the entrance to the cloaca. It enlarges and divides to form the collecting duct system, renal pelvis, ureters and major and minor calyces and grows towards and inside the metanephric mesenchyme.
103
Describe the formation of the nephrons.
The metanephrogenic blastema is formed from the nephrogenic cord and is a metanephric mass of mesenchyme, which gives rise to small metanephric vesicles. These vesicles elongate to form metanephric tubules which become invaginated by glomeruli, starting the formation of nephrons.
104
What is persistent foetal lobulation?
The foetal kidney s made up of five lobules. Failure of complete fusion results in persistent foetal lobules. This is not a clinically significant finding
105
What is a malpositioned/ectopic kidney?
Malrotated kidney can occur if the kidney fails to rotate and remains in its embryonic position. It is commonly associated with ectopic kidneys. Ectopic kidneys are when one or both of the kidneys are in an abnormal location. Occurs when the kidney fails to ascend into the abdomen. Most commonly found in the pelvis.
106
How does a horseshoe kidney form and what is the consequence?
When the lower poles of the kidney are fused. Happens when two metanephric blastemas come into contact as they ascend. Often found in the pelvis as the kidney can not migrate cranially due to the inferior mesenteric artery. Often an incidental finding as it may cause no symptoms and the collecting system will function normally. A third of horseshoe kidney patients develop other abnormalities.
107
How does duplication occur in the urogenital system?
Can be of the urinary tract such as a bifid ureter. This is caused by incomplete separation of the ureteric bud. Complete division of the ureteric bud can cause duplicate kidney with either a bifid ureter or two separate ureters. If two ureteric buds form there can be a third kidney however it is very rare.
108
What is renal agenesis and how does it occur?
The absence of one or both kidneys. Renal agenesis occurs when the ureteric buds fail to develop or the primordial of the ureters degenerate.
109
What is Autosomal dominant polycystic kidney disease ?
The most prevalent renal hereditary disease. These mutations affect the regulation of tubular and vascular development in the kidneys and other various structures. Cysts gradually increase in size, leading to ischaemia and pressure atrophy of intervening renal tissue resulting in gradual deterioration of renal function. Symptoms include include dangerous hypertension, pain in the back or sides, hematuria, recurrent urinary tract infections, kidney stones, and heart valve abnormalities.
110
What is oligohydramnios?
Insufficient amounts of amniotic fluid during pregnancy
111
What are some effects of oligohydramnios?
When there is too little amniotic fluid normal pressure that is exerted on the fetus during pregnancy can cause certain physical features such as distinctive facial features or skeletal abnormalities. When oligo-anhydramnios is present from early in pregnancy, the lungs are also underdeveloped (pulmonary hypoplasia)
112
What is Potter syndrome?
Oligohydramnios caused by absence of both kidneys (bilateral renal agenesis). This is sometimes referred to as classic Potter syndrome. It can also result from other conditions including polycystic kidney disease, malformed (dysplastic) or underdeveloped (hypoplastic) kidneys, and obstructive uropathy, in which urine cannot be voided from the bladder and builds up within the kidneys. Potter syndrome is usually fatal.
113
Associate kidney development with lung development.
Both kidney and lung development are driven by an epithelia-mesenchymal interaction.
The interaction between ureter anlage (epithelial tissue) and the metanephric blastem (mesenchyma) drives the process of forming the epithelial tree structure, interactive tissue induction, differentiation, polarization, migration, cell adhesion and finally the epithelia-mesenchymal transformation
The branching and growth of the lung epithelia is also under the direction of the mesenchyma on which it lies and the interaction is important for normal lung development.
114
Associate the close development of the renal with the reproductive system and thus the implications for concurrent congenital abnormalities
Reproductive congenital defects often occur with renal tract defects due to their proximity in development. They share the same embryological origin, the intermediate mesoderm along the posterior wall of the abdomen and both begin growth around the fourth week. Parts of the early urogenital system form parts of the genital system.
115
What early part of the urinal system forms part of the genitals?
Mesonephroi – second set of kidneys. Appear at the 4-week mark and function for four weeks. Their tubules become part of the male reproductive system. In females, the mesonephroi completely regress however, paramesonephric (Müllerian) ducts develop into the fallopian tube, uterus, cervix and upper part of the vagina.
116
What is hypospadia?
Can occur in the glans penis, shaft or scrotum. It arises from a closure disorder of the urethral groove
117
What is epispadia?
Much more rare than hypospadia. The urinary meatus is located on the upper side of the penis or clitoris. It arise from a disorder of the mesoderm cell migration during the 4th developmental week.
118
What is Mayer Rokitansky Kuster Hauser syndrome
Aka Mullerian agenisis. An absent or incomplete migration of the paramesonephric duct in the direction of the SUG is responsible for an atresia and/or complete or incomplete aplasia (failure of an organ to develop or function normally) of the uterus, which is usually associated with renal abnormalities. The kidneys may be abnormally formed or positioned, or one kidney may fail to develop (unilateral renal agenesis).
119
Comment on the development of the bladder.
The urinary bladder is derived from the upper part of the urogenital sinus and adjacent splanchnic (visceral) mesenchyme. As the bladder gets bigger the mesonephric ducts are absorbed and the ureters come to open separately into the bladder.
