Embryology Flashcards
How many spermatozoa reach the fallopian/uterine tube?
200-500 spermatozoa.
How long can spermatozoa survive in the female reproductive tract versus ovum?
Up to 5 days for spermatozoa. Ovum can only survive 24 hours if not fertilised.
Are spermatozoa moved more by their tails or uterine contractions?
Uterine contractions.
What happens when sperm penetrates the zona pellucida?
The zona pellucida changes so no more sperm enter the oocyte - prevents polyspermy.
What are the two types of molar pregnancy?
Complete mole and partial mole.
What is a complete molar pregnancy?
A single sperm fertilises egg that doesn’t contain any genetic material. Abnormal trophoblast develops, which causes placenta to form, but no embryo develops. Gives positive pregnancy test.
What is a partial molar pregnancy?
Normal egg cell fertilised by 2 spermatozoa. Embryo starts to develop but cannot survive. Gives positive pregnancy test.
What are the names of the stages of development from fertilisation to foetus?
Zygote
Morula (4 days)
Blastocyst (5-9 days)
Embryo (10-12 days)
Foetus (by 9 weeks)
What are the similarities between a zygote and a morula?
Both are surrounded by the zona pellucida and cells are tightly compacted.
Which has more cells, a zygote or a morula?
Morula.
Zygote has <16 cells. Morula has 16-32 cells.
What is the zona pellucida?
Glycoprotein coat surrounding the ovum.
When does the blastocyte hatch from the zona pellucida?
5-6 days after fertilisation, prior to implantation.
What cells make up the morula?
Blastomeres.
What two structures do the cells of the morula differentiate into?
Inner cell mass (ICM); will eventually develop into the embryo.
Trophoblast; outer cells which will develop into the placenta.
Implantation begins around day 6 after fertilisation. What are the 3 stages of implantation?
- Appopsition.
- Adhesion.
- Invasion.
How does maternal/foetal gas exchange occur?
Placental villi containing foetal blood protrude into the lacunae, where they are bathed in maternal blood.
The dorsal ventral axis is formed in the blastocyst when the inner cell mass flattens into a bilaminar disc formed of two layers.
What are these two layers?
The epiblast and the hypoblast.
Where does the amniotic cavity form?
Between the epiblast and the trophoblast.
Gastrulation begins in week 3 after fertilisation.
What two key developments occur in gastrulation?
The bilaminar disc develops into a trilaminar disc, establishing the three germ layers.
Anterior-posterior body axis develops.
During gastrulation epiblasts invaginate through the primitive streak. Three layers are then formed from the epiblast. What are these three layers?
Ectoderm.
Mesoderm.
Endoderm.
When does neurulation occur?
Week 4
What is the key process that occurs during neurulation?
Formation of the neural tube.
Describe how neurulation forms the neural tube in 4 key steps.
- Ectoderm thickens into neural plate.
- Plate bends to form a groove.
- Two sides of the groove fuse to form the neural tube.
- Neural tube detaches from ectoderm.
In embryological development, in which week is the primitive gut formed?
Week 4.
In embryological development, when are organ systems formed?
Weeks 3-8 of gestation.
In limb development, during which week do the hand and foot plates appear?
Week 6.
In embryological development, when do limb buds appear?
Week 4
When does the heart tube form in embryological development?
End of week 3 (cardiac looping occurs in week 4).
Vitamin A supplements in pregnancy can disrupt normal retinoic acid signalling, therefore is teratogenic.
Why is retinoic acid (vitamin A) concentration important during embryonic development?
Retinoic acid concentration is thought to provide positioning information for gut cells, which affects what cell types they differentiate into (higher concentration at distal end of gut).
During embryological development, digits form through apoptosis of some cells in the limb buds. In which week of development does this occur?
Digits are formed by week 8.
In embryological development, what do the hypoblasts become?
The yolk sac.
In embryonic development, from which layer do the coronary arteries develop from?
Ectoderm; the cardiac neural crest.
In embryonic development, from which layer does the myocardium develop?
Mesoderm (all types of muscle develop from the mesoderm).
What is the heart like at day 15 of embryonic development?
Tube-like heart, similar to a fish. First heart field; which will develop into left ventricle & atria, and second heart field; which will develop into right ventricle.
By what day of embryonic development does the heart start to resemble a mammalian heart?
Day 50.
What is a transcription factor and how might this relate to embryonic development of the heart?
Type of protein which when expressed “turns on/off” many other gene expressions.
If someone has a genetic variant which alters/deletes code for particular transcription factors which regulate embryonic heart development, could cause a malformed heart; congenital heart condition in baby or miscarriage.
What are the three stages of cardiac formation in the embryo?
- Formation of the primitive heart tube.
- Cardiac looping.
- Cardiac septation.
Why is Nodal important in embryological development and what is it?
Family of proteins, responsible for signalling developing cells to either move away or towards it; determines positioning e.g. left/right positioning of the heart.
What is the atrioventricular canal?
The initial gap between the embryonic single atrium and ventricle, which separates into two openings - eventually becoming the openings which situate the mitral & tricuspid valves.
What are the three types of capillaries?
Continuous (most common)
Fenestrated (kidney, small intestine, endocrine glands)
Discontinuous/sinusoidal (liver sinusoids, have larger openings than fenestrated)
When do the precursors of blood vessels form in the embryo, what are they called, and where do they form?
Day 17
Blood islands
Form in the yolk sac
When does vasculogenesis commence in the embryo?
Briefly describe the process.
Day 18
1. Blood islands = core of haemoblasts surrounded by endothelial cells.
2. Develop into angioblasts.
3. Angioblasts coalesce to form angioblastic cords throughout embryonic disc.
When and how does angiogenesis take place in embryonic development?
Day 18 onwards.
Driven by angiogenic growth factors.
Takes place by proliferation and sprouting. Other mesodermal cells are recruited to develop into smooth muscle intima media.
In embryonic development, do the 1st and 2nd aortic arch become coronary vessels?
No, they become minor head vessels.
In embryonic development, which aortic arch becomes the carotid arteries?
3rd aortic arch.
In embryonic development, what vessels derive from the 5th aortic arch?
Trick question! There is no 5th aortic arch!
In embryonic development, what forms from the left 6th aortic arch?
Ductus arteriosus (communication between pulmonary artery and aorta).
What are the four stages of lung development in embryology?
Embryonic
Pseudoglandular
Cannalicular
Alveolar
When is the alveolar stage of lung development?
At 25 weeks until term.
Which stage of lung development occurs at 16-25 weeks?
Cannalicular
Which stage of lung development occurs at 5-17 weeks?
Pseudoglandular
By what week in lung development do the lung buds enlarge to form right and left main bronchi (embryonic development)?
5th week.
What two developments occur during the pseudoglandular stage of lung development?
- Different cells/structures start to develop e.g. mucous glands.
- Angiogenesis (but pulmonary vessels will be constricted due to hypoxic environment - lungs are full of fluid).
In lung development, when do the respiratory bronchioles and alveolar ducts develop?
Cannalicular stage (16-25 weeks)
In what stage of lung development do the alveolar sacs and type 1/2 pneumocytes develop?
Alveolar stage (25 weeks until term).
When do the lungs finish developing?
3-5 years old.
How does pulmonary vasoconstriction ensure oxygenated blood flows from the right side of the heart to the left side of the heart?
Pulmonary vasoconstriction increases pressure in right side of the heart, so oxygenated blood (from placenta) flows through foramen ovale. Any remaining flows through ductus arteriosus.
What two vessels are joined by the ductus venosus?
The umbilical vein and the inferior vena cava.
What organ does the ductus venosus cause a proportion of blood to bypass?
The liver.
Which carries oxygenated blood from the placenta to the foetus, the umbilical artery or the umbilical vein?
The umbilical vein.
How does the ductus arteriosus close after birth?
The muscular wall contracts (mediated by bradykinin).
How is the foramen ovale closed after birth (3 steps)?
- First breath oxygenate the lungs, causing pulmonary vasodilation.
- Right side of the heart pressure drops.
- Foramen ovale closes as pressure in left side of the heart becomes higher than the right.
In foetal life, which structure shunts blood from the aorta to the pulmonary trunk?
The ductus arteriosus shunts blood from the aorta to the pulmonary trunk.
In embryonic divisions of the brain, what are the terms for the forebrain, midbrain, and hindbrain?
Forebrain = prosencephalon
Midbrain = Mesencephalon
Hindbrain = Rhombencephalon
In embryonic subdivisions of the brain, what forebrain subdivision does the cerebrum develop from?
Telencephalon
In embryonic subdivisions of the brain, what hindbrain subdivision do the pons and cerebellum develop from?
Metencephalon
In embryonic subdivisions of the brain, what hindbrain subdivision does the medulla oblongata develop from?
Myelencephalon
In embryonic subdivisions of the brain, what primary division does the diencephalon develop from?
Prosencephalon (forebrain).
During embryological development of the ear, two pharyngeal arches develop into six soft tissue swellings which eventually form the pinna.
What are these six swellings called?
The Hillocks of His
What does the brain of a 26/40 foetus look like (or premature baby born at 26 weeks)?
Smooth, no gyri or sulci.
In embryonic development, which layer does the central and peripheral nervous systems develop from?
Ectoderm.
How many weeks after conception does the neural tube close?
About 4 weeks (15-28 days).
In embryological development, in what week does the pronephros develop, and where does it develop?
Week 4
Cervical region
What happens to the pronephros by week 5 of embryological development?
It disappears!
In embryological development, in what week does the mesonephros develop, and where does it develop?
End of week 4
Develops off of pronephros, in the thoraco-lumbar region.
What is the function of the mesonephros?
Primitive urinary system (until weeks 10-12 when the metanephros begins working).
What does the mesonephros connect to in the embryo?
The cloaca
The mesonephros joins the cloaca, as does the hindgut. What orifice does the cloaca connect to, to excrete urine and faeces in the embryo?
Allantois
Where does the metanephros develop?
Pelvic region
The metanephric mesoderm/blastema releases growth factors which stimulates the mesonephric duct to develop a small bud. What is this bud called?
Ureteric bud
Reciprocal induction occurs between the ureteric bud and the metanephric mesoderm/blastema; this means they both release growth factors to stimulate development of each other. What do these two structures eventually become?
Ureteric bud = collecting system of kidney (ureter, renal pelvis, calyces, and collecting ducts)
Metanephric mesoderm/blastema = the kidneys!
In embryological development, the urorectal septum develops and separates the cloaca into two parts. What do these two parts become?
Anterior portion (urogenital sinus) becomes the bladder and urethra.
Posterior portion becomes the anal canal.
In the embryo, the cloaca drains into the allantois. What does the allantois develop into?
The urachus, which then becomes the median umbilical ligament.
In embryological development, when do we start swallowing?
Week 15
Which of the three primary germ layers (ectoderm, mesoderm and endoderm) do the paired genital ridges develop from?
Mesoderm
The primordial germ cells originate from the epiblast, then migrate through the primitive streak to the yolk sac.
At what week of embryological development do the primordial germ cells migrate to the paired genital ridges, causing primitive sex cords to form?
Week 6
What happens if the primordial germ cells do not migrate to the genital ridges in week 6 of development?
The gonads do not develop.
In males, what do the primitive sex cords (developed from the genital ridges) form?
Testis cords (which develop into testis)
In females, what do the primitive sex cords (developed from the genital ridges) form?
Cortical cords (which develop into the ovaries)
Initially the gonads are indifferent/undifferentiated. At what point of development do the gonads begin to differentiate depending on sex?
Week 7
What are the two pairs of genital ducts which are both present in the undifferentiated gonad?
Paramesonephric (Mullerian)
Mesonephric (Wolffian)
Which of the genital ducts develop into the epididymis, vas deferens, and seminal vesicle?
Mesonephric (Wolffian) ducts
Which of the genital ducts develop into the ovarian tube, uterus, and upper part of vagina?
Paramesonephric (Mullerian) ducts
What gene on the Y chromosome is crucial for driving development of gonad into testes?
SRY gene
What hormone in the male foetus, produced by Sertoli cells in the testis, is responsible for causing the paramesonephric ducts to regress?
Anti-Mullerian hormone
What hormone in the male foetus is responsible for driving development of mesonephric ducts and external genitalia?
Testosterone
What gene in females is required for ovarian development?
WNT4
What hormone in the female foetus drives development of paramesonephric ducts and external genitalia?
Oestrogen
How do the two paramesonephric ducts form the uterus, cervix, and upper vagina?
They fuse together.
What do the upper and lower parts of the vagina form from during foetal development?
Upper vagina = paramesonephric ducts
Lower vagina = urogenital sinus
What are the key structures in the genitals of the undifferentiated embryo?
Genital tubercle
Genital/urethral fold
Genital swelling
Urethral groove
Oestrogen and testosterone cause the undifferentiated genitals of the embryo to develop into female/male genitals.
What does the genital tubercle become in females and males?
Females: clitoris
Males: penis
Oestrogen and testosterone cause the undifferentiated genitals of the embryo to develop into female/male genitals.
What do the genital/urethral folds become in females and males?
Females: labia minora
Males: fuse to form penile urethra
Oestrogen and testosterone cause the undifferentiated genitals of the embryo to develop into female/male genitals.
What do the genital swellings become in females and males?
Females: labia majora
Males: scrotum
