Topic 6 - Embryology Flashcards
Lectures 18,19,26 & 27
What is Embryology?
The branch of biology that studies the prenatal development of gametes, fertilization, and development of embryos and fetuses.
Why is embryology important to us?
- Logical framework for adult anatomy
- Better understanding of pathology
- Anatomical variations
- Congenital malformations/anomalies and pathologies
Where do the gonads originate?
Posterior abdominal wall, they descend during development. In males -> scrotum. In females -> abdomen.
How do horseshoe kidneys occur?
The kidneys can fuse as they travel down during development.
What are the aspects of a human adult?
Anterior - front
Posterior - back
Superior - top
Interior - bottom
What are the aspects of a human embryo?
Ventral - front
Dorsal - back
Cranial - top
Caudal - bottom
What is a ‘lateral’ section?
Horizontal across the abdomen.
What is a ‘sagittal’ section?
Vertical down the middle from the front.
What is a ‘coronal’ section?
Vertical down the middle from the side.
What is a ‘menstrual age’?
Dates the pregnancy from the womans last menstrual period.
Full gestation 40 weeks.
Three equal trimesters.
Who is menstrual age used by?
Clinicians
What is a ‘fertilisation’ age?
Dated from exactly when the sperm fertilized the egg.
Full gestation 38 weeks.
Who uses fertilisation age?
Embryologists
What are the three periods of fertilisation age?
- Early Development
- Embryonic (organogenesis)
- Foetal
What happens in the foetal period of fertilisation age?
Growth and maturation of all the organs fromed in the embryonic period.
How are birth defects caused?
Genetics (meiosis and mitosis)
Environment (teratogens)
What can be the genetic causes for birth defects?
- Monogenic (defective gene on autosome)
- Chromosomal (numerical/structural)
What is Trisomy 21 and what does it cause?
Down’s Syndrome.
- growth retardation
- intellectual retardation
- craniofacial abnormalities
- congenital heart defects
What are the types of teratogens?
- Infectious (taxoplasmosis, rubella, hep B, herpes, cytomegalovirus, syphylis)
- Chemical (alcohol, thalidomide)
- Physical (radiation)
- Maternal disease (diabetes)
- Deficiency (folic acid)
When is the fetus most sensitive to terotogens?
During the embryonic period - around 5 weeks of gestation, as it is developing its organs. First prenatal visit tends to fall around 8 weeks.
How do infectious teratogens get to the embryo?
The diseases cross the placenta.
What are the infectious teratogens?
TORCH
- Toxoplosmosis
- Other (syphilis, hepatitis B)
- Rubella
- Cytomegalovirus (CMV)
- Herpes simplex virus (HSV)
What is toxoplasmosis?
A parasite usually contracted by the mother through raw meat/cat faeces during early pregnancy (organogenesis). Asymptomatic in adults.
What birth defects can toxoplasmosis cause in early pregnancy?
- inflammation of the retina and eye (micropthalmia)
- hearing loss
- enlarger liver/spleen
- hydrocephaly
- microcephaly
What is hydrocephaly?
Accumulation of fluid at the top of the head.
What is microcephaly?
A condition where the head circumference is smaller than normal.
What symptoms does Rubella cause as a birth defect?
-Cloudy cornea
-Intellectual disability
-Microcephaly
-Heart defects
Infection passes through placenta in the first 3 months of pregnancy
What is Cytomegalovirus?
Crosses the placenta, infection vie body fluids, usually symptomatic.
What does cytomegalovirus cause as a birth defect?
- Micropthalmia (inflammation of retina)
- Enlarged spleen/liver
- Mineral deposits on the brain
- Microcephaly
- Psychomotor retardation
What is the Varicella Zoster virus?
Chicken Pox
When is chickenpox most dangerous?
13-20 weeks of development or just before birth to 2 days postpartum.
What does chickenpox cause as a birth defect?
- Segmental skinloss/scarring
- Limb hypoplasia/paresis
- Microcephaly
- Visual defects
What is hypoplasia?
Underdevelopment or incomplete development of an organ
What is paresis?
A condition of muscular weakness caused by nerve damage or disease
What does the Zika virus cause as a birth defect?
- Microcephaly
- Severe cognitive disabilities
What is thalidomide?
Drug prescribed for morning sickness in the 1950s. Caused birth defects such as shortened or absent limbs. In some places now used to treat Leprosy and HIV.
What is Foetal Alcohol Syndrome associated with?
- Prenatal and postnatal growth retardation
- Intellectual disability
- Impaired motor ability and coordination
When is a fetus most sensitive to radiation?
First trimester
What can radiation cause as a birth defect?
- Microcephaly
- Mental and cognitive disabilities
- Haemopoietic malignancies and laukemia
What can Diabetes Mellitus cause as a birth defect?
- Macrosomia (big baby)
- Ventricular Septal Defects
- Spina bifida
- Renal Agenesis
What is spina bifida?
A birth defect where there is incomplete closing of the backbone and membranes around the spinal cord
What can a lack of folic acid in mothers diet cause as a birth defect?
- Spina bifida
- Anencephaly
By how much do OTC supplements reduce risk of malformation during pregnancy?
60%
What is anencephaly?
The absence of a major portion of the brain, skull, and scalp that occurs during embryonic development
What percentage of pregnancies are unplanned?
Around 50%.
What is fertilisation?
Fusion of male and female gametes to form a zygote
What is capacitation?
Sperm undergoes capacitation in the female reproductive tract as it is chemically altered as only capacitated sperm can fertilise an egg.
Where does fertilisation usually take place?
At the ampulla of the uterine tube
How does the acrosome reaction occur?
- capacitated sperm pass through corona radiata
- acrosome releases enzymes that allow sperm to penetrate zona pellucida
- sperm penetration initiates cortical reaction
- zona pellucida becomes impenetrable
How many cells is the morula?
16-32
Which part of the morula goes on to form what?
Inner cell mass -> embryo proper (embryoblasts)
Outer cell mass -> placenta (trophoblasts)
What is a cyst?
Fluid filled space
What does the blastocyst have to do in order to implant on the uterine wall?
Sheds the original zona pellucida
What is the blastocyst comprised of?
Embryoblast and Trophoblast
How is the blastocyst formed?
Morula fills with fluid to make the blastocyst
What does the Emryoblast differentiate into?
Hypoblast and Epiblast
What does the Trophoblast differentiate into?
Cytotrophoblast
Syncytiotrophoblast
How does the syncytiotrophoblast stop the immune system from recognizing the embryo as a foreign body?
- By chemically hiding the embryo from the immune system
- By losing cell surface membranes between cells, becoming multinucleated so the gaps between cells are too small for the WBC to get through.
What do syncytiotrophoblasts do?
- Aid implantation
- Establish uteroplacental circulation
- Protect embryo from mothers immune system
- Release hCG
What is the significance of the embryo becoming bilaminar?
- first stage of organisation of the body plan
- establishes dorsal/ventral axis of the embryo
- epiblast forms the dorsal surface
- hypoblast forms the ventral surface
Which axis of the embryo does the hypoblast form?
Ventral
Which axis of the embryo does the epiblast form?
Dorsal
How does the amniotic cavity form?
As a small fluid filled cavity in the epiblast.
When does the morula form?
Day 4
When does the blastocyst form?
Day 5
When does full implantation occur?
Day 9
When do the trophoblast and embryoblast differentiate?
Day 7
When does the amniotic cavity form?
Day 8
What occurs simultaneously to full implantation?
Mitigation of hypoblast cells to encase the blastocyst cavity, forming the Heuser’s membrane
How does the developing embryoblast receive nutrients without placental circulation?
Nutrients diffuse through the uterine gland into the trophoblastic lacuna
When does uteroplacental circulation occur?
10-11 Days
How is uteroplacental circulation established?
Syncytiotrophoblast erodes walls of maternal capillaries using digestive enzymes.
This may cause light spotting.
When does the formation of the Extraembryonic mesoderm occur?
10-11 Days
What is the extraembryonic mesoderm?
A layer of connective tissue derived from the yolk sac cells which forms between the primary yolk sac and cytotrophoblast. Eventually covers the amniotic cavity.
Which cavity forms in the extraembryonic mesoderm?
Chorionic cavity
When does the chorionic cavity form?
12-13 Days
When does the definitive yolk sac form?
12-13 Days
Which cells is the definitive yolk sac formed from?
Hypoblast
Which are the 3 most important embryonic cavities?
- Chorionic cavity
- Definitive yolk sac
- Amniotic cavity
What is the connecting stalk?
Attaches embryo to the support structure and will become the umbilical cord.
What occurs at the end of week 2 after fertilisation?
Amniotic cavity and secondary yolk are suspended in the chorionic cavity by a connecting stalk.
Migration of which cells form the definitive yolk sac?
Hypoblast
Which hormone does the syncytiotrophoblast produce?
hCG
What does hCG stand for?
Human Chorionic Gonadotrophin
What is the role of hCG?
Helps to maintain the endometrium and plays a role in maternal immunotolerance
What was the Bufo test?
Early pregnancy test - inject toad with female urine and if high hCG levels are present, the toad would ovulate within 24 hours
What is an ectopic pregnancy?
Implantation in an abnormal site - outside the uterine cavity
What percentage of pregnancies are ectopic?
2%
What percentage of pregnancy related deaths are due to an ectopic pregnancy?
9%
Where does fertilisation usually occur?
The ampulla
What is placenta previa?
A condition where the placenta is attached very low down on the uterine wall and perhaps right over the cervix.
What are the risks associated with placenta previa?
- Baby may lose blood supply
- Mother may haemorrhage
What percentage of ectopic pregnancies occur in the ampulla?
80%
What can an ectopic pregnancy in the right uterine tube be mistaken for?
Appendicitis - pain felt in a similar area
How does abdominal implantation occur?
There is a gap between the ovary and the uterine tube - therefore the egg can enter the abdomen
What is a lithopaedion?
When an abdominal pregnancy goes undiagnosed and the foetus dies, if it is too large to reabsorb the body will calcify it as a way of protecting the mother from necrotic tissue
What condition can give a false positive on a pregnancy test?
Hydatidiform mole releases hCG
How does a hydatidiform mole form?
Sperm fertilises an ‘empty’ egg lacking a nucleus - suggests paternal genes favour the formation of the trophoblast at the expense of the embryo (paternal imprinting)
What is fully established by the end of week 2 of development?
- bilaminar embryo in between amniotic cavity and secondary yolk sac
- fully implanted and established uteroplacental circulation
- dorsal/ventral axis established
