Introduction to Embryology Flashcards
What is embryology?
The study of how we develop from a single-celled organism, formed after fertilisation, into a multicellular organism
Why is embryology studied?
- as it gives a logical framework for adult anatomy
- it gives a better understanding of pathology
- identification of anatomical variations which may result from problems during development
What is pathology?
It is concerned with the diagnosis of disease by analysing body fluids and tissues
How is a horseshoe kidney formed?
The inferior parts of the kidney may fuse as they ascend upwards towards the posterior abdominal wall
What is a congenital malformation?
A birth defect caused when something along the developmental journey goes slightly wrong
Why does studying embryology help with identifying congenital malformations?
The developmental process is tightly regulated, so if something goes wrong, knowing the developmental journey allows you to see why this has happened
Why is different terminology used to describe the foetus?
It develops in a C-shape
What are the 4 words used to describe the regions of the foetus?
head end - cranial
tail end - caudal
front - ventral
back - dorsal
Why is pregnancy dated?
It allows you to work out the development stage of the embryo
How is menstrual age used to date pregnancy?
How long is the full gestation period?
It dates pregnancy from the woman’s last menstrual period
It is split into 3 equal trimesters and full gestation period is 40 weeks
How is fertilisation age used to date pregnancy?
How long is the full gestation period?
It dates pregnancy from the moment of fertilisation
Full gestation is 38 weeks
Why is the gestation period shorter when using fertilisation age?
The release of the oocyte occurs 2 weeks into the menstrual cycle
The oocyte cannot be fertilised until it is released
How is fertilisation age split into sections?
Early developmental stage occurs from 0 - 3 weeks
Embryonic stage occurs from 3 - 8 weeks
Foetal period occurs from 8 - 38 weeks
What happens during the early developmental stage?
Rapid cell division as the zygote divides
What happens during the embryonic stage?
It is also known as the organogenesis stage as the organs are being formed
What happens during the foetal period?
Growth and maturation of the organs and systems formed during the organogenesis period
How many births in the UK a year result in a congenital defect?
What is a congenital malformation?
It is a serious deleterious physical anomaly
It occurs in 1 in 44 births in the UK
What is a malformation syndrome?
This results from a typical combination of malformations affecting more than one body part
What factors can cause a congenital malformation?
18% of defects are caused by genetic problems
7% of defects are caused by environmental factors (teratogens)
25% of defects are a combination of both environmental and genetic factors
What is a teratogen?
An agent or factor that causes the malformation of an embryo
What does it mean if a congenital malformation is monogenic?
There is a single defective gene on an autosome
This can be inherited or can be spontaneous with no family history
What does it mean if a congenital malformation is chromosomal?
There could be a different number or structure of chromosomes
What are infectious agents that act as teratogens?
They are parasites or viruses that can cross the placenta
What does TORCH stand for in infectious agents that can cause congenital malformations?
Toxoplasmosis Other (e.g. hepatitis B, syphilis) Rubella Cytomegalovirus Herpes
What is toxoplasmosis caused by?
Where is this found?
The parasite, Toxoplasma gondii
It is found in cat faeces, undercooked meat and contaminated water
What chemical factors act as teratogens?
Any medical or recreational drugs
e.g. thalidomide and alcohol
What physical factors act as teratogens?
What maternal diseases act as teratogens?
Physical factors - X - rays
Maternal disease - Diabetes
What deficiency can cause congenital malformations?
Folic acid
What determines whether an environmental factor does or doesn’t affect the developing embryo?
It depends how far along in the development process the embryo is
It is most susceptible during the organogenesis period and less susceptible during the foetal period
Under what conditions will toxoplasmosis affect a developing embryo?
It will only affect the embryo if the mother contracts it when she is newly pregnant
It will not cause problems if she contracted it prior to conception
What does toxoplasmosis cause?
- inflammation of the retina and micropthalmia (small eyes)
- hearing loss
- enlarged liver and spleen
- hydrocephaly (fluid on the brain)
- microcephaly (small head/brain)
When does rubella cross the placenta to affect the foetus?
Infection passes over the placenta in the first 3 months of pregnancy
It can be prevented by the MMR vaccine
What does rubella cause?
- cloudy cornea
- intellectual disability due to problems with development of the CNS
- microcephaly
- heart defects
What does cytomegalovirus cause?
- inflammation of the retina and micropthalmia
- enlarged liver and spleen
- mineral/calcium deposits on the brain
- microcephaly
- psychomotor retardation
What do herpes simplex 1 and herpes simplex 2 cause?
Herpes simplex 1 causes cold sores
Herpes simplex 2 causes genital warts
What does Herpes Zoster cause?
When is this most dangerous to the foetus?
It causes chickenpox
This is the Varicella zoster virus and it is most dangerous between the 13th - 20th weeks of pregnancy
or just before birth, in the 2 days postpartum
What is the most common route of transmission of herpes to the foetus?
It is rare for it to travel up the reproductive tract
It is more common for the mother to transfer the herpes virus during birth
It can also be passed on if the mother contracts chickenpox during pregnancy
What does herpes virus cause in the developing foetus?
- segmental skin loss and scarring
- limb hypoplasia/paresis
- microcephaly
- visual defects
What does the Zika virus cause?
- fever, rash, joint pain and red eyes
- microcephaly
- severe cognitive disabilities
What was thalidomide given for in the 1950s?
What did it lead to?
It was given to treat morning sickness
It leads to shortened or absent limbs
What is thalidomide given to treat nowadays?
It is used to treat symptoms associated with leprosy and HIV
What are the symptoms of foetal alcohol syndrome?
- prenatal and postnatal growth retardation
- intellectual disability
- impaired motor ability and coordination
Why is alcohol consumption such an issue when it comes to congenital abnormalities?
50% of pregnancies are unplanned so many people continue to drink alcohol as they are unaware they are pregnant
What does radiation cause in the developing foetus?
Cell death or chromosome changes leading to…
- microcephaly
- mental and cognitive disabilities
- haemopoietic malignancies and leukaemia
What part of the foetus is most sensitive to radiation?
The central nervous system
The foetus is most sensitive during the first trimester
What does diabetes mellitus cause in the foetus?
- macrosomia - a larger than average baby
- ventricular septal defects
- spina bifida
- renal agenesis
What does folic acid deficiency cause?
Malformations in central nervous system formation
What is significant about folic acid deficiency?
Folic acid levels must be high for a number of weeks before fertilisation
If pregnancy is unplanned, folic acid levels may not be high enough
What does folic acid deficiency cause in the developing foetus?
- spina bifida
2. anencephaly - the centre of the brain fails to form correctly
What is gametogenesis?
The process in which cells undergo meiosis to form gametes
What are gametes?
Haploid cells that contain only one set of chromosomes and are made through meiosis
Where does sperm production take place?
Seminiferous tubules in the testes
These are kept separate from systemic circulation by the blood-testis barrier
What is the role of the blood-testis barrier?
It prevents hormones and components of the systemic circulation from affecting the developing sperm
Why must the blood-testis barrier prevent the immune system from recognising sperm as foreign?
Sperm are genetically different from male cells so will have different surface antigens
What are spermatogonia?
The initial pool of diploid cells that will divide by mitosis to form 2 identical cells
What is the difference between Type A1 spermatogonia and Type B spermatogonia?
A1 - replenish the pool of spermatogonia so the male remains fertile his whole life
B - these form mature sperm
What happens to the type B spermatogonia?
They divide by mitosis several times to form identical diploid cells linked by cytoplasm bridges
These are primary spermatocytes
What happens when the primary spermatocytes undergo meiosis?
Meiosis I produces 2 haploid cells - secondary spermatocytes
Meiosis II produces 4 haploid cells - spermatids
What happens during spermiation?
The cytoplasm bridges break down and the spermatids are released into the lumen of the seminiferous tubule
What happens to the spermatids once they are in the seminiferous tubule?
They travel along the tubule and differentiate into mature spermatozoa
When does the process of oogenesis begin?
What cells are present at this point?
Oogenesis begins in the foetus prior to birth
The cells that begin meiosis I before birth are primary oocytes
What are primary oocytes arranged to form?
They are arranged in the gonads in clusters
They are surrounded by flattened epithelial cells (follicular cells) to form a primordial follicle
What stage of meiosis are primary oocytes arrested in?
prophase stage of meiosis I
What happens during the pre-antral stage of oogenesis?
- primary oocyte grows dramatically whilst being arrested in meiosis I
- follicular cells grow and proliferate to form stratified cuboidal epithelium
- the cells have become granulosa cells
What do granulosa cells do?
They secrete glycoproteins to form the zona pellucida around the primary oocyte
What happens during the antral stage of oogenesis?
fluid-filled spaces form between granulosa cells
these will combine to form the antrum - a central fluid-filled space
The follicles are now secondary follicles
What happens during the pre-ovulatory stage of oogenesis?
The LH surge induces this phase and meiosis I is completed
2 haploid cells form within the follicle, but they are unequally sized
What cells are formed during the pre-ovulatory stage?
One daughter cell receives far less cytoplasm - first polar body
The other haploid cell is the secondary oocyte
What happens after the cells produced in the pre-ovulatory stage undergo meiosis II?
The first polar body gives rise to 2 polar bodies
The secondary oocyte arrests in metaphase of meiosis II
What do the conditions inside the female reproductive tract cause the sperm to undergo?
Capacitation
The cholesterol and glycoproteins are removed from the head, allowing the sperm to bind to the zona pellucida of the oocyte
When does the secondary oocyte complete meiosis II?
What does it form?
It completes meiosis II on fertilisation
This results in a fertilised egg and a third polar body
Where does fertilisation take place?
What must happen prior to this?
It occurs in the ampulla of the uterine tube
Fimbriae must sweep the oocyte into the uterine tube, as it is not connected to the ovary
What happens prior to the acrosome reaction?
The capacitated sperm pass through the corona radiate
what is the acrosome reaction?
The acrosome releases degradative enzymes that break down and allow the sperm to penetrate the zona pellucida
What happens once one sperm has penetrated the zona pellucida?
The cortical granules in the oocyte start the cortical reaction to prevent polyspermy
What is the result of the cortical reaction?
The zona pellucida becomes impenetrable
What does cleavage refer to?
Cell divisions that are not associated with significant growth of the embryo
What is a blastomere?
How is it formed?
It is a cell formed by cleavage of a fertilised ovum
After fertilisation, the zygote begins to divide
There is no change in size of the zygote as the blastomeres get smaller
What is a morula?
A ball of cells containing 16 - 32 cells
The cells begin to become organised
What is the inner cell mass of a morula made up from?
Embryoblast cells
These will go on to form the embryo itself
What is the outer cell mass of a morula made up from?
Trophoblast cells
These will go on to form the support structures needed by the embryo to survive
What is the blastocyst and when does it form?
It is a fluid-filled cavity which forms by day 5
What do embryoblasts and trophoblasts form in the blastocyst?
Trophoblasts form the thin layer around the outside
Embryoblasts form a compact inner cell mass
When does blastocyst formation begin?
When a fluid-filled cavity opens up in the morula
What is the blastocoel and how is it formed?
It is the inner fluid-filled blastocyst cavity
The trophoblast cells pump Na+ ions into the blastocyst, which causes water to follow through osmosis
This forms the blastocoel
On which day does the blastocyst implant into the uterine wall?
Why does this happen?
It implants into the uterine wall between days 5 and 6
It does this as it needs a blood supply as it cannot get enough oxygen and nutrients through osmosis
What is blastocyst hatching and why is it needed?
This is where the blastocyst loses the zona pellucida
This allows it to implant into the uterine wall
