1/2. Embryology Flashcards
What is embryology?
The study of embryos
What are the 3 stages of the development of a human?
- Pre-embryonic phase 0-3 weeks
- Embryonic phase 4-8 weeks
- Foetal phase 9-40 weeks
What are the parent cells called that produce sperm and ova?
- Spermatogonium
- Oogonium
What is the processes that produce sperm and ova called?
- Spermatogenesis
- Oogenesis
What is the end product of meiosis?
-4 unique sperm
OR
-1 ovum and 3 polar bodies
What happens during fertilisation
- Several sperm surround the ovum, only one penetrates.
- Pro nucleus of sperm enters and fuses with the pro nucleus of ovum to give a diploid cell called the zygote
What happens during week 1 of pre-embryonic phase.
- Zygote is formed; divides to form a blastocyst
- Moves through uterine tube to reach uterine cavity
What is the zygote?
- 1st cell in the body
- Diploid
Why are zygotes genetically unique?
It has 23 of the mothers and 23 of the fathers chromosomes
Where do the mitochondria cell organelles come from?
The mother
What happens to the zygote?
Cell divides by mitosis to form a solid ball of cells called the morula
What is an example of a mitochondrial inherited disease?
Lebers hereditary Optic Neuropathy which causes blindness
Why does a morula develop into a blastocyst?
- As the number and size of cells increases, getting nutrition to the central core of cells starts to become difficult.
- Therefore a cavity develops called the blastocystic cavity
What does a blastocyst look like?
- Cavity- blastocystic cavity
- Cells accumulate at one end to form an inner cell mass
- The outer lining is called the trophoblast
Endometrium
Lining of the womb
What aids in the movement of the zygote to uterus?
Uterine tubes have ciliated epithelium to help the ovum move through the reproductive system
How long does the first cell division take?
36 hours
What happens to the time of cell divisions?
Take less time with each successive division
What happens around day 5/6 of the pre-embryonic stage?
The blastula has formed and has reached the uterine cavity ready to implant
When might an ectopic pregnancy occur?
If the cilia function is abnormal
What happens during week 2 of the pre-embryonic phase?
- Implantation occurs in the uterine endometrial layer and placenta begins to develop
- Cells that later form the embryo form a bilaminar disc
- Sacs, membranes and cord to nourish the human conceptus start to form
What does the trophoblast ultimately form?
Chorion and develops finger like processes: chorionic villi
What does the chorion do?
- Implantation process
- Forms part of the placenta in due course
- Secretes human Chorionic gonadotropin (HCG)
What does HCG do?
Sends signal to the ovary to keep producing oestrogen and progesterone to ensure the endometrium is not shed
What is used to detect pregnancy?
HCG
What happens at around 7 days?
The blastocyst begins to burrow into the uterin wall
What plays an important role in burrowing?
The chorion
Decidua basalis
Part of the endometrium deep to the implanted conceptus
What happens to maternal blood and urine levels of HCG?
They increase till around 12 weeks gestation
What happens to the inner cell mass?
The cells of the inner cell mass form a 2-layered flat disc called the bilaminar disc
What 2 cavities form?
- Amniotic cavity
- Yolk sac
What are the main functions of the placenta?
- Foetal nutrition
- Transport of waste and gases
- Immune
Describe the foetal part and surface of the placenta.
Smooth with foetal blood vessels and end of umbilical cord
Describe the maternal part of the placenta.
Decidua basalis of endometrium. Rough and has maternal blood vessels
When does the placenta mature?
Matures by 18-20 weeks
How much does the placenta weigh?
1/6 of the foetus
How do fraternal/dizygotic twins occur?
- 2 ova release, 2 sperms which leads to 2 separate zygotes
- Different genetic makeups, 2 plancentae
- Can run in families
How do identical/monozygotic twins occur?
- 1 ovum, 1 sperm, 1 zygote initially which divides into 2 and each cell develops into a different embryo
- Same genetic make up share 1 placenta (might/might not share amniotic and chorionic sac)
Summarise the progress of the conceptus up till the end of week 2.
- Fertilisation
- Zygote
- Morula
- Blastocyst with inner cell mass
- Bilaminar disc with amniotic cavity above and yolk sac below
Summarise where everything takes place up until week 2.
- Morula reaches uterine cavity
- Blastocyst implants
- Outer cells form chorionic villi which help implantation
- Decidua basalis forms
- Placental formation begins
What occurs during week 3 of the pre-embryonic stage?
- Formation of germ layers
- Formation of neural tube
- Development of somites
- Early development of CVS
Gastrulation
Formation of germ layers
Neurulation
Formation of neural tubes
Primitive streak
Depression formed in the midline of the epiblast by the dipping in of cells
What is formed alongside the primitive streak?
The axis of the embryo
How are germ layers formed?
- Epiblast cells migrate into space between epiblast and hypoblast layers
- Cells then displace hypoblast
What are the 3 germ layers formed?
- Ectoderm
- Mesoderm
- Endoderm
What do the 3 germ layers form?
Trilaminar disc
When do cells become specialised?
When the germ layers are formed
Notochord
Solid tube of cells
How is the notochord formed?
Cells from the primitive streak sink down and the notochord is formed at the mesoderm
What does the notochord do?
Induces ectodermal cells in the midline to form a neural tube
How is the neural tube formed?
Neural plate sinks down
What does the neural tube do?
Induces the mesoderm to thicken
Once thickened what happens to the mesoderm?
It separates into 3 parts
What 3 parts does the mesoderm separate into?
- Paraxial mesoderm
- Intermediate mesoderm
- Lateral plate mesoderm
What happens to the lateral plate mesoderm?
It splits to form a somatic and splanchnic mesoderm
What is the space between the somatic and splanchnic mesoderm called?
Intraembryonic coelm
What divides to form somites?
Paraxial mesoderm
What becomes the urogenital system?
Intermediate plate mesoderm
What becomes the body cavity and coverings?
Lateral plate mesoderm
What occurs during weeks 4-8, embryonic stage?
-Folding into a tube completes
-Neural tube development
-Heart starts to beat on day 24
-Gut formation from endoderm
-Urogenital system formation from intermediate mesoderm
-Body cavities from lateral plate mesoderm
-43 pairs of somites form in the paraxial mesoderm and differentiate further
Limb buds form
-Neck development- pharyngeal arches
What are the 3 sections that each somite divides into?
- Dermatome
- Myotome
- Sclerotome
What do the dermatomes develop into?
Dermis of skin
What do the myotomes develop into?
Muscles
What do the sclerotomes develop into?
Bones including vertebrae
Teratology
Study of when things go wrong during development
Teratogens
Environmental factors that cause abnormal development
What are examples of things that can go wrong during pregnancy?
- Congenital rubella syndrome casued by contraction of German measles when pregnant
- Malformed limbs due to maternal use of Thalidomide
What are the causes of abnormal development?
- Unknown aetiology 60%
- Multifactorial 20%
- Environmental 10%
- Genetic 10%
What environmental factors are there?
- Drugs- prescription/other
- Alcohol
- Tobacco
- Infectious agents: ToRCH can transfer through placenta and affect developing embryo
- Others, e.g radiation
What does ToRCH stand for?
- Toxoplasma
- Rubella
- Cytomegalovirus
- Herpes
What genetic factors are there?
- Too many/too few chromosomes (Turner’s syndrome- 24 chromosomes single X ) (Down’s syndrome- 24 chromosomes- trisomy 21)
- Structural changes- deletions of genes, segments of chromosomes
What can cause problems related to genetic factors?
- Increased maternal age
- Damage from environmental factors such as radiation
How do the risks of teratogenesis differ with time?
- Weeks 1-2: high risk of death, low risk from teratogens
- Weeks 3-8: period of greatest sensitivity to teratogens
- Weeks 9-38: decreasing sensitivity to teratogens
What does the risk posed by a teratogen depend on?
- Exposure during critical periods of development
- Dosage of drug/chemical/ factor
- Genetic constitution of embryo i.e some more susceptible than others at equivalent doses
How can malformations be diagnosed prenatal?
- Blood- AFP
- Ultrasound scan- 12 week anomaly scan
- Invasive tests- chorionic villus sampling and amniocentesis
How can malformations be diagnosed postnatal?
- Hip stability
- Testes descent
- Fingers and toes
- Hearing
