Embryology Flashcards
The pre-embryonic phase of development occurs when?
Between 0-3 weeks
The embryonic phase occurs between what weeks?
4-8 weeks
The foetal phase of development occurs between…
9-40 weeks
What is made during Oogenesis?
1 ovum and 3 polar bodies
What is produced during spermatogenesis?
4 sperms
Genetic difference between ovums and sperm?
Ovum has 22 chromosomes and an X chromosome, sperm has 22 chromosomes and a X or Y chromosome
Are the products of meiosis - sperm and ovum, identical to each other of the same type, or genetically unique?
Unique - no two sperm are identical
When is a zygote formed?
When the (pro)nucleus of the sperm fuses with the (pro)nucleus of the ovum
Are zygotes diploid or haploid?
Diploid
What structures originate from the mother?
Mitochondria, cytoplasm and cytoplasmic cell organelles
What is the pathway of progression for a zygote?
Zygote to a Morula to a Blastocyte
How does a zygote split into a Morula?
Mitosis
What is a Morula?
A solid ball of cells
Give an example of an inherited mitochondrial disease?
Leber’s hereditary optic neuropathy, leads to blindness
What becomes difficult as the number and size of the morula increases?
Getting nutrition to the central core of cells is more difficult
How does the morula get around this difficulty?
A cavity develops called the blastocystic cavity and cells accumulate at one end to form an inner cell mass
What is the name given to the outer layer of cells surrounding a blastocyst cavity and inner cell mass?
Trophoblast
How long does the first cell division of the zygote take?
36 hours
As time progresses, do cell divisions get faster or slower?
Faster
Where in the body is the ovum fertilised?
Uterine tube
Where in the body does the ovum implant?
Uterine wall
How is the ovum transported from the ovary down the uterine tube?
Via ciliated epithelium and fimbriae at the end of the uterine tube
What is an ectopic pregnancy (pregnancy outside of the uterus) caused by?
Occurs if cilia function is abnormal and the ovum gets stuck in the unterine tube
When does the blastocyst form?
Day 5/6 of the pregnancy
What is the inner lining of the uterine cavity called?
Endometrium
Where does implantation of the ovum occur?
In the uterine endometrial layer
When does the placenta begin to develop
Roughly by day 6 of the 1st week
What do the cells that later form the embryo form?
A bilaminar disk
The trophoblast helps what form?
Sacs, membranes and umbilical cord
At roughly what day does the implantation of the blastocyte occur?
~ 7 days
What is a chorion?
A trophoblast which has divided and joined up with other cells
What does the chorion develop?
Finger like processes called chorionic villi
Why is the endometrium a good place for the blastocyte to implant into?
It has a good supply of blood and nutrients
What structures of the chorion help the blastocyst to burrow into the endometrium?
Villi
What layer of the uterus is the endometrium?
Inner layer
What are 3 functions of the chorion?
Implantation process (chorionic villi)
Forms part of the placenta in due course
Secretes human Chorionic gonadotropin (HCG)
What is Human Chorionic Gonadotropin used for?
To detect pregnancy - makes the body continue to produce estrogen and progesterone to stop the shedding of the endometrium
Once the blastocyst has implanted in the uterus, what is the endometrium in which the blastocyst has implanted known as?
Decidua basalis
The inner cell mass is flattened to form what two layers of cells?
Epiblast and hypoblast
Maternal blood and urine levels of HCG increase till….
Around 12 weeks of gestation
What is the layered flat disc formed by the epiblast and hypoblast layers known as?
Bilaminar disk
What does the flattening of the inner cell mass and the creation of the epiblast, hypoblast and the bilaminar disk create?
2 cavities - amniotic cavity and the yolk sac
What is most superior? Amniotic cavity or yolk sac?
Amniotic cavity
What is the allantoic cavity?
Cavity in which the waste products from the embryo are stored
What is the main functions of the placenta?
Foetal nutrition
Transport of waste and gases
Immune
Describe the foetal part of the placenta?
Smooth with foetal blood vessels and end of umbilical cord
What is the surface of the maternal part of the placenta?
The decidua basalis
Describe the Decidua basalis of endometrium?
Rough and has maternal blood vessels
What are the foetal and maternal surfaces of the placenta facing? Why are they named this way
Foetal part faces foetus and maternal faces mother
At what point in development does the placenta mature?
18-20 weeks
What causes fraternal/dizygotic twins?
2 different ova released, fertilised by 2 different sperm and forming 2 different zygotes
What causes identical/monozygotic twins?
1 ovum released, fertilised by 1 sperm, forms 1 zygote which then divides into 2
What develops during week 3?
Germ layers, neural tube, somites, early development of cardiovascular system
What is neurulation?
Formation of neural tube
What is gastrulation?
Formation of germ layers
What is the primitive streak?
Formed in the midline of the epiblast by the dipping in of cells (invagination)
What forms once the primitive streak is fully formed?
The axis of the embryo
What is the purpose of the axis?
Distinguishes between left and right and top and bottom so organs/body structures form in the right place
During gastrulation, what does the epiblast do?
Epiblast cells migrate into the space between the epiblast and the hypoblast layers, cells then replace the hypoblast
What are the germ layers formed in gastrulation?
ectoderm - mesoderm - endoderm
What germ layer is the outer layer?
Ectoderm
What germ layer is the inter layer?
Endoderm
Once the 3 germ layers are formed, cells are…
Specialised
How is the notochord formed?
Cells “sink” from the primitive groove from the enctoderm
What does the notochord induce?
Ectodermal cells in the midline to become thicker to form a neural tube
From what germ layer does the neural plate form?
Ectoderm
The neural tube induces the mesoderm to…
Thicken
What 3 parts does the mesoderm seperate into?
Paraxial mesoderm
Intermediate plate mesoderm
Lateral plate mesoderm
What does the lateral plate mesoderm split to form?
A somatic and splanchnic mesoderm
What is the name of the space between the somatic and the splanchnic mesoderm?
The intraembryonic coelom
What layer of the mesoderm forms the urogenital system?
Intermediate plate mesoderm
What layer of mesoderm forms somites?
Paraxial mesoderm
What layer of the mesoderm forms the body cavity and coverings?
Lateral plate mesoderm
What germ layer does the gut and respiratory stuff form from?
Endoderm
From inner to outer, what is the order of the mesoderm layers?
Paraxial - Intermediate plate - lateral plate
How are somites formed?
Via the segmentation of the paraaxial mesoderm - each segment is a somite
Between what weeks of development is the Organogenetic period?
4th-8th weeks
In what way does an embryo fold so that the ectoderm covers its full surface?
Laterally
What does each somite divide into?
3 things;
Dermatome
Myotome
Sclerotome
What does the dermatome form?
Dermis of skin
What does the myotome form?
Muscles
What does the sclerotome form?
Bones and vertebrae
During the lateral folding of the embryo - what does the lateral plate mesoderm split into?
A somatic mesoderm on top
Intra-embryonic cavity when the lateral folding is complete
Splanchnic mesoderm below/inner
What can the somatic, splanchnic and intra-embryonic coelom space form?
The pariteal, visceral pleua and the pleural cavity
What is teratology?
Study of when things go wrong in development
What are teratogens?
Environmental factors that cause abnormal development
What is confenital rubella syndrome caused by?
A contraction of german measles when pregnant
Give examples of environmental agents that can cause abnormal development?
Drugs - prescriptons/other
Alcohol/tobacco
Infectious agents that can transfer through placenta
Radiation
What type of infectious agents can affect growth?
ToRCH
Toxoplasma
Rubella
Cytomegalovirus
Herpes
Give examples of genetic factors.
Too many/too few chromosomes
Structural changes - deletion of genes/segments of chromosomes
Give an example of some syndromes caused by too many/too few syndrome?
Turner syndrome - 45 chromosomes, single X
Downs syndrome - 47 chromosomes, trisomy 21
What can cause genetic changes?
Increased maternal age
Damage from enviromental factors such as radiation
During weeks 1-2, what types of risk does the embryo face?
High risk of death
Low risk of teratogens
During weeks 3-8, what is significant?
It is the period where the foetus is most significant to teratogens
During weeks 9-38, what happens to teratogen sensitivity?
It decreases
What does the risk posed by a teratogen rely on?
Exposure during critical periods of development
Dosage of drug/chemical/factor
Genetic constitution of embryo ie some more susceptible than others at equivalent doses etc
How would you diagnose a malformation in the prenatal stage?
Blood - AFP
Ultrasound scan – 12 week anomaly scan
Invasive tests: chorionic villus sampling and amniocentesis
How would you diagnose a malformation in the postnatal stage?
Hip stability
Testes (descent)
Fingers and toes
Hearing
