Embryos - Embryology Basics Flashcards
What is the average foetal age
38 weeks from conception to birth is the average foetal age
What is the gestational age
Last menstrual period (2 weeks before ovulation) to birth = 40 weeks
What happens in the embryonic period
1st 8 weeks - all major organs are formed
What is the foetal period
The remaining 30 weeks - organs grow larger and become more complex
Fertilisation to implantation
Define ovulation
Where does the egg then travel
- Egg released into the peritoneal cavity
- Travels down fallopian tube in which fertilisation occurs
- At conception in fallopian tube, maternal and paternal genetic material join to form a human life - zygote
- Cell division occurs while travelling down the tube and into uterus
What happens to the zygote as it moves down the tube towards the uterus
When does this process occur
Divides repeatedly as it moves down the tube towards the uterus (cleavage)
WEEK 1 POST-CONCEPTION
Name the daughter cells of the zygote
Blastomeres
Define morula
The solid cluster of 12-16 blastomeres at about 72 hours
What happens on day 4 post-conception
60 cell morula enters uterus, taking up fluid & becoming BLASTOCYST
Describe the 2 distinct types of cells that make up the blastocyst
INNER CELL MASS - forms the embryo
TROPHOBLAST - layer of cells sorrounding the cavity which helps form the placenta
What happens to the blastocyst after conception
Floats for about 3 days
Implantation on about day 6 post-conception
- trophoblast eordes uterine wall
- takes 1 week to complete
How are monozygotic (identical) twins formed - at blastocyst level
If inner cell mass of a single blastocyst divides in the first week
Describe the events of week 2
- Inner cell mass divides into epiblast and hypoblast
- 2 fluid filled sacs form
Amniotic sac from epiblast
Yolk sac from hypoblast
- Bilaminae embryonic disc = area of contact - gives rise to the whole body
** only 2 layers of cells in week 2
What does the epiblast give rise to
Amniotic sac
What does the hypoblast give rise to
Yolk sac
Describe the events of week 3
- Bilaminar disc becomes a trilaminar disc
- 3 primary germ layers - all body tissues develop from these
Ectoderm
Skin
Nervous system
Mesoderm
Gut tube
Endoderm
Connective tissue
How are the 3 germ layers formed
- Primitive streak (groove) on dorsal surface of epiblast
- Gastrulation - invagination of epiblast cells
- Days 14-15 - they replace hypoblast, becoming the endoderm
- Day 16 - mesoderm formed in between
- Epiblast cells remain on surface, forming the ectoderm
What do the ectoderm and endoderm have in common
Epithelial tissues
What sort of tissue is the mesoderm
Mesenchyme tissue
Describe mesenchyme cells
Star shaped - do not attach to one another => migrate freely
When does the notochord form
Describe its formation
days 16-18
Primitive node epiblast cells invaginate and migrate anteriorly with some endoderm cells
Rod defining the body axis is formed
Future sight of the vertebral column
What is the nucleus pulposus
Centre of IV disc
Remnant of notochord
What is neurulation
Formation of spinal cord and brain begins
Neural plate → neural groove → neural tube
How does the process of neurulation progress
What supplement is important in this process
- Closure of neural tube begins at the end of wk 3 and complete by end of wk 4
- Folic acid is important for this step
- Extends cranially - brain, and caudally - spinal cord
- Neural crest, lateral ectodermal cells, pulled along and form sensory cells and other structures
What do the somites become & when
Bones and muscles
By end of WEEK 4
Where does the mesoderm differentiate to
When does this happen
Lateral to notochord - week 3
Extends cranially and caudally
What is the mesoderm divided into
SOMITES - 40 pairs of body segments by end of week 4
INTERMEDIATE MESODERM - lateral to somites
LATERAL PLATE - splits to form coelom (cavity)
Name the divisions of the mesodermal lateral plate and where they are located
- Somatic mesoderm - apposed to the ectoderm
- Splanchnic mesoderm - apposed to the endoderm
- Coelom in between will become the serous cavities of the ventral body cavity: peritoneal, pericardial, pleural
What does the coelom (in between the somatic and splanchnic mesoderm) become
The serous cavities of the ventral body cavity
Peritoneal
Pericardial
Pleural
When does folding begin
Week 4
Lots of growth of ectoderm
What happens at day 24
Embryo protrudes into amniotic cavity
What happens at day 23 of embryonic development
Begins to fold
Lateral folds will join ventrally
Spaces appear in mesoderm
Cylindrical human body plan @ day 28 VS cross section through abdomen of an adult
Major derivatives of the embryonic germ layers
When does the heart start pumping
29 days
embryo is 0.5cm in size
2 major ways that developmental abnormalities can occur
- Congenital (inherited or genetic)
- Maternal derived abnormalities
Trisomy 21
Down Syndrome - major chromosome abnormality
Trisomy 18 (also 9, 13, 15)
Edwards Syndrome
Possible reason for increased occurence of chromosomal abnormalities with age
The egg has been sitting “stagnant” for a long time
Define absolute risk
The rate of occurence of an abnormal phenotype among individuals exposed to the agent (e.g. foetal alcohol syndrome)
Relative risk
Ratio of the rate of the condition among the exposed and the non-exposed (e.g. smokers risk of having a low birth weight baby compared to non-smokers)
* A high relative risk may indicate a low absolute risk if the condition is rare
Overview of embryonic development
