Principles and overview of embryology Flashcards
From an embryological point of view, what is normally considered day 1 of pregnancy?
Day of conception or fertilisation of the egg
From a gynaecologists/obstetricians point of view, what is normally considered the first day of pregnancy?
1st day of last menstrual period
What is a blastocyst?
Structure with inner cell mass (ICM) (that has turned into a bilayer disc, epiblast and hypoblast). Occurs 9 days post-fertilisation (PF) and is ~1mm
What is an embryo?
Structure that is going to be a vertebrate but not clearly human. Forms 5-6 weeks post fertilisation and is around 1 cm
What is a foetus?
Recognisably human and occurs after 8w PF and is ~7cm
What is a conceptus?
Includes all the structures that develop from the fertilised egg including the foetus, placenta and all associated structures
What does totipotent mean?
Cell which has ability to turn into any other tissue of the conceptus
What does pluripotent mean?
Only applies to the ICM as the cells here ca produce any tissue within the body but not the placenta (cells of the placenta are derived from the trophectoderm)
What does multipotent mean?
Cells which can produce tissues of one particular germ cell type
What does unipotent mean?
Cells which can give rise to one major type of tissue
What is a trophectoderm?
Layer forming the outer layer of the blastocyst (after gastrulation) which gives rise to much of the placenta. The potency of these cells are complicated
What happens day 1-3 of pregnancy?
There are cleavage stages where the fertilised egg divides to the 8 cell stage
- This conceptus here relies on maternal RNA for growth
- The cells at this stage are totipotent and are completely equal to each other
What happens day 4 of pregnancy?
The morula forms which has 16-32 cells- intermediate stage of potency whereby totipotent cells become pluripotent
What happens day 5 of pregnancy?
The blastocyst forms
- Early stages of differentiation take place here
- The cells are pluripotent by this stage
What happens day 6 of pregnancy?
Attachment of blastocyst to decimalising endometrial epithelium
- This process involves sequential communication between embryonic and maternal tissues
What happens day 8-9 of pregnancy?
Implantation and formation of synctiotrophoblast (STB) contact with maternal tissue
- Formation of bilaminar embryonic disc from inner cell mass
What happens day 12-13 of pregnancy?
STB invasion and contact with maternal capillaries (2mm)
What happens week 3 of pregnancy?
Formation of trilaminar disc (mesoderm), CNS and somites. Blood vessel 3mm initiation. Formation of placental villi
What happens week 4 of pregnancy?
Closure of neural tube. Heart, face, arm initiated. Umbilical cord starts to grow
Elaboration of placental villi
What happens week 5 of pregnancy?
Face and limbs continue 5-8mm
What happens week 6 of pregnancy?
Face, ears, hands, feet, liver, bladder, gut and pancreas
10-14mm
What happens week 7 of pregnancy?
Face, ears, fingers and toes
17-22mm
What happens week 8 of pregnancy?
Lungs, liver and kidneys
28-30mm
What happens in terms of endocrinology after 8 weeks?
Placental endocrinology becomes dominant- rest of pregnancy switches from embryonic to foetal development
Over the period of foetal development, how does foetal weight increase throughout the trimesters?
50g in first trimester
1050g in 2nd trimester
2100g in 3rd trimester
Why is embryological development studying difficult?
There are multiple ethical issues
As most of development occurs in first few weeks and many women have irregular menstrual cycles, women may not know they’re pregnant until its too late to study these embryos
Embryos are also incredibly small
What are teratogens?
Compounds which change the structure of tissues/cells as they develop
How does response to teratogens change throughout pregnancy?
Different structures have varying responses to these factors e.g. CNS is quite susceptible over whole course of development whereas heart is only susceptible in first 3-6 weeks
What is commonly used to study embryology?
Animals are used as models
How are animals used as models?
Chicks were used to investigate limb development
Fish have been used to investigate the eye
Mice are the most common models, you can knock out or in genes to investigate effects of particular genes
What are the 4 main processes that cells undergo during embryological development?
Proliferation
Movement
Differentiation
Cell loss
What stimulates proliferation?
Growth factors
How do growth factors work?
They interact with receptors on target cells. Therefore changes in receptor expression can influence proliferation
What are the two main sorts of regulatory processes at start of development?
Autocrine
Paracrine
Not much endocrine as vasculature hasn’t developed
How do chemo-attractants work?
If they are produced in one part of the embryo, target cells that express the complementary cognate receptor to move towards the original cell
How do cells tend to move?
In an unregulated manner
In embryology, how is cells movement regulated?
It is facilitated by changes in tissue structure (i.e. remodelling of tissues and extracellular matrix)
MMPs and TIMPs are produced and activation of these proteases occurs
What is differentiation?
Change to express a specific function or change their specific function
In terms of proliferation, what happens in adults amongst cells when regulatory factor is introduced?
Cells closer to the regulatory factors (higher conc) proliferate quicker than cells further away
What is the mechanism of proliferation like in embryos?
The mechanism is more reliant on autocrine/paracrine regulators:
Cells which produce regulatory factors are at the surface of the tissue and therefore, proliferation is maintained from the tip
This leads cells at the base of the layer to proliferate much slower
This leads to formation of projections of cells as seen in limb bud
What is modified proliferation?
Where the process is under control of multiple factors (positive and negative)- leads to formation of asymmetric structures e.g. hand
