Embryology Flashcards
How many weeks is from conception to birth
38 weeks
What are the 3 periods of birth
- Pre - Embryonic period - 1st week
- Embryonic period - weeks 2 to 8
- Fetal period - weeks 9 to 39 ( 3 trimesters )
What happens during the Pre Embryonic period
Fertilization in ovary to Implantation in uterus = GLASTISIS
Not really an embryo - just a cluster of cells
What happens during the Embryonic period
- Multicellular Blastocyst to recognizable Vertebrate
- Organogenesis
- Developing embryo and creating something that looks like a liferform
What happens during the Fetal period
- Major growth of existing structures – everything increases in size + a few new ones/structures/organs produced
When do you become fertile
Not fertile until 2 weeks after last menstrual period – when you get pregnant
So its 38 weeks rather than 40 weeks
SO embryology occurs when pregnant women give birth- take date from last menstrual period
What is unique about whats produced at the end of the embryonic period
No matter what its coming from – human or mouse, we all roughly look the same at end of embryonic period - we’re just different sizes
What is the period where most organs are produced
Embryonic period
When was most of what we are created
During 1st 8 weeks
What happens during fetal period
Pretty much look like a human = end of embryonic period - have facial features
What are the cellular events that occur in embryonic development
- Cell Addition by Mitosis or ‘Cleavage’
(2) Cell Movement & Migration
(3) Programmed Cell Death or ‘Apoptosis’
What cell size is cell addition by mitosis or cleavage
From a single cell to >5 x 1012 cells
Doenst expand in size - only replicates
What happens during cell movement and migration
Forms layered, folded &/or tubular structures
Need those cells to move around and migrate to form different organs and structures in body – otherwise one lump of cell
What happens during programmed cell death or Apoptosis
Regression & elimination of tissues that are no longer required
Many times when cells and structures developed in developing embryo needed at that point of time but not needed when premature adult or human – develop and serve their function but those cells have to die off through apoptosis
Example of apoptosis in a tissue during embryonic development
- When eye developing – lens of eye coming and forming but at that time you have no aqueous humour production so there is no O2 and nutrients supply = blood vessel branching out of optic nevre – goes through cavity in middle of eye – vitreous and forms a basket of blood vessels – tunica vascularis lentis = provides o2 and nutrient supply for developing lens but when ciliary body matures , you get production of aqueous humour – that bathes lens with nutrietns and o2 so tunica vascularis lentis isn’t needed anymore so it dies – apoptosis
What is apoptosis
Programmed cell death - when the cell dies off
What happens during day 0
The zygote forms
Maternal and Paternal nuclei has fused
Sperm breaks down as it burrows its way inside - releasing its nucleus inside
What is the transparent capsule of the membrane called
Zona pellicuda - capsule/membrane that wasn’t around blasteocyst
How does a zygote form
Sperm or egg successfully found each other - sperm successfully eat its way through the membrane and the egg is implanted itself into the ovary
How many pairs of chromosomes in diploid
23
What happens during Day 1
Zygote divides/ cell splits to form 2 blastomeres
When it splits it doesnt double in size due to the 2 fused nuclei
Surrounded by membrane, separating inside capsule
What happens during week 1 - pre embryonic period
- Cleavage: daily series of binary cell divisions: blastomeres increase to 4, 8, 16 cells etc
- Moves down oviduct to uterine cavity, ‘wafted’ by cilia on oviduct epithelium - migration of cells and forming separate structures
- Instead of 1 cell dividing, both cells go through mitosis
What is produced at 3 days
Mast ball of cells - MORULA
What is cleavage
- Daily series of binary cell divisions
- 4 cells divide to create 8
- Every time you divide its exponential growth - v quick
How many days does it take for egg to get to Fallopian tube
7 days
Why are the eggs not stationary during week 1
As it goes through these divisions as ovaries are released into Fallopian tube
What is the Fallopian tube lined with and why
Ciliated columnar epithelial cells
Cilia is wafting - wants to move the egg down the Fallopian tube into the uterine cavity
How long does it take for egg to get to uterine cavity during 1st week of embryonic period
7 days
What is inside blastocyst
Internal cavity - filled with blasticium – fluid those cells are secreting
What happens during days 5 to 6
Blastocyst rearrange structure and segregate into 2 separate regions
What are the 2 sub regions of cells of the blastocyst
(1) Inner mass = Embryoblast
forms the Embryo (‘blast’: Gr,‘maker’)
(2) Outer layer = Trophoblast
forms the Placenta (‘tropho’: Gr,’feeder’) . Separated by fluid (blastocele)
What is embryoblast
Embryo maker – little spot of cells makes embryo
Inner cell mass
What is trophoblast
Feeder maker
All the cells around the outside
What happens during days 7 t o 10
- Zona pellucida ( translucent capsule ) ruptures = allows blasteocyst/trophoblast cells to invade uterine walls of uterus
- Trophoblast cells begin invading the epithelium & stroma of the maternal uterine wall
What is the placenta of trophoblast
- Outer layer
- Interphase between maternal bloody supply and embryo bloody supply – takes what it wants – o2 and nutrients from mothers blood supply and
And gives it to embryo – feeder
What happens during days 5 to 7
Blastocyst implanted into the uterine wall
Entering embryonic period - going into uterine wall
- Zona Pellucida ruptures - allows trophoblast cells to attach and migrate inbetween columnar epithelium of uterine wall
Where does embryo develop
Inside the mothers tissue - embeded into uterine stroma into connective tissue
NOT in uterus cavity
What happens during week 2
- Division of the Embryoblasts and Trophoblasts into 2 further cell types
- Cells proliferate
- Trophoblast cells buried deeper and deeper into uterine walls until whole blastocyst is inside the epithelium
- Membranes being broken down between cells – migrate as a big mass intothe stroma
What do Embryoblasts divide into
Into 2 layered disc/ cell types
- Epiblast,
- Hypoblast,
What do Trophoblasts divide into
Into 2 divisions/ cell types:
- Cytotrophoblast
- Syncytiotrophoblast
What is Epiblast
Outer layer of columnar cells
What is Hypoblast
Inner layer of cuboidal cells
Migrate around inside of cytotrophoblast - form Heusers membrane
What is Cytotrophoblast
Inner cells remain as single layer around the blastocyst – remain encompassing around developing embryoblast
- Encompasses around
What is Syncytiotrophoblast
- Outer cells lose their membranes, coalesce & infiltrate deeper – migrate further into stroma and embed developing embryo into uterine wall.
- As these cells migrate in, it forms a syncytian – lots of cells loose in their membrane and forming multiinuclated cells by their fusing - like skeletal muscle
What does all this cell divsion and migration need
Lot of nutrients, proteins, oxygen, minerals and energy
What develop each side of embryo during week 2
Cavities - formation of sacs - fluid filled bag with liquid
Epiblast forms sac on one side and Hypoblast forms sac on other side
Role of Amnioblast cells in the epiblast
- Form the amniotic sac, fluid & membranes - create a ring on sides
- Functions: homeostatic environment, mechanical shock-absorption, buoyancy - membrane doesn’t have too much pressure –can create cell death
, low resistance to movement – if mum falls over, baby doesn’t get slapped to sides - bones weak when devloping, protection from intrauterine infection – immune protection - barrier around developing embryo – any localized infections
What sac froms from epiblast
Amniotic sac
What sac froms from hypoblast
Yolk sac
What happens to cells in hypoblast
- Cells migrate along inside of the cytotrophoblast to form the Yolk sac (a.k.a., Heuser’s membrane)
- Functions: nutrients for early embryo, replaced later by placenta
Why does trophoblast penetrate deeper into maternal tissue
- Try and find a constant supply of oxygen and nutrients and then will develop the placenta
- Need another supply of nutrients reservoir for initial stages of embryology – get that from yolk sac so that hypoblast sends out cells (Heuser’s membrane) which wrap around inside of the cytotrophoblast and form yolk sac
What does yolk sac do
- Forms cavity in the middle
- Initial nutrient reservoir that embryo is laying down for initial stages of development - replaced later on by placenta
What happens during week 3
Gastrulation
What happens during gastrulation
- Primitive streak’ (midline groove) appears in caudal (tail) end of the epiblast
- Cells in the streak proliferate & ingress into Hypoblast
- Replacing original occupants (which die by apoptosis) & forming the Endoderm
- A 2nd wave of ingression by epiblast cells occurs in-between, forming a middle
layer, the Mesoderm - Remaining epiblast cells convert into the Ectoderm
Endo = within Meso = middle Ecto = external Derm = skin/layer
Why does gastrolation need to occur
Ball of cells with cavity in there turns into something that’s gonna start twisting and turning and forming structures that are more resembled future mammals
What is endoderm
Epiblast derivatives
What happens to remaining epiblast cells
Stop migrating down midline groove, change into the ectoderm
Formation of Primitive streak and what happens after
Epiblast cells migrate down at groove point and invadee this space that was taken uo by cells below them
- As they migrate down, hypoblast cells undergo apoptosis as they are no longer needed.
- Epiblast cells fill up space along bottom and differentiate into specific cell derivatives - endoderms
What is the point where the primitive streak forms
Where epiblast and hypoblast meet - interphase between yolk and amiotic sac
1 end head and 1 end tail - signalling within cellular structure - give polarity to whats developing
Summary of Gastrulation
- 1st wave of cells migrate down the midline and invade layer filled by hypoblasts
- Hypoblasts die off by apoptosis, leaving this space behind
- As epiblasts migrate in – reforms this layer forming endoderm
- Once layer reformed, cells continue to migrate in, fill space inbetween and become the mesoderm,
- Once these void is filled, these cells stop migrating and these cells differentiate into the ectoderm
Significance of the Germ Layers
- The cells in the 3 Germ Layers are no longer pluri-potent ‘stem cells’ capable of generating any cell type in the body, but have now become committed to producing only a restricted set of cells & tissues (known as their ‘derivatives’).
- This commitment to produce cells with a particular ‘fate’ involves inactivation of different sets of genes in the cells of each Germ Layer, so restricting potential to a limited range of possibilities the progeny they can give rise to.
Cell of outer ectoderm
Columnar epithelial cell
Cell of middle mesoderm
Fibroblast-like cells & fibres
Cell of Inner Endoderm
Cuboidal epithelial cells
What type of cells were the cells that created the different membranes in epiblast
Pluripotent - have a potential as a stem cell to prouce any single cell type throughout body
What happpens after you go throug gastoration and get formation of ectoderm, mesoderm and endoderm
The fate of the cells that are now within the 3 layers have definitive paths – cant form any single cell type – have specific lineage they can go and create – specific structures throughout body
The Germ Layer Derivatives
Endoderm - outer epithelia and nervous system
Mesoderm - everything in between including connective tissue
Endoderm - Inner Epithelia and some organs
What happens during week 4
- Germ disc grows & folds: tubular embryo
- Becomes clyindrical
- Start to get twisting
- Those germ layers - specific cell types start to differentiate and multiply and start to form different types of tissues
- Get foldings of Somites
What are somites
Clusters which will then develop into vertebrate – creating axial skeleton
What happens to Ectoderm
- Midline ( what it migrates into ) thickens & folds above
- Forms Neural Tube (precursor to brain, spinal cord) = can form n.s.
- Still continue to be your outer epithelia
What happens to Mesoderm
- Cells aggregate in groups ( groups of mesoderms cluster together and form precursor to different tissue types )
- Midline, compact: forms Somites, forerunners of the axial skeleton (e.g., vertebral column)
- Lateral ( away from midline ), migrate: forms scleratomes (non-axial skeleton – bones around periphery of body – arms,legs,fingers ), myotomes (muscle), dermatomes (dermis – connective tissue components of skin)
What happens to Endoderm
-Folds under – folds around on itself as it wants to form gut tube
Forms the inner Gut Tube
What happens at end of week 3
Disk is folding
What forms the vertebrate of spinal cord
Somites
What forms the neural tube
Foldings downwards at midline
What happens at week 5
A Recognizable Vertebrate
What is the Placenta
A vascular network with 2 separate parts
What are the 2 parts of the placenta
- Maternal blood vessels
- Embryonic blood vessels
Maternal blood vessels
- Spaces (lacunae) appear in syncytio-trophoblast merging and working its way through those capillaries/stroma
- Invaded by capillaries in uterine wall
- Supply O2 + nutrients & remove embryonic CO2 & waste products
Embryonic blood vessels
- Finger-like villi project from cyto-trophoblast on other side next to embryo – eventually form capillaries and those capillaries in cytotrophoblast will match up and fuse with capillaries/arteries and veins of developing embryo
- Inner cells at core of villi differentiate into capillaries
Why doesnt maternal - embryonic/fetal blood mix
Because haemoglobin in baby has higher affinity for O2 than the mother
What happens at week 8
Large and more foldings
Why do you need the placenta
Need a much larger reservoir of nutrients and O2 for the big events that are gonna go into the fetal life where everything expands in size
What comes out of the placenta
Finger like capillaries which run very closely to maternal blood capillaries – but they don’t fuse or touch
What does placenta join onto and how
Embryo through the embolical cord, blood flows from ebmbryo into placenta
What meets
Maternal blood supply meets embryonic blood supply
What do the capillaries go through in the maternal blood supply
Uterine stroma
How does embryo treat maternal capillaries
Embryo doensnt want to break maternal capillaries – untouched at the lacunae
It wants to work its way around them and fuse around them because it wants to keep those capillaries functioning – need those nutrients
What happens at week 9
Early Fetus, an incipient Human
Fetal development stages
- Week 12: ~10 cm long:
.External genitalia
.Disproportionate head growth - Week 24: ~20 cm long
.Face developed
.Increased body growth
.Limb movements - Week 27/28: ~ 25 cm long
. May survive premature birth, but hypothermia & lung collapse remain likely - cant regulate body temp yet - Week 38: 35-40 cm long
‘Hi, Mum’
What week can you start to tell the gender
12
Importance of taking Birth Histories when examining infants & children – END STAGE
- Was delivery normal & without injury?
Respiratory distress – less O2, risk factor for cerebral palsy – isn’t always obvious - Was the child premature?
.A Risk Factor for specific vision problems
.Mild (high myopia); Moderate (eye movements squint, nystagmus); Severe – (retinopathy: ROP)
(- Specific risks for Retinopathy of Prematurity Extreme prematurity, <28 weeks
.Duration of oxygen given on ventilator
.Blood transfusion required; occurrence of seizures
What happens if umbilical cord trapped
Cut off blood supply to fetus during labour = neuronal problems – brain damaged – effect on eyes
Retinopathy of Prematurity
If youre developing, your capillaries are growing on your retina – if youre exposed to high levels of O2 – that can diffuse straight through cornea = high levels of O2 = don’t need retinal capillaries to provide O2 = die off BUT problem is when baby comes out of high O2 chambers and back into normal air,your retina becomes hypoxic because it doesn’t have enough capillaries to feed all neurons of the retina – retina could end up as a screwed up ball inside your eye and lead to blindness
Abnormal devlopment
- Spontaneous embryonic abortion
- Birth defects
Spontaneous embryonic abortion
- Can occur after 1st period – miscarriage
- Major malformations incompatible with further development terminate ~50% of pregnancies – occur before women realise theyre pregnant
Birth defects
( if it does survive and you end up with developing features ) affect 5-10% live births – later on birth defects
- Embryonic defects ( 8 weeks ): can result in gross malformations, some preclude extra-uterine life – only live a few days
- Fetal defects: usually cause under or over -development of existing tissues/organs, so disabilities surmountable
Causes of abnormal devlopment
Genetic – from mother/father & environmental teratogens
What happens if problems during 1st 3 weeks
Devopling embryo cannot live = dies off – women back to normal mensutral cycle – not realise theyre pregnant
What is Toxoplasmosis
Caused by bacteria associated with cat poo
Examples of maternal exposure to toxic agents
1) Infectious: e.g., viruses & parasites
rubella, herpes, HIV & toxoplasmosis
Micro-ophthalmia & heart defects
(2) Physical: e.g. ionizing radiation
blindness, craniofacial & CNS defects – neuron deformity
(3) Chemical: e.g. too many to list
Thalidomide – chiral molecule – one isomer causes (anti-emetic): limb malformation – short arms and legs = got rid of morning sickness
Alcohol:, micro-encephaly with optic nerve hypoplasia & mental retardation – small head – everything is too small – brain underdevloped
Early vs. Late Teratogenic Effects
- Fetal teratogen (alcohol exposure) = Micro-Encephaly with smaller optic nerves (also different eyelid positions)
- Embryonic teratogen (exposure days 19-21) = Cyclopia or Synophthalmia
