Development & Ageing - Early Foetal Development Flashcards
What is fertilisation age?
Way of measuring foetal development, presumed to be the age from the start of fertilisation. We add one day to ovulation date as an assumption
What is a limitation of fertilisation age?
We don’t know when the precise age is unless we use IVF
What is gestational age?
Age beginning from the start of the last menstrual period
What is the difference in time between gestational age and fertilisation age?
Gestational age is 14 days longer than conceptional age
How can we accurately calculate gestation age?
Early obstetric ultrasound and reference to embryo size reference charts
What is Carnegie Staging?
From day 0-60, we can compare the embryo features to the 23 stages of development - allows us to compare development between species
What are the 3 main stages of development?
Embryogenic
Embryonic
Foetal
When is the embryogenic stage?
14-16 days after fertilisation
When is the embryonic stage?
16-50 days after fertilisation
When is the foetal stage?
50-270 days after fertilisation
What happens in the embryogenic stage?
We create 2 distinct cell populations:
- Pluripotent embryonic cells that contribute to foetus
- Extraembryonic cells which contribute to supporting structures e.g. placenta
What happens in the embryonic stage?
- Formation of 3 germ layers and differentiation of tissue types
- Body plan is established
What happens in the fetal stage?
- Major organs now present, but some will migrate to their final position
- Extensive growth and acquisition of foetal viability
Describe the steps from ovulated oocyte to eventually a blastocyst in the first few days of life
Ovulated oocyte Zygote (1 cell) Cleave stage embryo (2-8 cells) Morula (16+ cells) Blastocyst (200-300 cells)
When is the maternal-zygotic transition?
Occurs at the 4-8 stage
What happens before the maternal-zygotic transition?
Embryonic genes not transcribed therefore maternal mRNA and proteins which are synthesised pre-ovulation support embryo
What can lead to an impairment in development before the maternal-zygotic transition?
Failure to synthesis or store these maternal proteins
What occurs during the maternal-zygotic transition?
Embryonic genes are transcribed therefore there is an increase in embryonic protein synthesis, helping with organelle maturation e.g. mitochrondria and Golgi
When is compaction?
Around the 8 cell stage or later
What does compaction do?
Division within the zone pellucida means cells are compressed and the outer layer of cells become wedge shaped - these cells then connect via desmosomes and polarise with apical and basolateral membranes, creating a barrier for diffusion. The inner cell mass forms a separate population
How does the blastocyst form during compaction?
The inner cells and the outer cells reorganise to form a new blastocoel cavity, thus forming a blastocyst
What are the 2 cell populations formed in the blastocyst?
Inner cell mass (pluripotent stem cells)
Trophoectoderm
What is the zone pellucida?
Hard protein shell stopping polyspermy and protects early embryo
What is the blastocoel?
Fluid filled cavity formed osmotically by trophoblast pumping Na+ ions into cavity
What is the purpose of hatching?
Further development can occur upon implanting in the endometrium, however to do this we must escape the hard zone pellucida to allow further division
How does hatching occur?
Blastocyst secretes enzymes to digest zona pellucida.
Cellular contraction of the embryo to weaken a point of the ZP
When is peri-implantation?
7-9 days
What does the trophectoderm develop into?
Syncitiotrophoblast
Cytotrophoblast
What does the inner cell mass develop into?
Epiblast
Hypoblast
Purpose of syncitiotrophoblast?
Invade endometrium and degrade cells, break down capillaries to create interface between maternal blood and embryo
Purpose of cytotrophoblast?
Remain individual to supply cells to syncitiotrophoblast
Purpose of hypoblast?
Form yolk sac for gut development
Purpose of epiblast?
Foetal tissue derived from here
When does bi-laminar embryonic disc formation take place?
Day 12+
What does the syncitiotrophoblast secrete during disc formation?
hCG
How does the amniotic cavity form?
Some of the epiblast cells become separated from main cell block to form the amnio which is one of the extra-embryonic membranes. The space between the amnion and the epiblast = amniotic cavity
What do amnion cells do?
Secrete amniotic fluid
What is the result of disc formation? (ie what is it’s next stage?)
Embryo is now ready for gastrulation at in the embryonic stage
When does gastrulation occur?
3rd week (Days 15/16)
Describe the process of gastrulation
- A primitive streak forms at the caudal end of the epiblast, separating the cranial from caudal and left from right.
- Primitive node forms at the cranial end of the streak and moves towards caudal end thus forming primitive groove
- Epiblast cells migrate towards the groove and detach beneath to replace hypoblast cells via invagination
- Epiblast migration means that hypoblast cells are fully replaced by day 16 and we have the definitive endoderm.
- The remaining epiblast cells form the ectoderm, and the migrating cells in between stop migrating and from the mesoderm.
What is the end result of gastrulation?
3 germ layers
What is derived from the endoderm? (4)
GIT
Liver, pancreas
Lung
Thyroid
What is derived from the ectoderm? (3)
CNS, neural crest
Skin epithelia
Tooth enamel
What is derived from the mesoderm? (4)
Blood
Muscles
Gonads, kidney, adrenal cortex
Bone, cartilage
What is the notochord?
Rod like tube structure made of cartilaginous materials beneath the surface of the ectoderm
What is the purpose of the notochord?
In is an organising centre for foetal development:
- Mesoderm, especially muscle, development
- Releases growth factors for neurulation
What is the neural plate?
Thickened ectoderm on top of the embryo - forms the neural groove in response to signals from notochord
What is neurulation?
Formation of the neural tube
What is the first stage of neurulation?
Notochord sends signals to neural plate - it invaginate towards the notochord forming the neural groove.
2 areas of the plate then move up to form the neural fold which runs along the crania-caudal end of the axis
What is the second stage of neurulation?
Neural folds then move up and eventually fuse with each other to form a hollow neural tube.
The neural tube is overlaid with epidermis (ectoderm)
Neural crest cells migrate to tissues which differentiate
What is the third stage of neurulation?
Neural tube filled a few days later - the wall is made of neural plate and overlaid with epidermis derived from the ectoderm.
The tube is open at the head and tail end but is closed for CNS development
When does closure end in neurulation?
Head end - 23 days
Tail end - 27 days
What can happen if closure fails?
Spina bifida - usually affects lower spine due to failure to close at spinal end (varying severity)
Anencephaly - failure to close at head end - absence of skull and most of brain therefore much more drastic
What do neural crest cells do?
These are ectoderm derived and migrate to different tissues to help with the formation of different structures
What do cranial NC do?
Cranial nerves Glia Lower jaw Middle ear bones (ossicles) Facial cartilage
What do cardiac NC do?
Aortic arch/ pulmonary artery septum
Large artery wall musculoconnective tissue
What do trunk NC do?
Dorsal root ganglia Sympathetic ganglia Adrenal medulla Aortic nerve clusters Melanocytes
What do vagal and sacral NC do?
Parasympathethic ganglia and enteric nervous system ganglia
What happens as a result of neural crest migration/specification failure?
Diverse birth effects e.g. pigmentation disorders, deafness, cardiac and facial defects and failure to innervate the gut
What is somitogenesis?
Segmentation of the body axis
How are somites formed?
Formed from snychronised budding of paraxial mesoderm in pairs. It commences at the head and progresses down the embryo axis
How many somites do humans have?
44 pairs in total (1 pair every 90 minutes - species specific)
What 2 types of tissue do somites give?
Sclerotome
Dermatomyotome
What does the sclerotome form?
Vertebrae and rib cartilage
What does the dermatome form?
Dermis, some fat and connective tissue of the neck and trunk
What does the myotome form?
Forms muscle of the embryo
When is the gut tube formed?
Day 16
What are the 2 phases of gut tube formation?
Ventral folding (head and tail) Lateral folding (two sides of the embryo roll) This forms the yolk sac which in term forms the primitive gut
When is the metal heartbeat first detected?
6 weeks gestational age
How do the lungs form?
Lung bud of the endoderm adjacent to the foregut in the 4th week of development.
The lung bud splits into 2 at the end of the 4th week and progressively branches during development
How do XY gonads form?
Bipotnetial gonadal ridges: SRY gene directed to become Sertoli cells, triggering testis development, leydgi cell formation and testosterone production
How do XX gonads form?
No SRY means gonadal ridge cells adapt a granuloma cell fate and ovary development, this requires reinforcement by FOXL2
