S4 + S6 Embryology Flashcards
How many weeks is the pre-embryonic period? And what happens during this period?
2 weeks long
Cleavage (formation of morula) and implantation begins
What is the process when the ovum is travelling from the ovary to the uterus?
- Oocyte released from ovary
- Oocyte travels along Fallopian tube
- Oocyte is fertilised by sperm in the ampulla (of the Fallopian tube)
- Zygote travels to the uterus
What is the name of a fertilised oocyte?
Zygote
How long is an oocyte viable for?
1 day
How long is a sperm viable for?
Up to 3 days
What are the 6 steps that happen in week 1?
- Cleavage
- Zona pellucida
- Morula
- Compaction
- Hatching
- Implantation begins
What happens during cleavage?
This is the first mitotic division which begins 30 hours after fertilisation. It results in 2 blastomeres of equal size
What is a blastomere?
Each cell of the embryo formed after the first division to produce two cells until the embryo becomes a blastocyst
What is the zona pellucida?
A glycoproteins shell that keeps sperm away from the blastocytes.
What is the morula?
The stage when all cells are totipotent (capacity to become any cell type). Made up of 16 cells (blastomeres)
What is the compaction stage?
The formation of the 1st cavity and the blastocyst.
After compaction the cells are pluripotent (capacity to become many cell types)
What is a blastocyst?
A hollow sphere of cells formed from the morula through compaction - fluid is pumped in to create the blastocoele which results in inner and outer cell masses.
What is the hatching stage?
The blastocyst hatches from the zona pellucida so it is now free to enlarge and interact with the uterine surface for implantation
What happens as implantation begins?
There is now about 100 cells - 8 of these cells make the embryo, the rest (about 92) will contribute to development of feral membranes
What two layers is the outer cell mass made out of?
- Cytotrophoblast (single cell sheet)
2. Syncytiotrophoblast (multi-cell sheet)
What two layers make up the initial inner cell mass (bilaminar disk)?
- Epiblast
2. Hypoblast
What happens during week 2?
- Differentiation
- Conceptus implanted
- Cavity formation
What are the two cavities in week 2?
- Amniotic sac
2. Yolk sac
What suspends the cavities?
The connecting stalk (in the chorionic cavity)
What happens on day 11?
The primitive yolk sac membrane is pushed away from the cytotrophoblast layer by an acellular extraembryonic reticulum
What is the acellular extraembryonic reticulum converted in to? And how does this happen?
Extraembryonic mesoderm
By cell migration
What happens on day 9?
- at the embryonic pole, there’s rapid development of the synctiotrophoblast
- at the abembryonic pole, the primitive yolk sac is formed (the yolk sac membrane is in contact with the cytotrophoblast layer)
What happens on day 12?
- maternal sinusoids invaded by syncytiotrophoblast - lacunae become continuous with sinusoids - this is the start of uteroplacental circulation
- the uterine stroma prepares for support of the embryo
What is the uterine stroma?
Layer of connective tissue
What does the syncytiotrophoblast contain?
Lacunae
What happens on day 13?
Formation of the secondary yolk sac (the definitive yolk sac) - it pinches off from the primitive yolk sac
What happens on day 14?
Spaces in the extraembryonic mesoderm merge to form the chorionic cavity
What is the role of the outer cell mass?
Designed to produce the tissues supporting the embryo/foetus during development
What is the role of the inner cell mass?
Cells inside the blastocyst go on to form the embryo proper
What is another name for the inner cell mass?
Embryoblast
What are the 5 stages in early development?
- Fertilisation and implantation
- Gastrulation
- Neurulation
- Segmentation
- Folding
What 4 things does the mesoderm differentiate into?
- Notochord
- Somites
- Intermediate mesoderm
- Lateral plate
What is teratogenesis?
Major congenital malformation
When is teratogenesis the biggest risk?
Weeks 3 to 8 (embryonic period) as it’s the period of biggest change
When does the primitive streak (and primitive node) form?
In the 3rd week
What is the primitive streak?
A thickened region of the epiblast a the caudal end of the bilaminar disc. It is the site from which gastrulation proceeds
What happens during migration and invagination?
Epiblast cells migrate to the primitive streak and push through the tight junctions and displace the hypoblast layer, creating 3 layers
What is the trilaminar disc made up of?
- Ectoderm
- Mesoderm
- Endoderm
All from the epiblast
What does the ectoderm form?
- nervous system
- epidermis
Organs and structures that maintain contact with the outside world
What does the mesoderm form?
- muscle
- cartilage
- bone
- vascular system (incl. heart and vessels)
Supporting tissues
What does the endoderm form?
- epithelial lining of GI tract and respiratory tract
- parenchyma of glands
Internal structures
What is situs inversus? How does it occur?
A congenital condition when the major internal organs are reversed/mirror-image.
Occurs due to immotile cilia that develop during the embryonic period.
Usually no problems unless the person has both normal and mirror-image disposition.
What is responsible for establishing left and right asymmetry?
The primitive node - actions of ciliated cells at the node result in left-ward flow of signaling molecules which leads to a cascade of side-specific signals.
What is the notochord and what is its role?
It is a solid rod of cells running in the midline.
Has an important signalling role.
When does gastrulation occur?
In the 3rd week (start of the embryonic period)
What happens in gastrulation?
- bilaminar disc —> trilaminar disc
* formation of the 3 germ layers - ectoderm, mesoderm and endoderm
How does gastrulation occur?
Due to the formation of the primitive streak in the caudal epiblast which leads to the migration and invagination of epiblast cells
Why does gastrulation occur?
To ensure the correct placement of precursor tissues (left/right) to allow subsequent morphogeneis to occur
What does the notochord direct conversion of?
Conversion of the ectoderm to neuroectoderrm
What do notochord signals cause?
Causes the ectoderm to thicken and the edges to elevate out of the plane of the disc and curl towards each other - this creates the neural tube
What areas does the mesoderm differentiate into?
- Paraxial mesoderm
- Intermediate mesoderm
- Somatic mesoderm
- Splanchnic mesoderm
- Intraembryonic coelom
What does the intraembryonic coelom (cavity) allow?
The cavity in which organs of the body can grow
What does the splanchnic mesoderm differentiate into?
The viscera
What does the somatic mesoderm differentiate into?
The body connective tissue
What does the paraxial mesoderm form?
It forms somites
How many somites form? When do they start forming?
31 (42-44 initially but some are lost)
Day 20
What do somites undergo to give rise to sclerotome, myotome and dermatome?
Organised degeneration
What does the dermatome form?
Dermis
What does the myotome form?
Muscles
What does the sclerotome form?
Bones
What repeating structures does organisation of the mesoderm into somites give rise to?
- Vertebrae
- Ribs
- Intercostal muscles
- Spinal cord segments
What do somites guide?
Innervation
What two types of folding does the trilaminar disc undergo?
Cephalocaudal folding and lateral folding
Why does folding occur?
- It draws together the margins of the disc to pull the amniotic membrane around the disc
- creates a ventral body wall
- creates the primordium of the gut
- puts primordium of organs in correct places
- it creates a new cavity within the embryo
What has happened by the end of the fourth week?
- the nervous system has started to form
- segments have appeared - assigning specific tasks to specific cells
- embryo has folded - puts everything in the right place
