Embryology Flashcards
When is the pre-embryonic stage?
First two weeks of development
When is the embryonic period?
3rd to 8th weeks of development
When is the fetal period?
From the 9th week until birth
Oocyte travels from the ____ to the junction with the uterus (_____)
Oocyte travels from the ampulla, along the Fallopian tube, to the junction of the uterus (isthmus)
What happens during cleavage?
Cell division, creating two blastomeres initially then continued mitotic division leading to the formation of a morula, ~16 cells
How is a blastocyst formed?
Spaces between cells in the morula form one large space by compaction, (blastocoele or blastocyst cavity) creating a hollow sphere of cells
What is hatching?
Happens on day 5 of development, the blastocyst hatches from the zona pellucida, allowing it to enlarge and come into contact with the uterine surface for implantation
How do the cells change before and after compaction?
Before they are totipotent
After, they are pluripotent
What is the outer mass of cells?
The trophoblast, made up of two different types of cells:
- cytiotrophoblast
- syncytiotrophoblast
Trophoblast is a major component of fetal membranes, especially the placenta
What is the inner cell mass?
The embryoblast, becomes the embryo itself
Made up of:
-epiblast
-hypoblast
Epiblast and hypoblast make up the bilaminar disk
Which cavity is formed from spaces within the epiblast?
The amniotic cavity
What happens during implantation?
The uterine epithelial is breached
The conceptus implants within the uterine stroma
Maternal blood flow is established within the placenta, allowing support of the embryo from maternal circulation
The basic structural unit of materno-fetal exchange is established
What conditions are linked to implantation defects?
IUGR
Pre-eclampsia
Ectopic pregnancy
Placenta praevia
Ectopic pregnancy
Implantation at site other than uterine body (most commonly in the Fallopian tube)
Can be peritoneal or ovarian
Placenta praevia
Implantation in the lower uterine segment
Can cause haemorrhage in pregnancy
Requires C-section delivery
Which sac is formed from the blastocyst cavity, and how?
Primitive yolk sac
Formed from hypoblast endoderm and extraembryonic mesoderm
Formation of extraembryonic mesoderm
Primitive yolk sac membrane is pushed away from cytotrophoblast layer by an acellular extraembryonic reticulum
Reticulum is later converted to extraembryonic mesoderm by cell migration
How does uteroplacental circulation begin?
Maternal sinusoids invade synctiotrophoblast
Lacunae become continuous with sinusoids
Uteroplacental circulation begins
Secondary yolk sac
secondary yolk sac forms and pinches off from primitive yolk sac
Formation of the chronic cavity
Spaces within the extraembryonic mesoderm merge to form the chronic cavity
The embryo and its cavities are suspended by the connecting stalk, a column of mesoderm which is the future umbilical cord
Order of cavities, coelom, sacs and spaces
Blastocoele
-1st cavity, as a result of compaction
Amniotic sac
-formed from spaces within epiblast
Primitive yolk sac
-formed by hypoblast lining the blastocoele
Secondary yolk sac
-formed within primitive yolk sac
Extraembryonic coelom
- chorionic cavity
- formed from spaces within extraembryonic reticulum and mesoderm
Keys events of early development
Fertilisation and implantation
Gastrulation
Neurulation
Segmentation
Which all lead to
Folding
Gastrulation: When What How Why
When
-3rd week of development, start of embryonic period
What
- bilaminar disk converted to trilaminar disk
- 3 germ layers (ectoderm, mesoderm and endoderm)
- formation of notochord
How
- primitive streak forms in the caudal epiblast
- leads to migration and invagination of epiblast cells into streak
- displacement of hypoblast and creation of 3rd germ layer
Why
-ensure correct placement of precursor tissues to allow subsequent morphogenetic to take place
Derivatives of ectoderm, mesoderm and endoderm
Ectoderm
- organs & structures that maintain contact with outside world
- epidermis, CNS, peripheral nervous system
Mesoderm
- supporting tissues
- muscle, cartillage, bone, vascular system including heart and vessels
Endoderm
- internal structures
- epithelial lining of GI tract, respiratory tract, parenchyma of glands
Situs Inversus
- complete mirror image
- commonly results from immotile cilia
- no associated morbidity
- problems arise if both normal and mirror-image disposition
What drives left-right asymmetry
At primitive node, action of ciliates cells results in left-ward flow of signalling molecules
Ciliated cells beat and push cellular signalling molecules to left side
Differentiating left from right
Lack of action from cilia means down to chance which way signals go which can lead to situs invertus
Initiates side-specific signalling cascade
What is the notochord?
Solid rod of cells running in the midline with important signalling role
Directs conversion of overlying ectoderm to neuroectoderm
Notochord signals lead to formation of neural tube
What is neurulation
Creation of neural tube
Notochord signals cause overlying ectoderm to thicken, creating a slipper-shaped neural plate
Edges elevate out of the plane of the disk and curl towards each other, forming the neural tube
Zones of mesoderm
Paraxial mesoderm
Intermediate mesoderm
Somatic mesoderm
Splanchnic mesoderm
Intraembryonic coelom created between somatic and splanchnic mesoderm
Segmentation
Organisation of paraxial mesoderm into segments (somites) gives rise to repeating structures
- vertebrae
- ribs
- intercostal muscles
- spinal cord segments
Spinal cord segments line up with somites, so nerves emerging from spinal cord segments will supply somites and all derivatives
1st pair appear at day 20 in occipital region, more appear in craniocaudal sequence, 3 pairs a day until 42 - 44 pairs are present by end of week 5
Some disappear, leaving 31 pairs of somites
Somite derivatives
Dermatome
-dermis
Myotome
-muscles
Sclerotium
- syndetome (tendon)
- arthrotome (vertebral joints)
- endotome (endothelial cells, dorsal aorta)
- vertebrae and ribs
What is the buccopharyngeal membrane?
Site where going to create opening to oral cavity, site of future mouth
What causes embryonic folding?
growth and development of neural tube pushing down on caudal and cranial end and somites putting pressure laterally.
What is cephalocaudal folding
Head fold and tail fold
What does embryonic folding entail?
Cephalocaudal folding
Lateral folding
Results of folding
Draws together the margins of the disk
- creating a ventral body wall
- pulling amniotic membrane around the disk, embryo becomes suspended within amniotic sac
- pulling connecting stalk centrally
Creates the primordial of the gut, puts heart and primordial of the diaphragm in the right place
Creates a new cavity within the embryo
At the end of the 4th week…
The nervous system has started to form
Segments have appeared, assigning specific tasks to specific cells
Embryo has folded, putting everything in the right place
