BL Embryology Flashcards
What are the 5 key steps of embryonic development?
1- fertilisation + implantation
2- gastrulation
3- neurulation
4- segmentation
5- folding
When is the pre-embryonic period?
First 2 weeks
When is the embryonic period?
3-8 weeks
When is the foetal period?
9-38 weeks
Total period of gestation
38 weeks +/- 2
What is the length of a clinical pregnancy?
40 weeks?
What are the stages of the pre-embryonic period?
1- fertilisation
2- cleavage
3-compaction
4- hatching
5- implantation
Describe fertilisation
1- oocyte released from ovary
2- travels along fallopian tube under action of progesterone
3- sperm fertilised in ampulla
4- zygote formed
What is a zygote?
Fertilised oocyte
Where does fertilisation occur?
Ampulla of fallopian tube
Where is the ideal site for implantation?
Posterior uterine wall
What is a morula?
Solid ball of cells from a zygote
What is cleavage?
First division of zygote resulting in 2 blastomeres surrounded by Zona pellucida
Initiates a series of mitotic cell division which forms a morula
What is compaction?
Formation of a blastocyst
What is pre-implantation genetic diagnosis?
When a cell is safety removed from morula to be tested for serious hereditary conditions
Done before implantation of embryo in IVF
What happens in week 1?
1- cleavage
2- compaction
3- hatching
What is Zona pellucida?
Thick glycoproteins shell which surrounds blastomeres + protects from sperm after fertilisation
What is hatching?
Blastocyst leaves Zona pellucida + can interact with uterine surface to implant
Describe a blastocyst
Inner cell mass (embryoblast)
Outer cell mass (trophoblast)
Zona pellucida surrounding
Blastocyst cavity
What will the embryoblast develop into?
Embryo
What is a trophoblast?
Cells forming the outer layer of blastocyst that provide stability to blastocyst
What type of stem cells are before and after compaction?
Before - Totipotent
After - pluripotent
Why is week 2 called the week of twos?
Trophoblast splits into - syncytiotrophoblast
- cytotrophoblast
Embryoblast forms bilaminar disc - epiblast
- hypoblast
What provides O2 and nutrient to growing embryo?
Maternal blood
What does the hypoblast do?
Lines the yolk sac
What is the result of early embryonic period?
Foundations for development of body systems
What is needed in the early embryonic period?
Sufficient cells of right type in the right place
What happens in gastrulation?
Bilaminar disk»_space; trilaminar disk
Forms three germ layer - ectoderm, mesoderm + endoderm
Why does gastrulation occur?
To ensure correct placement of tissues to allow subsequent morphogenesis to occur
When does gastrulation occur?
Week 3
Embryonic period
How does gastrulation occur?
- Primitive streak forms in caudal epiblast
- migration + invagination of epiblast cells
- bilaminar disk»_space; trilaminar disk
What does the ectoderm derive into?
Organs+ structures that maintain contact with outside world
e.g. nervous system, epidermis
Where does the nervous system arise from?
Ectoderm
What does the mesoderm derive into?
Supporting tissues
e.g. muscles, cartilage, bone, vascular system
What does the heart arise from?
Mesoderm
What does the endoderm derive into?
Internal structures
*e.g. epithelial lining of GI tract, respiratory tract, parenchyma of glands
What are the types of mesoderm?
Somatic
Splanchnic
Paraxial
Intermediate
Where do somites arise form?
Paraxial mesoderm
What are the somites derivatives and what do they form?
Dermatome»_space; dermis
Myotome»_space; muscles
Sclerotome»_space; hard tissue, bone
What does folding achieve?
Draw together margins of disk
- creates ventral body wall
- pulls amniotic membrane around disk
- pulls connecting stalk centrally
Puts heart + diaphragm in right place
Creates new cavity within embryo
What occurs after 4 weeks?
Nervous system forming
Embryo folded»_space; everything in correct place
Segments appeared»_space; cells have specific tasks
Describe neurulation
- Notochord releases signals
- overlying ectoderm thickens > neuroectoderm
- mesoderm becomes Paraxial, intermediate, somatic or splanchnic mesoderm
Describe folding of embryo
Cephalcaudal
Head folds
Tail folds
Lateral folding
Forms primitive gut + amniotic sac
How does the neural tube form?
The neural plate forms paired neural folds that unite
How does the neural plate form?
Notochord induced thickening on the overlying ectoderm
How are holes left available for mouth and anus?
Gaps in mesoderm on trilaminar disc allow for holes to not be closed up
What does the neural tube become?
Central nervous system
What is the primitive streak?
Thin depression on the surface of ectoderm
What does the trophoblast split into?
Cytotrophoblast
Syncytiotrophoblast
What does the embryoblast split into?
Epiblast
Hypoblast
What is morphogensis?
Development of form and structure
Describe how the intraembryonic coelom is formed?
Splanchnic folds down + towards itself, meeting in the middle + jointing
Somatic layer folds down (lower than splanchnic layer) joining with itself, leaving a cavity
This cavity i the intraembryonic coelom
