Early Embryonic development 1 Flashcards
When is the embryonic period?
-3-8 weeks
When is the pre-embryonic period?
-1-2 weeks
When is the foetal period?
9-38 weeks
Which hormones stimulate oocyte release?
- Leutinising hormone
- Prostaglandin
By what physical mechanism is an oocyte released?
-Local muscular contractions in the ovarian wall extrudes the oocyte
What helps the oocyte enter the uterine tube?
-Fimbrae which sweep over the surface of the ovary
What happens after an oocyte has been released into the uterine tube?
-Oocyte travels along the uterine tube by peristaltic muscular contraction od the tube and by cilia in the mucosae
Where does fertilisation occur?
-In the ampillary region of the uterine tube (widest part, close to the ovary)
What is the name for a fertilised egg?
-Zygote
Where is the ideal site for implantation of a zygote?
-High up on the posterior uterine wall
What are the two main results of fertilisation?
- Restoration of diploid number by fusing of two haploid gametes
- determination of sex
How is the sex determined during fertilisation?
-The fertilising sperm is either carrying an x-chromosome or a y-chromosome
What is cleavage and what is its purpose?
- A series of mitotic divisions
- To increase cell number
What happens to the cells size as cleavage occurs?
-They become smaller
When does cleavage begin?
-approx. 30 hrs after fertilisation when the zygote has reached the two-cell stage
What potency are the cells after cleavage?
-Totipotent
What is the end product of cleavage?
-Morula
What is compaction?
-The process by which the morula form a compact ball of cells held together by tight junction with no spaces in between, maximising contact with one another
Why is compaction said to be the start of the first cavity?
-Compaction causes segregation of inner cells which communicate extensively by gap junctions from outer cells
To what does the inner cell mass give rise to?
-Embryoblast
To what does the outer cell mass give rise to?
-Trophoblast which later contributes to the placenta
What is the Zona Pellicuda and what is its function?
- A transient glycoprotein shell which surrounds a zygote created by a chemical change which occurs at fertilisation
- Keeps other spermatozoa on the outside preventing polyspermy
Define zygote
-Diploid cell resulting from the fusion of two haploid gametes
Define morula
-Solid ball of identical cells resulting from the division of a fertilised ovum
Define ovary
-Female reproductive organ in which ova/eggs are produced
Define fallopian tube
-Uterine tube which are female tubes which eggs travel from the ovary to the uterus
Define Uterus
- The womb
- Hollow muscular organ located in the pelvic cavity of female mammals in which a fertilised ovum implants
Define Cleavage
-Series of repeated mitotic cell divisions that occur in the ovum immediately after fertilisation up until the formation of the morula
When does blastocyst formation occur?
-At the same time as the morula is entering the uterine cavity (uterus/womb)
Describe blastocyst formation
- Fluid begins to penetrate zona pellicuda into the intracellular spaces of the inner cell mass
- The intracellular spaces become confluent, pushing the inner cell mass to one pole and a blastocyst cavity forms
Why is blastocyst formation known as the first differentiation stage?
- The inner cell mass develops into embryo and is known as the embryoblast
- The outer cell mass is strictly placenta and is known as the trophoblast
- The cells are now pluripotent
What is hatching?
-Blastocyst hatching from the zona pellicuda as the risk of polyspermi decreases
What is the purpose of hatching?
-The blastocyst becomes free to enlarge and can interact with the uterine surface to implant
What has happened to the endometrium during the menstrual cycle?
-It has become more suitable for the implantation of the conceptus
When does implantation occur?
-Day 6/7 after fertilisation
How many cells are in the conceptus at time of implantation and how are these made up?
- 107
- 8 make embryo
- 99 begin to develop the foetal membranes
What is the first stage of implantation?
-Attachment to epithelial cells which is mediated by selected integrins and ECM molecules
What does the trophoblast do at the beginning of implantation?
-Trophoblastic cells over the embryoblast pole begin to penetrate between the epithelial cells of the uterine mucosa
When does the development of the bilaminar disc occur?
-When the blastocyst is partially embedded in the endometrial stroma
Describe initial trophoblast differentiation
- Trophoblast develops into two layers
- Cytotrophoblast
- Syncytiotrohpblast
What is the cytotrophoblast?
-Protective inner layer of the trophoblast
What is the syncytiotrophoblast?
-Outer multinucleate layer of trophoblast that serves to invade the endometrium of the uterus
How does the syncytiotrophoblast develop?
-Mitotic divisions occur in the cytotrophoblast which migrate to the syncytiotrophoblast, fuse together and loose individual cell membranes
Describe the differentiation of the embryoblast to the bilaminar disc
- Embryoblast differentiates into two layers
- Hypoblast -> layer of small cuboidal cells adjacent to the blastocyst cavity
- Epiblast-> Layer of high columnar cells adjacent to the amniotic cavity
Where does the amniotic cavity develop from?
- A small cavity which appears in the epiblast
In the pre-embryonic period, what has priority?
-The placenta
Define embryoblast
-Cluster of cells segregated to one pole of the blastocyst from which the embryo develops
Define trophoblast
-Outer layer of cells surrounding the blastocyst from which placental tissues are derived
Define blastocyst
-Stage of embryogenesis at the time of implantation where outer trophoblast cells form a fluid filled sphere with a small group of embryoblast cells at one pole
Define implantation
-Attachment of the blastocyst to the epithelial lining of the uterus, penetration through the endomertrium occuring 6/7 days after fertilisation of the oocyte
When does bilaminar disc formation occur?
-Day 8
What is the purpose of implantation?
- Access to glands and vasculature to access O2 and nutrients
- To allow maternal blood flow in placenta
Once the conceptus is embedded in the stroma, what happens to the penetration defect?
- Closed by fibrin coagulum forming fibrin plug
- Surface epithelium almost covers the entire defect by day 11/12
Why can bleeding occur during this stage of preganancy?
-Implantation is very invasive
How does the syncytiotrophoblast develop throughout week 2 to provide uroplacental circulation?
- Considerable development, particularly at embryonic pole
- Vacuoles appear and fuse to form lacunae
- Large lacunae form an intercommunicating network
- Further cellular development leads to deeper erosion into stroma
- Erosion of endothelial lining of maternal sinusoids
- Development of uroplacental circulation
How does the support of the embryo change throughout week 2?
-Changes from histiotrophic (diffusion) to haemotrophic
How does the primitive yolk sac develop and when does this occur?
- Day 9/10
- At the embryonic pole, flattened cells originiating from the hypoblast line the inner surface of the cytiotrophoblast. This is the extracoelemic membrane
- The extracoelemic membrane together with the hypoblast form the primitive yolk sac
What is extraembryonic mesoderm? When and how does it develop?
Day 11/12
- Connective tissue which lies betweeen the extracoelemic membrane and the cytiotrophpblast
- New population of cells derived from the primitive yolk sace develop inbetween the extracoelemic membrane and the cytiotrophoblast
- Eventually this extraembryonic mesoderm fills the space between the trophoblast and the amniotic/yolk sac/extracoelemic membrane
How does the extraembryonic cavity form?
- Once extraembryonic mesoderm fills the space between the trophoblast and the amniotic sac/extracoelemic membrane, large cavities/vacuoles begin to appear
- These cavities become confluent and form the extraembryonic cavity; the trophoblast is surrounded by a thin layer of extraembryonic mesoderm
- Cavity surrounds the amniotic cavity and the primitive yolk sac except for the connecting stalk
What are primary villi and describe their development
- Units of foetal/maternal exchange
- Caused by the development of the cytotrophoblast
- Cells proliferate locally and penetrate into syncytium
- cellular columns surrounded by syncytium are villi
Describe the development of the definitive yolk sac and when does it occur?
- Day 13
- Hypoblast begins to produce additional cells
- Cells migrate and proliferate to form new cavities
- Cavities pinch off and merge at the opposite pole to form the definitive yolk sac
What is the chorionic cavity?
-The extraembryonic cavity once all other cavities in wekk 2 have formed
What is the connecting stalk?
-Extraembryonic mesoderm which connects the embryo and its specific cavities (Amniotic and secondary yolk) to the supporting trophoblast
What is Interuterine growth restriction?
-Growth of conceptus not supported as there is sub-optimal establishment of an implantation environment
What is Placenta Previa
- Implantation of the conceptus low down in the uterus such that development blocks the opening of the cervix
- Can cause haemorrhage and requires C section
What is an ectopic pregnancy?
- Implantation takes place outside the uterus; abdominal cavity, ovary, uterine tube
- Can become life threatening due to intra-abdominal haemorrhaging
