Embryology I Flashcards
What are the 3 phases of pre natal development?
Weeks 1-2: Fertilization and implantation
Weeks 3-8: Embryonic period
Week 9-birth: Fetal
Define gestation.
-Process or period of developing inside the womb between conception and birth
What is the risk/incidence of congenital anomalies at 4-6 weeks?
- 5 weeks, during the embryonic period you are at most risk of birth defects being induced.
The 1st week
Describe the process of Fertilisation. Including days.
What are the results of fertilisation?
1-Oocyte enters tube
2- Fertilisation…sperm penetrates corona radiata + zona pellucida > = fusion of the oocyte and sperm cell membranes
(12-24hrs)
3- Zygote forms when male and female pronuclei fuse
4- Spindle of 1st mitotic division
5- Initiation of cleavage (30hrs)
6- Eight cell stage
7- Morula ( 3days)
8- Hatching of early blastocyst out from zona pellucida so it can grow further (4.5-5 days)
9- Early phase of implantation (5.5-6 days)
> Restoration of the diploid number of chromosomes
Determination of chromosomal sex
Initiation of cleavage
1- What is cleavage?
2- What happens at the 8 cell stage?
- Specialised form of cell division
> Cytoplasm is cleaved into smaller partitions called blastomeres
> DNA is replicated but the daughter cells do not grow between mitotic divisions because the cells remain bounded by the zona pellucida, so there is simply no space for extra cytoplasm to be made. - Compaction is a process where tight junctions form between the outer cells (closest to zona pellucida) that seal off the inner cell mass:
> Inner cell mass = embryo proper
> Outer cell mass = trophoblast (of placenta)
The 1st week
1- Describe the Formation/Structure of a Blastocyst
2- Which tissues are derived from trophoblasts?
3- Which tissues are derived from embyroblasts?
1- As the morula approaches the uterine cavity, a fluid filled cavity starts to separate the outer cells from the inner cells and this cellular organisation is now called a blastocyst (Day 4-5)
2- Foetal contribution to Placenta
3- All embryonic tissue
The 1st week
1- At the time of implantation what is the fertilised egg now know as?
2- How does the body initially respond to the start of implantation?
1- Blastocyst
2- Increase in maturation of corpus luteum - which increase secretion of progesterone and oestrogen
> helps to maintain and increase thickness of the endometrium
The 1st week
How does the blastocyst implant? At what day?
Day 6
- Attachment to uterine epithelium
- Trophoblasts (invading tissue) differentiates into Syncytiotrophoblasts and Cytotrophoblasts:
> Multinucleated Syncytiotrophoblasts form a syncytium and actively invades the uterine epithelium cells and secrete hormones such as human chronic gonadotropin =hCG (maintains Corpus Luteum > produces progesterone > endometrial lining NOT SHED)
> Mononucleated Cytotrophoblasts maintain the outer layer of the blastocyst, provides a proliferative pool of cells that move into the syncytiotrophoblast
*Similar to growth of a tumor and can sometimes become unregulated.
Summarise what happens in week of 2s?
What does Extraembryonic mesoderm form?
- Extraembryonic somatic/parietal mesoderm lines the cytotrophoblast and covers the amnion. It also forms the connecting stalk that is the becomes the umbilical cord.
- Visceral extraembryonic mesoderm lines yolk sac.
The 2nd week “week of 2s”
Once the Blastocyst has implanted, it begins to further invade the epithelium until it is completely enclosed by the uterine tissue.
1- Following implantation (~1-2d after), Embryoblasts also differentiate into 2 layers. What are these? How do they form?
1- Day 7.5
-Under the influence of fibroblast growth factors (FGFs) at the early blastocyst stage, the pluripotent stem cells in the embryoblast differentiate into two types of cell
> Dorsal epiblast & Ventral hypoblast.
- Initially, these cells are scattered in the embryoblast, but near the time of implantation, they segregate according to their specification > dorsal–ventral polarity in the embryo.
- Around the time of implantation, some hypoblast cells specialize to form anterior visceral endoderm that will eventually migrate cranially > cranial–caudal embryonic axis is therefore established near the time of implantation
*Embryonic stem cells (ES cells)are derived from the inner cell mass of the embryo
How are the membranes formed:
How do we end up with 3 cavities, what are they?
1- Small cavity begins to appear within the epiblast which soon enlarges to become theamniotic cavity. Epiblast cells adjacent to the cytotrophoblast are calledamnioblasts; together with the rest of the epiblast, they line the amniotic cavity.
> Amnion = innermost membrane that encloses the embryo of a mammal, bird, or reptile
2-The original cavity of the blastocyst becomes the primitive yolk sac.
3- Chorionic space develops as a series of small spaces between the yolk sac and the trophoblast. These smaller cells gradually join up.
> Chorion = outermost membrane surrounding an embryo of a reptile, bird, or mammal. In mammals it contributes to the formation of the placenta.
The 2nd week “week of 2s” : FORMATION OF PLACENTA
1- What are trophoblastic lacunae? What day do they form?
2- When does the Lacunae fill with maternal blood?
3- How are primary villi formed?
4- How is secondary yolk sac formed?
5- What is the extraembryonic mesoderm?
6- Which two cavities are present day 13?
1- Day 9
lacunae (spaces) form in invading syncytiotrophoblasts which fill with maternal blood from maternal blood vessels in the uterine lining.
2- Day 12
3- Cytotrophoblast proliferate locally and penetrate into the syncytiotrophoblast, forming cellular columns surrounded by syncytium that are known asprimary villi.
4- Hypoblast produces additional cells that migrate along the inside of the exocoelomic membrane of the primitive yolk sac. These cells proliferate and gradually form a new cavity within the exocoelomic cavity that is now called thesecondaryordefinitive yolk sac, which is much smaller than the original.
5- Parietal (somatic) Layer that lines the cytotrophoblasts and covers amnion it also gives rise to the umbilical cord. Visceral (splanchnic) Layer lines yolk sac
6- Amnionic cavity above epiblast layer and the Secondary Yolk sac below hypoblast layer
What marks the end of week 2?
- Primitive streak
- At the caudal end of the embryo, a specialised proliferation site forms on the dorsal surface of the bilaminar disc.
*Primitive streak normally disappears by the end of the 4th week but sometimes remnants may persist and give rise to a sacroccoygeal teratoma (tumour).
What does the week of 3s involve?
Week 3 “week of 3s”
1- What is gastrulation? What does it form? What day does it occur?
2- What is the primitive streak?
3- What is the oropharyngeal membrane?
4- What is the primitive node?
5- What indicates that head/tail OR cranial/caudal axis is established?
1- Day 15
-Process of forming 3 primary germ cell layers from the epiblast.
> Arises as lateral epiblast cells proliferate + migrate towards the primitive groove to form endoderm and mesoderm.
2- Thickened band of epiblast cells > Forms a groove in the epiblast at caudal end of bilaminar disc stage. Where epiblast cells migrate from to form 2 of the germ layers
3- Eventually rupture (apoptosis) to communicate with the pharyngeal portion of the gut tube.
4- Nob like structure towards the top of the primitive streak. > The organiser – where notochord cells arise
5- Oropharyngeal membrane + primitive streak
Week 3
1- What does gastrulation transforms the embryonic disk from a bilaminar structure to..
2- Epiblast cells migrate to form 3 definite cell layers…
3- Hypoblasts are displaced and DO NOT CONTRIBUTE to the final tissues of the embryo…. Resultant embryo is ….
4- Describe the process of how the 3 germ cell layers are formed in Gastrulation.
1- Trilaminar structure - consists of three layers
2-
Ectoderm
Mesoderm
Endooderm
3- Trilaminar
4-
1.) First epiblast cells that begin to migrate + enter primitive groove and displace the hypoblast layer of cells. These form the definitive ENDODERM
2.) Second group of cells that migrate settle above the established endoderm and form MESODERM
3.) Remaining epiblasts form the ECTODERM
Describe the development of the notochord +
Why is the formation of the notochord important for development?
1- Prenotochordal cells migrate through the primitive streak and move cranially towards pre chordal plate between ectoderm and endoderm to form the notochordal plate which detaches from the endoderm to form the definitive notochord.
> Because these events occur in a cranial-to-caudal sequence, portions of the definitive notochord are established in the head region first.
2- Positioned centrally, notochord provides the embryo with axes (in both dorsal-ventral and right-left directions).
3- Secretes factors which signal to surrounding tissues – telling their fate and position. Especially important in CNS (+ others).
*The notochord is actually derived from endoderm, however, it ends up in the mesodermal layer
> Remnant = nucleus pulposus of IV disc.
What is the prechordal plate?
- Some cells migrate ahead of the notochord. These mesoderm cells form the prechordal plate that will assist in forebrain induction.
- Prechordal plate forms between the tip of the notochord and the oropharyngeal membrane and is derived from some of the first cells that migrate through the node in the midline and move in a cephalic direction.
1- What is neurulation?
2- Neurulation occurs at the same time as what process?
3- Describe the process of neurulation.
1- Process transforming the neural plate into the neural tube. From 3rd week
2- Gastrulation and primitive streak continue caudally
3-
1- Notochord releases growth factors = ectodermal cells to start proliferating
2- Forming neural plate which is induced cranially, thickening of the neural plate creating a v shaped neural groove allows for the plate to move downwards as the edges start moving towards one another. (involution)
3- Elevates itself above the ectoderm layer
4- Eventually neural folds fuse and forms the neural tube derived from the surface ectoderm
5- Neural tube forms underneath the ectoderm
*neural crest cells form on the neural folds
At what days do cranial and caudal neuropores close?
Fusion of the neural tube begins in the middle section of the body and the neurulation process completes:
Cranially = Day 25
Caudally = Day 28
Derivatives of Germ layers:
1- What does the endoderm derive?
2- What does the Mesoderm derive?
3- What does the ectoderm derive?
How does the spinal cord develop?
1- Cell bodies for sensory neurons are located in spinal ganglia outside the spinal cord (from crest cells)
2- Cell bodies for motor neurons differentiate in the ventral motor horns of the spinal cord (located inside the cord - nuclei)
3- Dorsal and ventral roots unite to form a spinal nerve at each intervertebral foramen.
- Dorsal roots carry sensory fibers;
- Ventral roots are motor fibers
- Spinal nerves have both motor and sensory fibers
4- Spinal nerves are very short and almost immediately divide into dorsal and ventral primary rami
By the end of 4 weeks, neural tube closure is complete, and three primary brain vesicles have formed: ….
prosencephalon (forebrain);
mesencephalon (midbrain);
and rhombencephalon (hindbrain).
1- How does the vertebral level of the spinal cord vary from a 3 month old to end of 5 months to a newborn?
2- When does the cauda equina form?
1>
- at 3 months the spinal cord ends at the bottom of the vertebral column
- at 5 months the vertebral column has grown and the spinal cord has been lifted to S1 level
- spinal cord ends in a newborn at ~ L3
2 - end of pregnancy/newborn ~ 9 months
1- What are ectopic pregnancies?
2- What is the clinical significance of ectopic pregnancy?
1- pregnancy that grow/implants outside of the uterine cavity
> Uterine tube most common
2- Can cause rupture = huge internal haemorrhage
most common cause of maternal death within the 1st trimester
1 - what is caudal dysgenesis? (mermaid syndrome)
2- what is Sacrococcygeal teratoma?
3- what happens if the caudal/posterior neuropore doesn’t close correctly?
4- What happens if the anterior neuropore doesn’t close correctly?
- What is the main risk factor for Anencephalpy?
1- - lack of gastrulation causing loss of mesoderm in the lumbar sacral region
- lack of definition between lower limb - fusion
2- gastrulation continues for too long
- this causes the formation of an congenital tumour in coccyx bone
- linked to physical reminence of the primitive streak which hasn’t closed correctly
3- Spine bifida, the spine doesn’t form properly/ in the correct position.
4- Anencephalpy - lack of scalp/skull means neural tissue is exposed to CSF causing degradation of the neural tissue = lack of brain.
> lack of folic acid pre-pregnancy and during pregnancy
Here are a series of diagrams that represent some of the different stages of the embryo during the first four weeks of development.
Order the following diagrams:
- G = First pair of cells after the first cleavage
- A = Morula
- C = Development of the blastocyst
- E = Implantation
- H = Formation of the membranes that surround the embryo
- F = Dorsal view of gastrulation to form the definitive layers of the embryo
- B = Formation of the neural groove
- D = Closure of the neural tube and development of somites.
