Embryology- introduction and week 1 of development, week 2 of development, week 3 – Trilaminar Disc Formation and Establishing Body Axis and Week 4 – Neurulation and Embryonic Folding Flashcards
1
Q
What end is the superior end?
A
Head
2
Q
What end is the inferior end?
A
Bottom
3
Q
What end is the anterior end?
A
Front
4
Q
What end is the posterior end?
A
Back
5
Q
What end is the cranial end of an embryo?
A
Head
6
Q
What end is the caudal end of an embryo?
A
Back/feet
7
Q
What end is the ventral end of an embryo?
A
Front
8
Q
What end is the dorsal end of an embryo?
A
Back
9
Q
Two ways in dating a pregnancy?
A
- Menstrual age- date pregnancy from women’s last menstrual period- 3 equal trimesters- 40 weeks
- Fertilisation age- split into 3- early development- rapid cell division, embryonic (organogenesis) period and foetal period where the systems mature- 38 weeks
10
Q
Human birth defects- genetic and environmental causes
A
Genetic causes:
Monogenic- defective gene on autosome
Chromosomal- problem during chromosome separation and misalignment
Environmental causes:
Teratogens- agent or factor which causes malformation of an embryo
Examples:
Infectious- TORCH Taxoplasmosis, Other (Hep. B. , Syphilis), Rubella, Cytomegalovirus and Herpes Chemical- Thalidimide and alcohol Deficiency- Folic acid Maternal disease- Diabetes Physical- Radiation
11
Q
When’s the most susceptible time for an embryo
A
3-8 weeks to teratogens
12
Q
Taxoplasmosis- what is it?
A
Parasite found in cat faeces
13
Q
Other- Zika Virus- what is it?
A
Mosquito, bodily fluids
14
Q
Rubella- what is it?
A
Infection passes over placenta
15
Q
Cytomegalovirus- what is it?
A
Virus that crosses the placenta
Infection via bodily fluids- blood, sweat and semen
16
Q
Herpes Virus- what is it?
A
Herpes Simplex- usually doesn’t travel through reproductive system- usually happens while giving birth if mother has herpes
Varicella zoster virus – Chickenpox
Most dangerous in between 13-20 weeks or just before birth to two days postpartum
17
Q
Chemical- Thalidomide- what is it? What does it do?
A
Prescribed for morning sickness
Shortened or absent limbs
Now used to treat leprosy and HIV
18
Q
Chemical- alcohol- foetal alcohol syndrome
A
Prenatal and postnatal growth retardation
Intellectual disability
Impaired motor ability and coordination- CNS
19
Q
Capacitation
A
Where the sperm (get the female reproductive system gets ready for fertilisation)- change chemical structure/ the shape of the head of the sperm for fertilisation
20
Q
What happens just after fertilisation?
A
- Fusion of male and female gametes to form zygote
- Capacitation of sperm
- Acrosome reaction
- Formation of zygote
- Fusion of pronuclei
21
Q
What is the acrosome reaction?
A
Acrosome releases degrative enzymes that allow sperm to penetrate zona pellucida (outer jacket of the oocyte). Sperms acrosome contains degrative enzymes. Once the capacitated sperm reaches the zona pellucida, the acrosome releases the degrative enzymes and breaks down the zona pellucida. The head of the sperm that contains the nucleus enters the oocyte and fertilises. This forms a zygote.
22
Q
What is the cortical reaction?
A
The cortical reaction shuts down the zygote, preventing a normal of sperm fertilising eggs (polyspermia). Zona pellucida becomes impenetrable – stops any sperm coming back in.
23
Q
Cleavage- rapid cell division- number of days? What happens?
A
1-3 days
After fertilisation zygote cells divide
No change in size of zygote. Blastomeres get smaller
After 24 hours the first cleavage occurs. Cells get smaller as cleavage occurs. Morula is formed- ball of cells.
24
Q
Formation of morula-number of days? How many cells? What is formed?
A
16-32 cells
Inner Cell Mass forms embryo proper (embryoblasts)- ones in the middle
Outer Cell Mass forms placenta (trophoblasts)- ones on the outside
Embryoblast form embryo and trophoblast form support structures such as the placenta.
25
Formation of blastocyst- number of days? How are the cells now arranged?
Day 5
-Formation of a fluid filled cavity by day 5
-Separates cells into a
• compact mass – Inner cell mass (embryoblasts)
• thinner outer layer -Outer cell mass (trophoblasts)
• Fluid filled space
26
Blastocyst Hatching and Initiating Implantation- number of days?
Days 5 and 6
Implantation- embryo now try to implant onto the uterine wall (endometrium(inner lining) of the uterus)
to get nutrients from the mother
27
Hatching
Blastocyst removes the Zona pellucida
28
What do trophoblasts and embryoblasts differentiate into?
Day 7
Trophoblasts differentiate into cytotrophoblasts and syncytiotrophoblasts. Forming the outer cell mass.
Embryoblasts differentiate into hypoblasts and epiblasts. Forming the inner cell mass/ bilaminar disc.
29
Importance of epiblast dorsal and ventral axis- what is the positioning?
The epiblast forms the dorsal surface of the embryo and the hypoblast forms the ventral surface.
Dorsal goes first into the uterine lining.
30
Formation of the amniotic cavity
Day 8
Amniotic cavity forms as a small fluid filled cavity in the epiblast.
Amniotic cavity formed between epiblast cells- this gets bigger and bigger until it engulfs the whole embryo.
31
Formation of the primary yolk sac
Full implantation- hypoblast cells begin to migrate around the blastocyst cavity.
Fully implanted embryo into the uterine lining.
Coagulation plug = ‘scab’ to heal uterine lining
The support structures cytotrophoblasts and syncytiotropohblast engulf the whole embryo.
The cytotrophoblasts membrane break down to form syncytiotropohblasts.
Cavity layered with hypoblast cells is now called primary yolk sac.
32
How does developing embryoblast receive nutrients?
Syncytiotrophblast bind to uterine gland and get nutrients from the mother by diffusion but not enough.
Maternal capillary formed (red circular thing on diagram) and the synctiotropoblasts break down the wall of maternal capillary- uteroplacental circulation.
Synctiotrophoblasts have gaps called trophoblastic lacuna, this is where the nutrients are before they get to the embryo. When get nutrients from uterine gland it is just going into the trophoblastic lacuna. Blood leaks into trophoblastic lacuna.
33
What day is the extraembryonic mesoderm form?
10-11 days
A new layer of connective tissue derived from the yolk sac cells forms between the hypoblast and cytotrophoblast – extraembryonic mesoderm
34
When does the extraembryonic mesoderm fully form?
11-12 days
Eventually it develops to cover the amniotic cavity too. Now extraembryonic mesoderm has formed all the way around the primary yok sac and amniotic cavity.
35
How is the chorionic cavity?
12-13 days
Cavity forms in the extraembryonic mesoderm.
This cavity is called the chorionic cavity. Now formed three cavities, primary yolk sac with migration of hypoblast cells, the amniotic cavity and now the chorionic cavity.
36
Formation of secondary (definitive) yolk sac
12-13 days
Simultaneously, a second wave of hypoblast cells migrate to form a secondary (definitive) yolk sac.
Hypoblast cells now begin to migrate again around the primary yolk sac and push the old hypoblast cells away and the primary yolk sac from the embryo. The definitive yolk sac is then formed.
This forms the secondary yolk sac.
37
The end of week 2 what forms?
Amniotic cavity and secondary yolk sac are suspended in the chorionic cavity by a connecting stalk.
38
How does pregnancy testing work?
Syncytiotrophoblast secretes a hormone – human chorionic gonadotrophin (hCG)- such high levels it is excreted in the urine which shows you are pregnant.
Helps maintain endometrium (the lining of the uterus)- normally shed every menstrual period and may play a role in maternal immunotolerance.
Measuring pregnancy
• Before – inject womans urine with hCG into frog if ovulate = pregnant woman
• Now – immunoassay records levels hCG
39
Where are abnormal implantation sites?
Embryo implanted wrong area
1. Abdominal cavity – mesentry
2. Uterine tube
3. Birthing canal – (internal os of uterus)
40
What is placenta previa?
Cervix is part of the birthing canal. The baby should pass through cervix and vagina. Placenta in upper lateral aspect still receiving blood and getting nutrients. Placenta previa- placenta covering cervix and can rupture endangering baby and mother-causing severe bleeding. Caesarean section can prevent this as it is covering the birthing canal.
41
What is an ectopic pregnancy?
Abnormal implantation outside the uterus
42
What is a tubal implantation?
The majority of ectopic pregnancies occur in the uterine tube (80% in the ampulla). Baby gets to a certain size and can longer be contained.
43
What is an abdominal implantation?
There is a gap between the ovary and uterine tube therefore fertilised egg can form between. Can implant anywhere in the abdominal cavity.
44
What is the lithopaedion?
Can occur when an abdominal pregnancy remains undiagnosed and the foetus dies- too big to resolve
The foetus is too large to be reabsorbed, instead it calcifies- makes bone out of it (litho = stone; paedion = child)
This is thought to protect the mother from the necrotic tissue of the foetus.
If it does implant in the abdominal cavity and finds a uteroplacental blood supply it can survive.
45
What is a hydatidiform mole?
Development of the trophoblast without any embryonic tissue.
Due to fertilisation of an ‘empty’ egg lacking a nucleus with a sperm- egg only contains paternal genes.
Suggests that paternal genes favour the formation of trophoblast at the expense of the embryo – parental imprinting.
Syncytiotrophoblast produces hCG giving a positive pregnancy test
46
What is gastrulation?
Days 14-15
Formation of the primitive streak
Cells of epiblast move towards the primitive streak
They then detach from the epiblast and insert into the hypoblast to replace the hypoblast cells
Successive migration leads to the formation of 3 germ layers
47
How does abnormal gastrulation affect the body?
Abnormal gastrulation can lead to R to L asymmetry of the body is reversed - situs inversus- right and left can be switched.
Complete situs inversus is often harmless but 20% of patients suffer from Kartagener syndrome – problem with cilia cells, genetic mutation. These cilia cells are needed to sweep the appropriate cells to the appropriate side to the body. Sweep cells that are forming the liver to the RHS- forming on the wrong side. This syndrome causes cells to switch either side- not sweeping cells left to right
Partial situs inversus e.g. dextrocardia- where heart is switched around (isolated right sided heart) is much rarer and associated with other malformations. If heart begin to switch can be a problem when heart switches around and ‘plumbing work’ doesn’t work- oxygenated blood to lungs, deoxygenated blood to the body. Only partial switches of some structures can create problems.
48
How does the definitive endoderm form?
The first germ layer
Primitive streak important as cells going to migrate through it.
Epiblast cells begin to migrate to primitive streak and come to primitive streak they move through it. Move to the yellow cell layer- the hypoblast cells.
Hypoblast cells start getting pushed out of the way until eventually they start replacing them. Hypoblast cells only transient as epiblast cells push them out the way and replace them.
Once replaced the first germ layer is formed- this is known as the definitive endoderm-the epiblast cells replacing the hypoblast cells.
The epiblast cells continue to migrate through the primitive streak and start forming a second layer.
49
Formation of mesoderm
14-15 days
The second germ layer.
Epiblast cells continue to move down through the primitive streak to form a middle layer in between the epiblast and definitive endoderm- the mesoderm
50
Formation of the ectoderm
14-15 days
The third germ layer.
The remaining epiblast cells become ectoderm.
This completes gastrulation.
51
What is the trilaminar embryonic disc?
Intraembryonic mesoderm is continuous with the extraembryonic mesoderm.
Trilaminar disc- comprised of the three germ layers.
52
Derivatives of germ layers- the ectoderm, mesoderm and endoderm
Ectoderm forms the CNS, PNS and epidermis of the skin.
Mesoderm forms muscles, blood vessels, part of the urinary and reproductive system.
Endoderm forms the epithelial lining, GI tract, glands and the rest of the urinary system.
53
Abnormal gastrulation- sironmelia
Mermaid syndrome – doesn’t develop enough mesoderm-if primitive streak regresses too early or cells don’t migrate
Insufficient mesoderm is formed in the caudal region of the embryo – abnormalities of urogenital system and lower limbs
54
Abnormal Gastrulation - Sacrococcygeal Teratoma
In the sacral region (bottom of spine). Primitive streak persists and stays around for too long so excessive migration of epiblast cells. Lot more mesoderm than we want and goes on to form a tumour.
Remnants of primitive streak may persist and give rise to a tumour = Sacrococcygeal Teratoma (SCT)
55
What is the notochordal process?
Primitive streak degenerate – cranial to caudal
| Primitive node forms notochordal process (hollow tube)
56
Formation of the notochord
The notochordal process fuses with endoderm to form flattened notochordal plate.
The notochordal plate separates from the endoderm and fuses to form solid notochord.
57
How is notochord I formed?
The notochord initiates neurulation, the first event in the development of the central nervous system.
Initiated by signals from the notochord that cause the overlying ectoderm to thicken and form the neural plate - induction
58
How is notochord II formed?
The notochord also initiates and organises the formation of the vertebrae from somites (from paraxial mesoderm). The notochord induces differentiation of the somites into 3 parts:
- Sclerotome – bone and cartilage
- Myotome – skeletal muscle
- Dermatome – dermis
59
Formation of the vertebrae
* The notochord organises the sclerotome
* Sclerotome cells surround the notochord to form the vertebral body and surround the neural tube to form the vertebral arch (spine)
60
What happens to the notochord?
Majority of notochord degenerates as bodies of vertebrae form
Theory is remnants as nucleus polposus intervertebral discs.
61
Abnormal induction of the Sclerotomes - Spinal defects- Spinal bifida occulta
Failure of induction of vertebral arch. Tuft of hair forms.
62
Abnormal induction of the sclerotomes - Spinal defects- Meningocele
Meninges cover spinal system normally – cerebrospinal fluid bathes spinal system. Failure of formation of vertebral arch leads to meninges permeating out – without neural tissue.
63
Abnormal induction of the sclerotomes- myoelomeningocele
Neural tissue protrude though the gap, known as myelomeingocele.
64
What is neurulation?
Week 4
Neurulation is the first event in the development of the central nervous system. Initiated by signals from the notochord that cause the overlying ectoderm to thicken and form the neural plate – induction.
65
How does neurlation work?
Week 4
Notochord is signalling to the ectoderm (blue). Cells replicate to form neuroectoderm.
Ectoderm starts to thicken and forms neural ectoderm and begins to elevate at the lateral edges to form a fold. Neural folds come closer together and fuse.
Neural folds are beginning to come closer together and form a groove.
The folds come in such close proximity they fuse to form a neural tube. Which forms the spinal cord and brain.
66
What is the induction of neurulation?
The notochord induces thickening of the overlying ectoderm to form neuroectoderm which gives rise to the neural plate.
The lateral edges of the neural plate elevate to form neural folds.
Fusion of the neural folds forming the neural tube.
Fusion of the neural folds begins in the cervical (neck) region (dotted red line) of the embryo and continues cranially and then caudally.
As the neural folds fuse, the tube separates from the surface ectoderm.
67
What day does the fusion of the anterior and posterior neuropores occur?
The anterior neuropore (cranial end) fuses on day 25.
| The posterior neuropore (caudal end) fuses on day 28.
68
What ends of the nervous system form the brain and the spinal cord?
* The cranial end of the neural tube will form the brain
| * The caudal end will form the spinal cord
69
What happens when the anterior neuropore doesn't form?
Anencephaly occurs.
The forebrain does not form – anterior neuropore open to cavity
• cognition (thinking), memory, speech, vision and hearing.
A brainstem is present:
• normally exposed- can be damaged after birth usually die
• heart and lungs and some reflexes
• brainstem has all our primary functions
70
What happens when the posterior neuropore to fuse?
* Neural plate present but no neural tube
* Spinal cord does not form properly leading to paralysis
* If posterior neuropore fails to fuse, neural plate tissue exposed to external environment- high risk of infection
71
What are neural tube defects?
Failure of formation of the neurone tube or the anterior and posterior neuropore to fuse is called a neural tube defect (NTD).
72
What are neural crest cells?
As neural folds elevate and fuse, cells at lateral edge separate from the neural tube – neural crest cells
Migrate laterally and ventrally and differentiating into a variety of structures throughout the body.
73
Defects in Neural Crest Cell Development
Neurofibromatosis – genetic condition caused by a mutation in neurofibromin 1 (NF1) gene
NF1 is a tumour suppressor gene-located in the neural crest cells that is ‘switched off’ in neurofibromatosis leading to increased cell division.
74
How is the embryo folded?
During the 4th week the embryo undergoes folding in two directions:
1) Craniocaudal folding- head to tail
2) Lateral folding-right and left side fold in midline
75
Process of folding the embryo- laterally (saggital folding)- part 1
Amniotic cavity getting bigger.
Amniotic cavity pushing down on lateral edges. Yolk sac is in yellow on ventral surface. It stays a similar size.
Amniotic cavity getting larger pushing sides of embryo together.
At level of midgut the lateral body walls come together but don’t fuse as yolk sac is in the way (amniotic cavity fusing in midline) – lateral edges don’t fuse. Lateral embryos come together and fuse in most regions. The connection between the yolk sac and the gut is known as the vitelline duct.
Above and below midgut it has fused. Tube goes to form the gut tube and continuous with the yolk sac.
76
Process of folding the embryo- laterally (saggital folding)- part 2
- Lateral plate mesoderm splits into 2 layers
1. Somatic layer – body wall, epidermis
2. Visceral layer – surround internal organs
- Endoderm + visceral lateral plate mesoderm fuse in midline form gut tube lining
- Somatic lateral plate mesoderm + ectoderm pushed down by amniotic cavity
• Fuse at ventral surface embryo
• Seal abdominal + thoracic cavities embryo
77
Embryonic Folding - Craniocaudal Folding
At head and tail end embryo is pushed together. The head (cranial) end through that amniotic cavity expanding they come close together but don’t fuse- form C shape.
78
Allantois
Connected to foetal bladder
79
Ventral body wall defects- Ectopia cordis
Ectopia cordis – lateral folds fail to fuse in thoracic region leaving heart outside the body
80
Gastroschisis
Lateral folds fail to fuse in abdominal region
Intestines outside the body wall.
At level of mid gut don’t have fusion. This is slightly above mid gut.
Silo bag- septic bag- baby develops more so can put back into baby when grows more.
