4. MECHANISMS OF DISEASE III: DURING EMBRYOGENESIS Flashcards
What are the two main periods of embryogenesis?
- EMBRYONIC PERIOD - First 8 weeks, when most of the organogenesis occurs
- FOETAL PERIOD - The remaining period iin utero which involves growth & modelling
What are the 7 processes of human development?
- FERTILISATION
- CLEAVAGE
- BLASTULATION
- GASTRULATION
5, NEURULATION - SOMITOGENESIS
- ORGANOGENESIS
What is cleavage?
- Cleavage refers to a series of mitotoc divisions without growth
- Zygote - 2 cell -> 4 cell -> Morula
- The zygote undergoes many cleavages as it travels through the uterine tube to the uterus before it implants at around 10 days
What is blastulation?
- Blastulation refers to the fromation of a blastocyst
- The Morula undergoes a process of compaction where the cells rearrange, getting closer to from a blastocyst
- Blastocyst consists of 32 - 46 cells at around day 4/5
What are the two cell layers of the blastocyst?
- Outer trophoblast
2. Inner embryoblast - gives rise to the inner cell mass
What is the blastocoele?
- The blastocoele is a fluid filled cavity which is located between the trophoblast & the embryoblast
- The trophoblast cells secrete fluid into cavity
- The blastocoele displaces the inner cell mass to one side
What two cell layers does the inner cell mass give rise to?
- Inner cell mass differentiates into:
1. EPIBLAST
2. HYPOBLAST
What is the bilaminar disc?
- The epiblast & hypoblast form the bilaminar disc
- The epiblast is located above the hypoblast
- Cells of the epiblast rearrange to from the amniotic cavity which is located in between the epiblast & hypoblast
What is gastrulation?
- Gastrualtion refers to the formation of the three germ layers
- The blastocyst/blastula is rearranged into a gastrula
What are the three germ layers?
- ECTODERM
- ENDODERM
- MESODERM
What is the primitive streak & how does it form?
- The primitive streak originates at teh caudal end of the epiblast & grows cranially
What is the primitive groove?
- The primitive groove is a shallow depression within the primitive streak
What happens during invagination of the epiblast?
- Cells of the epiblast ingress/invaginate through the primitive groove into the underlying hypblast
- The ingressing epiblast cells dipslace the hypoblast to give rise to the endoderm & primitive mesoderm
What two germ layers arise from the hypoblast?
- ENDODERM
2. MESODERM
What germ layer does the epiblast give rise to?
- ECTODERM
What is the trilaminar disc?
- Once the three germ layers have formed, the bilaminar disc becomes the trilaminar disc
What is neurualtion?
- Neurulation is the process by which the neural plate folds back on itself to form the neural tube beneath the ectoderm
How does the notochord form?
- Once the primitive streak has extended towards the cranial end, it begins regressing
- As the primitive streak regresses it lays down a chord like structure known as the NOTOCHORD underneath the ectoderm within the mesoderm
How does the neural plate from?
- The notochord releases extracellualr signals which causes the overlying ectoderm to form the NEURAL PLATE
- The neural plate is a thickening of the ectoderm
How does the neural plate become the neural tube?
- The neural folds of the neural palte fuse to form a ring which is the neural tube
- The neural tube begins by closing in the middle first, between the hindgut & spinal cord
- It then continues anteriorly & posteriorly until teh whole tube is fused
What is somitogenesis & which germ layer is involved in somitogenesis
- Somitogenesis is the process by which somites from
- The cells of the mesoderm differentiate into somites
- Somites are precursors of muscle & bones
What are neural crest cells?
- Neural crest cells are cells of the ectoderm which invaginate into the mesoderm
What is the puprose of body/embryonic folding?
- Embryonic folding allows organs to be enclosed
- Mesoderm & endoderm give ris to oragns which need to be enclosed
Waht two planes does embryonic folding occur on?
- HORIZONTAL PLANE
2. MEDIAN PLANE
What folds are produced through embryonic folding on the horizontal & median plane?
- Horizontal plane = two lateral folds
- Median palne = cranial & caudal body folds
What main structure does the endoderm differentiate into?
- Endoderm differentiates into Gi tract
- As embryonic folding occur, teh endoderm moves towards the midline & fuses to form the primitive gut tube
- The primitve gut tube forms the foregut, midgut & hindgut
What are the main events of embryonic folding?
- Heart & septum move from margin to centre
- Yolk sac, allantois & stalk make umbilical cord
- Prochordal & cloaclal paltes form gut tube
What are the 5 main events organogenesis?
- Organogenesis refers to teh development of organs
1. Development of somitic derivatives - bone, muscle
2. Development of sensory structures - eyes, ears
3. Limb formation - forelimbs then hindimbs
4. Formation of face structures - jaw, nose, tongue, palate
5. Formation of genital structure
What type of diseases can occur due to defects in embryogenesis?
- Congenital malformations or diseases
- If the defects occur earlier on, the resulting diseases will be more severe
What are 4 causes of disease?
- SINGLE GENE MUTATIONS - one gene mutation produces a characteristic effect (Mendelian disease)
- CHROMOSOMAL ANOMALIES - Abnormalities with chromosome number due to rearrangement e.g trisomy
- POLYGENIC DISORDER - several genes causing disease
- ENVIRONMENTAL - diet, infection, toxic compounds
What is multi-factorial aetiology?
- Congenital diseases have a multifactorial aetiology, meanining it’s caused by a combination of intrinsic & extrinsic causes
What are the desirable characteristics of a model organism to study embryogenesis?
- It would be unethical to study embryogenesis with a human embryo so other models need to be used
1. Represenattive/relevant
2. Accessibility/availability
3. Experimental manipulation
4. Gentics of organism are known
5. Cost/space
What model organisms can be used to study disease during embryogenesis?
- Drosophila
- Flat worms
- Vertebraes
- Zebrafish
What properties of zebrafish make it an ideal organism for study?
+ Embryos of zebrafish are transparent amking it easy to study changes
+ Cheap & easy to maintain
+ 70% of zebrafish genes are homologous with humans & 84% of disease genes
