Mechanism of disease during embryogenesis Flashcards
What periods are human development divided into?
Human development can be divided into two stages:
Embryonic period
• Up to the end of week 8
• Most of the organogenesis occurs in these first 8 weeks
Fetal period
• The remaining time in utero
• Involves growth and modelling, refinement or organs and tissues
What are the mechanisms of development during embryogenesis from the egg?
From Egg: • Fertilisation • Cleavage • Gastrulation • Neurulation and somitogenesis • Organogenesis • To give rise to adult form
What can be seen in the fertilised zygote?
- We can see the pronuclei
- It is covered by zona pellucida
- Inside are the polar bodies – remaining genetic material that has been extruded.
What happens when the egg is fertilised?
- Fertilisation occurs in the fallopian ducts
- It takes 10 days for the fertilised egg to get to the uterus, during this time the zygot will undergo cleavages to form the morula
- Then the re-organisation of these cells occurs to give rise to the blastocyte
Describe the cleavage stage?
- The fertilised zygote will form a 2 cell zygote and then a 4 cell zygote and so on
- Once is gets to 8-16 cells it is called a morula which is ready to undergo its first re-organisation known as compaction
- During compaction, cells in the embryo sort themselves, some remain in the exterior and some remain in the interior of the embryo.
- The outside cells will form cell contacts with the neighbouring cells. the basal lateral surface is in contact with the external environment
- All cells inside the embryo are apolar, all their sites are in contact with other cells and they don’t have any cell surface that is free
- The cells in the outside give rise to the tropfectedor and the cells in the inside give rise to the inner cell mass.
- The outside cells start pumping fluid into the embryo forming a fluid filled cavity inside called the blastocoele, this pushes the inner cell mass to one side and starts giving an asymmetry – now known as a blastocyst (32-64 cells).
- The blastocyst has reached the uterus and is ready to implant into the uterus and the inner cell mas gives rise to edge cells and the exterior cells giving rise to the hypoblast and epiblast.
- As the embryo full fully implants we can identify the bilaminar germ disk.
- There epiblast cells also give rise to the amniotic cavity
Describe gastrulation
- Gastrulation gives rise to form three layers of cells from the germ disk
- At one end from the bilaminar germ disk a groove forms on the epiblast: primitive groove
- Cells form the epiblast ingress through the groove and spread beneath the epiblast
- This displaces the hypoblast and forms the endoderm and mesoderm giving rise to the ectoderm.
Describe the start of neurulation
Ectoderm
• Specification of the central nervous system.
• The primitive streak will then start to ingress leaving structure behind such as the notochord. The notochord is a group of cells that secretes several extracellular molecules that tells the ectoderm to become neural tissue. The neural plate then becomes established.
• As the primitive streak regress the notochord gets extended posteriorly which instructs the overlying ectoderm to form the neural plate so we end up with the neural plate on top of the notochord
How does the neural plate give rise to form a tube called the neural tube?
The neural plate then gives rise to form a tube called the neural tube
• The neural plate starts to fold, and the edges come together
• This forms a tube underneath the ectoderm
• Closure of the neural tube doesn’t occur all at once. It starts as the between the hindbrain and spinal cord and progresses as a zip anteriorly and posteriorly to form a tube
What do the mesoderm cells form?
- The mesodermal cells become segmented called somites
* These somites give rise to the muscle, bones and tendons
How is neurulation is concomitant with other form-shaping (morphogenetic) processes, particularly gut formation and body folding (“silk purse” model)?
- The mesoderm and endoderm will give rise to the organs. So, they need to be enclosed. So the embryo fold on itself, known as body folding
- The septum and heart move form the margin to the centre
- The yolk sac, allantois and stalk make the umbilical cord
- Prochordal and cloacal plates delimit the gut tube
What are the subsets of Organogenesis?
• Differentiation of somitic derivatives o bones, muscles, tendons • Development of sensory organs o ears, eyes, olfactory pits • Limb formation o forelimbs first, hindlimbs next o establishment of pattern in the limbs: proximodistal, anterior-posterior, dorsal-ventral • Formation of face structures o jaws, nose, tongue, palate • Formation of genital structures
What are the desirable characteristics of a model organism
- Relevance/ representative
- Accessibility/ availability
- Experimental manipulation
- Genetics
- Cost/space
Define single gene mutations
Single gene mutations: refers to the cases when mutation in one particular gene will be enough to display a characteristic defect.
Define chromosomal abnormalities
Chromosomal anomalies: in some cases, rather than single mutations, whole chromosomal rearrangements are responsible for a disease. The most obvious examples are chromosomal trisomies, such as trisomy of chromosome 21, leading to Down syndrome.
Define polygenic disorders
Polygenic disorders: refers to cases where it is not just one gene affected, but several different genes simultaneously affected, what causes the disease.
