Lecture 8 Flashcards
How many neurons does the human brain have?
100 billion
How many connections does the human brain have?
2.5 x 10^15
How many transmitters does the human brain have?
40 known
Explain the overview of neural development
Sperm + Egg form single cell -> Zygote -> Blastocyst -> Gastrula -> Fetus -> Human
Explain human morula formation
- Formed by cell division from the zygote
- Contained within zona pellucida (a glycoprotein coat) (can’t expand) so no growth in size yet
- Resembles a mulberry
- In latter compaction stage individual cells become less distinct
Explain morula cells on day 4
Trophoblasts remained on the outside of embryoblasts
Cells secrete fluid
Outermost cells become tightly bound by compaction - forms inner cell mass
What forms on day 5 after conception
Blastocyst forms
Explain blastocysts
Have inner cell mass (embryoblast) that forms the embryo
Source of pluripotent embryonic stem cells that give rise to adult structures
Trophoblasts later forms the placenta
What forms primary germ layers
Gastrulation
Explain gastrulation
- Epiblast cells from inner cell mass converge at midline - ingress at primitive streak
- Two layers (epiblast and hypoblast) form three layers (ectoderm, mesoderm, and endoderm)
Gastrulation in the Chick Embryo
Hensen’s node is the ‘organiser’ for the process of infolding during gastrulation
- The order in which the cells enter to blastocoel through Hensen’s node determines their specification in the embryo
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What does the ectoderm form?
Skin, nails, hair, lens of eye, lining of internal and external ear, nose, sinuses, mouth, anus, tooth enamel, pituitary gland, mammary glands, and nervous system
What does the mesoderm form?
Muscles, bones, lymphatic tissue, spleen, blood cells, heart, lungs, and reproductive/excretory systems
What does the endoderm form?
Lining of lungs, tongue, tonsils, urethra, associated glands, bladder and digestive tract
Gastrulation in human embryo
Day 5: blastocyst (bilaminar = epiblast and hypoblast) embeds itself into
the endometrium (womb lining)
Day 14: primitive streak appears
Day 16: ingressing cells start to form the early mesoderm
What does the nervous system arise from?
Neuroectoderm
Organisers in different species
Birds - Henson’s node
Amphibians - Spemman’s organiser
Mammals - Primitive knot
How did Spemann and Mangold confirm the vital role of organisers in embryogenesis
Spemann & Gold – experiments to determine the localisation of the organiser in the embryo
Transplant of newt embryo tissue from one pigmented embryo into non-pigmented
Induction of two newt embryos fused in the middle
Resolved “preformation” and “epigenesis hypothesis
Preformation – embryos are just very small fully formed adults
Epigenesis – embryo generates new complexity as the organism develops
Basis of the Organiser: role of β-Catenin
Nieukwoop centre – small piece of tissue next to the Spemman’s organiser capable of inducing a new gastrulation site when transplanted into another embryo
Produces beta-catenin
Cells in vegetal pole degrade beta-catenin, but cells in animal pole do not – produces a beta-catenin diffusion gradient across the embryo
This promotes the action of beta-catenin in the Spemanns organiser
Beta-catenin binds to TCF3 to act as a transcription factor that activates the expression of genes such as twin and siamois that promote the synthesis of other “organiser” proteins
How does Wnt signalling regulate β-Catenin and Neural Development
The axin/GSK3/APC ‘destruction complex’ normally promotes proteolytic degradation of β-Catenin
Wnt signalling pathway passes signals from cell surface receptors (e.g. frizzled) to control DNA expression in the nucleus
Stimulation of receptors releases Dishevelled (Dsh) which inhibits complex
β-Catenin can now enter nucleus to form another complex that binds DNA to turn on the expression of key organiser genes
