Lecture 8

Question 1

How many neurons does the human brain have?

Answer 1

100 billion

Question 2

How many connections does the human brain have?

Answer 2

2.5 x 10^15

Question 3

How many transmitters does the human brain have?

Answer 3

40 known

Question 4

Explain the overview of neural development

Answer 4

Sperm + Egg form single cell -> Zygote -> Blastocyst -> Gastrula -> Fetus -> Human

Question 5

Explain human morula formation

Answer 5

• 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

Question 6

Explain morula cells on day 4

Answer 6

Trophoblasts remained on the outside of embryoblasts

Cells secrete fluid

Outermost cells become tightly bound by compaction - forms inner cell mass

Question 7

What forms on day 5 after conception

Answer 7

Blastocyst forms

Question 8

Explain blastocysts

Answer 8

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

Question 9

What forms primary germ layers

Answer 9

Gastrulation

Question 10

Explain gastrulation

Answer 10

• 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)

Question 11

Gastrulation in the Chick Embryo

Answer 11

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

-

Question 12

What does the ectoderm form?

Answer 12

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

Question 13

What does the mesoderm form?

Answer 13

Muscles, bones, lymphatic tissue, spleen, blood cells, heart, lungs, and reproductive/excretory systems

Question 14

What does the endoderm form?

Answer 14

Lining of lungs, tongue, tonsils, urethra, associated glands, bladder and digestive tract

Question 15

Gastrulation in human embryo

Answer 15

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

Question 16

What does the nervous system arise from?

Answer 16

Neuroectoderm

Question 17

Organisers in different species

Answer 17

Birds - Henson’s node

Amphibians - Spemman’s organiser

Mammals - Primitive knot

Question 18

How did Spemann and Mangold confirm the vital role of organisers in embryogenesis

Answer 18

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

Question 19

Basis of the Organiser: role of β-Catenin

Answer 19

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

Question 20

How does Wnt signalling regulate β-Catenin and Neural Development

Answer 20

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