Block B Lecture 1.2 - Embryonic Development

What are the 6 (roughly) steps of embryonic development?

1) making the stem cells

2) breaking symmetry

3a) making the two blast cell layers

3b) ‘gastrulation’

4) notochord formation

5) neurulation

6) somite formation and segmentation

Stage 1?

making the stem cells

Fertilastion?

-restores the diploid genome

-begins wiping out the memory of parents (clean slate so you don’t pass on affected genes)

Stages in early development?

-fertilisation (day 1)

-first cleavage

-2-cell stage

-4-cell stage

-8-cell stage

-morula

-128-cell blastocyst (final stage)

-implantation and placenta formations (day 7)

What happens during cleavage?

the fertilised egg undergoes a series of rapid cell divisions, remaining the same size

Stages involved in cleavage?

-2-cell

-4-cell

-8-cell

-morula

What is the end product of cleavage the same size as?

the orginal ‘zygote’

What is a blastocyst?

structure consisting 128 cells

Structure of blastocyst?

-inner cell mass (embryoblast)

-outer cell mass (trophoblast)

Inner cell mass (embryoblast)?

-goes on to form the embryo proper

-forms embryonic stem cells (pluripotent)

Outer cell mass (trophoblast)?

-this goes on to form the placenta

Pluripotent stem cells?

-theoretically can give rise to EVERY cell type in the animal body proper (except placenta)

-proliferate indefinitely

Where are pluripotent stem cells first recognised?

-in teratocarcinomas (form of cancer)

Stage 2?

breaking symmetry

What happens on day 7?

-blastocyst reaches the uterus & ‘hatches’ by shedding outer layer

-this exposes the bare trophoblast cells to the uterine wall

(uterine wall is prepared for implantation each month under hormonal control)

What happens after the blastocyst ‘hatches’?

-the exposed trophoblast begins to thicken as it begins to implant in the uterine wall & form the placenta

-embryo fully embedded after 10 days (17 days after fertilisation)

Stage 3a?

making the two blast cell layers

What happens after the trophoblast is implanted?

-the inner cell mass (ICM) (embryoblasts) pulls away from the trophoblast and forms a hollow amniotic cavity

What does the ICM then form?

a flat disc with two layers;

-epiblast & hypoblast

Epiblast?

-dorsal

-exposed to amniotic cavity (surrounding it)

Hypoblast?

-ventral

-faces the yolk sac

Stage 3b?

gastrulation

What happens during gastrulation?

-the epiblast layer undergoes a complex rearrangement to form the germ cell layers

Step 1 of gastrulation?

some of the epiblasts (ectoderm) cells begin to migrate toward the primitive streak

Step 2 of gastrulation?

the epiblasts (ectoderm) then move through the layer towards the hypoblast (primitive endoderm)

-the 1st cells through become the definitive endoderm

-the 2nd cells through form the intermediate layer the mesoderm

-those who don’t migrate form the ectoderm

What direction is the movement of the epiblast?

caudal to cranial (tail to head)

Ectoderm and Endoderm are?

-epithelial tissues (form sheets of tissue)

Mesoderm is a?

-mesenchymal tissue (star shaped cells that don’t attach to one another => migrate freely) (muscle & blood cells)

Stage 4?

notochord formation

Another wave?

epiblast migrate through the primitive pit forming a line towards the ‘head’

What type of germ cells form the notochord?

-mesoderm

What is the notochord?

it is a rod defining the body axis & future site if the vertebral column (organiser)

Where does the notochord form?

-under the ectoderm but on top of the endoderm

Stage 5?

neurulation

What is neurulation?

how the brain and spine form

-formation of the neural tube

What does the notochord do during neurulation?

-induces the overlying epiblast/ectoderm to fold in

Steps of neurulation?

-notochord signals overlying ectoderm to become the neural plate

-neural plate to neural groove to neural tube: pinched off into body

-neural groove formation

-associated neural crest cells

What does the mesoderm do next to the notochord threee regions?

differentiate into somites, intermediate mesoderm & lateral plate

Somites?

40 pairs of body segments (repeating units) form by end of week 4

Intermediate mesoderm?

just lateral to somites

Lateral plate?

splits to form coelom (‘body cavity’)

Closure of neural tube?

-begins at end of week 3; completed by end of week 4 (folic acid important for this step)

-’zipped’ cranially (eventually brain) & caudally (spinal cord)

What do the neural crest cells form?

sensory nerve cells and other structures

Spina bifida?

-condition caused by incomplete neural tube closure

-folic acid/folate supplements before and during pregnancy to prevent this

The formation of a fold at the top of the neural tube?

the appearance of the two hemispheres of the brain

Stage 6?

somite formation and segmentation

What begins to appear by the end of week 4?

body plan

-embryo undercutting is complete

What have somites done by the end of week 4?

divided into

-dermatome

-sclerotome

-myotome

=> segmentation in humans is clear when there is spinal injury

Where are somites found?

lateral to neural tube and notochord

Dermatome?

area of skin supplied by a single spinal nerve

Sclerotome?

gives rise to the vertebrae & ribs

Myotome?

muscle or group of muscles supplied by a specific spinal nerve