C2006/F2402 '06 -- Outline of Lecture #23. Power Point slides for this lecture are posted on Courseworks.

(c) 2006 by Alice Heicklen; Notes courtesy of Jeff Farrell. Last update 05/01/2006 01:52 PM

Muscle – separate cells (cytoplasm + nucleus) that fuse later cytoplasm

Spermatogenesis – never go through cytokinesis

To get ES stem cells, have to kill a viable embryo

would become human being in uterus

samples from umbilical cord have been frozen – hope these cells are still pluripotent

adult stem cells are studied because stem cells can be obtained without killing individual, such as bone marrow extracted from hip girdle


Ovaries – number of follicles

Thecal and granulosa cells in ovaries - XX

Oocyte, with granulosa surrounding, and thecal cells on outside

Oocyte filled with cytoplasm (yolk) full of ribosomes, polymerases, mitochondria, proteins, mRNA, etc.

Mammals - cell divides during embryogenesis and number of oocytes will not increase during lifetime

Xenopus can produce eggs each mating season

Germ cells spike before birth, and you lose them constantly afterwards

About 400 eggs ovulated during human lifetime

Oocytes stuck in Meiosis I until puberty

These eggs sit in Meiosis I for years...

Every cycle, 10 eggs begin to mature

Meiosis I unblocked, begin to mature

What is the trigger to develop?

Frog – longer days, warmer temperatures stimulates pituitary

Pituitary stimulates the ovaries to secrete estrogen which matures oocyte, and progesterone to break the stop in meiosis I

Progesterone activates c-mos, which activates one of the subunits of MPF (mitotic promoting factor)... protein from mitosis, now involved in meiosis

MPF = p34 (CDK) + Cyclin

1 of the 10 cells is ovulated

Paused in meiosis II

Fertilization releases block in meiosis II

Block released by CSF (cyostatic factor)

Sperm creates increase in cytoplasmic calcium (Ca2+) released from the ER

Calmodulin bound and activates proteases that digest CSF and allow meiosis to continue

Where did all of the egg’s cytoplasm come from?

Everytime it divides, egg keeps as much cytoplasm as possible (not equal division)

Polar bodies left-over – tiny, nuclei encase in membrane (contain almost no cytoplasm)

Differences between spermatogenesis and oogenesis – see table in Powerpoint




Ectoderm forms both epidermis and the neural tube

How does epithelial layer form a tube? Handout 24A

MTs allow cells to elongate

Once elongated, bands of MFs connected to adherens junctions will use myosin to contract the upper portion of the epithelial sheet (the apical domain)

Like the string on the top of a purse

Forms a pouch

Once two sides of pouch meet and join, form a new epithelial sheet, as well as a tube

Sheet will become epidermis

Tube will become CNS

Cells in between tube and sheet are “neural crest cells,” which become the PNS (as well as other parts of the body)

Formation of this tube is not restricted to neurogenesis – this is a common theme of development

Endocrine glands do just this

Heart forms a linear heart tube before looping and forming chambers

How do you induce this neural tube?

During gastrulation, some mesoderm (“the notochord”) ends up directly underneath this ectoderm

Causes (“induces”) it to create the neural tube

What molecules are being secreted or interacting between these tissues to cause the notochord to induce the epithelial layer above it to form a tube?

Is notochord inducing because it secretes some molecule that is responded to? (Paracrine signaling)

Are the cells themselves interacting through some contact (Juxtacrine signaling)

Put a filter between the two cell types – secreted molecules can get through, but cell contact cannot occur... can tell paracrine from juxtacrine signaling

Organs are made up of two kinds of tissue:

1. Epithelial

2. Mesenchyme – cells are NOT attached to their neighbors, but are individual cells

Not hanging out in space, though. Attached to ECM secreted by it and its neighbors

Just not hooked up to its neighbors via cell-cell junctions

Often it is the mesenchyme that induces the epithelium to create organs

Wing, Thigh, Foot mesenchyme induce different feathers or claw from the epidermis

Moving this mesenchyme around can cause epidermis to produce the wrong thing (i.e. thigh feathers on the wing by putting thigh mesenchyme under the wing epithelium)

Lens of the eye

Neural tube in head region will induce part of head ectoderm to form the lens of the eye

Common theme! Series of inductions generally occur... Notochord induces neural tube induces lens of the eye

Optic vesicle when under the ectoderm of the head induces an in pouching (same idea as the tube, but pinching off 360 degrees, rather than just in a line)

If cut off the optical vesicle, there's nothing to induce, so get no lens

If place foreign tissue between optic vesicle and head ectoderm, don't get lens

If place optic vesicle under trunk ectoderm, don't get a lens

Trunk ectoderm is not “competent” to receive the signal from the optic vesicle

That means it does not have proper receptors to respond to induction molecules produced by optic vesicle

Fgf8 and BMP4 are secreted by optic vesicle, which bind to tyrosine kinase receptors in target tissue

Tissue must be expressing the proper receptors for these ligands

TF Pax6 is expressed in the head

“Master regulator” – activates numerous downstream targets

Two of these are the RTKs that bind Fgf8 and BMP4

Master regulatory genes often result in histone modifications in regions of the chromatin

Ex: Lys 4 methylation makes chromatin looser and more transcriptionally active

Ex: Lys 27 methylation makes chromatin tighter and less transcriptionally active

In ES cell, Pax6 marked with both K4 and K27 – ready to go either way

In eye, Pax6 marked with just K4 – to turn it on

In eye, OTHER master regulators (that are not eye specific) are marked with just K27 – to turn them off

NY Times Science Article Summary

K4 Methylation = loosest euchromatin = transcriptionally active

K27 Methylation – loose euchromatin = transcriptionally inactive

  • Pax6
  • K4
  • +
  • +
  • -
  • Pax6
  • K27
  • +
  • -
  • +
  • Nkx2.5
  • K4
  • +
  • -
  • +
  • Nkx2.5
  • K27
  • +
  • +
  • -

What if you knock out Pax6 in the mouse?

No eye

No lens pit – has not been induced

Issues with optic vesicle

How do you know you need Pax6 in the head?

Use a WT and a Pax6 KO mouse

Pax6-/- optic vesicle with WT ectoderm, get lens

WT optic vesicle with Pax6-/- ectoderm, DON’T get a lens, because need Pax6 in the ectoderm to receive signal from the optic vesicle

Lens induces retina and pigmented epithelium behind it

Lens then induces the ectoderm that has pinched off over it to become the cornea

Reciprocal induction eventually gets us the eye

Why did head ectoderm express Pax6 to begin with?

Has to do with the various tissues it encountered on its way during gastrulation

In mammals, the first induction that starts this off is the sperm penetrating the egg

Gastrulation always occurs exactly opposite the site where the sperm entered

Serial inductions – everyone needs to be in the right place at the right time

In some other animals, this process is because of asymmetric division (and therefore distribution of cytoplasm)

In humans, if things don’t end up being in the perfect places, have some ways to compensate

Identical twins are one embryo that split in half. Can still recover from this – make two COMPLETE individuals even though lost cells

Through evolution, many inductive signaling molecules have been conserved

Transplant a chunk of frog embryo into newt gastrula

Frog belly to newt mouth place still becomes mouth

Newt belly to frog mouth place still becomes mouth

HOWEVER, the mouths looks different... end up with a newt whose mouth has a frog’s suckers ... end up with frog whose mouth has newt’s balancers

The STRUCTURAL genes downstream of the signaling molecules are different... Thus, because it is frog tissue, get a frog mouth in the newt. However, it’s mouth, because even tissue that is fated to become belly can respond to the mouth signaling molecules in the newt.

Pax6 conserved between humans and flies

Transplant mouse or human Pax6 onto fly, where wings should be, and can get eyes on the wings of flies

C. elegans - Not on Exam:

For those of you that are interested Vulva induction is described in figure 19.10 of Purves


NY Times Science Article Summary

K4 Methylation = loosest euchromatin = transcriptionally active

K27 Methylation – loose euchromatin = transcriptionally inactive

  • Pax6
  • K4
  • +
  • +
  • -
  • Pax6
  • K27
  • +
  • -
  • +
  • Nkx2.5
  • K4
  • +
  • -
  • +
  • Nkx2.5
  • K27
  • +
  • +
  • -