Oogenesis

Behind every successful embryo stands a hard-working mother.

Eggs are enormous cellscarrying nutrients forthe developing embryo

Why? Mass of egg must equal the mass of the embryo that emerges!

Even human eggsare huge cells

Eggs are enormous cellscarrying nutrients forthe developing embryo

• yolk=large stores of nutrients• large stores of macromolecules• most eggs surrounded by a coat or shell

The germ line is set aside early in development as a separate cell lineage

This can sometimes be visualized by the segregation of putative "determinants”as in C. elegans

Fertilized egg

"P granule"

mother cell of germline

= P4

P1HOW are P-granules segregated?

PAR proteins & microfilaments see p. 246 and Fig. 8.43

C. elegans:

P granulesare segregated at each divisionand make thecell that inherits them the germline

Figure 8.44 cell nuclei P-granules

Similar mechanisms lead to very early segregationof the germline in thefruit fly Drosophila

Germ cellprogenitors

A P

The Germline is also set aside in Humans

Oocytes arrest in prophase of meiosis I for up to 50 years!

Figure 19.23

How does a single egg cell make all the stuff needed to start development?

Fly Oogenesis

Nurse cells are mitotic sisters of the oocyte

Figure 19.4

Nurse cells synthesize macromolecules and pump them into the oocyte through the cytoplasmic bridges

oocyte

nurse cells

follicle cells

The Drosophila ovary is composed of somatic follicle cells, germ line nurse cells, and the oocyte

Most of the instructions for forming organisms are already in the egg!

Asymmetric distribution of egg contents influences embryonic development

Molecules synthesized by the nurse cells and specifically localized in the oocyte determine

the anterior/posterior axis of the embryo

Posterior Determinant

(nanos)

Anterior Determinant

(bicoid)

Oocyte

Specific mRNAs are bound by proteins and transported along microtubules by motor proteins to their destinations

Ventral follicle cells build a signal into the egg shell that determines the dorsal/ventral axis of the embryo

Ventral follicle cells

Oocyte signals follicle cells,follicle cells signal back to oocytecytoplasm

nucleus

gurken mRNA

PosteriorAnterior

Dorsal

Ventral

Torpedo receptor binds Gurken; signal inhibits Pipe in dorsal follicle cells

Pipe protein

See Fig. 9.11for more details

Pipe intiates a signaling pathway for activation of Dorsal transcription factor in ventral cells only

nucleus

Ventral signal

initiated by

Transmembrane receptor(Toll)

Signaling pathway

Transcription factor (Dorsal) relocalized to ventral nuclei of embryo activating new genes

Pipe

Localized mRNAs also influencevertebrate development

Vg1 mRNA

See also Figure 5.38

Human oogenesis:An example of hormonal control

of reproduction

Meiosis I

2 million present at birth ovulation

Cyclic changes in hormonal signals regulate the timing of femalereproduction

Most mammals-once/year

Humans- monthly

Release into the Fallopian tube

(Turns off FSH and LH!)

Cyclic changes in hormonal signals regulate the timing of femalereproduction

(Turns off FSH and LH!)

Release into the Fallopian tube

RU486- blocks progesterone receptors

Plan B-progestin (interferes with ovulation, fertilization, implantation)

Progesterone and estrogen birth control pills-prevent maturation of new ova

Anton van Leeuwenhoek

WNYC’s Radiolab clip from “Sperm” (12/2/08)By Jad Abumrad & Robert Krulwich

Figure 7.1 The Human Infant Preformed in the Sperm, as Depicted by

Nicolas Hartsoeker

(1694)

The sperm: a stripped downmachine for delivering DNA

Figure 7.2

Chemical cues from the egg attract sperm

In sea urchins, the cue is resact

Inject resact

Figure 7.9 Sperm chemotaxis

0 sec 20 sec 40 sec 90 sec

Step one:

Contact between the sperm and the eggshell

Step two:

Contact between the sperm and the egg’splasmamembrane

Fertilization: a multistepprocess

(zona pellucida=egg shell)

Sperm-eggshell contact triggers the acrosome reaction

Example 1-- the sea urchin

enzymes

Species specificity (equivalent to zona pellucida)

attracts/activates sperm

Figure 7.8

Sperm-eggshell contact triggers the acrosome reaction

Example 2: Mammals

Eggshell includes ZP3/Sperm have ZP3 receptor

Figure 7.8

BLOCKING POLYSPERMY

1. Fast/transient: prevent sperm fusion

2. Slow/permanent: removal of other bound sperm

A complex signal transduction pathway is activated in the egg by fertilization

Example-- the sea urchin egg

1. Na in+

2. Phospholipase C activated

InsP3DAG +

3. Ca release2+

Ca up 2+

5. Protein kinase C activated

PKC

H+

H+

6. pH increase

7. Increased protein synthesis

8. Initiate DNAreplication

4. Cortical granule release; blocks

polyspermy

Fast block1-3 secondslasts 1 min

Slow block20-60 secondspermanent

How do we know this?We can measure these events directly!

Figure 7.17 Membrane potential of sea urchin eggs before andAfter fertilization.

resting -70 (sperm can fuse with egg)

(sperm can’t fuse with egg)

+20 mV

Cortical granules:poised for release

15,000 cortical granules in a sea urchin egg• contain enzymes that clip the egg’s bindin receptor and any attached sperm• alter the vitelline envelope (zona pellucida in mammals so that ZP3 receptor on sperm cannot bind)

Blocking polyspermy: SLOW BLOCKFigure 7.6

Sperm-eggshell contact triggers the acrosome reaction

Example 2: Mammals

Sperm have ZP3 receptor

A wave of increased calcium can be visualizedmoving across the egg from the point of sperm entry

Figure 7.20

A complex signal transduction pathway is activated in the egg by fertilization

Example-- the sea urchin egg

1. Na in+

2. Phospholipase C activated

InsP3DAG +

3. Ca release2+

Ca up 2+

5. Protein kinase C activated

PKC

H+

H+

6. pH increase

7. Increased protein synthesis

8. Initiate DNAreplication

4. Cortical granule release; blocks

polyspermy

Fast block1-3 secondslasts 1 min

Slow block20-60 secondspermanent

7.4

200

7.0

7.2

6.8

pH

time (seconds)

intracellular pH

400 60010-8

10-7

10-6

intracellular

calcium

10-5

Ca2+