Germ Cells and FertilizationLecture 30 BSCI 420, 421 Nov 11, 2002

“The final aim of all love intrigues, be they comic or tragic,is the composition of the next generation”

- A. Schopenhauer

1. Sexual reproduction2. Meiosis3. Oogenesis4. Spermatogenesis5. Fertilization

1. Sexual reproduction

Has an advantage over asexual reproduction:

Increases diversity by producing offspring that are a mixture of the traits of the parents.

Is diversity an advantage?

If so,Why is diversity an advantage?

1. Meiosis

A pair of nuclear divisions that produce four different haploid cells, usually but not always gametes.

Gametes are haploid cells capable of fusing to produceA diploid cell called a Zygote.

Gametes can be of identical size and shape (isogamy).Or they can be different in size and or shape (anisogamy).

Meiosis vs mitosis

3. OogenesisIs gamatogenesis of the larger gamete called the egg or ovum.

Oogenesis

Mammalian (rabbit) oocytes (fig 24)

4. Spermatogenesis

Spermatozoon is a small, highly streamlined cell,specialized to deliver a nucleus to an egg far away, and to activate development of the egg.

Spermatogenesis

5. Fertilization

Involves:a. Activation of the sperm and the acrosomal reaction

b. Activation of the egg

1. The fast block to polyspermy

2. The slow block to polyspermy

3. Initiation of protein, RNA, & DNA synthesis

4. Fusion of the egg and sperm and male and femalepronuclei

a. Activation of the sperm and the acrosome reaction

Activation of the sperm:

Contact wZonaPellucida

AggregatesReceptors

Opens Na+Channels

Release ofV change opens Exocytosis lytic enzymesVS Ca2+ Ca2+ of AcrosomalChannels influx vesicle Exposure

of fertilins

b. Egg Activation: 1. Fast block to polyspermy

Sperm binding to Egg cell membraneBy fertilin and its receptor on egg

Receptor linked Na+ channels open

Na+ influx

Membrane potential change 0

Fast block to polyspermy-70 1 2

(Discovered by mV time, minE.E. Just contactAfro-Amer Dev.Biologist)

Starfish egg injected with the dye aequorin that fluorescesWhen the Ca2+ conc increases.

At time 0, the egg was fertilized, and a wave of Ca2+ sweepsOver the egg in about 1 min. initiating cortical granule Exocytosis and the slow block to polyspermy.

Slow Block to polyspermy

2. Slow block to polyspermy

Binding of fertilin to receptor on egg

Tyrosine kinase activation (both IP3 gated& Ca-Induced)

Phospholipase C-activation IP3 Ca2+ release

DAG production PKC activation Cortical granuleexocytosis

Stimulation of Slow block to3. protein syn polyspermy

RNA synDNA synthesis

4. Fusion of pronuclei

(MTs greenFl-aT;DNA blue)Fig 35

The merging of mammalian pronuclei after fertilization