Germ Cells• Specification• Migration• Meiosis• Gamete Formation

Germ Cell Specification: Germ Plasm• Theodor Boveri (1862-1915)• Centrifugation and displacement of first cleavage.

Germ cell specification: Drosophila• Pole cells• germ cell-less (gcl)• polar granule component (pgc)• Oskar, Vasa, Aubergine

Germ cell specification: Drosophila

Fig 16.4

Germ cell specification: Frogs/Fish• Germ Plasm located in

vegetal pole.• Nanos• Vasa

Germ cell specification: mammals• Wnts make cells competent

for BMPs• BMPs Induce blimp1 and

prdm14– Regulate Sox2, Nanog, and

Nanos3.

Cell Migration: Drosophila• Movement into posterior

midgut• Gut endoderm triggers

migration into visceral mesoderm

• PGCs split, each will associate with developing gonad.

• Germ cells migrate to gonads.– Attraction/repulsion

• HMG-CoA reductase

Cell Migration: Drosophila• Dpp- BMP2/4 homolog• Bag of Marbles (Bam)

Cell migration: Zebrafish• Sdf1 and CXCR4

Figure 16.12

Cell migration: Frogs

Fig. 16.13

Cell migration: Mammals

Fig. 16.14

Cell migration: Bird and Reptiles• Germinal crescent• Sdf1/CXCR4

Fig. 16.15 and 8.9

Cell migration: Bird and Reptiles• Germinal crescent• Sdf1/CXCR4

Fig. 16.15 and 8.9

Meiosis

• Chromatin condenses• Spindle forms• Nuclear membrane

breaks down• Homologous

chromosomes pair• Chromosomes attach

to spindle as tetrads

Prophase I

• While chromosomes are paired, crossing-over can occur

Homologous pairs form tetrads

Tetrad

• Tetrads line up at equator• Homologous chromosomes

face opposite poles

Metaphase I

• Homologous chromosomes separate• Pulled apart by spindle shortening• Sister chromatids stay together

Anaphase I

• Chromosomes reach poles• Nucleus may re-form• Spindle breaks down• Cell divides (cytokinesis)• Two haploid daughter cells

(N chromosomes)

Telophase I

Haploid cell

Haploid cell

• Daughters have one chromosome from each pair• Haploid: no homologous pairs• Still two copies of each chromosome (sister chromatids)

Meiosis I

diploid cell:N = 22N = 4

haploid cell:2 chromosomes

• Very similar to mitosis• Sister chromatids separate

Meiosis II

Telophase I Metaphase II Anaphase II Telophase II CytokinesisProphase IIInterphase II

Aa

a

A

a

A

A

A

a

a

b B B

b

bb

b

B B

B

• Four haploid gametes• One chromosome set each• Genetically different

Results of meiosis

Gamete Maturation: Frogs

• Vitellogenin• Progesterone

Fig. 16.22 and 16.25

Gametogenesis in MammalsFemale oogenesis• Meiosis initiated once in

finite population of cells• One gamete produced• Completion of meiosis

delayed months/years• Meiosis arrested in first

prophase.• Differentiation of gamete

occurs while diploid• All chromosomes exhibit

equivalent transcription

Male spermatogenesis• Meiosis initiated continuously

in mitotic stem cell• Four gametes produced• Meiosis completed in days or

weeks• Meiosis and differentiation

proceed continuously w/o cell cycle arrest

• Differentiation of gamete occurs while haploid

• Sex chromosomes excluded from recombination and transcription

Gametogenesis in Mammals

• Fig. 16.26

Gametogenesis in Mammals

• Fig 16.27

Spermiogenesis

Mammalian Oogenesis• Gonadotropin-releasing hormone (GRH)• Follicle –stimulating and lutenizing hormones (FSH and LH)