Lecture 13: Germ Cells, Meiosis, and Fertilization

terminally differentiated

• highly specialized

• post-mitotic

examples of terminally differentiated cells

neurons, egg, sperm

Early embryonic stages

zygote → blastula → gastrula

• totipotent → pluripotent → multipotent

totipotent cells

• gametes (sperm and egg)

• zygote

Why is the egg special?

• doesn’t give rise to other cells unless combine genetic material with sperm 

What does the egg have a lot of?

cytoplasm

• proteins, machinery, and mRNAs

The egg is _____ differentiated

terminally

• it reprograms the sperm cell nucleus to produce the totipotent zyote

Experimental Demonstration of the Reprogramming Capacity of Egg Cells

• John Gurdon 

• Can take a tadpole gut epithelial cell (specialized) or a cultured adult skin cell (differentiated), remove the nucleus and transfer it to eggs treated with UV radiation (DNA contents of egg cell destroyed), and a normal tadpole will develop 

  • The contents of the egg can reprogram somatic nuclei 

  • DNA can only come from the nucleus you injected into the egg

  • No sperm cell was used, so no fertilization occurred 

  • Genetic cloning   

primordial germ cells

• produce germ cells (sperm and oocytes)

• set aside early during development

Germline is ____

immortal

• A totipotent zygote can give rise to all cells, including more primordial germ cells, and so on 

Primordial Germ Cells are ___

diplod

• need the sperm and oocytes to be haploid so meiosis occurs

Development of the germline

1) specification

2) migration to gonad

3) proliferation, expansion

4) differentiation into eggs or sperm

• meiosis

What organisms specify PGCs by induction

mice, humans, some reptiles

PGCs specification through induction

• Receive cues from the environment to become PGCs

What happens after PGCs are specified?

• Then they migrate to the developing gonad to make testes or ovaries

• The development of gonads tells PGCs to differentiate into oocytes or sperm

What organisms specify PGCs by preformation (autonomous, intrinsic)

Drosophila, C.elegans, Xenopus, zebrafish

PGCs specification through preformation

• germ plasm is asymmetrically inherited to give rise to primordial germ cells 

• it becomes localized to give rise to PGCs

What is germ plasm in simple terms?

• Germ plasm is the hereditary, genetic material contained within germ cells

• Maternally inherited  

What is germ plasm in more detail terms

• Maternally derived proteins and RNA deposited in the oocyte that promote germline fate

• Highly concentrated and phase-separated RNA and protein condensates 

• Contains components that protect RNA and proteins from degradation 

• Contains transcriptional and translational repressors to keep somatic genes silenced 

• Don’t start transcribing the genome immediately in the zygote phase 

• Maintains expression of totipotency genes

• Asymmetrically inherited into a subset of cells that will become germline progenitors 

• Want to keep PGC from differentiating into something else 

Does the germ plasm keep the somatic genes on or silenced?

silenced

Germ cell specification by induction details

• no germ plasm in the oocyte

• BMP and WNT signals from one tissue signal to induce PGC fate in the adjacent tissue 

  • Activates expression of germ cell genes

• Maintains expression of totipotency genes

• Represses somatic genes

Are germ cells made from the gonad?

No, they travel there

• PGCs are specified, set aside, and then travel to the gonads

• in gonads, they differentiate into eggs or sperm

Germ cell differentiation

• In mammals, germ cells undergo sexual differentiation dependent on the gonad environment 

Sex-specific gonad differentiation

• If the Y chromosome is present, then Sry is made, which initiates the testes pathway 

• SRY is a transcription factor that leads to transcription of Sox9, which is another transcription factor that promotes testis formation and also leads to a pathway that inhibits β-catenin and the formation of ovaries

If you have a PGC that’s XX and ends up in a gonad that forms testes, what will happen

it will form sperm

spermatogenesis (making sperm)

• Meiosis is initiated continuously in a mitotically dividing stem cell population 

• Four haploid gametes are produced per meiosis

• Meiosis is completed in days or weeks

• Meiosis and differentiation proceed continuously without cell cycle arrest 

• Differentiation of gametes occurs while haploid, after meiosis ends  

• Sex chromosomes are excluded from recombination and transcription during the first meiotic prophase 

• Mitosis expands the progenitor population

  • Type A spermatogonia divide mitotically to maintain the stem cell pool and create Type B spermatogonia

• Incomplete cytokinesis 

  • During divisions, the developing cells remain connected by cytoplasmic bridges, allowing them to share gene products and develop synchronously

• PGCs colonize the gonad to form testes and make sperm

• Sperm cells contribute centrioles and DNA, but no cytoplasm, to the oocyte

• Meiosis begins in puberty

In spermatogenesis, meiosis is initiated _____

continuously in a mitotically dividing stem cell population

How many haploid cells produced per meiosis (Spermatogenesis)?

4 haploid gametes

How long does it take to complete meiosis (spermatogenesis)

days or weeks

Meiosis and differentiation proceed _____ (Spermatogenesis)

continuously without cell cycle arrest

Differentiation of gametes occurs while _____ (spermatogenesis)

haploid, after meiosis ends

sex chromosomes are ____ from recombination and transcription during Prophase 1 (Spermatogenesis)

excluded

_____ expands the _____ population (Spermatogenesis)

mitosis, progenitor

• Type A spermatogonia divide mitotically to maintain the stem cell pool and create Type B spermatogonia

Type A spermatogonia divide ____

mitotically to maintain the stem cell pool and create Type B spermatogonia

Cytokinesis is ______ (spermatogonia)

• incomplete

  • During divisions, the developing cells remain connected by cytoplasmic bridges, allowing them to share gene products and develop synchronously

PGCs colonize the ____ to form testes and make sperm

gonads

When does meiosis begin in males

puberty

Oogenesis

• Early embryonic specification of PGCs

• Mitosis with incomplete cytokinesis generates cysts

  • Bridges form communication between cells 

• Beginning of meiosis 1, oocytes stalled 

• When divisions happen, sister cells put their cytoplasmic content into one oocyte, while the 3 others die 

• Damaged materials are discarded in apoptotic sister cells for quality control 

• Process ensures quality over quantity

On the way to the developing gonads, PGCs undergo

active mitotic division

____ with _____ cytokinesis generates ______ (oogenesis)

mitosis, incomplete, cysts

• Bridges form communication between cells 

when are oocytes stalled 1st

meiosis 1

How many times is meiosis initiated in oocytes?

once in a finite population of cells

how many functional gametes produced per meiosis

1, other 3 polar bodies are destroyed

How long does it take to complete meiosis in oocytes?

completion of meiosis is delayed for months or years

when does differentiation of gametes occur (oocytes)

while diploid in first meiotic prophase

All chromosomes exhibit equivalent transcription and recombination during meiosis prophase for oogenesis or spermatogenesis?

spermatogenesis

end of meiosis 1, cells are

haploid (n)

synaptonemal complex

• a meiosis-specific, zipper-like protein structure that bridges paired homologous chromosomes

• Cohesin interacts with axial cores of the synaptonemal complex

• Cohesin holds the sister chromatids together 

• The synaptonemal complex allows sister chromatids to be held close together

Stages of Prophase I

1) Leptotene

2) Zygotene

3) pachytene

3) diplotene

4) Diakinesis

Leptotene

• Chromosomes begin to condense into visible, thin threads

• The maternal and paternal sister chromatids start to align

Zygotene

• Homologous chromosomes pair up in a process called synapsis

• The synaptonemal complex begins to assemble between them, acting like a zipper to hold the homologs together

Pachytene

• The synaptonemal complex is fully formed, and the paired chromosomes are called a bivalent (or tetrad)

• This is the stage where crossing occurs between non-sister chromatids

Diplotene

• Synaptonemal complex disassembles 

• The homologous chromosomes begin to separate

Diakinesis

• The homologs are completely unzipped 

How to visualize what’s going on during Prophase 1

electron or immunofluorescence (cohesin) microscopy

What does 3d structures illumination microscopy do?

can show the paired lateral elements of the synaptonemal complex for meiotic chromosomes at pachytene

After the synaptonemal complex has disassembled during diplotene, the homologs are held together by

chiasmata