03 gametogenesis

7/30/2019 03 gametogenesis

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Meiosis is the first step in gametogenesis: separation of homologous

chromosomes into haploid daughter cells

Spermatogonia and oogonia are the germ cells that will eventually develop

into the mature sperm or egg

Primary spermatocyte or oocyte: the first step in this development is the

duplication of homologous chromosomes to get ready for meiosis

Secondary spermatocyte or oocyte:

the first meiotic division separates

the homologous chromosomes from

each parent

Spermatids or eggs: the second

meiotic division separates the 2

chromatids and creates 4 haploid

cells

In males, this eventually produces 4sperm cells by the process of

spermiogenesis. In females, it

produces 1 egg and 3 polar bodies.

This allows the egg to retain more

cytoplasm to support early stages of

development


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Meiosis generates tremendous genetic diversity. How many different types

of gametes can be generated by an individual (male or female) with 23

different chromosomes?


3/18

More than 223 or 8,000,000 different gametes


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The timing of meiosis differs in females and males

In males, the spermatogonia enter meiosis and produce sperm from puberty

until death. The process of sperm production takes only a few weeks. Each

ejaculation has 100 to 500 million sperm.In females, this process is more complex. The first meiotic division starts

before birth but fails to proceed. It is eventually completed about one month

before ovulation in humans. In humans, the second meiotic division occurs

just before the actual process of fertilization occurs.

Thus, in females,the completion of

meiosis can be

delayed for over 50

years. This is not

always good.

Only I egg produced

In addition, all

meiosis is ended in

females at

menopause.


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Homologous chromosomes form the synaptonemal complex

which facilitates crossing over and genetic diversity

During meiosis, homologous

chromosomes join together in pairs to

form the synaptonemal complex.

Each pair of chromatids is connected by

axial proteins. The 2 homologous

chromosomes are held together closely

by central element proteins.

A recombination nodule forms thatcontains enzymes for cutting and

splicing DNA. Chromosomes are cut and

joined crosswise at points called

chiasmata, seen when they separate.

The exchange of genetic material is

evident when the chromosomes separate

This process is dangerous as it leads to

deletions and duplications of genetic

material. However, it is also valuable

because it increases genetic diversity

and facilitates evolution.


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Down syndrome is trisomy 21. It

results in short stature, roundface and mild to severe mental

retardation.

This is the failure of the 2

chromatids to separate during

meiosis 2. It results in one

oocyte receiving 2 instead of 1

chromatid. In older women, long

term association of chromatids

(i.e., over 50 years) results in the

axial proteins failure to separate.

Down syndrome occurs with a

frequency of 0.2% in women

under 30 but at 3% in those over

45 years of age.

In older women, failure of the synaptonemal complex

to separate properly can cause genetic disease


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Spermatogenesis occurs in the seminiferous tubules

The mammalian testes are divided into many lobules, and each lobule contains

many tiny seminiferous tubules. Sperm develop in an ordered fashion in these

tubules. Cells start to mature on the outside and move inward (towards the

lumen) as the become mature sperm.

Spermatogonia are the most primative cells. They differentiate as primaryspermatocyte secondary spermatid sperm are released into lumen.Sertoli cells are supporting cells that stretch from the lumen to the edge of

the tubule. They nourish the developing sperm. They form a blood-testis

barrier to control spermatogenesis (similar to the blood-brain barrier). These

cells also inhibit spermatogenesis before puberty and stimulate the process

after puberty.


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Spermiogenesis is the maturation process into sperm

The golgi vesicles combine to

form an acrosomal vesicle that

lies over the nucleus. Its full ofenzymes

Mitochondria start to localize next to the

flagella to provide ready energy

The nucleus condenses in size and is stabilized by

special proteins called protamines

The excess cytoplasm is pinched off as a residual body

(no need for organelles and cytoplasmic proteins)

Centosomes start to organize

microtubules into long flagella

Sperm are tiny, but highly specialized missiles for delivering the male genome:

Microfilaments shoot the acrosome into the egg to harpoon it and pull it in.

The acrosome has enzymes for breaking into the egg.

The midpiece has large numbers of mitochondria for horsepower.

The tail has a powerful flagellum for driving the sperm into the proximity of the

egg (in humans, through the uterus and up into the oviduct.


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Spermatogonia and oogonia are stem cells

What is a stem cell?

Stem cells have 3 properties: 1. They are undifferentiated cells

2. They have potential for self renewal3. They are able to undergo differentiation

to form committed progenitor cells (a

fancy word for all types of

differentiated adult cells such as

muscle, bone, skin, etc)


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The goal of oogenesis is to produce one egg

with massive amounts of cytoplasm

In many organisms, such as frogs and birds, the egg must contain all the

nutrients to support the entire process of embryonic development

In humans, the egg does not need to grow so large because the fertilized egg

only needs to support growth until it implants in the uterus. The placenta

then nourishes development.

In some organisms, such as

frogs, oocytes grow to extremely

large size and they have veryactive chromosomes that

synthesize large amounts of

RNA. In contrast to sperm which

are tiny cells, oocytes are among

the largest cells in the body.

Oocytes contain Lampbrush

chromosomes: look like brushes

that were used years ago to

clean lamps. Frog oocytes can

contain 200,000 times as many

ribosomes as a normal cell.


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Oocytes have a very small nucleus / cytoplasm ratio

Most normal cells have several times as much cytoplasm as nucleus. This

allows the nucleus to make enough mRNA and rRNA to keep up with the

cytoplasm and cell needs.

In some species, oocytes have a tremendously tiny nucleus to cytoplasm ratio.

They must have a large amount of cytoplasm and ribosomes to make all of the

proteins needed for embryonic development.

The nucleus is just not large enough to keep up and maintain enoughtranscription to generate all of the needed components. However, oocytes have

developed specializations to deal with this problem.

1. Ribosomal RNA genes are often amplified in oocytes. This allows more

templates to transcribe more rRNA.


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Specializations allow the egg to accumulate cytoplasm:

nurse cells allow oocytes of insects

to produce massive amounts of RNA

In Drosop hi la melanogaster, the oogonia are calledctyoblasts, and they undergo an unusual

specialization

They undergo multiple mitotic divisions, but fail to

undergo cytokinesis (cell division). Thus, they all

remain connected to the original cell as cytocytes

One of the lucky cytocysts becomes the oocyte

The other 15 become nurse cells. They make large

amounts of RNA and nutrients but they send it all to

the oocyte. This allows the oocyte to accumulate

massive amounts of cytoplasm to support

development (15 nuclei instead of 1).


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What does a flys ovary look like?


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Vitellogenesis is the process of producing the major yolk proteins

Yolk: animal eggs contain large amounts of protein, lipid, and glycogen to

nourish the embryo. These materials are collectively called yolk.

Yolk is minimal in animal eggs that sustain only the first portion of

embryogenesis (humans and many mammals that have a placenta need only

support cleavage for several days before implantation into the uterus).

However, yolk is stored in large amounts in the eggs of birds and reptiles

because their eggs have to support the entire process of development.

Yolk proteins are synthesized in the liver in vertebrates, or in the fat body of

insects (an analogous organ)

Animal vegetal polarity: In

eggs that have a lot of yolk, the

yolk is concentrated in the

vegetal pole. The animal pole

contains the nucleus andrelatively little yolk. The yolk in

the vegetal pole interferes with

cytokinesis during the process

of cleavage leading to

incomplete cleavage.


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Maturation processes prepare the oocyte

for ovulation and fertilization

Most oocytes of different species are arrested in the first meiotic division.

Oocyte maturation begins officially when this block is removed and meiosis

starts once again.1. The nuclear membrane breaks down and DNA starts to condense into

chromosomes

2. The permeability of the oocyte plasma membrane changes so it can

function outside of the ovary.

3. The plasma membrane develops receptors to interact with the sperm

Fertilization occurs at different stages of oocyte maturation:

How is oocyte maturation

initiated?


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Control of oocyte maturation has been studied extensively in frogs

Oocyte maturation is controlled by hormone interactions between the pituitary

and follicle cells. Pituitary gonadotropin hormone stimulates follicle cellsprogesterone triggers oocyte maturation by activating c-mosexpressionC-mosactivates maturation promoting factor, the same activity as M-phasepromoting factor, that is composed of cyclin B and cyclin dependent kinase 1

The exact mechanism isnt understood.

Ifc-mosis inactivated by antisense

oligonucleotides, no oocyte maturation

occurs. On the other hand, if extra c-mosis injected it triggers oocyte

maturation before it is ready.

MPF does many things, although the

exact pathways have yet to be found. It

causes breakdown of the nuclearenvelope by phosphorylating nuclear

lamins (proteins stabilizing the

envelope), it triggers changes in the

oocyte plasma membrane, it stimulates

ovulation, and it causes condensation

of chromosomes.


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Development of mammalian oocytes occurs within the ovary

The timing of oocyte maturation and ovulation varies in different

mammals. Ovulation can be stimulated by seasonal cues, the process of

mating, or in primates, by the monthly cycle regulated by hormones such

as estradiol, produced by the granulosa cells.

In the mammalian ovary, the oocytes are closely associated with somatic

cells called granulosa cells which aid oocyte maturation and ovulation.


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Eggs are protected by elaborate envelopes

Vitelline envelope: a glycoprotein layer covers the plasma membrane of all

eggs. This acts to protect the egg.

Eggs that are deposited in water have a jelly-like coating that surrounds the

egg (frogs eggs)

Eggs that are deposited on land have particularly elaborate envelopes. The

eggs of birds have a vitelline envelope, a fibrous layer, an outer layer of

albumin (egg white), and a shell composed of calcium carbonate. The outer

envelopes are synthesized in the oviduct after the egg has been fertilized.
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03 gametogenesis

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