Download Oogenesis

BIOLOGY 205/SECTION 7
DEVELOPMENT- LILJEGREN
Lecture 2
MODEL ORGANISMS USED TO STUDY DEVELOPMENT
1. A brief introduction to six model systems widely used in labs today
a. Why model organisms? Kit example
b. Megabase genome =1,000,000 (million) base pairs; Gigabase = 1 billion!
2. Rules of evidence
There are three main types of evidence that are accepted in all of experimental biology
when figuring out a gene’s function. It is important to KNOW THEM:
• Correlative Evidence
o You see a gene expressed in a tissue
o Weak, but relatively easy
• Loss of Function Evidence
o You knock out the gene, the tissue fails to form.
o Better, but a little harder.
• Gain of Function Evidence
o You express a gene in an inappropriate tissue, and transform it.
o Best, but hardest of all
Or to sum it all up, think of the motto: “Find it, Lose it, Move it”. [but use the formal terms
above in an exam situation!!]
Oogenesis
Behind every successful embryo stands a hard-working mother.
1. While both parents contribute DNA (the developmental blueprint), the mother
also contributes two key ingredients to the egg
a. a store of supplies providing for embryogenesis
b. asymmetric cues to establish embryo's anterior-posterior & dorsal-ventral axes
2. Eggs are assembled in a complex, multi-stage process called OOGENESIS.
a. Eggs are very large cells! They range from 100 µm (humans) to 1 mm (frogs,
fish, fruit flies) to 1-10 cm (birds and reptiles). So even human eggs, although
they seem small in comparison are 5x a typical somatic cell (20 µm)
b. Eggs contain large stores of nutrients, called the yolk. Allow at least earliest
embryogenesis & often entire process.
c. Eggs contain large stores of macromolecules - machinery to synthesize &
package DNA & assemble cells. (macromolecules include mitochondria, RNA/DNA
polymerases, ribosomes)
d. Eggs are enclosed in a coat or shell to:
i. protect contents from damage & desiccation
ii. regulate sperm entry.
3. In many animals eggs & sperm- the "germ cells"- arise from a special cell
lineage known as the germ line. Gonads also contain specialized somatic cells.
a. The germ line lineage often is set aside very early in development. Its
descendants are the only cells able to generate a complete animal. Often specific
determinants are segregated to specific embryonic cells which go on to form the
germ line. This is an example of the segregation of determinants we talked
about last time when we discussed how cells can asymmetrically receive info
from their parents=P granules in C.elegans.
[in figure, DNA stained blue using Hoescht dye, P-granules labeled with a fluorescent
antibody to a particular protein found in them]
Question - How does one single egg cell make all the stuff needed to start
development? Answer - with support from her friends... Lets look at how eggs
are assembled in flies
b. In Drosophila, mitotic sisters (nurse cells) synthesize macromolecules and
pump them into the egg. Germline stem cells divide 4 times, leading to 16 cells
interconnected by cytoplasmic bridges. So the nurse cells are mitotic sisters of
the oocyte.
c. Nurse cells make macromolecules that are delivered to the developing oocyte
through CYTOPLASMIC bridges (like gap junctions only larger!)
i. Components of protein, RNA, & DNA synthetic apparatus ie. ribosomes
ii. mRNAs differentially localized to particular parts of the oocyte. We’ll come
back to how asymmetric distribution of certain RNAs helps to establish the
anterior-posterior axis in the embryo in a minute.
d. Somatic cells called follicle cells form an epithelium that surrounds egg.
i. Produce nutrients that get pumped into the nurse cells and oocyte via GAP
JUNCTIONS
ii. Regulate import of other nutrients
iii. May secrete eggshell
iv. May regulate egg development by cell:cell signaling.
e. All this help also makes it easy to have asymmetric (uneven) distribution of egg
contents that can influence embryonic development.
4. Asymmetric distribution of egg contents influences embryonic development.
Example #1- Fruit fly oogenesis.
a. Molecules asymmetrically localized into one cell, the oocyte, before fertilization
b. Nurse cells deliver mRNAs to the developing oocyte through CYTOPLASMIC
bridges
i. mRNAs differentially localized to particular parts of the oocyte- the front
or anterior end (bicoid) and the back or posterior end (nanos). This
asymmetric distribution helps to establish the anterior-posterior axis in
the embryo.
c. Localized maternal mRNAs also help establish body axis in vertebrates. Ie. Vg1
mRNA is asymmetrically localized
d. Follicle cells also regulate egg development. Specific follicle cells exchange
signals with regions of the underlying oocyte, to help establish the future dorsalventral axis of the embryo.
e. For example, during development the nucleus of the oocyte travels to what will
be the dorsal side of the embryo. Gurken mRNA is transcribed and translated
into gurken protein by the oocyte but is localized only on the dorsal side. This
signal interacts with torpedo protein (produced by the follicle cells), which
causes these follicle cells to take on a dorsal cell fate, and inhibits pipe protein
synthesis in these cells. On the ventral side, the follicle cells don’t receive
Gurken (it doesn’t diffuse there), so they produce pipe, and a signal transduction
pathway is activated to cause ventral cell fate. [There is also evidence that
Gurken mRNA is also transported to the oocyte by the nurse cells, so this
suggests that some dorsal/ventral signaling cues are present before fertilization
and transcription of the Gurken oocyte genes.]
5. Physiological control of egg maturation- Example #2- Human oogenesis.
a. Primary oocytes mature to prophase of meiosis I during embryonic
development. They arrest at this stage, surrounded by a single layer of follicle
cells, or primordial follicle. {2 million present at birth but only ~400 used during
a woman’s lifetime}
b. A fraction mature to developing follicles: the oocyte is enlarged & has a shell
called the zona pellucida, & the follicle cells are multi-layered.
c. After puberty, monthly surges of Follicle-stimulating hormone (FSH) from
pituitary gland leads to growth of a group of developing follicles and the
formation of receptors on follicle cells that recognize Luteinizing hormone
(LH). Slightly later, the pituitary produces LH, which leads to follicle maturation
d. LH production regulated by environmental cues (e.g. day length) in most
mammals. Most mammals go through this cycle once/year, humans go through
it monthly.
e. Maturation is the completion of meiosis I & release of a single (usually) mature
follicle from surface of the ovary, down the fallopian tube for potential
fertilization.
f. Follicle cells then produce estrogen & progesterone which cause Uterine &
cervical changes, and feedback leads to a reduction in FSH.
g. What hormone do pregnancy tests measure? hCG (human chorionic
gonadotropin)
h. RU486, a drug used to terminate pregnancy, blocks progesterone receptors and
stops uterine wall from thickening and prevents implantation of the blastocyst.
The hormone progesterone is needed to support pregnancy, so this drug can be
effective up to 49 days.
Interesting misconceptions about sperm: Anton van Leeuwenhoek, the famous Dutch
microscopist who discovered sperm (1678) thought they were parasites living in the semen!
Nicolas Hartsoeker, another Dutch microscopist and co-discover of sperm, thought each
sperm contained a preformed human or “homunculus”. Leeuwenhoek believed that sperm
were like seeds and that females merely provided ‘the soil’.
6. SPERM are small and motile and made by the male.
a. Unlike eggs, Sperm do not need all the nutrients and building blocks, they
need to MOVE to get to the large sluggish eggs. So they develop differently.
b. SPERMATOGENESIS differs from oogenesis in several ways:
i. Cells enter meiosis CONTINUALLY from puberty on.
ii. Each cell that starts meiosis gives rise to 4 GAMETES, not just one
iii. Much of sperm differentiation occurs after meiosis is complete.
iv. Sperm differentiation makes a highly specialized structure with a head
and tail
v. it can move
vi. in the head is an acrosomal vesicle important for accessing the egg
NEXT LECTURE: sperm and egg hook up