Download Gametogenesis

Meiosis, sperm and egg production
Source: S. Gilbert Developmental Biology 7th Edition, Sinauer Associates Chapters 19 a and 7)
and as indicated
Last update 1/25/04 this replaces notes at the end of the gametogenesis section
From chapter 7 we discussed the homunculus (fig 7.1) and the ancient androcentric view of the
roles of eggs and sperm. Yet sperm, in mammals contribute very little material other than DNA
to the zygote. True, fertilization gets development started but in some species, parthenogenic
lizards for example even sperm are not required.
But in most species meiosis- the idea is to get haploid (most of the time) – is required.
many eggs or oocytes arrest in meiotic prophase (we looked at figure 7.5) or at other times in
meiosis and are not reactivated until fertilization.
Figure 7.2 modification of a mammalian germ cell to become a sperm. We discussed the
formation of the acrosome from the Golgi, the tail or flagellum elaborated by the centrioles, the
progressive reduction of cytoplasm and the rearrangement of mitochondria (ion) at the neck. In
sperm production, all of the meiotic products are used.
In comparison, eggs illustrated by the sea urchin egg in figure 7.4, have a lot more stuff in them
(and around them: see jelly coat that attracts sperm in this case). In many species only one of the
meiotic products becomes the ovum, the rest have their cytoplasm co-opted and become polar
bodies. In some species the polar bodies remain associated with the egg. (figure 19.22, meiosis
in mouse)
Mammalian Spermatogenesis (19.19)
Human males produce 10 12 to 10 13 sperm in a lifetime. Those not ejaculated are resorbed or
passed out in urine.
Sperm are produce in the seminiferous tubules, collected by the rete testis which coalesces into a
single very long coiled tube called the epididymis. Testicular sperm, in many species still have a
droplet of cytoplasm, frequently cannot swim and cannot fertilize an egg. Epididymal transport,
which can take as long as two weeks, matures the sperm, removes cytoplasmic droplet and adds
coating proteins to the surface, the sperm are now actively motile. When the sperm are exposed
to vaginal secretions and mechanical stress, they become capacitated, changing their swimming
pattern. It is only those sperm that have completed all these processes that actually can fertilize.
In the testicle, developing sperm are supported and nourished by Sertoli cells, bound together by
N cadherins (see biology 261) and an enzyme gactosyltransferase (we’ll talk more about this
type of interaction later).
In mammals PCG become type A1 spermatogonia. Under the direction of BMP8b secreted by
these same cells and accumulating until puberty, they divide probably twice to type A2
spermatogonia (one cell is replacement for A1). Divide to A3 and a4, A4 spermatogonia that
either replace themselves, die (apoptosis= programmed cell death) or differentiate into
intermediate spermatogonium. These then divide mitotically once to produce type B
spermatogonia, last cells to undergo mitosis. These generate primary spermatocytes that enter
meiosis. The transition to meiosis under control of glial derived neurotrophic growth factor.
Cohort of sperm mature connected by cytoplasmic bridges. (19.20)
1st meiotic division gives secondary spermatocytes, complete second division and are called
spermatids. Are haploid genetically, but functionally diploid due to these cytoplasmic bridges.
Process takes 65 days in human.
Spermeiogenesis : maturation of the sperm, generation of the acrosome and the flagellum,
cytoplasmic droplet remains. (see 7.2 again) Histones replaced by protamines, protamines are
basic proteins (about 50% arginine) that silence transcription
So until silenced by protamines, genes can be expressed in male meiosis, and in the haploid
condition. Also there are paternal effect mutations.
Oogenesis is widely different in different species! Developing ova are much more active in
terms of gene expression than sperm.
Life styles of ova: some species mature a cohort of oocytes every year, thus must replace stem
cells, for example sea urchins, frogs. Others such as humans mature only some in a lifetime. In
humans 7 million cells are produced by the 7 month of gestation, after this the number drops
precipitously (19.21) Remaining oogonia enter meiotic prophase, and are called primary oocytes,
progress until diplotene stage of meiotic prophase (see description of meiotic prophase stages
earlier in the chapter) and remain there until puberty. At adolescence, groups of oocyte resume
meiosis. About 400 oocytes will mature in a woman' life time.
Unequal divisions of the cytoplasm leading to the formation of the polar bodies (figure 19.22 ,
mouse)
A secondary oocyte is one that has formed its first polar body. An egg is not an egg until the
second polar body is generated, and that may take place at the time of fertilization.
The follicle develops under hormonal control (19.30). Rising estrogen levels mature the follicle.
Layers of follicle cells develop; zona pellucida (glycoprotein coat of the egg) and follicular fluid
expand as oocyte matures. Fluid filled follicle ruptures with FSH and LH spike, ovulation.
Follicle cells associated with egg are cumulus cells. The ovum is swept into the fimbriae of the
fallopian tubes. Follicle cells that remain collapse into corpus luteum that begins secreting
progesterone. Progesterone maintains uterine lining in “Luteal” phase, ready to receive
blastocyst. If the blastocyst does implant the corpus luteum persists to help maintain the uterine
lining during pregnancy. If no implantation, the corpus luteum regresses to become the corpus
albicans.
Figure 19.31 ovulation in the rabbit
We looked at the Vade Mecum sequence on follicle maturation.
I also noted that there is a Parascaris fertilization sequence that would be useful to review before
lab.
To begin on 1/26/04 postponed to 1/28/04
Miracle of Life video: human ovulation, sperm production, fertilization.
Amphibian oocyte maturation and vitellogeneis. 19.23 oocyte cohorts in the frog.
major yolk component is vitellogenen, which is produced in the liver and transported to the
follicle cells of the ovary, which "feed" it to the developing oocyte. This is split into phosvitin,
and lipovitellin inside the oocyte. These are packed in to yolk platelets that accumulate vegetally
in the egg, import occurs all over, but the vegetal yolk platelets do not move actively within the
egg. Cortical granules from Golgi and mitochondria are rapidly produced in large numbers
called a cloud. No new mitochondria will be made until after gastrulation begins
The other things that accumulate in the vegetal region are specific RNAs that specify "vegetal"
factors (see web site 19.10)Vg1 may be responsible for mesoderm determination (this region is
brought "inside" during gastrulation). The other RNAs may be responsible for keeping Vg1 at
the vegetal pole, see Vera They arrive very differently. Xcat2 encodes a protein related to
nanos (posterior determinant) in Dros.
Completion of meiosis (from arrest in pachytene) in amphibians is controlled by progesterone
that initiates a cascade that activates cyclins (see 19.24). Fertilization releases egg from
metaphase arrest. We won’t go into the details of this. Lots of things being made in eggs: figures
show ZPs in mice (19.25) histones (19.26) ribosomes in Xenopus (19.27 shows rRNA
transcription)
Drosophila oogenesis: (fig 19.28)
meroistic
16 cytocytes from connected by ring canals, 2 cells with 4 interacts can enter meiosis, only one
actually becomes the egg. This one has retained a large spectrin (cytoskeletal protein) containing
structure called the fusosome. The other 15 become nurse cells. Both actin filaments and
microtubules are important in maintaining the asymmetric distribution of essential gene products
into the egg and not the nurse cells. Eggs aren’t transcribing but nurse cells are.
Nurse cells feed RNAs to egg. (Figure shows actin, not RNAs) Polarities are set up at this time.