LAB EXERCISE 2 - Oogenesis

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ABI 3110 – Developmental Biology Laboratory

Laboratory Exercise No. 2
GAMETOGENESIS - OOGENESIS
The reproductive cells, called gametes, develop in the gonads. More specifically, spermatozoa
(or sperm) form in the seminiferous ampullae or seminiferous tubules of the testis, and ova (or eggs)
form in the follicles of the ovary. Two major processes occur during gametogenesis: (1) reduction
division, or meiosis, and (2) the acquisition of cellular specializations needed for fertilization and
embryogenesis.
In females, completion of meiosis I yields one secondary oocyte and the tiny first polar body;
completion of meiosis II yields a single haploid ovum and the second polar body (which is also haploid).
The first polar body may also undergo meiosis II to yield two tertiary polar bodies that are genetically
equivalent to the mature ovum, although they lack any reproductive future because their tiny size limits
the amount of yolk and other materials needed for normal development.
Unlike spermatozoa, eggs are large, spherical, nonmotile cells. Within any given species, a
mature egg, or ovum, is a larger cell than a spermatozoon chiefly because it contains the energy
reserves and other materials in its cytoplasm to initiate embryonic development. Beyond that
generalization, the sizes of vertebrate egg cells vary enormously: a human egg is about 0.15 mm in
diameter, whereas the egg of a coelacanth (Latimeria) reaches 90 mm in diameter. This 600-fold
difference in linear dimensions translates to a 200 million–fold increase in the total volume of materials
inside the egg! Ova develop in follicles within an ovary. All egg cells have, in addition to their plasma-
cell membrane, a primary egg cell membrane immediately surrounding them. This extracellular, largely
proteinaceous structure is secreted by the egg cell or by its surrounding follicle cells during oogenesis.
The primary egg cell membrane has different names in different groups of vertebrates. One frequently
used term is vitelline membrane. In mammals, the primary egg cell membrane is known as the zona
pellucida. In many actinopterygian fishes, it is toughened to withstand a harsh external environment
and is known as a chorion. The thick chorion of actinopterygian fishes has special openings, termed
micropyles, to admit spermatozoa. In some vertebrates, layers of follicle cells adhere to and surround
the primary egg cell membrane. In mammals these adhering follicle cells are known as the corona
radiata, and they pose an additional structural barrier that a spermatozoon must penetrate before
fertilizing the egg.
The cellular organization of an ovum can be exceedingly complex. The cytoplasm of egg cells
has a thick, gelatinous cortex, which is the zone of cytoplasm lying immediately adjacent to the egg’s
plasma membrane. The cortex often contains pigment granules and cortical granules, which are
important in fertilization, as described later. The interior cytoplasm of an egg cell may be more fluid
than its cortex, but it is still highly structured and packed with materials needed for development. Ova
contain variable amounts of yolk, which is composed primarily of protein, phospholipids, and neutral
fats. The process of yolk deposition in the egg cell is termed vitellogenesis, and depending on the
amount of yolk, vitellogenesis may require many months or even years. Yolk of vertebrate eggs is
usually synthesized exogenously (i.e., not in the oocyte itself) in the mother’s liver and delivered to the
ovary via the circulatory system. Once inside an egg cell, much of the yolk is organized into organelles
known as yolk platelets.
The amount of yolk in an ovum determines how long the embryo will be nourished by food
stored in the egg. There is little yolk in the microlecithal eggs (Gr., mikros –small; lekithos-yolk) of
amphioxus. These eggs hatch very soon into larvae that do not feed and that quickly metamorphose
into feeding juveniles. Nonteleostean actinopterygian fishes, lungfishes, and amphibians have an
intermediate amount of yolk in their mesolecithal eggs. Their eggs hatch into feeding larvae, which
then metamorphose into juveniles. The eggs of most teleost fishes are physically small but have a very
high proportion of yolk, which affects their cleavage. Like other actinopterygians, teleosts follow the
pattern of having feeding larvae. Much yolk is present in the large macrolecithal eggs of chondrichthyan
fishes, coelacanths, reptiles, birds, and monotreme mammals, the embryos of which develop into
miniature adults before hatching or birth. We also can describe the organization of the yolk itself. The
term plasmolecithal (Gr., plasmo = fluid) describes an ovum in which the yolk is in suspension in the
cytoplasm. In contrast is the descriptor telolecithal (Gr., telos = end), in which a great deal of yolk is
packaged into yolk platelets.
Materials in an egg are not distributed randomly but establish and follow gradients. The small
amount of yolk in the microlecithal egg of amphioxus is evenly distributed. In vertebrate eggs, yolk
usually is most concentrated toward one end, the vegetal pole, and least concentrated at the other
end, the animal pole. This gradient is particularly evident in macrolecithal eggs, where the egg nucleus
and most of the cytoplasm are restricted to the animal pole. The future anteroposterior axis of the
embryo is not the same as the egg axis but is related to it and is usually finally determined by the point
of entry of sperm. Often the anterior end of the embryo is determined about midway between the
animal pole and the equator of the egg.
In addition to yolk, vertebrate eggs are packaged with cytological necessities for early
development of the zygote. Billions of ribosomes (related to protein synthesis) and thousands of
mitochondria (related to ATP production) can be packed into a frog egg only a few millimeters in
diameter. These organelles provide for future protein synthetic and energy needs of the embryo. Some
mRNA transcripts of genes for later translation into proteins needed by the zygote are already
prepositioned in the egg cell’s cytoplasm. Still other information-containing molecules, such as the
germinal granules of frogs, already are localized to parts of the oocyte, where they will help determine
the differentiation of the germ cells in the adult.
At ovulation, an egg is discharged from its follicle and the ovary. A stimulus of some sort is
needed to activate the egg and initiate further development. Normally, the stimulus is sperm
penetration, but other chemical or physical stimuli are effective in some species, in which case the egg
develops parthenogenetically (i.e., without the contribution of the DNA of the sperm). If an egg is not
activated within a few hours of ovulation, its delicately balanced internal organization breaks down,
and the egg degenerates.
I. ILLUSTRATIONS
A. Observe the frog ovary and take note of the following structures: theca interna, theca externa (10x),
follicle cells, vitelline membrane, germinal vesicle, nucleolus, oocytes and oogonia (40x). Draw and label
the above-mentioned parts.
B. Draw and label the cross section of the cat ovary (using HPO). Label all the oogenic cells.
C. Draw the mature Graafian follicle and label the parts.
D. Label the parts of the human ovary illustration.

II. QUESTIONS FOR RESEARCH. Make sure to provide your references. Use CBE or APA format.
A. Make a table showing the differences of amphibian and mammalian ovaries and oogenesis.
Amphibia Mammalia
Ovary
• Structure
• Function
Oogenesis
• Oogonia
• Ooocyte
• Zona pellucida
• Antrum
• Menstrual Cycle
B. How do the eggs ovulated into the abdominal cavity find their way out of the frog body?
C. What are the exact roles of the theca interna and the follicle cells in the estrogen production?
D. The corpus luteum regresses in about 9 days. Why does it regress? Briefly discuss the process of
regression.
E. Provide the roles of two sex hormones produced by the female vertebrate body.
F. Briefly discuss the events that lead to only one or few oocytes being ovulated despite the many
oogenic cells that attempt to mature.

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ABI 3110 – Developmental Biology Laboratory

C. Draw the mature Graafian follicle and label the parts.

D. Label the parts of the human ovary illustration.

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