Finfish Breeding and Hatchery Management

Unit 2 - Sexual maturity, breeding season and development of gametes

2.3. Gamete maturation and release in females

Gametes develop and mature in the female fish through a series of stages, before being released. The stages are given below

1. Oogenesis

• Oogonia are the cells that give rise to oocytes through a continuopus process called oogenesis and are found throughout the life of a fish.
• Oocytes are produced from oogonia as a result of meiosis (reduction division).
• Afer the first meiosis the oocytes become surrounded by a layer of epithelial cells called the follicle.
• After this the oocyte enters a long stage of cytoplasmic growth.
• In fish that breed more than once, oogenesis goes on at various rates throughout the fishes life and is not under the control of hormones (GtH).

2. Primary oocytes growth

• The early development of the follicle and its oocyte is independent of pituitary GtH.
• The growth is due mainly to proliferation of cellular components.
• By the end of this stage, the typical teleost oocyte increases several hundred times in size to a diameter of 100-200µm and is called pre-vitellogenic oocyte.
• During the growth period, the follicle cells differentiate to form glandular granulose, separated from oocyte by a zona pellucida and surrounded by an outer theca.
• These cells play an important role in steroidogenesis.
• The primary growth process continues throughout the life of fish that are multiple spawners and previtellogenic oocytes are present in the ovary year round.
• If only the previtellogenic oocytes are present, the ovary is considered immature, which may be became of age or season.

3. Yolk vesicle formation

• In response to environmental cues- change in day length, temperature or rainfall, there will be a surge in the GtH levels that induce previtellogenic oocyte to develop further.
• The first sign of this stage is the appearance of yolk vesicles in the oocyte cytoplasm.
• There contains glycoproteins formed within the oocyte and will eventually become cortical alveoli (may be a source of energy for the embryo) that will be expelled into the previtlline space around the egg after fertilization.
• This process is some times referred to as “endogenous vitellogenesis.”
• Since there oocytes contain neither true yolk nor vitellogenin, it is better known as “yolk vesicle formation” which is triggered by GtH.

4. Vitellogenesis

• Sequestration of the phospholipids, vitellogenin (Vg) from the blood stream and accumulation of true yolk in yolk globules takes place after yolk vesicle formation. This represents the major growth of the oocytes and is known as vitellogenesis.
• It involves synthesis of Vg (yolk protein precursor) in the liver, its dlivery to the oocytes via bloodstream and uptake and chemical alterations to form yolk protein.
• GtH induces the thecal cells of the follicle to produce testosterone (T), which in turn is converted to an estrogen, 17β -estradiol (17βE2), in the granulosa cells.
• E2 travells to the liver in the blood and stimulates production of Vg, which travels to the oocyte, also by the blood stream.
• Oocytes sequester Vg as yolk protein in yolk globules and increase in size and this is facilitated by GtH.
• The plasma levels of GtH is high during this phase of maturation. T and E2 act on the pituitary in “feed back loops” to regulate GtH release.
• Vitellogenesis can be triggered and accelerated by environmental manipulation and hormonal manipulation.

5. Steroid switch and final maturation

• Final oocyte maturation in many warm water species is rapid- it usually takes less than 24hrs.
• It involves resumption of meiosis, migration of GV (Germinal vesicle) to the edge of the oocyte and GVBD (Germinal vesicle break down), an event that is useful in inducing the oocyte maturity.
• Meiosis then stops again and the oocyte is now mature, ready for expulsion from the follicle (ovulation).
• During this stage, the level of GtH increases and GtH stimulates the follicle to produce maturation inducing steroid (MIS) instead of estradiol.
• MIS is a form of progesterone called 17α - hydroxy progesterone (17α-OHP) or 17α, 20 β- dihydroxy progesterone (7α-20β-diOHP).
• At this stage the enzyme responsible for the production of E2 is inhibited. How this steroid switch is regulated is not known.
• The MIS induces a number of visible changes in the oocyte during final maturation.
• In addition to GV migration and breakdown, it causes an increase in oocyte diameter due to the uptake of water (hydration) in to the cytoplasm and changes in the appearance of the yolk.

6. Ovulation

• Final maturation is followed by ovulation- the release of egg from its follicle into the ovarian lumen and is ready to be expelled into the surrounding water for fertilization; ovulation is controlled by prostaglandin (PG) under the influence of GtH.
• PG is produced by the follicle and the oviduct; PG are cyclopentane fatty acids.
• Once ovulated the eggs of different species remain fertile within the ovary or body cavity for periods of <1 hr to several days.
• After this time the eggs become “overripe” and start to disintegrate.
• In fish, eggs remain fertile only briefly, (few hours), eg. Carps.
• Eggs remain fertile for days, eg. Salmonids
• Over-ripening of eggs held in the body after ovulation are highly temperature dependent.
• In hormone induced spawning, final maturation and spawning are the most important stages of reproduction.

Mature female with orange/yellow ovary