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Stages of Egg Development - Background

Edited from a publication by Dr. F. Velsen, Pacific Biological Station

Salmon are cold-blooded. Their body temperature remains close to that of the water in which they live. The salmon's ability to swim, the amount it eats, how fast it grows and the rate at which its eggs develop once they are deposited are all affected by water temperature. The lower the water temperature the longer it takes the eggs to hatch.

Egg development in all five species of Pacific salmon is similar. At a constant temperature of 10C the incubation period among eggs of the five species of salmon ranges from about 47 - 65 days.

There are thirty stages of embryonic development. These stages can be grouped into three phases.

Description of Developmental Stages

The egg is fertilized when one spermatozoan from the milt of the male finds and enters the micropyle, a narrow canal in the capsule. Thereafter, other spermatozoa are prevented from entering the micropyle by a series of events triggered by the entry of the first sperm. All eggs, fertilized and unfertilized, react to the water in which they are deposited. This action involves a swelling and increase in internal pressure (hardening), as water is taken up by the egg. The embryo begins to develop after fertilization. The development may be divided into three phases, cleavage, epiboly, and organogenesis. These phases are described in the following sections.

Cleavage

Cytoplasm is a colorless cell fluid. After fertilization the cytoplasm moves over the surface of the yolk. This fluid concentrates at the animal pole. The cytoplasm rounds up and rises slightly to form a hemispherical dome. This forms the first cell of the embryo. It is known as the blastodisc.

In the fertilized egg, cell division begins with the first cleavage of the blastodisc. It divides to form two cells. Each of the two new cells divide to form four cells. The next divisions form 8, 16, and 32 cells. The first divisions occur in the horizontal plane. After the 32 cell stage some divisions occur in the vertical plane and it is difficult to identify the cell numbers.

As cleavage continues the cells become smaller and smaller. After the first 32 cells are formed, the early and morula stages are formed. The morula begins to have a granular appearance. The individual cells may still be seen but become difficult to count. At this stage the future location of the embryo can be detected on one side of the disc.

Epiboly

The second phase begins with the formation of the early embryo. During this phase the cells formed in the cleavage phase begin to specialize to form tissues. A layer of cytoplasm called the periblast covers the yolk. The edge of the blastodisc expands and grows down over the surface of the yolk. The overgrowing edge, or germ ring, eventually envelops the yolk. This is part of the epiboly process.

The tissues formed by the germ ring overgrowth become first the yolk sac and later the tissues enclosing the body cavity of the young fish.

At the same time the organization of the embryo is becoming more clear. The head of the developing embryo is at the original animal pole. The beginning of the tail is near the border of the germ ring. Development at this phase can be seen in terms of germ ring overgrowth or epiboly. As epiboly nears completion, an area below the future tail remains open. This area, called the yolk plug, will get smaller in size and finally close. It will become the vent or anus of the developing embryo.

When the germinal layer has overgrown one third of the yolk (1/3 epiboly) the head can be seen. Some somites, future muscle tissues, in the trunk or body region are forming on either side of the spinal cord. The optic vesicles, the developing eyes, may be detected at one-half epiboly. By two-thirds epiboly the eyes are formed. The otic placodes which develop into balance organs are forming.

At the end of the second phase the cells formed during cleavage have turned into tissues which form the basic structure of the embryo.

Organogenesis

The third and final phase of development during egg incubation begins with the appearance of fins. The formation of the internal organs occur during this phase.

Immediately after the yolk plug closure the posterior end of the embryo extends to form the caudal bud. This 'bud' lifts free of the surface of the yolk. The caudal bud becomes the caudal or tail fin.

The circulatory system begins to develop during the third phase. The vitelline vein emerging from the left side of the head region encircles one-quarter of the yolk. The development of a network of blood vessels covering the surface of the yolk is called yolk vascularization. When three-quarters of the yolk surface has become vascularized the head is free, the eyes are fully pigmented, and the eggs are said to be "eyed".

Following the eyed stage blood vessels continue to develop on the yolk. The head of the embryo rests on, but is free of, the yolk sac. For the next few stages the operculum, or gill cover, begins to grow to cover the branchial arches on each side of the head. The bronchial arches will form the gills of the hatched fish. When all of the arches are covered, the paired fins formed, and the embryo fully pigmented, the embryo is complete. It does not hatch but remains in the capsule.

The embryo secretes a hatching enzyme. The enzyme digests the capsule from the inside. This enzyme weakens the capsule until it ruptures to free the now very active embryo. Once hatched, the embryo with its large yolk sac is called an alevin. Hatching usually is rapid once the capsule ruptures. However, all eggs do not hatch at one time, even though they have had identical temperatures. In a hatchery tray of eggs incubated at one temperature, the eggs may hatch over a period of a few days or more.

Stages of Egg Development

This table shows embryonic development in the sockeye egg. In the left column are 30 stages of development from fertilization to hatching and characteristics identifying each stage (Vernier 1969). These 30 stages are divided into three developmental phases: cleavage (cell division), epiboly (tissue formation), and organogenesis (organ formation). The three columns on the right show the times (hours, days, degree-days, or ATU's) to reach various stages at temperatures of 5C, 8C and 11C.

Stage 2	Stage 3	Stage 4
Stage 5	Stage 13

Stage 17 side view

Stage 17 top view

Stage 22

Stage 30

Hatching

As the embryo grows it starts to move as well. The first thing that is usually seen moving on an embryo is the heart - more accurately, the heart sac. This heart sac starts to contract. Then slowly it starts to move other parts of the body. Twitches are evident throughout the embryo. After the twitching of the body itself the tiny developing pelvic and pectoral fins begin twitching. The fanning motion of the fins is very important to the development of the embryo.

The hatching process is a result of a variety of activities that the embryo is going through. In addition to these activities the chemical and enzymatic readiness of the embryo to hatch is crucial to a successful hatch.

The embryo grows to such an extent that it needs more and more oxygen until finally it becomes desperate in its oxygen use. Through the increased need for oxygen, it starts to develop hatching enzyme glands. These glands are spread over the whole body but are concentrated primarily on the head and near the operculae (gill covers) on the side of the head. The hatching glands are triggered to release their contents when the oxygen shortage becomes acute. The glands spread their contents through the vitelline fluid that surrounds the embryo. The embryo assists this essential spreading of fluids by the fanning motion of the developing fins. The fanning produces the right mix of enzymes throughout the entire body of the embryo.

The enzymes within the zona radiata (capsule) gradually weaken the membrane. At the same time, the embryo is struggling for oxygen and finds the egg shell limiting. When the embryo increases its stretching and pushing movements it will burst the weakened membrane.

An embryo which has not been actively wriggling and squirming, because of a number of genetic or environmental reasons, will have released the hatching enzyme but the enzyme will not have been distributed throughout the body. The enzyme will have remained concentrated locally around the region of the head. These embryos may attempt to push and stretch but only their head will break through the shell. Since they are incapable of wriggling out of the capsule, they die - with only their heads out of the capsule.

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