Amphibian Reproduction

Publisher Summary

The Variety of Amphibian Reproduction
Eggs sectioned within the oviduct show the jelly as a transparent coating just outside the vitelline membrane. Publisher Summary The female reproductive system of amphibians, as in most vertebrates, consists of paired ovaries and oviducts. Amphibian eggs are protected by a jelly-like substance before hatching, unlike the hard or semi-hard shells of reptiles and birds. Select a subject to preview related courses: Upgrade to Premium to add all these features to your account! This means that the limited amount of energy available to the comatose frog is used in a more efficient manner.

Highlights

How Do Amphibians Reproduce?

There may be quite a few hours between the time of appearance of the first and the last eggs in the uteri. The maturation process can best be described as it begins, immediately after the normal breeding season in the spring. At this time the ovary has been freed of its several thousand mature eggs and contains only oogonia with no pigment and little, if any, yolk.

Even at this early stage each cluster of oogonia represents a future ovarian unit, consisting of many follicle cells and one ovum. There has been no way to determine which oogonium is to be selected for maturation into an ovum and which will give rise to the numerous follicle cells that act as nurse cells for the growing ovum.

It is clear, however, that both follicle cells and the ovum come from original oogonia. All ova develop from oogonia which divide repeatedly.

These pre-maturation germ cells divide by mitosis many times and then come to rest, during which process there is growth of some of them without nuclear division.

These become ova while those that fail to grow become follicle cells. However, there are pre-prophase changes of the nucleus of the prospective ovum comparable to the pre-prophase changes in spermatogenesis. The majority of oogonia, therefore, never mature into ova, but become follicle cells. The process of maturation involves contributions from the nucleus and the cytoplasm. First, chromatin nucleoli aid in the synthesis of yolk, and second, the breakdown of the germinal vesicle allows an intermingling of the nuclear and the cytoplasmic components.

Only a small portion of the germinal vesicle is involved in the maturation spindle so that it may be at this time that the nucleus exerts its initial influence on the cytoplasm.

All cytoplasmic differentiations must be initiated at a time when the hereditary influences of the nucleus are so intermingled with it. Growth Period to Primary Oocyte Stage. Growth is achieved largely by the accumulation of yolk. As soon as growth begins the cell no longer divides by mitosis and is known as an oocyte rather than an oogonium.

The growth process is aided by the centrosome, which is found to one side of the nucleus, and around which gather the granules or yolk platelets. The chromatin filaments become achromatic and the nucleoli increase in number, by fragmentation, and become more chromatic.

Many of the nucleoli, which are concentrations of nucleo-protein, pass through the nuclear membrane into the surrounding cytoplasm during this period. It is not clear whether this occurs through further fragmentation of the nucleoli into particles of microscopic or sub-microscopic size, and then their ejection through the nuclear membrane. It may occur by the loss of identity and chromatic properties by possible chemical change and subsequent diffusion of the liquid form through the membrane to be resynthesized on the cytoplasmic side of the membrane.

During the growth of the oocyte, further nucleoli appear within the nucleus, only to fragment and later to pass out into the cytoplasm.

The presence of chromatic nucleoli in the cytoplasm is closely associated with the accumulation deposition of yolk. The granules within the cytoplasm extruded fragments of nucleoli function as centers of yolk accumulation and have therefore been named "yolk nuclei. It is not a true cell nucleus.

The centrosome and other granular centers lose their identity and the yolk granules then become scattered throughout the cytoplasm. The source of all yolk for the growing ova is originally the digested food of the female.

This nutrition is carried to the ovary by way of the blood system and conveyed to the nurse or follicle cells and thence to the oocyte. The yolk is at first aggregated around yolk nuclei, then concentrated to one side of the nucleus. Finally it assumes a ring shape around the nucleus between an inner and an outer zone of cytoplasm. Subsequently the nucleus is pushed to one side by the ever-increasing mass of yolk so that eventually there is an axial gradient of concentration of oval yolk platelets from one side of the egg to the other.

The smaller platelets are found in the vicinity of the nucleus, in the animal hemisphere. The larger platelets are located toward the vegetal hemisphere. There is an increase averaging from to per cent in the total lipoid substance, neutral fat, total fatty acids, total cholesterol, ester cholesterol, free cholesterol and phospholipin content of the ovaries of Rana pipiens occurring during the production and growth of ova Boyd, The primary oocyte may show a slight flattening of the surface directly above the region of the nucleus.

These growth changes and the unequal distribution of pigment, yolk, and cytoplasm are the first indications of polarity or a gradient system within the egg. When the polarity is well established, the cytoplasm, the superficial melanin or black pigment, and the nucleus are all at the animal hemisphere pole. The light colored yolk is more concentrated toward the vegetal pole.

The egg is then regarded as a telolecithal egg. During this phase of egg maturation there is a drain on the metabolism of the frog which requires an excess of food intake because the materials for egg growth must be synthesized from nutritional elements received from the vascular system of the female.

For Rana pipiens this period of most active feeding comes during the summer when the natural foods, insects, worms, etc. During the growth of the oocyte in general there are important changes occurring within the nucleus germinal vesicle of the egg. Thirteen pairs of chromosomes may be seen in synizesis contraction , converging toward the centrosome at the "yolk nucleus" stage. A little later the nuclear membrane develops sac-like bulges, the nucleoli are scattered, and there is a colloidal chromosome core which almost fills the entire nucleus.

The chromosomes themselves are small and almost invisible. When the primary oocyte is about half its ultimate size, there appear definite sacs on the nuclear surface. The fragmented nucleoli are located at the periphery of the lobulated nuclear membrane, and the chromosome frames have become relatively large.

The chromosomes, by this time, have reached their maximum length and possess large lateral loops. Finally, in the fully grown nucleus of the primary oocyte the nuclear sacs are very prominent, and the nucleoli appear in clusters in the center of the egg, surrounding the chromosome frame. This frame is a gel structure which gives rise to the first maturation spindle, containing 13 pairs of slightly contracted chromosomes. These structural features can be observed in the living germinal vesicle if it is removed from the oocyte and placed in isotonic and balanced salt medium, omitting the calcium ion.

A minute amount of NaHoPO, is added to shift the pH toward the acid side, which makes the chromosomes the more visible. Or, the chromosomes may be.

Stained with crystal violet in a calcium-free medium. Amphibian cells are among the largest in the animal kingdom and the frog's egg nucleus is large enough to see with the naked eye.

It can be removed with considerable ease and examined beneath the binocular microscope. Before the time of hibernation the eggs that are to be ovulated for the next spring are in the fully grown primary oocyte stage, having their full complement of yolk, cytoplasm, and pigment.

Externally more than one-half of the egg appears densely black, due to surface pigment granules, while the rest is creamy white. The nucleus is prepared for the maturation divisions. Such an egg measures about 1. The surface layer of the amphibian egg is formed before fertilization and it is definitely not hyaline, as it is in some Invertebrate eggs.

It contains many small yolk grains and irregular accumulations of spherical, black pigment granules. With each cleavage, subsequent to fertilization, this superficial coat is divided between the blastomeres, being an integral part of the living cell.

There is no clear-cut demarcation between this surface coat and the inner cytoplasm and yolk. It is believed that these growth changes of the egg are under the influence of the basophilic cells of the anterior pituitary gland, which cells are greater in number at this time than at any other.

During the growth period the vitelline membrane appears on the surface of the oocyte as a thin, transparent, non-living, and closely adherent membrane. It is formed presumably by a secretion from the egg itself, aided by the surrounding follicle cells.

It appears to be similar in all respects to the membrane of the same name found around the eggs of all vertebrates. Ovulation, or the liberation of the egg from the ovary, is brought about by a sex-stimulating hormone from the anterior pituitary gland. Just before and during the normal spring breeding period there is a temporary increase in the relative number of acidophilic cells in the anterior pituitary.

It is believed that this is not coincidental but a causal factor in sex behavior in the frog. However, until such time as extracts of specific cell types can be made, this will be difficult to prove conclusively.

Attempts on the part of the male to achieve amplexus are resisted by the female not sexually stimulated. However, such a female can be made to accept the male by injecting the female with whole anterior pituitary glands from other frogs. It is very probable that environmental factors such as light, temperature, and food may act through the endocrine system to prepare the frogs for breeding when they reach the swampy marshes in the early spring, after protracted hibernation.

The pituitaries of the hibernating frogs do contain the sex-stimulating factor, but apparently to a lesser degree than the glands of frogs approaching the breeding season. The injection of 6 glands from adult female frogs will cause an adult female of Rana pipiens to ovulate as early as the last week in August, some 8 months before the normal breeding period.

One or two such glands will accomplish the same results if used early in. Another explanation for this may be offered, namely that the ovary itself may become more sensitive to such stimulation as the breeding season approaches. The ovulation process itself consists of the rupture and the emergence of eggs from their individual follicles.

The surface of the egg, separated from the body cavity by only the non-vascular theca externa, is first ruptured and then the egg slowly emerges through the small opening. Since the egg is known to contain a peptic-like enzyme, it is believed that the pituitary hormone may activate this enzyme to digest away the tight and non-vascular covering. Then by stimulation of the smooth muscle fibers of the cyst wall theca interna the process of emergence is completed.

The relation of the pituitary to smooth muscle activity has long been established clinically. It is true that the ovarian stroma shows undulating contractions at all seasons, irrespective of sex activity. An egg will emerge at any time from a surgically ruptured follicle. If an ovulating female is etherized and the body cavity is opened, the ovary may be removed and placed in amphibian Ringer's solution and the ovulation process may be observed directly.

This will go on for several hours after all connections with the nerve and blood supply are cut off. From the initial rupture of the theca externa until the egg drops free into the body cavity there is a lapse of from 4 to 10 minutes at ordinary laboratory temperatures.

The first maturation division occurs at the time of ovulation. The heterotypic chromosomes i. Movement of the chromosomes is identical with that found in ordinary mitosis. The outermost group of telophase chromosomes are pinched off, with a small amount of cytoplasm and no yolk, to comprise the first polar body. The innermost telophasic group of chromosomes remain within a clear i.

These changes occur as the egg leaves the ovary and before it reaches the oviduct. Possibly the same forces which bring about follicular rupture also influence this maturation process. The second maturation division begins without any intermediate rest period for the chromosomes, at about the time the egg enters the oviduct. There may be a variation in time up to 2 hours for eggs to reach the ostium, depending upon the region of the body cavity into which they are liberated.

Thus the stage of maturation of different eggs within the oviduct may vary considerably. There is a longitudinal division of the chromosomes of the egg which are lined up in metaphase on the second maturation spindle, the axis of which is at right angles to the egg surface. Since the spindle is primarily protoplasmic, and is made up in part of fibers which may be contractile , the space occupied by the spindle will be free of yolk.

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First Name Name is required. Last Name Name is required. Phone Number Don't worry. We won't call unless you want us to. Phone number is required. Initially the colour of the ovary is yellowish with small black spots. The lumen of the ovary is part of the coelom. It is filled with coelomic fluid. During the breeding season the wall of the ovary becomes studded with a large number of ovarian follicles.

Each Ovarian Follicle contains a developing Egg. The ovarian follicles project towards the lumen of ovary. Such an ovary greatly enlarges. It attains black color with light yellow spots. Each oviduct is a long narrow and highly coiled tube. It is divided into three parts in accordance with its structure and functions.

The anterior end of the oviduct forms a wide and fringed oviducal funnel. The ovoiducal funnel is located on the dorsal side of the lung. The margin and inner surface of the oviducal funnel is lined by ciliated epithelium. The oviducal funnel leads into the oviduct. This oviduct is straight and thin-walled for a short distance.

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