How Do Amphibians Reproduce?

Sexual Reproduction

Book - The Frog Its Reproduction and Development 3
This is the synaptene stage. In other projects Wikimedia Commons Wikispecies. The toxic secretions from their skin was used in brewing evil potions, but was also put to use to create magical cures for human and livestock ailments. Surrounding the entire egg is a non-living surface coat, also containing pigment. Some species of anurans hybridize readily. 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. These body cavity eggs are often quite distorted, due to the fact that the ovulation process involves a rupture of the follicle and forcing out of the egg from a very muscular follicle.

Salamanders and Newts

Amphibian Reproduction

How Do Amphibians Reproduce? Quick Answer Amphibians reproduce by laying eggs, but the fertilization process differs based on the type of amphibian. What Animals Undergo Metamorphosis? How Do Angiosperms Reproduce? Full Answer Unlike mammals that give birth to live babies, amphibians must lay their eggs in a suitable environment in order to reproduce. Learn more about Animal Reproduction. How Do Parakeets Reproduce? This coiled oviduct runs posteriorly along the outer side of the kidney.

The hinder portion of the oviduct becomes very thin walled. It is sac-like and is called ovisac. The ovisac opens of the posterior end in the dorsal wall of the cloaca by its individual apertures lying anteriorly to the openings of ureters.

The cloaca opens to the exterior by a cloacal aperture at the posterior end of the body. The release of ovum in female is termed as spawning. Subscribe to Post Comments [ Atom ]. Subscribe to Posts [ Atom ]. Wednesday, October 31, Reproductive system of frogs. Reproductive system of frogs. The male frog and the female frog can be distinguished even by their external morphological characters.

In the male frog the principal reproductive organs are a pair of testes and the accessory reproductive.

In the female frog the principal reproductive organs are a pair of ovaries and the accessory reproductive organs are: At January 10, at 8: Contributors Badam Smith WebGuru. The thickness of this tissue is much reduced immediately after breeding or pituitary stimulation. The interstitial tissue is continuous with the covering of the testes known as the tunica albuginea, and the whole testis is enclosed in the thin peritoneal epithelium.

Shortly after the normal breeding season in the spring for Rana pipiens, the spermatogonium, which has ceased all mitotic activity, enters upon a period of rest but not inactivity.

During this period the nucleus passes through a sequence of complex changes which represent an extended prophase. This is in anticipation of the two maturation divisions that finally produce the haploid spermatid which metamorphoses into a spermatozoon. The nucleus of the spermatogonium contains chromatin which appears as relatively coarse lumps distributed widely over an achromatic reticulum.

Both the cytoplasm and the nucleus grow and the chromatic granules become finely divided and arranged into contiguous rows, bound by an achromatic thread, together known as chromosomes. This is the leptotene stage of spermatogenesis. Shortly the chromosomes become arranged in pairs which converge toward that side of the nucleus where the centrosome is found.

The opposite ends of the paired chromosomes merge into the general reticulum. This is the synaptene stage. The chromatin granules become telescoped together on the filaments so that the aggregated granules, known as chromosomes, appear much shorter and thicker. Pairs of chromosomes become intertwined and the loose terminal ends become coiled and tangled together. This is the contraction or synizesis stage.

Then the members of the various pairs become laterally parabiotically fused. While there is no actual reduction in total chromatin, there is a temporary and an apparent but not real reduction in the total number of chromosomes to the haploid condition, because of this fusion. There is no actual reduction in the total amount of chromatin material, nor is there any permanent reduction at this stage in the number of chromosomes.

Their identity is lost only temporarily. This is known as the pachytene stage. The members of each pair then separate again. It must be remembered, however, that a the separation need not be along the original line of fusion and that b an exchange of homologous sections of the chromosomes may occur without any cytological evidence.

In any case, the diploid number of chromosomes reappears and this is then known as the diplotene stage. During these changes in the chromatin material of the nucleus, the volume of the nucleus and the cytoplasm are considerably increased, the nuclear membrane breaks down, and the chromosomes assume bizarre shapes and various sizes. They may be paired, curved, or straight; "V" and "C" and reversed "L" shapes, figure 8's, and grouped as tetrads.

This is known as the diakinesis stage. The chromosomes are then lined up on a spindle in anticipation of the first of the two maturation divisions. Spermatogenesis in the frog is seasonal and is completed within the testes. The walls of the seminiferous tubules produce spermatogonia which go through mitotic divisions and then the series of nuclear changes described above without mitosis. This results in the appearance, toward the lumen of each tubule, of clusters of mature spermatozoa.

By the time of hibernation October all the spermatozoa that are to become available for the following spring breeding season will have matured. At this time the testis will exhibit only these spermatozoa and relatively few spermatogonia, without the intervening maturation stages.

The spermatogonia are found close to the basement membrane of the seminiferous tubule. These then await their turn to undergo the maturation changes necessary for the production of spermatozoa which will be ready for the breeding season a year and a half thereafter. The elongated and filamentous tails of the clustered mature spermatozoa project into the lumen of each seminiferous tubule.

If one studies the July or August testis, which organ is then in its height of spermatogenetic activity, he can find all the stages of maturation from the spermatogonium to the spermatozoon. The spermatogonia are always located around the periphery of the seminiferous tubule and are small, closely packed cells, each with a granular, oval nucleus.

In between the spermatogonia may be found occasional very large cells, the primary spermatocytes. These tend to be irregularly spherical, possessing large and vesicular nuclei. The cells are so large that they may be seen under low power magnification X of the microscope.

Apparently they divide to form secondary spermatocytes almost immediately, for they are so few and far between. The secondary spermatocytes which develop as the result of the first division are about half the size of the primaries, and lie toward the lumen of the tubule.

They generally have a darkly staining nucleus, and the cytoplasm may be tapered toward one side. The spermatid, following another division, is even smaller and possesses a condensed nucleus of irregular shape. Clusters of spermatids appear as clusters of granules, the dark nucleus being almost as small as the cross section of a sperm head.

The metamorphic stages from spermatid to spermatozoon are difficult to identify with ordinary magnification, and are often confused with the spermatids themselves.

During this change the inner of two spermatid centrioles passes into the nucleus while the outer one gives rise to the tail-like flagellum. The mature spermatozoon averages about 0.

It has an elongated, solid-staining head nucleus with an anterior acrosome, pointing outwardly toward the periphery of the seminiferous tubule. The short middle piece generally is not visible but the tail appears as a gray filamentous extension into the lumen, about four or more times the length of the sperm head. In any cross section of the testis, bundles of sperm heads or tails may be cut at right angles or tangentially, giving misleading suggestions of structure.

The mature spermatozoon is dependent upon external sources of nutrition so that it joins from 25 to 40 other spermatozoa, all of whose heads may be seen converging into the cytoplasm of a relatively large, columnar-type basal cell known as the Sertoli cell.

This is functionally a nurse cell, supplying nutriment to the clusters of mature spermatozoa until such time as they may be liberated through the genital tract to function in fertilization. In observing a section of the summer testis of the frog under low power magnification, it is readily apparent that each seminiferous tubule may contain all the stages of maturation and that each stage is found in a cluster or group within the tubule. Each group of similar cells is derived presumably from a single original spermatogonium, by the processes of mitosis and meiosis.

This is reminiscent of the condition found in the grasshopper Rhomaleum testis. Maturation of the germ cells occurs in groups so that when the spermatid stage is reached, the tips of the metamorphosing spermatozoon heads are all gathered together into the cytoplasm of the Sertoli cell.

Spermatozoa may remain thus throughout the entire period of hibernation only to be liberated under the influence of sex-stimulating hormones during the early spring. These spermatozoa are functionally mature, as can be demonstrated by dissecting them from the testes and using them to fertilize frogs' eggs artificially at any time from late in August until the normal breeding season in April or May.

It has been proved definitely that the anterior pituitary hormone causes the release of the mature spermatozoa from the testis. But this hormone also releases other maturation stages. It is therefore probable that there are smooth muscle fibers, either among the interstitial cells or in the tunica albuginea of the testes, which fibers contract to force the spermatozoa from the seminiferous tubules.

It would be as difficult to physiologically demonstrate the presence of these fibers in the testis as it is simple to demonstrate them in the contracting cyst wall of the ovary.

Responding to sex stimulation, the spermatozoa become free from their Sertoli cells and are forced from the lumen of the seminiferous tubule into the related collecting tubule. These collecting tubules are small and are lined with closely packed cuboidal cells. They join the vasa efferentia which leave the testis to pass between the folds of the mesorchium and thence into the Malpighian corpuscles of the kidney. From this point the spermatozoa pass by way of the excretory ducts, the uriniferous tubules, and into the mesonephric duct ureter which may be found attached to the lateral margin of the kidney.

Within the excretory system the spermatozoa are immotile, due to the slightly acid environment. They are carried passively down the ureter to the slight dilation near the cloaca, known as the seminal vesicle. Within the vesicle the spermatozoa are stored briefly in clusters until amplexus and oviposition occur. At oviposition the male ejaculates the spermatozoa into the neutral or slightly alkaline water where they are activated and then are able to fertilize the eggs as they emerge from the cloaca of the female.

During the normal breeding season amplexus is achieved as the females reach the ponds where the males are emitting their sex calls.

During amplexus there are definite muscular ejaculatory movements on the part of the male frog, coinciding with oviposition on the part of the female. Amplexus may be maintained by the male for many days, even with dead females. As soon as the eggs are laid and the male has shed his sperm, he goes through a brief weaving motion of the body and then releases his grip to swim away. The frogs completely neglect the newly laid eggs.

In the male frog the ureter is not directly connected with the bladder, as it is in higher vertebrates. It is possible that the bladder in the Anura may be an accessory respiratory and hydrating organ, particularly in the toads, where water may be stored during migrations onto land.

The male frog also has a duct, homologous to the oviduct of the female, known as the "rudimentary oviduct" or Miillerian duct. This duct normally has no lumen, and is very much reduced in size so that it may be difficult to locate.

There is experimental evidence that this duct may be truly a vestigial oviduct since it responds to ovarian or female sex hormones by enlarging and acquiring a lumen. At the anterior end of the testes of some Anura e. This structure is said to respond to the removal of the adjacent testis or to the injection of female sex hormones by enlarging to become structurally like an ovary. Occasionally isolated ova have been found within the seminiferous tubules of an otherwise normal testis, suggesting the similar origin and the fundamental similarity of the testis and the ovary.

Finally, attached to the anterior end of the testis of the hibernating frog may be seen finger-like fat bodies corpora adiposa which represent stored nutrition for the long period of hibernation, and for the pre-breeding season when food is scarce.

Under the microscope these fat bodies appear as clusters of vacuolated cells, and are not to be confused with the mesorchium. It is believed that they, as well as the gonads, arise from the genital ridges of the early embryo.

The fat bodies tend to be reduced immediately after the breeding season, only to be built up again as the time for hibernation approaches. The mature female frog is generally larger than the male of the same age and species, the Rana pipiens female measuring from 60 to 1 10 mm.

The sexually mature female has a body length of at least 70 mm. It can be identified by the absence, at any season, of the dark thumb pad; the inability to produce lateral cheek pouches resulting from the croaking reaction; a flabby and distended abdomen; and the presence of peritoneal cilia.

These cilia are developed in the female in response to the prior development and secretion of ovarian hormones. The ovaries of the frog are paired, multi-lobed organs, attached to the dorsal body wall by a double-layered extension of the peritoneum known as the mesovarium. This peritoneum continues around the entire ovary as the theca externa. Each lobe of the ovary is hollow and its cavity is continuous with the other 7 to 12 lobes.

The ovaries of the female are found in the same relative position as the testes of the male but the peritoneum extends from the dorso-mesial wall rather than from the kidneys, as in the male. The size of the ovary varies with the seasons more than does the size of the testis.

From late summer until the spring breeding season the paired ovaries will fill the body cavity and will often distend the body wall. The mature eggs are highly pigmented on the surface of the animal pole, so that the ovary has a speckled appearance of black pigment and white yolk, representing the animal and the vegetal hemispheres of the eggs.

There is no appreciable change in the size of the ovary during hibernation, nor is there any observable cytological change in the ova. However, if a female is forced to retain her eggs beyond the normal breeding period by isolating her from males or by keeping her in a warm environment and without food, the ova will begin to deteriorate cytolize within the ovary.

Immediately after the spring breeding season, when the female discharges thousands of mature ova, the remaining ovary with its oogonia to be developed for the following year is so small that it is sometimes difficult to locate.

There is no pigment in the tissue of the ovary in the stroma or in the immature ova , and each growing oocyte appears as a small white sphere of protoplasm contained within its individual follicle sac.

The histology of the ovary shows that within its outer peritoneal covering, the theca externa, are suspended thousands of individual sacs, each made up of another membrane, the theca interna or cyst wall, which contains smooth muscle fibers.

Wednesday, October 31, 2012