Reproduction in Vertebrates | Essay | Vertebrates | Reproductive Biology

In this essay we will discuss about the process of reproduction in vertebrates.

Reproduction in Vertebrates

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Essay Contents:

1. Essay on the Process of Reproduction in Fishes
2. Essay on the Process of Reproduction in Amphibia
3. Essay on the Process of Reproduction in Reptilia
4. Essay on the Process of Reproduction in Birds
5. Essay on the Process of Reproduction in Mammals

Essay # 1. Process of Reproduction in Fishes:

Most teleosts, such as plaice or trout produce a large number of yolky eggs. The stimulus for ripening of gonads is supplied by pituitary hormones which depend for their secretion on rhythmical environmental changes. Nearly all fishes show anti-natant spawning (against the current) migrations, so that they actually release eggs up-current of the feeding grounds, which allow the larvae to drift down towards the latter.

Majority of marine fishes, except herring, have pelagic or floating eggs, while most river fish make nests or secure their eggs in some way to prevent them from drifting downstream. To produce coordinated mating, elaborate shoaling rituals may take place (e.g. cod) or there may be patterns of behaviour between pairs (e.g. stickleback) ensuring the release of gametes at the same time.

Fishes, on the whole, show little parental care for their offspring and as a result mortality rate of young is very high.

There are some exceptions to the above reproductive methods. The elaboration of nests occurs in certain river fish and as in the case of stickleback, this provision may extend to protection of the young after hatching. There are species of catfish and species of sea-horse, which retain their young in their mouths or in the brood pouches, while others such as the guppy are viviparous and the eggs develop within the body of the female.

This latter system is well developed among the elasmobranchs (e.g. Squalus sps.) and it involves internal fertilization helped by claspers of the male, and the female lays a small number of eggs with a quantity of yolk to be produced. In the dogfish, Scyliorhinus canicula internal fertilization is followed by the secretion of a protective case around the eggs and they are attached by its trailing threads to objects on the sea bed.

In some elasmobranchs an ovoviviparous condition has evolved with some degree of assimilation of nutrients from the uterine wall secretions of the mother. Such is the case in Mustelus vulgaris, whose developing embryos take up mucoproteins, fat and monosaccharide sugar. The uterine secretions also contain urea, which forms an important part of embryo’s environment.

Some elasmobranchs have true viviparity and a placenta is formed between the wall of the uterus and the yolk sac. In fishes such as Mustelus laevis, the weight of the embryo increases several hundred per cent during development. Where a placenta is formed, the wall of the uterus may have large number of villi and projections from the yolk sac burrow into this to make a large surface bound between parent and embryo.

Transfer of substances from the female elasmobranch to its developing young is associated with a severe loss in weight of the maternal liver during gestation. This is true in fish which form a placenta and those that secrete nutritive fluids. Loss of liver weight does not occur in mammals during gestation.

Essay # 2. Process of Reproduction in Amphibia:

Amphibians are stimulated by increasing daylight and other factors. The endocrines of the frogs, which control reproduction, become active during the later part of the hibernation. Metabolic changes occur and the quantities of stored fat decrease, while the gonads enlarge and ripen.

Most frogs return to water for reproduction and they may migrate several miles to find their original spawning ground. The croaking of the male frogs and the enlarged belly of the females are two of the stimuli which lead mating, accomplished by the male grasping the female with his nuptial pads.

Fertilization of the eggs takes place externally and the sperms must penetrate the egg rapidly before its coating of albumen swells. In the male the sperms pass down from the testes via the anterior part of the kidney and the Wolffian ducts. Both sexes have a cloaca where genital as well as excretory products are passed to the exterior. All these features as well as the need to return to water indicate the primitive nature of amphibian reproduction.

Once laid, the eggs have limited protection of their albumen coating and the fact that the black pigment they contain has a bitter taste. The larval amphibians or tadpoles are well adapted to life in water, having respiratory and locomotory systems as well as sensory adaptations, such as the lateral line, which are similar to those of fishes.

While lack of parental care is typical of the majority of amphibians, there are a good number of exceptions. The main adverse feature of the reproductive system described above is its necessity for external water. Some tree-frogs live together making hollows quite above the ground where rain water collects and into which they can lay eggs. Among the most remarkable adaptations are those of the female midwife toads which have a number of holes in their back into which they push the eggs after fertilization and in these pockets the tadpoles develop.

Essay # 3. Process of Reproduction in Reptilia:

With the more efficient colonisation of the land achieved by the reptiles we see many changes from the amphibian condition. Fishes and amphibians are classed as anamniotes, as they do not possess the amnion characteristic of the reproductive mechanism of reptiles, birds and mammals which are classed as amniotes.

The amnion is an extra-embryonic membrane evolved in conjunction with shelled egg and it facilitates the embryo to develop within a stable fluid environment. In conjunction with the amnion a further extraembryonic membrane, the allantois, develops. This has a role in respiratory exchanges and excretion.

As with anamniotes there is yolk enclosed in a sac and this provides the raw material for the developing embryo. The albumen of the egg contains water and porous shell allows gaseous exchange, so that at the level of the reptiles a great deal of provision is made for both the nutrition and protection of the developing offspring.

Snakes guard their eggs and turtles and crocodiles, among others, bury their eggs to provide them with an uniform and protected environment. A few lizards and some snakes have developed viviparity but there is no exchange of materials between mother and offspring.

Essay # 4. Process of Reproduction in Birds:

The eggs of a bird are much the same as those of the reptiles from which the birds evolved, but there is a wider range of colour and shape. Birds with open nests usually have camouflaged eggs (e.g. Plover), while those with concealed or domed nests have white eggs (e.g. owl).

There is a general tendency to reduce the number of eggs laid in the more advanced birds; thus the ostrich has some 80 eggs, while the highly successful fulmar has only one; but on the whole birds lay far fewer eggs than do reptiles. Recent work has shown that the number of eggs laid by a bird is related to the number of offsprings that the parent can successfully look after. Variations exist within a few species according to the date of nesting, latitude and immediate ecological conditions.

Parental care is well developed over the whole reproductive period and the male and female tend to cooperate in making of the nest, incubation of the eggs and subsequent care and feeding of the young. Nest sites are chosen to give maximum protection from predators and for insulation soft materials such as moss are used.

The actual position of the nest in regard to other members of the species is also important. Land birds have quite extensive territories which they defend and which have effect of dispersing a given species efficiently in an area. Sea-birds have their nests closely placed for protection against predators but still regard the small area around their nests as territory.

Birds being homeiothermic, it is essential that they maintain their eggs at constant temperature. The construction of the nest assists in this and the female and also sometimes the male develops an increased blood supply to the skin as well as losing feathers from her breast. She also develops the drive to incubate, which is very strong and can be clearly seen in the behaviour of a ‘broody’ hen.

After hatching, the youngs are usually cared for by both parents, the gape of the fledging’s- beak acting as the releasor to the parents, feeding responses. An interesting subsidiary hormonal effect is by the pituitary lactogen which causes the secretion of ‘milk’ from the regeon’s crop. Above scheme illustrates how many birds cooperate over a period of months for reproductive purposes.

Essay # 5. Process of Reproduction in Mammals:

Together with the development of the cerebral hemispheres and complex behaviour, the mammals owe their success to the efficiency of their reproductive system. In this class of vertebrates, we find protection of the developing young and subsequent care of the offspring exceedingly highly developed.

The egg, which still goes through a similar gastrulation to the reptile and bird egg, is quite small in the placental mammals, although the shelled eggs of the primitive monotremes are large. Mammalian eggs have reduced yolk because they are retained in the female body and nourished by her tissues. For this purpose, the oviducts in female are modified to form a specialised and muscular region, the uterus where the embryodevelops. The wall of uterus establishes contact with the foetal membranes of the developing embryo, the placenta which serves various important physiological functions.

The placenta forms from the extra-embryonic membrane, the chorion, coming into close contact with the lining of uterus. Later, the allantois grows out from the foetal endoderm and fusing with chorion gives rise to the chorioallantoic placenta. The inner wall of the chorion and the outer lining of the allantois are lined with mesoderm and in this blood vessels arise forming the umbilical artery and vein.

The foetal heart derives blood from the umbilical artery and between foetal and maternal circulations a countercurrent exchange system may develop. Food, water and oxygen are passed from the maternal circulation and carbon dioxide and other metabolities are returned.

There has been a tendency to reduce the layers involved in the placenta in many orders of mammalia; thus, in the maternal epithelium, connective tissue and endothelium are not present in the placenta and the foetal tissues project directly into the blood sinuses.

In such placentae the uptake of ions and presumably other substances is greatly enhanced and in this respect they are more efficient than the placentae with many layers. Some of the intervening layers are lost during development in different mammalian orders to increase the intimacy between maternal and foetal tissues and to increase efficiency of functioning of placenta.

The organs of sex in both the male and the female animals mature at a definite age that varies in different species. At this time, which is called puberty the reproductive functions commence. Sex desire is aroused a mating occurs. The reproductive period extends throughout the greater past of the animal’s life.

It ends with atrophic changes in the sex organs and the gradual suppression of sexual activity. In the human species, puberty occurs from 13 to 15 years of age, being usually a little earlier in girls than in boys. The secondary sexual characters such as development of mammary glands and appearance of hair on the pubic region occur.

The placentals, depending on the type of sexual cycle, fall into two categories, namely:

(i) Estrous, and

(ii) Menstrual.

The first category includes those which breed only in specific period of the year when ovulation takes place and the animals break the desire of sexual mating. The breeding period is always followed by the resting phase. These animals may breed once, twice or many times in a year, every breeding period being followed by a resting period and accordingly the estrous animals may be mono-, bi- or polyestrous.

The second category breeds throughout the year and ovulation occurs every month periodically alternating from the two ovaries. In these animals, breeding phase may be followed either by pregnancy or by the discharge of unfertilized ovum, the endometrial tissue and blood in the form of menstrual fluid.

Estrous Cycle:

In the majority of animal species, the female will receive the male at certain periods of the year. These so-called mating seasons are characterized by animal breeders as ‘heat’ and by physiologists as estrus. The sexual phases immediately preceding estrus are called prosestrus; that following it, postestrus. The changes during proestrus consist of swelling and increased vascularity of the vulva, and vagina.

The uterus becomes enlarged and its glands hypertrophy. In some animals, at this lime, bleeding occurs from the uterus and appears externally. The changes of postesrus are preparatory in nature, the female organs being brought into a condition suitable for the reception of the male and the fertilization of ovum.

The Graffian follicles in the ovary undergo maturation. Estrus itself is the period during which ovulation occurs and the female mates with male. The changes during postestrus are anticipatory to the implantation of the fertilized ovum in the uterus.

The uterine mucosa hypertrophies and its glands show an increase in secretory activity. During postestrus the corpus luteum develops. The uterine changes in postestrus resemble those taking place during pregnancy which, indeed, are an extension or continuation of the form.

Thus, the changes at the different phases of estrus cycle are as follows:

1. Proestrus:

Uterus and vagina become congested and secrete a clear sanguineous fluid. Vaginal epithelium proliferates. This is caused by estrogens secreted by the maturing follicles. Vaginal smear shows large number of nucleated cells broken off from the proliferating vaginal epithelium.

2. Estrus (Heat Period):

This is the period of heat, only when female receives the male. Congestion of the uterus becomes maximum. The vaginal epithelium thickens further and the superficial layers are fully keratinised. Ovulation takes place at this time, so that impregnation is possible. If fertilization takes place, placenta forms and pregnancy begins. If not, it passes on to the next phase. Vaginal smear shows large number of keratinised cells.

3. Postestrus (Leuteal Phase):

Changes initiated in the previous stage proceed still further due to the action of progesterone, secreted by the newly formed corpora lutea. In the absence of pregnancy, corpora lutea degenerate and the changes of the generative organs subside. In monoestrus animals (bitch), the hypertrophied mucosa breaks down and is discharged. Vaginal smears show large number of neutrophils.

4. Anestrus:

This is the resting asexual period. In monoestrus animals, it lasts up to the next mating season, and is known as anestrus. In polyestrus animals, the resting interval is short, up to the next cycle and is called diestrus. In rats it lasts for 4-5 days. Vaginal smear shows degenerating leucocytes and denuded epithelium.

The phases of the estrus cycle in a monoestrous animal are shown in the following scheme: