Answer of Question of Reproduction & Development

Answer of Question of Reproduction & Development

Introduction

Reproduction is a universal occurrence in all living organisms Both asexual and exual reproductions occur in the animal kingdom.

Asexual reproduction produce offspring whose genes all come from a single parent. ission, budding, and fragmentation with regeneration are mechanisms of asexual production in various invertebrates.

Sexual reproduction requires the fusion of male and female gametes to form a iploid zygote. The production of offspring with varying genotypes and phenotypes may nhance reproductive success in fluctuating environment:

Animals may reproduce exclusively sexually or asexually, or they may alternate etween the two, depending on environmental conditions. Variations on these two modes re made possible through parthenogenesis, hermaphroditism, and sequential ermaphroditism.

In sexual reproduction, gametes unite in the external environment (external rtilization) or within the body of female (internal fertilization).

Diverse reproductive systems have evolved in the animal kingdom. Human reproduction involves intricate anatomy and complex behavior.

Human male reproductive anatomy consists of internal organs and external enitalia, the scrotum and penis. The gonads, or testes reside in the cool environment of t e scrotum. They possess endocrine interstitial cells surrounding sperm—forming

miniferous tubules that successively lead into the epididymis, vas deferens, ejaculatory ct, and urethra, which exits at the tip of the penis. Accessory glands add secretions to t e semen.

Before a human male can mature and function sexually, special regulatory rmones (FSH, GnRH, inhibin, LH and testosterone) must function.

The reproductive roles of the human female are more complex than those of the ale. Not only do females produce eggs, but after fertilization, they also nourish, carry, d protect the developing embryo. They may also nourish the infant for a time after it is

The female reproductive system consists of two ovaries, two uterine tubes, a uterus, gine, and external genitalia. The mammary glands contained in the paired breasts oduce milk for the new born baby.

The human female is fertile for only a few days each month, and the pattern of rmone secretion is intricately related to the cyclical release of a secondary oocyte from t e ovary. Various hormones regulate the menstrual and ovarian cycles.

‘Pregnancy sets a new series of physiological events into motion that are directed to housing, protecting and nourishing the embryo.

The development of a human may be divided into prenatal and postnatal periods.

Human pregnancy can be divided into three trimesters. Organogenesis is completed by eight weeks

Placenta is the organ that sustains the embryo and fetus through out the pregnancy.

Birth, or parturition, results from strong, rhythmic uterine contractions that -bring about the three stages of labor dilation of the cervix, expulsion of the baby and delivery of the placenta. Positive feedback involving the hormones estrogen and oxytocin, and prostaglandins, regulate labor.

Lactation includes both milk secretions (production) by the mammary glands and milk release from the breasts.

Answers to the Questions

Q.1.  Define asexual reproduction and describe forms of asexual reproduction in invertebrates.

Ans. Asexual reproduction is the creation of offspring whose genes all come from one parent without the fusion of gametes that is eggs or sperm. In most cases, asexual reproduction relies entirely on mitotic cell division. Offspring produced by asexual reproduction all have the same genotype (unless mutations occur) and are called clones.

Asexual, reproduction appears in many invertebrate phyla, such as cnidarians, bryozoans, annelids, echinoderms. and hemichordates. In animal phyla in which asexual reproduction occurs, most members also employ sexual reproduction. In these groups, asexual, reproduction ensures rapid increase in numbers when differentiation of the organism has not advanced to the point of forming gametes. The basic forms of asexual reproduction are fission, budding, gemmulation. fragmentation and parthenogenesis

Fission

Protists and some multicellular animals (cnidarians, annelids) may reproduce by fission. Fission (L.Fissio, the act of splitting) is the division of one cell, body, or body part into two.     this process. the cell  pinches in two by inward furrowing of the plasma membrane. Binary fission is common among bacteria and protozoa

 

binary fission the body of the parent divides by mitosis into two approximately equal parts, each of which grows into an individual similar to the parent. Binary fission may be lengthwise, as in flagellate protozoa, or transverse, as in ciliate protozoa. In multiple fission the nucleus divides repeatedly before division of the cytoplasm, producing many daughter cells simultaneously. Spore formation, called sporogony, is a form of multiple fission common among some parasitic protozoa, for example, malarial parasites. Fig. 7.1.

Budding:

Budding (L.bud, a small protuberance) is an unequal division of an organism. The new individual arises as an outgrowth (bud) from the parent, develops organs like those of the parent and then detaches itself. If the buds remain attached to the parent, they form a colony. Budding occurs in several animal phyla and is especially prominent in cnidarians, tunicates and sponges. Fig. 7.2.

Gemmulation:

Gemmulation is the formation of a new individual from an aggregation of cells surrounded by a resistant capsule, called a gemmule. In many freshwater sponges, gemmules develop in the fall and survive the winter in the dried or frozen body of the parent. In the spring, the enclosed cells become active, emerge from the capsule, and grow into a new sponge. Fig. 7.3.

Fragmentation:

Fragmentation, is the breaking of the body into several pieces, some or all of which develop into complete adults. For an animal to reproduce this way, fragmentation must be accompanied by regeneration, i.e., the regrowth of lost body parts. Reproduction by fragmentation and regeneration occurs in many sponges, cnidarians, polychaete annelids, and tunicates.

Parthenogenesis:

Parthenogenesis (Gr.parthenos, virgin + genesis, production) is a spontaneous activation of a mature egg, followed by normal egg divisions and subsequent embryonic development. In fact, mature eggs of species that do not undergo parthenogenesis can sometimes be activated  to develop without fertilization by pricking them with a needle, or by exposing them to high concentrations of calcium, or by altering their temperature.

Parthenogenesis in invertebrates occurs in certain flatworms, rotifers, roundworms, insects, lobsters etc. Parthenogenesis has a role in the social organization of certain species of bees, wasps, and ants. Male honeybees, or drones, are produced parthenogenetically, whereas females, both sterile workers and reproductive females (queens), develop from fertilized eggs.

Q.2. What are some advantages of asexual reproduction in Invertebrates? What are some disadvantages?

Ans. Advantages of asexual

Reproduction:

Asexual reproduction has several potential advantages. For instance, (1) it enables animals that live in isolation to produce offspring without locating mates. (2) It may also allow many offspring to be produced in a short period of time, which is ideal for colonizing a habitat rapidly. (3) Theoretically, asexual reproduction is most advantageous in stable, favorable environments because it perpetuates successful genotypes precisely.

Disadvantages:

Without the tremendous genetic variability bestowed by meiosis and sexual processes, however, a population of genetically identical animals stands a great increased chance of being devasted by a single disease or environmental insult, such as a long drought. A given line of asexually reproducing animals can cope with a changing environment only through the relatively rare spontaneous mutations (alterations^, in genetic material) that prove to be beneficial. Paradoxically, however, most mutations are detrimental or lethal, and herein lies one of the greatest disadvantages of asexual reproduction.

Q.3. What is the difference between asexual and sexual reproduction.

Q.4. Define two alternatives to bisexual reproduction-hermaphroditism and parthenogenesis. What is the difference between ameiotic and meiotic parthenogenesis?

Ans. Hermaphroditism

Hermaphroditism (Gr.hermaphroditos an organism with the attributes of both sexes) occurs when an animal has both functional male and female reproductive systems. This dual sexuality is sometimes called the monoecious (Gr. monos, single + oikos, house) condition. Many sessile burrowing, or endoparasific invertebrate animales e g most flatworms, some hydroids and annelids, and all barnacles and pulmonates (snails) and a few vertebrates (some fishes), are hermaphroditic. Some hermaphrodites fertilize themselves, but most avoid self-fertilization by exchanging germ cells with another member of the some species. An advantage is that with every individual producing eggs, a hermaphroditic species could potentially produce twice as many offspring as could a dioecious species, in which half the individuals are nonproductive males.

Another variation of hermaphroditism, sequential hermaphroditism occurs when an animal is one sex during one phase of its life cycle and the opposite sex during another phase. Hermaphrodites are either protogynous or protandrous. In protandary, an animal is a male during its early life history and a female later in the life history. The reverse is true for protogynous animals. A change in the sex ratio of a population is one factor that can induce sequential hermaphroditism, which is common in oyster.

Parthenogenesis

Parthenogenesis (“virgin origin”) is the development of an embryo from an unfertilized egg or one in which the male and female nuclei fail to unite following fertilization. Parthenogenesis may be ameiotic or meiotic.

Ameiotic parthenogenesis: In ameiofic parthenogenesis, no meiosis occurs, and the egg is formed by mitotic cell division. This “asexual” form of parthenogenesis is known to occur in some species of flatworms, rotifers, crustaceans, insects, and probably others. In these cases, the offspring are clones of the parents because, without meiosis, the parent’s chromosomal complement is passed intact to offspring.

Meiotic parthenogenesis: In meiotic parthenogenesis a haploid ovum is formed by meiosis, and it may or may not be activated by the influence of a male. For example, in some species of fishes, a female may be inseminated by a male of the same or related species, but the sperm serves only to activate the egg; the male’s genome is.rejected before it can penetrate the egg. In several species of flatworms, rotifers, annelids, ‘mites, and insects, the haploid egg begins development spontaneously; no males are required to stimulate activation of an ovum. The diploid condition is restored by chromosomal duplication.

The disadvantage of parthenogenesis is that if the environment should suddenly change, as it often does, parthenogenetic species have limited capacity to shift gene combinations to adapt to the new conditions.

Q.5.  Define major patterns of fertilization and describe the environmental and

behavioral requirements of each pattern.

Ans. The mechanisms of fertilization; the union of sperm and egg, play an important part in sexual reproduction. The two major patterns of fertilization that have evolved are external and internal fertilization.

External fertilization: In external fertilization, eggs are shed by the female and fertilized by the male in the environment. Because external fertilization requires an environment where an egg can develop without desiccation or heat stress, it occurs almost exclusively in moist habitats. Many aquatic invertebrates simply shed their eggs and sperm into the surroundings, and fertilization occurs without the parents actually making physical contact. Timing is crucial to ensure that mature sperms encounter ripe eggs. Environmental cues such as temperature or day length may cause all the individuals of a population to release gametes at once, or pheromones from one individual releasing gametes may trigger gamete release in others.

Most fishes and amphibians that use ekternal fertilization exhibit specific mating behaviors, resulting in one male fertilizing the eggs of one female. Courtship behavior is a mutual trigger for the release of gametes, with two effects: The probability of successful fertilization is increased, and the choice of mates may be somewhat selective.

Internal fertilization: It occurs when sperms are deposited in (or nearby) the female reproductive tract, and egg and sperm unite within her body. Internal fertilization requires cooperative behavior, leading to copulation. In some cases, uncharacteristic sexual behavior is eliminated by natural selection in a direct manner; for example, female spiders will eat males if specific reproductive signals are not followed during mating. Internal fertilization also requires sophisticated reproductive systems. Copulatory organs for the delivery of sperm and receptacles for its storage and transport to the eggs must be present

Q.6. Define sexual reproduction. What are some advantages and disadvantages of sexual reproduction?

Ans. Sexual Reproduction:

Sexual reproduction in animals is the creation of offspring by the fusion of two haploid gametes (sperm and egg) to form a diploid zygote. Gametes are formed by meiosis, and sexual reproduction usually involves two parents, both contributing genes to the offspring. The offspring of sexual union are somewhat different from their parents and siblings-they have genetic diversity.

Advantages of Sexual Reproduction

New combinations of traits can arise more rapidly in sexual reproducing animals because of genetic recombinations. The resulting genetic diversity or variability increases the chances of the species surviving sudden environmental changes. Furthermore, variation is the foundation of evolution. In contrast to the way asexually reproducing populations tend to retain mutations; sexually reproducing populations tend to eliminate deleterious and lethal mutations.

Many biologists believe that sexual reproduction, with its breakup and recombination of genomes keeps producing novel genotypes that in times of environmental changes may survive and reproduce, whereas most others die. Variability, advocates of this viewpoint argue, is sexual reproduction’s trump card.

Disadvantages of Sexual Reproduction

Sexual reproduction also has some disadvantages. For example, sexual reproduction is complicated, requires more time, and uses much more energy than asexual reproduction. Mating partners must come together and coordinate their activities to produce young.

Many biologists believe that an even more troublesome problem is the “cost of meiosis”. A female that reproduce asexually passes all of her genes to her offspring. But when she reproduces sexually the genome is divided during meiosis and only half her genes flow to the next generation. Another cost is wastage in production of males, many of which fail to reproduce and thus consume resources that could be applied to production of females. In addition, many of the gametes that are released are not fertilized, leading to a significant waste of metabolic effort.

Q.7. What reproductive strategy developed in each of the following to increase the chances of survival: (a) bony fishes, (b) amphibians, and (c) reptiles?

Ans. (a) Bony fishes

(1)  Fishes are.well known for their high potential fecundity, with most species releasing thousands to millions of eggs and sperm annually. Males and females come together in great schools and release vast numbers of gametes into the water to drift with the current.

(2)  Fish species have reproductive methods, structures, and an attendant physiology that have allowed them to adapt to a great variety of aquatic conditions. For example, unlike the minute, buoyant, transparent eggs of pelagic marine teleosts, those of many near shore bottom dwelling (benthic) species are larger, typically yolky, non buoyant, and adhesive. Some bury their eggs, many attach them to vegetation, some deposit them in nests, and some even incubate them in their mouths. Many benthic spawners guard their eggs.

Freshwater fishes that do provide some form of egg care, produce fewer, larger eggs that enjoy a better chance for survival.

(b)Amphibians:

The reproductive strategies in amphibians are much more diverse than those observed in other groups of vertebrates. In each of three living orders of amphibia (caecilians, salamanders, anurans) are trends toward terrestriality. The variety of these adaptations is especially noteworthy in anurans. These reproductive adaptations have been viewed as pioneering evolutionary experiments in the conquest of terrestrial environments by vertebrates. Noreworthy is the evolution of direct development of terrestrial eggs, ovoviviparity viviparity and parental care that have been important in successful invasion of mountainous environments by amphibians. Eggs are laid in large masses; usually anchored to vegetation. Migration of frogs and toads^. is correlated with their breeding habits. Males usually return to a pond or stream before females,

which they then attract by their calls. Some salamanders also have a strong homing instinct, returing each year to reproduce in the same pool, to which they are guided by olfactory cues.

(C)Reptiles:

The reptilian system includes shelled, desiccation — resistant eggs. These eggs had the three basic embryonic membranes that still characterize the mammalian embryo, as well as flat embryo that developed and underwent gastrulation atop a huge yolk mass.

Q.8. How do bird eggs and amphibian eggs differ?

Ans. The frog egg has no shell, and it dehydrates quickly in dry air. Fertilization is external in most species of amphibians, with the male grasping the female (amplexus), and spilling his sperm over the eggs as the female shed them. Amphibians generally lay their eggs in ponds or swamps or at least in moist environments. Some species lay vast numbers of eggs, and mortality is high. In contrast are species that display various types of parental care and that lay relatively few eggs. In bird’s egg, a shell is present which prevents desiccation of the egg, which can therefore be laid in a

dry place. Specialized membranes within the egg function in gas exchange, waste storage, and transfer of stored nutrients to the embryo. These are called extraembryonic membranes because they are not part of the body of the developing animal, although they develop from tissue layers that grow out from the embryo. The amniotic egg is named for one of these membranes, the amnion, which encloses a compartment of amniotic fluid that bathes the embryo and acts as a hydraulic shock absorber. The egg shells in birds are much thicker than those of reptiles. Thicker shells permit birds to sit on their eggs and warm isri them. This brooding, or incubation, hastens embryo development. Figure 7.4.

Q.9.  Explain in detail how evolution in reproduction has taken place in

vertebrate classes.

Ans. Almost all vertebrates reproduce sexually, only a few lizards and fishes normally reproduce parthenogenetically. Sexual reproduction evolved among aquatic animals and then spread to the land as animals became terrestrial.

Fishes

Most fishes favor a simple theme: they are dioecious, with external fertilization and external development of the eggs and embryos (oviparity). Soon after the egg of an oviparous species is laid and fertilized, it takes up water and the outer layer hardens. Cleavage follows, and the blastoderm forms, sitting astride a relatively enormous yolk mass. Soon the yolk mass is enclosed by the developing blastoderm, which then begins to assume a fish-like shape. The fish hatches as a larva carrying a semitransparent sac of yolk, which provides its food supply until the mouth and digestive tract have developed. After a period of growth the larva undergoes a metamorphosis, especially dramatic in many marine fishes. Body shape is refashioned, fin and color patterns change, and the animal becomes a juvenile bearing the unmistakable definitive body form of its species.

Unlike birds and mammals, which stop growing after reaching adult size, most fishes after attaining reproductive maturity continue to grow for as long as they live. This may be a selective advantage, since the larger the fish, the more gametes are produces and the greater its contribution to future generations. Thus, to ensure reproductive success in fishes:

1. there is large number of eggs
2. the fertilized egg develops rapidly, and
3. the young achieve maturity within a short time.

Amphibians

Amphibians (frogs, salamanders, and the like) evolved from fish, and they too generally use external fertilization, they must therefore return to the water or to a very moist place on land to lay their eggs. Some salamanders have evolved a behavioral sequence in which the male releases a membranous packet (spermatophore) containing sperm that the female picks up with her cloaca. These amphibians have thus evolved a primitive type of internal fertilization, and some of them mate on land, but their eggs must still be laid in very moist places.

Depending on the species, either males or females may incubate eggs on their back (Pipe), in the mouth, or eggs and tadpoles even in the stomach (Rheobatrachtis females). Certain tropical tree frogs stir their egg masses into moist foamy nests that resist zirying. There are also itve-bearing amphibians (

eggs in the female reproductive tract, where embryos can develop without drying out. The developmental period is much longer in amphibians than in fishes, although the eggs do not contain appreciably more yolk. An evolutionary adaptation present in amphibians is the presence of two periods of development: larval and adult stages. The aquatic larval stage develops rapidly, and the animal spends much the Erne eating and growing. After reaching a sufficient size, the larval form undergoes a developmental transition called metamorphosis into the adult (often in terrestrial forms. Many amphibians exhibit complex and diverse social behavior, especially during their breeding seasons. Frogs are usually quiet creatures, but many species fill the air with their mating calls during the breeding season. Males may vocalize to defend breeding territory or to attract females. In some terrestrial species, migrations to specific breeding sites may involve vocal communication, celestial navigation, or chemical signaling. Reptiles

The reptiles, evolved from ancestral amphibians. They were the first vertebrates to be fully emancipated from the ancestral dependence on the aquatic environment for reproduction. The evolutionary adaptations found in reptiles are:

1. They use internal fertilization, and lay eggs enclosed_ in tough membranes and shells. Since internal fertilization entails much less wastage of egg cells than external fertilization, only a few egg cells are released during each reproductive season.
2. Many reptiles are oviparous. Others are ovoviviparous. Viviparity in reptiles is limited to squamates, and has evolved at least 100 separate times.
3. The shelled egg and extraembryonic membrane is also first seen in reptiles.

These adaptations allowed reptiles to lay eggs in dry places without danger of desiccation. As the embryo develops, the extraembryonic chorion and amnion help protect it, the later by creating a fluid—filled sac for the embryo. The allantois permits gas exchange and stores excretory products. Complete development can occur within the shell. The young hatches as a lung — breathing juvenile. The appearance of the shelled egg widened the division between evolving amphibians and reptiles and, probably more than any other adaptation, contributed to the evolutionary establishment of reptiles. Fig. 7.4. Birds Birds have retained the important adaptations for life on land that evolved in the early reptiles. The evolutionary adaptations found in birds are:

1. Males simply deposit semen against the cloaca for internal fertilization. Some water fowls and ostriches possess intromittent organs.
2. All birds are oviparous, and the eggshells are much thicker than those of reptiles, and
3. Extensive parental care and feeding of young are more common among birds than fishes, amphibians, or reptiles.

Fertilization takes place in the upper oviduct several hours before the layers of albumen, shell membranes, and shell are added. Sperm remain alive in the female oviduct for many days, after a single mating. The thicker shells of eggs permit birds to sit on their eggs and warm them. This brooding, or incubation, hastens embryo development. Most birds build some form of nest in which to rear their young. Newly hatched birds are of two types: precocial and altricial. Fig. 7.5. Precocial young, such as quail, fowl, .ducks, and most water birds, are covered with down when

hatched and can run or swim as soon as their plumage is dry. Altricial ones, on the other hand, are naked and helpless at birth and remain in the nest for a week or more. Young of both types require care from parents for some time after hatching. They must be fed, guarded, and protected against rain and sun. Parents of altricial species must carry food to their young almost constantly, for most young birds will eat more than their weight each day. This enormous food consumption explains the rapid growth of the young and their quick exit from the nest.

Mammals:

Because mammals are descendants of one of three lineages that originated with a common amniote ancestor, they inherited the amniotic egg. The earliest mammals were egg layers, and even today some mammals retain this primitive character; the monotremes (duck — billed platypus and spiny anteater) lay large yolk eggs that closely resemble bird’s eggs. After hatching, the young suck milk from the fur of the mother around openings of the mammary glands. Thus in monotrems there is no gestation. All other mammals are viviparous, and mammalian viviparity was major evolutionary adaptation and it has taken two forms.

1. The marsupials develop the ability to nourish their young in a puch after a short gestation inside the female.
2. The placentals retain the young inside the female, where the mother norishes them by means of a placenta.

In marsupials (such as opossums and kangaroos), the embryos develop for a time within the mother’s uterus. But the embryo does not “take roof’ in the uterine wall, and consequently it receives little nourishment from the mother before birth. The young of marsupials are therefore born immature and are sheltered in a pouch in the mother’s abdominal wall and nourished with milk. Fig. 7.6.

All other mammals, composing 94% of class mammalia, are placental mammals. In placentals, the reproductive investment is in prolonged gestation, unlike marsupials in which the reproductive investment is in prolonged lactation. The embryo remains in the uterus, nourished by food supplied through a chorioallantoic type of placenta, an intimate connection between mother and young. Even after birth. mammals continue to nourish their young. Mammary glands are a unique mammalian adaptation that permits the female to nourish the young with milk that she produces Some mammals nurture their young until adulthood, when they are able to mate and fend for themselves Mammalian reproductive behavior also contributes to the transmission and evolution of culture that is the key to the evolution of the human species.

Q.10. What does the human male reproductive system consists of?

Ans. The reproductive role of the human male is to produce sperms and deliver them to the vagina of the female. This function requires the following structures:

1. Two testes that produce sperms and the male sex hormone, testosterone.
2. Accessory glands and tubes that furnish a fluid for carrying the sperms to the penis. This fluid, together with the sperm, is called semen.
3. Accessory ducts that store and carry secretions from the testes and accessory glands to the penis.
4. A penis that deposits semen in the vagina during sexual intercourse. Q.11. Describe in detail the production and transport of sperm in human male.

Ans. The male gonads or sex organs, are the testes — oval glandular structures that form in the dorsal portion of the abdominal cavity from the same embryonic tissue that gives rise to the ovaries in females.

Scrotum

In the human male the testes descend about the time of birth from their points of origin into the scrotal sac (scrotum), a pouch whose cavity is initially continuous with the abdominal cavity via a passageway called the inguinal canal. After the testes have descended through the inguinal canal into scrotum, the canal is slowly plugged by growth of connective tissue, so that the scrotal and abdominal cavities are no longer continuous. The temperature in a scrotum is about 2°C below that in the abdominal cavity. The lower temperature is necessary for active sperm production and survival.

Testes

Each testis contains over eight hundred tightly coiled seminiferous tubules, which produce thousands of sperms each second in healthy young men. The walls of the seminiferous tubules are lined with two types of cells.

1. Spermatogenic cells, which give rise to sperms, and

Sustentacular cells, which nourish the sperms as they form, also secrete a fluid (as well as the hormone inhibin) into the tubules to provide a liquid medium for the sperms. Between the seminiferous tubules are clusters of endocrine cells, called interstitial cells (Leydig cells), that produce testosterone. Inhibin and testosterone, both are androgens, the male sex hormones. Duct system

A system of tubes carries the sperm, that the testes produce, to the penis. The seminiferous tubules merge into a network of tiny tubules called the rete testis (L.rete, net), which merges into a coiled tube called the epididymis. The epididymis has three main functions:

1. It stores sperm until they are mature and ready to be ejaculated.
2. It contains smooth muscle that helps propel the sperm toward the penis by peristaltic contractions, and
3. It serves as a duct system for sperm to pass from the testis to the ductus deferens.

The ductus deferens (formerly called the vas deferens or sperm duct) is the dilated continuation of the epididymis. Continuing upward after leaving the scrotum, the ductus deferens passes through the lower part of the abdominal wall via the inguinal canal. The ductus deferens then passes around the urinary bladder and enlarges to form the ampulla. The ampulla stores some sperm until they are ejaculated. Distal to the ampulla, the ductus deferens becomes the ejaculatory duct. The urethra is the final ein-tion of the reproductive duct system. Fig. 7.7. Accessory Glands Three sets of accessory glands add secretions to the semen, the fluid that is ejaculated:

1. A pair of seminal vesicles contributes about 60% of the total volume of the semen. The fluid from the seminal vesicles is thick and clear, containing mucus, amino acids, vitamin C, prostaglandins, and large amount of fructose (sugar), which provides energy for the sperm and helps to neutralize the natural protective acidity of the vagina. (The pH of vagina is about 3 to 4, bui sperm motility is enhanced when it increases to about 6).
2. The prostate gland is the largest of the semen secreting glands. It secretes its products directly into the urethra through several small ducts. Prostatic fluid is thin and milky, contains several enzymes, cholesterol, buffering salts and phospholipids.
3. The bulbourethral glands are a pair of small glands along the urethra below the prostate. Before ejaculation they secrete a clear, alkaline and viscous fluid that lubricates the urethra to facilitate the ejaculation of semen. Bulbourethral fluid also carries some sperm released before ejaculation, which is one reason for the high failure rate of the withdrawal method of birth control.

Penis

The penis has two functions,

1. It carries urine through the urethra to the outside during urination, and
2. It transports semen through the urethra during ejaculation.
3. The human penis is composed of three cylindrical strands of spongy erectile tissue derived from modified veins and capillaries: two corpora cavernosa and the corpus spongiosum. The corpus spongisum extends beyond the corpora cavernosa and becomes the expanded tip of the penis called the glans penis. The human glans is covered by a fold of skin called the foreskin, or prepuce, which may be removed by circumcision. Fig. 7.7

Sperm

A mature human sperm consists of a head, midpiece, and tail. The head contains the haploid nucleus, which is mostly DNA. The acrosome, a cap over most of the head, contains an enzyme called acrosin that assists the sperm in penetrating the outer layer surrounding a secondary oocyte. The sperm tall contains an array of microtubules that bend to • produce whip like movements. The spiral mitochondria in the midpiece supply the ATP necessary for these movements.

Q.12. What are the major constituents of semen? What are the functions of seminal fluids?

Ans. Semen is a mixture of seminal fluid and sperm cells. The seminal fluid is secreted by three sets of glands: the seminal vesicles secrete water, fructose, prostaglandins, and vitamin C; the prostate, secrete water enzymes, cholesterol, buffering salts, and phosphohpids; and the bulbouretheral glands secrete a clear, alkaline fluid. The average human ejaculation produces 3 to 4m1 of semen and contains 300 to 400 million sperms.

The seminal fluid has a variety of functions:

1. It serves as a vehicle for transport of sperm.
2. It lubricates the passage through which the sperm must travel.
3. As an effective buffered fluid, it helps protect the sperm from the harmful effects of the acids in the female genital tract.
4. It contains much sugar (mostly fructose) which the active sperm can use as a source of energy.

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