REPRODUCTION AND DEVELOPMENT IN ANIMALS 1

Reproduction and its type

Reproduction : Reproduction is the ability of living organism to produce a new generation of living individuals similar to themselves.

Basic features of reproduction : All organisms reproduce. Modes of reproduction vary in different organisms. However, all modes have certain common basic features. These are

(1) Replication of DNA. This is the molecular basis of reproduction.

(2) Cell division, only mitotic, or both mitotic and meiotic. This is cytological basis of reproduction.

(3) Formation of reproductive bodies or units.

(4) Development of reproductive bodies into offspring.

Types of reproduction : These are of two main types

(1) Asexual (Non-gametic)            (2) Sexual (gametic)

(1) Asexual reproduction

Definition : Production of offspring by a single parent without the formation and fusion of gametes is called asexual reproduction. The young one receives all its genes from one parent.

Asexual reproduction is also known as agamogenesis or agamogeny. It involves only mitotic cell divisions, and also termed somatogenic reproduction. Asexual reproduction produces identical offspring commonly referred to as a clone. Today, the scientists have been able to produce clones of multicellular animals (e.g., boar calf names as Frosty, and Finn Dorset lamb named as the famous Dolly) artificially in the laboratory.

Occurrence : Asexual reproduction occurs in protozoans and some lower animals such as sponges, coelentrates, certain worms and tunicates. It is absent among the higher non-vertibrates and all vertibrates.

Types : Asexual reproduction takes place in five principal ways :

(i) Binary fission : Binary fission is the division of the parent into two small, nearly equalized daughter individuals. During binary fission nuclear divisions or karyokinesis, always followed by division of cytoplasm or cytokinesis. Examples – Protozoans (Amoeba, Euglena etc.) Bacteria and Planarians.

Modes of binary fission : In Binary fission, the nucleus divides first and the cytoplasm next. Subsequently, the mother cell splits into two equal sized daughter halves or cells. There are three modes of binary fission.

(a) Simple binary fission : If the plane of cytoplasmic division passes through any direction, the fission is called simple fission. Example – Amoeba.

 

(b) Transverse binary fission : If the plane of cytoplasmic division coincides with the transverse axis of the individual, the fission is termed transverse binary division. Example – Paramecium and Planaria.

(c) Longitudinal binary fission : If the plane of cytoplasmic division concides with the longitudinal axis of the individual. This kind of fission is designated as longitudinal binary fission. Example  Euglena and vorticella.

Binary fission involves mitosis only and consequently, the resultant offspring’s are genetically identical to the parent and each other.

(ii) Multiple fission : Multiple fission is the division of the parent into many small daughter individuals simultaneously. Examples – Multiple fission occurs in many protozoans such as Plasmodium, Amoeba and Monocystis, Foraminifera.

 

Mode of multiple fission : Sometimes, the nucleus divides several times by amitosis to produce many nuclei, is not followed immediately by cytokinesis. Later, each nucleus gathers a small amount of cytoplasm around it and the mother individual splits into many tiny daughter cells.

In course of time, each of these daughter cells starts a free life and transforms into an adult individuals. This kind of fission is called multiple fission.

(a) Encystation : In response to unfavourable living condition, an Amoeba withdraws its pseudopodia and secretes a three-layered hard covering or cyst around itself. This phenomenon is termed as encystation.

(b) Sporulation : During favourable condition, the encysted Amoeba divides by multiple fission and produces many minute amoebae or pseudopodiospores; the cyst wall burst out, and the spores are liberated in the surrounding medium to grow up into many amoebae. This phenomenon is known as sporulation.

(c) Schizogony : It is a type of multiple fission present in plasmodium. Schizogonies are of two type. Liver schizogony and RBC schizogony.

(iii) Plasmotomy : Plasmotomy is the division of a multinucleate protozoan into several small, multinucleate daughters without nuclear division. The daughters grow and regain the normal number of nuclei by nuclear divisions. It takes place in Opalina and Pelomyxa.

(iv) Budding : Formation of a daughter individual from a small projection, the bud, arising on the parent body is called budding. It is a common method of asexual reproduction. In budding new individual form by mitosis. Examples – Budding occurs in some protozoans and certain lower animals such as sponges (Scypha), coelenterates (Hydra), annelids (Chaetopterus) and tunicates (Salpa).

Types of budding : There are two types of budding

(a) Exogenous or External budding : Initially, a small outgrowth of the parent’s body develops into a miniature individual. It then separates from the mother to lead a free life. This type of budding is recognised as exogenous budding. Example – Hydra.

 

(b) Endogenous or Internal budding : In fresh water sponges (e.g., Spongilla) and marine sponge (e.g., Sycon), the parent individual releases a specialised mass of cells enclosed in a common opaque envelope, called the gemmule, on germination. Each gemmule gives rise to an offspring gemmules are thought to be internal buds. This type of budding recognised as endogenous budding. Example – Sycon and Spongilla.

(v) Fragmentation : It is the breaking up of an animal’s body into two or more pieces, each of which grows into a new individual. Examples – It occurs in the flatworm, microstomum.

Special asexual reproductive bodies : Archaeocytes of sponges are totipotent cells. They take part in the formation of gemmules. Gemmules form new sponges.

Reproductive units in asexual reproduction : Reproductive units vary in different forms of asexual reproduction. These are entire parent bodies in binary and multiple fission’s and are small parts of parent body in budding and fragmentation. An asexual reproductive unit is called blastos.

Characteristics of asexual reproduction : All forms of asexual reproduction have certain common basic features. These are under :

(i) A single parent produces offspring, that is, asexual reproduction is uniparental.

(ii) Gametes are not formed.

(iii) Cell divisions are only mitotic.

(iv) The new individuals formed are usually genetically identical to the parent. Variability, if it occurs, is restricted to mutation only.

(v) Multiplication occurs rapidly.

(vi) The offspring are often formed in large numbers near the parent.

Significance of asexual reproduction : Asexual reproduction brings multiplication of the species only. It does not play a role in evolution as no variation is introduced into the new individuals formed by it. Asexual reproduction is theoretically most advantageous in stable, favourable environment because it perpetuates successful genotypes precisely.

(2) Sexual reproduction

Definition : It is the production of offspring usually by two parents, male and female. Involving four processes :

(i) Formation of special haploid cells, the gametes, by meiosis. (Gametogenesis)

(ii) Fusion of the gametes in pairs, forming diploid cells, the zygotes (Fertilization)

(iii) Repeated mitotic divisions of zygotes to form embryos (Embryogenesis)

(iv) Growth of Embryos into a new individuals (Development)

• Sexual reproduction is also called syngenesis.

Occurrence : Sexual reproduction occurs nearly in all animals, including those which reproduce asexually. Some protozoans, such as Amoeba, Euglena lack sexual reproduction. In male and female, and the difference between them is determined genetically. In sexual reproduction offsprings resemble the parent.

Types : Sexual reproduction is of two main types –

(i) Amphigony : It involves the complete and permanent fusion of two gametes from differents or from the same parent to form a composite cells, the zygote. It is further of two kinds :

(a) Syngamy : It involves the fusion of two entire gametes to form a zygote. The fusion nucleus of zygote called synkaryon. It is further of two types with regard to the source of fusing gametes :

• Endogamy : It involves self-fertilization, e., the fusion of two gametes of the same parent. It is, thus uniparental. e.g., Taenia.
• Exogamy : It involves cross-fertilization, e., the fusion of two gametes formed by different parents. It is, thus, biparental. e.g., frog rabbit and man. Syngamy is also of two kinds :
• Isogamy : The fusing gametes are similar morphologically as in Monocystis (a protozoans). Such gametes are known as isogametes, and their fusion is termed isogamy. Although the isogametes are similar in structure, they have behavioural differences. g., forminifera, monocystis.
• Anisogamy or Heterogamy : The fusing gametes are different in form, size structure and behaviour as in frog and humans. Such gametes are known as anisogametes, or heterogametes, and their fusion is termed anisogamy, or heterogamy. g., Plasmodium, vertebrates.

Special forms of syngamy : These are three special forms of syngamy :

(a) Neoteny : Neoteny refers to “retention of a larval or embryonic trait in adult body”. e.g., Retention of larval gills in some adult salamander.

(b) Paedogenesis or paedomorphosis : Paedogenesis is refers to “development of gonads and production of young ones by larva”. e.g., salamander Axolotl larva,  liver fluke Redia larva, and gall fly.

(c) Polyembryony : The blastomeres formed by division of the zygote separate in early stages of development, each producing in a complete individual (fasciola liver fluke). Armadillo regularly produces 4-8 young ones per zygote. Identical twins in human beings is another example.

(ii) Conjugation : Some acellular protist animals (e.g., Paramaecium) exhibit sexual reproduction by forming male and female gamete nuclei, which they exchange through temporary cytoplasmic bridge; later, the cytoplasmic bridge disappears and the gamete nucleus of one individual fuses with that of the other to form zygote nuclei. This mode of sexual reproduction is known as conjugation.

Parthenogenesis (Virgin birth)

It is a modification of sexual reproduction in which an egg develops into a complete offspring without fertilization. It is monoparental. Parthenogenesis was discovered by Bonnet (1745).

Occurrence : Parthenogenesis is found in many non vertebrates such as rotifers, aphids, bees and crustaceans. It also occur in a few vertebrates.

Types : Parthenogenesis is of two main types :

(1) Natural parthenogenesis : It is a regular phenomenon in the life history at some animals. It may be three type.

(i) Complete (Obligatory) parthenogenesis : Males are absent, females develop parthenogenetically, e.g., rotifers, Typhlina brahmina (small lizard, 15 cm long), Lacerta saxicola-armeniaca (Caucasian Rock Lizard), Cnemidophorus (Whiptail Lizards of America).

(ii) Incomplete (cyclic) parthenogenesis : Some animals have both sexual and parthenogenetic individuals, which may alternate. In these animals, female can produce unfertilized or fertilized eggs, depending upon environmental conditions. In Daphnia, a fresh water crustacean, female lays unfertilized eggs that develop parthenogenetically under favourable conditions, and fertilized eggs during times of environmental stress.

In honeybee, unfertilized eggs develop into male bees (drones) with haploid cells, and fertilized eggs give rise to females (queen bees and worker bees) with diploid cells.

(iii) Paedogenetic parthenogenesis : In certain insects, larvae lay eggs which develop parthenogenetically into a new generation of larvae. Parthenogenesis in larvae is called paedogenesis.

(2) Artificial parthenogenesis : Eggs of certain animals, such as annelids, mollusks, starfish, frog, hen, rabbit, etc., can be induced to develop parthenogenetically by artificial stimuli. Artificial stimuli may be (i) physical, viz., prick of a needle, electric shock, change in temperature or pH; or (ii) chemical such as addition of urea, fatty acids, ether, chloroform, to water.

On the basis of chromosome sets parthenogenesis is of two types –

(i) Arrhenotoky (Haploid parthenogenesis) : Haploid eggs grow to form haploid males e.g., Arachnids, some insects (honey bees).

(ii) Thelotoky (Diploid parthenogenesis) : Diploid eggs grow without fertilization in to diploid individuals, generally females. e.g., Gall fly.

Advantages of parthenogenesis

(i) This avoids the wastage of germplasm as sperms and ova. Adult organism is devoted exclusively to feeding and reproduction so is a mode of high reproduction e.g., aphids.

(ii) There is no chance of separation of useful combination of genes by crossing over and are transmitted as such.

(iii) The offsprings are exactly similar to parents.

(iv) Haploid parthenogenesis is the direct proof of chromosomal theory of sex-determination.

Disadvantages of parthenogenesis : It stops the chances of new combinations of genes and thus avoids selection in population. It decreases the chances of adaptability followed by extinction.

Reproductive unit in sexual reproduction

The reproductive units in sexual reproduction are specialised cells called gametes. The gametes are generally of two kinds –

(1) Microgametes or Spermatozoa

(2) Macrogametes or Ova

Both are well developed for their role in reproduction. The male gametes are mostly minute and motile so that they may swim to the female gametes for fertilization. The female gametes are usually large, non motile and often have a store of food to nourish the developing embryo.

Maintenance of chromosome number : The gametes are usually formed by meiotic divisions. Therefore, they are haploid, i.e., have halved or reduced (n) number of chromosomes. In sexual reproduction, the male and female gametes fuse to form a single cell, the zygote formed by the fusion of two haploid gametes in naturally diploid, i.e., has double or normal number (2n) of chromosomes.

Reproduction pattern

(1) External fertilization and External development : This pattern is found in many aquatic animals, such as Obelia, Nereis, all bony fishes and frogs. Parents release sperms and eggs into the surrounding water, where fertilization occurs and zygotes develop into offspring.

(2) Internal fertilization and External development : Sperms are passed from the male into the female with an intromittent organ, such as claspers in male shark, hemi penis in lizard, snake, crocodile and penis in mammals, or otherwise, for example, by cloacal apposition in birds, with modified arm in cuttle fish. Internal fertilization has several advantages.

Animals with internal fertilization usually produce fewer zygotes because of protection provided by egg shells or internal development.

(3) Internal fertilization and Internal development : Internal development provides additional advantages to the embryo. The mother’s body provides exactly the right chemical conditions and, in mammals, warmth and nourishment also. As the mother carries the embryo wherever she goes, it is not vulnerable to predators who attack externally developing eggs.

Characteristics of sexual reproduction : Sexual reproduction has the following important basic features –

(i) It is generally biparental.

(ii) It involves formation of male and female gametes.

(iii) Mostly there is fusion of male and female gametes (fertilization).

(iv) Cell divisions are meiotic during gamete formation and mitotic during development of zygote into an offspring.

(v) The offspring are not genetically identical to the parents. They show variation as they receive characters (chromosomes) from two different parents. Sexual reproduction is, thus, a source of variety in population.

Significance of sexual reproduction : Sexual reproduction has a dual significance for the species :

(i) It results in multiplication and perpetuation of the species.

(ii) It contributes to evolution of the species by introducing variation in a population much more rapidly than asexual reproduction.

Table : 8.3-1 Difference between sexual and asexual reproduction

S.No.	Asexual reproduction	Sexual reproduction
1.	It is always uniparental.	It is generally biparental.
2.	It invariably results in increase in the number of individuals.	It may not result in increase in the number of individuals.
3.	Gametes are not formed.	It always involves the formation and fusion of gametes.
4.	There is no fertilization.	Fertilization generally occurs.
5.	It involves only mitotic cell divisions.	It involves meiotic divisions during gamete formation and mitotic divisions during development of zygote into an offspring.
6.	Daughter individuals are genetically identical to the parent.	Daughter individuals genetically differ from the parents.
7.	It occurs in only lower invertebrates and lower chordates.	It occurs nearly in all animals.
8.	It contributes little to evolution.	It contributes to evolution by introducing variation in offspring.
9.	It often causes rapid increase in number.	It causes slower increase in number.

Blastogenesis and Embryogenesis : Development of the offspring from reproductive units, such as buds or fragments, in asexual reproduction is called blastogenesis. Development of the embryo from the zygote in sexual reproduction is termed embryogenesis.

Unisexual or dioecious : Organism in which the two sexes occur in different individuals, e.g., humans, mammals, birds, lizards.

Bisexual / Hermaphrodite or monoecious : Organism in which the two types of sex organs (testes and ovaries) occur in the same individual, e.g., Earthworm, Taenia, Leech, Fasciola, Myxine, Herdmania.

Deviations in the reproductive strategies : Although asexual and sexual reproductions are the two major trends of breeding, many deviations are also observed in the reproductive strategies of animals. One such variation in reproductive strategy in hermaphroditism, found in tapeworms and earthworms. Tapeworms are self-fertilising; the sperm produced in the testes of one individual can fertilise the eggs produced by the same individual. The earthworms employ cross, fertilisation; the sperm of one individual fertilises the eggs of the other.

Sexual dimorphism : Differentiation in morphology of the two sexes of the same species is called sexual dimorphism. Example – Ascaris, Oryctolagus and humans etc.

Human reproductive system

In human beings, reproduction takes place by sexual method and the sexes are separate.

(1) Sex organs : Human are unisexual. The reproductive system of each sex consists of many organs. The latter are distinguishable into primary and secondary sex organs. Besides these, there are some accessory sex characters –

(i) Primary sex organs : Gonads which form gametes are called primary sex organs – testis (plural testes) in males and ovary (plural ovaries) in females. Testis produces sperms and secrets testosterone. Ovary produces ova. Maturing Graffian follicles secrete estrogens.

(ii) Secondary sex organs : Sex organs, glands and ducts which do not produce gametes but are otherwise essential for sexual reproduction are known as secondary sex organs. In human male reproductive system, the secondary sex organs are vasa efferentia, epididymes, vasa deferentia, ejaculatory ducts, urethra, accessory sex glands are prostate glands, Cowper’s glands seminal vesicle and penis. Secondary sex organs of a human female include fallopian tubes, uterus, vagina, external genitalia, Bartholin’s gland and mammary glands are accessory sex glands.

(iii) Accessory / External / Secondary sex characters : They are traits which do not have any direct role in reproduction but provide specific features and structures to the two sexes. The important external / accessory sex characters of human male are beard, moustaches, body hair on shoulder and chest, pubic hair on both lateral and vertical directions, comparatively more height with more muscular body, larynx apparent externally, voice low pitched with breathing more by means of diaphragm. The important accessory sex character of human females are high pitched voice, breast, broader pelvis, lateral pubic hair, rounded body contours with more subcutaneous fat in thighs, buttocks and face and sternal breathing.

• Puberty : Beginning of sexual maturity or ability to reproduce is known as puberty. Puberty occurs at the age of 10 – 14 years in girls and 13 – 15 years in boys.

(2) Characteristics of human reproduction :

(i) Human beings are non-seasonal breeders.

(ii) There is no oestrus / heat.

(iii) In human females the ability to produce young ones begins at menarche (beginning of menses) and ends at menopause (stoppage of menses).

(iv) In human females the reproductive phase has 28 day repeated menstrual cycle.

(v) Fertilization is internal.

(vi) There is vivipary, i.e., giving birth to young ones.

(vii) Foetus develops inside uterus and is nourished by joint special structure called placenta.

(viii) Infants can be fed on mother’s milk.

(ix) Parental care is very well developed.

Male reproductive system

The male reproductive system consists of a scrotum, a pair of testes, vasa efferentia, a pair of epididymis, a pair of vasa deferentia, a pair of ejaculatory ducts, a urethra, a penis and certain accessory sex glands.

Reproductive organs

(1) Scrotum : The scrotum is a pouch of pigmented skin arising from the lower abdominal wall and hanging between the legs. The testes originate in the abdominal but latter, during the seventh month of development, descend permanently into the respective scrotal sac through passages termed inguinal canal.

 

A spermatic cord connects testis with abdominal cavity. It consists of connective tissue that encloses an artery, a vein, a lymph vessel, a nerve, cremaster muscle and a vas deferens. A testis rests in it chamber over pad called gubernaculum.

The scrotal sac of male homologous to female’s labia majora.

• Variations in position of testes : In some mammals (lion, bull, horse), the testes remain permanently in the scrotum and keep functioning throughout the year as in man. In certain seasonally breeding mammals, such as bat, otter and llama, (Insectivora, Tubulidentata and most Rodentia) the testes enlarge, become functional, and descent into the scrotum in the breeding season, but thereafter ascent into the abdominal cavity, and become reduced and inactive. In a few cases (elephant, whale, seal) the testes remain permanently in the abdomen as the body temperature is low enough for sperm maturation. Scrotum is absent in such cases. Scrotum is in front of penis in Kangaroo.

 

(2) Human Testes : The testes are the primary sex organs. They are about 4 – 5 cm long, 2.5 cm wide and 3 cm thick. They are suspended in the scrotal sacs by spermatic cords. Each testes weights about 10-15 gms.

Each testis has three coverings – tunica vaginalis, tunica albuginea and tunica vasculosa.

In growth of the tunica albuginea, called septa, divide the testis into some 200 to 300 lobules. Each testicular lobule contains 1 – 3 highly convoluted seminiferous tubules, blood vessels and nerve embedded in loose connective tissue. A total of about 750 seminiferous tubules occur in each testis.

Each seminiferous tubules is lined by germinal epithelium, seminiferous tubules is the site of spermatogenesis. The process occurs in waves along the length of the tubule, taking about 9 weeks (63 days) to complete in man. Seminiferous tubules contain 3 types of cells –

(i) Germ cells : Germ cells or primordial germ cells arise from yolk sac endoderm and enter the testes early in development. These are spermatogenic cells, by mitotic divisions, produce spermatogonia into the lumen of the seminiferous tubule. The spermatogonia grow into primary spermatocytes which undergo meiosis, producing haploid cells, first secondary spermatocytes and then spermatids. Spermatids differentiate by a process of spermiogenesis into dimorphic haploid sperm (containing X or Y chromosome). Mature spermatozoa lie free in the cavity of the seminiferous tubules.

(ii) Somatic cells / Sertoli cells / Sustentacular cells / Nurse cells : These are supportive nutritive and secrete a polypeptide hormone called inhibin and a steroid estradiol which interferes with spermatogenic activity and kinetics of sperm production.

(iii) Leydig cells ( = Interstitial cell) : Leyding cells endocrine cell of testes which lie in the form of clusters or singly in the interstitium (=space between seminiferous tubules).

These are secrete a sex steroids called androgen by using cholesterol. The cells contain a rich repertoire of enzymes which facilitate formation of pathways for steroid biosynthesis and biotransformation. These enzymes are called steroid-dehydrogenases.

• Rete testis : This is a plexiform arrangement (Network) of space supported by highly vascular collagenous connective tissue. It is lined by squamous epithelial cells some of which bear flagella whose activity assists in forward migration of testicular sperm (which are immotile at this stage).

The seminiferous tubules open into rete testis.

(3) Vasa efferentia : Rete testis is connected to caput epididymis by 12 – 20 fine tubules called vasa efferentia or ductuli efferentes. Their lining epithelium is ciliated for conducting sperms.

Tubuli recti, rete testis and ductuli efferents constitutes an intertesticular genital duct system. The cells of vasa efferens are columnar ciliated.

(4) Epididymis : From rete testis sperms moves into a series of coiled efferent ducts in epididymis that empty into a single tube called ductus epididymis present inside epididymis as highly coiled tube, measures about 6 m (20 ft) in length. It is lined by pseudostratified columnar epithelia.

Epididymes has 3 parts

(i) Upper part (Heads) : Caput epididymis or globus major.

(ii) Middle part : Corpus epididymis or globus normal.

(iii) Basal part (Tail) : Cauda epididymis or globus minor.

In epididymis the sperms are stored for a few hours to a few days till sent out through ejaculation.

The epididymis shows peristaltic and segmenting contraction at intervals to push the spermatozoa away from the testis.

Testis and epididymis are together called testicle.

(5) Vasa deferentia (Singular-vas deferens) : The vas deferens is a continuation of the cauda epididymis. It is about 45cm. long and is slightly coiled at first but becomes straight as it enters the abdominal cavity through the inguinal canal.

Vasa deferentia (ducti deferentes) conduct sperms from epididymis to urethra and lined by pseudostratified columnar epithelia.

Surgical interference (vasectomy) of vas deferens ensure successful non-reversible male contraception.

Table : 8.3-2 Difference between Vasa efferentia and Vasa deferentia

S.No.	Vasa efferentia	Vasa deferentia
1.	Arise from the rete testes.	Arise from the cauda epididymis.
2.	Vary from 15 to 20 in number.	Are only 2 in number.
3.	Are fine and convoluteds	Are thick slightly coiled in the scrotum, straight in the abdomen
4.	Lining bears many ciliated cells.	Lining has sterocilia on many cells.
5.	Carry spermatozoa from rete testes to caput epididymis	Carry spermatozoa from cauda epididymis to ejaculatory ducts.

(6) Ejaculatory ducts : They are short (2 cm) straight muscular tubes each formed by union of a vas deferens and duct of seminal vesicle where ejaculate is formed by mixing of sperms with secretion of seminal vesicle. The two ejaculatory ducts join the urethra within prostate gland.

(7) Urethra : It is the urinary duct leading from the bladder. Urethra passes through prostate gland, urinogenital diaphragm, and penis. From the point it is joined by ejaculatory ducts, it carries urine as well as spermatozoa and secretions of the seminal vesicles. It also receives secretion of the prostate and cowper’s glands. Urethra is some 20 cm long and passes through the penis. The urethra has 4 regions –

(i) Urinary urethra : It carries only urine.

(ii) Prostatic urethra : It is a short proximal part which is surrounded by prostate gland.

(iii) Membranous urethra : It is a short middle part, without any covering, is smallest part of urethra.

(iv) Penile urethra : It is a long distal part that passes through the penis, also known as spongy uethra.

The penile part is also called spongiose urethra because it lies inside corpus spongiosum.

(8) Penis : The penis is an erectile copulatory organ. It consist of a long shaft that enlarges to form an expanded tip, the glans penis. It is covered by a loose, retractable fold of skin, the prepuce or foreskin. Under the skin, the penis contains three columns of erectile tissue : two cylinders of the corpora cavernosa of the penis, placed dorsally, and one cylinder, the corpus spongiosum, along the ventral side. The corpora cavernosa of the penis and the urethra are covered by dense connective tissue, the tunica albuginea. Both urine and semen are carried out of the body through the penis. Corpus spongiosum contains the spongy urethra. Margins of glands penis known as corona.

The penis of opposum, bandicoot etc. is doubled branched.

• Sperm storage : Sperms are stored for the most part in the vasa efferentia, epididymes and proximal parts of vasa deferentia.

Accessory sex glands : The substances secreted by the accessory, sex glands help in reproduction these are –

(1) Seminal vesicles : The seminal vesicles are long pouches with muscular wall; they secrete spermatozoa activating substances, such as fructose, citrate, inositol, prostaglandins and  several proteins, sperms use fructose as a respiratory substrate. Seminal fluid maintains viability and motility of sperms.

Seminal vesicle secretes a alkaline, nutritive fluid which forms main part i.e., 60 % of the semen. It is also called uterus-masculinus. It forms from the mullerian duct of the embryo. In females, these ducts form the ovi-ducts. The seminal vesicle do not store sperms. Seminal vesicles are found between urinary bladder and rectum.

• Test for rape : Fructose, which is present in the seminal fluid and is not produced anywhere else in the body, provides a forensic test for rape. Its presence in the female’s genital tract confirms sexual intercourse.

(2) Prostate gland : The prostate gland surrounds the first portion of the urethra. This gland secretes an slightly acidic fluid  (pH about 6.5) which forms 25% part of the semen. The secretion nourish and activates the spermatozoa to swim. It is essential for sperm motility (removal causes sterlity).

In the secretion of prostate–gland citric acid, calcium and phosphate, Fibrinogen and Fibrinolysin is present. The secretion of the prostate gland combines with the secretion of seminal vesicle and so the semen gets coagulated. In the coagulated semen, the mobility of sperms is reduced and so their energy is conserved. After sometime due to fibrinolysins, semen again liquefies and in this semen now the sperms can move.

(3) Cowper’s glands : These are also termed as Bulbourethral glands. 1^st pair of Cowper’s glands are attached to urethra. They secrete alkaline mucus which is discharged into the spongy part of urethra. The mucus lubricates the reproductive tract. This serves to neutralize any acid of urine remaining in the urethra. Secretion of Cowper’s glands is produced before the ejaculation of semen.

Secretion of Cowper’s glands carries some spermatozoa released before ejaculation. This is one of the reasons for the high failure rate of the withdrawal method of birth control.

(4) Perineal or Rectal glands : These are found both in males and females during the breeding season, these glands secrete and odoriferous liquid which has pheromones or Ectohormones in it. Its smell attracts the animal of opposite sex, found in herbivorous and carnivorous mammals.

In man, Perineal or Rectal glands are absent.

(5) Other glands : Prepuce contains preputial glands which produce a sebaceous substance which together with desquamated epidermal cells forms a whitish, pasty, foul-smelling accumulation, called smegma, about the base of the glans penis beneath the prepuce.

Semen : The products of the testes (spermatozoa) and prostate gland, alongwith fluid from the seminal vesicle, are collectively knows as semen. It is a milky, viscus and alkaline (pH 7.2 – 7.7) fluid ejaculated by male reproductive system during orgasm. The volume of ejaculate varies from person to person. Abstinence play a role in this. Each ejaculate measures 3.5 ml and contains 50 – 150 million sperm/ml i.e. 250 million – 525 million (average – 400 million).

The life span of human sperm after ejaculation is 24 – 48 hrs. Crayopreservation enhances the longevity of sperm. The rate of active moment of sperm is 1.5 – 3.0 mm per minute in uterine endometrium.

Semen has chemicals for nourishing the sperms (e.g., – fructose), neutralizing the acidity of urethra and vagina (e.g., – bicarbonate), stimulating movements in female tract (e.g., – prostaglandins). pH of semen – 7.2 – 7.7.

A person with a sperm count below 20 million will be physiologically sterlile. Fusion of defective sperm (e.g., 22+xy) with ovum causes many birth defects e.g., klinefelter’s syndrome.

Hormonal control of male reproductive system : The growth, maintenance and functions of secondary sex organs (epididymis, vasa deferentia, accessory glands and penis) are under the control of testosterone hormone secreted by Leydig’s cells of testis, while those of seminiferous tubules and Leydig’s cells are controlled by Follicular Stimulating Hormone (FSH) and Interstitial Cells Stimulating Hormone (ICSH) of anterior pituitary lobe respectively.

Onset of puberty in the male : Puberty is the period when reproductive organs become functional. It is triggered by the secretion of the hormone testosterone in the testes. This hormone brings about growth and maturation of the secondary sex organs and development of the accessory sex characters. The latter induce :

(1) Enlargement of the penis and scrotum.

(2) Broadening of the shoulders.

(3) Growth of body and facial hairs.

(4) Deepening of the voice duce enlargement of layrnx and thickening of vocal-cords.

(5) Increased development of musculature and bones.

(6) Increase in height so characteristic of male puberty.

Male sex act

The male sex act involves 3 phases :

(1)  Erection : Erection of the penis is caused by rush of arterial blood into the empty sinuses of its spongy tissue on sexual excitement. As the spongy tissue distends, it compress the veins, inhibiting the flow of blood out of the tissue. Filling of tissue with blood is called vasocongenstion.

(2) Copulation : Mucus from the urethral glands, Cowper’s glands and vaginal glands provides lubrication for copulation. Friction due to rhythmic movements of sexual intercourse stimulate the sensory cells of the glans penis. This stimulation releases semen into the proximal part of urethra by contraction of reproductive glands and ducts. This process is called emission. Then the rhythmic, wavelike contractions of the muscles at the base of the penis cause forceful discharge, called ejaculation, of semen into the vagina. One ejaculate (about 3 ml.) contains 200 to 400 million spermatozoa. Ejaculation marks the climax of copulation.

Orgasm : At the peak of sexual stimulation, pleasurable sensation, called orgasm. It occurs usually last only a few seconds.

(3) Subsidence of erection : After ejaculation, the arterioles to the penis contract, reducing the blood flow to the penis, and erection subsides. This often takes a few minutes.

Disorders of male reproductive system

Only a few are mentioned.

(1) Prostatomegaly (Prostatic hypertrophy) : This is enlargement of prostate gland. If often occurs in old age. The enlarged gland may block the urethra, causing frequent night urination (nocturia) or difficult or painful micturition. Prostate cancer is very common in men. It is treated surgically or with drugs.

 

(2) Impotence : This is inability of the male to achieve and or maintain erection of the penis long enough to engage in or complete copulation.

(3) Sterility : Inability of the male’s sperm to fertilized the ovum, it may or may not be associated with impotence. Sterility also results from immobility and morphological abnormality of the sperms, and from low sperm count in the semen.

Female reproductive system

The female reproductive system consists of a pair of ovaries, a pair of fallopian tubes, uterus, vagina, external genitalia or vulva and breasts.

Reproductive organ

(1) Ovaries : Ovaries are the primary sex organs of female.

The ovaries are almond shaped bodies, about 3 cm long, 1.5 cm wide and 1 cm thick. The ovaries, like the testes, have both an exocrine function (production of ova) and an endocrine role (secretion of female sex hormones : estrogen and progesterone). After menopause, the ovaries become small and lose follicles.

Each ovary is located close to the lateral walls of the pelvic cavity, being suspended from the dorsal body wall just behind the kidney, by a section of peritonium, the mesovarium.

Each ovary is a compact or solid organ, consisting of an outer cortex and inner medulla. The stroma of the cortical region is composed of spindle shaped fibroblasts. A poorly delineated dense connective tissue layer, the tunica albuginea, covers the cortex. It imparts the whitish colour to the ovary. Located outside the tunica albuginea, the germinal epithelium, formed of simple squamous or cuboidal epethelial cells, covers the surface of the ovary.

(2) Fallopian tubes / Uterine tubes / Oviducts : Each ovary is located in front of a funnel shaped opening of the uterus, the oviduct. The oviduct is a muscular tube, measuring about 12 cm in length. Its lumen is lined by ciliated epithelium.

Oviducts develop from the mullarian duct of the embryo. It conveys the egg from the ovary to the uterus, and provides the appropriate environment for its fertilization. It is supported by a double fold of peritoneum called mesosalpinx. The wall of oviduct is made of three layers :

(i) Serosa : It is the outermost layer of visceral-peritoneum.

(ii) Muscle-layer : The middle layer of the oviduct is made up of unstriped-muscle.

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