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Unit 6 Urinogenital System Fig. 6.12: Male and Female reproductive systems in humans. (^) 201 You will recall that anamniotes include cartilaginous and bony fishes and amphibians while amniotes include the reptiles, birds and mammals.
THE GENITAL SYSTEM
The genital system or the reproductive system and is made up of the gonads, gonoducts and genital openings. The gonads as you have read are mesodermal in origin. They are called testes (sing. testis) in males and ovaries (sing. ovary) in female. Both the testes and the ovaries produce gametes and secrete hormones and are termed as primary sex organs. The early development of the genital organs is the same in both sexes, this is the sexually indifferent stage. For some time the sex organs of the embryo remain in an indifferent stage, during which the development of the gonads and their ducts proceed considerably far without any indication of whether the embryo would develop into a male or female. Finally the embryo develops its specific sex features, presumably due to hormonal activity. Unpaired gonads are present in cyclostomes (jawless fish), certain fish as well as female birds of most species. This is the result of either fusion of paired gonads or the unilateral degeneration of one gonad. Evidence of metamerism (repetition of organs in segments) of gonads is only found is the primitive chordate amphioxus and in no other vertebrate. In vertebrates the gonads called the ovaries in females and the testes in males are attached to the dorsal body wall by mesentery like bands of tissue, called the mesorchium in the males and mesovarium in the females (Fig.6.12). In most vertebrates the gametes ova in females and spermatozoa or sperms in males are produced by the ovaries and testes (sing:testis) respectively. In males the spermatozoa are transported outside the body by means of vas deferens while in females they are transported by oviducts. In a few forms of primitive vertebrates, like cyclostomes the ducts are absent in both sexes. In them the gametes escape from the body cavity through genital pores. The vas deferens of the male genital system of the anaminote vertebrates are usually the mesonephric or the Wolffian ducts which also serve to transport urinary wastes from the opisthonephric or mesonephric kidneys which function in these animals either during embryonic or adult life (in male). In amniotes in which the metanephros is the functional kidney and in
Block 2 Functional Anatomy of Vertebrates-II 202 which deliver full fledged young ones) e.g. mammals. which the mesonephros degenerates, the Wolffian duct of the male on each side of the testes persists to become the male genital duct and is referred to as vas deferens or ductus deferens. In vertebrates, such as amphibians, birds reptiles and egg laying mammals a distinct cloaca (derived, from the Latin meaning ―sewer‖), is present into which the reproductive ducts open posteriorly. The cloaca is a common chamber into which intestinal, reproductive, and excretory canals empty. In all the placental mammals however, the cloaca is absent; instead the urinogenital system has its own opening separate from the anal opening and the cloacal region becomes modified so that in females the reproductive ducts called the oviducts open separately to the outside or as in the males, the reproductive ducts called the vas deferens join the excretory ducts to emerge as a common opening termed as the urinogenital opening. In many aquatic vertebrates, fertilisation is external while in all terrestrial vertebrates except for anuran amphibians and even in many aquatic species it is internal. In some animals transport of spermatozoa from male to female is brought about by apposition of the cloaca (cloaca: sing) of the two sexes in most animals. However, several males have copulatory organs which are used in an intromittent manner to deposit the spermatozoa into the reproductive tract of the female. Various types of copulatory organs are found in the vertebrate groups. In both sexes all the structures or organs which help to convey the germ cells or products of the primary sex organs like the reproductive ducts are termed as accessory sex organs. The other accessory sex organs include the, glands associated with the primary sex organs and the intromittent sex organs. Secondary sex characters are indirectly concerned with sex but play a part in the reproductive scheme. Sexual differences in secondary sex characters such as plumage, body size and strength, as well as vocal apparatus are only indirectly related to reproduction. Functions of the Genital System The primary function of the genital system is production of gametes, the sperms and the eggs respectively by the male and female gonads respectively under the influence of hypothalamo – pituitary hormones. Gonoducts or genital ducts of the genital system transmit gametes to the place most suitable for fertilisation. It may be water in which eggs are laid by the female and sperms are dropped on them by the male (external fertilisation) or sperms may be discharged just as far into the female tract where egg and sperm can meet (internal fertilisation). Transport of sperms into female body is specially performed by accessory reproductive organs or intromittent organs, to ensure fertilisation. The female genital system is different from that of the male. It is specialised for retention of: (i) egg (in case of egg laying or oviparous organisms), e.g. fish, amphibia, reptiles and birds (ii) the fertilised egg with the developing embryo (in case of ovoviviparous organisms which lay the egg after embryo has developed to a certain stage) e.g. certain snakes or (iii) the developing embryo (in case of viviparous organisms
Block 2 Functional Anatomy of Vertebrates-II sperms and sertoli cell. 204 seminiferous tubules in which the primordial germ cells develop into mature sperm and are conveyed to the outside by means of the vas deferens ducts. In others, such as cyclostomes, fish and urodele amphibians (ii) the testes consists of rounded cavities called seminiferous ampullae or spermatic ampullae or spermatic cysts. Both the elongated tubules, and rounded ampullae and at the embryonic stage consist of solid masses of cells which later develop the cavities or lumina. Seminiferous Tubules The testes in anurans, reptiles, birds and mammals and even some teleosts (bony fishes), are composed largely of seminiferous tubules which are coiled tubes and whose walls contain cells that produce the sperms (Fig. 6.13 a). (b) (c) (^) (d) Fig. 6.13: Mammalian testis (a) Longitudinal section through testis showing the epididymis which is the passage through which the sperms travel to the penis (b) an enlargement of the boxed area in (a) showing the details of seminiferous tubules and rete testis (c) section of testes showing seminiferous tubules and a group of Leydig cells between the tubules (d) enlargement of the boxed area in (c) showing developing
Unit 6 Urinogenital System are usually less than 10 in number. The kidney tubules or vasa efferentia drain (^205) Each paired testis is surrounded by a capsule, the tunica albuginae (Fig.6. a) which grows inwards in many mammals including humans, to separate the inner cavity into compartments that contain the seminiferous tubules. This arrangement allows the packing in of an extensive quantity of germinal epithelium into a small space (Fig. 6.13 a). The seminiferous tubules are surrounded by interstitial cells of Leydig , which secrete testosterone. The seminiferous tubules are lined with multilayered germinal epithelium which contain the germinal cells called the spermatogonia cells and supportive cells called the Sertoli cells which are nutritive in function and in which the heads of the maturing sperms are embedded (Fig. 6.13 b & c). Sertoli cells also produce, most of the fluid in which the sperms are suspended, while leaving the testis. The seminiferous tubules are inconspicuous and the germinal epithelium is inactive in immature males and between breeding season in the adult males of those species which have specific breeding seasons. In some species however, spermatogenesis (Fig. 6.13 d) or production of sperms in adult males proceed at a variable pace throughout the year. An active germinal epithelium may exhibit all stages of developing sperms. The lumen or tubule cavity contains the tails of many sperms whose heads are embedded in the Sertoli cells, free sperms and fluid (Fig. 6.13c). In mammals in any single zone along a tubule, all sperm are at the same stage of maturation; adjacent zones contain different generation of sperms and a period of sperm formation and discharge is followed by interval of inactivity. Mature sperms move from the lumen of the seminiferous tubules into the straight tubules known as tubuli recti and from there to a fine network of tubules called rete testes and finally through 15-20 efferent ductules into the anterior head of the epididymus (see Fig. 6.13 a again). The process by which the spermatogonial cells in the seminiferous tubules develop into mature sperms is known as spermatogenesis, You will learn more about this process in Block 3, Volume 2 of this course Male Genital Duct The male genital ducts which in most vertebrates serve to transport spermatozoa to the outside of the body are as mentioned earlier, are the Wolffian ducts, and have been formed in the developing mesonephros kidneys. You will recall that the name Wolffian duct is used for the kidney duct in anamniotes. Although the male genital ducts are similar to the female genital ducts, the male genital ducts have a more complex history and organisation. The original function of the genital ducts as you will recall, is the elimination of urinary wastes. In a number of fishes and amphibians certain tubules of the kidney become modified into genital ducts and are employed in carrying spermatozoa from the testis to the Wolffian ducts. They are referred to as efferent ductules or efferent ducts. The Wolffian ducts as genital ducts are named as the ductus deferens or vas deferens. In all amniotes, the mature sperms from the testes are collected by the rete testes which are then drained by a number of small ducts called the efferent ducts or vasa efferentia. The vasa efferentia are modified kidney tubules and
Unit 6 Urinogenital System rudimentary testicular tissue. In most fishes except in very primitive forms, the (^207) An ovarian follicle consists of an oo External Male Genitalia: Intromittent Organs In most aquatic forms external fertilisation takes place and water provides the medium by which the spermatozoa reach the eggs. In terrestrial forms, however, a liquid environment is needed to transport the spermatozoa so internal fertilization is the rule. The necessary fluids are produced by both male and female vertebrates. In a number of terrestrial vertebrates spermatozoa are transferred from male to female by cloacal apposition but in most terrestrial forms, and even in many aquatic species where fertilisation is internal the males develop serxual organs called intromittent or copulatory organs which are used by the males during internal fertilization to introduce the sperms suspended in seminal fluid into the female tract. In fishes the copulatory organs usually consist of modified fins. In snakes and lizards paired hemipenis are present; but in turtles, crocodilians, certain birds and all mammals, a single penis is used for this purpose. In all mammals except monotremes. The urethra arises from the urinary bladder and passes through the penis. It thus serves as a passage for both urinary and seminal fluids. SAQ 6 Fill in the blanks FEMALE GENITAL SYSTEM OF VERTEBRATES The genital system in vertebrate female (Fig. 6.14 a) consist of: Paired gonads – ovaries (sometimes one due to degeneration) Paired oviducts Single female genital opening Ovary Ovaries lie within the body cavity and are suspended by the dorsal mesentry, the mesovarium, through which blood and lymph vessels and nerves pass. Primitive vertebrate ovaries are found in hagfish, in which a mesentry-like fold of gonadal tissue stretches across nearly the entire length of the body cavity. In the hagfish, the unique feature is that the functional ovarian tissue occupies only the anterior half of the gonadal mass while the rear part contains the
Block 2 Functional Anatomy of Vertebrates-II 208^ uterus and vagina; (b) cut away of ovary showing internal structures. (a) (b) Mülerian duct: either of a pair of embryonic ducts parallel to the Wolffian ducts and giving rise in the female to the fallopian tubes, uterus, cervix, and upper portion of the vagina—called also paramesonephric duct. ovaries are similarly elongated and large in comparison to body size. In tetrapods except for mammals the ovaries are usually confined to the middle third or half of the body cavity, particularly during the non breeding seasons. The ovaries of mammals undergo moderate caudal displacement in order to be located between the kidney and the pelvis and are much smaller in comparison to body size. The shape of an ovary depends on many factors like whether one egg or thousands are discharged (ovulated), whether the eggs are immature or ripe; whether mature eggs are small or large, or whether pigments occur in the egg cytoplasm, such as those responsible for yellow yolk. The appearance of the ovary is also affected by other factors such as the season of the year in seasonal breeders, as ovary enlarges during the breeding seasons and diminishes in size between seasons, the age of the animal (whether juvenile or reproductively active or senile especially in birds and mammals) and the fate of the ovulated egg, follicles or sacs. Morphologically the ovary is of two types- compact or saccular. Most vertebrates have a compact ovary. Mammalian ovary is a typical example of the compact type. We will study the genital system and ovary of the female human being as an example of the genital system. In Fig. 6.14a you can see the female genital system of human female in which the paired ovaries are of compact type. In Fig. 6.14 b the internal structures of the female human ovary are seen in its longitudinal section. Each ovary is a compact structure which is covered by a germinal epithelium that is continuous with the peritoneum lining of the body cavity. Beneath the epithelium, is a layer of condensed tissue composed of short connective- tissue fibers with fusiform cells between them which forms the capsule of the ovary known as the tunica albuginea. The tunica albuginea of the ovary which is considerably thinner than that surrounding the testis. Internal to tunica albuginea is located an external cortex. The internal medulla is located underneath the external cortex Fig. 6.14: Mammalian female reproductive system. (a) showing oviduct, ovary,
Block 2 Functional Anatomy of Vertebrates-II 210 the tip and one tip enters one lateral vaginal canal to discharge the semen. Uterus Uterus is the muscular middle part of the oviduct (refer to Fig. 6.14 a). Its muscular portion is formed of the myometrium. The inner lining of the uterus is called the endometrium. The endometrium becomes highly vascularised before the blastocyst stage when the developing embryo gets attached to the uterus. In all mammals the uterus narrows to form the posterior cervix (Fig. 6.14a). Lips of the cervix enclose the uteri opening or ‗ os uteri ‘ through which sperms rise upto the upper part of oviduct for fertilisation. The cervix dilates during delivery of the baby. Uterus shows a number of variations in mammals. In primitive mammals like monotremes (egg laying mammals) and marsupials there are two uteri ( duplex uterus ). In most mammals however, the distal parts of the two uteri are fused together to give a bipartite or bicornuate uterus. In higher primates there is a complete fusion of the two uteri to form a single simplex uterus as seen in humans (Fig. 6.15). Fig. 6.15: Diagram showing the fusion of posterior ends of paired uteri in females of placental mammals. (The uterus and part of the vagina have been cut open.) Vagina The uterus leads through the narrow cervix into the vagina. Vagina is the fused terminal part of the Müllarian duct. It opens into urinogenital sinus or is extended to open directly to the outside. It is a muscular distensible tube which receives the penis during mating. It has convolutions on it which are called vaginal rugae. Vagina opens to the exterior by the female genital opening. Vagina is absent in egg laying mammals. Marsupials, have paired vagina that open into urinogenital sinus. To match the two vagina, male penis is forked at
Unit 6 Urinogenital System these glands (Fig. 6.16). (^211) Female Accessory Glands You have already read in subsection 6.7.3 about the various accessory glands associated with the male genital system. You will similarly find that accessory glands are also associated with the female genital system of many vertebrates. Oviducts of many female vertebrates have glands called the albumin gland that coat the egg with albumin. Other glands associated with the oviduct are the shell glands that cover the egg with shell material; mucous cell or oviducal tubular glands that secrete a jelly like material. Amniotes that lay large eggs may have in the oviduct mucous secreting glands called vaginal mucous gland that coat the egg prior to expulsion, possibly to lubricate it. Some fishes have adhesive glands that coat the eggs with a sticky secretion so that the eggs can adhere together or to appropriate objects. In some vertebrates that retain the developing embryos within the body, special glands of the oviduct evolve in order to nourish the young. These glands secrete into the oviduct so that young present there absorb or ingest the secretion. This secretion is called uterine milk or embryotrophe. External Female Genitalia In females the external genitalia in comparison to males is feebly developed. Maximum development of female genitalia is in the Order Primates of mammals. In human females it is well developed and is described below: External Genitalia of Human Female The external genital organs of the female are termed as vulva. The external genitalia of the human female consists of an outermost structure called labia majora which is a pair of skin folds and contains adipose tissue. Within the cleft formed by these folds are the labia minora , which are a smaller pair of skin folds that are highly vascularised but have no fatty tissue. At the anterior end of the vulva, these two interior skin folds, partly enclose the clitoris ,which is a small organ for sexual stimulation. The opening of the urethra is about midway between the clitoris and vaginal opening. The vaginal opening also called the vestibule of the vagina is located behind the urinary meatues and is much larger than the urinary opening. It is covered by a thin mucous membrane the hymen. Bartholin‟s glands (or bulbovestibular glands) or greater vestibular glands are two bean shaped glands, one on either side of the vaginal opening also called vestibule into which they secrete a lubricating fluid. Bartholin glands are homologous to the male bulbourethral glands. A group of tiny mucous glands, the lesser vestibular glands also called the Skenes glands open into the vestibule as well. Mammary Apparatus Mammals are named so because of the characteristic presence of the mammary apparatus, which includes (i) mammary glands , (ii) elevated nipples which are the outlets for secretion of these glands and (iii) breasts or mammae which are the integumentary swellings due to localised presence of
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