Study Notes on Human Reproduction

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  • Humans are sexually reproducing and viviparous organisms.
  • Their reproductive events include formation of gametes (gametogenesis) , i.e. sperms in males and ovum in females, transfer of sperms into the female genital tract (insemination) and fusion of male and female gametes (fertilization) leading to formation of zygote.
  • This is followed by formation and development of blastocyst and its attachment to the uterine wall (implantation), embryonic development (gestation) and delivery of the baby (parturition).
  • There are remarkable differences between the reproductive events in the male and in the female, I puberty for example, sperm formation continues even in old men, but formation of ovum ceases in women around the age of fifty years.

THE MALE REPRODUCTIVE SYSTEM

  • The male reproductive system is located in the pelvic region.
  • It includes a pair of testes, along with accessory ducts, glands and the external genitalia.

 

The Testes

  • The testes are situated outside the abdominal cavity within a pouch called scrotum.
  • The scrotum helps in maintaining the low temperature of the testes (2 – 2.5°C lower than the normal internal body temperature) which is necessary for spermatogenesis.
  • Whenever the outside temperature is low, these contract to move the testes close to the abdominal or pelvic cavity. When outside temperature is high, these relax moving the testes away.
  • In adults, each testis is oval in shape, with a length of about 4 to 5 cm and width of about 2 to 3 cm.
  • The testis is covered by a dense covering. They are enclosed in an outer tough capsule of collagenous connective tissue, the tunica albuginea.
  • Each testis has about 250 compartments called testicular lobules,

  • These compartments contain highly coiled tubules called seminiferous tubules.

  • Each lobule contains one to three seminiferous tubules in which the sperms are produced.
  • Each seminiferous tubule is lined on its inside by two types of cells called male germ cells (spermatogonia) and Sertoli cells.
  • Spermatogonia lining these tubules give rise to spermatozoa which are released into the lumen of the tubule
  • In between spermatogenic cells, Sertoli or sustentacular or nurse cells are present which provide nourishment to developing spermatozoa and regulate spermatogenesis by releasing inhibitin to check FSH over-activity.
  • Groups of polyhedral cells called Interstitial cells of Leydig, are located in the connective tissue around the seminiferous tubules.They constitute the endocrine tissue of the testis.
  • Leydig cells synthesise and secrete testicular hormones called Androgens into the blood.
  • Seminiferous tubules unite to form several straight tubules called tubuli recti which open into irregular cavities in the posterior part called rete testis.
  • Several tubes called vasa efferentia arise from it and conduct spermatozoa out of the testis.
  • Tubuli recti, rete testis and ductuli efferentes form the intra-testicular genital duct system.
  • The extratesticular duct system consists of tubes which conduct sperms from the testes to the outside.
  • It starts with ducts known as vasa efferentia. From each testis, 10-12 vasa efferentia confluent to form a folded and coiled tube called epididymis behind each testis.
  • The epididymis consists of three parts: (i) Caput (ii) Corpus (iii) Cauda.
  • The epididymis stores the sperms temporarily.
  • The epididymis leads to vas deferens that ascends to the abdomen and loops over the urinary bladder. It receives a duct from seminal vesicle and opens into urethra as the ejaculatory duct

Urethera

  • Male urethera provides a common pathway for the flow of urine and semen.
  • The urethra originates from the urinary bladder and extends through the penis to its external opening called urethral meatus

Penis

  • This is the copulatory organ of man.
  • It is a cylindrical, erectile, pendulotus organ suspended from the pubic region in front of scrotum.
  • It is made up of special tissue that helps in erection of the penis to facilitate insemination.
  • The enlarged end of penis called the glans penis is covered by a loose fold of skin called foreskin.

Accessory Glands of male

  • It include paired seminal vesicles, a prostate and paired bulbourethral glands.
  • Secretions of these glands constitute the seminal plasma which is rich in fructose, calcium and certain enzymes.
  • The secretions of bulbourethral glands also helps in the lubrication of the penis

Female reproductive system

  • The female reproductive system consists of a pair of ovaries, a duct system consisting of a pair of fallopian tubes (oviducts), a uterus, cervix and vagina, a pair of mammary glands

Ovaries

  • The ovary is the primary female sex organ.
  • It produces ova and secretes the female sex hormones, estrogens and progesterone which are responsible for the development of secondary female sex characters and cause marked cyclic changes in the uterine endometrium.
  • The human ovaries are small, almond-like flattened bodies, about 2 to 4 cm in length and is connected to the pelvic wall and uterus by ligaments.
  • Ovaries are located near kidneys and remain attached to the lower abdominal cavity through mesovarium.
  • Each ovary is covered by a thin epithelium which encloses the ovarian stroma.
  • The stroma is divided into two zones – a peripheral cortex and an inner medulla.
  • The cortex contains numerous spherical or oval, sac-like masses of cells known as ovarian follicles
  • The medulla consists of loose connective tissue, elastic fibres, numerous blood vessels and some smooth muscle fibres.

Fallopian Tubes (Oviducts)

  • These are one pair of long (10 to 12 cm), ciliated, muscular and tubular structures which extend from the periphery of each ovary to the uterus.
  • Each oviduct is differentiated into three parts:

(1)Infundibulum : The part of oviduct closer to the ovary is the funnel shaped infundibulum. The edges of infundibulum possess finger-like projections called fimbriae. Fimbriae help in the collection of the ovum after ovulation.

(2) Ampulla : The infundibulum leads to a wider part of the oviduct called ampulla.

(3) Isthmus : It is the last and narrow part having narrow lumen that links to the uterus.

  • The tube is involved in conduction of the ovum or zygote towards the uterus by peristalsis and ciliary action.
  • It is also the site of fertilization. (Fertilization occurs at the junction of ampulla and isthmus).

Uterus (Hystera/Womb)

  • It is a large hollow, muscular, highly vascular and inverted pear shaped structure present in the pelvis between the bladder and rectum.
  • It is suspended by a mesentery, the mesometrium. It has the following three parts.
  1. Cervix :
  • It is lower, narrow part which opens in body of uterus by internal os and in vagina below by external os.
  • It is mainly formed of the most powerful sphincter muscles in the body. The cavity of the cervix is called Cervical canal which along with vagina forms the birth canal.

Wall of uterus : The wall of uterus is formed of outer peritoneal layer, perimetrium; middle muscular myometrium of smooth muscle fibres, and inner highly vascular and glandular endometrium.

It is the site of foetal growth during pregnancy. It also takes part in placenta formation and expulsion of thebaby during parturition.

Vagina

  • It is a long (8.5 cm), fibro-muscular tube. It extends backward in front of rectum and anal canal from cervix to the vestibule.
  • Vaginal orifice is covered partially by a membranous diaphragm called hymen. The hymen is often ruptured during the first coitus (intercourse). However, it can also be broken by a sudden fall or jolt, insertion of a vaginal tampon, active participation in some sports like horseback riding, cycling etc.

External Genitalia

  • The external genital structures of the female reproductive system are collectively called the vulva.
  • The female external genitalia or vulva includes mons pubis, labia majora, labia minora, hymen and clitoris.
  • Mons Pubis is a cushion of fatty tissue covered by skin and pubic hair.
  • The labia majora are fleshy folds of skin, which extend down from mons pubis and surround the vaginal opening.
  • The labia minora are paired folds of tissue in the form of lips under the labia majora. The opening of vagina is often covered partially by a menorane called hymen.
  • The clitoris is a tiny finger-like structure which lies at the upper junction of the two labia minola above the urethral opening.

Accessory Glands

Mammary Glands/Breasts

  • There are a pair of rounded prominences present over the pectoralis major muscles on the front wall of the thorax. These remain in rudimentary form in male. In females, these remain undeveloped till puberty.
  • The glandular tissue comprises about 15-20 lobes in each breast. Each lobe is made up of a number of lobules. Each lobule is composed of grapelike clusters of milk secreting glands termed alveoli.
  • When milk is produced it passes from the alveoli into the mammary tubules and then into the mammary ducts.
  • Several mammary ducts join to form a wider mammary ampulla which is connected to lactiferous duct through which milk is sucked out.

GAMETOGENESIS

  • The primary sex organs – the testis in the males and the ovaries in the females, produce gametes i.e. sperms and ovum respectively, by the process called gametogenesis.

Spermatogenesis

  • In testis, the immature male germ cells, spermatogonia produce sperms by a process spermatogenesis that begins at puberty.
  • Spermatogenesis occurs in four stages: (i) Spermatocytogenesis, ii) Meiosis-l, (ii)Meiosis-Il and (iv) Spermiogenesis.
  1. Spermatocytogenesis : In spermatocytogenesis, the spermatogonia present on the inside wall of the seminiferous tubules multiply by mitotic division and increase in numbers. Each spermatogonia is diploid containing 46 chromosomes. Some spermatogonia undergo changes they grow, increase in size by accumulating nourishing materials and are called primary spermatocytes which periodically undergo meiosis and others remain as spermatogonia.
  2. Meiosis-l : A primary spermatocyte is diploid, (2n) with 44 + XY (total 46) chromosomes. It completes the first meiotic division (reduction division) leading to the formation of two equal, haploid cells called secondary spermatocytes, which have only 23 chromosomes each i.e. 22 + X or 22 + Y.
  • Meiosis-ll: The secondary spermatocytes undergo the second meiotic division to produce four equal haploid spermatids. The number of chromosomes in each spermatid is 23.
  1. Spermiogenesis : Transformation of spermatid into sperm is termed spermiogenesis.
  • A spermatid is non-motile and heavy. It has organelles like mitochondria, Golgi bodies, centrioles, nucleus etc.
  • During spermiogenesis, the weight of gamete is reduced along with the development of locomotory structures.
  • Nucleus becomes compact forming the major part of head of spermatozoa. Golgi complex of spermatid gives rise to
  • The two centrioles of the spermatids become arranged one after the other behind the nucleus.
  • Mitochondria from different parts of spermatid get arranged in the middle piece around axial filament.
  • Much of the cytoplasm of a spermatid is lost. It forms a thin layer around middle piece.
  • A typical mammalian sperm is flagellated, consisting of four parts namely head, neck middle piece and tail. After spermiogenesis the sperm heads become embedded in the Sertoli cellls, and are finally released from the seminiferous tubules by the process called spermiation.

Hormonal Control of Male Reproductive System

  • Spermatogenesis starts at the age of puberty due to significant increase in the secretion of gonadotropin releasing hormone (GnRH) from hypothalamus.
  • The increased levels of GnRH then acts at the anterior pituitary gland and stimulates secretion of two gonadotropins- luteinising hormone (LH) and follicle stimulating hormone (FSH)
  • LH acts at the Leydig cells and stimulates synthesis and secretion of androgens.
  • Androgens, in turn, stimulate the process of spermatogenesis.
  • FSH acts on the Sertoli cells and stimulates secretion of some factors which help in the process of Spermiogenesis.
  • Sertoli cells also secrete another protein, hormones called inhibin, which suppresses FSH synthesis. So, FSH along with testosterone stimulate the sperm production in the seminiferous tubules.

Structure of Mature Sperm

  • Mature sperm cell consists of a head, a neck, a middle piece and a tail.
  • A plasma membrane envelops the whole body of sperm.
  • The sperm head contains a very little cytoplasm, an elongated haploid nucleus, the anterior portion of which is covered by a cap-like structure, acrosome.
  • The acrosome is filled with enzymes that help in fertilisation of ovum.
  • These enzymes called sperm lysins that dissolve the membranes enveloping the ovum and help the sperm cell to enter the ovum.
  • Acrosome is derived from golgi apparatus. Its membrane extends down the outer surface of nucleus.
  • The short neck, contains two distinct granules the proximal and distal centrioles.
  • The proximal centriole plays a crucial role during the first cleavage of the fertilised ovum.
  • The distal centriole gives rise to the axial filament of the long tail of the sperm.
  • The middle piece possesses numerous mitochondria (25 to 30 arranged spirally) which produce energy for the movement of tail that facilitates sperm motility essential for fertilisation, that is why it is called as the power house of the sperm.
  • The tail is made up of a central axial filament surrounded by a small amount of cytoplasm and cell membrane as external sheath. The sperms move by swimming at the rate of 1.5 to 3 mm per minute to reach the site of fertilisation within 30 minutes.
  • The human male ejaculates semen in female tract during coitus, containing 200 to 300 million sperms.
  • For normal fertility, at least 60 percent of these sperms must have normal shape and size and at least 40 percent of them must show vigorous motility.
  • Sperms released from seminiferous tubules, are transported by the accessory ducts, secretions of epididymis, vas deferens, seminal vesicle and prostate are essential for maturation and motility of sperms.
  • The functions of the male accessory ducts and glands are maintained by the testicular hormones (androgens).

Oogenesis

  • The process of formation of a mature female gamete is called oogenesis which is markedly different from spermatogenesis.
  • Oogenesis is initiated during the embryonic development stage when a couple of million gamete mother cells (oogonia) are formed within each fetal ovary; no more oogonia are formed or added after birth.
  • An ovarian follicle consists of an oocyte, surrounded by one or more layers of follicular (flat epithelial) cells.
  • A large number of these follicles degenerate from birth to puberty. Degeneration of ovarian follicles is called follicular atresia and their disposal is done by phagocytes. Therefore, at puberty only 60,000 to 80,000 primary follicles are left in each ovary. With the onset of puberty, a primary follicle begins to mature with each ovarian cycle
  • The primary oocyte within the tertiary follicle grows in size and completes its first meiotic division at puberty. It is an unequal division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body.
  • The secondary oocyte retains the bulk of nutrient rich cytoplasm of the primary oocyte.
  • The tertiary follicle changes into the mature follicle or Graafian follicle.
  • The secondary oocyte forms a new membrane called Zona pellucida surrounding it.
  • This thick coat of zona pellucida is composed of glycoproteins and synthesised by oocyte.

MENSTRUAL CYCLE

  • The reproductive cycle in the female primates (e.g., monkeys, apes and human beings) is called menstrual cycle.
  • The first menstruation begins at puberty and is called
  • In human females, menstruation is repeated at an average interval of about 28/29 days, and the cycle of events from one menstruation till next one is called the menstrual cycle
  • One ovum is released (ovulation) during the middle of each menstrual cycle of 28 days.

Phases of menstrual cycle

(i)Menstrual phase

(ii)Follicular phase

(iii) Ovulation

(iv) Luteal/Secretory phase

(i)Menstrual phase

  • Menstrual flow occurs in this phase and it lasts for 3 to 4 days.
  • This flow results due to the breakdown of endometrial lining of the uterus and its blood vessels which forms a liquid and flows out through the vagina.
  • Menstruation usually occurs about 14 days after ovulation, if the released ovum is not fertilised. Lack of menstruation may be indicative of pregnancy
  • The total amount of blood discharged in one cycle is 30 to 50 ml.

 (ii) Follicular phase

  • The menstrual phase is followed by the follicular phase.
  • During this phase, primary follicles in ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of uterus regenerates through proliferation.
  • These changes in the ovary and the uterus are induced by changes in the levels of pituitary gonadotropins and ovarian hormones.
  • Secretion of gonadotropins (LH and FSH) increases gradually during the follicular phase, and stimulates follicular development and secretion of estrogens by the growing follicles.
  • FSH hormone stimulates follicular growth.
  • The follicular cells secrete estrogen, a sex hormone that also aids in the growth of the follicle.
  • Estrogen hormone stimulates mitotic divisions of the cells in the lining of uterus, and helps to repair the broken tissue and blood vessels.
  • It also causes the thickening of the endometrium
  • Both FSH and LH attain a peak level in the middle of each cycle, on 14th day of 28th day cycle.
  • During this phase, the estrogen level in the blood continues to rise until it reaches the peak and the Graafian follicle moves to the surface of ovary.
  • The elevated estrogen levels acts as positive feedback mechanism by stimulating the anterior lobe of pituitary to secrete luteinising (LH) hormone, which initiates the next stage of menstrual cycle.
  • Rapid secretion of LH leading to its maximum level during the mid-cycle called as LH surge induces the rupture of Graafian follicle and thereby the release of ovum (ovulation).

(iii) Ovulation :

  • LH induces ovulation which usually occurs on 14th day in the 28 days cycle.
  • The Graafian follicle ruptures and secondary oocyte (ovum) is released.
  • Day of ovulation = Number of days in M cycle -14

(iv) Luteal phase/Secretory phase:

  • Following ovulation, an egg is swept into the fallopian tube, where it awaits fertilisation as it travels through the tube towards uterus, The egg has stored nutrients to survive about 24 hours.
  • The ovulatory phase is followed by luteal phase during which the remaining parts of Graafian follicle fransform as Corpus luteum in the ovary.
  • Corpus luteum secretes large amounts of progesterone which is essential for maintenance of endometrium which is thickened by estrogen
  • In luteal phase, the endometrium further thickens due to estrogen hormone also secreted by corpus luteum.
  • In the absence of fertilisation, the corpus luteum degenerates; the level of progesterone hormone will fall.
  • This causes disintegration of the endometrium leading to menstruation, marking a new cycle.
  • In human beings, menstrual cycles ceases around 50 years of age, termed as

FERTILISATION AND IMPLANTATION

  • During copulation (coitus) semen is released by the penis into the vagina of female, called
  • A human sperm can live for many weeks in male genital duct
  • Once ejaculated in the semen, it lives only for 48 to 72 hours outside the body.
  • Sperms move in the liquid medium secreted by female genital tract at a speed of1.5-3.0 mm/minute. Prostaglandins of semen help in the movement of spermatozoa
  • The process of fusion of a sperm with an ovum is called Fertilization.
  • One has to remember that the sex of the baby has been decided at this stage itself
  • As we know the chromosome pattern in the human female is XX and that in the male is XY. Therefore, all the haploid gametes produced by the female (ova) have the sex chromosome X whereas in the male gametes (sperms) the sex chromosome could be either X or Y, hence, 50 per cent of sperms carry the X chromosome while the other 50 per cent carry the Y.
  • After fusion of the male and female gametes the zygote would carry either XX or XY depending on whether the sperm carrying X or Y fertilised the ovum. The zygote carrying XX would develop into a female baby and XY would form a male
  • That is why, scientifically it is correct to say that the sex of the baby is determined by the father and not by the mother

Embryonic developement 

It includes cleavage, blastulation, implantation, gastrulation and organogenesis.

Cleavage

  • First cleavage is completed after 30 hours of fertilization,in which a furrow passes from animal vegetal axis as well as centre of zygote
  • Then the second cleavage divides the zygote completely into the blastomeres (Holoblastic cleavage)
  • It is completed earlier in one of the two blastomeres resulting in a transient 3-celled stage.
  • Third cleavage is horizontal forming 8 blastomeres, I

Morula

  • Cleavage results in a solid ball of cells, Morula having 8-16 cells.
  • Zona pellucida still forms the outer cover. Morula undergoes compaction.
  • Morula descends slowly towards uterus in 4-6 days and corona radiata detaches during this period.

Blastulation or Blastocyst Formation :

  • Endometrium secretes a nutrient fluid and its mucosal cells become enlarged with stored nutrients.
  • As the morula enters uterus, it gets a rich supply of nutrients.
  • Outer peripheral cells enlarge and flatten further. They form trophoblast
  • Trophoblast cells secrete a fluid into the interior creating a cavity called blastocoel.
  • An inner group of cells attached to trophoblast called the inner cell mass.

Implantation

  • The trophoblast layer then gets attached to the endometrium and the inner cell mass gets differentiated as the embryo.
  • After attachment, the uterine cells divide rapidly and covers the blastocyst.
  • As a result, the blastocyst becomes embedded in the endometrium of the uterus . This is called implantation and it leads to pregnancy

PREGNANCY AND EMBRYONIC DEVELOPMENT

  • After implantation, finger-like projections appear on the trophoblast called chorionic villi which are surrounded by the uterine tissue and maternal blood.
  • The chorionic villi and uterine tissue become interdigitated with each other and jointly form a structural and functional unit between developing embryo (foetus) and maternal body called placenta
  • The placenta facilitate the supply of oxygen and nutrients to the embryo and also removal of carbon dioxide and excretory/waste materials produced by the embryo.
  • The placenta is connected to the embryo through an umbilical cord which helps in the transport of substances to and from the embryo.
  • Placenta also acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens, progestogens, etc.
  • In the later phase of pregnancy, a hormone called relaxin is also secreted by the ovary.
  • It should be noted that that hCG, hPL and relaxin are produced in women only during pregnancy.
  • In addition, during pregnancy the levels of other hormones like estrogens, progestogens, cortisol, prolactin, thyroxine, etc., are increased several folds in the maternal blood.
  • Increased production of these hormones is essential for supporting the fetal growth, metabolic changes in the mother and maintenance of pregnancy.
  • Immediately after implantation, the inner cell mass (embryo) differentiates
  • The human foetus within the uterus into an outer layer called ectoderm and an inner layer called endoderm.
  • A mesoderm soon appears between the ectoderm and the endoderm. These three layers give rise to all tissues (organs) in adults.
  • It needs to be mentioned here that the inner cell mass contains certain cells called stem cells which have the potency to give rise to all the tissues and organs.

  • The human pregnancy lasts 9 months.
  • In human beings, after one month of pregnancy, the embryo’s heart is formed.
  • The first sign of growing foetus may be noticed by listening to the heart sound carefully through the stethoscope.
  • By the end of the second month of pregnancy, the foetus develops limbs and digits. By the end of 12 weeks (first trimester), most of the major organ systems are formed, for example, the limbs and external genital organs are well-developed.
  • The first movements of the foetus and appearance of hair on the head are usually observed during the fifth month.
  • By the end of 24 weeks (second trimester), the body is covered with fine hair, eye-lids separate, and eyelashes are formed.
  • By the end of nine months of pregnancy, the foetus is fully developed and is ready for delivery

PARTURITION AND LACTATION

  • The average duration of human pregnancy is about 9 months which is called the gestation period.
  • Vigorous contraction of the uterus at the end of pregnancy causes expulsion/delivery of the foetus. This process of delivery of the foetus (childbirth) is called parturition.
  • Parturition is induced by a complex neuroendocrine mechanism.
  • The signals for parturition originate from the fully developed fetus and the placenta which induce mild uterine contractions called foetal ejection reflex.
  • This triggers release of oxytocin from the maternal pituitary.
  • Oxytocin acts on the uterine muscle and causes stronger uterine contractions, which in turn stimulates further secretion of oxytocin.
  • The stimulatory reflex between the uterine contraction and oxytocin secretion continues resulting in stronger and stronger contractions.
  • This leads to expulsion of the baby out of the uterus through the birth canal – parturition. Soon after the infant is delivered, the placenta is also expelled out of the uterus.
  • The mammary glands of the female undergo differentiation during pregnancy and starts producing milk towards the end of pregnancy by the process called lactation.
  • This helps the mother in feeding the newborn. The milk produced during the initial few days of lactation is called colostrum which contains several antibodies absolutely essential to develop resistance for the new-born babies.
  • Breast-feeding during the initial period of infant growth is recommended by doctors for bringing up a healthy baby.