11 Introduction to physiology of human reproduction

Gautam Kshatriya and Suniti Yadav

epgp books

 

 

 

LEARNING OBJECTIVES

  • To understand the primary and secondary human reproductive organs.
  • To understand the role of hormones in the formation of male and female gametes.
  • To understand the process of oogenesis in females and spermatogenesis in males.
  • To understand the role of each reproductive organ in facilitating fertilization.
  • To understand the process of fertilization.

    CONTENTS

 

INTRODUCTION

1.      FEMALE REPRODUCTIVE SYSTEM

a.      External genital organs or vulva

b.      Vagina

c.       Ovaries

          i.      Ovarian Cycle

         ii.      Oogenesis

d.      Fallopian tubes

e.       Uterus and Cervix

f.        Breasts

2.      MALE REPRODUCTIVE SYSTEM

a.      Scrotum

b.      Testes

         i.      Sertoli cells

        ii.      Germ/Sperm cells

       iii.      Spermatogenesis

c.       Epididymis

d.      Glands

         i.      Seminal vesicles

        ii.      Prostate

       iii.      Bulbourethral Glands

e.       Penis

f.        Testosterone

 

INTRODUCTION

 

‘Reproductive fitness’ is one of the governing factors for the survival of any species, the task for which is assigned to reproductive system. Not all individuals may reproduce but some individuals need to produce offspring to maintain the continuity or survival of species. Broadly four functions are allocated to the reproductive system –

1.      to produce the basic unit of reproduction i.e. gametes

2.      to sustain and transport these gametes to their right place during the process

3.      to help sustain the viable offspring (or fetus).

4.      to produce hormones that help in proper functioning/maturation of reproductive organs

 

These functions are divided between ‘primary’ and ‘secondary/accessory’, reproductive organs. Primary reproductive organs (also called as gonads) are responsible for the production of the gametes i.e. egg and sperm cells and hormones. The hormones produced by the primary reproductive organs help in the maturation and regulation of normal physiology of the reproductive system and development of sexual characteristics.

 

The ‘secondary reproductive organs’ assist in transportation of the gametes and sustainability (or nurture) of the offspring (fetus) and these include ducts, glands and other organs such as uterus and penis.

 

1. FEMALE REPRODUCTIVE SYSTEM

 

The female reproductive system is assigned to produce egg or the ova and reproductive hormones, transport the ova to the site where it may be fertilized by the sperm. Apart from this, it also has an additional task of supporting the developing fetus and delivering it to the outside world.

Figure1. Section of the female reproductive system in humans

(Source: https://kullabs.com/classes/subjects/units/lessons/notes/note-detail/5641)

 

The female reproductive system is located primarily inside the pelvic cavity. The female reproductive system includes the ovaries, fallopian tubes, uterus, vagina, accessory glands, and external genital organs (Figure1).

 

1.1. External genital organs

 

The external female genital organs or the vulva is located outside the pelvic cavity. The vulva collectively comprises of mons pubis, labia majora, labia minora, clitoris, hymen and Bartholin’s glands (Figure2). The vulva contains numerous sweat and oil glands and the secretions from these glands help in sexual arousal.

 

Figure2. Frontal view of vulva (Source – American Cancer Society, 2017)

 

1.2 Vagina

 

The vagina is a muscular canal that serves as an entry point for the sperm during sexual intercourse and an exit point from the uterus during menstrual flow and child birth. The vagina is surrounded by vulva on the outside.

 

1.3 Ovaries

The ovaries are the female gonads that produce the egg/ovum. Ovum is the mature female germ cell ready for reproduction or ready to receive the sperm whereas the immature ovum is called as oocyte. The group of an oocyte and its supporting cells is called a follicle. The production of egg cells in the ovary is called as oogenesis whereas the release of the mature ovarian follicle (which is ready to receive sperm) is called as ovulation.

 

1.3.1     Ovarian Cycle

The ovarian cycle is a set of conventional changes during the women’s reproductive years, which occur in the oocytes and ovarian follicles over a span of approximately 28 days. The cycle includes two interrelated processes: oogenesis (the production of ovum) and folliculogenesis (the growth and development of ovarian follicles).

 

1.3.2     Oogenesis

The process of gamete formation in females is called as oogenesis and begins with the formation of oogonia during fetal development. The oogonia form the primary oocytes in the fetal ovary prior to birth. The primary oocytes are arrested in the meiosis-I stage and resume at the beginning of puberty, continuing till menopause. The release of oocyte from the ovary marks the initiation of reproductive maturity for a female from puberty. Ovulation occurs in a vicious cycle of approximately 28 days. Prior to ovulation, an increase in the luteinizing hormone initiates the resumption of meiosis in a primary oocyte, therefore producing a secondary oocyte (Figure3).

Figure3. Gametogenesis in a female: Oogenesis

(Source: http://biologyandmedicineanimation.blogspot.com/2014/12/oogenesis.html)

 

During this cell division during meiosis, identical cells are not formed (as in meiosis). The cytoplasm is divided unequally, resulting into a larger cell – the secondary oocyte and a much smaller cell called the polar body, which eventually disintegrates. Therefore, at the end of the meiosis cycle of an oocyte, four cells are formed, but only one survives and leaves the ovary during ovulation. Meiosis of the secondary oocyte is completed only if a sperm successfully penetrates the secondary oocyte. Meiosis-II is resumed and one haploid ovum (produced at the instant of fertilization by a sperm – haploid) becomes the first cell of the zygote (diploid) (Figure4).

Figure4. The processes of meiosis, fertilization, and mitosis that maintains diploidy in a zygote

 

(Source: http://sphweb.bumc.bu.edu/otlt/MPH-Modules/PH/DNA-Genetics/DNA-Genetics_print.html) The large amount of cytoplasm in the ovum provides necessary nutrients to the newly formed zygote

 

during the period between fertilization in the fallopian tube and implantation in the uterus. The sperm contributes only the DNA (genetic material) during fertilization and not the cytoplasm.

 

1.4    Fallopian Tubes or Uterine Ducts

 

The fallopian tubes or uterine tubes or oviducts serve as channel for the transfer of oocyte from the ovary to the uterus. With each ovary transcends a fallopian tube that connects to the uterus, though the tubes are not directly connected to it. After the ovulation, the secondary oocyte is released into the peritoneal cavity, from where the fallopian tube receives the oocyte. The inner layer of the fallopian ducts is lined with the mucus-secreting ciliated cells. The ciliated cells beat in the direction of the uterus and facilitate the movement of oocyte. High concentrations of estrogen hormone during the time of ovulation induce muscular contractions along the entire length of the fallopian tubes. These contractions coordinate movement and eventually the oocyte is pulled into the interior of the fallopian tube. During fertilization, the sperm meets the egg while it is still actually moving through the ampulla of the fallopian tube. If fertilization is successful, the resulting zygote starts dividing and makes its way through the fallopian tube and gets transplanted in the uterus at a 72-cell stage. If the ovum is not fertilized, it degrades in fallopian tube or uterus, where it may be discarded during the next menstrual period (Figure5).

 

Figure5. Fallopian tubes where fertilization occurs and the movement of fertilized egg towards uterus (Source: https://www.quora.com/)

 

1.5   The Uterus and Cervix

 

The uterus is the muscular organ where the 72-cell zygote or the blastula is implanted. The uterus nourishes and supports the growing embryo during the pregnancy till parturition. The uterine wall is made up of three layers. The outermost layer is called perimetrium, which consists of epithelial tissue that covers the exterior of the uterus. The middle layer, or myometrium, is a thick layer of smooth muscle responsible for uterine contractions. The innermost layer is the endometrium which lines the lumen of the uterus and is shed during the menstrual flow.

 

During follicular phase of the ovarian cycle, estrogen is secreted by mature follicles. At the same time, the endometrium thickens and prepares itself for potential implantation. After ovulation, progesterone hormone increases during the luteal phase and maintains the thickness of the endometrium. Till the time functional corpus luteum is present, the endometrium is prepared for any implantation. If implantation occurs, signals are sent to corpus luteum to maintain endometrium by secreting progesterone and if the embryo does not implant itself, this signal ceases and declines the production of progesterone, thus marking an end to the luteal phase. This result in endometrium thinning and the spiral arteries of the endometrium constrict and rupture, thereby preventing oxygenated blood from reaching the endometrial tissue. As a result, endometrial tissue dies and endometrial lining along with white blood cells are shed through the vagina during menstruation.

 

The cervix is the anterior opening of the uterus that connects to the vagina. It is through cervix that the full developed fetus is expelled out which comes out of the body through the vaginal opening.

 

1.6 The Breasts

 

The breasts are located far from the other reproductive organs but they are considered to be accessory organs of the female reproductive system. The function of the breasts is to supply milk to the growing infant immediately after birth. The process of milk production in a female is called as lactation. Breast milk is produced by mammary glands which are modified sweat glands. During the hormonal changes in menstrual cycle, breast tissues may swell or become tender under the influence of estrogen and progesterone. After the implantation of the fetus in the uterus, the mammary tissues also may enlarge.

 

2. MALE REPRODUCTIVE SYSTEM

 

The transport of male gametes i.e. sperm, since its production in the male genital organ is certainly quite a journey. The function of male reproductive system is to produce the male gamete and transfer it to the female reproductive tract. Unlike females, the male reproductive system is located outside the body, around the pelvic region. The human male reproductive system consists of organs whose function is to contribute towards reproductive process and constitutes a pair of testes, several accessory organs and conductory ducts that aid in the process of sperm maturation, transport and seminal components to the penis, which delivers the sperm to the female reproductive tract (Figure6).

 

Figure6. Male reproductive system

(Source: http://www.webmd.com/men/male-reproductive-system)

    2.1 Scrotum

 

The sperm producing organ i.e. testes are placed in muscular sack like structure called as scrotum that extends from the body behind the penis. The scrotum is located outside the body and it keeps the testes at almost 3-13oC below the core body temperature for efficient production and viability of the sperm.

 

2.2 Testes

The testes (singular – testis) are the male gonads and the primary site for the production of sperm and androgens (male sex hormone -testosterone) and remain active throughout the life. The produced sperm is released into the lumen of seminiferous tubules, from where the sperm pass into a duct called epididymis via rete testes. Inside the seminiferous tubules, there are supporting cells called as sertoli cells and developing germ cells or sperm cells (Figure7).

 

 

Figure7. Seminiferous tubule where sertoli and sperm cells are released into the seminiferous lumen (Source: http://reproductionandbreeding.weebly.com/oogenesis-and-spermatogenesis.html)

 

2.2.1     Sertoli cells

The sertoli or sustentacular cells are elongate branching cells that secrete signaling molecules that promote sperm production. The tight junction between sertoli cells helps create blood-testis barrier and keeps blood borne substances from germ cells.

 

2.2.2 Germ cells

The basement membrane of the tubule is lined by the most immature cells i.e. spermatogonia (singular– spermatogonium). The spermatogonia divide to produce primary spermatocytes and secondary spermatocytes, which mature into next stage to form spermatids and finally produce sperm. The process of the formation of a mature sperm from spermatogonia is called as spermatogenesis.

 

2.2.3 Spermatogenesis

Spermatogenesis occurs in the seminiferous tubules in testes and continues throughout the life of a male after puberty. A complete cycle of the formation of a mature sperm cell from spermatogonia takes approximately 64 days, though this time is not synchronized across all seminiferous tubules.

 

Spermatogenesis begins with a mitotic division of diploid spermatogonia and the daughter cells of this undergo meiotic division resulting into a haploid male gamete (Figure8).

 

Figure8. Spermatogenesis in human male

(Source: http://www.biology-pages.info/S/Sexual_Reproduction.html)

 

Spermatogonial stem cell undergoes a mitotic division resulting into two identical, diploid daughter cells, one of which remains a spermatogonia and the other becomes a primary spermatocyte. The primary spermatocyte undergoes meiotic-I division like mitosis and produces two identical diploid cells called as secondary spermatocyte. These secondary spermatocytes undergo meiosis-II division and produce four haploid cells called as spermatids. The spermatids undergo a process of maturation i.e. spermiogenesis, with reduced cytoplasm and further transformation into a sperm. The fifth and last stage of spermatogenesis in the portion of the tubule nearest the lumen, result into spermatozoa or sperm which is eventually released into the lumen, moves into a duct called epididymis.

 

2.3 Sperm Transport

The sperm released into the seminiferous tubules is transported into the ductular epididymis and then into penis to be put into the female reproductive tract.

 

2.3.1 Epididymis

The immotile sperm cells are moved to epididymis, where the maturation of sperm cells takes place. The sperm enters the head of epididymis and move along by the contraction of smooth muscles that line epididymal tubes. During this course of movement, the sperm matures further and becomes motile. The mature sperm cells are then stored in the tail of the epididymis until ejaculation occurs through the penis.

 

2.4 Glands

 

Sperm make up only 5 percent of the final volume of semen, the thick, milky fluid that a male ejaculates. The bulk of semen is produced by three accessory glands of the male reproductive system – the seminal vesicles, the prostate, and the bulbourethral glands.

 

2.4.1 Seminal Vesicles

The seminal vesicles are aired glands that contribute approximately 60 percent to the semen volume. The semen from seminal vesicles contains fructose, which is used by the sperm mitochondria to generate ATP during its movement through the female reproductive tract. This fluid that contains both sperm and secretions from seminal vesicles is then transported to the next structure of male reproductive system i.e. prostate gland.

 

2.4.2 Prostate Gland

The prostate gland secretes a milky alkaline fluid to the passing seminal fluid (or semen), which is critical for the coagulation and de-coagulation of the semen following ejaculation. This temporary thickening of semen helps retain it in the female reproductive tract and provides time to the sperm for utilization of fructose provided by seminal vesicle secretions. When the semen regains its fluid state, the sperm can then pass into the female reproductive tract.

 

2.4.3 Bulbourethral Glands

The paired bulbourethral glands (or Cowper’s glands) add to the final volume of the semen. It releases a thick, salty fluid (lubrication proteins) that helps in the lubrication of the male urethral end and the female vagina. It also helps to clean any urine residues from the penile urethra. The fluid from these glands is released only after a male becomes sexually aroused and shortly before the release of semen. This fluid is also called as pre-ejaculate.

 

2.5 Penis

 

The penis is the male organ of copulation (sexual intercourse). It is flaccid for actions such as urination, but turgid and rod-like during sexual arousal. During sexual arousal, it becomes erect and this erectness allows it to penetrate into the female vagina and deposit semen into the female reproductive tract. The other end of penis towards exterior called as glans penis has high concentration of nerve endings influences the likelihood of ejaculation. Penile erection is a result of vasodialation because of more arterial blood flowing into penis than leaving through the veins. There is an increased blood flow to the penis resulting into penile erection.

 

2.6 Testosterone

 

Testosterone or the male sex hormone, an androgen, is a steroid hormone produced by Leydig cells in the testes and activates characteristic physical changes in men and initiates spermatogenesis. Low levels of testosterone may lead to male infertility.

 

Small amounts of testosterone are also secreted in females, most of which is converted to estradiol – the female sex hormone. A small amount of testosterone is also secreted by the adrenal glands in both males and females to maintain blood pressure homeostasis.

 

SUMMARY

  • Male reproductive system is located exterior to the human body and female reproductive system is placed inside the body.
  • Gametes are the reproductive units that fuse to form an offspring. The organs that produce gametes are called gonads. Male gametes are called sperm and female gametes are called ovum or egg.
  • The process of formation of male and female gametes is called spermatogenesis and oogenesis, respectively.
  • The primary site for the production of sperm is testes and of ovum is ovary.
  • Female sex hormone is called estradiol and male sex hormone is testosterone.
  • Vagina serves as an entry point for the sperm and exit point for the fetus.
  • The male organ of copulation or intercourse is penis.
  • Fertilization occurs in fallopian tubes and implantation occurs in the uterus.
  • Semen consists of sperm and seminal fluid that helps in lubrication of the urethral end in male and vagina in females during copulation.
  • Termination of menstrual cycle in females is called menopause and marks the end of reproductive lifespan.
  • In males, the production of sperm continues throughout the lifespan but reduces after an age of 35 years.
  • Female infertility if marked by cessation of viable ovum production due to imbalance in LH, FSH and progesterone levels whereas in males low levels of testosterone may lead to infertility.
you can view video on Introduction to physiology of human reproduction

    Glossary

Primary reproductive organs – Also called as gonads, responsible for the production of the gametes. 

 

econdary reproductive organs – Assist in transportation of the gametes and sustainability of the fetus.

 

Vulva – External female genital organ that comprises of mons pubis, labia majora, labia minora, clitoris, hymen and Bartholin’s glands.

 

Vagina – Muscular organ that serves as entry point for sperm and exit point for fetus.

 

Ovaries – Female gonads that produce ovum.

 

Oogenesis – Process of formation and maturation ovum in the ovary.

 

Ovulation – Release of the mature ovarian follicle (which is ready to receive sperm).

 

Ovarian Cycle – A cycle of 28 days marked by beginning of endometrium shedding and blood flow.

 

Fallopian Tubes – Ducts where fertilization takes place.

 

Uterus – Muscular structure where implantation takes place.

 

Cervix – Opening from where fetus is expelled out into vagina.

 

Breasts – Functions to supply milk produced by mammary glands to the infant.

 

Scrotum – Muscular bag that contains testes in males.

 

Testes – Male gonads that are responsible for sperm production.

 

Spermatogenesis – Process of formation and maturation of sperm cells.

 

Epididymis – Duct that carries sperm to the penis and where sperm gains maturity and becomes mobile.

 

Seminal Vesicle – The gland that releases seminal fluid containing fructose.

 

Prostate gland – Secretes fluid that helps in coagulation of semen.

 

Bulbourethral gland – Also called Cowper’s gland, secretes fluid that helps in lubrication of urethral end in males and vagina in females during intercourse.

 

Testosterone – Male sex hormone.

 

REFERENCES

 

American Cancer Society, 2017. Available at – https://www.cancer.org/cancer/vulvar-cancer/about/what-is-vulvar-cancer.html

 

WEB REFERNCES

 

https://kullabs.com/classes/subjects/units/lessons/notes/note-detail/5641

 

http://biologyandmedicineanimation.blogspot.com/2014/12/oogenesis.html

 

http://sphweb.bumc.bu.edu/otlt/MPH-Modules/PH/DNA-Genetics/DNA-Genetics_print.html

 

https://www.quora.com/

 

http://www.webmd.com/men/male-reproductive-system

 

http://reproductionandbreeding.weebly.com/oogenesis-and-spermatogenesis.html http://www.biology-pages.info/S/Sexual_Reproduction.html

 

SUGGESTED READING

 

Essentials of Anatomy and Physiology. Fourth Edition. Valerie C. Scanlon and Tina Sanders.

 

Human Anatomy. Sixth Edition. Van De Graaff.

 

Heffner, L. J., & Schuster, D. J. (2006). Reproductive system at a glance (2nd ed.). Ames, IA: Blackwell Publishing Professional.