Human Reproduction

Human Reproduction

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Female Reproductive System 

The parts of the female reproductive system include: 

  1. The Ovaries (Female Gonads) 
  2. Oviducts or the uterine tubes or fallopian tubes 
  3. The Uterus 
  4. The Vagina 
  5. External Organs are collectively called the vulva, or pudendum. 
  6. The mammary glands are considered part of both the integumentary system and the female reproductive system. 

1. Ovaries

  • The ovaries (female gonads) are paired glands. Each ovary resembles in size and shapes that of unshelled almond.
  • Each ovary has two important functions: 
    1. Production of gametes, secondary oocytes that develop into mature ova (eggs) after fertilization, and 
    2. Production of hormones- progesterone, estrogens, inhibin, and relaxin.

2. Uterine Tubes 

  • Two uterine (fallopian) tubes, or oviducts, extend laterally from the uterus.
  • The tubes, measure about 10 cm (4 in.) long.
  • The important function of these tubes is to provide a route for sperm to reach an ovum and transport secondary oocytes and fertilized ova from the ovaries to the uterus. 

3. Uterus

  • It is located between the urinary bladder and the rectum.
  • It is the size and shape of an inverted pear. 

Functions of Uterus:

  • It provides a pathway for sperm deposited in the vagina to reach the uterine tubes.
  • It is the site of implantation of a fertilized ovum, development of the fetus during pregnancy, and labor.
  • During reproductive cycles when implantation does not occur, the uterus is the source of menstrual flow.
Human female reproductive system 
Fig.1: Human female reproductive system 

4. Vagina 

  • The vagina is a tubular, 10 cm long fibromuscular canal lined with mucous membrane that extends from the exterior of the body to the uterine cervix.
  • It is located between the urinary bladder and the rectum. 

Functions of Vagina: 

  • It is the receptacle for the penis during sexual intercourse.
  • It is the outlet for menstrual flow.
  • It is the passageway for childbirth. 

5. Vulva 

The term vulva, or pudendum, refers to the external genitals of the female. The vulva is composed of the following parts: 

  • Mons pubis: It is anterior to the vaginal and urethral openings. It is an elevation of adipose tissue covered by skin and coarse pubic hair that cushions the pubic symphysis. 
  • Labia majora: From the mons pubis, two longitudinal folds of skin, called labia majora, extend inferiorly and posteriorly. 
  • The clitoris is a small cylindrical mass composed of two small erectile bodies, the Corpora cavernosa, and numerous nerves and blood vessels.
  • Vestibule: The region between the labia minora is the vestibule.
  • The vaginal orifice, the opening of the vagina to the exterior, occupies the greater portion of the vestibule and is bordered by the hymen.
  • Anterior to the vaginal orifice and posterior to the clitoris is the external urethral orifice, the opening of the urethra to the exterior.

Oogenesis and Follicular Development 

The formation of gametes in the ovaries is termed oogenesis. Oogenesis occurs in essentially the same manner as spermatogenesis; meiosis takes place and the resulting germ cells undergo maturation.

Oogenesis
Fig.2: Oogenesis

The Female Reproductive Cycle 

  • The female reproductive cycle is controlled by hormonal changes.
  • It involves the ovarian and uterine cycles.
    1. Ovarian cycle: It consists of a sequence of actions in the ovaries that occur during and after the maturation of the oocyte.
    2. Menstrual (uterine) cycle: It consists of a simultaneous sequence of changes in the uterine endometrium preparing it for the arrival of the fertilized ovum.

Male Reproductive System 

The parts of the male reproductive system are: 

  1. The testes
  2. A system of ducts: 
    • Epididymis 
    • Ductus deferens 
    • Ejaculatory ducts 
    • Urethra 
  3. Accessory sex glands 
    • Seminal vesicles 
    • Prostate gland 
    • Bulbourethral glands 
  4. The penis 
  5. The scrotum
Human male reproductive system
Fig.3: Human male reproductive system

1. Testes

  • The testes are paired oval glands located in the scrotum.
  • Each testis measures about 5 cm long and 2.5 cm in diameter and has a mass of 10–15 grams.
  • Each testis consists of internal compartments called lobules. Each of the 200–300 lobules contains one to three tightly coiled tubules and seminiferous tubules.
  • Sperms are produced in the seminiferous tubules. 

2. System of Ducts 

Ducts of the Testis: The sperm and fluid are pushed along the lumen of seminiferous tubules and then into a series of very short ducts called straight tubules.

  • The straight tubules lead to a network of ducts in the testis called the rete testis.
  • From the rete testis, sperm move into a series of coiled efferent ducts in the epididymis that empty into a single tube called the ductus epididymis. 

Epididymis

  • The epididymis is a comma-shaped organ about 4 cm long that lies along the posterior border of each testis.
  • Each epididymis consists mostly of the tightly coiled ductus epididymis. 

Ductus Deferens 

  • Within the tail of the epididymis, the ductus epididymis becomes less convoluted, and its diameter increases. Beyond this point, the duct is known as the ductus deferens or vas deferens. 

Spermatic Cord

  • The spermatic cord is a supporting structure of the male reproductive system that ascends out of the scrotum. 

Ejaculatory Ducts 

  • Each ejaculatory duct is about 2 cm long and is formed by the union of the duct from the seminal vesicle and the ampulla of the ductus (vas) deferens.
  • The ejaculatory ducts form just superior to the base (superior portion) of the prostate and pass inferiorly and anteriorly through the prostate. 

Urethra

  • In males, the urethra is the shared terminal duct of the reproductive and urinary systems; it serves as a passageway for both semen and urine.
  • About 20 cm long, it passes through the prostate, the deep muscles of the perineum, and the penis.

3. Accessory Sex Glands 

  • The ducts of the male reproductive system store and transport sperm cells, but the accessory sex glands secrete most of the liquid portion of semen.
  • The accessory sex glands include: 
    1. The seminal vesicles 
    2. The prostate 
    3. The bulbourethral glands

Seminal Vesicles 

  • The paired seminal vesicles or seminal glands are convoluted pouch-like structures, about 5 cm in length, lying posterior to the base of the urinary bladder.
  • Through the seminal vesicle ducts, they secrete an alkaline, viscous fluid that contains fructose (a monosaccharide sugar), prostaglandins, and clotting proteins that are different from those in blood. 

Prostate Gland

  • The prostate is a single, doughnut-shaped gland about the size of a golf ball.
  • It measures about 4 cm from side to side, about 3 cm from top to bottom, and about 2 cm (0.8 in.) from front to back.
  • It is inferior to the urinary bladder and surrounds the prostatic urethra.
  • The prostate secretes a milky, slightly acidic fluid (pH about 6.5) that contains several substances like Citric acid, proteolytic enzymes, such as pepsinogen, lysozyme, amylase, and hyaluronidase. 

Bulbourethral Glands

  • The paired bulbourethral glands, or Cowper’s glands, are about the size of peas.
  • They are located inferior to the prostate on either side of the membranous urethra within the deep muscles of the perineum, and their ducts open into the spongy urethra.

4. Penis

  • The penis contains the urethra and is a passageway for the ejaculation of semen and the excretion of urine.
  • It is cylindrical and consists of a body, glans penis, and a root. 

5. Scrotum

  • The scrotum (bag), the supporting structure for the testes, consists of loose skin and an underlying subcutaneous layer that hangs from the root (attached portion) of the penis.

Spermatogenesis  

  • The process by which the seminiferous tubules of the testes produce sperm is called spermatogenesis.
  • The total duration of spermatogenesis in humans is 65–75 days.
  • Spermatogenesis starts with the spermatogonia, which contains the diploid (2n) number of chromosomes.
Spermatogenesis
Fig.4: Spermatogenesis
  • The final stage of spermatogenesis spermiogenesis is the development of haploid spermatids into sperm.
  • Each spermatid becomes a single sperm cell.
  • Sperm then enter the lumen of the seminiferous tubule. Fluid secreted by Sertoli cells pushes sperm along their way, toward the ducts of the testes. At this point, sperm are not yet able to swim.

Menstrual Cycle

  • It is the regular natural change of the female reproductive system. It is the reason for pregnancy and required for the production of ovocytes and also for the preparation of the uterus for pregnancy. Pregnancy is only possible due to the menstrual cycle.
  • It normally starts between twelve and fifteen years of age. The frequency of the menstrual cycle is after 21 to 40 days and an average after every 28 days.
  • Menstruation (bleeding) usually lasts for 2 to 7 days (average four days).
  • Common symptoms of menstruations are acne, tender breasts, bloating (inflation), and feeling of tiredness.
  • It is governed by hormonal changes. The menstrual cycle is divided into three phases based on events in the ovary i.e. ovarian cycle or in the uterus i.e. uterine cycle.
  • Again the ovarian cycle consists of the follicular phase.
    1. Ovulation
    2. Luteal phase
  • While uterine cycle consists of
    1. Menstruations
    2. Proliferative phase
    3. Secretory phase 

Gradual increase in amounts of estrogen in the follicular phase, due to stimulation, the discharge of blood (menses) flow stop and the lining of the uterus thickens. Follicles in the ovary begin developing under the influence of a complex interplay of hormones and after several days one becomes dominant. The dominant follicle releases an ovocyte in an event called ovulation. The ovocyte lives only for 24 hours or less without fertilization, while the remains of the dominant follicle in the ovary become a corpus – lubeum. The corpus luteum has the main function to produce large amounts of progesterone. Due to progesterone the uterine–lining changes to prepare for the potential implantation of an embryo to establish a pregnancy.  However, if implantation does not occur within two weeks (approximately) the corpus luteum will involute reputing a sharp drop in levels of both progesterone and estrogen.  Hormone drop causes the uterus to shed its lining in a process known as menstruation (menses).

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