Reproduction in Human

REPRODUCTION IN HUMAN

The humans reproduce sexually. Reproduction in humans can be studied in two parts (a) Reproductive system, and (b) Fertilization, Pregnancy and development of the embryo.

(a) Reproductive System

The puberty is the name given to the changes that occur in boys and girls as they grow up. Mostly these changes occur between the age of 10 to 14 year, and these are brought about by certain hormones. During puberty the body grows rapidly, and both primary and secondary reproductive organs grow and become mature. Along with these changes, secondary sex character also start appearing. It is also to be noted that in males, sexual maturity is attained at the age of 13-14 years and in females at the age of 11-13 years. This leads ultimately to a stage when the child becomes an adolescent.

The term adolescence comes from the Latin verb Adolescere, meaning ‘to grow into maturity.’ In this sense ‘adolescence is a process rather than a time period, a process of achieving the attitudes and beliefs needed for effective participation in society. The World Health Organization (WHO) defines adolescence as the period from 10 to 19 years of age characterized by developments and changes in physical, psychological and social areas.

During adolescence, the secondary sexual characters that develop are as follows: In males they include deepening of voice, widening of shoulders, muscular body, appearance of beard and moustache, growth of axillary and pubic hair, enlargement of external genital organs.

In females, the changes include growth of axillary and pubic hair, widening of pelvis and hip enlargement of breasts and initiation of the menstrual cycle. Sexual maturation is a very significant stage in one’s life; hence it is necessary to maintain the health and hygiene of the reproductive organs during this stage. The reproductive organs are described and illustrated below.

Male Reproductive System : The reproductive system in male consists of the following organs –  a pair of testes, a pair of epididymis, a pair of vasa differentia (singular: vas deferens), urethra, penis and accessory glands.

(i) Testes

Testes (Singular: Testis) are the male gonads. In an adult male, each testis is approximately 4-5 cm long and about 12 g in weight. These are extra-abdominal, that is, present outside the abdomen in pouch made up of skin and connective tissue called scrotal sac or scrotum that hangs in the region between the legs.

The scrotum acts as a thermoregulator. It helps in maintaining the temperature of testes of about 2-3 C lower than the body temperature. This temperature is suitable for the development of sperms.

Anatomically, each testis is encased in a capsule of white fibrous connective tissue – called tunica albuginea. Each testis has several highly coiled tubules called seminiferous tubules where the sperms are produced. Between the seminiferous tubules is the connective tissue, which contains clumps of interstitial cells, also called Leydig cells. These cells secrete testosterone the male sex hormone. Testosterone maintains the primary and secondary sexual characteristics in males.

(ii) Epididymis

It is a long highly coiled tube which remains attached to the testis and lies within the scrotal sac. Epididymis stores spermatozoa (sperms) and serves a passage for their transport from the testis.

(iii) Vas Deferens (sperm duct)

Each epididymis continues as vas deferens. It enters the abdominal cavity passes over the urinary bladder and joins the duct of seminal vesicle to form the ejaculatory duct. The ejaculatory duct opens into the urethra.

(iv) Urethra

The urethra in males is about 15-20 cm long and is differentiated into three parts – an anterior prostatic part which passes through the prostate gland; a middle membranous part; and a posterior penile part which passes through the copulatory organ, the penis. Urethra functions as a passage for both semen and urine.

(v) Penis

Penis is a cylindrical, spongy, muscular and a highly vascular copulatory organ in males. The urethra runs through it centrally and serves as a common passage for urine and semen. During sexual excitement, the spongy tissue gets filled up with blood, making it erect and stiff. Externally, the penis is covered by skin. The tip of the penis is soft and highly sensitive. It is called glans penis. It is covered by lose fold of skin called prepuce which can be retracted.

Function

Seminiferous to Produce sperms epididymis Stores sperms in a viable but immobile state

Sperm duct (Contractions help in the passage of sperm into urethra during ejaculation)

Seminal vesicle secrete fluid which activates and nourishes sperms

Urethra contractions expel semen from penis during ejaculation (urethra also carries urine to exterior) Penis.

Contains spongy tissue and serves as the copulatory organ

Accessory glands

The Accessory glands include seminal vesicles, prostate glands and Cowper’s glands.

Seminal vesicles

A pair of seminal vesicles are present at the base of the urinary bladder. The seminal vesicles store sperms that descend from the testis and secrete seminal fluid. The seminal seminal vesicles store sperms that seminal fluid. The seminal 80 percent of the ejaculate (semen thrown out of the penis).

Prostrate gland

Prostrate gland surrounds the first part of the urethra. It secretes an alkaline fluid which is discharged into the urethra. This fluid keeps the sperms alive and helps them to swim vigorously secretion of prostrate gland forms about 5-30 percent of the ejaculate.

Cowper’s glands or Bulbourethral glands.

These are paired glands that lie below the prostate gland and join the urethra at a short distance from that of the prostrate glands. Cowper’s glands secrete a white, viscous, alkaline secretion resembling mucous which acts as a lubricant.

Spermatozoa and semen

The Spermatozoa are male gametes produced by the testes. Structurally, human sperm has three main parts – Head, Neck and Tail. The tip of sperm is covered by acrosome, which helps the sperm to penetrate inside the egg during fertilization. Tail of Spermatozoa are immotile when stored in the epididymis but get activated, and motile by the secretions from the accessory reproductive glands in males. The secretions of various accessory glands along with sperms form the semen. The sperm are released in millions. In one ejaculation 200,000,000 (2 x 108) sperms are discharged. Sperms when introduced into the vagina of the female move with the speed of 2 mm/minutes in the female.

The Female Reproductive System : The Female Reproductive system consists of the following organs. A pair of ovaries, a pair of fallopian tubes, uterus, vagina and external genitalia.

(i) Ovaries 

There are a pair of ovaries, which lie in the lower part of the abdominal cavity, one on each side of the body. Ovaries produce ova and also secrete female sex hormones, oestrogen and progesterone. The process of formation of egg in the ovary is known as oogenesis. If a section of the ovary is cut, eggs at various stages of maturing can be seen. Each egg begins as a primary follicle. Follicular cells then cover the egg and a cavity called antrum is formed. This is the mature egg called Graafian follicle. The egg then gets released (ovulation) from the ovary leaving the empty follicle called corpus luteum.

(ii) Fallopian Tube (oviducts)

There are two oviducts (or fallopian tubes) in female reproductive system. Each oviduct is about 10-15 cm long. The proximal funnel-shaped end of each oviduct lies near the ovary and is called infundibulum. Its margin bears finger-like projections called fimbriae. Each infundibulum continues as a thin and coiled tube called oviduct or Fallopian tube. Both Fallopian tubes open into the uterus.

(iii) Uterus

The uterus is a pear-shaped, muscular, thick-walled organ. It is about 7cm long, 5 cm broad, and 2.5 cm thick. The wall of the uterus comprises of three coats – the innermost endometrium, middle myometrium, and outermost perimetrium. The endometrium layer is richly supplied with blood vessels. There is a sphinctoc muscle that closes the lower end of the uterus where it joins the vagina.

(iv) Vagina

Vagina is the organ where the penis is inserted during coitus (sexual act) for the discharge of semen. It is a muscular tube about 7-10 cm in length. It serves as the birth canal during child birth and also acts as a duct for the passage of uterine excretions and menstrual flow. The Vagina opens to the outside by an opening. The opening of vagina is normally obstructed in a virgin female by a perforated membrane, the hymen. In a human female, the urethra and the genital duct have separate opening.

Does an irregular menstrual cycle cause any problems?

When a girl begins menstruating, it may take sometime for her periods to become regular. Also, sometimes her menstrual cycle may become irregular, and her periods may be delayed or may occur earlier than the expected date because of illness or mental tension such as stress or depression. Irregularities in the menstrual cycle are quite common among young girls who have just begun to menstruate. However, if one does not menstruate and one is worried about it, then it is important to consult a doctor or a health worker.

How does one maintain hygiene during the menstrual period?

• Daily bathing along with regular/ daily washing of the genital area is essential.
• Sanitary pads and/ or cloths used should be changed at least twice a day, if not more frequently.
• It is important to maintain menstrual hygiene in order to reduce the risk of contracting an infection of the female reproductive tract.
• If pads or napkins are not changed frequently, the old blood begins to smell. This may lead to social embarrassment.
• Home-made sanitary napkins should be washed thoroughly with hot water and soap, and should be dried in a sunny and airy place. They should be stored in a clean and dry place.
• Moderate exercise and sufficient rest are also important.

Menstrual cycle in Human Females

In a human female, the fertility period extends from the age of puberty, i.e. about 12-13 years up to menopause, i.e. 45-50 years. The stage of puberty is marked by the appearance of secondary sexual characteristics.

The onset of menstruation in a female is called menarche. It starts at an age of about 11-13 years. The permanent stoppage of menstruation in a female is called menopause. It occurs at an age of about 45-50 years. At the time of menopause, ovulation and menstruation stop and the reproductive organs decrease in size.

Between puberty and menopause, the female reproductive system passes through a regular monthly sequence of events called the menstrual cycle.

During menstrual cycle an ovum is matured and released once every 28 days. However, many a times, due to some reasons this period may increase or decrease. The menstrual cycle starts with the menstrual flow during which the cellular lining of the uterus, with blood flow, shed off. This process continues for 3-4 days. From the 5th upto the 13th day of the onset of menstrual cycle, growth and maturation of the Graafian follicle takes place. Graafian follicle is the final stage in the maturation of an ovum and a mass of cells surrounding it. The Graafian follicle also produces a hormone, oestrogen, which stimulates the uterus to prepare itself to receive the ovum. The cells lining the uterus grow rapidly and develop a dense network of blood vessels.

Ovulation takes place 13-14 days after the onset of menstruation. The Graafian follicle ruptures to release the ovum. The cells of the ruptured follicle from the corpus luteum which secretes the hormone, progesterone. The ovum reaches the uterus via the fallopian tube on the 13th or 14th day and remains there up to the 16th day (for 48-72 hours). If the ovum does not receive any sperm during this period it starts degenerating. At the end of the 28th day ovum is rejected along with the uterine lining. This marks the start of slow disintegration of the thickened lining of the uterus and the next menstrual cycle.

What happens to the menstrual cycle if the ovum receives sperm and fertilization occurs? If the ovum receives sperm and gets fertilized, menstruation (and ovulation) cease for as long as the woman is pregnant. This is because progesterone is produced continuously first by the corpus luteum (which persists in the ovary) and later by the placenta.

Fertilization and implantation

Spermatozoa remain viable in the female genital tract from 24 to 72 hours. For fertilisation, sperms are introduced into the female body. One sperm fuses with the ovum in the fallopian tube. If the ovum happens to meet a sperm, the two unite to form a zygote. 13-14 days after onset of menstruation are most favourable for conception (pregnancy). The zygote immediately begins to divide and passes down the fallopian tube to the uterus and fixes itself to the wall of the uterus. This fixing of the embryo in the wall of the uterus is called implantation and the female is said to be pregnant. Implantation takes place about a weed after fertilization.

Placenta

Placenta is an association between maternal and foetal tissue meant for some extremely important physiological exchange. The developing embryo is attached to the uterus by a tissue called placenta. Umbilical cord is a tough structure that serves as the blood vascular connection between the foetus (developing embryo) and uterine wall. From the first few weeks of development, the embryo is enclosed in a sac called amnion which is filled with amniotic fluid. Amniotic fluid acts as a shock-absorber and helps to protect the embryo from damage of the placenta.

Placenta serves as a tissue through which oxygen and food are supplied from the maternal blood to the foetus. It also transports carbon dioxide and excretory waste from the foetal blood to the maternal blood.

What happen during childbirth?

• The uterus undergoes occasional contraction (labour).
• The amnion bursts and the amniotic fluid is discharged.
• The uterus contracts vigorously, expelling the baby.
• The baby’s lungs start functioning and the baby takes its first breath.
• The umbilical c d is tied and cut.
• Afterbirth the placenta gets discharged.
• The breast start producing milk.

Usually only one ovum is released by an ovary in every reproductive cycle. Then this ovum receives sperm and gets fertilized, one baby is born to the mother. But sometimes two eggs may be released and fertilized by this different sperms. Such siblings are called fraternal twins who may be brother and sister or brother – brother, or sister-sister. But in certain cases, only one egg is released and gets fertilized. After this, it divides into two, and the two cells then separate and start developing independently into two separate individuals. They are identical in all respects and called identical twins. They are always of the same sex.

Siamese twins

The twins produced from one egg which fail to separate are called Siamese twins. The first case of Siamese twins was of twin boys born to a Chinese mother in Siam, (now Thailand) in 1811. These were joined at the thoracic region. These were joined at the thoracic region. These twins lived up to an age of 65 years. The Siamese twins can sometimes be surgically separated. However, it depends upon the extent of their joining.

NEW MEDICAL TECHNIQUES IN REPRODUCTION

There are many new medical techniques in the field of reproduction to help infertile males and females produce babies. Some of these are given here.

Test Tube Babies

Some women their oviducts (Fallopian tubes) get blocked. This prevents the ova from being fertilized. This problem can be overcome by the test tube baby technique. In this technique, one or more ripe ova are sucked from a woman’s ovaries using a special syringe. These ova are placed in a dish containing sperms from her male partner under optimum conditions for a few hours. Sperms fertilize the ova which form an embryo. One embryo is then inserted into the woman’s uterus where there is a chance it will implant and develop into a baby.

Artificial Insemination

Human semen can be rapidly frozen using liquid nitrogen and stored in sperm banks for several years without losing its fertile condition. It is then thawed, and introduced into a woman by means of a syringe at a time when ovulation takes place. This is called Artificial Insemination.

Fertility drugs

In some women ovaries fail to develop the Graafian follicles needed to release ripe ova into their reproductive tract. It is now possible to artificially stimulate follicle production by injecting sterile women with a fertility drug containing FSH (follicle stimulating hormone) obtained from animals..