Human Reproduction | Zoology Optional Notes for UPSC PDF Download

Introduction

In this unit we are going to understand about reproductive biology of males and females, the biological factors related to human fertility and about menarche and menopause. We will also study other bio events related to human fertility. Now let us try to understand about the phenomenon of reproduction among human beings. Reproduction is one of the essential characteristics of life. It is illustrated in its primitive form by the action of single –celled amoeba in dividing into two. Most of the cells of the human body have the same power of division by virtue of which growth and repair are possible. Reproduction in man and other higher animals is a complex process involving the existence of two sexes, both of which play their respective roles in the formation of a new individual, i.e. offspring. The reproductive organs of the male and female differ in anatomical structure and arrangement, each having functional specificities required for reproduction. The function of the male organs is to form spermatozoa or sperms and implant & 52 Human Growth and Development them within the female so that they can meet the ova. The female organs are adapted to form ova or eggs which, if fertilized by spermatozoa, remain in the cavity of the uterus. Here an embryo or fetus is formed and is retained until the individual grown in the uterus is capable of a separate and independent existence.

The Reproduction System

Evolutionary biology clearly indicates that the sexes are separate among human beings. As such the reproductive systems are separate and function independently. Both male and female reproductive systems function with the help of different hormones secreted by different glands, thereby they are responsible for different functions and carry out reproductive process in a successful manner.

Reproductive Physiology Of Male And Female

Before actually knowing about the reproductive physiology of males and females it is better to understand the different reproductive organs involved in it. In this sub unit we can discuss about the reproductive physiology of male and female briefly. Human beings are bi-sexual organisms wherein sexes are separate and as such they have separate sex organs and these sex organs are specialised for carrying out certain functions. In both the sexes, different organs are meant for different functions in reproductive process. In subsequent sub-units we are going to discuss about the structure of male and female sex-organs, the related endocrine secretions and their functions in the process of reproduction in brief.

Organs Of Male Reproductive System

The male reproductive system includes the primary sex organs and accessory sex organs. Primary sex organs are testes and the accessory sex organs are seminal vesicles, prostate gland, urethra and penis.

Testis

Testis is the primary male sex organ or male gonad. It corresponds with ovary in females. There are two testes (singular = testis) in almost all the species. Each testis contains about 900 coiled tubules known as seminiferous tubules. The seminiferous tubules produce sperms. The sperms enter the vas deferens, which form the epididymis. It is continued as vas deferens.

Seminal Vesicles

The seminal vesicles are accessory sex organs in males, which are situated on either side of prostate. Secretions of seminal vesicles are emptied into ampulla of vas deferens. The enlarged portion of vas deferens is called ampulla. The ampulla of the vas deferens is continued as ejaculatory duct, which passes through prostate to form internal urethra.

Prostate Gland

Prostate gland is also an accessory sex organ formed by numerous secretory glands. Secretion from prostrate glands follows the path to utriculus prostaticus and is then emptied into internal urethra.

Urethra

Urethra has two parts namely, internal urethra and external urethra. Internal urethra is the continuation of ejaculatory duct. Internal urethra passes through penis as external urethra. Urethra contains mucus glands throughout its length, which are called glands of litter. The bilateral bulbourethral glands also open into the urethra.

Penis

Penis is the male genital organ formed of three erectile tissue masses, i.e., a paired corpora cavernosa and an unpaired corpus spongiosum. The urethra passes through penis and opens to the exterior and the spongiosum surrounds the urethra and terminates distally to form glans penis.

Structure of Testis

The testes are ovoid or walnut shaped bodies having the organisation of compound tubular gland. Both the testes are located in the sac like structure called scrotum.

Functions of Testis

The gametogenic function and Endocrine function. The production of gamete cells is called the gametogenic function. Spermatogenesis is the process by which spermatozoa are developed from the primitive germ cells in the testis known as spermatogonia.

Organs Of Female Reproductive System

Now we can try to understand about the female reproductive organs; specifically their structure and functions. The female sex organs are situated in the pelvis and for purposes of description may be divided into: i) Internal organs: uterus, ovaries, fallopian (uterine) tubes, vagina ii) External organs: mons veneris, labia majora and minora, clitoris, hymen iii) Secondary organs: the breast or mamae (mammary glands).

The Ovary

There are two ovaries (female gonads), right and left, lying on each side of the upper pelvic cavity situated against the pelvic wall near the uterus. Each is about the size of a large almond and is attached to the posterior aspect of the broad ligament of the uterus by a fold of peritoneum. It lies immediately below the fallopian tube which forms an arch over the top of the ovary and ends just below its lateral margin.

The Fallopian Tube

The fallopian tubes, two in number, named after the 16th century Anatomist, Fallopins, are about 10 cm (4 inch) in length and lie in the upper margin of each broad ligament of the uterus, thus being surrounded by peritoneum. The outer end of the tube is expanded and has an opening into the peritoneal cavity. This is surrounded by a number of fringes – like processes, the fimbriae, which lie close to the lateral part of the ovary.

The Uterus

The uterus (womb) is a hollow, pear-shaped organ situated in the pelvic cavity above the urinary bladder and in front of the rectum. It has thick muscular walls and a small central cavity. In the nulliparous women (those who have never borne a child) it measures about 7.5 cm (3 in) in length, 5 cm (2 in) in width and 1.75 cm (1 in) in thickness. In multiparous (those who have previously borne children) the uterus is still larger and its shape remains variable.

The Vagina

This is a canal with muscular walls 8-10 cm (3-4 in) long which passes in a downward and forward direction from the cervix of the uterus to its lower orifice in the vulva. It is normally collapsed, and the length and diameter of the vagina increased during sexual arousal.

The Breasts or Mammary Glands

The two breasts are glands which are accessory to the genital system, that is, they take no parts in the actual process of reproduction. They are present in an undeveloped form in the female before puberty as also in the male. The fully developed female breast, while varying considerably in size, is circular in outline and approximately hemispherical in shape.

Physiology Of Male Reproductive Process

Now we can discuss the physiology of male reproductive process. In the last sub unit i.e., 4.4 it is clear about the way how the process of spermatogenesis takes place and now we will see the role of hormones in the process.

The Hormones which are necessary for spermatogenesis are:

• Testosterone not only stimulates the process of spermatogenesis but is also necessary for the formation of secondary spermatocyte from primary spermatocyte.
• Follicle stimulating hormone (FSH) in addition to testosterone is also necessary for the stimulation of the process of spermatogenesis.
• Luteinizing hormone (LH) is essential for the secretion of testosterone from Leydig cells.
• Estrogen secreted by Sertoli cells is also necessary for the process of spermatogenesis.
• Growth hormone (GH) is essential for the general metabolic processes in testis. It is also necessary for the proliferation of spermatogonia.

Physiology Of Female Reproductive Process

Before actually having a glance on female reproductive process we shall try to know about the functions of the female reproductive organs. The functions of the female reproductive organs are directed to the following ends:

• The formation of ova or ovulation.
• The preparation of the uterus to receive the fertilized ovum.
• The retention of the fertilized ovum within the cavity of the uterus until a mature foetus is formed, capable of leading an independent existence, i.e. pregnancy.
• The expulsion of the mature foetus, i.e. labour or parturition.

Ovulation

The ovary contains many thousands of eggs or ova which lie dormant until the onset of puberty. Active changes then take place in the ovary which results in the periodic discharge of an ovum at intervals of a month. A graafian follicle is a small cystic sac containing fluid and having the ovum attached to its wall, which comes gradually to the surface of the ovary and ruptures about two weeks after the commencement of the last menstrual period. The ovum therefore actually passes into the peritoneal cavity but is soon caught up in the fimbriae of the fallopian tube which closely surround the ovary. By the action of the ciliated epithelium of the fallopian tube, the ovum is carried slowly towards the cavity of the uterus. Within or little over ten days this stage of journey of the ovum towards the uterine cavity is completed. The ovum is either fertilized, in which case it becomes embedded in the wall of the uterus and commences to grow into an embryo; or else it is discharged unfertilized from the uterus in the menstrual flow. Certain changes take place in a graafian follicle after its rupture and it becomes a solid yellowish body called the corpus luteum. This body goes on developing until the next menstrual period, when it gradually disappears and is replaced by fibrous tissue. If the ovum is fertilized, however, the corpus luteum persists throughout pregnancy and, it will be recalled, acts as a gland of internal secretion, producing the hormone progesterone.

Menstruation

Menstruation is a function of the uterus established at puberty (average age, 12 to 13 years) as a result of ovarian activity and consists of the periodic discharge of blood from its cavity. It occurs on an average of every twenty-eight days until the menopause or climacteric is reached, and lasts for three to five days. The amount of fluid, which consists of blood, mucin and epithelial cells, varies between 90-200 ml (3-7 fl. Oz). Menstruation ceases during pregnancy and is often not reestablished until lactation is completed. The purpose of the monthly cycle is to prepare the mucous membrane of the uterus (endometrium) to receive a fertilized ovum. The endometrium undergoes constant changes between one menstrual period and another and these changes are made in preparation to receive the fertilized ovum. They are largely brought about by the Follicle-Stimulating (FSH) and the Luteinizing (LH) hormones secreted by the pituitary gland, and by estrogen and progesterone secreted by the ovary. Menstruation is really a clearing up of these changes in the endometrium when no fertilized ovum has arrived, and therefore in this sense it gives the endometrium an opportunity to make a fresh preparation. These changes are described as the menstrual cycle and may be conveniently divided in the following way: 1) The secretory (pre-menstrual) phase, lasting for about 14 days before the period, during which the endometrium becomes thickened and congested and is in a state of preparedness to receive a fertilized ovum. 2) The menstruation period (three to five days) in which some of the epithelium of the uterine mucosa is shed and is accompanied by bleeding. In other words, no fertilized ovum has been received and the work of preparation has been useless. 3) The stage of repair begins in the third or fourth days of the menstrual cycle. 4) The growth phase starts on the fourth day and continues up to fourteenth day before the next secretory phase.

Biological Aspects Of Human Fertility

Fertility is generally indicated by the actual reproductive performance of a woman or group of women. At the same time, we should know about the other related term ‘fecundity’. Fecundity is the biological potential, i.e. the physiological capacity for reproduction. The absence of this potential is known as infecundity. Fertility can be described as the phenomenon of giving birth to children. Both biological and non-biological factors play an important role in determining fertility. The onset of menarche in proper time, regular production of healthy ova, the production of a sufficient number of healthy sperm among the males, etc. are important factors. The other biological factors influencing fertility are health and disease, food habits, etc. Furthermore, genetic factors play an important role in human fertility as well.

Relevance of Menarche

Menarche is the first menstrual cycle, or first menstrual bleeding, in human females. From both social and medical perspectives it is often considered the central event of female puberty, as it signals the possibility of fertility. Girls experience menarche within a range of different ages after attaining puberty. The timing of menarche is influenced by female biology, as well as genetic and environmental factors, especially nutritional factors. The average age of menarche has declined over the last century but the magnitude of the decline and the factors responsible remain subjects of contention. Menarche is the culmination of a series of physiological and anatomical processes of puberty. During this period, a number of physiological changes occur in the girl’s body wherein secondary sexual characters appear. Menarche as a discrete event is thought to be relatively a chance result of the gradual thickening of the endometrium induced by rising but fluctuating pubertal estrogen. Puberty signals the onset of adult sexual life, and menarche means the inception of menstruation. At the start of about 8 years and usually terminating at the onset of menstruation between ages 11 and 16 years i.e. at an average age of 13 years, there is a gradual increase in gonadotropic hormone secretion by the pituitary causing the period of puberty. In the female, as in the male, the infantile pituitary gland and ovaries are capable of full function if appropriately stimulated.

Relevance of Menopause

Menopause is a period of permanent termination of the primary functions of the human ovaries, which are the ripening and release of ova and the release of hormones that cause both the creation of the uterine lining and later detaching of the uterine lining (a.k.a. the menses). Menopause is generally experienced in women during their midlife, which is late 40s or early 50s. This marks the end of the fertile phase of a woman’s life. There is a major decline in the production of female hormones by the ovaries during the change from reproductive to non-reproductive phase. It is not abrupt but phases over a period of years and is accepted to be a natural consequence of ageing. However, variations in the transition phase have been observed amongst women. In some women, the accompanying signs and effects can significantly disrupt their daily activities and their sense of well-being. In addition, menopause at a younger age is experienced by women who have some sort of functional disorder that affects their reproductive system (i.e., endometriosis, polycystic ovary syndrome, cancer of the reproductive organ). These functional disorders considerably hasten the menopausal process and result in health problems, both physical and emotional, in the affected woman.

Other Bio-Events To Fertility

Here we can discuss the effects of a few biological factors on human fertility. These factors include contraception, abortion, and sterilization. All these factors are so important that they need careful study. In every society, nowadays there is a tendency that family should be small, population explosion should be checked.

Contraception

Contraceptive practices affect fertility by decreasing the chance of conception. There is considerable variation in the effectiveness of practicing contraceptive methods as projected in theory and while in actual use. Effective rates of more than 95% are reported using contemporary methods such as oral pills and intrauterine devices. Older methods such as condoms and diaphragm can be more than 90 per cent effective when used regularly and correctly, but their average use effectiveness is lower because of irregular or incorrect use. Natural methods of contraception viz. withdrawal or abstinence are also in use with a variable degree of effectiveness in human groups.

Abortion

Induced abortion diminishes fertility by terminating pregnancy not by affecting fecundability. The practice of abortion in human societies dates back to ages and is rather common in some settings. Statistics reveal that the officially registered percentage of pregnancies terminated by abortion is more than one-third in some countries and substantial numbers of unregistered abortions are perhaps prevalent even in countries where they report a very low rate.

Sterilization

The complete elimination of fecundability can be brought about by sterilization. The surgical procedures of tubectomy and vasectomy have become common in diverse nations and cultures. In the USA, voluntary sterilization has become the most prevalent single means of regulating fertility, typically adopted by couples who have achieved their desired family size. In India, sterilization has been encouraged on occasion by various government-run incentive programs.