Contents
Developmental biology, a branch of Biology Topics, examines the processes of growth and differentiation.
What are the Secondary Sexual characteristics in the Human Male and Female respectively?
There are two types of sexual characteristics (or sex characteristics) in human beings: Primary sexual characteristics and Secondary sexual characteristics. The sexual characteristics which are present at birth are called primary sexual characteristics. Primary sexual characteristics include internal and external sex organs which are present in babies at the time of their birth. The primary sexual characteristics in máles (or boys) are:
Testes, Penis and Seminal vesicles, etc. The primary sexual characteristics in females (or girls) are : Ovaries, Oviducts, Uterus and Vagina, etc. The prinzarl,I sexual characteristics arc directly involved in reproduction.
The sexual characteristics controlled by hormones which distinguish between sexually mature males and females (sexually mature boys and girls) but are not directly involved in reproduction, are called secondary sexual characteristics. In secondary sexual characteristics, the body parts (other than sex organs) develop special features which make it easier to distinguish a boy from a girl.
For example, the growth of facial hair (like moustache and beard) in boys is a secondary sexual characteristic which helps to distinguish between a mature boy and a girl (because facial hair do not grow in girls). Similarly, the development of breasts in girls is a secondary sexual characteristic which helps to distinguish a girl from a boy (because boys do not develop breasts). The secondary sexual characteristics start developing at the time of puberty and continue to develop through the period of adolescence.
The main secondary sexual characteristics in males (or boys) are the following:
- Hair grow on face (in the form of moustache and beard) in boys.
- Shoulders and chest broaden (become wider) in boys.
- A deeper voice (or low pitched voice) in boys.
- Adam’s Apple develops in front of throat (or neck) in boys.
The secondary sexual characteristics in boys are produced by the male sex hormone called ‘testosterone’ made in testes. The main secondary sexual characteristics in females (or girls) are the following:
- Development of breasts in girls.
- Hips broaden and become more curved and prominent in girls.
- A shrill voice (or high pitched voice) in girls.
The secondary sexual characteristics in girls are produced by the female sex hormone called ‘estrogen’ made in ovaries. We will now discuss the role of hormones in initiating reproductive functions.
Hormones
Hormones are the chemical substances which co-ordinate the activities of living organisms (including human beings), and also their growth. Hormones are made and secreted by specialised tissues in the body called ‘endocrine glands’. The hormones are poured directly into the blood and carried throughout the body by the blood circulatory system. The hormones act on ‘specific tissues’ or ‘specific organs’ in the body called ‘target sites’. A hormone is produced by an endocrine gland in one part of human body but causes a particular effect in another part of the body.
Hormones are of many different types and perform different functions in the body. The hormones involved in the development and control of the reproductive organs and secondary sexual characteristics are called sex hormones. The two common sex hormones are testosterone and estrogen (Estrogen is also spelled as oestrogen).
Exocrine Glands and Endocrine Glands
A gland is a structure which secretes a specific substance (or substances) in the body. A gland is made up of ‘a group of cells’ or ‘tissue’. There are two types of glands in our body : exocrine glands and endocrine glands. A gland which secretes its product into a duct (or tube) is called an exocrine gland. So, exocrine glands are the glands having ducts. For example, the salivary gland secretes its product (saliva) into a duct called ‘salivary duct’, therefore, salivary gland is an exocrine gland. Sweat glands and sebaceous glands (or oil glands) are also exocrine glands which release their secretions through ducts.
A gland which does not have a duct and secretes its product directly into the blood stream, is called an endocrine gland. Thus, endocrine glands are ductless glands. An endocrine gland makes (and secretes) a chemical substance called ‘hormone’. Some of the examples of endocrine glands (or ductless glands) in our body are pituitary gland, thyroid gland and adrenal glands. Some of the glands in our body function both, as endocrine glands as well as exocrine glands. For example, pancreas, testes and ovaries function as endocrine glands as well as exocrine glands. Endocrine glands release their hormones directly into the blood stream of a person. These hormones reach the concerned body part (called target site) through the blood and act on it.
Role of Hormones in Initiating Reproductive Functions
The hormones play an important role in initiating the reproductive functions of adolescents. This is because the changes which occur at adolescence are controlled by hormones. In other words, the onset of puberty is controlled by hormones. Puberty begins with a sudden increase in the production of hormones in the body which cause a number of changes in boys and girls.
We have already studied that testes and ovaries are the real reproductive organs in humans. The testes and ovaries produce sex cells or gametes (sperms, and eggs or ova). In addition to producing gametes (sperms and eggs), testes and ovaries produce sex hormones which bring about changes in the bodies of boys (males) and girls (females) at puberty, and make their reproductive systems functional.
The testes make male sex hormone called testosterone. The ovaries make female sex hormone called estrogen. The production of these sex hormones is under the control of another hormone (called gonadotropic hormone) secreted by pituitary gland. The production of sex hormones ‘testosterone’ and ‘estrogen’ increases dramatically at the stage of human growth called puberty. This rise in the level of sex hormones is responsible for the changes in the bodies of adolescent boys and girls at puberty.
(a) The hormones from pituitary gland stimulate testes to release male sex hormone ‘testosterone’ at the onset of puberty in boys. Testosterone hormone performs the following functions :
- Testosterone hormone produces male secondary sexual characteristics in boys at puberty (such as deeper voice; growth of facial hair like moustache and beard ; broad shoulders and chest; Adam’s Apple ; and more muscles).
- Testosterone hormone causes the growth and development of male sex organs at puberty.
- Testosterone hormone causes growth spurt’ in boys at puberty.
(b) The hormones from pituitary gland stimulate ovaries to release the female sex hormone ‘estrogen’ at the onset of puberty in girls. Estrogen hormone performs the following functions :
- Estrogen hormone produces female secondary sexual characteristics in girls at puberty (such as shrill voice ; development of breasts ; and broader hips).
- Estrogen hormone causes the growth and development of female sex organs at puberty.
- Estrogen hormone brings about the monthly preparation of uterus for pregnancy.
The pituitary gland secretes many hormones. One of these hormones stimulates the formation of sperms in testes and maturation of egg cells (or ova) in ovaries. A yet another hormone develops the mammary glands (milk producing glands) inside the breasts.
Reproductive Phase of Life in Humans
Adolescents become capable of reproduction at puberty when their testes and ovaries begin to produce gametes (sperms and eggs). Adolescent boys grow and become men. Adolescent girls grow and become women. In men, the capacity to produce male gametes (or sperms) usually lasts throughout life. But in women, the capacity to produce female gametes (eggs or ova) lasts only up to about 45 to 50 years of age.
Due to this, the reproductive phase of life in men {or males) lasts much longer than in women {or females). We will now discuss the reproductive phase of life in human females (or women) in detail. In females (or women), the reproductive phase of life begins at puberty (10 to 12 years of age) and generally lasts till the age of approximately 45 to 50 years.
(a) With the onset of puberty, the eggs (or ova) begin to mature in the ovaries of a woman. One mature egg (or ovum) is released by one of the ovaries of the woman once in about 28 to 30 days. During this period, the inner lining of uterus grows and becomes thick and spongy, and prepares itself to receive the fertilised egg (see Figure).
So, in case the fertilisation of egg cell occurs by a sperm, the fertilised egg cell begins to divide to form an embryo. The embryo then gets embedded in the thick uterus lining. This results in pregnancy which ultimately leads to the birth of a baby. We will now describe what happens if the fertilisation of egg cell does not take place.
(b) If fertilisation does not occur (due to lack of sperm), then the egg released by the ovary dies within a few days and the thick lining breaks down (because it is no longer required). Since the thick uterus lining contains a lot of blood vessels, therefore, the breaking down (or disintegration) of uterus lining produces blood alongwith other tissues. This blood and other tissues come out of vagina of woman in the form of a bleeding’. The bleeding from the uterus which occurs in a woman (or mature girl) every month (if the egg cell has not been fertilised) is called menstrual flow or menstruation (see Figure).
Menstruation occurs once in about 28 to 30 days (which is almost a month). Menstruation occurs every 28 to 30 days because ovulation (release of egg or ovum by the ovary of woman) occurs after every 28 to 30 days. In everyday language, menstruation is called ‘monthly periods’ or just ‘periods’. It is also called ‘menses. Menstruation (or periods) usually lasts for about 3 to 5 days in a month.
The first menstruation (or menstrual flow) begins at puberty (when the girl or woman is around 10 to 12 years of age). The first occurrence of menstruation (or periods) at puberty is called menarche. Menarche is the beginning of the reproductive life of a girl (or woman). In other words, menarche is the time from which a girl (or woman) becomes capable of having a baby. Menstruation stops temporarily when a woman becomes pregnant.
Menstruation restarts after the birth of the baby. Menstruation stops permanently when a woman reaches the age of about 45 to 50 years. With the permanent stoppage of menstruation, a woman loses her ability to bear children. The permanent stoppage of menstruation (or periods) in a woman is called menopause. Menopause occurs in women at the age of about 45 to 50 years. A woman stops ovulating at menopause and can no longer become pregnant. Menopause is the end of the reproductive life of a woman. We can now say that the reproductive life of a woman starts at menarche and ends at menopause.
Menstrual Cycle
The process of ovulation and menstruation in women is called menstrual cycle (because it occurs again and again after a fixed time period). The menstrual cycle is a period of about 28 to 30 days during which an egg cell (or ovum) matures, the mature egg cell is released by the ovary, thickening of uterus lining takes place, and finally the uterus lining breaks down causing bleeding in women (if the egg cell has not been fertilised). Initially, the menstrual cycle in girls may be irregular (it may not be of 28 to 30 days). It becomes regular after some time. Menstrual cycle in women is controlled by hormones. We will now discuss the determination of sex in humans which involves the role of sex chromosomes.
Sex Chromosomes
Chromosomes are present inside the nucleus of every cell of the human body. All the human beings have 23 pairs of chromosomes in the nuclei of their normal body cells. Out of these 23 pairs of chromosomes, one pair of chromosomes (or two chromosomes) are called sex chromosomes. The two chromosomes that determine the sex of an offspring (or baby) are called sex chromosomes. The two sex chromosomes are named X chromosome and Y chromosome. The gametes (or sex cells) are not normal body cells, they are special cells called reproductive cells.
The gametes contain only 23 single chromosomes and hence they have only one of the two sex chromosomes (X or Y) that exist in the normal body cells. In females (or women), all the gametes or all the egg cells (ova) contain X chromosomes. Females (or women) have no Y chromosomes in their gametes (egg cells or ova). On the other hand, in males (or men), half of gametes (or sperms) have X chromsomes and half of gametes (or sperms) have Y chromosomes. The baby developed from XX combination of sex chromosomes in zygote is a girl. And the baby developed from XY combination of sex chromosomes in zygote will be a boy. Let us discuss this in detail.
How is the Sex of a Baby Determined
When a woman gives birth to a baby it can be a boy (male) or a girl (female). The sex of a baby to be born is decided at the moment the egg cell of woman (mother) gets fertilised by the sperm of man (father) and pregnancy occurs. This is because the instructions for determining the sex of baby (whether it will be a boy or girl) are present in the sex chromosomes which are in the nucleus of the fertilised egg called zygote. The sex of baby is determined by the type of sex chromosomes present in the fertilised egg (or zygote) from which the baby develops. This will become more clear from the following discussion.
There are two types of sex chromosomes : X chromosomes and Y chromosomes.
- A female (woman or mother) has only X chromosomes in all her gametes called eggs (or ova). This means that all the female gametes called eggs (or ova) have only X chromosomes.
- A male (man or father ) has X chromosomes as well as Y chromosomes in male gametes called sperms. Actually, half the male gametes or sperms have X chromosomes and the other half of male gametes or sperms have Y chromosomes.
The sex of a child depends on what happens at fertilisation :
(a) If a sperm carrying X chromosome fertilises an egg cell (or ovum) which carries X chromosome, then the zygote formed will have XX combination of sex chromosomes due to which the child born will be a girl (or female) (see Figure).
(b) If a sperm carrying Y chromosome fertilises an egg cell (or ovum) which carries X chromosome, then the zygote formed will have XY combination of sex chromosomes due to which the child born will be a boy (or male) (see Figure).
Thus, the sex of unborn child (or unborn baby) depends on whether the zygote formed at the time of fertilisation has XX combination of sex chromosomes or XY combination of sex chromosomes. XX combination of sex chromosomes makes the child a girl whereas XY combination of sex chromosomes makes the child a boy. Please note that it is the sperm of man which determines the sex of the child. This is because half of the sperms have X chromosomes and the other half have Y chromosomes.
Thus, there is a 50 percent chance of a boy and a 50 percent chance of a girl being born to the parents. This is why the human population is roughly half males and half females. The egg cells (or ova) of woman cannot decide the sex of the child because all the egg cells (or ova) contain the same sex chromosome, X chromosome.
From the above discussion we conclude that if the father (man or husband) contributes X sex chromosome at fertilisation through his sperm, the baby born will be a girl. On the other hand, if the father (man or husband) contributes a Y sex chromosome at fertilisation through his sperm, then the baby born will be a boy. This means that it is the sex chromosome contributed by father (man or husband) which decides the sex of the baby which the mother (woman or wife) will give birth to. Thus, father (man or husband) is responsible for the sex of the baby (boy or girl) which is born.
The belief that mother (woman or wife) is responsible for the sex of her baby is absolutely wrong. In many ignorant Indian families, the mother (woman or wife) is held responsible for the birth of a girl child and unnecessarily harassed. Such people should understand that it is the husband who is responsible for the birth of a girl child (and not his wife). Moreover, a girl is no less than a boy.