Growth and Development

Sexual reproduction involves the fusion of two sets of DNA to form a zygote. Development is the series of changes in a zygote, which becomes an adult. The stage of multicellular organism that develops from a zygote is called embryo. In seed plants, the embryo is part of the seed. The study of development of embryo is called embryology. 

Growth is an increase in the amount of living material in an organism. Growth is a fundamental characteristic of all living organisms. The process of development is so closely linked with growth that the phrase growth and development is commonly used to describe the processes which are normally thought of as growth. Development could be described as an increase in complexity, and growth as the permanent and irreversible increase in size that occurs in an organism. 

Starting with an individual cell, growth of a multicellular organism can be divided into three phases. 

(a) Cell Division: An increase in cell number as a result of mitosis mitosis and cell division. 

(b) Cell Enlargement: An irreversible increase in the cell size as a result of the uptake of water or the synthesis of living material. 

(c) Cell Differentiation: It is the specialization of cells in its broad sense, growth also includes this phase of cell development. 

Sexually Transmitted Diseases (STDs)

Sexually transmitted diseases are caused by organisms ranging from viruses to arthropods, however, only gonorrhea, syphilis, AIDS, and Herpes simplex are discussed here.

It is caused by the gram positive bacterium Neisseria gonorrhoeae. There is no blood test to diagnose gonorrhea. In male, typical symptoms are pain upon urination and a thick greenish yellow urethra! discharge. If a baby is exposed during birth, an eye infection leading to blindness can result. Gonorrhea can spread to internal parts of the body, causing heart damage or arthritis. It is transmitted through direct contact or sexual contact. It is treated using antibiotics penicillin or tetracycline.

It is caused by a spirochete bacterium Treponemn pallidum. It has three stages, which are typically separated by latent periods. In the primary stage, a hard chancre (ulcerated sore with hard edges) appears. In the secondary stage, rash appears all over the body.

During the tertiary stage, syphilis may affect the cardiovascular and nervous system. It damages reproductive organs, eyes, bones joints, central nervous system, heart and skin. Sexual contact is the major source of its dissemination. Syphilis is a very devastating disease. Control depends on prompt and adequate treatment of all cases be treated with antibiotic therapy.

The organism that causes acquired immunodeficiency syndrome (AIDS) is a virus called human immunodeficiency virus (HIV). HIV attacks the type of lymphocyte (a type of WBCs) known as helper T cells. These cells stimulate the activities of B lymphocytes, which produce antibodies. After an infection sets in, the person becomes more susceptible to other types of infection, as helper T cells begin to decline in number. Symptom of AIDS has three stages of infection called category A, B and C. During category A stage, which may last about a year, there may be no symptoms, but the individual may pass the infection. During category B stage, which may last six to eight years, the lymph nodes swell and there may be weight loss, night sweats, fever, diarrhea. Category C stage is full blown AIDS characterized by nervous disorders and by development of an opportunist disorders such as unusual type of pneumonia etc. AIDS is transmitted by sexual contact with an infected person, through infected dental equipment used needles, shaving blades and through transfusion of infected blood or blood clotting factors.

Medicine like AZT is used to cure AIDS. The best way is “prevention is better than cure”.

Genital Herpes
It is caused by herpes simplex virus type 2. It is transmitted by sexual contact causing infection of the genitalia. It produces genital soreness and ulcers in the infected areas. Exposure to herpes in females can cause an infection in the new born, which leads to neurological disorders, damage to eye and even death.

Reproduction of a Placental Mammal - Man

The highest order of protection of the offspring is seen in man. It is evident from the internal development, birth and later care and feeding of the young.

Male Reproductive System
The reproductive function of the male is the production of sperm cells and their transfer to the female. Male reproductive system consists of:

(a) Testes
(b) Accessory Glands
(c) Penis

The paired testes (singular, testis) are formed inside the body cavity, but they descend shortly before birth into an external sac between the thigh, the scrotum. The testes contain about 15 to 20 coiled seminiferous tubules which produce sperm cells or spermatozoa at a daily rate of a billion or so in young man. The actual combined length of the seminiferous tubules in both testes is about 225 meters.

Male Reproductive System

Accessory Ducts and Glands
Several structures are associated with the testes. Applied directly to each testes in an epididymis (Greek: upon the twins). It is a coiled tube. It would be about six meters long if it is stretched. The haploid sperm cells are stored in the epididymis.

All the seminiferous of the testis lead into the epididymis, which then fuses into the major seminal duct the vasdeferen (or ductus deferens). Vasdeferens receives secretions from the seminal vesicles and the prostate gland as it passes alongside them to the urethra.

The urethra is the final tube, which leads through the penis and transport sperms outside the male body. During ejaculation, fluids from the seminal vesicles and prostrate gland are secreted into the deferens, and these secretions plus the sperm cells make up the semen. Seminal fluid enhances sperm motility in various ways.

Testis And Sperm
With the onset of sexual excitement, the cowpers glands secrete alkaline fluid into the urethra to neutralize the acidity of any remaining urine. These fluids also act as a lubricant within the urethra to facilitate the ejaculation of semen. The function of the penis is two fold. It carries urine through the urethra to the outside and it transports semen through the urethra during ejaculation.

Hormonal Regulation in Male
Among the seminiferous tubules in the testes, are small masses of interstitial cells which secrete male sex hormones, especially testosterone. Testosterone affects (1)the production of sperm cells (2) the development of the sex organs (3)the appearance of secondary male sex characters.

FSH and LH produced by the pituitary gland are chiefly responsible for the stimulation of spermatogenesis and testosterone secretion in the testes.

Human Female Reproductive System
Human Female Reproductive System 
Sexuality in females is more complex than it is in males. Not only do the females have to produce egg, but after fertilization they must nourish, carry and protect the developing embryo.

They must also nourish it for a time after it is born. Besides all these, females have a monthly menstrual cycle. Female reproductive system consists of:

(i) Ovaries (ii) Ducts (iii) External Genitalia.

The egg-producing organs are the paired ovaries, elongated bodies, about 5cm long at maturity.

Ovaries have two functions (1) they release eggs (ova) fairly on regular monthly schedule (2) they produce female sex hormones.

The ovaries contain follicles, the actual centre of egg production. Each follicle contains a potential ovum and follicles are always present in several stages of development.

Infundibulum: Each ovary is partially covered by a funnel shaped infundibulum, into which the egg passes when it is released from the ovary. From the infundibulum an egg can pass down the fallopian tube (or oviduct or uterine tube) which leads to the uterus or wombs.

The fallopian tube terminates in the uterus, a pear shaped and sized organ located directly behind the urinary bladder. The uterus leads downward to the vagina a muscle-lined tube about 8-10 cm long. A fold of skin called the hymen partially blocks the vaginal entrance. The vaginal canal ends at the cervix or the neck of the uterus.

Fertilization: Fertilization of the ovum usually takes place in fallopian tube or oviduct, near the ovary.

implantation of zygote: The zygote enters the uterus. It is implanted in the wall of the uterus, and undergoes further development.

Placenta: The embryo establishes a close contact with the uterine tissue through which exchange of materials takes place. This connective tissue is called placenta. Together with the chorion, the allantois helps to form the placenta.

Gestation period: It is called pregnancy. This period is usually of 280 days in human female.

Umbilical cord: The edges of the amniotic folds come together around the stalk of the allantois and yolk sac and form a tube, which leads from the embryo to the placenta. This tube is called the umbilical cord. It contains the very important blood vessels that connect the embryo with the placenta.

During pregnancy the uterus usually increase about 24 times. The process of child birth or labor is started and controlled by a variety of hormonal events. Birth has been found to be brought about by a change in hormones produced by the placenta and also by a hormone produced by the pituitary glands. Oxytocin brings about the contractions of the uterus. The process of child birth or parturition is divided into 3 stages:


Dilation: During the first stage of childbirth, the cervix dilates or becomes larger. This is the longest stage of childbirth. The opening of the cervix is initially quite small. It must enlarge from a diameter of one to two centimeters to 10 to 11 cm before the fetus can leave the uterus.

Dilation begins when the muscles of the uterus begin to contract. These muscle actions of the uterus gradually force the fetus and amniotic sac downwards. At first the contractions are mild and occur at 20 minutes apart. Later, the contractions become stronger and may occur as close as a minute apart. Often towards the end of this stage, the amniotic sac breaks releasing the amniotic fluid.

Expulsion: The expulsion stage begins when the cervix is completely dilated and ends with the birth of the baby. This stage is characterized by strong and frequent contractions of the uterus. The expulsion of the fetus from the uterus to the outside of the mother's body usually takes from half an hour to two hours.

Placental stage: In the last stage of birth, also known as after birth, contractions of the uterus dislodge the placenta from the wall of the uterus. Continued contractions expel blood, other tissue, fluids and the placenta through the vagina. The placenta is squeezed as it passes out of the body. This action forces blood from the placental blood vessels into the body of the newborn baby. After the umbilical cord stops pulsating, the cord is tied and then cut by a doctor. The placental stage is from ten to fifteen minutes.

Usually at about the ninth month after the conception near the end of the human gestation period the head is turned downwards towards the vagina. At birth, therefore the head usually comes out first. However, very rarely the feet may appear first; causing complications. This results in breech. In other types of severe complications, for example if baby has been found to have a weak heart cesarean section may by performed in which the abdominal and uterine wall is cut to remove the baby.

Placental remains: The contraction and relaxation movements of the uterus continue for some time after the birth, as a result the placental remains are expelled out.

The three stages of birth 
Mammary Glands
There is a pair of mammary glands. They secrete milk and are accessory organs to the female reproductive system. They are converging to the nipple and opening on its surface in large numbers. The lymph nodes consist of many lobes. The lobes are divided into lobules.

During pregnancy secretions of estrogen's and progesterone cause the lobules to develop. The actual milk producing hormone is prolactin.

Milk: It is produced by the mammary gland just at the time of child birth. It contains a special lymph like fluid known as colostrum. It is rich in antibodies. The true milk is not secreted till about the third day after child birth. Usually the baby is fed on the mother's milk up to two years. The baby may be fed on other sources of milk if required. As soon as the mother stops feeding the baby, her reproductive cycle begins again. Sometimes the reproductive cycles initiate even when the mother is breast (mammary gland) feeding.

Hormonal Control of Female Reproductive Tract

Estrous: For the proper timing of internal fertilization, in many animals the female informs the male that she is ready to mate. When the female is in the estrous, or “heat” (becomes sexually receptive to male), she undergoes physical and behavioral changes. These changes coincide with the maturation of eggs, within the ovary.

Estrous Cycle: The physiological changes occurring from one period of estrous to the next are called estrous cycle. It is controlled by hormones. The timing of the estrous cycle varies in different animals e.g. rats: 5 days, bitch: every 6 months. Estrous cycle occurs in many animals. Example: rats, mice, cats, dogs.

Menstruation: The periodic shedding of blood and tissue from the uterine lining the endometrium is called menstruation.

Menstrual cycle: Unlike many other mammals, eggs of the human female mature quite often. Human females do not undergo a seasonal estrous cycle as dogs or cats do. About once every twenty-eight days, a mature egg is released from an ovary. At the same time certain uterine changes occur to prepare for a possible pregnancy. This monthly egg maturation and uterine preparation is called menstrual cycle.

The Role of the Ovary
The ovary consists of two types of cells (a) germ cells (b) somatic cells. Germ cells give rise to the ova(eggs). Somatic cells are the follicle cells.

Follicle Cells: These are concerned with support and nutrition of the developing ova. The follicle cells secrete estrogen. It is a female sex hormone. Estrogen acts on the uterus. It causes the endometrium of the uterus to divide rapidly. Some of the cells of endometrium become glandular in nature. They start secreting proteinaceous secretions. When the embryo reach the uterus, it starts taking up glandular type proteinaceous secretion. The wall of the uterus become thick and more blood vessels develop in it due to estrogen.

Ovary and follicles
The ovum is surrounded by follicle. The ovulation takes place. As a result follicle ruptures. After rupture the follicle is converted into a mass of specialized tissue called corpus luteum. If the discharged ovum is fertilized, the corpus luteum remains till the placenta is formed later it gradually turns into a scar tissue. If the ovum is not fertilized, the corpus luteum degenerates; and turns into scar tissue. The corpus luteum is cream in color (corpus luteum means yellow body). Its cells become glandular. They secrete a hormone called progesterone.

Functions of Progesterone: It prepares myometrium, endometrium and muscles of the uterine wall for the implantation of zygote.

Menstrual Cycle
There is periodic shedding of blood and tissue from the endometrium of the uterus. It is called menstruation. (The Latin word for “month” is menses). Cycle is defined as a succession of events repeated regularly within a given period of time. So the occurrence of menstruation at regular intervals is called menstrual cycle. Generally it is repeated after every 28 days.
However, the number of days is variable in different individuals or within the same individual at different times.

Menstrual cycle can be divided into four stages:

(a) Follicular stage (b) Ovulation stage (c) Luteal stage (d) Menstrual stage

Follicular Stage: One or sometimes more than one egg and follicles start growing in size. The egg is surrounded by follicle. The follicles arrange themselves in layers. So a large cavity appears between the follicle cells and the developing egg. Some of the follicle cells secrete estrogen. It affects the growth of the soft tissues of the uterus. Primarily estrogen affect the internal lining of the uterus. So the uterine tissue becomes spongy in nature.

Ovulation stage: At about the midpoint of twenty eight to thirty days cycle, under the influence of Luteinizing hormone, ovulation takes place. The ovum then passes to the fallopian tube (or oviduct).

Luteal stage: After ovulation, continued LH stimulation causes the follicle cells to secrete glandular cells. These glandular cells constitute the corpus luteum. The maturing corpus luteum secretes progesterone. Progesterone acts on the uterus by stimulating vascularization (blood vessel development), glandular development and glycogen accumulation in the endometrium. As a result the uterine epithelium becomes folded, and the endometrium becomes soft and spongy.

Menstrual stage: In a typical menstrual cycle, when a pregnancy has not begun, the LH level falls. It causes the corpus luteum to begin degenerate by 21st or 22nd day. As the corpus luteum is degenerating, its hormones output decreases. The thickened spongy part of the uterus starts a breakdown, the menstruation begins. During this phase the broken down soft tissues of the uterus and considerable amount of blood is released and passes out of the vagina. The day on which menstruation begins is commonly designated as “day one” of the menstrual cycle. Menstruation is a clear signal that pregnancy has not begun. The menstrual stage lasts for 5-6 days. The cycle is thus completed. The uterus then enters into the next menstrual cycle.

Age for Menstruation: It begins usually at the age of 12-14 in the females. It is variable.

Menopause: Between the ages of approximately 43 and early 50 a woman loses her ability to produce ova. The menstrual cycle ceases. Her breasts, genitalia and uterus all decrease in size. Sexual desire may also be reduced.

Menstrual Cycle 

Role of Pituitary Gland

Gonadotropic: The pituitary hormones that play an important role in the reproductive cycle in the female are commonly called as gonadotropic. The most important Gonadotropic are:

(a) Follicle stimulating hormone.
(b) Luteinizing hormone.

Both these hormones affect the ovary. The FSH affects the developing oocyte. Oocyte is the cell that will give rise to the ovum. The follicle also increases due to FSFI (Female Sexual Function Index). The mature follicles are called graafian follicle. The LH at this stage causes ovulation. The follicle cells surrounding the developing oocyte secretes estrogen. So both FSH and LH cause secretion of estrogen in an indirect way. Estrogen causes structural and functional changes in the uterus.

Luteotropic Hormone
It is secreted by pituitary gland. LTH is also called prolactin. It stimulates the development and secretion of milk in the mammary gland. LTH also stimulates the corpus luteum to secrete progesterone.

Role of Placenta
Some cells of the placenta become specialized. These cells secrete hormones. The hormones are like the pituitary hormone in function.

Chorionic Gonadotropin
This hormone is secreted by chorion of the placenta. Its function is similar to LH and LTH. Due to CG the activity of the corpus luteum is maintained within the ovary. So there is secretion of progesterone for a considerable period of time. The placenta then starts secreting its own progesterone. At this stage the secretion of ovarian progesterone ceases.

Functions: Major functions of human estrogen and progesterone during pregnancy are.


(1)        Growth of mammary glands.

(2)        Inhibits FSH release.

(3)        Inhibits prolactin release.

(4)        Prevents infection in uterus.

(5)        Increases size of uterine muscle cells.

(6)        Increases sensitivity of myometrium to oxytocin.


(1)        Growth of mammary glands.

(2)        Inhibits FSH release.

(3)        Inhibits prolactin release.

(4)        Inhibits contraction of myometrium.

The hypothalamus produces GnRH (gonadotropic releasing hormone). GnRH stimulates the anterior pituitary to produce FSH (follicle-stimulating hormone) and LH (luteinizing hormone). FSH stimulates the follicle to produce estrogen, and LH stimulates the corpus luteum to produce progesterone.

Estrogen and progesterone affect the sex organs (e.g., Uterus) and the secondary sex characteristics and exert feedback control over the hypothalamus and the anterior pituitary.

Role of hormones on testes and ovary
Test Tube Babies
In vitro (glass wares) fertilization (IVF) is commonly known as the test tube baby technique. It was first devised in 1978, as a means of helping woman with blocked oviduct. The technique involves fertilizing one or more eggs outside the body, and "then transferring the fertilized egg, known as preembryos; back into the uterus. The main stages of IVF are:

(a) Stimulation of the ovaries with fertility drugs to produce several eggs.

(b) Collecting the mature eggs

(c) Fertilizing the egg in the laboratory.

(d) Culture of the pre-embryo.

(e) Embryo transfer to the uterus.

Protection of Embryo

The developing embryo is provided with many special structures which protect the embryo from physical injury. These structures are present in all the embryos whether they are developing inside or outside the body.

Protective Coats of Embryos

These are special structures to protect the embryo. These structures are in the form of membranes or coats, it can be best seen by studying a developing chick (birds) embryo.

Protection By Internal Development

In most of the mammals development takes place in the reproductive tract (uterus). It provides a safe and moist environment for the development of the embryo.

Lower Vertebrates

In some lower vertebrate e.g. sharks and some reptiles, the embryo develops in the uterus. There is no biological contact between the tissue of the mother and the embryo. The nutrition for the developing embryo is provided by the egg. The uterus provides only the safe environment for the developing embryo.


The mammals are divided into three sub classes:

1- Prototheria
2- Metatheria
3- Eutheria

Duckbill Platypus
Prototheria: These are eggs laying mammals e.g. Duckbill platypus. Duckbill platypus produces an egg very much like the egg of a reptile. The egg is not laid immediately after it is formed. It is retained for a time in the body of the female. Then it is laid in a nest and hatched. With the hatching, the mammary glands of the mother secrete milk. The young ones are fed on the mammary glands. In this way the neonates i.e. the new born get the food. This is a very important step in the care of the young ones by the mammals.


Metatheria: The pouched mammals. e.g. Opossum and Kangaroos etc. The eggs are small having little yolk. The yolk is only enough for the development of very small and immature individuals. Early development occurs within the body of the female. Then the tiny, almost helpless infants emerge out of the female's body. They crawl up the female's belly and slip into the pouch. Here the infants are sheltered and kept warm. There are mammary glands in the pouch. Each infant, feeds on the mother's milk as it continues its development. The young ones stay in the pouch till they grow and become independent of the mother.

Eutheria: The placental mammals e.g. Cow, Goat, Man etc. A placenta is a structure that develops between the developing embryo and uterus of the female. Exchange of nutrition and waste materials takes place through the placenta. The eggs have a negligible amount of yolk. So the female provides the needed food materials to the embryo through the placenta. The young ones when born are fed on the milk secreted by the mammary glands. Thus the placental mammals provide all possible nourishment and protection to the embryos.

Unisexual and Bisexual Animals

The male and female gametes in most animals are usually produced by the male and female animal, respectively. Such animals are called unisexual or dioecious.

Animals with both ovaries and testes in the same individual i.e. male and female gametes are produced by the same organism. Such animals are called bisexual or monoecious or hermaphrodite e.g. Liver fluke, earthworm, leach, tapeworm. Hermaphrodites double the reproductive capacity since all individuals can produce egg. In case of endoparasites such as tapeworm, hermaphrodites permit self-fertilization which is essential for such organisms.


After internal fertilization external development takes place e.g. reptiles and birds. They lay shelled eggs to protect the developing embryo. Such animals are called oviparous. Some animals are Ovoviviparous that their eggs are retained in the body until they hatch. Then fully developed offspring are released, e.g. Oysters, Sea horse, Garter snake, Pit vipers, Duckbill platypus.


Mammals are particularly viviparous. Here fertilization and development is internal, i.e. inside the female body. The female gives birth to the young ones, and the nutrients needed for development are supplied by the mother.

Viviparity represents ultimate caring for zygote and embryo. The placenta is a complex structure derived in part from the chorion, which first appeared in shelled egg. Its evolution allowed the developing offspring to exchange materials with the mother.