This unit takes you through the sexual reproduction system in animals especially humans.


Reproduction is the giving rise to new viable offspring by the parents.  In animals, especially mammals, reproduction is sexual, in which gametes from parents fuse together to form a zygote that develops into an embryo (foetus) which in turn grows and develops into a new individual.

Sexual reproduction involves the fusion of two gametes.

Sperm and eggs are sometimes produced by the same individual and sometimes produced by different individuals.

The sexes of vertebrates are separate but some vertebrates can change sex and others are both male and female.


Sexual reproduction promotes variation because offspring inherit genes from two different parents.

In addition, when an individual produces gametes, crossing-over and independent assortment mix genes from that individuals parents. Gametes are haploid; they contain some genes from the individuals mother and some from the father.

Variation is advantageous to species in fluctuating environments.

Sexual reproduction is seen in nearly all animals. In animals reproduction also involves production of gametes that are haploid cells. In unicellular organisms like the protozoans, the gamete-producing individuals are called gametocytes. The gametes may be similar – isogametes as in Monocystis or dissimilar – heterogametes as in Plasmodium. In the latter the gametocytes are:

  • Macrogametocyte (female)
  • Microgametocyte (male)

In multicellular organisms, the gametes are produced by the reproductive organs. The male gametes called the sperms are produced by the male reproductive part and the female gametes called the eggs are produced by the female reproductive part. Production of sperms is called spermatogenesis and production of eggs is called oogenesis. In most cases the gametes are produced by different individuals. However, in some cases like that of earthworm, the same individual produces both the types of gametes. It is called a hermaphrodite. However, fertilisation is between the gametes of different individuals.

Most of the animals are unisexual with an individual only producing either male or female gametes. The male gamete of one individual fertilizes with the female gamete of another individual to produce the zygote. The fertilisation may be external or internal.



External fertilisation takes place in animals like the fish and frog where the eggs are released from the body of the females into the water outside.

frog fertilisation process in detail

Frog, Ovulation, Accumulation of Eggs in Uterus, Spawning and External fertilisation

These eggs are then fertilised by the sperms produced by the male species. The fishes and frogs are oviparous, that is they lay eggs.


Organisms like birds, insects, reptiles are also oviparous. However, in these organisms the fertilisation is internal. Internal fertilisation occurs in mammals also.

In internal fertilisation the sperms are released into the body of the females during copulation. The resulting zygote develops into a young one. Thus mammals are viviparous, that is, they give birth to young ones.

This type of fertilization enables animals to reproduce in a terrestrial environment because it enables sperm to swim to the egg and it prevents gametes from drying out.

The male often has a copulatory organ (a penis) for transferring sperm.


Animals can be grouped into those which give birth to living offspring and those which lay eggs that eventually hatch into offspring. Those animals which give birth to live offspring are called live-bearing or viviparous. Those animals which lay eggs are called egg-laying or oviparous. The difference is in the place where the offspring develops before it is born.

Below is a table of the vertebrates which shows which groups are viviparous and which groups are oviparous.

MAMMALS Only a few primitive egg-laying mammals exist. They live in Australia and New Guinea. e.g. spiny anteater and duck-billed platypus. Nearly all mammals  e.g. mouse, human, cat, dog, bear, kangaroo and dolphin.
BIRDS All birds. e.g. robin, penguin, parrot, sparrow and eagle. None
REPTILES Most reptiles are egg-layers. e.g. crocodile, turtle and cobra. Some lizards and snakes are live-bearing. e.g. vipers.
AMPHIBIANS Nearly all. e.g. frog, toad and A few species of frogs living in South America and West Africa are live-bearers.
FISH Most species. e.g. herring, salmon and trout. Quite a few species are live-bearers. e.g. sharks and guppies.


Amongst the invertebrates there are many which are oviparous but a few are viviparous such as sea anemones and aphids.


A species of animal usually exists in two types or sexes called males and females. Each sex has its role to play in reproduction.

When an animal gives birth or lays eggs we notice that it is always the female which does this. In some cases, such as the sea horse and the midwife toad, the male appears to give birth to the offspring. In these exceptions the male has been given the eggs to look after but it is still the female animal that lays the eggs in the first place.

 animal cell

To produce an offspring two special cells are needed. A cell is a microscopic part of an organism’s body. The bodies of animals are made of millions of cells. There are many different sorts of cells in a body, each having specialized functions. Some are found in the blood transporting oxygen, others are found in the walls of our stomach producing juices to digest our food. To reproduce, animals make special sex cells. In the male animal these sex cells are called sperm cells or sperms and in the female animal they are called egg cells or eggs.

Sperm Cells

sperm cell

Sperm cells are very small but they are very specialized too. The sperm cells of different species of animal are all about the same size, about 60µm long (µm = micrometres ; this is a thousandth of a millimetre). Over 11 000 sperm cells could fit on a pin head! All sperms have a head and a tail called a flagellum. They use the flagellum to swim through liquids. The head of the sperm is very important because it contains the nucleus.

The nucleus of the cell is the control centre. In sex cells the nucleus carries half of the information needed for reproduction (like the plans needed to construct a house).

Sperm cells are made in very large numbers by special organs in the male’s body called testes (sing. testis). In most animals the testes are carried inside the male’s abdomen but in mammals they are kept in a sack of skin called sperm duct the scrotum outside the abdomen.

Egg Cells

egg cell

Egg cells are produced by female animals in special organs called ovaries (sing. ovary). These are found inside the abdomen of all female animals.

Egg cells are much bigger and simpler than sperm cells in their structure. They do, however, vary a lot from one species of animal to another. For example the egg of a human is only 0,1 mm wide but the egg of a chicken is 20 mm wide. Even so the human egg cell is still nearly 50 times wider than a sperm cell. The reason for the difference in size is that the egg has a large food supply stored inside it called yolk. Yolk is the yellow part of a chicken’s egg. The egg also has its own nucleus which carries the other half of the information needed for reproduction.

Because eggs are bigger than sperms they are not produced in such large numbers. Even so animals which do not take care of their offspring, such as fishes and frogs, will lay a large number of eggs. Animals which take care of their offspring, such as mammals and birds, will produce less eggs.










  • Mating and Fertilization
  • Reproduction in Birds
  • Reproduction in Mammals
  • Different Ways of Growing
  • Chapter Summary (useful for revison)
  • Top of Page



male reproduction

Sperm are produced in the seminiferous tubules of the testes but mature in the epididymides (sing. epididymis). They are stored in the epididymides and vas deferens.

The penis contains 3 masses of spongy erectile tissue with distensible blood spaces that produce erection when blood flow in the veins is inhibited. The spongy erectile tissues become engorged with blood. Stimulation is provided by the parasympathetic nervous system.

An orgasm occurs as rhythmic muscular contractions compress the urethra and expel the semen. This expulsion is termed ejaculation.


The prostate gland produces a secretion that buffers the vaginal pH, which is normally 3.5 to 4.

The seminal vesicles add fructose to nourish the sperm and prostaglandins to promote contractions in the female.

The bulbourethral gland (cowpers gland) produces mucus which lubricates the penis.




The testes are kept slightly cooler than body temperature because they are suspended outside the body within the scrotum.

Each testis contains 250 to 300 lobules, and each of these contains one to three tightly coiled seminiferous tubules. Sperm are produced within the seminiferous tubules.

The Leydig cells, located between the tubules, function to secrete testosterone.

Sertoli cells are located within the tubules. They support, nourish and regulate (stimulate) cells that form sperm.


Testosterone is secreted by Leydig cells in the testes. These cells are located outside the seminiferous tubules.

Testosterone is also necessary for the development and function of the male reproductive tract.

The formation of sperm (called spermatogenesis) is stimulated by testosterone.

It promotes the development and maintenance of sexual behavior.

It stimulates the development of secondary sexual characteristics such as beard growth, deepening of the voice, etc. and has growth-promoting effects.

Negative Feedback Regulation of Sex Hormones

Testosterone inhibits the hypothalamus and anterior pituitary. Inhibin, secreted by Sertoli cells also inhibits the hypothalamus and anterior pituitary.






Sperm are produced in the seminiferous tubules in the testes. After meiosis is complete, sperm move from the lumen of the seminiferous tubule to the epididymis, where they become motile.

Sperm production starts at puberty.

Spermatogenesis is a continuous process. An average male produces approximately 1,000 sperm/second (30 billion/year).

Each ejaculation should contain 200-300 million sperm.

The testes are located outside the body because spermatogenesis requires a temperature cooler than the body core.



The head of a sperm contains the nucleus and acrosome. The acrosome contains enzymes that play a role in breaking down the barrier surrounding the egg.

The middle piece contains mitochondria needed to generate ATP necessary to operate the flagella.

The tail contains microtubules arranged in the 9+2 arrangement typical of eukaryotic cells.





Gametes are produced by ovaries. The ovaries are suspended in the abdominal cavity by ligaments and mesenteries.

Oviducts serve as passageways from ovaries to uterus. They are not connected to the ovaries.

The uterus lies in a nearly horizontally position on top of the bladder. The myometrium is a muscle layer; the endometrium is the lining. The outer layer of endometrium is shed during menstruation.

The cervix is muscular sphincter between uterus and vagina.

The vagina is a muscular structure that functions to receive the penis and also as the birth canal. It opens between urethra and anus. Bacteria in the vagina convert glycogen (a carbohydrate) to lactic acid, making it slightly acidic.

The clitoris is homologous with the glans penis in males; it contains erectile tissue and a glans.

Labia are folds of skin near the opening of the vagina. labia majora are fleshy and covered with pubic hair. They are homologous with the scrotum in the male. Labia minora are homologous with the shaft of the penis in male.

The external genitalia of females are the vulva.

Oogenesis: Formation of Egg Cells


Primary oocytes begin to form in females before they are born (during the 3rd month of embryonic development). Development stops before the first meiotic division is completed. Females are born with all the primary oocytes that they will ever possess.

The first meiotic division is completed at about the time of ovulation; secondary oocytes are therefore released at ovulation, not eggs.

The second meiotic division occurs after fertilization.

An infant may have as many as 2 million primary oocytes.

Approximately 300,000-400,000 primary oocytes remain at time of puberty; 400 mature over the reproductive years.

Ovarian Follicle

The oocyte and cells surrounding it are the follicle.

The follicle moves to the surface and erupts, releasing the oocyte.

After ovulation, the follicle develops into a corpus luteum, which lasts only about 10 days if the secondary oocyte is not fertilized.

The secondary oocyte is swept into the oviduct by beating cilia and movement of the oviduct. Cilia and peristalsis move the oocyte through the oviduct.

Fertilization occurs in oviduct.

The secondary oocyte must be fertilized within 36 hours. Sperm survive in the female reproductive tract for approximately 48 hours. It takes 3 days to reach uterus.

Below: Rabbit ovary X 40

rabbit ovary


The Role of Hormones in Females

Sex hormones produce secondary sexual characteristics.

Most female mammals come into “heat” or estrus and mating occurs at this time.

Humans and other primates have a menstrual cycle; there is no period of estrus nor is there a mating season.

During the menstrual cycle, the uterine lining thickens to prepare for pregnancy but is sloughed off if pregnancy does not occur. These events repeat themselves approximately every twenty-eight days.


Ovulation occurs about two weeks after the onset of menstruation. In response to a sudden surge of LH, the follicle ruptures and discharges a secondary ocyte. This is swept into the open end of the fallopian tube and begins to move slowly down it.

Copulation and Fertilization

For fertilization to occur, sperm must be deposited in the vagina within a few (5) days before or on the day of ovulation. Sperm transfer is accomplished by copulation. Sexual excitation dilates the arterioles supplying blood to the penis. Blood accumulates in three cylindrical spongy sinuses that run lengthwise through the penis. The resulting pressure causes the penis to enlarge and erect and thus able to penetrate the vagina.

Movement of the penis back and forth within the vagina causes sexual tension to increase to the point of ejaculation. Contraction of the walls of each vas deferens propels the sperm along. Fluid is added to the sperm by the seminal vesicles, Cowper’s glands, and the prostate gland.  These fluids provide

  • a source of energy
  • an alkaline environment to activate the sperm, and
  • perhaps in other ways provide an optimum chemical environment for them.

The mixture of sperm and accessory fluids is called semen. It passes through the urethra and is expelled into the vagina.

Physiological changes occur in the female as well as the male in response to sexual excitement, although these are not as readily apparent. In contrast to the male, however, such responses are not a prerequisite for copulation and fertilization to occur.

Once deposited within the vagina, the sperm proceed on their journey into and through the uterus and on up into the fallopian tubes. It is here that fertilization may occur if an “egg” is present (strictly speaking, it is still a secondary oocyte until after completion of meiosis II).

Although sperm can swim several millimeters each second, their trip to and through the fallopian tubes may be assisted by muscular contraction of the walls of the uterus and the tubes. The trip is fraught with heavy mortality. An average human ejaculate contains over one hundred million sperm, but only a few dozen complete the journey, arriving within 15 minutes of ejaculation. And of these, only one will succeed in fertilizing the egg.

Sperm swim towards the egg by chemotaxis following a gradient of progesterone secreted by cells surrounding the egg. Progesterone opens CatSper (“cation sperm”) channels in the plasma membrane surrounding the anterior portion of the sperm tail. This allows an influx of Ca2+ ions which causes the flagellum to beat more rapidly and vigorously.

Fertilization begins with the binding of a sperm head to the glycoprotein coating of the egg (called the zona pellucida). Exocytosis of the acrosome at the tip of the sperm head releases enzymes that digest a path through the zona and enable the sperm head to bind to the plasma membrane of the egg. Fusion of their respective membranes allows the entire contents of the sperm to be drawn into the cytosol of the egg. (Even though the sperm’s mitochondria enter the egg, they are almost always destroyed — by autophagy — and do not contribute their genes to the embryo. So human mitochondrial DNA is almost always inherited from mothers only.)

Within moments, enzymes released from the egg cytosol act on the zona making it less permeable to other sperm that arrive. The egg also sheds the receptor molecules (called Juno) to which the successful sperm had bound so that any later arrivals will not be able to enter the egg.

Soon the nucleus of the successful sperm enlarges into the male pronucleus. At the same time, the egg (secondary oocyte) completes meiosis II forming a second polar body and the female pronucleus.

The male and female pronuclei move toward each other while duplicating their DNA in S phase. Their nuclear envelopes disintegrate. A spindle is formed (following replication of the sperm’s centriole), and a full set of dyads assembles on it. The fertilized egg or zygote is now ready for its first mitosis. When this is done, 2 cells — each with a diploid set of chromosomes — are formed.

In sea urchins, at least, the block to additional sperm entry and the fusion of the pronuclei are triggered by nitric oxide generated in the egg by the sperm acrosome.


Development begins while the fertilized egg is still within the fallopian tube. Repeated mitotic divisions produces a solid ball of cells called a morula. Further mitosis and some migration of cells converts this into a hollow ball of cells called the blastocyst. Approximately one week after fertilization, the blastocyst embeds itself in the thickened wall of the uterus, a process called implantation, and pregnancy is established.


The blastocyst produces two major collections of cells:

  • Three or four blastocyst cells develop into the inner cell mass, which will form
    • 3 extraembryonic membranes: amnion, yolk sac, and (a vestigial) allantois and
    • in about 2 months, become the fetus and, ultimately, the baby.
  • The remaining 100 or so cells form the trophoblast, which will develop into the chorion that will go on to make up most of the placenta. All the extraembryonic membranes play vital roles during development but will be discarded at the time of birth.

The placenta grows tightly fused to the wall of the uterus. Its blood vessels, supplied by the fetal heart, are literally bathed in the mother’s blood. Although there is normally no mixing of the two blood supplies, the placenta does facilitate the transfer of a variety of materials between the fetus and the mother.

  • receiving food
  • receiving oxygen and discharging carbon dioxide
  • discharging urea and other wastes
  • receiving antibodies (chiefly of the IgG class). These remain for weeks after birth, protecting the baby from the diseases to which the mother is immune.

But the placenta is not simply a transfer device. Using raw materials from the mother’s blood, it synthesizes large quantities of proteins and also some hormones.

The umbilical cord connects the fetus to the placenta. It receives deoxygenated blood from the iliac arteries of the fetus and returns oxygenated blood to the liver and on to the inferior vena cava.The metabolic activity of the placenta is almost as great as that of the fetus itself.

Because its lungs are not functioning, circulation in the fetus differs dramatically from that of the baby after birth. While within the uterus, blood pumped by the right ventricle bypasses the lungs by flowing through the foramen ovale and the ductus arteriosus.

Although the blood in the placenta is in close contact with the mother’s blood in the uterus, intermingling of their blood does not normally occur. However, some of the blood cells of the fetus usually do escape into the mother’s circulation — where they have been known to survive for decades. This raises the possibility of doing prenatal diagnosis of genetic disorders by sampling the mother’s blood rather than having to rely on the more invasive procedures of amniocentesis and chorionic villus sampling (CVS).

Fragments of fetal DNA (~ 300 bp long) from apoptotic cells of the placenta are also found in the mother’s plasma as early as 5 weeks after implantation. These can be tested for various forms of aneuploidy, e.g. the trisomy 21 of Down syndrome

Far rarer is the leakage of mother’s blood cells into the fetus. However, it does occur. A few pregnant women with leukemia or lymphoma have transferred the malignancy to their fetus. Some babies have also acquired melanoma from the transplacental passage of these highly-malignant cells from their mother.

During the first 2 months of pregnancy, the basic structure of the baby is being formed. This involves cell division, cell migration, and the differentiation of cells into the many types found in the baby. During this period, the developing baby — called an embryo — is very sensitive to anything that interferes with the steps involved. Virus infection of the mother, e.g., by rubella (“German measles”) virus or exposure to certain chemicals may cause malformations in the developing embryo. Such agents are called teratogens (“monster-forming”). The tranquilizer, thalidomide, taken by many pregnant European women between 1954 and 1962, turned out to be a potent teratogen and was responsible for the birth of several thousand deformed babies.

After about two months, all the systems of the baby have been formed, at least in a rudimentary way. From then on, development of the fetus, as it is now called, is primarily a matter of growth and minor structural modifications. The fetus is less susceptible to teratogens than is the embryo.

Pregnancy involves a complex interplay of hormones. These are described in a separate page.

The placenta is an allograft

One of the greatest unsolved mysteries in immunology is how the placenta survives for 9 months without being rejected by the mother’s immune system. Every cell of the placenta carries the father’s genome (a haploid set of his chromosomes); including one of his #6 chromosomes where the genes for the major histocompatibility antigens (HLA) are located.

Yet it thrives.Thus the placenta is immunologically as foreign to the mother as a kidney transplant would be.

Despite a half-century of research, the mechanism for this immunologically privileged status remains uncertain. But one thing is clear:

The mother is not intrinsically tolerant of the father’s antigens.

Some evidence:

  • She will promptly reject a skin transplant from the father.
  • She develops antibodies against his histocompatibility antigens expressed by the fetus. In fact, women who have borne several children by the same father are often excellent sources of anti-HLA serum for use in tissue typing.






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