The primary goal of sex is to merge the sperm and egg (fertilization) to make a baby. In many organisms,
sex occurs outside of the body. For example, in most fish or amphibians, females lay eggs somewhere (usually on the sea/river
bed), the male comes along and sprays the eggs with sperm and fertilization takes place.
In reptiles and mammals (including
humans), fertilization takes place inside the body of the female (internal fertilization). This technique increases the chances
of successful sexual reproduction. Because we use internal fertilization, our sexual organs are specialized for this purpose.
Let's take a closer look at the sexual organs in males and females.
Male Sex Organs
From the outside, the
male has two visible sex organs, the testes and penis. The testes (singular: testis) are the primary male sexual organs in
that they make sperm and produce testosterone. The sperm cell is the male sex cell (gamete). Testosterone is the hormone that
causes male secondary sex characteristics such as facial and pubic hair, thickened vocal cords and developed muscles.
testes are housed outside of the main part of the male's body, in a sac called the scrotum. This location is important because
in order for the sperm to develop properly, they must be kept at a slightly lower temperature (95 to 97 degrees Fahrenheit,
35 to 36 degrees Celsius) than normal body temperature (98.6 F, 37 C).
The immature sperm travel from each testis
to a coiled tube on the outer surface of each testis called the epididymis, where they mature in about 20 days. The sperm
exit the body through the penis.
The penis is made of soft, spongy tissue (see How Viagra Works for details). When
engorged with blood during sexual excitation and intercourse, the spongy tissue stiffens and causes the penis to become erect,
which is important for the penis's main function -- to place the sperm inside the female.
What is meiosis?
You have probably learned about meiosis in a previous class, but I will review
it a little bit here... There are two types of cell division. The type of cell division other than meiosis is
mitosis and it goes on all over the body. Mitosis is cell division that starts with one cell and ends with two identical
cells. The starting cell is called the parent cell and the ending cells are called the daughter cells. In mitosis,
the daughter cells are not only identical to each other, but they are also identical to the parent cell. We use mitosis
all over our bodies for repair after injury, replacement of cells as some get worn and die, and growth. Mitosis is
also used for embryonic development from the single, fertilized egg to the multicellular fetus and neonate.
Meiosis, on the other hand, is only used for making gametes! Why do we need a special type of cell division just for
making gametes? There are two ways to answer this question. The first is that when we produce our next generation,
we do not wish to make identical individuals to ourselves... instead, we make unique individuals, different from either parent.
In order to do that, we have to be able to mix up our own genetic information into new combinations; it is these new combinations
that come together during fertilization, producing an entirely unique, new individual. Mitosis does not allow for any
mixing up of genetic information-- instead, it preserves genetic information perfectly to make identical daughter cells.
Meiosis does the opposite-- meiosis jumbles up our genetic information into new combinations.
The second reason that we have to use meiosis is because we need our spermatozoa and ova to contain only half of the normal
amount of genetic information. Keep in mind that during fertilization, the spermatozoan and ovum fuse together into
one, combined cell (called the zygote). That means that their genetic information also comes together. When each
gamete has half of the normal amount of genetic information, they can come together and make a whole. From that point
forward the embryo (and then fetus and then neonate...) has a new set of genetic information, half from each parent, that
is preserved for all the cells of its forming body.
How does meiosis occur?
Meiosis begins with
one cell, called a germ cell (because it specifically makes gametes). As this germ cell prepares to divide, it replicates
its genetic material, in the same way that this occurs before mitosis. The germ cell is going to have to go through
two cell divisions in order to get from having double the genetic information within it to having only half the normal amount
of genetic information within it.
Our cells have two copies of every type of chromosome.
Chromosome number is typically represented by "N." We have two of every chromosome because we get one of each chromosome
type from our mothers and one from our fathers. Therefore, we have "2N" chromosomes, which just means two of every type.
N = 23 in humans, since we have 23 different types of chromosomes... and because we are 2N, we actually have 46 chromosomes
in every cell. Organisms and cells with 2 copies of every chromosome type are called diploid, while those with only
one of every chromosome type are called haploid.
Use the diagram to follow the description
in the next paragraph.
Before our germ cells undergo meiosis, they proceed through DNA replication,
during which every chromosome is copied. This means that we go from 2N to 2 x (2N), or 4N. The germ cells then
proceed through two cell divisions: Meiosis I and Meiosis II. During meiosis I, the genetic information splits
so that each cell has 2 x N instead of 2 x 2N. During meiosis II, the genetic information splits again, leaving each
gamete with only N chromosomes. In humans, that would mean that each gamete has 23 chromosomes, and only one of each
type (not 2N, but only N). When a spermatozoan and an ovum fuse, the 23 chromosomes from the spermatozoan join up with
the 23 chromosomes from the ovum to make the zygote, which now has 46 chromosomes. Note that the 46 chromosomes in the
zygote are actually 2 sets of 23 chromosomes or 2N (not 46 completely different chromosomes).
Another way to discuss this is to say that our germ cells are diploid (2N), and that they make haploid gametes (N) through
There are many steps to each meiotic division (interphase, prophase, metaphase, anaphase,
and telophase). I will assume that you remember enough about each of these phases to be able to use these terms in other
web pages. If not, review them a little bit... you can review them in mitosis because they look the same in meiosis
as they do in mitosis.
Gamete production differs between males and females
Both males and females undergo meiosis to produce their gametes. However, males have to produce very high numbers of
gametes while females only need to produce up to one gamete per month. Therefore, the specific ways that gametes are
made in the two sexes differ. The production of gametes in general is called gametogenesis. Gametogenesis in males
is more specifically referred to as spermatogenesis, while gametogenesis in females is called oogenesis. These two
types of gametogenesis are explained on the webpages linked from this page.
The final stage of the first prophase of meiosis. All four chromatids of a tetrad are fully visible and homologous chromosomes
start to move away from one another except at chiasmata.
A specialised form of nuclear division in which there two successive nuclear divisions (meiosis I and II) without any
chromosome replication between them. Each division can be divided into 4 phases similar to those of mitosis pro, meta, ana
and telophase). Meiosis reduces the starting number of 4n chromosomes in the parent cell to n in each of the 4 daughter cells.
Each cell receives only one of each homologous chromosome pair, with the maternal and paternal chromosomes being distributed
randomly between the cells. This is vital for the segregation of genes. During the prophase of meiosis I (classically divided
into stages: Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis), homologous chromosomes pair to form bivalents, thus
allowing crossing over, the physical exchange of chromatid segments. This results in the recombination of genes. Meiosis occurs
during the formation of gametes in animals, which are thus haploid and fertilization gives a diploid egg. In plants meiosis
leads to the formation of the spore by the sporophyte generation.
Germ cell development in the testis (including mitosis, meiosis, spermiogenesis), Sertoli cells, other cell types
Spermatogenesis is an elaborate process of cell differentiation starting with a non-differentiated spermatogonial germinal
stem cell and terminating with a fully differentiated highly specialized motile cell called a spermatozoon (Fig. 1). The
formation of spermatozoa takes place within narrow coiled seminiferous tubules which form the bulk of the testis. Each seminiferous
tubule, approximately half a millimeter in diameter, may be close to one meter in length. These tubules have a central fluid-filled
lumen and a wall called the seminiferous epithelium composed of germinal cells and of somatic cells, the Sertoli cells,
which support and nourish the germinal cells.
Spermatogenesis may be subdivided into three main phases, each involving a class of germinal cells.
First phase: The spermatogonia are immature germinal cells located at the base of the seminiferous epithelium. In man,
there are three types of spermatogonia: the pale type A spermatogonia or Ap, the dark type A spermatogonia or Ad, and the
type B spermatogonia. The Ap spermatogonia divide by mitosis and give rise either to new type Ap cells or to the more differentiated
type B spermatogonia. Thus, the Ap cells may be thought of as self-renewing stem cells since they can produce both new Ap
stem cells and a new class of type B spermatogonia. The Ad spermatogonia, which rarely divide in normal adults, are tentatively
considered as dormant reserve stem cells. The type B spermatogonia produced by the Ap cells all divide by mitosis to yield
differentiated spermatocytes. Thus, the spermatogonial population not only maintains itself, but continuously yields crops
Second Phase: Spermatocytes are ceIls which are unique in undergoing two successive special cell divisions, the so-called
reductional or meiotic divisions, that produce, the spermatids. These cells have exactly half the number of chromosomes
contained by the nuclei of cells that compose the rest of the body. Spermatids are said to be haploid while somatic cells
are diploid. In man, somatic cells contain 46 chromosomes and spermatids and spermatozoa contain 23 chromosomes. The fusion
of an haploid spermatozoon with an equally haploid ovum restores the diploid number of chromosomes in the cells of the embryo.
Because there are two meiotic divisions, there are two generations of spermatocytes: primary and secondary. At an early
or preleptotene stage, the nuclei of primary spermatocytes replicate their DNA content. Fine filamentous chromosomes subsequently
appear in the nucleus and the cells are at the leptotene stage. Soon after, homologous chromosomal filaments approximate
each other and form close pairs, a phenomenon called synapsis, and the cell is at the zygotene stage. Then each chromosomal
pair shortens and thickens and the chromosomes assume the pachytene configuration. The spermatocytes go through an early,
mid and late pachytene stage during which the cell and its nucleus progressively increase in volume. The pachytene nucleus
also has a prominent nucleolus indicating that these nuclei are actively synthesizing ribosomal RNA which enters the cytoplasm
and contributes to the active protein synthesis observed in these cells. Following the long pachytene stage, the primary
spermatocytes rapidly complete their first meiotic division going through metaphase, anaphase and telophase during which
the homologous chromosomes separate and migrate to the poles of the cell which then splits to form two daughter cells called
secondary spermatocytes. These cells undergo a second maturation division after a short interphase which, this time, is
not accompanied by DNA replication. During this division, the chromosomes (of which there is an haploid number) split in
half and each half reaches the nucleus of the daughter cells, which are now referred to as the spermatids. Thus spermatocytes,
through complex regulatory mechanisms and elaborate cell division processes, are converted to haploid spermatids. This process
of meiosis is covered in more detail in the following chapter.
Third phase: The newly formed spermatid, a small spheroidal cell, undergoes a dramatic metamorphosis referred to as spermiogenesis.
The nucleus progressively elongates as its chromatin condenses, and gradually takes on the flattened and pointed paddle
shape that characterizes the head of human spermatozoa. The Golgi apparatus elaborates a secretory-like granule which gradually
grows to produce a cap-like structure, the acrosome, over the nuclear membrane. This structure contains the hydrolytic
enzymes necessary for the fertilization of the ovum and partly covers the nucleus of the spermatozoon. The centrioles reach
the membrane of the nucleus at the pole opposite to that occupied by the acrosome, bind to it and initiate the formation
of the contractile components of the tail, i.e. the microtubules that form the axoneme. The mitochondria migrate toward
a segment of the growing tail and form the mitochondrial sheath, which constitutes the respiratory organ of the spermatozoon.
The spermatid bulk of cytoplasm is eventually discarded as the residual body, which is phagocytosed and eliminated from
the seminiferous epithelium by the Sertoli cell.
Thus, the spermatozoon is a streamlined cell 60 mum long with a head and a tail completely encased in a cell membrane.
The head is composed of a small compact nucleus covered by the acrosome. The tail is made up of the contractile axoneme
associated with other complex cytoskeletal elements and is partly covered by mitochondria. This cell will continue to develop
and mature during its transit through the epididymis.
The complexity of the whole process of spermatogenesis explains its marked sensitivity to toxic substances or hormonal
imbalance. In addition, many abnormal and degenerating germinal cells are observed during spermatogenesis in normal men.
Spermatogenesis takes approximately 60 days, a duration that does not appear to vary from one individual to the next.
Spermatogenesis is possibly one of the most complex processes of cell differentiation taking place in the tissues of
adult individuals. Many of its facets remain to be studied and clarified at the molecular level.
Dym M. The male reproductive system. In: Weiss L, ed. Histology. Cell and Tissue Biology, fifth edition. New York, Amsterdam,
Oxford: Elsevier Biomedical; 1983:1000-1053.
Desjardins C, Ewing LL, eds. Cell and Molecular Biology of the Testis. New York, Oxford: Oxford University Press, Inc.,
What factors determine the sex of an individual?
X, Y, SRY (loci, genes), sequence of events in development of normal male
One can view mammalian sex determination as occurring in three steps. First is the establishment of chromosomal sex.
This occurs at fertilization when either an X- or Y-bearing sperm fertilizes an X-bearing oocyte giving rise to an XX or
XY zygote. Second is the establishment of the primary sexual characteristics: the testes or ovaries. In XY fetuses, the
fetal gonads differentiate into testes; in XX fetuses, ovaries form. Third is the establishment of the secondary sexual
characteristics which is dependent upon hormones secreted by the gonads.
How does sexual differentiation occur?
A fetus is initially sexually indifferent and has the primordia for both the male and female accessory sex organs, the
Wolffian and Mullerian ducts, respectively. In the 1940s, Jost demonstrated that the male phenotype is imposed on a fetus
that would inherently develop into a female. Jost surgically removed the testes from fetal male rabbits at a stage when Wolffian
and Mullerian ducts were present and then allowed fetal development to proceed in utero. When Jost examined the fetuses
at a later date, the castrated males were phenotypic females (Fig. 1). Jost concluded that the fetus is programmed to develop
into a female. However, if testes are present they secrete two factors that override the female program and masculinize
the fetus. The first factor, secreted by Leydig cells, is testosterone which induces the Wolffian ducts to differentiate
into the epididymides, vas deferens, and seminal vesicles. Male external genitalia form when the cells of the urogenital
tubercle metabolize testosterone into dihydrotestosterone which induces the development of the penis and scrotum. The second
factor, secreted by Sertoli cells, is Mullerian inhibiting substance (anti-Mullerian hormone), which induces the Mullerian
ducts to regress. In the absence of these two factors the Wolffian ducts degenerate and the Mullerian ducts develop into
the oviducts, uterus, cervix, and upper vagina.
What induces development of the gonads into testes or ovaries?
It was initially assumed that humans had a sex determining mechanism similar to the well studied fruitfly, Drosophila,
since in both species males are XY and females are XX. In the fruitfly, sex is determined by the ratio of the number of
X chromosomes to autosomal sets such that an XXY individual is female and an XO is male. However, in 1959, the identification
of an XXY male patient with Klinefelter syndrome, an XO female patient with Turner syndrome, and an XO female mouse suggested
that, in mammals, the Y induces testes development. Cytogeneticists have since identified individuals with varying numbers
of X or Y chromosomes. All individuals who had at least one Y chromosome had testes and a male phenotype, irrespective of
the number of X chromosomes. The locus on the human Y that induces testes development was designated the testes-determining
How does the Y chromosome control masculinization?
By correlating deletions on the Y with the presence or absence of testes and by studying XX males which carry a tiny
portion of the Y on one of their X chromosomes, investigators mapped TDF to a 35-kb region of the Y short arm. Cloning of
this region resulted in the identification of a gene designated sex-determining region Y (SRY). Convincing evidence that
SRY is the testis-determining gene was obtained when a 14.6-kb genomic sequence of the mouse SRY locus was shown to be capable
of inducing XX fetuses to develop into males in transgenic experiments.
The hypothesis is that SRY is a master regulatory gene that initiates a cascade of gene interactions that transforms
the fetal gonad into a testis (Fig. 2). SRY encodes a member of the High Mobility Group-1/-2 (HMG) protein family whose
members are characterized by an 80-amino acid DNA-binding motif called the HMG domain. Several HMG proteins, including SRY,
are transcription factors that recognize and bind a specific DNA target sequence and cause the bound DNA to bend into an
angle. The SRY target sequence has been identified in the promoter region of genes controlling sexual differentiation such
as Mullerian inhibiting substance and P450 aromatase, an enzyme that converts testosterone to estradiol. Furthermore it is
present in the promoter region of SRY itself suggesting a positive feedback loop.
It took over three decades from the recognization of the Y as testis-determining to the identification of SRY as TDF.
The cloning of SRY is undoubtedly a milestone in our understanding of mammalian sex determination. The difficult job of
deciphering how SRY regulates transcription and identifying the genes upstream and downstream of SRY in the sex determination
cascade must now be addressed.
Affara NA. Sex and the single Y. BioEssays 1991;13(9):475-478.
Koopman P, Gubbay J, Vivian N, Goodfellow P, Lovell-Badge R. Male development of chromosomally female mice transgenic
for SRY. Nature 1991;351:117-121.
McLaren A. Sex determination in mammals. Trends in Genetics 1988;4(6):153-157.
McLaren A. What makes a man a man? Nature 1990;346:216-217.