Spermatogenesis

Spermatogenesis is a process of developing male gametes, known as sperm within the male
reproductive organs, the testes. In this process, each sperm (haploid,
containing a single copy of each chromosome. In order to create the haploid gamete, a cell
undergoes the process of meiosis in which the genome is replicated and divided twice to
produce four haploid gametes.
This process generally occurs in the seminiferous tubules of the testes following different
stages. It is followed by maturation in the epididymis where they are secreted in the form of
semen along with glandular secretions. This process begins during puberty and ends only
when the individual dies. The complete process of spermatogenesis in males are carried out
by the actions of Leydig cells, hypothalamus, and pituitary gland The quantity of these
sperms gradually reduces with the age and finally leads to infertility.
spermatogenesis, the origin and development of the sperm cells within the
male reproductive organs, the testes. The testes are composed of numerous thin tightly
coiled tubules known as the seminiferous tubules; the sperm cells are produced within the
walls of the tubules. Within the walls of the tubules, also, are many randomly scattered cells,
called Sertoli cells, that function to support and nourish the immature sperm cells by giving
them nutrients and blood products. As the young germ cells grow, the Sertoli cells help to
transport them from the outer surface of the seminiferous tubule to the central channel of
the tubule.

human sperm cells

Sperm cells are continually being produced by the testes, but not all areas of the seminiferous
tubules produce sperm cells at the same time. One immature germ cell takes as long as 74
days to reach final maturation, and during this growth process there are intermittent resting
phases.

The immature cells (called spermatogonia) are all derived from cells called stem cells in the
outer wall of the seminiferous tubules. The stem cells are composed almost entirely of
nuclear material. (The nucleus of the cell is the portion containing the chromosomes.) The
stem cells begin their process by multiplying in the process of cell duplication known
as mitosis. Half of the new cells from this initial crop go on to become the future sperm cells,
and the other half remain as stem cells so that there is a constant source of additional germ
cells. Spermatogonia destined to develop into mature sperm cells are known as primary
sperm cells. These move from the outer portion of the seminiferous tubule to a more central
location and attach themselves around the Sertoli cells. The primary sperm cells then
develop somewhat by increasing the amount of cytoplasm (substances outside of the
nucleus) and structures called organelles within the cytoplasm. After a resting phase the
primary cells divide into a form called a secondary sperm cell. During this cell division there
is a splitting of the nuclear material. In the nucleus of the primary sperm cells there are 46
chromosomes; in each of the secondary sperm cells there are only 23 chromosomes, as there
are in the egg. When the egg and sperm combine and their chromosomes unite, the
characteristics of both individuals blend and the new organism starts to grow.

The secondary sperm cell still must mature before it can fertilize an egg; maturation entails
certain changes in the shape and form of the sperm cell. The nuclear material becomes more
condensed and oval in shape; this area develops as the head of the sperm. The head is
covered partially by a cap, called the acrosome, which is important in helping the sperm to
gain entry into the egg. Attached to the opposite end of the head is the tailpiece. The tail
is derived from the secondary sperm cell’s cytoplasm. In the mature sperm, it consists of a
long, slender bundle of filaments that propel the sperm by their undulating movement. Once
the sperm has matured, it is transported through the long seminiferous tubules and stored
in the epididymis of the testes until it is ready to leave the male body. Spermiogenesis
This process is the final phase of spermatogenesis. Spermatids mature and form
spermatozoa, fully differentiated sperm cells. This stage ends when the mature cells leave
the germinal epithelium. These free cells, at this point, are called spermatozoa. Spermatozoa
have a unique shape, which is essential in their movement to the female gamete. The
condensed nucleus and the presence of an acrosome needed to establish contact with the
female gamete provide them with their unique shape. They exhibit extensive motility as they
contain a connected flagellum. As they transform into mature sperm, spermatids undergo a
wide array of morphological changes. It takes about 30 to 40 days for spermatogenic stem
cells to generate spermatozoa. Spermatogenesis is a process that occurs in the seminiferous
tubules, and many internal and external factors regulate it. Intrinsic regulation requires the
production and release of testosterone, neuroendocrine substances, and growth factors
secreted by Leydig cells. They communicate with nearby Leydig cells, blood vessels,
peritubular tissue of the seminiferous tubules, and Sertoli cells. They maintain the
trophic factors of these cells and participate in the regulation of peristalsis in the
seminiferous tubules. These growth factors and neuroendocrine substances influence the
contractility of myofibroblasts to ensure the proper transport of spermatozoa. Intrinsic
factors also play a vital role in regulating the flow of blood in the intertubular
microvasculature. This process is quite intricate and is mainly investigational in laboratory
animals. In humans, it is still unclear.
Extrinsic regulation of spermatogenesis requires stimuli from the hypothalamus and
hypophysis. The hypophysis awaits a signal from the hypothalamus to begin the release of
LH (luteinizing hormone). This signal is the pulsatile secretion of the hormone GnRH
(gonadotropin-releasing hormone). The release of LH stimulates the Leydig cells to produce
testosterone. Testosterone has a significant effect on spermatogenesis as well as other
functions throughout the body. Sertoli cells are stimulated by FSH (follicle-stimulating
hormone), an important signal that allows for the maturation of germ cells. Sertoli cells
secrete inhibin that is involved in the feedback mechanism.

Process
Spermatogenesis involves the following three complex integrated processes initiated at the
onset of puberty:
Meiosis
Male fertility requires the formation of millions of gametes. Production of functional gametes
or sex cells requires a single cell to undergo cell division to reduce the number of
chromosomes by half. This process is known as meiosis. Meiosis occurs twice, thereby
creating four daughter haploid cells, also known as spermatids. For every diploid
spermatocyte, meiotic divisions produce four haploid spermatids.
Mitotic cell division: diploid spermatogenic stem cells differentiate into two sets of diploid
primary spermatocytes, also known as tetraploid cells. The largest germ cells found in the
germinal epithelium are the primary spermatocytes, containing the largest cell nuclei.
Meiotic Cell Division I
The division begins with the leptotene stage of prophase. The stage takes place in the
germinal epithelium, more specifically, the basal compartment. Spermatocytes enter the ad
luminal compartment after reaching the Sertoli cell barrier. Here, further prophase stages
continue, and these include the zygotene, pachytene, and diplotene stages. These stages see
DNA reduplication, chromosomes condensation, and homologous chromosomes pairing, as
crossing over will occur. Each set of diploid primary spermatocytes differentiates into two
haploid secondary spermatocytes, where the total number of chromosomes gets reduced to
half.

Meiotic Cell Division II

Each haploid secondary spermatocyte differentiates into two haploid spermatids, therefore,
resulting in four haploid cells. This process occurs quickly, and no DNA replication takes place.

scrotum

scrotum, in the male reproductive system, a thin external sac of skin that is divided into two
compartments; each compartment contains one of the two testes, the glands that
produce sperm, and one of the epididymides, where the sperm is stored. The scrotum is
a unique anatomical feature of humans and certain other species of land-dwelling mammals.
It is continuous with the skin of the lower abdomen and is located directly behind
the penis and in front of the anus. The scrotal wall is a thin layer of skin lined with smooth
muscle tissue (dartos fascia). The skin contains more pigment than that of surrounding areas
and has many sebaceous (oil-producing) glands and sweat glands, as well as some hair. The
two compartments of the scrotum are distinguished externally by a middle ridge called
the raphe. Internally, the raphe connects to a muscular partition, the septum, which serves
to divide the scrotum into its two areas.

The function of the scrotum is to protect the testes and to keep them at a temperature several
degrees below the normal body temperature. The scrotum thus protrudes from the body
wall: moreover, it contracts from cold, exercise, or sexual stimulation and expands and
relaxes when warm. When contracted, it conserves heat; while relaxed it is smooth and
elongated, permitting the circulation of air that effects cooling. The relatively cool
temperature of the scrotum is thought to be important for the production of viable sperm.

The muscle tone of the scrotum becomes weakened and relaxed in older men.
In animals whose scrotum is always tight against the body, as in rats, boars, and stallions, the
testes are cooled by the intricate blood system that surrounds them. Failure of the scrotum
to cool the testes, which occurs during high fevers or, in some animals, during the hot
summer months, causes temporary sterility.

Epididyme

Epididyme, either of a pair of elongated crescent-shaped structures attached to each of the

two male reproductive organs, the testes. Sperm cells produced in the testes are
transported to the epididymes, where they mature and are stored. Each epididymis has
three regions, called, respectively, the head, body, and tail. The head is the uppermost and
largest part of the epididymis; it lies on the top surface of the testis. The body is attached to
the anal side of the testis and extends the length of the gland. The smallest region is the tail,
which begins at the point of separation of the epididymis from the testis. Sperm cells
mature primarily in the head and body of the epididymis and are stored in the tail.

The epididymis receives sperm from the tubules in the mediastinum testis, the region in
the testis in which all its sperm-producing tubules converge and empty. Leading from the
mediastinum to the head of the epididymis are 15–20 small, tightly coiled ducts called
the ductuli efferentes. The cells lining the ductuli have pigment granules, secretory
granules, and cilia (hairlike structures). In the head region of the epididymis, all the ductuli
efferentes connect to one large vessel, the ductus epididymidis. This duct is also extremely
coiled, being about 4 to 5 m (13 to 16 feet) long when stretched out. The ductus
epididymidis extends through both the body and the tail region of the epididymis. In the
tail region it becomes thicker, less coiled, and larger in diameter. As it emerges from the
end of the epididymis, it straightens out to form the ductus deferens.

During ejaculation, sperm are propelled through the ductuli efferentes and ductus
epididymidis in two ways. First, the muscle tissue, by contracting, narrows the ducts,
propelling the sperm. Second, the cilia located in the ductuli efferentes can propel sperm by
their continual swaying motions. As sperm pass through the various ducts, they acquire
small amounts of fluids that help to keep them alive. These secretions include high
concentrations of potassium, sodium, and a substance known as
glycerylphosphorylcholine, which is an energy source for sperm. sexual reproduction, the
production of new organisms by the combination of genetic information of two individuals
of different sexes. In most species the genetic information is carried on chromosomes in the
nucleus of reproductive cells called gametes, which then fuse to form a diploid zygote. The
zygote develops into a new individual. Sexual reproduction is the dominant form of
reproduction in living beings.

Sexual reproduction allows for the reshuffling of genetic material, both within and between
individuals of one generation, resulting in the potential for an extraordinary array of
offspring, each with a genetic makeup different from that of its parents. In the sexual
reproduction of all organisms except bacteria, there is one common feature: haploid,
uninucleate gametes are produced that join in fertilization to form a diploid, uninucleate
zygote. At some later stage in the life history of the organism, the chromosome number is
again reduced by meiosis to form the next generation of gametes. The gametes may be
equal in size (isogamy), or one may be slightly larger than the other (anisogamy); the
majority of forms have a large egg and a minute sperm (oogamy). The sperm are usually
motile and the egg passive, except in higher plants, in which the sperm nuclei are carried
in pollen grains that attach to the stigma (a female structure) of the flower and send out
germ tubes that grow down to the egg nucleus in the ovary.
Testis

Testis, plural testes, also called testicle, in animals, the organ that produces sperm, the
male reproductive cell, and androgens, the male hormones. In humans the testes occur as a
pair of oval-shaped organs. They are contained within the scrotal sac, which is located
directly behind the penis and in front of the anus.

Anatomy of the testes

In humans each testis weighs about 25 grams (0.875 ounce) and is 4–5 cm (1.6–2.0 inches)
long and 2–3 cm (0.8–1.2 inches) in diameter. Each is covered by a fibrous capsule called the
tunica albuginea and is divided by partitions of fibrous tissue from the tunica albuginea into
200 to 400 wedge-shaped sections, or lobes. Within each lobe are 3 to 10 coiled tubules,
called seminiferous tubules, which produce the sperm cells. The partitions between the
lobes and the seminiferous tubules both converge in one area near the anal side of each testis
to form what is called the mediastinum testis.

The testes contain germ cells that differentiate into mature spermatozoa, supporting cells
called Sertoli cells, and testosterone-producing cells called Leydig (interstitial) cells. The
germ cells migrate to the fetal testes from the embryonic yolk sac. The Sertoli cells, which
are interspersed between the germinal epithelial cells within the seminiferous tubules,
are analogous to the granulosa cells in the ovary, and the Leydig cells, which are located
beneath the tunica albuginea, in the septal walls, and between the tubules, are analogous to
the hormone-secreting interstitial cells of the ovary. The Leydig cells are irregularly shaped
and commonly have more than one nucleus. Frequently they contain fat droplets, pigment
granules, and crystalline structures; the Leydig cells vary greatly in number and appearance
among the various animal species. They are surrounded by numerous blood and lymphatic
vessels, as well as by nerve fibres.

The embryonic differentiation of the primitive, indifferent gonad into either the testes or the
ovaries is determined by the presence or absence of genes carried on the
Y chromosome. Testosterone and its potent derivative, dihydrotestosterone, play key roles
in the formation of male genitalia in the fetus during the first trimester of gestation but do
not play a role in the actual formation of the testes. The testes are formed in the abdominal
cavity and descend into the scrotum during the seventh month of gestation, when they are
stimulated by androgens. About 2 percent of newborn boys have an undescended testis at
birth, but this condition often corrects itself by the age of three months. The production of
testosterone by the fetal testes is stimulated by human chorionic gonadotropin,
a hormone secreted by the placenta. Within a few weeks following birth, testosterone
secretion ceases, and the cells within the testes remain undeveloped during early childhood;
during adolescence, gonadotropic hormones from the pituitary gland at the base of
the brain stimulate the development of tissue, and the testes become capable of producing
sperm and androgens.

Sperm

sperm, also called spermatozoon, plural spermatozoa, male reproductive cell, produced
by most animals. With the exception of nematode worms, decapods (e.g., crayfish),
diplopods (e.g., millipedes), and mites, sperm are flagellated; that is, they have a whiplike
tail. In higher vertebrates, especially mammals, sperm are produced in the testes. The sperm
unites with (fertilizes) an ovum (egg) of the female to produce a new offspring. Mature
sperm have two distinguishable parts, a head and a tail.

The head of the sperm varies in shape for each animal species. In humans it is flattened and
almond-shaped, four to five micrometres long and two to three micrometres wide (there are
about 25,000 micrometres in an inch). The head portion is mainly a cell nucleus; it consists
of genetic substances, called chromosomes, which are responsible for transmitting specific
characteristics of an individual, such as the colour of eyes, hair, and skin. In each body cell of
healthy humans, there are 46 chromosomes, which are responsible for the individual’s
general physical makeup. The sperm cells have only 23 chromosomes, or half of the usual
number. When a sperm cell unites with the ovum, which also has 23 chromosomes, the
resulting 46 chromosomes determine the offspring’s characteristics. The sperm cells also
carry the X or Y chromosome that determines the sex of the future child.
Covering the head of the sperm is a cap known as the acrosome, which contains enzymes
that help sperm to enter an egg. Only one sperm fertilizes each egg, even though 300,000,000
to 400,000,000 sperm are contained in an average ejaculation. Each egg and sperm produced
has slightly different genetic information carried in the chromosomes; this accounts for the
differences and similarities between children of the same parents.

A small middle portion of the sperm contains the mitochondria. The tail of the sperm,
sometimes called the flagellum, is a slender, hairlike bundle of filaments that connects to the
head and middle portion. The tail is about 50 micrometres long; its thickness of one
micrometer near the mitochondria gradually diminishes to less than one-half micrometre at
the end of the tail. The tail gives the sperm cell movement. It whips and undulates so that the
cell can travel to the egg. Following sperm deposition in the female reproductive tract,
activation of tail movement is suppressed until the sperm is carried to within a relatively
short distance of the egg. This gives the sperm an increased chance of reaching the egg before
exhausting its energy supplies.

The activation of tail movements is part of the process of capacitation, in which the sperm
undergoes a series of cellular changes that enables its participation in fertilization. A
fundamental change that occurs during capacitation is alkalinization of sperm cytoplasm, in
which the intracellular pH levels increase, particularly in the flagellum. This process, which
is driven by the rapid movement of protons out of the cell through ion channels on the
flagellum, underlies tail activation. Proton channels on sperm flagella are primed for opening
by the presence in the female reproductive tract of a substance known as anandamide, which
is thought to occur in high concentrations near the egg. Upon reaching an egg, enzymes
contained within the sperm acrosome are activated, enabling the sperm to traverse the thick
coat surrounding the egg (the zona pellucida); this process is known as the acrosome
reaction. The membrane of the sperm cell then fuses with that of the egg, and the
sperm nucleus is conveyed into the egg.

Sperm deposited in the reproductive tract of the female that do not reach the egg die. Sperm
cells may live in the human body for two or three days after mating. Sperm may also be
stored in a frozen state for months or years and still retain their capacity to fertilize eggs
when thawed.

Gamete

gamete, sex, or reproductive, cell containing only one set of dissimilar chromosomes, or half
the genetic material necessary to form a complete organism (i.e., haploid). Gametes are
formed through meiosis (reduction division), in which a germ cell undergoes two fissions,
resulting in the production of four gametes. During fertilization, male and female gametes
fuse, producing a diploid (i.e., containing paired chromosomes) zygote.
oogenesis

oogenesis, in the human female reproductive system, growth process in which the
primary egg cell (or ovum) becomes a mature ovum. In any one human generation, the
egg’s development starts before the female that carries it is even born; 8 to 20 weeks after
the fetus has started to grow, cells that are to become mature ova have been multiplying,
and by the time that the female is born, all of the egg cells that the ovaries will release
during the active reproductive years of the female are already present in the ovaries. These
cells, known as the primary ova, number around 400,000. The primary ova remain
dormant until just prior to ovulation, when an egg is released from the ovary. Some egg
cells may not mature for 40 years; others degenerate and never mature.

The egg cell remains as a primary ovum until the time for its release from the ovary arrives.
The egg then undergoes a cell division. The nucleus splits so that half of its chromosomes
go to one cell and half to another. One of these two new cells is usually larger than the other
and is known as the secondary ovum; the smaller cell is known as a polar body. The
secondary ovum grows in the ovary until it reaches maturation; it then breaks loose and is
carried into the fallopian tubes. Once in the fallopian tubes, the secondary egg cell is
suitable for fertilization by the male sperm cells.

zygote

zygote, fertilized egg cell that results from the union of a female gamete (egg, or ovum) with
a male gamete (sperm). In the embryonic development of humans and other animals, the
zygote stage is brief and is followed by cleavage, when the single cell becomes subdivided
into smaller cells.

The zygote represents the first stage in the development of a genetically unique organism.
The zygote is endowed with genes from two parents, and thus it is diploid (carrying two sets
of chromosomes). The joining of haploid gametes to produce a diploid zygote is a common
feature in the sexual reproduction of all organisms except bacteria.

The zygote contains all the essential factors for development, but they exist solely as an
encoded set of instructions localized in the genes of chromosomes. In fact, the genes of the
new zygote are not activated to produce proteins until several cell divisions into cleavage.
During cleavage the relatively enormous zygote directly subdivides into many smaller cells
of conventional size through the process of mitosis (ordinary cell proliferation by division).
These smaller cells, called blastomeres, are suitable as early building units for the future
organism.

In humans, identical twins develop from a zygote that splits into two separate cell masses at
a relatively early stage in its growth. These two masses, which are genetically identical to
each other, go on to become embryos. Fraternal twins, by contrast, develop from two
separate zygotes (two separate eggs fertilized by two different sperm).
ovulation

ovulation, release of a mature egg from the female ovary; the release enables the egg to be
fertilized by the male sperm cells. Normally, in humans, only one egg is released at one time;
occasionally, two or more erupt during the menstrual cycle. The egg erupts from
the ovary on the 14th to 16th day of the approximately 28-day menstrual cycle. If not
fertilized, the egg is passed from the reproductive tract during menstrual bleeding, which
starts about two weeks after ovulation. Occasionally, cycles occur in which an egg is not
released; these are called anovulatory cycles.

Prior to eruption from the ovary, an egg first must grow and mature. Until stimulated to
grow, the primary egg cell passes through a period of dormancy that may last several years.
The egg cell is surrounded by a capsule of cells known as the follicle. The follicular wall
serves as a protective casing around the egg and also provides a suitable environment for
egg development. As the follicle ripens, the cell wall thickens and a fluid is secreted to
surround the egg. The follicle migrates from within the ovary’s deeper tissue to the outer
wall. Once the follicle reaches the surface of the ovary, the follicular wall thins. Pressure
caused by the follicle and fluid against the ovary’s surface causes bulging of the ovarian wall.
When the follicle ruptures, the egg and fluid are released along with some torn patches of
tissue. The cells, fluid, and egg are directed into the nearby fallopian tube, which serves as a
passageway by which the egg reaches the uterus and as a site for fertilization of the released
egg by sperm.

The hormones that stimulate ovulation are produced in the pituitary gland; these are known
as the follicle-stimulating hormone and luteinizing hormone. After the egg leaves the ovary,
the walls of the follicle again close, and the space that was occupied by the egg begins to fill
with new cells known as the corpus luteum. The corpus luteum secretes the
female hormone progesterone, which helps to keep the uterine wall receptive to a fertilized
egg. If the egg is not fertilized, the corpus luteum stops secreting progesterone about nine
days after ovulation. If the egg becomes fertilized, progesterone continues to be secreted,
first by the corpus luteum and then by the placenta, until the child is born. Progesterone
blocks the release of more hormones from the pituitary gland, so that further ovulation does
not normally occur during pregnancy. See also menstruation; oogenesis.

ovary
ovary, in zoology, female reproductive organ in which sex cells (eggs, or ova) are produced.
The usually paired ovaries of female vertebrates produce both the sex cells and
the hormones necessary for reproduction. In some invertebrate groups, such as
coelenterates (cnidarians), formation of ovaries is associated with the seasons. Many
invertebrates have both ovaries and testes in one animal, and some species undergo sex
reversal.
Ovary function and anatomy

The primary function of the ovaries is to nurture and prepare oocytes (eggs) for the process
of ovulation (rupture and release of the mature egg from the ovary). Once an egg is released,
it migrates down a fallopian tube to the uterus. While in the fallopian tube, an egg may be
penetrated and fertilized by a sperm. If an egg becomes fertilized, it will implant in the wall
of the uterus. The processes of ovulation and fertilization are controlled largely by cells in
the ovaries that produce and secrete hormones. These hormones also are essential for
female sexual development and are necessary to sustain a pregnancy. In humans they also
serve to regulate the menstrual cycle (periodic shedding of the uterine lining).

The ovaries of newborns and young girls are a mass of elongated tissue located in the pelvic
girdle; as the female reaches adolescence, the ovaries gradually enlarge and change their
shape. The adult ovaries are almond-shaped, and their surface is usually uneven and has
areas of scar tissue. They are about 4 cm (1.6 inches) long, 2 cm (0.8 inch) wide, and 1.5 cm
(0.6 inch) thick; the two ovaries weigh 4–8 grams (0.14–0.3 ounce). The ovaries are held in
place by several ligaments (bands of fibrous connective tissue), including the
broad ligament, the suspensory ligament, and the ovarian ligament. Each ovary consists of
an outer cortex, which contains the follicles, oocytes, and some interstitial cells, and an inner
medulla, which contains additional interstitial cells, fibrous tissue, blood vessels, lymphatic
ducts, and nerves. (See also human reproductive system.)

Follicular development
The follicles, which are hollow balls of cells, contain immature eggs and are present in the
ovaries at birth; there are usually 150,000 to 500,000 follicles at that time. By the beginning
of a woman’s reproductive life, the number of immature follicles has fallen to about 34,000,
and this number continues to fall thereafter. As a woman ages, the follicles gradually
diminish in number until, at menopause and the cessation of reproductive function, the few
remaining follicles degenerate. During the active childbearing years, normally between ages
13 and 50, only 300 to 400 of the follicles undergo maturation. At the beginning of
each menstrual cycle, known as the early follicular phase, several follicles enlarge
and migrate from the cortex toward the outer surface of the ovary. The cells lining the follicle
multiply to form a layer known as the zona granulosa, and a cavity forms within this zone.
The stromal and interstitial cells that surround the follicle arrange themselves concentrically
to form a theca (an enclosing sheath) around the zona granulosa. One or sometimes more of
the follicles are selected for further growth and maturation. The mature follicles, known as
Graafian follicles, may reach 30 mm (about 1.2 inches) in diameter before they rupture.
The interstitial cells, especially those in the theca, produce mainly the hormones known
as androgens. Within the granulosa cells these androgens
are converted to estrogens (estradiol and estrone), the major ovarian hormones. The fluid in
the cavity bathing the oocyte contains high concentrations of estrogens and other steroid
hormones (progesterone and androgens), as well as enzymes and bioactive proteins. This
phase of the menstrual cycle, during which follicular development occurs, lasts about two
weeks.

At the end of the follicular phase of the menstrual cycle, one or occasionally two (or even
more) mature follicles at the surface of the ovary rupture and release the egg. The egg then
enters a fallopian tube to be carried to the uterus. After the follicle ruptures, the granulosa
and theca cells fill the lumen of the follicle, forming the corpus luteum. The corpus luteum
produces large amounts of progesterone for about two weeks. If at the end of that time the
egg has not been fertilized, the corpus luteum involutes (becomes smaller) and becomes a
whitish scar mass, known as a corpus albicans. As the corpus luteum disappears,
progesterone levels fall, and the uterine lining is shed through the process of menstruation,
thereby passing the unfertilized egg from the body. However, if fertilization occurs, the
corpus luteum continues to produce large amounts of progesterone for several months and
will remain in the ovary until the end of the pregnancy. Progesterone helps the fertilized egg
to secure itself to the uterus and to develop into an embryo. The processes of follicular
development, ovulation, and formation and function of the corpus luteum are controlled
by gonadotropins known as follicle-stimulating hormone (FSH) and luteinizing
hormone (LH), both of which are secreted from the pituitary gland.
Regulation of ovarian function
The ovaries secrete both estrogen and progesterone into the bloodstream, and thus they are
important endocrine glands. Before the onset of puberty the ovaries are quiescent, and the
cortex of each ovary contains only immature follicles. Puberty begins with pulsatile
nocturnal secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus.
Nocturnal pulses are initiated at least in part by increasing body size, which may cause an
increase in the secretion of leptin (from the Greek leptos, meaning “thin”; a
protein hormone important for regulation of reproduction, metabolism, and body weight),
which in turn stimulates the secretion of GnRH. Pulsatile secretion of GnRH activates the
gonadotroph cells of the anterior pituitary, resulting in pulses of secretion of moderate
quantities of FSH and of significant quantities of LH. In time, pulsatile secretion of GnRH and
pulsatile secretion of the gonadotropins occur continuously. Increasing secretion of
gonadotropins leads to increasing production of estrogens by the ovaries.
Estrogens stimulate the development of secondary sex characteristics and the maturation of
ovarian follicles. Increased secretion of estrogens normally occurs between ages 8 and 14 in
girls.

With continued maturation of the hypothalamus, pituitary, and ovaries, the cyclic
hypothalamic-pituitary-ovarian activity characteristic of adult women begins. During the
first days of the menstrual cycle, secretion of FSH increases, causing the maturation of
follicles as described above. As follicles mature, they secrete more estradiol (the most potent
of the estrogens), which is paralleled by an increase in the secretion of LH. Increased
secretion of LH stimulates the secretion of more estradiol and a small amount of
progesterone that then trigger a transient surge in LH secretion and to a lesser extent FSH
secretion, causing rupture of the mature Graafian follicle. The surge in LH secretion can be
readily detected in the urine, providing a means whereby women can determine if they have
ovulated and therefore are potentially fertile.

The follicular phase of the cycle ends at the time of ovulation. Serum LH, FSH, and estradiol
concentrations then decrease considerably, and the corpus luteum begins to produce some
estrogen and large quantities of progesterone. This is known as the luteal phase of the
menstrual cycle, which lasts until the corpus luteum degenerates (luteolysis) and estradiol
and progesterone production decreases. The decreasing serum estrogen and progesterone
concentrations result in constriction of uterine arteries, thus interrupting the delivery of
oxygen and nutrients to the endometrium. The endometrium is then sloughed off, causing
the vaginal bleeding characteristic of menstruation. A new menstrual cycle then begins.

The normal menstrual cycle is typically divided into a follicular phase of about 14 days,
during which the endometrium proliferates, and a luteal phase of about 14 days, which
culminates with the endometrial lining being sloughed off. Thus, the two phases are
separated by ovulation on the one hand and by menstruation on the other hand. The phases
vary in length by several days in different women and sometimes in the same woman.
Variations in cycle length are most common in the first years after menarche (the first
menstrual cycle) and just before menopause (when menstruation ceases).

The changing serum estrogen and progesterone concentrations during the menstrual cycle
have several other effects. Basal body temperature fluctuates little during the follicular phase
of the menstrual cycle but increases abruptly after ovulation. This increase parallels the
postovulatory increase in serum progesterone concentrations and is caused by an effect of
progesterone on the temperature-regulating centres in the brain. The decrease in serum
estradiol and progesterone concentrations near the end of the cycle may be accompanied by
changes in mood and activity and by an increase in fluid retention. The changes initiated by
the decrease in secretion of estradiol and progesterone comprise the symptoms
of premenstrual syndrome, although the relationship between hormonal changes and these
symptoms is unclear.

After menopause, the ovaries shrink in size and usually consist of old fibrous tissue. The
production of estrogen drops considerably but does not totally cease.

ovum, plural ova, in human physiology, single cell released from either of the female
reproductive organs, the ovaries, which is capable of developing into a new organism when
fertilized (united) with a sperm cell.

The outer surface of each ovary is covered by a layer of cells (germinal epithelium); these
surround the immature egg cells, which are present in the ovaries from the time of birth. A
hollow ball of cells, the follicle, encompasses each ovum. Within the follicle the ovum
gradually matures (see oogenesis). It takes about four months for a follicle to develop once it
is activated. Some follicles lie dormant for 40 years before they mature; others degenerate
and never develop. During child-bearing years, 300 to 400 follicles mature and emit eggs
capable of being fertilized. By the time a woman reaches menopause, most remaining
follicles have degenerated.

A follicle-stimulating hormone, secreted into the bloodstream by the pituitary, causes ovum
growth. After the egg matures, a second hormone from the pituitary, luteinizing hormone, is
liberated; this causes the egg’s release, called ovulation (q.v.).

As the ovum develops, the walls of the follicle expand by adding new cells. The follicle and
ovum slowly migrate through the tissue of the ovary until they cause a bulge in the surface
of the organ. The hollow cavity between the egg and the follicular wall usually contains a
fluid secreted by the follicular cells. This keeps the ovum moist and provides a suitable
growing environment. When the follicle ruptures, the egg is released from the ovary and is
then captured and guided by the fallopian tubes. Muscular contractions of the fallopian tubes
move the egg to the cavity of the uterus.

The ovum itself has a central nucleus that contains the female’s genetic material; this, with
the genetic material in the sperm cell, determines the inherited characteristics of the child.
Surrounding the nucleus is a cell plasma, or yolk, that contains nutritional elements essential
to the developing egg cell.

If an egg does not become fertilized within 24 hours of its eruption, it begins to degenerate.
After the egg is fertilized it undergoes a series of cell divisions. If at an early stage of
its development the fertilized egg splits into two parts that continue to grow, identical twins
will result; incomplete division will result in Siamese twins, born physically joined. Fraternal
twins result when two separate eggs are released and independently fertilized.

What is menstruation?

Menstruation is the monthly shedding of the lining of your uterus. Menstruation is also
known by the terms menses, menstrual period, menstrual cycle or period. Menstrual blood
— which is partly blood and partly tissue from the inside of your uterus — flows from your
uterus through your cervix and out of your body through your vagina.

Menstruation is driven by hormones. Hormones are chemical messengers in your body.

Your pituitary gland (in your brain) and your ovaries (part of your reproductive system)
make and release certain hormones at certain times during your menstrual cycle.

These hormones cause the lining of your uterus to thicken. This happens so that if a
pregnancy would occur, an egg can implant into your uterine lining. Hormones also cause
your ovaries to release an egg (ovulation). The egg moves down your fallopian tubes, where
it waits for sperm. If a sperm doesn’t fertilize that egg, pregnancy doesn’t occur. The lining
of your uterus breaks down and sheds. This is your period.
What is a menstrual cycle?

The menstrual cycle is a term to describe the sequence of events that

occur in your body as it prepares for the possibility of pregnancy each
month. Your menstrual cycle is the time from the first day of your
menstrual period until the first day of your next menstrual period.
Every person’s cycle is slightly different, but the process is the same.

What are the four phases of the menstrual cycle?

The rise and fall of your hormones trigger the steps in your menstrual
cycle. Your hormones cause the organs of your reproductive tract to
respond in certain ways. The specific events that occur during your
menstrual cycle are:
Phases of the menstrual cycle

There are four main phases of the menstrual cycle.

1. Menstruation

Menstruation is commonly known as a period. When you menstruate, your uterus lining
sheds and flows out of your vagina. Your period contains blood, mucus and some cells from
the lining of your uterus. The average length of a period is three to seven days.

Sanitary pads, tampons, period underwear or menstrual cups can be used to absorb your
period. Pads and tampons need to be changed regularly (preferably every three to four
hours) and menstrual cups should be changed every eight to 12 hours.

2. The follicular phase

The follicular phase starts on the first day of your period and lasts for 13 to 14 days, ending
in ovulation. The pituitary gland in the brain releases a hormone to stimulate the production
of follicles on the surface of an ovary. Usually, only one follicle will mature into an egg. This
can happen from day 10 of your cycle. During this phase, your uterus lining also thickens in
preparation for pregnancy.
3. Ovulation

Ovulation is when a mature egg is released from an ovary and moves along a fallopian tube
towards your uterus. This usually happens once each month, about two weeks before your
next period. Ovulation can last from 16 to 32 hours.

It is possible to get pregnant in the five days before ovulation and on the day of ovulation,
but it’s more likely in the three days leading up to and including ovulation. Once the egg is
released, it will survive up to 24 hours. If sperm reaches the egg during this time, you may
get pregnant.

4. The luteal phase

After ovulation, cells in the ovary (the corpus luteum), release progesterone and a small
amount of oestrogen. This causes the lining of the uterus to thicken in preparation for
pregnancy.

If a fertilised egg implants in the lining of the uterus, the corpus luteum continues to produce
progesterone, which maintains the thickened lining of the uterus.

If pregnancy does not occur, the corpus luteum dies, progesterone levels drop, the uterus
lining sheds and the period begins again.
What are symptoms of getting your period?

Some people experience symptoms of menstruation and others don’t. The intensity of these
symptoms can also vary. The most common symptom is cramps. The cramping you feel in
your pelvic area is your uterus contracting to release its lining.
Other signs you’re getting your period are:

• Mood changes.
• Trouble sleeping.
• Headache.
• Food cravings.
• Bloating.
• Breast tenderness.
• Acne.

Menopause

Menopause is the permanent end of menstrual periods and thus of fertility.

For up to several years before and just after menopause, estrogen levels fluctuate widely,
periods become irregular, and symptoms (such as hot flashes) may occur.
After menopause, bone density decreases.
Menopause is diagnosed when a woman has not had a period for 1 year, but blood tests
may be done to confirm it.
Certain measures, including hormone therapy and other drugs, can lessen symptoms.
During the reproductive years, menstrual periods usually occur in approximately monthly
cycles, with an egg released from the ovary (ovulation) about 2 weeks after the first day of
a period. For this cycle to occur regularly, the ovaries must produce enough of the
hormones estrogen and progesterone.
Menopause occurs because as women age, the ovaries stop
producing estrogen and progesterone. During the years before menopause, production
of estrogen and progesterone begins to fluctuate, and menstrual periods and ovulation
occur less often. Eventually, menstrual periods and ovulation end permanently, and
pregnancy can no longer occur naturally. A woman’s last period can be identified only later,
after she has had no periods for at least 1 year. (Women who do not wish to become
pregnant should use birth control until 1 year has passed since their last menstrual period.)
Perimenopause refers to the several years before and the 1 year after the last menstrual
period. How many years of perimenopause precede the last menstrual period varies
greatly. During perimenopause, estrogen and progesterone levels fluctuate widely. These
fluctuations are thought to cause the menopausal symptoms experienced by many women
in their 40s.
The menopausal transition is the part of perimenopause that leads up to the last menstrual
period. It is characterized by changes in the pattern of menstrual periods. The menopausal
transition lasts from 4 to 8 years. It lasts longer in women who smoke and in women who
were younger when it began.
Postmenopause refers to the time after the last menstrual period.

Symptoms of Menopause
Perimenopause symptoms
During perimenopause, symptoms may be nonexistent, mild, moderate, or severe.
Symptoms may last from 6 months to about 10 years, sometimes longer.
Irregular menstrual periods may be the first symptom of perimenopause. Typically, periods
occur more often, then less often, but any pattern is possible. Periods may be shorter or
longer, lighter or heavier. They may not occur for months, then become regular again. In
some women, periods occur regularly until menopause.
Hot flashes affect 75 to 85% of women. They usually begin before periods stop. They last
for an average of almost 7 1/2 years but can last more than 10 years. Usually, hot flashes
become milder and occur less frequently as time passes.
What causes hot flashes is unknown. But it may involve a resetting of the brain's
thermostat (the hypothalamus), which controls body temperature. As a result, very small
increases in temperature can make women feel hot. Hot flashes may be related to
fluctuations in hormone levels. There is no convincing evidence that alcoholic beverages
trigger hot flashes.
During a hot flash, blood vessels near the skin surface widen (dilate). As a result, blood flow
increases, causing the skin, especially on the head and neck, to become red and warm
(flushed). Women feel warm or hot, and perspiration may be profuse. Hot flashes are
sometimes called hot flushes because the face can become red.
A hot flash lasts from 30 seconds to 5 minutes and may be followed by chills. Night sweats
are hot flashes that occur at night.

Other symptoms may occur around the time of menopause. The changes in hormone
levels that occur at this time may contribute to the following:
• Breast tenderness

• Moodiness

• Worsening of migraines that occur just before, during, or just after menstrual
periods (menstrual migraines)