|
Formation of the Male Sex Cells: Male Anatomy & Spermatogenesis
Introduction
The male genitalia is designed to produce sperm and deliver them
to the proper site to ensure fertilization for the continuation
of the species. From our point of view, we simply want to understand
the basic organization of the male sexual anatomy as a prelude
to understanding the events that occur during fertilization. In
the male genital tract, the sperm are not only formed but are
also subjected to certain modifications and environmental conditions
that are an essential prelude to successful fertilization.
Development of External Male Genitalia
Click on the link to download the movie Download
Movie
TDF & Sexual Determination
The determination of sex begins at fertilization (Males XY, females XX). At the "Indeterminate Stage" of sexual development (~10 Weeks in above figure) the genital ridge is bipotential. It can develop either into male or female genitalia. A gene on the Y chromosome that is not present on the X chromosome leads to the production of testis determination factor (TDF). As its name implies TDF causes each genital ridge to develop into a testis. In the absence of TDF, the genital ridge develops into an ovary. Development of the male and female genital organs involves many other events that are under the control of testosterone and other hormones.
Male genital tract is designed to:
Produce sperm
Store sperm
Release sperm as required
Add regulatory components to sperm
Add fluid
The Urogenital System of the Human Male
Sperm are produced in the testes
The scrotum holds the testes outside of the body
Together they lower the temperature for spermatogenesis
Why does sperm formation need lower temperatures? No other
differentiation/development
event requires this
From the Testes to Ejaculation: The Fate of Sperm Cells
Sperm spend their life in the male body in a continuous series
of tubes much like the plumbing (water pipes) in a house. As seen in the right-hand panel of the above figure, spermatogenesis,
occurs in the testes after which the full formed sperm leave the
seminiferous tubules to enter the epididymus (blue arrow) where
they remain stored until the appropriate moment. Certain secretions
are released in the epididymus that alter the sperms environment
and its surface characteristics, among other things. In preparation
for ejaculation, the sperm follow the route demonstrated by the
red arrows first leaving the epididymus to enter the vasa efferentia.
The sperm move along each vas efferens and enter the vas deferens.
As the sperm move along the vas deferens materials are added to
the sperm and to the extracellular fluid (forming semen) that
surrounds them. The prostate gland and seminal vesicles add fluid
and nutrients at the time of sperm ejaculation providing most
of the volume of the ejaculate. This complex of fluid and sperm
cells travels up the urethra to be released externally to the
male (last red arrow).
Glands & Secretions
Epididymus: stores sperm from seminiferous tubules; adds
secretions; adds surface glycoprotein to sperm Prostate gland
& seminal vesicles: add fluid, nutrients, etc. at time
of ejaculation of the semen
Foreskin & HIV Receptors
Throughout the course, some interesting topics that are related
to our areas of discussion--but not always central to them--will
be brought up to add to our understanding of our lives. This is
one example. There has been a lot of discussion about the topic
of circumcision (removal of the foreskin--usually in newborns
or young babies) in males. Recent scientific evidence indicates
that the foreskin contains receptors for the HIV virus, thus making
uncircumcised males more susceptible to contracting the virus
(and thus developing AIDS). In 2006, WHO issued statement advising that circumcision could save millions of lives in Africa. As with any such situation, cultural issues make the implementation of circumcision difficult.
Another point of interest that that the cells from the foreskin
have proven to be very valuable for study on cell function in humans.
Clearly the use of these cells will have even greater potential
with this new discovery.
The Organization of the Testes
The testes is made of: seminiferous tubules, interstitial
cells, blood vessels and extracellular material
Spermatogenesis occurs in seminiferous tubules
Spermatogenesis occurs from basement membrane towards the
lumen of tubules
The Testis & Spermatogenesis
The following diagram shows the organization of the events of
spermatogenesis within the testis. The earliest stages are closest
to the outer wall of the seminiferous tubule while the most mature
stages are near its lumen.
The Sequence of Events in Spermatogenesis
In the next lecture we will deal with the details of spermatogenesis.
Here we simply want to introduce the subject and indicate how
it is organized in a general way. Since the seminiferous tubules
are, as their name specifies, small tubes, they appear like the
cross-section of a pipe or hose. Instead of the wall being made
up of copper (pipe) or rubber (hose), the wall is made up of cells
and extracellular material or basement membrane as indicated above.
The interstitial cells and extracellular matrix lies on the outside
of the seminiferous tubule wall where it is in contact with non-reproductive
supportive tissues, blood vessels and nerves. Spermatogenesis
occurs on the inside of the wall where the stages of spermatogenesis
are lined up with the earliest stages adjacent the basement membrane.
Thus the spermatogonia (originally derived from the PGCs) that
have now increased in number lie closest to the basement membrane.
Spermatogenesis occurs in contact with special non-sexual, Sertoli
Cells that also adhere to the basement membrane. Successive stages
lie further and further away from the basement membrane in the
following order: Primary spermatocytes, secondary spermatocytes,
spermatids and spermatozoa. Some of these stages can also sub-divided
into other sub-categories of cell types that don't concern us
in this course.
Sertoli Cell Functions
Surround developing sperm cells
Synchronize the events of spermatogenesis
Secrete critical proteins (e.g., growth factors, androgen-binding
protein, inhibin) that are important for testes function and
spermatogenesis
Secretion of tubule fluid
Phagocytosis of residual bodies so cellular constituents
can be recycled
The Human Sperm Cell
The ultimate goal of spermatogenesis is the production of spermatozoa
or sperm cells. To fully understand the complexity and significance
of this process, we first should examine exactly what the sperm
cell looks like and the function of each of its components. Later
we'll re-examine this information in more detail when we discuss
fertilization. There are three main regions in the sperm cells:
Head: Front end where the haploid nucleus resides
covered by a single membrane sac (bag) of enzymes. A pair of
centrioles lies just behind the sperm nucleus.
Midpiece: The middle region of the sperm mainly
dominated by a spiral complex of mitocondria that are fused
together.
Tail: This is essentially a flagellum with some
specializations not seen in non-mammalian flagella.
Major Components of Sperm Cells &
Their General Functions
Each of the components of the sperm has a specific function. Their
overall job is to get their payload, the haploid male nucleus,
into the egg cytoplasm where it can fuse with the female pronucleus
to form the diploid, zygote nucleus.
Sperm Cell Membrane: contains receptors for recognizing
the egg. Remember, all cells are completely surrounded by a cell
membrane (sometimes called the plasma membrane). Plant cells
and many microbes have cell walls--animal cells do not
have cell walls.
Acrosome: A specialized lysosome containing digestive
enzymes for penetration of the material surrounding the egg.
Nucleus: haploid, gametic nucleus will fuse with
haploid egg nucleus forming 2N zygote nucleus.
Centrioles: organize axoneme (microtubules) of
the flagellum.
Flagellum: whip-like "tail" of sperm functions
in sperm motility; motive force for sperm involves sliding of
axonemal microtubles as is typical for all flagella and cilia
in all organisms that possess them.
Mitochondria: provide energy (ATP) for sperm motility
|
