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Formation of the Male Sex Cells: Spermiogenesis
Lecture Outline
First, let's take a step back and understand that, although there
are major differences, the four phases of gametogenesis are fundamental
to both oogenesis and spermatogenesis.
Four Phases of Gametogenesis
Formation & Migration of PGCs
Mitotic Increase in Germ Cell Numbers
Meiotic Reduction in DNA/Chromosome Content
Differentiation & Maturation
While the early events are similar in males and females (phase
1 & 2), the later stages differ dramatically. First, the germ
cell number does not undergo dramatic apoptosis as occurs in the
female but instead stem cells (spermatogonia) remain proliferative
(mitotic) throughout life. Meiosis is a sequential and continuous
process and doesn't stop and start in response to hormonal and
other influences as occurs in the egg. Like the egg the mature
sperm cells will be haploid. Unlike the egg. Differentiation occurs
after meiosis is complete and maturation occurs during the latter
stage of spermatogenesis.
Spermatogenesis
Process of sperm formation
Occurs in testes
Begins at puberty
Continuous process
Continues throughout life: Males always fertile (80+ year
olds have become fathers)
The Events of Spermatogenesis
As with oogenesis, there are a number of sequential stages that
have been identified during spermatogenesis. They are listed here
with some of the attributes that define each sage:
Spermatogonia
(2N): somatic like cells; stem cells; reproduce
by mitosis
1o Spermatocytes:
Growth phase; the cells increase ~2x in size; double their DNA
content to 4C; this stage represents the the start meiosis
2o Spermatocytes:
result of 1st Meiotic Division; smaller than 1o spermatocytes
Spermatid:
result of 2nd Meiotic Division; smaller than 2o spermatocytes;
dense nucleus; differentiate into sperm
Spermatozoa:
fully differentiated sperm
Now, let's look at a diagram starting with the migration of
primordial germ cells and finishing with spermatozoa.
Spermiogenesis
Spermiogenesis
refer to the differentiation of the spermatid into the functional
spermatozoan
Spermatid:
non-motile, round, non-specialized
Spermatozoan:
motile, elongate, specialized components, special surface molecules
and characteristics
Events of Spermiogenesis
Nuclear Shaping & Condensation
occurs due to changes in chromatin packaging (change
from somatic histones to sperm-specific histones).
Formation of flagellar tail results
due to elongation of microtubules (9+2 axoneme) from distal
centriole at the base of the nucleus.
Formation of Acrosome (from
the golgi) at the front end of the sperm
Rearrangement of organelles
(e.g., mitochodria, centrioles); Mitochondria fuse and form
spiral around anterior portion of axoneme of flagellum in
the midpiece region.
Most of the cytoplasm is lost.
It is shed as the Residual
Body
Cell Differentiation
While there are many ways to define "cell differentiation" one
of the simplest, yet most accurate is simply the specialization
of a cell for a specific role. Thus the gametes are cells that
are specialized for the role of fertilization. The end product
of cell differentiation is commonly the result of processes
that begin with changes in gene activity leading to alterations
in cellular structure and function which then are usually visible
(e.g., under a microscope) as physical changes in the cell.
Thus it is important to understand the following terms and to
use them accurately when you are discussing the process of cell
differentiation. Let's use the simple red blood cell (RBC) as
an example since we'll also be using these terms to discuss sperm
differentiation shortly.
Morphological
Differentiation: changes in the way the cell
looks (e.g., RBC is round)
Biochemical
Differentiation: appearance of new biochemical
components such as proteins (e.g., Hemoglobin in RBC)
Molecular
Differentiation: regulation of specific genes
that leads to biochemical changes and then morphological changes
(e.g., transcription of Globin genes in RBC)
It should be noted that you could also discuss other types
of differentiation (e.g., physiological differentiation if you
were discussing changes in physiology of the cells in question).
Cell Differentiation: A Simple Diagram
So, let's summarize how this would apply to Sperm
Cell Differentiation as shown in the table that follows:
Now let's look in a bit more detail at a few of the events that
occur during spermiogenesis so we can apply this information to
a deeper understanding the actual processes and complexity of differentiation.
Sperm Membrane is specialized for Fertilization
The sperm must fuse with the egg, this involves at least three
sequential events:
Sperm-egg recognition
Sperm-egg binding
Sperm-egg fusion
Thus, the sperm and the egg must have complementary recognition
molecules (receptor & ligand) for these events. Since the
acrosome will exocytose from the front of the sperm this region
of the sperm cell membrane must also have special attributes for
fusion of the acrosome with cell membrane.
Domains in Sperm Membrane
The following images taken with a Phase contrast/fluorescence
microscope reveal some domains on the human sperm surface. The
upper images show the fluorescence patterns when the sperm are
stained with three different antibodies that bind to three unique
surface proteins of unknown function that specifically localize
(as seen by red fluorescence) to the front of the sperm head (1st
pic), the posterior of the sperm head (2nd pic) or the sperm tail
(3rd pic). The lower pictures show the sperm under fluorescence
microscopy which verifies the localization of the staining in
the indicated regions revealing that the sperm cell has different
domains that contain different antigenic regions.
Formation of the Acrosome
The acrosome contains the digestive enzymes that will permit the
sperm to penetrate the egg. It is a specialized lysosome and, as
such, forms from the golgi. Burgos & Fawcett (1955) did some
of the initial ultrastructural studies of acrosome formation by
studying the process in the cat. The quality of their work still
stands today as shown in the following picture. First note the
changes in the acrosome. The fusion of smaller vesicles produced
by the golgi and the appearance of condensed material within the
acrosome. Notice how the material gets organized at the sperm
tip and is followed by the dissolution of the golgi. Then notice
the changes that occur in the nucleus because we'll look at that
next.
Nuclear Morphogenesis
You should have noticed that the nucleus underwent some changes
as well. It changed shape (elongated) and became more dense due
to chromatin condensation.
Each species of animal has a species-specific sperm morphology which
often involves the shape, size and density of the nucleus. The change
in nuclear morphology is referred to as nuclear morphogenesis. In
humans spermatogenesis, nuclear morphogenesis involves a number
of changes, the significance of which are still under analysis.
The following are important points to remember:
The Nucleus changes shape and density giving it a specific
morphology
Chromatin is repackaged to protect it from environment (UV,
chemicals)
Histone & DNA constitute chromatin of somatic cells;
nucleosomal organization
Sperm specific histone proteins appear during spermiogenesis;
non-nucleosomal organization allows tighter DNA packing
Protamines
Nuclear morphogenesis is driven, at least in part, by changes
in the way the sperm chromatin is packaged in the nucleus. The
following ultrastructural pictures reveal this change. In the early stages of spermatogenesis, somatic cells package their DNA with somatic histones (basic proteins) that results in the
formation of nucleosomes (N) which are seen throughout the nucleus.
This gives chromatin the appearance of "beads on a string" (like
a pearl necklace). During spermiogenesis, the somatic histones
(N) are replaced with protamines (n) which don't form nucleosomes
and so allow a tighter packing of the DNA.
The Sperm Tail
The sperm tail provides motility for the sperm. The tail consists
of a central axoneme of microtubules and accessory proteins essential
for flagellar movement. Surrounding the axoneme are some dense
structures (at least they appear that way in the EM) called accessory
fibers. The following picture shows a mammalian sperm cell with
the typical 9+2 structure of microtubules that make up the axoneme
(Phillips & Fawcett, 1955). Because this material was negatively
stained you can actually see the outlines of the tubulin molecules
in each microtubule. The flagellar membrane is not visible.
Kartagener’s Syndrome
Also known as Immotile
cilia syndrome
Patients have problems with respiratory tract (since it
is lined with cilia)
Patients produce normal numbers of sperm but the sperm
are not motile--they can't swim
Morphologically nothing appeared to be wrong with these
sperm cells until they were examined under the electron microscope
(EM).
EM: Sperm lacking dynein arms of A subfibres in flagellar
axoneme
Structure of the Normal Sperm Axoneme
The sperm axoneme is surrounded by the cell membrane which is
referred to as the flagellar membrane in this part of the sperm.
The axoneme consists of the 9 + 2 arrangement of microtubles plus
associated molecules (e.g., dynein) that in the presence of ATP
allows the microtubules to slide thus making the microtubule bend.
Dynein is a motor protein that is an ATPase (breaks down ATP to
release the energy stored in the terminal phosphate bond). The
dynein molecules hydrolyze ATP and undergo shape changes in the
presence of ATP to move the microtubules past each other. The
central sheath and doublet and the spokes, as well as other yet
uncharacterized molecules, all seem to play a role in how the
tail moves when the microtubules slide.
Sperm Axoneme: Normal Vs. Kartagener’s
Syndrome
Patients with Kartagener's Syndrome lack the dynein arms and thus
cannot use ATP to cause the sliding of the microtubules. Thus
the sperm are immotile.
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