Spermatogenesis PPT

SPERMATOGENESIS
DEVELOPMENTAL BIOLOGY
Learning Outcomes
•Describe the processes of human spermatogenesis
•Discuss the differentiations that take place on the cells undergoing
spermatogenesis
•Explain the components and significance of a semen
•Describe the anatomy of the different parts of a sperm cell and their respective
functions
Spermatogenesis
•The process by which male gametes, called sperm are created
•In humans, spermatogenesis takes 65 to 75 days
•Each sperm is haploid
• Only contain one copy of each chromosome
• In order to create haploid gametes, cells must go through the process of
meiosis
Spermatogenesis
•In humans, spermatogenesis takes place in the seminiferous tubules (ST)
• An intricate system of tubules in the testes
• The ST of an adult human male can sometimes a hundred million sperm per
day
•To produce huge amount of sperm, the ST contain population of dividing, self-
sustaining germ cells
• Cells that become gametes
Spermatogonia
•Spermatogonia are types of stem cells
•When they undergo mitosis, some spermatogonia remain near the basement
membrane of the ST in an undifferentiated state
• Serve as a reservoir of cells for future cell division and subsequent sperm
production
•The rest of the spermatogonia lose contact with the basement membrane and
differentiate into primary spermatocytes
Spermatogonia
•Spermatogonia remain dormant until puberty
•Always in contact with the basal lamina of the tubule
•Everyday, around 25 million spermatogonia enter meiosis and become primary
spermatocyte (2n)
Diploid (2n)
Seminiferous Tubules (ST)
•These tubules are enclosed by a thick basal lamina and surrounded by 3 to 4
layers of smooth muscle cells (or myoid cells)
•The insides of the tubules are lined with seminiferous epithelium
Leydig Cells
•ST converge and form a highly convolute structures known as the epididymis
•In the wall of the ST, there is the interstitial space where blood capillaries and
Leydig cells are found
Leydig Cells
•Found in the interstitium
•Release testosterone which stimulates the stem cell to differentiate into the
diploid primary spermatocyte
• These then undergo meiosis I to produce the secondary spermatocyte
• The secondary spermatocyte undergoes meiosis II to form the spermatid
which ultimately matures into the sperm cell
Spermatogenesis
Primary Spermatocytes
•Each primary spermatocyte replicates its DNA and then meiosis begins
•In meiosis I, homologous pairs of chromosomes line up at the metaphase plate,
and crossing over occurs
•The meiotic spindle pulls one (duplicated) chromosome of each pair to an
opposite pole of the dividing cell
•The two cells formed by meiosis I are called secondary spermatocytes
Primary Spermatocytes
•They immediately enter the prophase of the first meiotic division, which is
extremely prolonged (about 22 days)
•A large number of primary spermatocytes is always visible in cross-sections
through seminiferous tubules
Secondary Spermatocyte
•Each secondary spermatocyte has 23 chromosomes, the haploid number (n)
•Each chromosome within a secondary spermatocyte, however, is made up of
two chromatids (two copies of the DNA) still attached by a centromere
•No replication of DNA occurs in the secondary spermatocytes
Haploid (n)
Spermatids
•In meiosis II, the chromosomes line up in single file along the metaphase plate,
and the two chromatids of each chromosome separate
•The four haploid cells resulting from meiosis II are called spermatids
• Once the meiosis II is completed, but before the haploid cells had the chance to
differentiate into actual sperm, the cells are called spermatids
•A single primary spermatocyte therefore produces four spermatids via two
rounds of cell division (meiosis I and meiosis II)
Spermatids
•These spermatids, with the help of nourishing cells called sertoli cells, eventually
differentiate into sperm cells
•When the spermatids interact with the sertoli cells, the sertoli cells not only
give the cells proper nutrients but also phagocitize the cytoplasm from the
spermatid to produce the sperm cell
Spermatids
•These spermatids now have the correct number of chromosomes to be
functional gametes
• But they do not have the physical characteristics of sperm yet that allow them
to swim to swim to the ovum and fertilize it
Haploid (n)
Cytoplasmic Separation
•As spermatogenic cells proliferate, they fail to complete cytoplasmic separation
(cytokinesis)
•The cells remain in contact via cytoplasmic bridges through their entire
development
•This pattern of development most likely accounts for the synchronized
production of sperm in any given area of the seminiferous tubule
Cytoplasmic Separation
•It may also have survival value in that half of the sperm contain an X
chromosome and half contain a Y chromosome
•The larger X chromosome may carry genes needed for spermatogenesis that
are lacking on the smaller Y chromosome
Spermatozoa
•The chromatin condenses during the maturation of the spermatids into
spermatozoa, and the nucleus becomes smaller and stains darker
Sperm Cells
•The final stage of spermatogenesis, spermiogenesis, is the development of
haploid spermatids into sperm cell
•No cell division occurs in spermiogenesis; each spermatid becomes a single
sperm cell
•During this process, spherical spermatids transform into elongated, slender
sperm cell
Sperm Cells
•An acrosome forms atop the nucleus, which condenses and elongates, a
flagellum develops, and mitochondria multiply
•Sustentacular cells dispose of the excess cytoplasm that sloughs off
Spermiation
•Finally, sperm are released from their connections to sustentacular cells, an
event known as spermiation
•Sperm then enter the lumen of the seminiferous tubule
•Fluid secreted by sustentacular cells pushes sperm along their way, toward the
ducts of the testes
Sperm Cell
•Once the sperm cells are formed, they are released in the luminar cavity of the
ST and then travels end up in the epididymis
•Once inside the epididymis, this is where they mature into sperm cells
Sperm Cell
•It is also in the epididymis where the sperm cells are stored before they are
released in the outside environment
Sperm Cell
Sperm Cell
•A sperm has three basic components
•The nucleus which contains the haploid genome
• the main job of the sperm is to get the haploid genome into the ovum
•The acrosome is a structure that forms a cap over most of the nucleus of the
sperm cell
• Its main job is to penetrate the outer layer of the ovum so that the sperm can
get inside
Sperm Cell
•The flagellum is a long whip-like cellular appendage that is used for locomotion
• A sperm cell uses its flagellum in a whip-like fashion, lashing it back and forth, to
propel the sperm forward
•Where does the sperm cell get all the energy to propel?
• The middle portion of the sperm is filled with mitochondria that provide the
ATP that powers the sperm for locomotion
Semen
•A mixture of sperm and seminal fluid, a liquid that consists of the secretions of
the seminiferous tubules, seminal vesicles, prostate, and bulbourethral glands
Semen
Semen
•The volume of semen in a typical ejaculation is 2.5–5 milliliters (mL), with 50–
150 million sperm per mL
•When the number falls below 20 million/mL, the male is likely to be infertile
Semen
•Despite the slight acidity of prostatic fluid, semen has a slightly alkaline pH of
7.2–7.7 due to the higher pH and larger volume of fluid from the seminal
vesicles
•The prostatic secretion gives semen a milky appearance, and fluids from the
seminal vesicles and bulbourethral glands give it a sticky consistency
•Seminal fluid provides sperm with a transportation medium, nutrients, and
protection from the hostile acidic environment of the male’s urethra and the
female’s vagina
Semen
•Once ejaculated, liquid semen coagulates within 5 minutes due to the presence
of clotting proteins from the seminal vesicles
•After about 10 to 20 minutes, semen reliquefies because prostate-specific
antigen (PSA) and other proteolytic enzymes produced by the prostate break
down the clot
•Abnormal or delayed liquefaction of clotted semen may cause complete or
partial immobilization of sperm
Summary
•Spermatogenesis takes 65 to 75 days in humans
•It is a complex process that involves two rounds of meiosis (I and II)
•Sperm cells are the end product of spermatogenesis
•Semen is a mixture of sperm and seminal fluid
References
Tortora, G. & Derrickson, B. (2017). Principles of Anatomy and Physiology (15th
Edition). John Wiley & Sons, Inc. Danvers, MA

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SPERMATOGENESIS

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