Spermatogenesis pdf - D. Sumeya

Slide 1

Gametogenesis

Dr. Sumeya

Slide 2

Gametogenesis

–

gamete formation by meiosis.

Spermatogenesis –

Sperm formation (1N)

Oogenesis –

Egg formation (1N)

Slide 3

• Primordial germ cells (PGCs)



Gametes are derived from (PGCs) that are formed
in epiblast during

week

& that move to wall of

yolk sac.



During

4th week

, these cells begin to migrate by

amoeboid movement from yolk sac toward
developing gonads, where they arrive by

end of

5th week

Slide 4

Slide 5

Primary spermatocyte

Secondary

spermatocytes, 2N

Spermatids, 1N

Sperm, 1N

Meiosis

morphogenesis

Spermatogenesis

Slide 6

Spermatogenesis

Maturation of Sperm Begins at Puberty

•Spermatogenesis, which begins at puberty, includes all of the
events by which spermatogonia are transformed into
spermatozoa.
•It can be divided into 3 phases :
•

a. spermatocytosis

•b. meiosis
•C. spermiogenesis

•At birth, germ cells in the male infant can be recognized in the sex
cords of the testis as large, pale cells surrounded by supporting
cells . Supporting cells, which are derived from the surface
epithelium of the testis become sustentacular cells, or Sertoli
cells

Slide 7

Spermatogenesis

occurs in testes

Slide 8

Cross section

through testes’ tubes

Slide 9

sperm

Slide 10

• Shortly before puberty, the sex cords acquire a lumen

and become the

seminiferous tubules

. At about the

same time, PGCs give rise to spermatogonial stem cells.
At regular intervals, cells emerge from this stem cell
population to form

type A spermatogonia

, and their

production marks the initiation of spermatogenesis.

• Type A cells undergo a limited number of mitotic

divisions to form clones of cells. The last cell division
produces

type B spermatogonia

, which then divide to

form

primary spermatocytes

Slide 11

• spermatocytes then enter a prolonged prophase (22

days) followed by rapid completion of meiosis I and
formation of

secondary spermatocytes

. During the

second meiotic division, these cells immediately begin
to form haploid

spermatids .

Throughout this series of events, from the time type A

cells leave the stem cell population to formation of
spermatids, cytokinesis is incomplete, so that
successive cell generations are joined by cytoplasmic
bridges.

Slide 12

• Furthermore, spermatogonia and spermatids remain

embedded in deep recesses of Sertoli cells throughout
their development). In this manner, Sertoli cells support
and protect the germ cells, participate in their nutrition,
and assist in the release of mature spermatozoa.

• Spermatogenesis is regulated by LH production by the

pituitary gland. LH binds to receptors on Leydig cells and
stimulates testosterone production, which in turn binds to
Sertoli cells to promote spermatogenesis.

Follicle-

stimulating hormone (FSH)

is also essential because its

binding to Sertoli cells stimulates testicular fluid production
and synthesis of intracellular androgen receptor proteins.

Slide 13

Primary
Spermatocyte

Secondary
Spermatocyte

Spermatid

Sperm

flagella

Slide 14

Sperm morphogenesis (change of shape)

nucleus

mitochondrion

Golgi complex

Slide 15

Spermiogenesis

• The series of changes resulting in the transformation of

spermatids into spermatozoa is

spermiogenesis

. These

changes include

• (1) formation of the

acrosome

, which covers half of the nuclear

surface and contains enzymes to assist in penetration of the egg

and its surrounding layers during fertilization
• (2) condensation of the nucleus;
• (3) formation of neck, middle piece, and tail
• (4) shedding of most of the cytoplasm as residual bodies that

are phagocytized by Sertoli cells.

• In humans, the time required for a spermatogonium to develop

into a mature spermatozoon is approximately 74 days, and
approximately 300 million sperm cells are produced daily.

Slide 16

• When fully formed, spermatozoa enter the lumen of

seminiferous tubules. From there, they are pushed
toward the epididymis by contractile elements in the
wall of the seminiferous tubules. Although initially only
slightly motile, spermatozoa obtain full motility in the
epididymis.