Gametogenesis 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

Obgective

• This lecture discuss the steps of sperm

formation from primordial germ cells to
mature spermatozoa, hormonal control and
abnormality of spermatogenesis

Slide 6

Primary spermatocyte

Secondary

spermatocytes, 2N

Spermatids, 1N

Sperm, 1N

Meiosis

morphogenesis

Spermatogenesis

Slide 7

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 8

Spermatogenesis

occurs in testes

Slide 9

Cross section

through testes’ tubes

Slide 10

sperm

Slide 11

• 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 12

• 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 13

• 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 14

Primary
Spermatocyte

Secondary
Spermatocyte

Spermatid

Sperm

flagella

Slide 15

Sperm morphogenesis (change of shape)

nucleus

mitochondrion

Golgi complex

Slide 16

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 17

• 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.