Spermatogenesis pptx - D. Sumeya

Gametogenesis

Dr. Sumeya

Gametogenesis – gamete formation by meiosis.

Spermatogenesis – Sperm formation (1N)
Oogenesis – Egg formation (1N)

Primordial germ cells (PGCs)

Gametes are derived from (PGCs) that are formed in epiblast during 2nd 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

Primary spermatocyte

Secondary spermatocytes, 2N
Spermatids, 1N
Sperm, 1N

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Meiosis

morphogenesis
Spermatogenesis

• 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

Spermatogenesis occurs in testes

Cross section through testes’ tubes

sperm

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

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.

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.

Primary Spermatocyte

Secondary Spermatocyte

Spermatid

Sperm

flagella

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Sperm morphogenesis (change of shape)

nucleus
mitochondrion
Golgi complex

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.

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.

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.