1. © SSER Ltd.

2. Spermatogenesis
Spermatogenesis is a continuous process that begins at puberty and usually continues throughout life
Spermatogenesis begins with the mitotic division of the primordial germ cells that form the outer layer of the wall of the seminiferous tubules
Cell growth and meiosis follow, and as cell division proceeds, the daughter cells move towards the lumen of the tubule
Spermatogenesis is initiated and controlled by the hormone testosterone (secreted by specialised cells of the testes) and FSH (secreted by the pituitary gland)

3. sertoli cell (nourishes the maturing spermatozoa)
germinal epithelium (location of primordial germ cells that begin to divide by mitosis at puberty)
spermatozoa resulting from proliferation of primordial germ cells, followed by growth, meiosis and maturation
sertoli cell (nourishes the maturing spermatozoa)
interstitial cells (Leydig cells) that secrete the hormone testosterone

4. Multiplication Phase Growth Phase Maturation Phase
Diploid primordial germ cells, that form the outer layer of the seminiferous tubules, divide repeatedly by mitosis to produce diploid spermatogonia
Multiplication
Phase
Diploid
Spermatogonium
(2n)
Growth
Phase
Each spermatogonium grows to form a diploid primary spermatocyte
Each primary spermatocyte undergoes Meiosis I and produces two haploid secondary spermatocytes
Maturation
Phase
Meiosis I n
Secondary spermatocytes
undergo Meiosis II to
produce spermatids
(four from each
primary spermatocyte)
Meiosis II n
Spermatids differentiate into mature spermatozoa

5. mature spermatozoa (n)
primordial
germ cell
spermatogonium
(2n)
primary
spermatocyte
(2n)
secondary
spermatocyte
(n)
differentiating
spermatids (n)
sertoli cell
mature spermatozoa (n)
Differentiating spermatids attach themselves to sertoli cells (‘nurse cells’) which provide nourishment for the developing sperm

6. This schematic drawing shows the positions of the layers of cells within the seminiferous tubule as they undergo spermatogenesis
germinal epithelium
(primordial germ
cells; 2n)
sertoli cell
spermatogonia (2n)
primary spermatocytes (2n)
lumen of seminiferous tubule
secondary
spermatocytes
(n)
spermatids (n)

7. Oogenesis resumes at puberty and continues
Oogenesis begins during embryonic life when primordial germ cells, forming the germinal epithelium of the ovary, begin to proliferate; daughter cells grow and enter prophase I of meiosis, remaining in this stage until puberty
Oogenesis resumes at puberty and continues
until the menopause
In contrast to spermatogenesis, the meiotic divisions of oogenesis are unequal cytoplasmic divisions; the daughter cells are of unequal size although each contains the haploid number of chromosomes; the smaller cell is a polar body

8. Multiplication Phase Growth Phase Maturation Phase
Diploid primordial germ cells that form the germinal epithelium of the fetal ovary, divide repeatedly by mitosis to produce diploid oogonia
Multiplication
Phase E m b r y o n i c L f e
Diploid oogonia produced in groups just under the surface of the ovary
In contrast to spermatogenesis, only one oogonium from each group normally grows into a diploid primary oocyte;
each primary oocyte then enters prophase of Meiosis I and remains in this arrested state until puberty
Growth
Phase
Meiosis I involves an unequal cytoplasmic division such that the polar body acts purely as a vehicle for one half of the chromosomes
Meiosis I completed n
polar body (n)
Maturation
Phase
As puberty begins, a primary oocyte completes Meiosis I each month to produce a secondary oocyte and a polar body
Meiosis II
ovum (n)
polar body (n)
unequal cytoplasmic
division
The secondary oocyte undergoes Meiosis II at fertilisation to produce an ovum and a second polar body