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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)
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
Diploid primordial germ cells, that form the outer layer of the seminiferous tubules, divide repeatedly by mitosis to produce diploid spermatogonia
Diploid Spermatogonium(2n)
MultiplicationPhase
Each spermatogonium grows to form a diploid primary spermatocyte
GrowthPhase
Each primary spermatocyte undergoes Meiosis I and produces two haploid secondary spermatocytesSecondary spermatocytesundergo Meiosis II toproduce spermatids(four from each primary spermatocyte)
Meiosis I
n nMeiosis II
n nn nSpermatids differentiate into mature spermatozoa
MaturationPhase
primordialgerm cellspermatogonium(2n)
primaryspermatocyte (2n)secondaryspermatocyte (n)sertoli cell
mature spermatozoa (n)differentiating spermatids (n)Differentiating spermatids
attach themselves to sertoli cells (‘nurse cells’)
which provide nourishment for the
developing sperm
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)
spermatogonia (2n)primary spermatocytes (2n)secondaryspermatocytes (n)
spermatids (n)
sertoli cell
lumen of seminiferous tubule
Oogenesis
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
Diploid primordial germ cells that form the germinal epithelium of the fetal ovary, divide repeatedly by mitosis to produce diploid oogonia
Diploid oogonia produced in groups just under the surface of the ovary
MultiplicationPhase
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
GrowthPhase
Embryonic
Life
MaturationPhase
As puberty begins, a primary oocyte completes Meiosis I each month to produce a secondary oocyte and a polar body
Meiosis I completed
n polar body (n)
Meiosis I involves an unequal cytoplasmic division such that the
polar body acts purely as a vehicle for one half
of the chromosomes
The secondary oocyte undergoes Meiosis II at fertilisation to produce an ovum and a second polar body
Meiosis II
ovum (n) polar body (n)
unequal cytoplasmicdivision