© SSER Ltd.

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