CASE 1
1. How does gametogenesis work in men and women? (oogenesis &
spermatogenesis + mitosis and meiosis)
Gametogenesis → The maturation of primordial germ cells into mature
gametes that are ready for fertilization.
These primordial germ cells (PGCs) are
present in the endothelium of the yolk sac and
stimulate formation of the gonads.
Then, in the gonads, the PGCs will undergo
further mitotic divisions, which is called
gametogenesis.
NOTE → Once present in the gonads, the
PGCs are often called Gonocytes.
The timing of this process of maturation is
however different in males and females.
• Males → No mitosis from 6th week of embryonic development. Only in the puberty
they will differentiate into spermatogonia. These
undergo meiosis and mature into spermatozoa which
are produced from puberty until death.
• Females → Undergo more mitotic divisions after
invested by somatic support cells, where they will
differentiate in oogonia. In 5th month of fetal
development:
o All oogonia start with meiosis until they are
primary oocytes.
o During puberty → each month few ovarian
follicles resume this development BUT only
one will mature into a secondary oocyte
which will then ovulate.
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Meiosis → A specialized process of cell division that
occurs only in the germ line.
In meiosis, a diploid germ cell erplicates its DNA (becoming
diploid, 4N) and undergoes two successive, qualitatively different
nuclear and cell divisions to yield four haploid, 1N offspring.
• In Males → The cell divisions of meiosis are equal
and yield four identical spermatozoa.
• In females → The meiotic cell divisions are
dramatically unequal and yield one single, massive,
haploid definitive oocyte and three minute,
nonfunctional haploid polar bodies.
,There are two meiotic divisions:
1. First meiotic division → DNA replication and recombination. Two haploid, 2N
daughter cells are made.
o Each chromosomal DNA molecule has to be replicated → diploid cell goes
from 2N to 4N. The produced cells are called primary oocytes and
primary spermatocytes.
▪ After DNA replication, each chromosome consists of 2 chromatids
joint together by the centromere. NOTE → each chromatid is
double stranded DNA.
o Prophase → Chromosomes condense into compact, double stranded
structure. Later, the homologous chromosome pairs match up to form a
joint structure called a chiasma (four chromatids, two centromeres and
two chromosomes).
▪ In this phase → crossing over can occur, which results in
recombination of the genetic material.
o Metaphase I → Four stranded chiasmas are on the equator of a spindle
apparatus.
o Anaphase I → One double-stranded chromosome of each homologous
pair is distributed to each of the two daughter nuclei.
▪ NOTE → The two chromatids of each chromosome remain
together. SO daughter cells are 2N.
o The cell undergoes cytokinesis, where it divides into two cells. This
produces:
▪ Two secondary spermatocytes in the male
▪ One secondary oocyte and a first polar body in the female.
2. First meiotic division → Double-stranded chromosomes divide. Four haploid, 1N
daughter cells are made.
o NO DNA replication takes place!
o Prophase II → The Double-stranded chromosomes condense.
o Metaphase II → The chromosomes line up and the centromeres replicate.
o Anaphase II → The double-stranded chromosomes are divided into two
single-stranded chromosomes which are distributed to each of the
daughter nuclei.
o The cells undergo cytokinesis, where one cell divides into two cells. This
produces:
▪ Two definitive spermatocytes → also called spermatids. SO in
total four from each germ cell that starts meiosis.
▪ In females this is again unequal. One large definitive oocyte is
formed together with another polar body.
,Spermatogenesis
During puberty → the testes begins to secrete the steroid
hormone testosterone. This will stimulate the
commencement of spermatogenesis (along with many
other effects).
Testosterone induces the differentiation of Sertoli cells into
a system of seminiferous tubules. The PGCs will divide
several times by mitosis into spermatogonia.
→ These spermatogonia are located under the
basement membrane surrounding the seminiferous
tubules.
The spermatogenesis takes place while the spermatogonia
are translocated from one to the other side of the
seminiferous epithelium. Then four spermatids are
formed.
HOWEVER, these spermatids have to undergo dramatic
changes before they reach the lumen. This process is
called spermiogenesis.
• The cytoplasm shrinks dramatically
• A tail is formed out of microtubules = flagellum
• The head is formed which contains the DNA.
• Between the head and the tail are a lot of
mitochondria so the sperm cell has enough energy
to move to the egg cell → midpiece.
Spermatogenesis takes place continuously from puberty to
death. One cycle of spermatogenesis takes about 64 days.
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Oogenesis
When female germ cells are invested by somatic support cells they first undergo several
mitotic divisions before they differentiate into oogonia.
→ After 12 weeks of embryotic development the oogonia will enter the first meiotic
prophase, where they become dormant primary oocytes.
• The nucleus of these cells are very large and watery, and called a germinal
vesicle.
• Around the cells a single-
layered, squamous
capsule of epithelial follicle
cells enclose the primary
oocyte → The capsule +
primary oocyte =
primordial follicle.
During puberty (until women enters
menopause) → monthly cycles in the
secretion of hypothalamic, pituitary and
ovarian hormones control a menstrual
cycle which results each month in the
production of a female gamete.
, Before a menstruation cycle, the follicular epithelium of ±12 primordial follicles thicken →
converting the squamous cells into cuboidal cells.
• Primary follicles are formed.
o The follicle cells secrete a thin layer of acellular material, forming a layer
called the zona pellucida. This barrier has intercellular junctions from oocyte
to follicle cells.
• Just before ovulation, the primary follicles make a multilayered capsule of follicle
cells, making the growing follicle.
• Most of these growing follicles will not develop and thus degenerate. Others will
enlarge because FSH levels rise. They will take up fluid and develop a fluid-filled
cavity called the antrum. The antral vesicular follicles are formed.
o The surrounding of the follicles differentiate into an inner and outer layer, that
become vascularized.
• Eventually, only one
growing follicle has
primacy and continues
to grow larger. The
large, swollen follicle is
called a mature
graafian (vesicular)
follicle.
NOTE → Still NO meiosis has
taken place.
Only 15 hours before the
ovulation, the germinal vesicle
of the oocyte breaks down. At
20 hours the metaphase of the
first meiotic division starts.
→ Then, the secondary oocyte and first polar body will be made. Only 3 hours
before ovulation, it will begin the second meiotic metaphase, and also forming
another polar body (or 2,
this depends).
