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Larsen human embryology summary CH1-3
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This is a summary of the first three chapters of Larsen human embryology.
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Larsens human embryologyCH 1
Between the fourth and sixth of gestation primordial germ cells (pale cytoplasm and
rounded shape) travel form the yolk sac via gut tube to the dorsal wall. They undergo mitosis
during this migration and if one gets lost somewhere along the way it could give rise to a
teratoma later in life. The migration is directed by chemokines, specifically sdf1 and its
receptor Cxcr4.
On arrival the PGCs stimulate the creation of somatic support cells, the proliferation of these
cells creates a swelling just medial of each mesonephros → genital ridges.
The somatic support cells give rise to the ovarian follicles in women and Sertoli cells and
testis cords in men.
Gametogenesis
After this the first
difference between men
and women starts. The
PCGs remain dormant
until puberty for men.
When puberty hits they
differentiate in to
spermatogonia (diploid,
2N) and undergo meiosis
and become spermatids
(haploid, 1N).
For women however, this
all starts way earlier.
Directly after the
cooperation with the
somatic support cell
undergo some mitosis
and differentiate into
oogonia(diploid, 2N). In
the fifth month oogonia
start meiosis but enters
meiotic arrest at the first
meiotic prophase
creating primary oocytes
(diploid, 4N). That’s
when the big sleep starts
for women, when they
hit puberty each month
a few ovarian follicles
will resume
, development resulting in just one secondary oocyte (haploid, 2N) which ovulates and waits
to be fertilized.
The difference is because of the inhibition of meiosis in men, if a PCG fails to reach the
gonad it will become and oocyte even in men. The inhibition might be a signaling factor
produced by the Sertoli cells.
In males meiosis leads to four spermatids, in females meiosis leads to one oocyte and three
polar bodies.
Spermatogenesis
Puberty → testes begin to secrete increase amounts of testosterone resulting in growth of
the testes, maturation of the seminiferous tubules and the start of spermatogenesis.
Testosterone → Sertoli cells → they differentiate into a system seminiferous tubules. →
PCGs resume development →
spermatogonia (diploid 2N)
These spermatogonia are found in
between the Sertoli cells in pockets.
The Sertoli cells are connected to
each other by tight junctions
creating a blood-testis barrier which
makes it safe for the
spermatogonia.
The spermatogonia then pass from
the basal to the luminal side of the
seminiferous epithelium while
spermatogenesis takes place. This
results in four spermatids for every
spermatogonia at luminal side. At
the luminal side the last connection
with the Sertoli cells is breached
and the spermatids is released in
the tubule lumen (spermiation).
It then moves to the tail of the epididymis to be stored for ejaculation. During ejaculation it
travels through the vas deferens and into the uterus and oviduct, where it can survive for
the three days. Just before fertilization changes in the acrosome take place to ensure that it
can release the enzymes to break through Zona Pellucida. (capacitation)
Sperm cell:
- Head → enzymes (in acrosome) and DNA
- Midpiece → mitochondria
- Tail → microtubules
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