Human Reproduction ❮ 207theepiblastand the hypoblast.The hypoblast gives rise to the yolk sac, which produces
the embryo’s first blood cells. In birds and reptiles, the yolk sac provides nutrients to the
embryo. In humans, the placentafills this role.
The epiblast develops into the three germ layers of the embryo: the endoderm,the
mesoderm,and the ectoderm.Endoderm:inner germ layer; gives rise to the inner lining of the gut and the digestive
system, liver, thyroid, lungs, and bladder.
Mesoderm:intermediate germ layer; gives rise to muscle, the circulatory system, reproductive
system, excretory organs, bones, and connective tissues of the gut and exterior of the body.
Ectoderm:outer germ layer; gives rise to nervous system and skin, hair, and nails.The separation of cells into the three primary germ layers sets the stage for cellular differ-
entiation by which different cells develop into different structures with different functions.
As far as this specific structural and functional differentiation is concerned, keep your focus
on the basic development of the nervous system.
The human nervous system derives primarily from the ectoderm, but the mesoderm
contributes a structure known as the notochord,which serves to support the body. In
vertebrates, this is present only in the embryo. The cells of the ectoderm that lie above the
notochord form the neural plate,which becomes the neural groove,which eventually
becomes the neural tube.This neural tube later gives rise to the central nervous system.
One other term you should be familiar with in the development of the mammalian embryo
is the somite,which gives rise to the muscles and vertebrae in mammals.
There are four extraembryonic structures necessary for the healthy development of the
embryo:- Yolk sac:derived from the hypoblast; site of early blood cell creation in humans. Source
of nutrients for bird and reptile embryos. - Chorion:formed from the trophoblast; the outer membrane of the embryo. Site of
implantation onto the endometrium. Contributes to formation of the placentain
mammals. - Allantois:mammalian waste transporter. Later it becomes the umbilical cord,which car-
ries oxygen, food, and wastes (including CO 2 ) back and forth from placenta to embryo. - Amnion:formed from epiblast. Surrounds fluid-filled cushion that protects the devel-
oping embryo. Present in birds, lizards, and humans, to name only a few.
How Do Cells Know What to Do?
How do the various cells of the developing embryo differentiate into cells with different
functions if they come from the same parent cell? As mentioned earlier, not every cell
receives the same amount of cytoplasm during the cleavage divisions. It is thought that this
asymmetric distribution of cytoplasm plays a role in the differentiation of the daughter
cells. Cells containing different organelles or other cytoplasmic components are able to per-
form different functions. Two other factors, induction and homeotic genes, contribute to
cellular differentiation.
Inductionis the influence of one group of cells on the development of another
through physical contact or chemical signaling. Just in case you are asked to write an essay
on induction, it is good to know a bit about the experiments of the German embryologist
Hans Spemann. His experiments revealed that the notochord induces cells of the dorsal
ectoderm to develop into the neural plate. When cells from the notochord of an embryoLindsay (12th
grade): “Take
the time to learn
these. Know
which layer pro-
duces what. It’s
worth a point on
the exam.”BIG IDEA 2.E.1
The timing and
coordination of
specific events are
necessary for normal
development.
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