Germ Layers
Trilaminar disc
In the third
week of development the embryonic trilaminar disc is formed, giving the embryo
three germ layers: ectoderm, mesoderm and endoderm (see
Figure 14.4). From these germ layers almost all of the structures of the embryo
will develop.
The ectoderm
will form the external surface of the embryo: the epidermis of the skin (Figure
15.1). The dermis is formed from the mesoderm layer. Melanocytes, the cells
that give the skin its pigment, are derived from neural crest cells. These
cells are themselves ectodermal and are involved in the development of a range
of structures (see Chapters 17 and 18).
The nervous
system is also formed from ectoderm (Figure 15.1), as we see when we study
neurulation (see Chapter 17). This probably reflects the evolutionary
internalisation of sensory apparatus. Simpler, early animals had external
sensory apparatus that allowed the animal to sense nutrients, chemicals, light,
and so on. This apparatus developed from ectoderm, the external layer. In
humans much of this sensory apparatus remains external to some extent (the
retina, touch, temperature and pain senses in the skin), but the nervous system
that has evolved is now located internally.
The mesoderm
is a major contributor to the embryo and its cells are used to build the bones,
cartilage and connective tissues of the skeleton, striated skeletal muscle,
smooth muscle, most of the cardiovascular system and lymphatic system, the
reproductive system, kidneys, the suprarenal cortex, ureters, the linings of
body cavities such as the peritoneum, the dermis of the skin and the spleen the
cardiovascular and immune systems rmed in the bone marrow are also derived from
mesoderm.
So the
remaind e formed from endoderm. What is left? Epithelia derived from endoderm
line internal passages exposed to external substances, including the
gastrointestinal tract, the lungs and respiratory tracts (Figure 15.3). Glands
that open into the gastrointestinal tract and the glandular cells of organs
associated with the gastrointestinal tract, such as the pancreas and liver, are
also derived from the endoderm (Figure 15.3).
The
epithelia of the urethra and bladder come from endoderm cells, as do the
tonsils, the thymus, the thyroid gland and the parathy- roid glands (Figure 15.3).
You can find out how these latter structures form in the pharyngeal arch
chapters (see Chapters 40–43).
Endoderm and
ectoderm meet at the openings of the mouth and anus. Thus, the oral cavity and
part of the pharynx have an epithelium derived from ectoderm, and the remainder
of the pharynx has a lining derived from endoderm. The same thing occurs at the
anus, and this has important anatomical ramifications for the development of
the vasculature there, with respect to portosystemic venous anastomoses for
example.
The germ
layers should not be confused with germ cells. Germ cells migrate from the yolk
sac through the gut tube and dorsal mesentery into the dorsal mesenchyme of the
embryo (see Chapter 38). Here they differentiate to form gametes; either
oocytes or spermatocytes.
A gamete is
a reproductive cell with a haploid (half) set of chromosomes that will combine
with another gamete during fertilisation to produce a new cell with a full,
diploid complement of chromosomes (see Chapter 10). That cell (zygote) will
become the embryo and its supporting structures.
A gamete,
then, is an example of a cell that does not develop from embryonic ectoderm,
endoderm or mesoderm.
Clinical relevance
With the
meeting of the ectoderm and endoderm near the anal opening of the gut tube, the
rectum develops with links partly to the ectoderm and partly to the endoderm.
The superior part of the rectum (endoderm, gut tube) drains blood back to the
liver via the superior rectal vein and subsequently the inferior mesenteric
vein and the portal vein. The inferior part of the rectum (ectoderm, not gut
tube) drains blood via the inferior and middle rectal veins to pelvic veins,
iliac veins, the inferior vena cava and thus into the systemic circulation.
Venous anastomoses (links) exist between the superior, middle and inferior
rectal veins. An impedance to the flow of blood through the liver from the
portal vein to the inferior vena cava will cause the blood to find an alternate
route, one example of which are the rectal portosystemic venous anastomoses.
The veins here stretch and enlarge, causing haemorrhoids.
Germ cell tumours are growths that develop
from germ cells.
They may
occur within or outside the gonads, possibly from aberrant or normal migration,
and can be congenital.
Germ cell
tumours of different types exist, including teratomas. A teratoma may
form structures of any of the three germ layers, uding thyroid, liver or
lung tissues, or occasionally hair, teeth or bone.
