Download Embryology

Dentistry Department; 2nd grade
Dijlah University College
Embryology
Lec.5 Lab. 5
Dr. Wasan Adnan
Third to Eighth Weeks
Embryonic Period
The embryonic period or period of organogenesis, occurs from the
third to the eighth weeks of development and is the time when each of
the three germ layers; ectoderm, mesoderm, and endoderm, gives rise
to a number of specific tissues and organs. By the end of this period the
organ systems are established making the external features noticeable by
2nd month.
Derivatives of the Ectodermal Germ Layer
At the beginning of the third week of development, the ectodermal
germ layer has the shape of a disc that is broader in the cephalic than the
caudal region. Appearance of the notochord and prechordal mesoderm
induces the overlying ectoderm to thicken and form the neural plate.
Cells of the plate make up the neuroectoderm and their induction
represents the initial event in the process of neurulation.
Fig. 1. A. Dorsal view of a 16-day presomite embryo. The primitive streak and primitive node
are visible. B. Dorsal view of an 18-day presomite embryo. The embryo is pear-shaped, with
its cephalic region somewhat broader than its caudal end.
1
Dentistry Department; 2nd grade
Dijlah University College
Fig. 1. C. Dorsal view of an 18-day human embryo. Note the primitive node and, extending
forward from it, the notochord. The yolk sac has a somewhat mottled appearance. The length
of the embryo is 1.25 mm, and the greatest width is 0.68 mm.
NEURULATION
It’s the process of formation of the neural tube. It starts at the end of the
3rd week by elevation of the margin of the neural plate to form the
neural folds with the neural groove in the mid region. These folds fuse
to form tube with open end. They are called cranial neuropore and
caudal neuropore respectively. When they close, the neural tube
represent the central nervous system and consist of a dilated cephalic
region called the brain vesicles and a narrow long caudal part called
spinal cord. As the neural folds elevate and fuse, cells at the lateral
border or crest of the neuroectoderm begin to dissociate from their
neighbors. This cell population, the neural crest, will undergo an
epithelial-to-mesenchymal transition as it leaves the neuroectoderm by
active migration and displacement to enter the underlying mesoderm.
(Mesoderm refers to cells derived from the epiblast and extraembryonic
tissues.
In general terms, the ectodermal germ layer gives rise to
organs and structures that maintain contact with the outside world:
2
Dentistry Department; 2nd grade
Dijlah University College
(a) The central nervous system.
(b) The peripheral nervous system.
(c) The sensory epithelium of the ear, nose, and eye.
(d) The epidermis, including the hair and nails. In addition, it gives rise
to subcutaneous glands, the mammary glands, the pituitary gland,
and enamel of the teeth.
Fig. 2 A. Dorsal view of a late presomite embryo (approximately 19 days). The amnion has
been removed, and the neural plate is clearly visible. B. Dorsal view of a human embryo at 19
days. C. Dorsal view of an embryo at approximately 20 days showing somites and formation
of the neural groove and neural folds. D. Dorsal view of a human embryo at 20 days.
3
Dentistry Department; 2nd grade
Dijlah University College
Fig. 3 A. Dorsal view of an embryo at approximately day 22. Seven distinct somites are
visible on each side of the neural tube. B. Dorsal view of a human embryo at 21 days. C.
Dorsal view of an embryo at approximately day 23. Note the pericardial bulge on each side of
the midline in the cephalic part of the embryo. D. Dorsal view of a human embryo at 23 days.
4
Dentistry Department; 2nd grade
Dijlah University College
Fig.4 A. Lateral view of a 14-somite embryo (approximately 25 days). Note the bulging pericardial
area and the first and second pharyngeal arches. B. The left side of a 25-somite embryo approximately
28 days old. The first three pharyngeal arches and lens and otic placodes are visible.
Fig.5. Formation and migration of neural crest cells in the spinal cord. A,B. Crest cells form at the tips
of neural folds and do not migrate away from this region until neural tube closure is complete. C. After
migration, crest cells contribute to a heterogeneous array of structures, including dorsal root ganglia,
sympathetic chain ganglia, adrenal medulla, and other tissues. D. In a scanning electron micrograph,
crest cells at the top of the closed neural tube can be seen migrating away from this area.
5
Dentistry Department; 2nd grade
Dijlah University College
Neural Crest Derivatives
*Connective
tissue and bones of the face and skull.
*Cranial nerve ganglia.
*C- cells of the thyroid gland.
*Conotruncal septum in the heart.
*Odontoblasts
*Dermis in face and neck.
*Spinal (dorsal root) ganglia.
*Sympathetic chain and preaortic ganglia.
*Parasympathetic ganglia of the gastrointestinal tract.
*Adrenal medulla.
*Schwann cells.
*Glial cells.
*Meninges (forebrain)
*Melanocytes.
*Smooth muscles in blood vessels of face and forebrain.
Derivatives of the Mesodermal Germ Layer
Important components of the mesodermal germ layer are
(a) Paraxial mesoderm
(b) Intermediate mesoderm and
(c) Lateral plate mesoderm.
6
Dentistry Department; 2nd grade
Dijlah University College
(a)Paraxial mesoderm:
By the beginning of the third week, paraxial mesoderm begins to be
organized into segments. These segments, known as somitomeres, first
appear in the cephalic region of the embryo, and their formation proceeds
cephalocaudally. From the occipital region caudally, somitomeres further
organize into somites. The first pair of somites arises in the occipital
region of the embryo at approximately the 20th day of development. At
the end of the fifth week, 42 to 44 pairs are present, somites form the
axial skeleton. Somites give rise to the myotome (muscle tissue),
sclerotome (cartilage and bone), and dermatome (subcutaneous tissue of
the skin), which are all supporting tissues of the body. Each myotome
and dermatome has its own segmental nerve supply somites and
pharyngeal arches give the embryo its characteristic appearance.
(b)Intermediate mesoderm: It gives the following:
1-Urogenital structure (kidney, gonads and their ducts but not the bladder.
2-Spleen
3-Adrenal cortex of the suprarenal glands.
(c)Lateral plate mesoderm: It divides into two layers
1-Somatic (parietal) layer: Lines the intraembryonic cavity.
2-Splanchnic (visceral) layer: Surrounds the internal organs.
7
Dentistry Department; 2nd grade
Dijlah University College
Fig. 6. Transverse sections showing development of the mesodermal germ layer. A. Day 17.
B. Day 19. C. Day 20. D. Day 21. The thin mesodermal sheet gives rise to paraxial mesoderm
(future somites), intermediate mesoderm (future excretory units), and the lateral plate, which
is split into parietal and visceral mesoderm layers lining the intraembryonic cavity.
Fig. 7. Cross section through the somites and neural tube showing the organization of the
paraxial mesoderm into somites and intermediate and lateral plate mesoderm.
8
Dentistry Department; 2nd grade
Dijlah University College
Blood and blood vessels:
Mesoderm also gives rise to the vascular system, that is, the heart,
arteries, veins, lymph vessels, and all blood and lymph cells. Although
the first blood cells arise in blood islands in the wall of the yolk sac, this
population is transitory. The definitive hematopoietic stem cells are
derived from mesoderm surrounding the aorta. These cells colonize the
liver, which becomes the major hematopoietic organ of the embryo and
fetus from approximately the second to seventh months of development.
Stem cells from the liver colonize the bone marrow, the definitive bloodforming tissue, in the seventh month of gestation, and thereafter, the liver
loses its blood-forming function.
Derivatives of the Endodermal Germ Layer
The endoderm forms the followings:
1-Epithelial lining of GIT, respiratory tract, urinary bladder.
2-Parenchyma of the thyroid, parathyroid glands, liver and pancreas.
3-Epithelial lining of the tympanic cavity and auditory tube.
As a result of formation of organ systems and rapid growth of the
central nervous system, the initial flat embryonic disc begins to folds
cephalocaudally, establishing the head and tail folds. The disc also folds
transversely (lateral folds), establishing the rounded body form.
Connection with the yolk sac and placenta is maintained through the
vitelline duct and umbilical cord, respectively.
9
Dentistry Department; 2nd grade
Dijlah University College
10
Dentistry Department; 2nd grade
Dijlah University College
Note that the cardiogenic mes region and so are innervated from
11
Dentistry Department; 2nd grade
Dijlah University College
12