Embryology

1. Embryonic stage
Themba Hospital FCOG(SA) Part 1 Tutorials
By Dr N.E Manana

2. Embryonic stage
• Period of organogenesis, occurs from the third to the eighth weeks of
development
• 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 the embryonic period, the main organ systems have
been established

3. 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
in 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.

5. Neurulation
• Is the process whereby the neural plate forms the neural tube
• By the end of the third week, the lateral edges of the neural plate become
elevated to form neural folds, and the depressed midregion forms the
neural groove
• Gradually, the neural folds approach each other in the midline, where they
fuse
• Fusion begins in the cervical region (fifth somite) and proceeds cranially
and caudally
• Until fusion is complete, the cephalic and caudal ends of the neural tube
communicate with the amniotic cavity by way of the anterior (cranial) and
posterior (caudal) neuropores, respectively

7. Neurulation
• Closure of the cranial neuropore occurs at approximately day 25 (18- to 20-
somite stage)
• The posterior neuropore closes at day 28 (25-somite stage)
• Neurulation is then complete, and the central nervous system is
represented by a closed tubular structure
• With a narrow caudal portion, the spinal cord, and a much broader
cephalic portion characterized by a number of dilations, the brain vesicles

9. Neural crest cells
• Cells at the lateral border of the neuroectoderm begin to dissociate from
their neighbours.
• This cell population, the neural crest, they undergo an epithelial-to-
mesenchymal transition
• Leave by active migration and displacement to enter the underlying
mesoderm
(1) Dorsal pathway through the dermis form melanocytes
(2) Ventral pathway through the anterior half of each somite to become
sensory ganglia, sympathetic and enteric neurons, Schwann’s cells, and cells
of the adrenal medulla

10. Placodes
• By the time the neural tube is closed, two bilateral ectodermal
thickenings, the otic placodes and the lens placodes, become visible
in the cephalic region of the embryo
• the otic placodes invaginate and form the otic vesicles, which will
develop into structures needed for hearing and maintenance of
equilibrium
• At approximately the same time, the lens placodes appear. These
placodes also invaginate and, during the fifth week, form the lenses
of the eyes

12. Ectodermal germ layer
Organs and structures that maintain contact with the outside world:
● The central nervous system;
● The peripheral nervous system;
● The sensory epithelium of the ear, nose, and eye; and
● The epidermis, including the hair and nails.
● Subcutaneous glands,
● The mammary glands,
● The pituitary gland, ● And enamel of the teeth

13. Mesodermal germ layer
• Initially form a sheet of tissue on each side of the midline
• Cells close to the midline proliferate and form a thickened plate,
paraxial mesoderm
• The lateral mesoderm remains thin and is known as the lateral plate,
which divides into two layers somatic or parietal mesoderm layer
and the splanchnic or visceral mesoderm layer
• Intermediate mesoderm connects paraxial and the lateral plate
mesoderm
• Intermediate mesoderm differentiates into urogenital structures.

15. Mesodermal germ layer
• By the beginning of the third week , paraxial mesoderm begins to be organized into
segments(somitomeres), cephalocaudally.
• Approximately three pairs per day until end of the fifth week
• There are 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8 to 10 coccygeal
pairs.
• The first occipital and the last five to seven coccygeal somites later disappear, while the
remaining somites form the axial skeleton
• Ventrolateral edge migrate into the parietal layer of lateral plate mesoderm to form most
of the musculature for the body wall

17. Blood and Blood vessels
• The first blood islands appear in mesoderm surrounding the wall of
the yolk sac at 3 weeks of development and slightly later in lateral
plate mesoderm and other regions
• Hemangioblasts, a common precursor for vessel and blood cell
formation.
• Hematopoietic stem cells are derived from mesoderm surrounding
the aorta
• These cells colonize the liver, which becomes the major
hematopoietic organ of the embryo
• From the 7th month they colonise the bone marrow

19. Endodermal germ layer
• The gastrointestinal tract is the main organ system derived from the
endodermal germ layer
• The tube is divided into three regions: the foregut, midgut, and hindgut
• Oropharyngeal membrane : This membrane separates the stomadeum, the
primitive oral cavity derived from ectoderm, from the pharynx, a part of
the foregut derived from endoderm (4w)
• Cloacal membrane : This membrane separates the upper part of the anal
canal, derived from endoderm, from the lower part, called the
proctodeum, which is formed by an invaginating pit lined by ectoderm(7w)
• This germ layer covers the ventral surface of the embryo

20. Endodermal germ layer
Endoderm gives rise to:
● The epithelial lining of the respiratory tract
● The parenchyma of the thyroid, parathyroids, liver, and pancreas
● The reticular stroma of the tonsils and the thymus
● The epithelial lining of the urinary bladder and the urethra
● The epithelial lining of the tympanic cavity and auditory tube

22. Thank you
• Read on External appearance and Summary of key event