04/12/2023 1

3rd Week of development and derivatives of germ layers

ByDr. Abdul Waheed Ansari

Chairperson & Prof. Anatomy,RAKCODS.RAKMHSU.

04/12/2023 2

Learning outcomes of the topic

• What are the embryonic changes occurring during 3rd week of intrauterine life ( I.U.L.)?

• Formation of primitive streak with the cranial end of future embryo and caudal end defined

• Gastrulation and creation of the third germ layer from the epiblast cells.

• Neurulation is the formation of neural tube from the epiblast cells.

• Formation of neural crest cells (N.C.C.)• Derivatives of all germ layers.

04/12/2023 3

The bilaminar germ disk changes into trilaminar germ disk

• The primitive streak is formed on the surface of the embryonic disk.

• The ectoblast cells migrate through the primitive streak and form the mesoblast.

• At this stage the embryonic disk is oval -shaped and the ectoblast is bathed in amniotic fluid.

• From the 17th day a thickening of the embryonic disk begins around the median line along the rostro-caudal axis.

• This median structure (primitive streak) lengthens until it occupies roughly half the embryo.

04/12/2023 4

At the anterior end, a groove forms in the ectoblast (primitive groove).The cranial region is strengthened by the epiblast cells and so forms the primitive pit with the primitive node (Hensen's node in birds).

• The head of the embryo will form at the extremity of the embryonic disk near the primitive pit.

• The dorsal germinal layer becomes the ectoblast/-derm.• The middle germinal layer (3rd germinal layer) becomes the

mesoblast/-derm.• The ventral germinal layer receives the name endoblast/-derm. • The epiblast cells form the ectoblast, the mesoblast and the

intraembryonic endoblast.• The hypoblast cells give rise to the extra embryonic endoblast

(the umbilical vesicle and the allantois).

04/12/2023 5

The embryonic disk 3rd week of I.U.L.• 1. Primitive groove

• 2. Primitive pit

• 3. Primitive node

• 4. Oropharyngeal membrane

• 5. Cardial plate

• 6. Sectional edge of amniotic membrane• 7. Mesoderm• . • 8. Endoderm

• 9. Future cloacal membrane

• NB -1+2+3 primitive streak

04/12/2023 6

Gastrulation is the emergence of mesoderm from the epiblast cells

• Third germinal layer, the intraembryonic mesoblast.

• The mesoblast cells wander in all directions: laterally, cranially and caudally.

• This middle germinal layer lies between the definitive endoblast and epiblast.

• Exceptions are the cloacal membrane as well as the pharyngeal membrane, where the ectoderm and endoderm lie directly opposite each other.

1. Primitive groove2. Epiblast3. Extraembryonic mesoblast4. Definitive endoblast5. Invading epiblastic cells forming the intraembryonic mesoblast.6. Hypoblast

04/12/2023 7

04/12/2023 8

Notochord is formed during III week of development

• From the epiblasts that make up the floor of the amnion cavity.

• The notochord forms the first longitudinal midline axis around which the vertebral bodies are organized and is the basis for the axial skeleton.

• The notochord induces the formation of the neural plate (neurulation), synchronizing the development of the neural tube.

04/12/2023 9

Neurulation• The embryonic ectoderm over

the developing notochord thickens to form a neural plate .

• The ectoderm of the plate is called neuroectoderm and eventually gives rise to the central nervous system (brain and spinal cord).

• Neuroectodermal cells at the lateral edge of the neural plate -do not become part of the tube but form a neural crest over the neural tube and give rise to the neural crest cells.

• By the end of week 3, the neural folds move together, fuse, and convert the neural plate into the neural tube.

• Closure begins in the middle of the embryo and progresses toward both cephalic and caudal ends. It begins on day 21.

• The anterior or cranial neuropore closes in week 4 (day 26), whereas the posterior or caudal neuropore closes near day 28.

04/12/2023 10

Defects in the closure of neural pore will give rise to

• Anencephaly – when the anterior neuropore does not close, the brain will not develop and the fetus will be born with out brain inside the cranial cavity.

• Spina bifida – there will be a wide open gap at the lower end of spinal cord and it will be exposed out as a vesicle, as the posterior neuropore doesn't close.

04/12/2023 11

Neural tube formation and neural crest cells migration

04/12/2023 12

The allantois

• Appears on day 16 as a small, fingerlike out pouching or diverticulum from the caudal wall of the yolk sac.

• It remains small in the human embryo, is involved with early blood formation, and is related to the development of the urinary bladder.

04/12/2023 13

The primary germ layers

• endoderm, mesoderm, and ectoderm are formed and organized in their proper locations during gastrulation.

• Endoderm, the most internal germ layer, forms the lining of the gut and other internal organs.

• Ectoderm, the most exterior germ layer, forms skin, brain, the nervous system, and other external tissues.

• Mesoderm, the middle germ layer, forms muscle, the skeletal system, and the circulatory system.

04/12/2023 14

The neural crest cells

04/12/2023 15

Ectoderm derivatives

• The central nervous system;• The peripheral nervous system;• The sensory epithelium of the ear, nose and eye;• The epidermis, hair and nails; and• The subcutaneous, mammary and pituitary

gland; • The enamel of teeth.

04/12/2023 16

04/12/2023 17

References

• http://labspace.open.ac.uk/mod/resource/view.php?id=285192

• http://cro.sagepub.com/content/14/2/78.figures-only

• http://www.embryology.ch/anglais/hdisqueembry/triderm01.html