Download Embryology

Embryology
24/6/2012
Summary Of Major Events Of The Second Week Development:
1- The completion of implantation. Embryo is fully implanted in the endometrium.
2- The production of the bilaminar germ disk. i.e. the inner cell mass (embryoblast)
differentiates into epiblast above (columnar) and hypoblast below (cuboidal).
Now Embryo = 2 Disks
3- Formation of the extra-embryonic structures such as : the amniotic cavity , the
amnion , the yolk sac (temporarily extra embryonic) , the chorionic sac , the
connecting stalk (future umbilical cord).
4- Appearance of the primary chorionic villi which are formed when the mesoderm
“pushes” the cytotrophoblast which in turn “pushes” the syncytiotrophoblast.
**Therefore the inner coating of the villi consists of the mesoderm and the outer
coating consists of the syncytiotrophoblast and the trophoblastic lacunae.
P.S. These events take place at the embryonic pole.
Major Events Of The Third Week :
1-Development of the primitive streak (First sign of Gastrulation)
2-Development of the Notochord.
3-Formation of the trilaminar germ disk.
4-Beginning of formation of neural tube.
P.S. Neural plates grow forming a neural groove. When both sides of the neural groove
are brought close enough together for their tips to touch a neural tube forms.
P.S. Remember that the ectoderm gives rise to the nervous system as well as the
epidermis of the skin.
Derivatives Of The Mesodermal Germ Layer:
Part of the mesoderm nearest to the notochord (axis) is called the Paraxial Mesoderm ,
found on both sides of the notochord (left and right)
P.S. Remember that the mesoderm ,unlike the ectoderm and endoderm, is not
continuous. It is interrupted by the notochord.
Paraxial Mesoderm develops into 3 major parts :
1- Medial Mesoderm
2- Intermediate Mesoderm
3- Lateral Mesoderm
1-The Medial Mesoderm
- Pushes the ectoderm upward to give Somites
- As the embryo develops the number of Somites increases until it reaches 44-45
somites.
– When the embryo is completely developed, the Somites give rise to:
1- Sclerotomes which eventually differentiate into the bone of the 34 vertebrae.
P.S. Vertebral Column = Vertebral Disk (from notochord) + Vertebrae ( from Somites)
2-Myotomes : Muscles
3-Dermatomes : Skin
** 10 Somites vanish when the embryonic tail is lost during embryogenesis.
P.S. Some texts call the Medial Mesoderm Paraxial Mesoderm because its near the
notochord.
2-The Intermediate Mesoderm
-It develops into the Urogenital System.
***Lateral Mesoderm will be discussed later on as it depends on other topics.
The Folding Of The Embryo…
- Folding occurs by differential growth of tissues.
-An embryo folds in 2 manners :
1-Caudocephalic Folding : Folding of the Head and the Tail.
2-Lateral Folding (Transverse ) : changes embryo from disk to tube-like.
The Yolk sac , as a result of lateral folding , moves inward thus forming the primitive gut.
Why Does The Embryo Fold??
1-Extensive and rapid growth of the cranial end of the neural tube to support the
immense amount of life skills a child has to acquire during the first 2 years of life.
Therefore The Head Folds Forward.
2-Enlargement of the Amnion.
General External features present at 4 weeks:
1-Brain
2-Brachial arches : gill-like structures which later contribute to the development of the
face.
3-Buccopharangeal membrane : which gives rise to the mouth.
4-Somites.
5-Primordia of the Heart.
6-Upper limb buds.
7-Primordia of the Liver.
8-Lower limb buds.
9-Primordia of the nose.
P.S. This is a very important stage because it sets the foundations for almost all body
systems.
P.S. Primordial = Primary.
P.S. Here we are talking about the intra-embryonic mesoderm. The extra-embryonic
mesoderm lines the yolk sac, amnion, connecting stalk, and the cytotrophoblast.
3-The Lateral Mesoderm:
-Divides into two layers :
1-Parietal Layer
2-Visceral Layer
-Together they form the lateral body wall folds which in turn work wwith the head
(cephalic) and tail (caudal) folds to close the ventral body.
!! The Body Cavity Is Now A Closed Tube !!
The Parietal Layer:
-
Overlies the ectoderm.
-
Gives rise to:
1- Dermis of skin.
2-bones and connective tissue of limbs.
3-the sternum (found along midline)
4-mesoderm cells lining the intra-embryonic cavity.
5-Sclerotomes and muscle recurser cells that migrate into the parietal layer of
the lateral plate mesoderm to form costal cartilages, limb muscles, and most of
the body wall muscles.
P.S. The intra-embryonic cavity gives rise to:
1-Pleural Cavity
2-Pericardial Cavity
3-Peritoneal Cavity
*** All 3 cavities are lined with cells derived from the parietal layer of the lateral
mesoderm.
The Visceral Layer:
-Covers endoderm which gives rise to internal organs.
-Lines the yolk sac which eventually gives rise to the digestive system.
****** The Digestive System Is Covered By Visceral Mesoderm While Its Wall Is Covered
By The Parietal Layer Of The Mesoderm ******
N.B. Dr. Amjad said that the topic of the derivatives of the parietal and visceral layers is
a very common source of questions.
Hope this was much help. Please bear in mind that you always have to go back to the
slides and sometimes the book to supplement your info with graphics and extra facts. I
tried my best to write everything as exact as possible.
Ayya AL-Rashdan.