Lecture 35- Third Week Of IUL

Question 1

What is the first major event of week 3- and arguably the MOST important part of human development?

Answer 1

Gastrulation- meaning the formation of the gut
Formation of the 3 germ layers (Endo, meek, ecto)
Provides changes in the fate of the germ layers (differentiation) and start organogenesis
Establishing body axis (cranial- caudal left-right, etc)

Question 2

Formation of x defined for the first time the major body axes.
What is x, and what body axes are they?

Answer 2

The primitive streak
Cranial caudal (head-tail)
Dorsal ventral (back belly)
Medial lateral (left right)

Question 3

Describe how Epiblast cells are induced to migrate and form the germ layers
And which week does this occur?

Answer 3

The primitive streak is formed; which is a longitudinal midline structure in the Epiblast near the caudal end of the bilaminar disk
This PS moves through the midline of the Epiblast, (from caudal end - cranial) which makes the Epiblast cells migrate from the midline down towards the Hypoblast cells, and towards the lateral ends creating a primitive groove where they migrate.
The Hypoblast cells are displaced and the first layer is now the Endoderm.
Afterwords, the Epiblast form another layer while migration creating the Mesoderrm
The Epiblast that remain, are now termed Ectoderm.
Week 2

Question 4

True/False The Hypoblast is the source of all 3 germ layers.

Answer 4

False. The EPIBLAST is the source of all 3 germ layers.

Question 5

What ar ethe 2 membranes that have no mesoderm layers during the process of gastrulation?

Answer 5

The oropharyngeal membrane (cranial end and will become opening of mouth)
Cloacal membrane (caudal end and will become opening of anal canal)

Question 6

Notochord will form from the x and is on the y

Answer 6

Notochord will from the primitive streak and is on the midline

Question 7

What is the function of the notochord?

Answer 7

Forms the basis of the longitudinal axis of the embryo (cranial and caudal) it comes out from the primitive node
Gives the embryo rigidity
Gives signals for the development of the CNS

Question 8

What is the fate of the primitive streak?

Answer 8

Diminishes in size and becomes an insignificant structure. Disappears by the end of the 4th week

Question 9

What is sacrococygeal teratoma?

Answer 9

A disease caused by the remanence of the primitive streak
Causes a tumor to be formed which contains tissues from all germ layers as the primitive streak is remaint of Epiblast (pluripotent)
Only occurs in the sacrococygeal region
Good prognosis

Question 10

What is caudal dysgenesis?

Answer 10

A group of syndromes that occur because of INSUFFICIENT mesoderm formation in the caudal region of the embryo.

Question 11

What are some characteristics of caudal dysgenesis?

Answer 11

Hypoplasia (poor formation of tissues and organs) and fusion of lower limbs
Anomalies of the lumber and sacral vertebrae
Agenesis of the kidney and urinary tract
Agenesis or internal genital organs EXCEPT GONADS
Imperforated anus
In EXTREME CASES - Sirenomelia (deficiency in caudal development leads to fusion of lower limb buds during early development

Question 12

What is neurulation?

Answer 12

The process by which the neural plate, neural folds, and neural tube form.

Question 13

Describe neurulation, and state which week it occurs?

Answer 13

The notochord, lying exactly in the midline, sends signals to the ectodermal cells in the midline to thicken. This will form the neural plate. As the neural plate progresses, the lateral ends elevate, forming neural groove.
The neural folds approach the midline where they fuse
forming neural tube.
The fusion begins at the cervical region, and starts moving cranial and caudally.
The 2 ends (anterior neuropore and posterior neuropore) both do not close initially
Week 3 (finishes at end day 21)

Question 14

When does the anterior and posterior neuropore close?

Answer 14

Anterior neuropore - 25 (4th week)

Posterior neuropore- 28 (end of 4th week)

Question 15

Give some examples of neural tube defects and their causes

Answer 15

Anencephaly- open brain and lack of skull vault (anterior neuropore did not close on day 25) V Poor prognosis
Myelomeningocele - open spinal chord with meningeal cyst (posterior neuropore did not close on day 28)
Spina bifida occulta -asymptomatic condition in which vertebra are not completely closed
Meningocele- protrusion of the meninges

Question 16

How can 70% of neural defects be prevented?

Answer 16

By taking folic .4mg of folic acid daily beginning 3 months before conception and taking it throughout pregnancy.
If the family has a history of NTD increase dose to 1mg

Question 17

What are the 3 main subdivisions fo the mesodermal germ layer?

Answer 17

Paraxial mesoderm
Intermediate mesoderm
Lateral plate mesoderm

Question 18

In the 17th day, cells close to the midline for, x, known as the y, which will later form somitws

Answer 18

Cells close to the midline form a thickened plate known as paraxial mesoderm

Question 19

On the 19th day, more laterally, the mesoderm layer x, and is known as y

Answer 19

Laterally, the mesoderm layer remains thin and is known as the lateral plate mesoderm

Question 20

The x connects the paraxial and lateral plate mesoderm

Answer 20

Intermediate mesoderm

Question 21

What is the function of the paraxial mesoderm?

Answer 21

Forms somites (starting on the 20th day) (3rd week) start on the cephalic end, and continue cephalo caudally; 3 pairs/day. By the end of the 5th week 42-44 pairs!
The somires ads divided into :
Scelerotome: cells migrate medically to surround ribs and give rise to vertebrae ribs
Dermatome: cells migrate to the under surface of the ectoderm and give rise to dermis and subcutaneous tissue
Myotome: give rise to SKELETAL muscles of body wall and limbs (biceps,triceps)

Question 22

What is the function of the intermediate mesodermal germ layer?

Answer 22

Gives rise to the urinary system and gonads (kidney, bladder, urethra, tests, etc)

Question 23

What is the function of the lateral plate mesodermal layer?

Answer 23

Small cavities appear in the lateral space mesoderm, which merge and form a large cavity called INTRAembryonic coelom.
The Intraembryonic coelom splits the lateral plate mesoderm into 2 parts;
Somatic mesoderm- which give rise to bones, ligaments, blood vessels, abs connective tissue of the lungs and the parietal layer of the serous membranes.
Splanchnic mesoderm- formed the SMOOTH muscles of Wall of the GIT and respiratory organs of the visceral layer of the serous membranes

Question 24

Blood cells and blood vessels arise from which of the 3germ layers?

Answer 24

Mesoderm

Question 25

X gives rise to the pericardial, pleural, and peritoneal cavities

Answer 25

The intraembryonic coelom

Question 26

How does the intraembryonic coelom form?

Answer 26

When the somatic layer, which is in contact with the surface of the ectoderm forms the body wall, and the splanchnic layer, which is in contact with the endodermal tube (gut)forms the smooth muscles of the gut wall (4th week)
Basically when the somatic and splanchnic layers split

Question 27

Between the 17th and 28th days, growth of the brain vesicles and lengthening of the neural tube cause both through head and tail regions to move x forming y body folding
Simultaneously, the sides of the embryo begin to fold z, to form p

Answer 27

The growth of the brain vesicles and lengthening fo the neural tube causes both the head and tail regions to move ventrally forming craniocaudal body folding
Simultaneously, the sides of the embryo begin to fold ventrally to form 2 lateral body folding

Question 28

Craniocaudal and lateral body wall folding results in what to fuse?

Answer 28

It makes the ectoderm, mesoderm, the future coelmic cavities and the ectoderm from the opposite sides to fuse.

Question 29

The process of craniocajdal and lateral folding transforms the embryo from a flat disk to x

Answer 29

It transforms the embryo from a flat disc to 3D tube within a tube.(outer ectoderm and inner ectoderm (gut)
The embryo grows more rapidly than the yolk sac, and the cavity remains continuous with the developing gut tube through the narrowing vitelline duct.

Question 30

Give examples of body structures that arise from the ectoderm

Answer 30

All nervous tissue
Epidermis of skin
Hair follicles, arrector pili muscles, nails, epithelium of skin glands, mammary glands 
Lens, cornea. And internal eye muscles
Internal and external ear

Question 31

The umbilical cord is developed from what?

Answer 31

Unsplit extraembryonic mesoderm

Question 32

The gut is derived from what?

Answer 32

The yolk sac

Question 33

What induces the formation of the neural tube?

Answer 33

The notochord

Question 34

What arises from the mesoderm?

Answer 34

All skeletal and cardiac muscle tissue
Most smooth muscle tissue
Cartilage, bone, and connective tissues
Blood, red bone marrow, lymphatic tissue
Blood and lymphatic vessels
Dermis of the skin
Fibrous tonic and vascular tonic of the eye 
Kidney and ureters
Adrenal cortex
Gonads and genital ducts (EXCEPT GERM CELLS)
Dura matter

Question 35

What arises from the endoderm?

Answer 35

Epithelial lining of the GI tract and epithelial of its glands (except oral cavity and anal canal)
Epithelial limit. Of urinary bladder, gallbladder, and liver
Epithelial lining of pharynx], auditory tubes (eustachian),tonsils,tymphanic cavity, larynx, trachea, bronchi and lungs
Epithelium of thyroid gland, parathyroid gland, pancreas, and thymus
Epithelial lining of prostate and bulbourethra, glands, vagina, vestibule, urethra
Gametes (sperm and oocytes)