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Question 1

EVENTS from 3rd to 8th week (embryonic peroid)?

Answer 1

• Dervitives of ectoderm and process of Neurulation • Dervitives of mesoderm • Development of Intraembryonic Coelom and formation of body cavties • Folding of the embryo • Dervitives of endodermand fromation of gut tube

Question 2

What happend During embryonic period (period of organogenesis) ?
And By the end of the embryonic period?

Answer 2

 During embryonic period (period of organogenesis) 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, the major features of the external body
form recognizable by the end of the second month.

Question 3

1. At which week does the median thickening in the ectoderm form the neural plate?
   
   2. What type of epithelium is involved in forming the neural plate?
   3. What structures are formed from the neural plate?

Answer 3

1. At the third week, median thickening in the ectoderm forms the neural plate.
   
   2. The neuroepithelium is involved in forming the neural plate.
   3. The neural groove and neural folds are formed from the neural plate.

Question 4

1. What is formed by the fusion of neural folds?
   
   2. What do the cranial and caudal ends of the neural groove form?
   3. When do the anterior and posterior neuropores close?

Answer 4

1. Neural tube.
   
   2. Anterior and posterior neuropores.
   3. The anterior neuropore closes at the 25th day, and the posterior neuropore closes at the 28th day.

Question 5

1. What happens to the neural tube after the closure of the neuropores?
   
   2. What does the cranial part of the neural tube form?
   3. What does the caudal part of the neural tube form?

Answer 5

1. After the closure of the neuropores, the neural tube is completely separated from (buried under) the surface ectoderm.
   
   2. The cranial part of the neural tube forms the brain and its ventricles.
   3. The caudal part of the neural tube forms the spinal cord and its central canal.

Question 6

What happens to the lateral margins of the neural plate that are not incorporated into the neural tube?

Answer 6

The lateral margins of the neural plate that are not incorporated into the neural tube will form the neural crest.

Question 7

1. During which week does the formation of the neural tube (neurulation) begin?
   
   2. What cells thicken and form the neural plate, and under whose inductive influence does this occur?
   3. What does the neural plate show after its formation?
   4. How is the neural tube formed, and what happens to the surface ectoderm during this process?

Answer 7

1. Formation of the neural tube (neurulation) begins in the 3rd week.
   
   2. Surface ectodermal cells dorsal to the notochord (neuroectoderm) thicken and form the neural plate under the inductive influence of the notochord.
   3. The neural plate shows neural folds (edges) and a neural groove in between.
   4. The neural folds fuse in the midline, converting the neural groove into a neural tube and restoring the continuity of the surface ectoderm.

Question 8

5. Where does fusion of the neural folds begin, and in which directions does it proceed?
   
   6. Through what structures does the neural tube communicate with the amniotic cavity, and when do they close?
   7. What happens to the cells from the margins of the neural plate that are not incorporated into the neural tube?
   8. What does the cephalic part of the neural tube form?
   9. What does the caudal part of the neural tube form?

Answer 8

5. Fusion begins cervically and proceeds both cranially and caudally.
   
   6. The neural tube communicates with the amniotic cavity through cranial (anterior) and caudal (posterior) neuropores, which close on the 25th and 28th day, respectively.
   7. The cells from the margins of the neural plate that do not become incorporated into the neural tube form the neural crest.
   8. The cephalic part of the neural tube forms the brain.
   9. The caudal part of the neural tube forms the spinal cord.

Question 9

1. What results when neural tube closure fails to occur?
   
   2. What condition occurs if the neural tube fails to close in the cranial region?
   3. What condition occurs if the neural tube fails to close anywhere from the cervical region caudally?

Answer 9

1. Neural tube defects (NTDs) result when neural tube closure fails to occur.
   
   2. If the neural tube fails to close in the cranial region, it results in anencephaly.
   3. If the neural tube fails to close anywhere from the cervical region caudally, it results in spina bifida.

Question 10

1. At what time does the flat embryonic disc start folding ventrally?
   
   2. What are the two types of folds in the embryonic disc?
   3. In which directions does the embryonic disc fold simultaneously?
   4. What causes the folding of the embryonic disc?
   5. What contributes to the formation of the head and tail folds?
   6. What contributes to the formation of the two lateral folds?

Answer 10

1. At the time of formation of somites, by the 4th week, the flat embryonic disc starts folding ventrally.
   
   2. The two types of folds are the head and tail folds and the two lateral folds.
   3. The embryonic disc folds in two directions simultaneously:
      • In the cephalocaudal direction, forming the head fold and tail fold.
      • In the lateral direction, forming two lateral folds.
   4. The causes of folding are:
      • Extensive rapid growth of the cranial end of the neural tube.
      • Faster growth of the axial part of the embryonic disc than its periphery.
      • Enlargement of the amniotic cavity.
   5. The head and tail folds are formed by folding in the cephalocaudal direction.
   6. The two lateral folds are formed by folding in the lateral direction.

Question 11

1. What happens to the flat embryonic disc during folding?
   
   2. What happens to the amniotic cavity as a result of folding?
   3. What occurs to the yolk sac during folding?
   4. What are the parts of the primitive gut, and how are they organized?
   5. Which structure closes the foregut?
   6. Which structure closes the hindgut?
   7. How does the midgut communicate with the yolk sac outside the embryo?

Answer 11

1. The flat embryonic disc becomes cylindrical, and the embryo acquires its characteristic form.
   
   2. The amniotic cavity, which was dorsal, now surrounds the embryo almost completely.
   3. The yolk sac is absorbed into the body to form the primitive gut.
   4. The parts of the primitive gut are:
      • Foregut: the most cranial part.
      • Hindgut: the most caudal part.
      • Midgut: the middle part.
   5. The foregut is closed by the buccopharyngeal membrane.
   6. The hindgut is closed by the cloacal membrane.
   7. The midgut communicates with the definitive yolk sac (outside the embryo) through the yolk sac stalk (vitelline duct).

Question 12

1. What happens to the position of the buccopharyngeal membrane relative to the pericardial cavity after folding?
   
   2. Where do the pericardium and heart lie after folding?
   3. What change occurs to the position of the septum transversum relative to the pericardial cavity after folding?
   4. What happens to the position of the cloacal membrane relative to the connecting stalk after folding?
   5. What happens to the connecting stalk’s position and what does it form after folding?

Answer 12

1. The buccopharyngeal membrane, which was caudal to the pericardial cavity, becomes cranial to it after folding.
   
   2. The pericardium and heart come to lie on the ventral aspect of the embryo.
   3. The septum transversum, which was cranial to the pericardial cavity, becomes caudal to it after folding.
   4. The cloacal membrane, which was cranial to the connecting stalk, comes to occupy the caudal end of the embryo after folding.
   5. The connecting stalk, which was attached to the caudal end of the embryonic disc, comes to lie on the ventral aspect of the embryo and forms the umbilical cord.

Question 13

1. What does the epidermis of the skin and its appendages include?
   
   2. What are the orifices lined by the surface ectoderm?
   3. Which glands are derived from the surface ectoderm?

Answer 13

Surface Ectoderm:
1. The epidermis of the skin includes hairs and nails.
2. Orifices lined by the surface ectoderm include:
• The mouth, enamel of teeth, lower half of the anal canal.
• Parts of the ear, nose, and eye.
3. Glands derived from the surface ectoderm include subcutaneous glands, mammary glands, salivary glands, and the anterior pituitary.

Question 14

4. What structures are derived from the neural tube?
5. What does the posterior pituitary originate from?

Answer 14

Neural Ectoderm:
4. Structures derived from the neural tube include the CNS (brain and spinal cord), part of the PNS (motor nerve fibers), and the posterior pituitary.
5. The posterior pituitary originates from the neural ectoderm.

Question 15

6. What is the definition of neural crest cells?
7. Why are neural crest cells important?
8. What are the derivatives of the neural crest related to the peripheral nervous system (PNS)?
9. What glial cells are derived from the neural crest?
10. Which meninges are derived from the neural crest?
11. What pigment-producing cells are derived from the neural crest?
12. What part of the adrenal gland is derived from the neural crest?
13. What skeletal elements are derived from the neural crest?

Answer 15

Neural Crest:
6. Neural crest cells are cells derived from the edges of neural folds that migrate to give rise to many organs and tissues.
7. Neural crest cells are important because they are referred to as the fourth germ layer and are involved in one-third of all birth defects and many cancers, such as melanomas and neuroblastomas.
8. Derivatives of the neural crest related to the PNS include:
• Sensory ganglia (cranial ganglia and dorsal root ganglia) and sensory nerve fibers.
• Autonomic ganglia and postganglionic nerves of the autonomic nervous system.
• Schwann cells.

9.	Glial cells derived from the neural crest include all except microglia (which are mesodermal).
10.	The meninges derived from the neural crest are the pia mater and arachnoid mater (the dura mater is mesodermal).
11.	Melanocytes are derived from the neural crest.
12.	The adrenal medulla is derived from the neural crest (the adrenal cortex is mesodermal).
13.	Skeletal elements in the head and neck are derived from the neural crest.

Question 16

DERIVATIVES OF NEURAL CREST بإختصار?

Answer 16

3 G: Sensory Ganglia, Autonomic ganglia, Glial cells. 2 S: Schwan cells, Skeletal elements. 2 M: Melanocytes, Meninges. 1 A: Adrenal medulla.

Question 17

1. What are the main structures derived from the endoderm, including the epithelial linings and specific organs?
   
   2. Which glands are derived from the endoderm?

Answer 17

1. The endoderm gives rise to the epithelial lining of the following structures:
   • Gastrointestinal tract (GIT).
   • Respiratory tract (except the nose and paranasal sinuses, which come from the ectoderm).
   • Parts of the urinary bladder and urethra.
   • Tympanic cavity and auditory tube.
   
   2. The glands derived from the endoderm include the parenchyma of the thyroid and parathyroid glands.

Question 18

What changes occur in the external appearance of the embryo by the end of the second month of development?
2. What is the length of the embryo at the end of the second month of development?
3. Which major structures are evident in the embryo by the end of the second month?

Answer 18

1. By the end of the second month of development, the external appearance of the embryo changes due to an increase in head size and the formation of the limbs, face, ears, nose, and eyes.
   
   2. The length of the embryo at the end of the second month of development is 28 mm.
   3. By the end of the second month, major internal organs are evident in the embryo.