Embryology Organogenesis

Question 1

What are the derivatives of the ectoderm?

Answer 1

• Central nervous system
• Peripheral nervous system
• Epidermis, Hair, Nails
• Sensory epithelium (nose, ear, eye)

Question 2

What are the derivatives of the mesoderm?

Answer 2

paraxial division: Skull, muscles, vertebrae

intermediate division: urogenital system

Lateral plate: serous membranes around organs (visceral layer) , body wall & limbs (parietal layer)

Question 3

What are the derivatives of the endoderm?

Answer 3

Gut tube and its derivatives: glands, lungs, liver, gallbladder, pancreas

Question 4

During week 3, the outer ectoderm undergoes neurulation. What are the three germ layers not our creative?

Answer 4

• Surface ectoderm (integumentary system)
• Neural tube (CNS)
• Neural crest cells (PNS)

Question 5

What structures of the central nervous system are derived from the neural tube?

Answer 5

• Brain
• Spinal cord
• Posterior pituitary

Question 6

Define the neural plate.

Answer 6

Pear-shaped thickening of ectoderm induced by the notochord and prechordal mesoderm.

(forms the CNS)

Question 7

How is the neural tube formed?

Answer 7

The lateral edges (neural folds) of the neural plate approach each other and fuse (cranially to caudally) from a cervical region

Question 8

Define neuropores. When do they close? What do they form?

Answer 8

• Partially incomplete fusion of the neural folds
• Day 25
• Cephalically forms the brain and the spinal cord caudally

Question 9

Define neural crest.

Answer 9

Special Band of ectodermal cells on the neural fold that migrate into the mesoderm, proliferate, and form important structures.

Question 10

Which structures are neural crest derivatives?

Answer 10

• Spinal ganglia
• Sensory ganglia of cranial nerves V, VII, IX, X
• Autonomic ganglia
• Adrenal medulla
• Schwann cells
• Connective tissue of the anterior part of the skull and meninges
• Melanocytes
• C cells of the thyroid gland
• conotruncal septum of the heart

Question 11

Answer 11

1. axial mesoderm becomes the notochord
2. paraxial mesoderm becomes somites
3. Intermediate mesoderm becomes the reproductive and urinary system
4. Lateral plate music becomes splanchnic and somatic divisions

Question 12

What are the paraxial mesoderm somites?

Answer 12

• Sclerotome (vertebrae, ribs, occipital bone)
• Somite dermatome (skin over spine in epaxial region)
• Syndetome (tendons)
• Myotome (skeletal muscle)

Question 13

Which tissues are derived from the somatic lateral plate mesoderm (SoLPM)?

Answer 13

Connective tissue, bones and smooth muscle

Question 14

Which tissues are derived from the splanchnic lateral plate mesoderm (SpLPM)?

Answer 14

• Connective tissue and smooth muscle associated with the visceral inner tube
• Endothelium of blood vessels arteries and veins
• cardiac muscle

Question 15

How does blood vessel formation occur?

Answer 15

• In the extraembryonic mesoderm surrounding the yolk sac at 3 weeks, then migrates.
• Later, it forms within the lateral plate mesoderm of the embryo.

Question 16

GI tract is formed due to what process?

Answer 16

lateral and cephalocaudal folding of the fetal trilaminar disc.

(somites grow down and pinch off gute tube and also pull the amnion around the embryo)

Question 17

What causes cephalocaudal folding of the fetal trilaminar disc?

Answer 17

• Growth of brain ventricles
• lengthening of embryonic axis

(closes the body wall around the umbilical ring, forming the Primitive umbilical cord from the connecting stalk and the vitelline duct (yolk stalk)).

Question 18

What sites in the uterus are considered normal sites for implantation?

Answer 18

Fundus (posterior, usually) AND body

Question 19

Upon implantation what happens to the trophoblast?

Answer 19

It differentiates into two layers:

1. the syncytiotrophoblast (outer layer)
2. the cytotrophoblast (inner layer)

Question 20

Define syncytiotrophoblast.

Answer 20

• Secretes human chorionic gonadotropin
• The outer layer which erodes endometrium and allows implantation

Question 21

What does the cytotrophoblast become?

Answer 21

Syncytiotrophoblast

Question 22

What does the fetus rely on for nourishment until the establishment of a placenta in week 12?

Answer 22

Embrytroph and the yolk sac

Question 23

Describe how the primitive uteroplacental circulation is established.

Answer 23

1. Syncytiotrophoblast invades endometrium
2. Lacuna are formed in the uterine wall and filled with embryotroph
3. Embryo invades deeper into the uterine wall and is completely embedded and covered by day 12

Question 24

Define embrytroph.

Answer 24

Ruptured uterine capillaries and glands.

(Lacunae in the syncitiotrophoblast are filled with embryotroph, which nourishes the fetus until placenta is established)

Question 25

How is the chorionic cavity formed and on what day does this occur?

Answer 25

Day 12 Develops within extraembryonic mesoderm (between the primitive yolk sac and the cytotrophoblast). Secondary yolk sac is pinched off from the primary yolk sac. (the chorion surrounds the chorionic cavity. It is formed by the extra embryonic mesoderm plus two layers of trophoblast the amnion and the yolk sac)

Question 26

What are the components of the chorion.

Answer 26

Extraembryonic mesoderm plus two layers of trophoblast (syncytiotrophoblast + cytotrophoblast) (the amnion and yolk sac remain attached to the chorion across the chorionic cavity by the connecting stalk)

Question 27

What are the extraembryonic membranes and their functions?

Answer 27

* *Amnion**: supports and nourishes fetus, contains amniotic fluid * *Primary yolk sac**: nourishes embryo early in development * *Chorion**: protective function, fetal contribution to placenta (there are also secondary and tertiary sacs, the tertiary yolk sac becomes incorporated into the migut)

Question 28

What is happening on *day 14* of embryonic development?

Answer 28

1. 3 membranes present: amnion, exocoelomic membrane w/primary yolk sac. 2. fully implanted into uterine wall 3. Nourishment from embryotroph found in lacunae

Question 29

What are the functions of the placenta?

Answer 29

* Metabolism * Transport of gases and nutrients * Endocrine secretions (hCG)

Question 30

When does the placenta begin forming and when is it mature?

Answer 30

* Week 2 * Week 20

Question 31

What are the two parts of the placenta?

Answer 31

* Decidua: maternal portion * Chorion: fetal portion

Question 32

Name the three distinct regions of the Decidua.

Answer 32

* Decidua basalis * Decidua capsularis * Decidua parietalis (Functional layer of the gravid endometrium)

Question 33

Is the chorionic cavity located in relation to the embryo?

Answer 33

The chorionic cavity surrounds the embryo, amniotic cavity, and yolk sac

Question 34

How is the chorion developed?

Answer 34

**Chorionic villi (primary, secondary and tertiary)**; projections of the chorion into the uterine endometrium (cover the chorion until week 8 when they are replaced by a smooth chorion)

Question 35

Describe the primary villi.

Answer 35

Consists of syncitiotrophoblast over a core of cytotrophoblast (week 2)

Question 36

Describe secondary villi (chorionic development)

Answer 36

Has a core of extraembryonic mesoderm

Question 37

Describe tertiary villi (chorionic development).

Answer 37

Core is invaded by umbilical blood vessels. (week 3)

Question 38

How is maternal and fetal blood circulation transferred?

Answer 38

***_Indirectly_*** via diffusion across the placental membrane.

Question 39

What are the three adaptations that constitute a mature placenta?

Answer 39

1. Thin chorionic membrane (regression of cytotrophoblast) 2. Large continuous intervillous space 3. Extensive branching of chorionic villi (the last two increase surface area)

Question 40

In the mature uteroplacental circulation, where is maternal blood found and where is fetal blood found?

Answer 40

* Intervillous spaces (derived from primitive lacunae) * Villous capillaries

Question 41

What supplies the intervillous spaces (maternal blood in mature uteroplacental circulation)?

Answer 41

Spiral endometrial arteries (drained by endometrial veins)

Question 42

Blood is carried from the placenta to the fetus by the ____ and returned to the placenta by the \_\_\_\_.

Answer 42

* umbilical vein * umbilical arteries