Unit 2.L2-Pharyngeal Apparatus, the Gut tube & Urogential System

Question 1

By Day 28, what is formed as paired structures and where can they be seen?

Answer 1

The Pharyngeal Apparatus/arches (or branchial arches) forms as paired structures, externally visible on the ventrolateral side of the early brain/head

Question 2

What does the Pharyneal Apparatus comprise of? (7)

Answer 2

1. Pharyngeal Arches
2. Pharyngeal Grooves
3. Pharyngeal Pouches (cannot be seen from the outside; endodermal side)
4. Pharyngeal Membranes
5. Pharyngeal Nerves
6. Pharyngeal Muscles
7. Pharyngeal Artery

Question 3

What contributes to the formation of the Pharyngeal Apparatus?

Answer 3

All 3 germ layers, Ectoderm, Mesoderm & Endoderm contribute to the formation of the pharyngeal apparatus

Question 4

How many arches form from the Pharyngeal Apparatus?

Answer 4

Five arches form (1,2,3,4, & 6)

NOTE: Humans only form 5 arches, Arch V is absent in humans

Question 5

How many arches are externally visible and what are they? Which arches fuse?

Answer 5

• Only 4 are externally visible (1,2,3 & 4/6)
• 4 & 6 fused

Question 6

What forms the skeleton of the arch?

Answer 6

A cartilaginous rod forms the skeleton of the arch.

Question 7

What differentiates into the muscle of head/neck of the arch?

Answer 7

Mesoderm differentiates into muscles of head /neck.

Question 8

What supplies blood/nutrition to each arch?

Answer 8

Each Aortic Arch Artery supplies blood/nutrition.

Question 9

Sensory/motor nerves dervied from where, invade what in each arch?

Answer 9

Sensory/motor nerves derived from neuroectoderm of the brain invade the mucosa/muscles in each arch

Question 10

When and how does the first pair of arches appear in the developing embryo?

Answer 10

By day 23 (3 weeks) the first pair of arches, the primordial jaw appears as surface elevations lateral to the developing pharynx (dorsal view)

Question 11

During the emergence of the Pharyngeal Apparatus (Day 23-28) what develops by:

• Day 24
• Day 26
• Day 28

Week 3

Answer 11

• Day 24: 1st & 2nd Pharyngeal arch, 1st pharyngeal groove & optic placode
• Day 26: 1st, 2nd, & 3rd Pharyngeal arch, Optic pit
• Day 28: 1st, 2nd, 3rd & 4th Pharyngeal arch, 1st & 2nd pharyngeal groove

All lateral views

Question 12

If you rotate the embryo to the Ventral/Facial view what is developing simultaneously to the Pharyngeal arches at Day 23, 26 & 28.

Week 3

Answer 12

• Day 23: Oropharyngeal membrane, maxillary prominence, mandibular prominence
• Day 26: Maxillary prominence, mandibular prominence, Ruptured Oropharyngeal membrane, heart prominence
• Day 28: Stomodeum, Nasal placode

Question 13

• Which germ layer does the Pharyngeal arch originate form?
• Pharyngeal pouches?
• Pharyngeal membrane?

Answer 13

• Mesoderm→Arches
• Endoderm→Pouches
• Mesoderm,Endoderm & Ectoderm→Membrane

NOTE: Ectoderm→Clefts/Groove

Question 14

What are the components of a developing Pharyngeal Apparatus? (8)

Answer 14

1. Mesoderm→Arches
2. Endoderm→Pouches
3. Ectoderm→Clefts/Groove
4. Pharyngeal Membrane
5. An aortic arch artery, supplying blood
6. A cartilaginous rod forming the skeleton
7. Arch nerve to mucosa/muscles
8. A muscular component (not seen)

Question 15

By day 28, what develops along side the Pharyngeal arches? (2)

Answer 15

• The tongue buds (bilaterally)-gives rise to your tongue
• Laryngotracheal groove (between the arches)-gives rise to your larynx and trachea

Question 16

What gives rise to the larynx and trachea?

Answer 16

Laryngotracheal groove (between the arches)

Question 17

Between 4-8 weeks what is formed? What cells play an important role in the formation?

Answer 17

• The pouches/face forms
• Neural crest cells migrate into the pouches

Question 18

When do the facial proportions develop?

Answer 18

9th wk-birth: The facial proportions develop in the fetal period

Question 19

When does a definitive face form? And what causes it?

Answer 19

In childhood at puberty: After development of teeth & paranasal sinuses, a definitive face forms

Question 20

What causes the change in your facial structures during puberty?

Answer 20

Paranasal sinuses

Question 21

What is the origin of the core mesenchyme of the pharyngeal apparatus? What is the core mesenchyme covered by?

4.5 week (Day 32)

Answer 21

. Paraxial mesoderm→Core mesenchyme: Covered by ectoderm + endoderm

Question 22

What is the origin of the Arch Endothelial cells and what do they eventually form?

4.5 week (Day 32)

Answer 22

Lateral mesoderm→Arch Endothelial cells→Invasive angioblasts, forming the vasculature (gets attached to the aortic arches)

Arch endothelial cells surround the heart

Question 23

By day 32, Neural crest migrate and largely form what? (2)

Answer 23

• Arch mesenchyme
• Connective tissue (cartilage, bone, muscles, ligaments & dermis) in 1,2,3,4 arch

Question 24

Ventrally where does the blood supply to the pharyngeal apparatus arise from? Where does it pass?

Answer 24

Ventrally: Arteries arises from the Truncus arteriosus of the primordial heart & passes around the primordial pharynx, supplying blood to the pharyngeal apparatus

Question 25

Dorsally, where does the blood supply to the pharyngeal apparatus arise from? Where does it enter?

Answer 25

Dorsally: Aortic Arch Arteries **connects & enters the Dorsal Aorta** on the other side

Question 26

The Pharyngeal arch cartilage derivates of the **1st Arch** gives rise to which structures? (ventrally and dorsally)

Answer 26

* Ventrally: **Meckel’s cartilage**→**Ossified** (membranous bone)→**maxillary/mandibular prominences** * Dorsally: **Middle ear bones (malleus, incus)**

Question 27

The Pharyngeal arch cartilage derivates of the **2nd Arch** gives rise to which structures? (4)

Answer 27

* Stapes * Styloid process * Lesser cornu/horn of hyoid bone * Superior body of hyoid bone

Question 28

The Pharyngeal arch cartilage derivates of the **3rd Arch** gives rise to which structures? (2)

Answer 28

* Greater cornu/horn of hyoid * Inferior body of hyoid bone

Question 29

The Pharyngeal arch cartilage derivates of the **4th Arch** gives rise to which structures? "Too Come And Count Cute Things"

Answer 29

* Neck Cartilages: Thyroid, Cricoid, Arytenoid, Corniculate, Cuneiform & Triticeal ## Footnote NOTE: Thymus, thyroid, parathyroid comes from the 3rd apparatus but Thyroid cartllage comes from the 4th

Question 30

By Week 4, the 1st Pharyngeal Arch has what type of depression and what does it form?

Answer 30

A **ventral depression** →**stomodeum** (future mouth) is **contagious with the gut tube**

Question 31

The 1st Arches flank the stomedum (future mouth) and divides into what?

Answer 31

Flank the stomodeum & divide: **dorsal maxillary process & ventral mandibular process**

Question 32

Ventrally, the Mandibular process of the 1st Arch becomes what? And what is it eventually replaced with?

Answer 32

Ventrally: Mandibular process→**Meckel’s cartilage** (NCC origin), which is replaced by **intramembranous bony mandible**

Question 33

Dorsally, the Mandible process of the 1st Arch forms what?

Answer 33

Dorsally: Mandible forms **incus & malleus (middle ear bones)**

Question 34

List the structures derived from the 1st pharyngeal arch: * Bones * Ligaments * Muscles * Nerve * Blood supply

Answer 34

* Bones: Incus, mallelus, maxilla, mandible (also squamous part of the temporal lobe, zygomatic bone, palatine bones) * Ligaments: **Sphenomandibular ligaments,anterior ligament of malleus** * Muscles: **Muscles of mastication (temporalis, masseter,ptyerygoids), anterior belly of diagastrc, mylehyoid, tensor vali palatini and tensor tympani** * Nerve: **Trigemenal nerve (CN V)** (Maxillary and mandibular branches-V2 & V3) * Blood supply: **Maxillary artery** (first branch from the first aortic arch)

Question 35

By week 5, the 1st Pharyngeal groove forms what part of the ear?

Answer 35

Week 5: 1st groove starts forming the **external auditory meatus of the ear** ## Footnote Reminder the groove is of ectodermal origin

Question 36

By week 5, what happens to the 2nd, 3rd, & 4th Pharyngeal groove and why?

Answer 36

Grooves 2,3 & 4 fuse and disapper because the growth in the mesodern is so fast ## Footnote Fusing grooves causes a lacunae remaining in the mesoderm

Question 37

When the 1st Pharyngeal Pouch forms a lengthy ingrown what does it become? | Endodermal origin

Answer 37

Forms a lengthy ingrowth→**tubotympanic recess** (endodermal origin) ## Footnote Remember: The pharyngeal pouch has an endodermal surface so the ingrowth of tubotympanic recess is endodermal

Question 38

What does the Tubotympanic recess (of the 1st Pharyngeal pouch) fuse with? And what does it form?

Answer 38

Tubotympanic recess ,which **proximally** fuses with **pharyngeal structures** to form the **Pharyngotympanic tube (Middle ear cavity + Eustachian tube)** & connects to the **nasopharynx**

Question 39

The junction of the External acoustic meatus + tubotympanic recess forms what? And include it's function? | 1st Pharyngeal pouch

Answer 39

Junction of External acoustic meatus + tubotympanic recess forms the **“tympanic membrane (Ear drum)”; separates outer & middle ear** ## Footnote External acoustic meatus (ectodermal origin) Tubotympanic recess (endodermal origin)

Question 40

What is the function of the Eustachian tube? | 1st Pharyngeal pouch ## Footnote Part of the pharyngotympanic tube w/ middle ear cavity

Answer 40

Equalizes atmospheric pressure in the middle ear

Question 41

How does the 2nd Pharyngeal Arch grow? And what does it cover? What does it form?

Answer 41

Arch II grows **rapidly and inferiorly** to cover the **smaller arches** (3&4) forming the **operculum**. This growth forms a **‘lid’ over other arches** and creates the smooth covering of the neck

Question 42

What is formed from the 2nd Arch connective tissue? (6)

Answer 42

* **Stapes bone/stapedius muscle** (smallest bone & muscle; 1mm), are formed from the 2nd arch connective tissue. * **Bony styloid process, the lesser cornu (horn) of the hyoid bone** & **the stylohyoid ligament** are formed by 2nd arch connective tissue

Question 43

What is the nerve of the 2nd Arch and what does it innervate?

Answer 43

2nd arch **facial nerve (CN VII)** innervate the **muscle and tongue mucosa**

Question 44

List the structures formed from the 2nd Pharyngeal Arch: * Bones * Ligaments * Muscles * Nerve * Blood supply

Answer 44

* Bones: Stapes, styloid process of temporal bone, lesser horn and superior part of the body of the hyoid bone * Ligaments: Stylohyoid ligament * Muscles: Muscles of facial expression, stapediusm mylohyoid, posterior belly of diagastric, auricular, buccinator, platysma * Nerve: Facial nerve (CN VII) * Blood supply: Hyoid artery (foetal), stapedial artery (foetal)

Question 45

How does the 2nd Pharyngeal Groove get isolated? What does the isolation form?

Answer 45

2nd Pharyngeal groove gets isolated by **growth of the second arch**, which forms the **operculum** (flap; evolutionarily conserved- gills or smooth neck)

Question 46

# 2nd Pharyngeal Groove What does the Operculum fuse with and what does it form? ## Footnote Operculum formed by 2nd arch

Answer 46

Operculum fuses with **3rd & 4th grooves** & forms the **cervical sinus** lined with **ectodermal epithelia** →Later forms glands

Question 47

# 2nd Pharyngeal Groove What is the Cervical sinus lined with and what does it later form?

Answer 47

Lined with **ectodermal epithelia** →**Later forms glands**

Question 48

# 2nd Pharyngeal Groove What happens to the Cervical sinus later on and what do the ectodermal cells lining it form?

Answer 48

Later, the **Cervical sinus disappears** & the **Ectodermal cells** → **Hassall’s corpuscles** found in the **thymus gland**

Question 49

# 2nd Pharyngeal Groove Where are Hassall's corpsuscle found?

Answer 49

Thymus gland

Question 50

The 2nd Pharyngeal Pouch endodermal epithelium proliferates and invades the mesenchyme to form what?

Answer 50

2nd Pharyngeal pouch endodermal epithelium **proliferates and invades** into the **mesenchyme** to form **lymphoid tissue**→**Palatine tonsils**

Question 51

What forms the Palatine tonsils from the 2nd Pharyngeal Pouch?

Answer 51

Palatine tonsils form (**mesodermal lymphoid tissue + crypts lined by endodermal epithelia**) ## Footnote * Lymphoid tissue form lymphocytes * Endodermal Epithelium invades the mesoderm as crypts

Question 52

What does the 2nd Pharyngeal Pouch endodermal epithelium form?

Answer 52

2nd Pharyngeal pouch endodermal epithelium **proliferates and form Tonsillar crypts**

Question 53

#2nd Pharyngeal Pouch Where do the Tonsillar crypts invade and what do they form?

Answer 53

The Tonsillar crypts invade into the **neck mesenchyme**, which form **lymphoid tissue**

Question 54

At Birth: Palatine tonsils are formed by two components: 1____________ +2________________

Answer 54

At Birth: Palatine tonsils are formed of two components: 1. **Mesodermal lymphoid tissue** + 2. **Crypts lined by endodermal epithelia**

Question 55

What does the 3rd Pharyngeal connective tissue form?

Answer 55

3rd arch connective tissue forms: **Greater horn (cornu)** & **inferior parts of the hyoid bone**

Question 56

What does the 3rd Pharyngeal mesoderm form?

Answer 56

3rd arch mesoderm forms the **Stylopharyngeus muscle**

Question 57

What is the 3rd Arch innvervate by? And what does the nerve innervate?

Answer 57

* 3rd Arch innervated by the **glossopharyngeal nerve (CN IX)** * CN IX, also innervates **pharyngeal plexus in the Palate + Pharynx**

Question 58

What are the 3rd Aortic Arch ateries?

Answer 58

3rd aortic arch artery→ **Adult common & internal carotid arteries**

Question 59

What happens to the 3rd & 4th Pharyngeal grooves and what disappers?

Answer 59

3rd & 4th Pharyngeal groove **fuse**, & the **cervical sinus disappears**

Question 60

What does the 3rd pouch endoderm contribute to?

Answer 60

3rd pouch endoderm contributes to **neck endocrine glands**

Question 61

List the structures formed from the 3rd Pharyngeal Arch: * Bones * Muscles * Nerve * Blood supply

Answer 61

* Bones: Greater horn and the inferior part of the body of the hyoid bone * Muscles: Stylopharyngeus muscle * Nerve: Glosspharyngeal nerve (CN IX) * Blood supply: Common carotid artery and the first part of the internal carotid artery

Question 62

What happens with the 4th and 6th Pharyngeal Arches and what structure do they become?

Answer 62

4th/6th arches **fuse**→common **neck structure**

Question 63

The 4th/6th arches connective tissues fuse and form what?

Answer 63

4/6th arch connective tissue fuse→**Laryngeal Cartilages (Thyroid, Cricoid, Arytenoid, Corniculate & Cuneiform cartilage)**; but **NOT the epiglottis**

Question 64

What does the 4th/6th arch connective tissue NOT form?

Answer 64

Epiglottis

Question 65

What creates phonation? (3)

Answer 65

the **Constrictors of the pharynx, palate (levator veli palatine) & Cricothyroid** (phonation) ##Footnote Originate from the 4/6th arch mesoderm

Question 66

6th arch forms the intristic muscle of what?

Answer 66

6th arch→intrinsic muscles of the **larynx**

Question 67

What innervates the 4/6th arches?

Answer 67

**Vagus nerve (CN X)** innervate 4/6th arches in **TWO BRANCHES**: * **4th arch→superior laryngeal branch of the Vagus (CN X)** * **6th arch→recurrent laryngeal nerve CN X** that **innervate the larynx**

Question 68

The superior laryngeal branch of the Vagus (CN X) innervates what branch?

Answer 68

4th Arch

Question 69

The recurrent laryngeal nerve CN X innverates what?

Answer 69

6th arch; Larynx

Question 70

List the structures formed from the 4th/6th Pharyngeal Arch: * Cartilage * Muscles * Nerve * Blood supply

Answer 70

* Cartilage: Thyroid, cricoid, arytenoid, corniculate, cuneiform Muscles: * Arch IV: cricothyroid, levator veli palatini and the pharyngeal constrictors * Arch VI: intrisic muscle of the larynx (except cricothroid) Nerve * Arch IV: Superior laryngeal, branch of vagus nerve * Arch VI: Reccurent laryngeal, branch of the vagus nerve Blood supply (At this level in the embryo we see differences in the cardiovascular system develop bte left and right sides) * Arch IV: Left-aortic arch; right-subclavian artery * Arch VI: left-pulmonary artery and ductus arterious; right-pulmonary artery

Question 71

What causes Superfical face distribution?

Answer 71

**Two Caudal Trigeminal branches of 1st pouch (CN V)** →Superficial facial distribution.

Question 72

List the distribution of nerves from all arches

Answer 72

* 1st Pharyngeal Arch: Trigemenal (CN V)→V2 (Maxillary division), V3 (Mandibular) * 2nd Pharyngeal Arch: Facial (CN VII) * 3rd Pharyngeal Arch: Glosspharyngeal (CN IX) * 4th Pharyngeal Arch: Vagus (CN X)

Question 73

Which pouch does the primary lymphoid organ develop and what is it made of?

Answer 73

* Primary lymphoid organ develops from **3rd pouch** * Made of **(endoderm + mesoderm + neural crest cells)**

Question 74

Explain the steps for the formation of the Thymus * What descends into the mesenchyme? Cords form what? * What fills the septa and develop into what? * The cervical sinus ectodermal cells converts to?

Answer 74

1. **Epithelial tubes/solid cords descend** into the mesenchyme. Cords form **reticulated thymus lobules**, trap the **mesenchyme** & form **septa** 2. **Lymphocytes fill the septa** & develop into **thymocytes (precursor T cells)** 3. The **cervical sinus ectodermal cells** converts to **Thymus “Hassall’s corpuscles” (cytokines for T-cell maturation)**

Question 75

What is the structure of the Parathyroid gland?

Answer 75

2 Superior (top of thyroid) + 2 Inferior glands (Thymus) ## Footnote NOTE: Thyroid forms forms from both pouch 3 and 4 but 4 is critical for the superior parathyroid

Question 76

Explain the steps for the formation of the Parathyroid gland * What happens at week 5? * What secretes PTH? What does it regulate? * 4th pouch ventrally forms what? And produces what? * What is differentiate from neural crest cell?

Answer 76

1. Week 5: **Dorsal epithelium** of **3 & 4th pouches** proliferate & form small **dorsal nodules** & become **vascularized** 2. **Parathyroid CHIEF CELLS (PRINCIPAL CELLS)** differentiate & **secrete parathyroid hormone (PTH)**, regulating fetal **Ca++ metabolism & bone formation** 3. 4th pouch ventrally forms the **ultimopharyngeal body**, which **fuses with the thyroid gland** & produce the **PARAFOLLICULAR CELLS (C cells that produce Calcitonin)** 4. **C CELLS** differentiate from neural crest cells .

Question 77

By Week 5, what does the nasal placodes form?

Answer 77

The Nasal placodes form **nasal pits→nasal sacs**

Question 78

What deepens the nasal pits and what does it eventually form? | Week 5

Answer 78

**Nasal mesenchymal prominences** deepen the pits→Primordial nasal sacs→ **Nostrils** ## Footnote The primordial nasal sacs are ectodermal in origin but are formed by mesoderm surrounding the nasal pit and pushing to form sac like structure

Question 79

What seperates the nasal and oral cavity?

Answer 79

The  **Oronasal membrane** separates the nasal & oral cavity ## Footnote Oronasal membrane formed by ectodermal cell

Question 80

By week 6, what happens to the oronasal membrane and what does it connect?

Answer 80

Oronasal membrane **ruptures** (lacunae)→**connects the nasal and oral cavities**

Question 81

By Week 7, what connects the oral cavity to nasal pharynx and what is formed?

Answer 81

**Primordial choana** connects oral cavity to nasal pharynx, and an **epithelial plug forms**; (lost by wk-16). ## Footnote Plug fromed to keep fluid from entering nasal cavity

Question 82

What type of fibers from the brain enter the nasal cavity? | Week 7

Answer 82

**Olfactory nerve fibers** from the brain come into nasal cavity

Question 83

What palates forms during Week 12?

Answer 83

**Palates (hard primary + secondary + soft)** forms ## Footnote Plug still present

Question 84

Week 12: * What matures from choana? * What does the Ectodermal epithelium (nasal roof) differentiate into? * Where do the Olfactory nerve fibers connect?

Answer 84

* **Nasal conchae** matures from choana. Ectodermal epithelium (nasal roof)→differentiate into **olfactory epithelium**→**olfactory receptor cells** Olfactory nerve fibers (axons) connect **dorsally** into the **olfactory bulbs of the brain** ## Footnote Nasal conchae: flap like structure

Question 85

Which paranasal sinus develops in late fetal stage? When do the others form? | Week 12

Answer 85

**Only Maxillary sinuses** develop in late fetal life; **others form after birth**

Question 86

When is the Palatogenesis phase? And when is the most critcial period?

Answer 86

* **Week 6-12: Palatogenesis phase** * **Critical (6th-9th week)** (If there are environmental factors like alcohol, tabacoo fetus will develop.a cleft palate)

Question 87

When is the formation of the complete palate primorida? What is formed?

Answer 87

**9-12th week**: Complete palate primordia of (**Primary + Secondary palate**)

Question 88

Where does the Primary palate (Median palatine process) originate from?

Answer 88

Primary palate (Median palatine process): Originates from the **inter-maxillary segment** (mesoderm migrating and condensing)

Question 89

How is the Inter-maxillary segment formed? | Origin of the Primary palate (median palatine process)

Answer 89

Formed by **merging** of the  **medial nasal prominences** (a wedge-shaped mass of mesenchyme) between the  **maxillary prominences**

Question 90

What develops later to give rise to the primary palate or small hard palate?

Answer 90

Later, **primordium of premaxillary part of the maxilla** develops giving rise to **primary palate or small hard palate**

Question 91

Explain the formation of the Secondary Palate (Definitive Plate): * Week 6-12: What emerges from the maxillary prominences?

Answer 91

Week 6-12: **Two mesenchymal projections** emerge laterally from the maxillary prominences, called the **lateral palatine processes (palatal shelves)**

Question 92

Explain the formation of the Secondary Palate (Definitive Plate): * Week 6: Where are the palatal shelves located? * Week 7: What elongates? * Week 8: What helps fuse the two palatal shelves? * Week 12: How is the hard palate formed?

Answer 92

* Week-6: the palatal shelves lie **inferomedially to the tongue** * Week-7: **Jaws elongate** & pull the **tongue lower** Week-8: **Hyaluronic acid** in the **palatine mesenchyme** helps fuse the two palatal shelves. Week-12: Bone from the maxillae moves into the palatine shelves, forming **palatine bones**, which forms the hard palate.

Question 93

After the fromation of the hard palate what remains dorsally? And what does it fuse with?

Answer 93

Dorsally: **Soft palate** remains & fuses with the **Uvula**

Question 94

What are the (3) functions of the Uvlva?

Answer 94

1. Guards the nasal passage 2. Makes saliva, lubricates throat 3. Critical for phonation; highly evolved

Question 95

# Respiratory Primordium By 4th week, what forms the Laryngotracheal groove?

Answer 95

4th week: a **median endodermal outgrowth** in the **floor of the 4th pouch of the anterior foregut** forms the Laryngotracheal groove

Question 96

# Respiratory Primordium By the 5th week, what does the Laryngotracheal groove (4/6 pouch endoderm) undergo? What does it form and later gives rise to?

Answer 96

5th week: Laryngotracheal groove (4/6 pouch endoderm) undergoes **ventral evagination**, forming a **pouch-like laryngotracheal diverticulum (lung bud)**, which later gives rise to the **Tracheobronchial tree** ## Footnote Ventral evagination into the mesoderm

Question 97

# Respiratory Primordium What does the Tracheobronchial tree endoderm give rise to? (4)

Answer 97

Tracheobronchial tree endoderm * **Pulmonary epithelium** * **Larynx** * **Trachea** * **Bronchi**

Question 98

# Respiratory Primordium What does the Tracheobronchial (splanchnic) mesoderm surround? And give rise to?

Answer 98

The tracheobronchial tree splanchnic mesoderm **surrounding the foregut diverticulum** gives rise to the **connective tissue, cartilage, and smooth muscle** of the lung

Question 99

At the end of the 5th week: * _____________develops, partitioning the foregut into _____________ and _______________ * The __________ (endodermal tube + surrounding mesenchyme), undergo _________ to give rise to ___________ & -_____________ bronchial buds.

Answer 99

* **Tracheoesophageal septum** develops, partitioning the foregut into **esophagus** (dorsal) and **laryngotracheal tube** (ventral) * The **Respiratory bud** (endodermal tube + surrounding mesenchyme), undergo **branching** to give rise to **primary & secondary** bronchial buds.

Question 100

# Progressive stages in the development of the trachea By week 4, the endoderm tube gives rise to?

Answer 100

The endoderm tube gives rise to the **pulmonary epithelium**

Question 101

By week 12: * What appears in the trachea? * What does the surrounding mesenchyme form in the trachea? (3)

Answer 101

* 12 weeks: **Epithelial glands (goblet/mucus cells & ciliary cells)** of the trachea appear * 12 wks: The surrounding mesenchyme forms the **connective tissue, muscle, & Cartilaginous rings** of the trachea

Question 102

Development of bronchi, Pleura and Lung Lobes 1. What invades the mesenchyme and where does it emerge? 2. The buds soon divide into what? 3. Two buds grow into what? 4. Which bronchial buds develop and what forms?

Answer 102

1. A **respiratory bud** invades the mesenchyme & emerges at the **caudal end** of the **laryngotracheal diverticulum** 2. The bud soon divides into **two outpouchings: Two primary bronchial buds** 3. Two buds grow **laterally** into the **pericardioperitoneal canals (primordial pleural cavities)** 4. **Secondary/Tertiary** bronchial buds develop & **Visceral pleura** & **Pleural cavity** is formed

Question 103

# Sucessive Development of bronchial buds, bronchi, & lung lobes * The main bronchi branches into? * The Superior Lobar bronchus are what lobes and branches? * The Inferior lobar bronchus are what lobes and branches?

Answer 103

* The **main bronchi**→Secondary bronchi→**Lobar, segmental, and intra-segmental branches** * The **Superior Lobar bronchus**→Right & Left Upper lobes→Segmental/intra-segmental branches * The **Inferior Lobar bronchus**→Right Middle and Right Lower lobe+Left Lower lobe→Segmental/intra-segmental branches

Question 104

# Sucessive Development of bronchial buds, bronchi, & lung lobes * At Week 7, how many segmental branches are in the Right and Left Lung? What do they give rise to? * At Week 24, what is formed and how many order of branches form it? * At Postnatal, what is formed and how many order of branches form it?

Answer 104

* Week 7: Segmental branches (10 in R.Lung) & (8 in L.Lung)→**Bronchopulmonary segments** * Week 24: **17 orders** of branches form **Respiratory bronchioles** * Postnatal: **7 orders** of branches form **Terminal airways** (up to 8 to 10 yrs of age)

Question 105

Lung mesenchyme surround all the tubes and forms what? (3)

Answer 105

* Bronchial smooth muscle * Connective tissue & pulmonary fibroblasts * Vasculature & capillaries

Question 106

What are 4 stages of Lung Maturation?

Answer 106

1. **Pseudoglandular Stage** (6-16 Wks) 2. **Canalicular Stage** (16-26 Wks) 3. **Terminal Sac Stage** (24 wks-Late fetal period) 4. **Alveolar Stage** (Late fetal period-8 yrs)

Question 107

1. What does the Pseudoglandular Stage resemble? 2. In the Canalicular Stage, what does the terminal bronchioles differenitate into? What becomes possible at week 26?

Answer 107

1. **Pseudoglandular Stage: 6 -16 Wks**: (**resemble exocrine glands**) (Epithelial lining mesoderm→NO gas exchange) 2. **Canalicular Stage: 16-26 wks** * **Terminal bronchioles →Respiratory bronchioles→ Primordial alveolar ducts→ Primordial alveoli** with **vascularization** At 26 weeks: **Respiration possible due to surfactant & vasculature!!** (Premature babies survive!!)

Question 108

3. Terminal Sac Stage: * What is there many of? * What allows blood-air gaseous exchange? * What type of pneumocytes are present and include function? * What is the function of surfactants?

Answer 108

Terminal Sac Stage: 24 Wks-Late Fetal Period: * **Many primordial alveoli** * Blood–air gaseous exchange via **epithelial sacs/endothelial capillaries** * **Type I pneumocytes (gas exchange)** & **type II pneumocytes (make surfactant)** * **Surfactants reduce surface tension** & **prevent alveolar collapse** (atelectasis)

Question 109

4. Alveolar stage: * Thin type I pneumocytes form what? That eventually becomes what?

Answer 109

**Alveolar Stage: Late Fetal Period – 8yrs** * Thin type I pneumocytes  form a **alveolocapillary membrane in alveolar sacs** → **terminal alveolar ducts for respiration**

Question 110

95% of Mature alveoli develop when?

Answer 110

Develop **postnatally** from terminal alveolar ducts

Question 111

What are the 3 regions defined by aortic blood supply of the early gut?

Answer 111

* Celiac trunk: foregut * Superior mesenteric artery: midgut * Interior mesenteric artery: hindgut

Question 112

# Development of the Esophagus * Where does the esophagus develop from? * What partitions the trachea from the espohagus and what does it form? * Where is the striated muscle of the superior part of the esophagus derived from? And what does it form? * Where is the smooth muscle of the inferior part of the esophagus coming from? * What innervates both esophageal muscles?

Answer 112

1. Esophagus develops from the **foregut**, **caudal to the pharynx** & **dorsal to the lungs** 2. The partitioning of the trachea from the esophagus by the **tracheoesophageal septum** forms a **distinct esophageal tube** 3. **Striated muscles**  derived from **4 & 6th Arch** forms the **muscularis externa** on **superior part**. 4. **Smooth muscles** of the **inferior part** come from surrounding **splanchnic mesenchyme** 5. Both Esophageal Muscles are innervated by branches of the **vagus nerves (CN X)** ## Footnote * Striated muscle: paraxial mesoderm * Smooth muscle: lateral mesoderm

Question 113

# Development of the Stomach, Duodenum, Liver & Pancreas During the 4th Week: * Foregut dilates caudal to esophagus and forms what? * What is caudal to the stomach * Hepatic diverticulum is near which two structures that forms what?

Answer 113

4th week: * **Foregut dilates** caudal to esophagus→**primordial fusiform Stomach** * **Duodenum (foregut & part of midgut)** caudal to the stomach * **Hepatic diverticulum** near **septum transversum & ventral mesogastrium**, forms: **Liver** (cranially), **Gallbladder** & **Bile duct** (caudally)

Question 114

# Development of the Stomach, Duodenum, Liver & Pancreas During the 5th Week: * What emerges? * What creates the Greater omentum? * What creates the Lesser omentum? Where does it attach?

Answer 114

* **Pancreatic buds (Dorsal & Ventral)** & **Hepatic Cords & Gall bladder** emerge * **Dorsal mesentery (dorsal mesogastrium)** will expand →**Greater omentum** (where the stomach grows) * **Ventral mesentery (ventral mesogastrium)**→ **Lesser omentum**, attaching the **liver** ## Footnote Septum transversum moves from cranial to ventral Ventral mesogastrium becomes a pocket

Question 115

What allows all the organs to be fixed for growth?

Answer 115

Ventral and Dorsal Mesogastrium

Question 116

# Early Development of Stomach, Duodenum, Liver and Pancreas By Week 6: * Dorsal stomach grows faster to form what? * Duodenum grows and froms what? * What structures fuse dorsally? * On Day 42, foregut rotation (clockwise) moves what?

Answer 116

* **Dorsal stomach grows faster** to form **dorsal greater curvature of the stomach** (lesser ventrally) * **Duodenum grows** & forms a **C-shaped Ventral-loop** * **Two Pancreatic buds** fuse dorsally (ventral pancreatic duct rotates dorsally) * Note: Day 42: **Foregut rotation** moves the **bile duct to open posterior** to the duodenum & pancreas ## Footnote Duodenum is the axis of rotation (superior side-foregut, posterior side-midgut) Liver flips but stays on the same side

Question 117

# Development of the Pancreas & Foregut rotation 5th-8th Week: * Dorsal pancreatic bud grows how and where? * Ventral pancreatic bud grows how and where? * The two 90° rotations brings what structures together?

Answer 117

* **Dorsal pancreatic bud**: Grows early, **cranially** in the **dorsal mesentery** * **Ventral pancreatic bud**: Grows later, **caudally** in the **ventral mesentery** * Two 90° rotations: bring **ventral pancreas** + **bile duct** dorsally to **fuse with dorsal pancreas & its duct**

Question 118

# Development of the Pancreas & Foregut rotation 5th-8th Week: * Ventral pancreatic duct forms what to fuse with what? * Dorsal pancreatic duct forms what to fuse with what? * 10% have 2nd-Opening into what?

Answer 118

* Ventral pancreatic duct→**main pancreatic duct, fusing with the bile duct** * Dorsal pancreatic duct→ **accessory pancreatic duct fuses with the main pancreatic duct** * 10% have 2nd-Opening into the **minor duodenal papilla** (90% Dorsal duct closes and ventral becomes the main pancreatic duct)

Question 119

Development of the Liver * Hepatic diverticulum invades into what? What expands? * What does the septum transversum fuses with? * Ventrally, liver is attached to what? * Dorsally, liver is attached to what? * Liver attach to stomach via what? * Liver attach to duodenum via what?

Answer 119

* **Hepatic diverticulum invades** into the **septum transversum** & the **peritoneal cavity** expands. * The **septum transversum fuses** with the **ventral mesentery (mesogastrium)**. * Ventrally, liver is attached to the **parietal peritoneum** by the **Falciform ligament**. * Dorsally, liver is attached to the **Lesser omentum** (ventral mesentery of stomach/duodenum) * Attached to **stomach→ Hepatogastric ligament** * Attached to **duodenum →Hepatoduodenal ligament** ## Footnote Falciform ligament (ventral mesentry)

Question 120

Liver Development together with the Biliary Apparatus * The hepatocytes (endoderm derived cells) form what? * Hepatic cords anastomose with endothelium-lined spaces form what? * Kupffer cells (from septum transversum) form what and where? * What happens at 6th week? * Bile formation by what and when? Secreted and opened in what? * Cystic duct of gall bladder opens in what? To form what? * What is the biliary apparatus?

Answer 120

* The **hepatocytes** (endoderm derived cells) form **hepatic cords** * Hepatic cords anastomose with endothelium-lined spaces→**hepatic sinusoids** * **Kupffer cells (from septum transversum)** form **macrophages** in **hepatic sinusoids.** * **Hematopoiesis** →**6th week** in the **hepatic sinusoids** * **Bile formation** by hepatocytes→**12th week** & is secreted in **bile canaliculi**, opening in the **bile duct** * **Cystic duct of gall bladder** →open in the **hepatic duct** to form the **bile duct** * **Biliary Apparatus**: (Hepatic ducts + Gall bladder + Cystic duct + Bile duct)

Question 121

# Spleen in a rotating foregut What is formed by the dorsal megastrium in the 5th week?

Answer 121

5-Week: **Spleen**  as a **vascular lymphatic organ**, formed by dorsal mesogastrium cells

Question 122

By week 5, the stomach rotates how? What is position of the liver a spleeen? And what is fromed d/t the rotation?

Answer 122

The **stomach rotates clockwise**: the **liver→right** & **spleen→left**, forming the **Gatrosplenic ligament** ventrally & **Splenorenal ligament** dorsally ## Footnote * Before rotation all organs method were in a linear position (liver most ventral, spleen most dorsal) * Gastrosplenic ligament: Spleen to stomach * Splenorenal ligament: Spleen to mesonphros (rudimentary kidney) * Ligaments all dorsal mesentry

Question 123

# Retroperitoneal position of the pancreas By week 6, what allows the retroperitoneal positioning of the pancreas?

Answer 123

6-Week: **Left surface** of the dorsal mesogastrium **fuses** with the **peritoneum over the left kidney**, allowing **retroperitoneal positioning** of the pancreas. (kidney behind pancreas) ## Footnote Easily fuses with the body wall d/t mesodermal origin of both the wall and pancreas (kidney also fuses)

Question 124

What does the Splenic mesenchyme differentiate into? (3)

Answer 124

**Splenic mesenchyme** differentiate→**the capsule, connective tissue framework**, & **spleen parenchyma**

Question 125

What is the function of the Spleen?

Answer 125

For **hematopoiesis** in **fetal life**; & retains the potential to form blood cells in adults too!

Question 126

What are the Midgut derivatives (7)? And where does the midgut end?

Answer 126

* Distal duodenum * Opening of the bile duct * Small intestine * Cecum * Appendix * Ascending colon * Right 2/3rd of the transverse colon * It ends where the branches of the **superior mesenteric artery ends**

Question 127

When and how does Umbilical Herniation occur?

Answer 127

**Day 36**: 1. Growing midgut forms a **ventral U-shaped loop** of **intestine (midgut loop**) in the extraembryonic coelom (proximal umbilical cord), **connecting to Yolk sac via the Omphaloenteric duct** 2. **Three 90° rotations = 270° rotations** on the **superior mesenteric artery axis herniates the midgut intestine** (into the omphalenteric duct) 3. **10th week: Retraction** of Intestinal Loops 4. **Cecum descends down** to get correct colon orientation.

Question 128

Fixation of Midgut Intestines: * The enlarged transverse colon presses what? * Which organs become retroperitoneal? * What is the Dorsal intestinal mesentery bound to? What does it attach to? * What other organs become retroperitoneal? * Greater Omentum mesentery fuse with what? * Other derivatives of the midgut loop (jejunum & ileum) retain what?

Answer 128

1. The **enlarged transverse colon presses** the **duodenum & pancreas** against the **posterior (dorsal)** abdominal **(parietal) wall** 2. Thus, **duodenum, pancreas**→**Retroperitonal** 3. Also, **Dorsal intestinal mesentery bound to colon** attaches to the **posterior abdominal wall** 4. So, the **Ascending colon+Descending colon→ Retroperitoneal** 5. The **Greater Omentum mesentery** fuse with the **Transverse colon mesentery**, fixing it 6. Other derivatives of the midgut loop (**jejunum & ileum)** **retain their mesenteries** (fan-shaped) and **do not attach to the abdominal / body wall**

Question 129

What organs retain in their mesenteries and do not attach to the abdominal/body wall?

Answer 129

jejunum & ileum | Midgut derivatives

Question 130

Midgut: Cecum and Appendix * What happens at week 6? * What is the Appendix? * How does the appendix positioned?

Answer 130

1. Week 6: The **Cecal swelling (Cecum+Appendix)** at the caudal limb of the midgut loop. 2. **Appendix**: A long tube arising at the **distal end of cecum**, grows in the fetus & after birth. 3. **Posterior to the cecum (64%); Reterocecal Appendix** 4. **Anterior to the cecum (36%); Retrocolic or Reteropelvic Appendix (1%)**

Question 131

Hindgut: Partitioning of the Cloaca & Anal Canal * What is the cloaca? * Cloaca (_ lining) is in contact with what? forming what? * Allantois attaches to what? * What degenerates as the rectum forms?

Answer 131

1. Cloaca is a chamber into which the **hindgut** and **allantois** empty waste. 2. **Cloaca (endodermal lining)** is in contact with the **surface ectoderm**, forming a **two layered cloacal membrane (endoderm + ectoderm of anal pit)** 3. **Allantois** (finger-like diverticulum) attaches to the **cloaca ventrally**. 4. **Post-anal portion** degenerates as the rectum forms.

Question 132

Hindgut: Partitioning of the Cloaca & Anal Canal * What develops in the angle between the allantois and hindgut? * Uro-rectal septum partitions the Cloaca into 3 Endodermal parts: * Cloacal membrane ruptures by what? what is temporarily closed? * Mesenchymal cells do what? And form what?

Answer 132

5. A **wedge of mesenchyme, the Uro-rectal septum**(mesoderm origin), develops in the angle between the allantois and hindgut. 6. **Uro-rectal septum** partitions the Cloaca into **3 Endodermal parts: (1) Rectum; (2) Superior Anal canal; (3) Urogenital sinus (Urinary bladder)** 7. **Cloacal membrane ruptures** by **apoptosis**; **anorectal lumen** is temporarily closed by an **epithelial (endodermal origin) plug** 8.**Mesenchymal cells** proliferate & ** push surface ectoderm** around the epithelial anal plug & **recanalization occurs by apoptosis of epithelial plug**; this forms the **Ectodermal Anal Pit**

Question 133

# Anal Canal * Superior 2/3rd of anal canal is what? * Inferior 1/3rd develops from what? * Hindgut & Anal pit are separated by what? * About 2 cm superior to the anus is what? * Hindgut: has what type of epithelium? * Above Hilton’s white line: * Below Hilton’s white line: * The muscular wall of the anal canal are derived from what?

Answer 133

* **Superior 2/3rd** of anal canal→**Hindgut** * **Inferior 1/3rd** develops from the  **Anal pit** * Hindgut & Anal pit are separated by a **Pectinate line**. * About **2 cm superior to the anus** is the **“Anocutaneous line” (Hilton’s white line).** * **Hindgut**: Has **Simple Columnar epithelium** * **Above Hilton’s white line**: Anal Pit has **Stratified squamous epithelium** * **Below Hilton’s white line**: Anal pit near the anus is **Keratinized Stratified squamous epithelium** with **sweat glands & sebaceous glands** * The **Muscular Wall of the anal canal** are derived from **splanchnic mesenchyme**

Question 134

Urogenital System: * Day 18: the urogenital system develops from what? * Day 28: longitudinal elevation of Intermediate mesoderm, the urogenital ridge, flanks what? * Urogenital ridge gives rise to (2)?

Answer 134

* Day 18: the **urogenital system** develops from **intermediate mesenchyme** * Day 28: A **longitudinal elevation** of **Intermediate mesoderm**, the **urogenital ridge**, flanks the **dorsal aorta** * **Urogenital ridge** gives rise to: 1. **Urinary system (nephrogenic cords)** 2. **Genital System (Gonadal ridge)**

Question 135

Function of: 1. Kidneys: 2. Ureters: 3. Urinary bladder: 4. Urethra:

Answer 135

1. **Kidneys**: produce and **excrete** urine. 2. **Ureters**: **transport** urine: kidneys→urinary bladder 3. **Urinary bladder**: **stores** urine temporarily. 4. **Urethra**: **discharges** urine: bladder→outside

Question 136

Explain the 3 sets of Nephroi Formation

Answer 136

1. The **Pronephroi** (**4th week**: rudimentary & **degenerates**) 2. The **Mesonephroi (Fetal (5-12th Wk)**: functions briefly & **disappears)** 3. The **Metanephroi** (Forms the **TWO Permanent Kidneys**) flanking the dorsal Aorta. | All originate from the Intermediate Mesoderm ## Footnote NOTE: Mesonephric duct remains →male reproductive system

Question 137

What is the overlap of mesonephroi and metanephroi?

Answer 137

* Over: 9-12 week of mesonephroni+metanephroni * After 12th week, metanephroi creates permanent kidneys and forms urinary bud

Question 138

Mesonephric duct remain as what?

Answer 138

male reproductive system

Question 139

* What is Mesonephroni (intermim kidney) role? * What structures do they consist of? * Where do the tubules open into? Where are they dervied? * Where do the Mesonephroni ducts open into? * When do they degrade?What remains? * What do the Mesonephric tubules eventually form?

Answer 139

**Mesonephroi: Day 28**: Elongated excretory **interim kidneys** before permanent kidney start functioning (**remains for 8 weeks**) 1. **Glomeruli + Mesonephric tubules** 2. Tubules open into **bilateral mesonephric ducts**, derived from **pronephric ducts** 3. Mesonephric ducts open into the **cloaca** 4. **Mesonephroi degenerate by week 12; Mes. Tubules remain** 5. **Mesonephric Tubules→”Efferent ductules” of the testes, which connects the epididymis to the testis**

Question 140

Development & Function of the Mesonephroi * By 5 weeks, what does the Nephrogenic cords form? * By Week 8, what structures fuse? Week 11 * The expanded medial end (cup) of the mesonephric tubule is invaginated by what? And forms what? * Aorta-> Afferent vessels bring blood to where? * Efferent vassals-> takes back filtered blood to where? * Fluid waste goes where? * What is formed at the gonadial ridge?

Answer 140

* 5-week: **Nephrogenic cords** form **mesonephric tubules & mesonephric duct** * Week 8: **Mesonephric tubules** & **mesonephric duct** fuse. Week 11: * The expanded medial end (cup) of the mesonephric tubule is invaginated by **blood vessels** coming from the **aorta** to form a **glomerular capsule**. * **Aorta**→**Afferent vessels** bring blood to the capsule * **Efferent vassals**→takes back **filtered blood**→**Posterior Cardinal Vein** * The **fluid waste**→**Mesonephric duct→Cloaca.** * **Gonad Primordia are formed at the gonadal ridge.**

Question 141

* What are the primordia of the permanent kidneys? * What happens of week 5 of kidneys? * Urine formation continues when? * Where is urine excreted into what?

Answer 141

* **Metanephroi** are the **primordia of the permanent kidneys** * **5th week**: Begin to develop; **functional by 9th week**(Overlap!!). * **Urine formation** continues **throughout fetal life**. * Urine is **excreted** into the **amniotic cavity** – part of the amniotic fluid.

Question 142

Kidneys develop from two sources. What are they?

Answer 142

* The **ureteric bud (is metanephric diverticulum –originate from mesonephric duct)** →intermediate mesoderm * The **metanephric blastema** (metanephric mass of **mesenchyme**)-> cap meschyme outside ## Footnote BLADDER IS ENDODERMAL

Question 143

Ureteric Bud branches are? List the different structures?

Answer 143

**Ureteric Bud branches→collecting parts of the adult kidney**: * **Ureter, Renal pelvis, Calyses (Major & Minor Calyx) & Collecting tubules**.

Question 144

Ureteric Bud invades the what and induces what?

Answer 144

**Ureteric Bud** invades the **intermediate mesoderm** and induces the **metanephric blastema** (metanephric mass of mesenchyme) to form many **NEPHRONS**, linking **collecting tubules (derived from ureteric bud)**

Question 145

Development of Nephrons (structural & functional units of the kidney): * When does nephrogensis begin? * Metanephric tubules of blastema (primordial nephrons), connect with what and to form what? * Cap mesenchym goes through what and forms what? * The proximal tubule gets invaginated by what?

Answer 145

1. **Nephrogenesis** begins by **week 8** 2. **Metanephric tubules of blastema (primordial nephrons)**, connect with the **collecting tubules** to form **uriniferous tubules** 3. **Cap mesenchyme**-> **mesenchymal-to-epithelial transition (EMT)** & form **Nephron epithelium** 4. The proximal tubule gets invaginated by glomeruli & become cup shaped

Question 146

What is the nephron consisted of? (3)

Answer 146

* **Nephron loop (Henle Loop)** * Glomerulus * Glomerular (Bowman Capsule)

Question 147

What are the positonal changes of the kidney?

Answer 147

1. **Primordial kidneys** lie **close to each other** in the pelvis (ventral to the sacrum) 2. As the abdomen and pelvis grow, the **kidneys gradually ascend** to the abdomen, **protected by the rib-cage** & **move farther apart**. 3. As they move up the **renal arteries** change their blood supply: **(1) Common iliac arteries to (2) Abdominal aorta to (3) new branches from the Aorta** as it ascends. 4. The **Ureters elongate** as the **Urinary bladder remains fixed** in the pelvic region

Question 148

Development of Gonads are formed from what?

Answer 148

cells from the umbilical vesicle

Question 149

* The gonads (testes or ovaries) produce what? * The gonads are derived from three sources:

Answer 149

* The **gonads (testes or ovaries)** produce sex cells (sperms or oocytes). * The gonads are derived from three sources: 1. **Mesothelium**: **mesodermal epithelium** lining the **posterior abdominal wall** 2. **Underlying mesenchyme** (embryonic connective tissue) 3. **Primordial germ cells** (PGCs; endodermal undifferentiated sex cells)

Question 150

Explain the Indifferent Gonads development until Week 7

Answer 150

* Day 24: **Primordial germ cells (PGCs)** from the **Umbilical vesicle (Yolk Sac)** migrate via the **dorsal mesentery** into **both Urogenital ridges or Gonadal ridges** * A medial **bulge in the mesonephros** is the **gonadal ridge** or **primordium of gonad**

Question 151

Gonadal Cortex and Medulla containing PGCs: * Migration of PGCs via what? to where? * Mesodermal epithelium (abdominal wall) covers what and forms what? * Gonadal cords grow where? * The indifferent gonads (male or female) now consist of what?

Answer 151

1. **Migration of PGCs** via **hind gut dorsal mesentery** to the **gonadal ridge** 2. **Mesodermal epithelium (abdominal wall)** covers the PGCs & form the **gonadal cords**. 3. **Gonadal cords** grow in the underlying mesenchyme **(Medulla)** 4. The indifferent gonads (male or female) now consist of an **external cortex (epithelial layer)** & an **internal medulla**.

Question 152

XY embryo (Male): * What gene is present * Gonadal cords differentiate into what? * The medulla differentiates into what?

Answer 152

* The **Y-Chromosome** with an **SRY-gene (sex- determining region on the Y chromosome)** make **TDF (Testis Determining Factor)**, which determines **testicular differentiation** * **Gonadal cords differentiate into Seminiferous Cords ->Seminiferous tubules with PGCs** * The **medulla differentiates into a testis**, and the **cortex regresses**

Question 153

XX embryo (Female): * What type of pathway? * The Cortex differentiates into what?

Answer 153

* **Default Pathway** (no Y-chromosome) * The **Cortex differentiates into an Ovary (Oocytes resides)**, and the **medulla regresses**.

Question 154

XY embryo (Male): Development of Testes: * Seminiferous cords condense, branch, anastomose to form what? * Seminiferous cords detach from what? * Seminiferous cords go into? * What does sertoli cells do? * Rete testis connects with? * Mesonephric duct becomes what? * Mature testis where?

Answer 154

* **Seminiferous cords** condense, branch, anastomose to form the **rete testis** * **Seminiferous cords** detach from the cortex as the **fibrous tunica albuginea** develops. * **Seminiferous cords-> seminiferous tubules; surrounding mesenchyme** forms **Leydig cells** (produced by medulla that has mesodermal cells); **secrete testosterone**, inducing masculine differentiation * **Sertoli cells (produced by cortex epithelium)** nourishes the **Spermatogonia** (derived from **PGCs**)-> **SPERMS** * **Rete testis** connects with **mesonephric tubules**, which then becomes **efferent ductules**. * **Mesonephric duct->duct of the epididymis** * Mature testis: Out of the body before birth; (cooler by 40C) * No sperms till puberty.

Question 155

XX embryo (Female): Development of Ovaries: * No Y, thus X chromosome genes determine what? * Gonadal cords form what? * Cortical cords extend where? * Cords break into what? * Oogonium-> _ -> _ * The ovary separates from what? what is it suspended by?

Answer 155

* No Y, thus X chromosome genes determine ovarian development. * **Gonadal cords** form a **rudimentary rete ovarii-**> **degenerates**. * **Cortical cords** extend in the mesenchyme & **incorporate PGCs**. * **Cords break**->**primordial ovarian follicles**, each with a PGC; **(Oogonium)**. **No Oogonia from postnatally**. * **Oogonium->Primary Oocyte→OVUM** * The **ovary separates** form the **regressing mesonephros** & is suspended by **Ovarian ligament** & the surrounding mesentery, the **Mesovarium**.