Semester 2

Question 1

1) Pattern formations of neural tube

Answer 1

• Neural tube will develop to the CNS
• The embryo’s precursor to the CNS, which comprises the brain and spinal cord. The neural groove gradually deepens as the neural folds become elevated, and ultimately the folds meet and coalesce in the middle line and convert the groove into the closed neural tube.
• The centre of the tube is the neural canal.
• Four neural tube subdivisions each eventually develop into distinct regions of the central nervous system by the division of neuroepithelial cells: The prosencephalon, the mesencephalon, the rhombencephalon and the spinal cord.
• For a short time, the neural tube is open both cranially and caudally. These openings, called neuropores, close during the fourth week in the human.
• Morphogens in the developments
  
  • Sonic hedgehog (SHH) is secreted by the notochord and the floorplate.
  • Bone morphogenetic proteins (BMP) are secreted by the roofplate
  • These opposing morphogens gradients determine dorsal-ventral cell fates.

Question 2

2) Placodal ectoderm and derivates.

Answer 2

• A neurogenic placode is an area of thickening of the epithelium in the embryonic head ectoderm layer that gives rise to neurons and other structures of the sensory nervous system.
• Derivates
  
  • lens placode
    
    • develops into the lens vesicle and later the lens.
  • nasal placode
    
    • the forerunners of the external nares and nasopharyngeal epithelium.
  • olfactory placode
    
    • ultimately provides the sensory nerves for the olfactory region of the nasal mucosa.
  • otic placode
    
    • invaginates and is a major contributor to the internal ear.

Question 3

3) Development of ear and eye

Answer 3

The internal ear develops in week 4 from a thickening of the surface ectoderm called the otic placode.

The otic placode invaginates into the connective tissue (mesenchyme) adjacent to the rhombencephalon and becomes the otic vesicle. The otic vesicle divides into utricular and saccular portions.

1. Utricular portion of the otic vesicle gives rise to the following:
   
   1. Utricle contains the sensory hair cells and otoliths of the macula utriculi. The utricle responds to linear acceleration and the force of gravity.
   2. Semicircular ducts contain the sensory hair cells of the cristae ampullares. They respond to angular acceleration.
   3. Vestibular ganglion of cranial nerve VIII lies at the base of the internal auditory meatus.
   4. Endolymphatic duct and sac is a membranous duct that connects the saccule to the utricle and terminates in a blind sac beneath the dura. The endolymphatic sac absorbs endolymph.
2. Saccular portion of the otic vesicle gives rise to the following:
   
   1. Saccule contains the sensory hair cells and otoliths of the macula sacculi. The saccule responds to linear acceleration and the force of gravity.
   2. Cochlear duct (organ of Corti) is involved in hearing. This duct has pitch (tonopic) local-ization whereby high-frequency sound waves (20,000 Hz) are detected at the base and low-frequency sound waves (20 Hz) are detected at the apex.
   3. Spiral ganglion of CN VIII lies in the modiolus of the bony labyrinth.

THE MEMBRANOUS AND BONY LABYRINTHS

1. The membranous labyrinth consists of all the structures derived from the otic vesicle.
2. The membranous labyrinth is initially surrounded by neural crest cells that form a connective tissue (mesenchyme) covering. This connective tissue becomes cartilaginous and then ossifies to become the bony labyrinth of the temporal bone.
3. This sets up the interesting anatomical relationship by which the membranous labyrinth is suspended (or floats) within the bony labyrinth by perilymph.

Development of the Eye

• Starts at the beginning of the 4th week.
• Bilateral diverticulum from forebrain at level of diencephalon.
• This diverticulum forms optic vesicle and optic stalk.
• Optic vesicle will be optic cup which form retina, iris and ciliary body. the mouth of the cup will form pupil.
• Inner layer of the cup forms nervous layer of retina ehile the outer layer forms the pigmented layer of retina.
• Optic stalk forms optic nerve and invaginated by hyaloid artery which will be central retinal artery.

Question 4

4) Derivates of neural crest, development of peripheral nervous system

Answer 4

• Neural crest cells are a transient, multipotent, migratory cell population
• Gives rise to a diverse cell lineage including
  
  • Mesectoderm, examples
    
    • Dental papillae
    • Craniofacial cartilage and bone
  • Endocrine cells, example
    
    • chromaffin cells of the adrenal medulla
  • Peripheral nervous system, examples
    
    • Sensory neurons and glia of the dorsal root ganglia
    • Schwann cells of all peripheral nerves.
  • Melanocytes and iris pigment cells
• Neural crest cells originating from different positions along the anterior-posterior axis develop into various tissues. These regions of neural crest can be divided into four main functional domains
  
  • cranial neural crest
  • trunk neural crest
  • vagal and sacral neural crest
  • cardiac neural crest.
• The peripheral nervous system (PNS) is the part of the nervous system that consists of the nerves and ganglia outside of the brain and spinal cord.
  
  • Cranial, spinal and visceral nerves
  • Cranial, spinal and autonomic ganglia
  • The main function of the PNS is to connect the CNS to the limbs and organs, essentially serving as a communication relay going back and forth between the brain and the extremities.
  • Develops mostly from the neural crest

Question 5

5) Somitogenesis, paraxial mesoderm.

Answer 5

• Paraxial mesoderm is a thick plate of mesoderm located on each side of the midline. Paraxial mesoderm becomes organized into segments known as somitomeres, which form in a craniocaudal sequence. Somitomeres 1–7 do not form somites but contribute mesoderm to the pharyngeal arches.
• The remaining somitomeres further condense in a craniocaudal sequence to form 42–44 pairs of somites.
  
  • The first pair of somites forms on day 20, and new somites appear at a rate of 3 per day.
  • The caudal-most somites eventually disappear to give a final count of approximately 35 pairs of somites.
  • The number of somites is one of the criteria for determining the age of the embryo.
• Somites further differentiate into the following components:
  
  1. Sclerotome forms the cartilage and bone components of the vertebral column.
  2. Myotome forms epimeric and hypomeric muscles.
  3. Dermatome forms dermis and subcutaneous area of skin.

Question 6

6) Epithelial-mesenchymal interaction I. Lung and glands development. Tooth development

Answer 6

Epithelial-mesenchymal interaction II. Lung and glands development.

• On Week 4, Respiratory Diverticulum (Lung Bud) appears as an outgrowth of the Foregut.
  
  • This appearance depends on an increase in Retinoic Acid (RA)
  • Increase in RA causes upregulation of transcription factor TBX4, expressed at the endoderm of the gut tube adjacent to lung buds.
• Because of Retinoic Acid, following structures are entirely of Endodermal origin:
  
  • Larynx;
  • Trachea;
  • Bronchi;
  • internal epithelial lining.
• Following structures are of Splanchnic Mesodermal origin:
  
  • Cartilaginous;
  • Muscular;
  • Connective tissue;
  • components of Trachea and Lungs.
• Initially Lung Buds are in open communication with Foregut
  
  • Separation by Tracheoesophageal Ridges
    
    • Following this separation, two lateral Bronchial Buds form at the beginning of 5th Week.
    • The right buds forms 3 secondary bronchi and the left bud forms 2, corresponding to the Lobes.
  • These ridges fuse to form Tracheoesophageal Septum, separating Foregut into
    
    • Dorsal Portion: Esophagus
    • Ventral Portion: Trachea and Lung Buds
• Larnygeal Orifice of Respiratory Primordium maintains communication with the Pharynx.
• These buds expand into the body cavity through Pericardioperitoneal Canals.
  
  • Pleuroperitoneal and Pleuropericardial folds separate Pericardioperitoneal canals from peritoneal and pericardial cavities.
• The mesoderm which covers outside of the lung develops into Visceral Pleura.
• The somatic mesoderm layer covering the body wall from the inside, become the Parietal Pleura.
  
  • Hence, epithelial-mesodermal interaction in the development of this organ.
• Following the development of the fetus, bronchi divide in a dichotomous fashion for 17 generations.
  
  • Additional six divisions occur post-natally to complete the development.
• Signals for development and the branching of bronchi are;
  
  • Fibroblast Growth Factors: of Mesodermal Origin

Development of the Teeth

• Teeth develop from an interaction between oral epithelium and mesenchyme from the Neural Crest.
• A Dental Lamina is formed on Week 6 by the epithelial lining of oral cavity.
• From this lamina, Dental Buds (10 on each jaw) arise.
• These buds invaginate to form the Cap Stage in the development of tooth, comprising of:
  
  • Outer Dental Epithelium: Later becoming Odontoblast to produce Dentin and continuously during life, Predentin.
  • Inner Dental Epithelium: Later becoming Ameloblasts to produce Enamel deposited in the teeth.
  • Stellate Reticulum
• At this time, the mesenchyme from Neural Crest forms the Dental Papilla.
• After a while, tooth enters the Bell Stage, during which odontoblasts retreat into dental papilla, leaving behind a Dental Process.
• Remaining cells of the Dental Papilla (Neural Crest) form the Pulp of the Tooth.
• A cluster of Ameloblasts form a structure called Enamel Knot to regulate the Development of Tooth.
• Dental Epithelia come together to penetrate the underlying mesenchyme, forming the Epithelial Root Sheath. A layer of dentin surrounds this sheath, narrowing it to a point where only vasculature and nerves pass into the pulp from this Root Canal.
• Mesenchymal cells in contact with the dentin surrounding the sheath, differentiate into:
  
  • Cementoblasts: to produce Cementum, a specialized bone.
  • From Cementum: Periodontal Ligament develops.
• At 6 – 24 months after birth, deciduous teeth erupt.
• Buds for the permanent teeth under the deciduous teeth begin developing on the third month of development and stay dormant until 6th postnatal year.

Question 7

7) Epithelial-mesenchymal interaction II. Molecular genetics of kidney development.

Answer 7

1. The kidney is formed via reciprocal interactions between two precursor tissues derived form the intermediate mesoderm: the Wolffian duct and the Metanephric mesenchyme (MM)
2. MM-derived signals, mainly the glial-derived neurotrphic factor (GDNF), induce an outgrowth from the Wolffian duct, termed the Ureteric bud (UB). The UB then secretes WNT9b, attracting MM cells.
3. MM cells condense around the tips of the branching UB, forming the condensed/cap mesenchyme (CM).
4. The CM expresses a unique combination of genes (red) and the mesenchymal marker, vimentin. The CM contains the kidney stem cells and is capable of self-renewal.
5. CM cells start to produce WNT4, which acts in an autocrine fashion, leading to epithelialization of the cells. The cells sequentially form the pretubular aggregate, renal vesicle, C-, and S- shaped bodies, and finally the mature nephron.
6. The cells derived from the CM form most of the nephron body (from glomerulus to distal tubule), whereas the UB-derived cells form the collecting duct.

http://openi.nlm.nih.gov/detailedresult.php?img=2996087_stem0028-1649-f1&req=4

Question 8

8) Development of pancreas and liver

Answer 8

• Derives from the end of the foregut (endoderm origin)
• Liver:
  
  • The hepatic diverticulum is formed from the ventral surface of the foregut. This splits into 2 parts:
    
    • Hepatic cords: these grow rapidly and form the liver
    • The other part becomes the gall bladder and cystic duct
  • Molecular embryology:
    
    • See image
• Pancrease:
  
  • Develops from a dorsal and a ventral pancreatic bud, which will fuse together as the foregut rotates. The dorsal bud will form most of the pancreas, while the ventral bud will become the uncinated process.
  • The pancreatic duct will fuse with the bile duct.
  • It starts intraperitoneal, but ends up as retroperitoneal.
  • Molecular embryology:
    
    • Pdx1: differentiates the endoderm into pancreatic tissue, and promots growth
    • Ngn3: islet differentiation
    • Ptf1a: Acini differentiation
    • Hnf6: Ductal differentiation

Question 9

9) Early development of the heart

Answer 9

The heart derives from embryonic mesodermal germ-layer cells that differentiate after gastrulation into mesothelium, endothelium, and myocardium. Mesothelial pericardium forms the outer lining of the heart. The inner lining of the heart, lymphatic and blood vessels, develop from endothelium.[4]

Endocardial tubes
In the splanchnopleuric mesenchyme on either side of the neural plate, a horseshoe-shaped area develops as the cardiogenic region. This has formed from cardiac myoblasts and blood islands as forerunners of blood cells and vessels.

By day 19, an endocardial tube begins to develop in each side of this region. These two tubes grow and by the third week have converged towards each other to merge, using programmed cell death to form a single tube, the tubular heart.

From splanchnopleuric mesenchyme, the cardiogenic region develops cranially and laterally to the neural plate. In this area, two separate angiogenic cell clusters form on either side and coalesce to form the endocardial tubes. As embryonic folding continues, the two endocardial tubes are pushed into the thoracic cavity, where they begin to fuse together, and this is completed at about 22 days.

At around 18 to 19 days after fertilisation, the heart begins to form. This early development is critical for subsequent embryonic and prenatal development.

The heart is the first functional organ to develop and starts to beat and pump blood at around day 21 or 22. The heart begins to develop near the head of the embryo in the cardiogenic area. Following cell signalling, two strands or cords begin to form in the cardiogenic region As these form, a lumen develops within them, at which point, they are referred to as endocardial tubes. At the same time that the tubes are forming other major heart components are also being formed. The two tubes migrate together and fuse to form a single primitive heart tube, the tubular heart which quickly forms five distinct regions.

From head to tail, these are the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and the sinus venosus.

Question 10

10) Vasculogenesis, AGM and early hemopoiesis.

Answer 10

(A) The emergence of both the endothelial and hematopoietic lineages takes place during vasculogenesis from a common precursor, the hemangioblast.

(B) Shortly thereafter, a subset of specialized endothelial cells retains the ability to give rise to and/or support hematopoiesis, particularly in the dorsal aorta in association with the aortic- gonado-mesonephros (AGM) region.

(C) In the adult bone marrow, hematopoiesis proceeds from pluripotent hematopoietic stem cells (PHSC), which give rise to common myeloid progenitors (CMP) and common lymphoid progenitors (LP). In addition, some reports have documented the formation of long- term endothelial cells (ECs) from hematopoietic progenitors. The common myeloid progenitor further differentiates into megakaryocyte-erythroid progenitor (MEP) and granulocyte-macrophage progenitor (GMP).

The first definitive multipotent hematopoietic stem cells (HSCs) are generated within the embryonic aorta- gonad-mesonephros (AGM) region. The AGM extends from the umbilicus to the anterior limb bud of the human embryo and contains the dorsal aorta. Within the dorsal aorta, a cluster of CD34+ hematopoietic cells is associated with the ventral floor of the aorta.

Question 11

11) Development of limbs and skin

Answer 11

Limbs

• Starts at the end of 4th week. Derives from lateral somatic mesoderm
  
  • ​Rapid proliferation forms the limb bud.
• As the limb bud continues to grow out, the distal ends will flatten and form foot/hand plates.
  
  • Tissue between the developing digits will disappear, due to apoptosis.
• Chondrification centers form in week 5. This starts to ossify by week 7.
• Myogenic precursor cells migrate to the limb buds and differentiate into myoblasts. They will develop in 2 compartments: flexor and extensor compartments.
• Motor axons and neural crest cells (Schwann cells) grow into the bud at week 5.
• Limb rotation:
  
  • Upper limb: 90 degrees laterally (elbows facing dorsally)
  • Lower limb: 90 degrees medially (knees facing ventrally)
• Molecular embryology: Axial patterning
  
  • Proximal-distal: Hox signalling and FGF signalling
  • Anterior posterior (radial/ulnar): ZPA zone produces Shh on the ulnar side. o Dorsal-ventral: Wnt7 signalling

Skin

• 4 weeks - simple ectoderm epithelium over mesenchyme.
• 1-3 months
  
  • ectoderm - germinative (basal) cell repeated division of generates stratified epithelium.
  • mesoderm - differentiates into connective tissue and blood vessels.
• week 11 - (GA week 13) blood vessels visible in the early fetal skin, small blood vessels in the upper papillary region and larger vessels in the deep reticular dermis.
• 4 months
  
  • Basal cell - proliferation generates folds in basement membrane.
  • Neural crest cells - melanoblasts migrate into epithelium. These are the future melanocyte pigment cell of the skin.
  • Embryonic connective tissue- differentiates into dermis, a loose ct layer over a dense ct layer. Beneath the dense ct layer is another loose ct layer that will form the subcutaneous layer.
  • Ectoderm contributes to nails, hair follictles and glands.
  • Nails form as thickening of ectoderm epidermis at the tips of fingers and toes. These form germinative cells of nail field.
  • Cords of these cells extend into mesoderm forming epithelial columns. These form hair follicles, sebaceous and sweat glands.
• 5 months
  
  • Hair growth initiated at base of cord, lateral outgrowths form associated sebaceous glands.
  • Other cords elongate and coil to form sweat glands.
  • Cords in mammary region branch as they elongate to form mammary glands. These glands will complete development in females at puberty. Functional maturity only occurs in late pregnancy.
• Embryonic and Fetal Epidermis
  
  • Electron Micrographs of the Developing Human Epidermis[6]
• Dermis
  
  • Somite Components - The underlying connective tissue layers of the skin (dermis and hypodermis) arise from the dermatome component of the developing somite.

Question 12

12) Gonadal development and sex determination

Answer 12

Sex determination

• Initially, both sexes have Wolffian and Mullerian ducts. However, the SRY gene, found on the males’ Y chromosome, and the AMH gene causes the Mullerian duct to regress and the Wolffian duct to develop. In females, the Wolfian duct regresses in absence of testosterone.

Gonad development

• From 3 sources: mesothelium, mesenchyme, germ cells
• The indifferent gonads develop as a thickening of the mesonephros. At this point, the gonad has a cortex and medulla
  
  • In testes, the cortex regresses
  • In ovaries, the medulla regresses
• SRY gene triggers testes differentiation. Lack of SRY (lack of testosterone) triggers ovary differentiation.

Genital development

• External development
• Genital duct development