Development of Eye and Heart | Zoology Optional Notes for UPSC PDF Download

Development of Eye

• Neural tube interacts with cranial ectodermal placodes.
• Interaction leads to formation of principal sensory organs of the skull.
• Cranial ectodermal placodes are series of epidermal thickenings.

The mechanism of eye development:

• Head ectoderm interacts with involuting endoderm and mesoderm during gastrulation.
• Interaction predisposes head ectoderm towards lens formation.
• Optic vesicle, emerging from diencephalon, places lens in relation to retina.
• Optic vesicle contact triggers development of lens placode, later forming lens.
• Optic vesicle transforms into two-walled optic cup with differentiating layers.
• Outer layer differentiates into pigmented retina.
• Inner layer undergoes fast cell division, producing various retinal cells.
• Retinal ganglion cells carry electrical signals to the brain via optic nerve.
• Anterior tip of neural plate expresses transcription factors Six3, Pax6, and Rx1.
• Optic vesicles arise from this domain.
• Pax6 crucial for lens and retina growth, shared feature in photoreceptive cells.
• Sonic hedgehog secretion divides single eye field into bilateral fields.
• Alterations in sonic hedgehog gene or protein processing can lead to cyclopia.
• Sonic hedgehog protein from prechordal plate suppresses Pax6 expression, dividing the eye field.

Neural differentiation of the retina:

• Neural retina develops into layered array of various neuronal types.
• Similarity to cerebral and cerebellar cortices observed.
• Layers include rod and cone photoreceptor cells, ganglion cell bodies, and bipolar interneurons.
• Horizontal neurons conduct electrical impulses within the retina plane.
• Amacrine neurons lack long axons.
• Müller glial cells maintain retina integrity.
• Striated, laminar pattern formed during early retinal development.
• Pattern created through cell division, migration, and differential cell death.
• Single neuroblast precursor cell from retinal germinal layer can give rise to at least three different types of neurons or two different types of neurons and a glial cell.

Differentiation of lens and cornea:

• Lens placode folds and touches fresh ectoderm during lens development.
• Lens vesicle induces ectoderm to form transparent cornea.
• Proper curvature of cornea crucial for focusing light onto retina.
• Intraocular fluid pressure essential for maintaining cornea curvature.
• Ring of scleral bones, likely from neural crest, maintains intraocular pressure.
• Changes in cell structure, shape, and production of crystallins needed for lens tissue differentiation.
• Crystallins, lens-specific proteins, crucial for lens transparency and light direction onto retina.
• Inner cells of lens vesicle expand and transform into lens fibers under neural retina influence.
• Lens fibers produce crystallins, filling cells and leading to nucleus extrusion.
• Crystallin-synthesizing fibers cover gap between two layers of lens vesicle.
• Anterior lens vesicle cells form ongoing germinal epithelium.
• Dividing cells move towards vesicle's equator, lengthen, and transform.

• Lens consists of three distinct zones:
  • Anterior zone: Contains proliferating epithelial cells.
  • Equatorial zone: Characterized by cellular elongation.
  • Posterior and middle zone: Comprised of fiber cells containing crystalline.
• Ongoing laying down of fibers maintains this arrangement throughout animal's lifetime.
• Transition from epithelial cell to lens fiber in adult chicken takes two years.
• Iris, a pigmented and skeletal tissue, located in front of lens.
• Iris muscles regulate pupil's size.
• Part of iris derived from ectodermal layer, unlike other body muscles from mesoderm.
• Optic cup section forming iris continuous with neural retina but does not produce photoreceptors.

Development of Heart

• Circulatory system is a significant achievement of lateral plate mesoderm.
• It comprises heart, blood cells, and intricate network of blood vessels.
• Nourishes growing vertebrate embryo.
• Heart is the first functional organ and circulatory system is the first functional component of embryo.
• Splanchnic mesoderm sections on both sides of body specialize for heart development.
• Interaction with surrounding tissue leads to specialization and formation of vertebrate heart.

Specification of heart tissue and fusion of heart rudiments:

• Amniote vertebrate embryos are flattened discs.
• Lateral plate mesoderm doesn't fully cover yolk sac.
• Early primitive streak, slightly posterior to Hensen's node, is where cardiogenic mesoderm originates.
• Cardiogenic mesoderm cells divide through the streak.
• Division generates two clusters of mesodermal cells lateral to Hensen's node.
• These clusters are the source of cells forming endothelium lining of heart, cushion cells of valves, Purkinje conducting fibers, and atrial/ventricular muscles.

• Presumptive heart cells migrate anteriorly between ectoderm and endoderm in chick embryo at 18-20 hours old.
• Migration occurs while maintaining close contact with endodermal surface.
• Migration halts at lateral walls of anterior gut tube.
• Direction of movement guided by foregut endoderm.
• Cardiac mesoderm moves opposite to rotation of heart region endoderm about embryo.
• Anterior-to-posterior concentration gradient of fibronectin in endoderm directs migration.
• Some cardiogenic cells designated by endoderm and primitive streak develop into heart muscles.
• Cerberus and unidentified substance (presumably BMP2) induce cardiogenic cells to produce Nkx2-5 transcription factor in anterior endoderm.
• Nkx2-5 directs mesoderm to develop into cardiac tissue, triggering production of other transcription factors (GATA, MEF2 families).
• Transcription factors stimulate expression of genes coding for proteins specific to heart muscle.
• Ventricular cells become defined before atrial cells.
• Advancing heart-forming primordia experience separate cell differentiation.
• Cells display N-cadherin on apices and unite to form epithelium as they move.
• Endocardium formed when small population of cells detach from epithelium and downregulate N-cadherin.
• Myocardium comprised of epithelial cells develops into heart's pumping muscles.
  
• Endocardial cells produce numerous heart valves and regulate neural tissue positioning in heart.
• They secrete proteins controlling myocardial growth.
• Splanchnic mesoderm folds inward during neurulation to form foregut.
• Myocardium becomes one tube as movement pulls two cardiac tubes together.
• "Cardio Bifida" results in development of distinct hearts on each side of body.
• Previously paired coelomic chambers combine to form body cavity housing heart.
• Fusion of coelomic chambers occurs around 29 hours in chick development.
• Apertures of vitelline veins in heart formed by unfused posterior sections of endocardium.
• Veins transport nutrients from yolk sac to sinus venosus.
• Blood enters atrial portion of heart through flap resembling valve.
• Truncus arteriosus contractions accelerate blood flow into aorta.
• Heart begins beating while primordia are still joining.
• Sinus venosus acts as pacemaker for contractions.
• Wave of muscular contraction propagates from where contractions start in tubular heart.
• Heart pumps blood before intricate valve system fully develops.
• Heart muscle cells contract naturally.
• By fourth day, chick embryo's ECG similar to adult's.
• Contractions in embryo controlled by electrical inputs from medulla oblongata via vagus nerve.

Looping and formation of heart chambers:

• Three-day-old chick embryo has two-chambered heart with atrium and ventricle.
• Blood cycle observed with blood entering lower chamber and pumped out through aorta.
• Heart's looping changes original anterior-posterior polarity to right-left polarity.
• Right ventricle area located ahead of left ventricle area due to looping.
• Left-right patterning proteins necessary for looping process.
• Nkx2-5 controls Hand1 and Hand2 transcription factors in heart primordium.
• Hand1 limited to left ventricle and Hand2 to right once looping begins.
• Disruption of Hand proteins leads to improper ventricle development.
• Pitx-2 transcription factor crucial for correct cardiac looping, activated on left side of lateral plate mesoderm.
• Pitx-2 may control expression of proteins like lectin to manage physical strain of heart components.
• Xin gene mediates cytoskeletal alterations required for heart looping, activated by transcription factors Nkx2-5 and MEF2C.
• Various transcription factors limited to specific regions of heart tube define atrium-ventricle separation.
• Myocardium cells release substance (likely transforming growth factor-β3) triggering endocardium cells to separate and enter hyaluronate-rich cardiac jelly, partitioning tube into distinct atrium and ventricle.