Retinal Synaptogenesis in Retinal Synaptogenesis in MammalsMammals

Jon A. WagnonJon A. Wagnon

The Human EyeThe Human Eye

Cross-section of the EyeCross-section of the Eye

Cross-section of the FoveaCross-section of the Fovea

Beginning of SynaptogenesisBeginning of Synaptogenesis

Neuron contacts target cellNeuron contacts target cell Synapse formation beginsSynapse formation begins

Macaque MonkeyMacaque Monkey

Genus Genus MacacaMacaca Common research subjectCommon research subject Similar eye formation to HumansSimilar eye formation to Humans 165 to 175 days gestation (humans 165 to 175 days gestation (humans

are approximately 40 weeks)are approximately 40 weeks) Slow gestation allows for greater Slow gestation allows for greater

resolution of developmentresolution of development

Macaque Monkey (cont.)Macaque Monkey (cont.)

Retinal is cone dense at the fovea Retinal is cone dense at the fovea and rod dense at the periphery, and rod dense at the periphery, therefore development of rods and therefore development of rods and cones can be examined separatelycones can be examined separately

Fovea FormationFovea Formation

Fetal Day (fd) 55 in Fetal Day (fd) 55 in MacacaMacaca Fetal Week (fw) 11 in HumansFetal Week (fw) 11 in Humans

Synaptic Formation in Synaptic Formation in the Inner Retinathe Inner Retina

Tridiated Thymadine experiments Tridiated Thymadine experiments show that all neurons and Müller show that all neurons and Müller cells are post mitotic and formed cells are post mitotic and formed between fd36-60between fd36-60

Morphological differential begins Morphological differential begins after mitosis endsafter mitosis ends

Synaptic Formation in the Synaptic Formation in the Inner Retina (cont.)Inner Retina (cont.)

Early formation of the Inner Plexiform Early formation of the Inner Plexiform Layer (IPL) begins between fd55-85Layer (IPL) begins between fd55-85

Between fd85-132 IPL becomes more Between fd85-132 IPL becomes more compactcompact

Number of membrane junctions not Number of membrane junctions not associated with vesicles drops with timeassociated with vesicles drops with time

No synapses in periphery until fd55-78No synapses in periphery until fd55-78 Eye opening does not cue to stop Eye opening does not cue to stop

synaptogenesis (Fisher, 1979)synaptogenesis (Fisher, 1979)

Synaptic Formation in the Synaptic Formation in the Inner Retina (cont.)Inner Retina (cont.)

There are three phases of IPL There are three phases of IPL synaptogenesis in mouse (Fisher, 1979b)synaptogenesis in mouse (Fisher, 1979b)1. Day 3 to 101. Day 3 to 10– conventional synapses are producedconventional synapses are produced

2. Day 11 to 152. Day 11 to 15– ribbon synapses are produced and conventional ribbon synapses are produced and conventional

synapse production increasessynapse production increases

3. Day 15 3. Day 15 – sharp reduction in the rate of ribbon and sharp reduction in the rate of ribbon and

conventional synapse productionconventional synapse production

Synaptic Formation in the Synaptic Formation in the Outer RetinaOuter Retina

Outer Plexiform Layer (OPL)Outer Plexiform Layer (OPL) Can be distinguished shortly before Can be distinguished shortly before

ribbon formationribbon formation Cone synapses develop before rod Cone synapses develop before rod

synapsessynapses Development begins at fd60-65Development begins at fd60-65 OPL synapses appear after IPL OPL synapses appear after IPL

synapsessynapses

Synaptic Formation in the Synaptic Formation in the Outer Retina (cont.)Outer Retina (cont.)

In humansIn humans– cone synapses: fw12cone synapses: fw12– rod synapses: fw18rod synapses: fw18

Synaptic Formation in the Synaptic Formation in the Outer Retina (cont.)Outer Retina (cont.)

Opsin formation Opsin formation follows follows synaptogenesissynaptogenesis– humans: 2 months humans: 2 months

laterlater– monkey: 2 weeks monkey: 2 weeks

laterlater

Synaptic DevelopmentSynaptic Development

Rates and Rates and times of times of development development change with change with ageage

SynapsesSynapses

Ribbon synapse:Ribbon synapse:– contains a “ribbon”contains a “ribbon”– the ribbon may appear floating or the ribbon may appear floating or

following an axonfollowing an axon– associated only with bipolar cellsassociated only with bipolar cells

Conventional synapse:Conventional synapse:– associated with bipolar, amacrine, associated with bipolar, amacrine,

and ganglion cellsand ganglion cells

Process of SynaptogenesisProcess of Synaptogenesis

1. Patches of dense filamentous membrane first 1. Patches of dense filamentous membrane first appear on the dendrites of ganglion (G) cellsappear on the dendrites of ganglion (G) cells

2. Membrane densities on ganglion cell dendrites 2. Membrane densities on ganglion cell dendrites then become opposed to amacrine (A) cell then become opposed to amacrine (A) cell processes still lacking their own membrane processes still lacking their own membrane densities and vesiclesdensities and vesicles

3. A cell processes acquire membrane 3. A cell processes acquire membrane specializations associated with vesciles at the specializations associated with vesciles at the sites apposing G cell dendritessites apposing G cell dendrites

Process of SynaptogenesisProcess of Synaptogenesis

4. Pairs of A cell processes form A4. Pairs of A cell processes form AA subtypes of A subtypes of conventional synapsesconventional synapses

5. Monad ribbon synapses are established between 5. Monad ribbon synapses are established between bipolar (B) and G or A cell processesbipolar (B) and G or A cell processes

6. Dyad ribbon synapses (B 6. Dyad ribbon synapses (B GG; B GG; B GA; B GA; B AA) AA) are formedare formed

7. Processes of some A cells form a feedback circuit 7. Processes of some A cells form a feedback circuit with B cell axons (A with B cell axons (A B) B)

Ribbon SynapsesRibbon Synapses

Detected in the periphery at fd99Detected in the periphery at fd99 In cone-dominated areas, develop In cone-dominated areas, develop

before conventional synapsesbefore conventional synapses In species with are entirely cone-In species with are entirely cone-

dominated (e.g. tree shrew, chick), dominated (e.g. tree shrew, chick), develop before conventional synapsesdevelop before conventional synapses

Ribbon synapse of the OPL appear Ribbon synapse of the OPL appear prior to those of the IPLprior to those of the IPL

Ribbon Synapses (cont.)Ribbon Synapses (cont.)

Ribbons of the OPL are produced with Ribbons of the OPL are produced with no centro-peripheral gradient (Maslimno centro-peripheral gradient (Maslim

Plateau at a density of 5.5/100Plateau at a density of 5.5/100mm22

In humans ribbons are distributed In humans ribbons are distributed fairly evenly throughout with the fairly evenly throughout with the slight suggestion of four broadly slight suggestion of four broadly overlapping bands (Koontz & overlapping bands (Koontz & Hendrickson, 1987)Hendrickson, 1987)

Ribbon Synapses (cont.)Ribbon Synapses (cont.)

Most ribbons connect to only one Most ribbons connect to only one postsynaptic cell, either amacrine or postsynaptic cell, either amacrine or ganglionicganglionic

There are significantly more ribbons There are significantly more ribbons connecting bipolar (B) to ganglion (G) in connecting bipolar (B) to ganglion (G) in the fovea (91%) than in the periphery the fovea (91%) than in the periphery (66%) suggesting that there is more (66%) suggesting that there is more amacrine cell processing in the amacrine cell processing in the periphery (Koontz & Hendrickson, 1987)periphery (Koontz & Hendrickson, 1987)

Ribbon Synapses (cont.)Ribbon Synapses (cont.)

Ribbon terminals Ribbon terminals containing containing glycogenlike glycogenlike granules are granules are concentrated in concentrated in the outer half of the outer half of the IPLthe IPL

Conventional SynapsesConventional Synapses

Sharp increase beginning at fd88Sharp increase beginning at fd88 Detected in the periphery at fd78Detected in the periphery at fd78 Result from the aggregation of synaptic Result from the aggregation of synaptic

vesicles on one side of junctions the vesicles on one side of junctions the first existed as symmetrical membrane first existed as symmetrical membrane densities without vesiclesdensities without vesicles

In rod-dominated areas, develop before In rod-dominated areas, develop before ribbon synapsesribbon synapses

Conventional Synapses Conventional Synapses (cont.)(cont.)

In species with no cone-dominated In species with no cone-dominated areas (e.g. ferret, mouse, rabbit, rat, areas (e.g. ferret, mouse, rabbit, rat, guinea pig, cat) develop before ribbon guinea pig, cat) develop before ribbon synapsessynapses

Continued rise until after birthContinued rise until after birth Dark rearing leads to an increase in Dark rearing leads to an increase in

conventional synapses (Fisher, 1979a)conventional synapses (Fisher, 1979a) Plateau at 15/100 Plateau at 15/100 mm22

Conventional Synapses Conventional Synapses (cont.)(cont.)

Conventional Synapses band at varying Conventional Synapses band at varying levels of the IPL depending on the levels of the IPL depending on the speciesspecies

Synaptic Formation Related Synaptic Formation Related to IPL depthto IPL depth

Synaptogenesis begins in the outer Synaptogenesis begins in the outer IPLIPL– may be related to the length that the may be related to the length that the

axon needs to growaxon needs to grow– OFF bipolar cells may form before ON OFF bipolar cells may form before ON

cells giving spatial and temporal cells giving spatial and temporal dominancedominance

ConclusionsConclusions

Bipolar cells produce ribbon synapsesBipolar cells produce ribbon synapses Amicrine, Ganglion, and Bipolar cells Amicrine, Ganglion, and Bipolar cells

produce conventional synapsesproduce conventional synapses The type of photoreceptor dominance The type of photoreceptor dominance

determines synaptic developmentdetermines synaptic development Synaptogenesis precedes opsin Synaptogenesis precedes opsin

formationformation