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TitleThe Developing Marsupial: Models for Biomedical Research
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Page 2

c. H. Tyndale-Biscoe and
P. A. Janssens (Eds.)

The Developing
Models for Biomedical Research

With 102 Illustrations

Berlin Heidelberg New York
London Paris Tokyo

Page 126

118 S. A. Dunlop et al.

a Primary Visual Nuclei • •

Fig. 9.1. a Diagram of the mammalian primary visual system. Axons (A) from retinal ganglion
cells (RGC) enter the nerve fibre layer (NFL) and converge at the optic nerve head (ONH). Most
fibres cross at the chiasm (CH) to the contralateral optic tract (OT) whilst some enter ipsilaterally
before terminating in the primary visual centres. b Transverse section through an adult S.brachy-
urus retina showing the nerve fibre layer (NFL), retinal ganglion cell layer (RGCL), inner plexi-
form layer (IPL), inner nuclear layer (INL), outer plexiform layer (OPL) and outer nuclear layer
(ONL). Cresyl violet, scale bar 20 11m

in the outer nuclear layer transduce incoming light into electrical impulses; inter-
estingly, the cones of marsupials are unique amongst mammals in possessing oil
droplets (O'Day 1936, Walls 1939, Braekevelt 1973). Visual information is then re-
layed via the inner nuclear layer to the ganglion cell layer. Axons arise from gan-
glion cells and converge at the optic disc to form the optic nerve which connects
the eye to the brain (Fig. 9.1 a). Most fibres cross to the other side of the brain at
the chiasm to form the optic tract before terminating in the primary visual centres
(Fig. 9.1 a). The degree of decussation of fibres at the chiasm is probably related to
the extent of binocular overlap (Lund 1978).

The retina of some marsupials is essentially avascular, relying on a prominent
choroidal circulation to supply nutrients and oxygen by diffusion (Johnson 1901,
Chase 1982). However, in a number of macropodids and phalangerids, fine capil-
laries arise from the optic disc and radiate a short distance across the nerve fibre
layer (Johnson 1901; Fig.9.2d,e). In dasyurids, the retinal circulation is more
extensive (Harman 1982). The degree of retinal circulation also varies in eutheri-
ans from networks which cover the entire retina in human, monkey, cat and rat to
the avascular retina of guinea pig; rabbits provide an interesting example of par-
tial vascularization with vessels being confined to the myelinated epiretinal strip
(Chase 1982, Stone and Dreher 1987).

Several aspects of the marsupial primary visual pathway and its development
will be reviewed. Events occurring during the formation of cell density gradients
in the retinal ganglion cell layer will be described. Also, maturation of the optic
nerve and fibre projections to the primary visual centres will be discussed.

Page 127

Development of the Marsupial Primary Visual Pathway 119

Fig. 9.2. a Transverse section through the head of an embryo of S. brachyurns at Day 21 of preg·
nancy, showing the neural tube (NT), which has yet to close (arrow). The optic vesicles (OV)
extend from the ventro·lateral portion of the neural tube towards the overlying skin ectoderm (E).
Scale bar 100 Ilm. b Transverse section through the optic cup (OC) of an embryo of S .brachyurns
at Day 24 of pregnancy. The inner and outer layers of the cup form the neural retina (R) and pig·
ment epithelium (PE) respectively. The lens (L) has separated from the skin ectoderm (E). Scale
bar 100 Ilm. c Diagram of the eye of adult and Day 7 1 M.fuliginosus, sectioned through the optic
nerve head (ONH). Liens; Ccornea; Ttemporal; Nnasal. Scale bar 1 cm. d and e capillary loops
extending away from the optic nerve head (arrow). (a and b modified from Harman and Beazley

Cell Density Gradients in the Ganglion Cell Layer

The retinal ganglion cell layer is composed of two main classes of neuron, Retinal
ganglion cells project an axon into the visual centres via the optic nerve. Displaced
amacrines comprise the second cell type and are intrinsic neurons which lack an

1 Throughout this book 'Day', unless qualified, refers to days since birth. Eds.

Page 251


panting in response to heat 159
parturition 26
passive immunity 164, 190, 196-198
pelage development 157
pepsinogen 164
Perameles nasuta 10,31,205
peramelids 30, 37
Petaurus brevieeps 50
phallus 206-208
pharynx 19
Phaseogale calura 109,110
Phaseolaretos cinereus 50
phosphoenolpyruvate carboxykinase 167-169
pituitary vasopressin 183, 184
placenta 31
Planigale tenuirostris 39
plasma proteins 111-115
polygyny 37
pons of newborn 15
potassium concentration in milk 49
Potorous tridaetylus 10,38,42, 160, 175, 205
pouch, anatomy of 201,202

bacterial flora 134, 196
development 206-208
humidity 133
respiratory gas levels 133
temperature 174,175

pouch exit 158, 173
primary visual pathway 117, 118
primary visual system 127, 130
progesterone in lactation 58,69, 73-75
prolactin, during lactation 59, 60, 62-65, 77

effect of sucking stimulus 63
in milk protein synthesis 71,84
mammary receptors 58, 80-93

pronephros 176-177
prostate 207
protein synthesis in mammary explants 79
proteins in cerebrospinal fluid 115

in milk 47, 70, 77, 84, 162
Pseudoeheirus peregrinus 39
pulmonary systemic shunts, closure at birth

pyruvate kinase 167, 170

rate of cooling of pouch young 154
relative medullary thickness 186
REM sleep 159, 160
reproductive investment 35
respiration, cutaneous 137

development of chemical control 141
respiratory system in newborn 140
retina, cones and oil droplets 118

neurogenesis in 123 - 127
vasculature 118

retinal ganglion cell layer 119, 125
retinal ganglion cells 119-123
rhinarium 11

Subject Index

s-adenosyl methionine, effect on prolactin
receptors 80

salivation in response to heat 159
Sareophilus harrisii 8-23,50,209
scrotum 201-208
Sertoli cell 200, 201
Setonix braehyurus, bacteria in pouch 134,196

development of immune system 191-195
development of visual system 119-130
dominance hierarchy 37
milk iron content 49
milk protein 75
oxygen consumption during development

sex allocation 35-37
sex chromosomes, extra-gonadal effects 209
sex reversal 207
sexual differentiation 200
sexual dimorphism 37-39,91
sialic acid in milk 172
skin vascularity of newborn 137
sleep and thermoregulation 159
small intestine 20
Sminthopsis erassieaudata 4, 7
sodium concentration in milk 49
somatosensory system 96
spleen 191
staging system for brain development 88
standard metabolic rate 148,149
stomach 163
sucking, effects on lactation 62, 83
superior colliculus 131
surface area of pouch young 154
surfactant 24, 135

Tachyglossus aculeatus 45,47,52
Tarsipes rostratus 39
taste bud of newborn 17, 18
teat denervation, effects on lactation 61
teat development 206
telencephalon 102
testis migration 200, 201
testosterone 200, 207
thermal incompatibility of mother and young

thermogenesis 155

non-shivering 157
thermoregulation of young 153,156,159,174
Thylogale thetis 38
thymectomy, effects of 193 -195
thymus 191
thyroid hormones, effect on metabolic rate

in lactation 73-77

tidal volumes in newborn 140
time to maturity 28, 31, 39, 40
tissue metabolism 152

Page 252

Subject Index

tongue 20
touch in newborn 16
transferrin in milk 77, 198
trehalase 168
Trichosurus vulpecula 42,48,50,65,109, 122,

trigeminal nerve 15-16,86,95
trypsin 166
Turner syndrome 209

urine, ammonia 170-172
concentration 177 - 183
osmotic stimulators 185
pH 170-172
urea 170-172

urogenital sinus 206-208

vasa recta 187
vasopressin 182-185
vibrissae, nerve connections 97
visual projections 131
visual streak 120-122
vitamins in milk 51
Vombatus ursinus 42, 50

weaning, metabolism during 171
whey protein 77
Wolffian duct 201-208

y chromosome 200


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