staging reference

MONODELPHIS STAGING REFERENCE SERIES

All photographs taken immediately after removing embryo from uterus, with no preparation. Ages are mean age, calculated from time of mating. See data on age/stage relations at end of series.

Stage 22

Approximately 10.1 days after mating

first somites
onset of neural crest migration
preotic and otic sulci present
notochord visible in midline groove

Stage 23

Approximately 10.25 days

first arch mesenchyme condensing into recognizable mass
first appearance of optic pit (slit like)
~ 4 somites
no heart tubes
no sensory anlagen

STAGE 24

Approximately 10.54 days

no contact of neural folds
optic pits and otic placode visible; optic pits rounded
6-8 somites
forebrain = diencephalon only (identified by optic pit) – thin flap projection anteriorly on embryo
migration of second arch neural crest
robust mass of mesenchyme/neural crest in future 1^st arch/frontonasal region

STAGE 25

Approximately 10.6 days

first contact of neural folds; closed up to otic capsule
fusion of heart tube
12-13 somites
optic vesicles project ventrally
otic placodes well-formed cups
migration of post-otic neural crest
first evidence of cephalic flexure
rhombomeres clear
spread of neural crest into frontonasal processes
first and second arches clear

STAGE 26

Approximately 10.75 days

definitive cephalic flexure
no closure of neural fold anterior to myelencephalon
head fold in amnion, to about level of otic placode
forelimb thickening
maxillary process begins to differentiate
optic pits begin to project laterally (diencephalic floor has expanded)
olfactory placode

STAGE 27

Approximately 10.8

cervical flexure near level of cervical spinal cord.
15-18 somites
major cranial neural crest migration complete
otocyst still open; olfactory pit appears
neural tube open anterior to otic capsule
eye begins to appear on lateral surface of embryo; beginnings of lens induction (check)
amniotic head fold back to cervical region
forelimb bud broad fin-like ridge
3rd arch distinct
maxillary process differentiated

STAGE 28

Approximately 11.3 days

otocyst and anterior neuropore closed
head fold of amnion extends to forelimb bud
no deep naso-oral groove
maxillary process beginning to fuse with naso-frontal process
primary lumbar flexure -- i.e. a dorsal flexure in lumbar region
forelimb bud becomes bud rather than ridge (longer than wide)
lens placode beginning
first three pharyngeal arches well defined
up to 25 somites

Stage 29

Approximately 11.7 days

distinct nasal-oral grove
tail bud
club-like enlargement of distal part of forelimb
hind limb ridge; not yet bud
pharyngeal membrane breaks down
trachea separates from esophagus
cranial ganglia send processes to CNS
otocyst gets endolympahtic duct

stage 30

Approximately 11.75

secondary lumbar flexure (sharply flexed ventrally)
flattening of forelimb club to paddle
hindlimb bud
posterior neuropore closed
nasal-oral groove
sensory ganglia connected to spinal cord
motor branches going out to limb buds
all cranial nerves present except motor to eye

Stage 31

Approximately 12.2 days

loss of nasal-oral groove
frontal processes
auricular tubercles
fusion of mandibular and hyoid arches
prominent digital ridges on forelimbs
hind limb still bud stage
motor nerves to eye appear

Stage 32

Approximately 13.4 days

hind limb club like
definite digital buds on forelimb
no eye folds
cartilage appearing
pigment in eye

Stage 33

Approximately 13.75 days

upper and lower eyelid folds
clear digits apparent
mouth open, tongue protruding
digits not present on hind limb
lateral palatine processes
membrane bones in maxilla and mandible

Birth

Approximately 14.5 days

Age/stage relation based on documented mating (data as of July, 2002)

Stage	number	mean age	min	max

20	1	10.00
22	3	10.11	10.00	10.33
23	4	10.28	10.25	10.29
24	4	10.54	10.25	10.75
25	12	10.61	10.50	11.13
26	8	10.74	10.54	10.83
27	5	10.81	10.54	11.00
28	8	11.38	11.00	11.75
29	2	11.75	11.71	11.79
30	7	11.77	11.33	12.67
31	3	12.21	11.50	12.75
32	3	13.44	13.38	13.50
33	3	13.78	13.50	14.25