UNSW Embryology

Day

Theiler Stage

Event

1

Theiler Stage 1

Theiler Stage 2

Fertilization

Dividing egg stage 2-4 cells. Zona pellucida present. First cleavage occurs at about 24 hours. Embryonic age = 1 dpc (range 1-2.5 dpc)

2

Theiler Stage 3

Morula (early to fully compacted) 4-16 cells. Zona pellucida present. Usually found in the oviduct towards the utero-tubal junction. Embryonic age = 2 dpc (range 1-3.5 dpc)

3

Theiler Stage 4

Blastocyst (ICM apparent) 16-40 compacted cells. Zona pellucida present. Embryo progresses from morula to the blastocyst. Early evidence of the blastocoelic cavity.

In the blastocyst stage (zona-intact) there is a distinct inner cell mass and an outer layer of trophectoderm cells. Usually located in the uterine lumen. Embryonic age = 3 dpc (range 2-4 dpc)

4

Theiler Stage 5

Theiler Stage 6

Blastocyst (Zona pellucida absent) Zona free blastocyst. Invariably located within the uterine lumen.

Embryonic age = 4 dpc (range 3-5.5 dpc)

Theiler Stage 6 - Attachment of blastocyst. Blastocyst implants, first evidence of embryonic endoderm cells covering the blastocoelic surface of the inner cell mass. Embryonic age = 4.5 dpc (range 4-5.5 dpc)

5

Theiler Stage 7

Implantation and formation of egg cylinder. Ectoplacental cone appears. Rapid increase in the number of inner cell mass cells leading to the formation of the epiblast with subsequent growth to form the egg cylinder. The proximal or visceral cells (opposite side from the trophoblastic cap) are cuboidal in shape. Primary endoderm lines the mural trophectoderm. Embryonic age = 5 dpc (range 4.5-6 dpc)

6

Theiler Stage 8

Theiler Stage 9

Differentiation of egg cylinder. Implantation site 2x3mm.The maternal tissue is invaded by trophoblast (primary) giant cells and the ectoplacental cone is invaded by maternal blood. Differentiation of the egg cylinder into embryonic and extra-embryonic regions and the formation of the pro-amniotic cavity. Reichert's membrane, which is non-cellular and secreted by the distal endoderm, first appears. Embryonic age = 6 dpc (range 5-6.5 dpc)

Stage 9a Advanced Endometrial Reaction. Advanced egg-cylinder stage with the first evidence of an embryonic axis. Clear morphological distinction between the embryonic and extra-embryonic ectoderm. The ectoplacental cone is further invaded by maternal blood and the original lumen of the uterine crypt has disappeared. Equivalent Downs and Davies Stage : PS (pre-streak)

Stage 9b Advanced Endometrial Reaction. Late in this stage gastrulation begins, producing the first mesodermal cells. Equivalent Downs and Davies Stage : ES (early streak)

7

Theiler Stage 10

Theiler Stage 11

Stage 10a Amnion. Tissue at the posterior end of the primitive streak bulges into the pro-amniotic cavity and forms the amniotic fold (Equivalent Downs and Davies stage: MS, mid-streak)

Stage 10b Amnion. In the mesoderm of the posterior amniotic fold small cavities coalesce to form a single cavity, the exocoelom Embryonic age = 7.0 dpc (range 6.5-7.5 dpc)

Stage 10c . Amnion. The allantoic bud first appears, gastrulation continues and the node becomes visible. Embryonic age = 7.0 dpc (range 6.5-7.5 dpc) (Equivalent Downs and Davies stages: MS - LS, mid-streak to late streak)

Stage 11a Neural Plate, Presomite stage The amniotic cavity is now sealed off into three distinct cavities - the amniotic cavity, the exocoelom and the ectoplacental cleft. The neural plate is defined anteriorly and the head process is developing. In the midline, subjacent to the neural groove, the notochodal plate is visible. Embryonic age = 7.5 dpc (range 7.25-8 dpc) Equivalent Downs and Davies stages: OB-EB (no allantoic bud to early allantoic bud); LB-EHF-LHF (late allantoic bud to early head fold to late head fold)

Stage 11b Neural Plate, Presomite stage. The allantoic bud elongates. Embryonic age = 7.5 dpc (range 7.25-8 dpc) . Equivalent Downs & Davies stages: OB-EB (no allantoic bud to early allantoic bud); LB-EHF-LHF (late allantoic bud to early head fold to late head fold)

Stage 11c Neural Plate, Presomite stage. The rostral part of the neural plate begins to enlarge to form the head folds. The neural groove is visible. Embryonic age = 7.5 dpc (range 7.25-8 dpc) Equivalent Downs and Davies stages: OB-EB (no allantoic bud to early allantoic bud); LB-EHF-LHF (late allantoic bud to early head fold to late head fold)

Stage11d Neural Plate, Presomite stage. Head folds continue to enlarge and the foregut pocket begins to form. Embryonic age = 7.5 dpc (range 7.25-8 dpc) ; LB-EHF-LHF (late allantoic bud to early head fold to late head fold) . Equivalent Downs & Davies stages: OB-EB (no allantoic bud to early allantoic bud)

Day

Stage

Event

8

Theiler Stage 12

Theiler Stage 13

Theiler Stage 12a First Somites Unturned embryo with first appearance of somite pairs 1-4 somites. The allantois extends further into the exocoelom and the maxillary components of the 1st branchial arch become prominent. The preotic sulcus is visible in the 2-3 somite embryo. The cardiogenic plate begins to form and the foregut pocket is clearly visible. Embryonic age = 8 dpc (range 7.5-8.75 dpc) 1-7 somite pairs

Theiler Stage 12b First Somites Unturned embryo with first appearance of somite pairs 5-7 somites. The headfolds are particularly prominent and neural closure occurs in the region of the 4th and 5th somites, extending in both directions from this site. The optic placodes are first evident and become indented to form the optic pits. The heart rudiment develops rapidly. The allantois contacts the chorion at the end of this stage. Absent: The 2nd branchial arch and >7 somites. Embryonic age = 8 dpc (range 7.5-8.75 dpc) 1-7 somite pairs

Theiler Stage 13 Turning of the embryo. This is a short period with turning initiated in embryos with 6-8 pairs of somites and usually completed in embryos with 14-16 pairs of somites. The first branchial arch has maxillary and mandibular components but the maxillary process is not visible until later (TS16). A second branchial arch is now evident. There is evidence of regionalisation of the heart and the neural tube is closed from a point opposite the outflow tract to the proximal part of the tail. Absent: 3rd branchial arch. Embryonic age = 8.5 dpc (range 8-9.25 dpc) 8-12 somite pairs

9

Theiler Stage 14

Theiler Stage 15

Formation and closure of anterior neuropore. The rostral extremity of the neural tube closes in embryos with usually about 15-18 somite pairs and defines this stage. The otic pit becomes progressively more indented but not closed, the mandibular process of the 1st branchial arch is clearly visible. The 3rd branchial arch becomes visible late in the stage. An increasingly prominent ridge on the lateral body wall, approximately at the level of the 8th-12th somite, indicates the site of the future forelimb bud. Absent: forelimb bud. Embryonic age = 9 dpc (range 8.5-9.75 dpc) 13-20 somite pairs

Formation of posterior neuropore, forelimb bud. The posterior neuropore forms and the condensation of the forelimb bud becomes apparent near the 8th-12th somite pairs. A distinct condensation of the hind limb bud appears just at the end of the stage.The forebrain vesicle subdivides into telencephalic and diencephalic vesicles. Absent: hindlimb bud, Rathke's pouch. Embryonic age = 9.5 dpc (range 9-10.25 dpc) 21-29 somite pairs

10

Theiler Stage 16

Theiler Stage 17

Closure of posterior neuropore. Hind limb bud and tail bud. The hind limb bud becomes visible at the level of the 23rd-28th somites. The tail bud appears as a short stump and the 3rd and 4th branchial arches are distinctly concave. Rathke's pouch and the nasal processes start to form. At the end of this stage the posterior neuropore begins to close. Absent: thin and long tail. Embryonic age = 10 dpc (range 9.5-10.75 dpc) 30-34 somite pairs

Stage 17 - Deep Lens Indentation The most obvious distinguishing features are the deepening of the lens pit, with a narrowing of its outer pore-like opening, and the first appearance of the physiological umbilical hernia. The 1st branchial arch is conspicuously divided into maxillary and mandibular components. There is advanced development of the brain tube and the tail elongates and thins. Absent: nasal pits. Embryonic age = 10.5 dpc (range 10-11.25 dpc) 35-39 somite pairs

Theiler Stage 18

Theiler Stage 19

Closure of Lens Vesicle. The primary externally recognisable feature is the progressive closure of the lens vesicle. The somites in the cervical region are no longer visible and the rapid growth of the brain is striking. The nasal pits start to form. Absent: auditory hillocks, anterior footplate. Embryonic age = 11 dpc (range 10.5-11.25 dpc) 40-44 somite pairs

Stage 19 - Lens vesicle completely separated from surface. The lens vesicle becomes completely closed and detached from the ectoderm. The peripheral margins of the eye become well defined. The forelimbs are seen to be divided into two regions, the proximal part consisting of the future limb-girdle and 'arm' and the more peripheral part which forms a circular or paddle-shaped 'handplate' (anterior footplate). The medial and lateral margins of the otic pit are coming together reducing the entrance to a narrow slit and the auditory hillocks become visible. Absent: retinal pigmentation, signs of 'fingers'. Embryonic age = 11.5 dpc (range 11-12.25 dpc) 45-47 somite pairs

Gonad - 11.5 - coelomic epithelium basement membrane is discontinuous, supporting cell migration. 11.2-11.4 - (somite 15-17 stages) coelomic epithelial cells of both sexes migrated into the gonad. In XY gonads, the migrating coelomic epithelial cells became Sertoli cells, as well as interstitial cells. This ability of the coelomic epithelium to give rise to Sertoli cells was developmentally regulated. 11.5-11.7 - (somite 18-20 stages) coelomic epithelial cells no longer became Sertoli cells. Instead, cells that migrated into the gonad stayed outside testis cords, in the interstitium.

12

Theiler Stage 20

Earliest signs of fingers. The 'handplate' (anterior footplate) is no longer circular but develops angles which correspond to the future digits. The posterior footplate is also distinguishable from the lower part of the leg. It is possible to see the pigmentation of the pigmented layer of the retina through the transparent cornea. The tongue and brain vesicles are clearly visible. Absent:5 rows of whiskers, indented handplate. Embryonic age = 12 dpc (range 11.5-13 dpc) 48-51 somite pairs

Gonad - 12.5 - (somite 30 stage) cell migration finished. Coelomic epithelium basement membrane thickens to form the tunica albuginea.

13

Theiler Stage 21

Anterior footplate indented, marked pinna. The distal borders of the anterior and posterior footplates are now indented and the digit widths and locations can be discerned. The 'elbow' and 'wrist' are now identifiable. The pinna rapidly develops and forms a crest at right angles to the head. Five rows of vibrissae are visible as well as a prominant hair follicle over the eye and another over the ear. The lens vesicle has lost its lumen. The physiological umbilical hernia is prominent. Absent: hair follicles, distally separate fingers. Embryonic age = 13 dpc (range 12.5-14) 52-55 somite pairs

14

Theiler Stage 22

Fingers separate distally. Individual 'fingers' are visible in the anterior footplate and there are deep indentations between the 'toes' which are not yet separated.The long bones of the limbs are present and there are hair follicles in the pectoral, pelvic and trunk regions. The pinna is turned forwards and the umbilical hernia is conspicuous. Absent: hair follicles in the cephalic region. Embryonic age = 14 dpc (range 13.5-15 dpc) 56-60 somite pairs

Day

Stage

Event

15

Theiler Stage 23

Toes separate. The 'toes' separate and are clearly divergent, not becoming parallel until later. Hair follicles are present in the cephalic region but not at the periphery of the vibrissae. The pinna covers more than half of the external auditory meatus and the eyelids are still open. Absent: nail primordia, 'fingers' 2-5 parallel. Embryonic age = 15 dpc >60 somite pairs

16

Theiler Stage 24

Reposition of umbilical hernia 'Fingers' 2-5 are nearly parallel. Nail primordia are visible on the 'toes'. The eyelids have fused in most cases by the end of the stage and the pinna almost completely covers the external auditory meatus. The umbilical hernia is disappearing and there is a corresponding increase in the size of the peritoneal sac. Absent: 'fingers' and 'toes' joined together. Embryonic age = 16 dpc > 60 Somite pairs   

17

Theiler Stage 25

Skin wrinkled The skin has thickened and formed wrinkles and the subcutaneous veins are less visible. The 'fingers' and 'toes' have become parallel and the umbilical hernia has disappeared. The eyelids have fused. Whiskers are just visible. Absent: ear extending over auditory meatus, long whiskers. Embryonic age = 17 dpc

18

Theiler Stage 26

Long whiskers The whiskers that were present at stage 25 are definitely longer and the skin has thickened. The pinna is larger and such that virtually none of the lumen of the auditory meatus is visible. The eyes are barely visible through the closed eyelids. Embryonic age = 18 dpc

19

Theiler Stage 27

Theiler Stage 28

New born Mouse

Post-natal development

20

21

Data For Carnegie Stages Comparison Graph (Species/Days)

Species

Stage

Human

Days

20

22

24

28

30

33

36

40

42

44

48

52

54

55

58

Mouse

Days

9

9.5

10

10.5

11

11.5

12

12.5

13

13.5

14

14.5

15

15.5

16

Rat

Days

10.5

11

11.5

12

12.5

13

13.5

14

14.5

15

15.5

16

16.5

17

17.5