8/4/2019 Events in Embryology - Grouped by Time

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Events in Embryology

[Note: cf. is used to refer you to another item in the same or a separate list for comparison of tworelated items. For example, (cf. 3rdWeek: 4)refers you to the 4th item in the list of the 3rd Week.]

1st Week 4th Week

1) (3 days after fertilization): morulaformed

1) (end): primitive streak beginsdegenerating and invagination stops

22) (during 4th-7th weeks): heartdivides into typical four-chamberedstructure

2) (6th day): implantation of blastocystin uterine endometrium begins

2) (end): caudal germ cells begindifferentiation (cf. 3rdWeek: 4)

3) (25 days): cranial neuropore closes 23) (in) stomach appears as afusiform dilation

3rd Week 4) (27 days): caudal neuropore closes

1) (of): the most characteristic event isgastrulation

5) (in): buccopharyngeal membraneruptures (cf. 7thWeek: 5)

24) (beginning): pronephros isrepresented by 7-10 solid cell groupsin the cervical region

2) (16th day): allantois appears 6) (end): embryo has 28 somites

3) (15-16 days): primitive streak clearly

visible as a narrow groove with slightlybulging regions on either side

7) (end of): sclerotome cells form

mesenchyme

25) (end): all indications of pronephric

system disappeared

4) (middle): cephalic germ cells begindifferentiating (cf. 4thWeek: 2)

8) (end of): limb buds become visible 26) (early): first excretory ducts ofmesonephros appear

5) (beginning): primary villi are present 9) (during): slerotome cells migratearound spinal cord and notochord andfuse with those of opposing somites

27) (4th-5th weeks): pharyngeal archesappear

6) (end): tertiary villi are formed 10) (during): diaphragm lies oppositecervical somites (cf. 6thWeek: 3)

28) (end): stomodeum with pair ofpharyngeal arches are present

7) (19-20 days): embryo is (still)attached by connecting stalk

11) (23rd day): heart tube continuesto elongate and bend (loop)

8) (beginning): ectoderm is disc-shaped 12) (28th day): looping of heart iscomplete

9) (end): neural folds and groove areformed

13) (middle): sinus venosus receivesblood from right and left sinus horns

10) (17 days): mesoderm differentiatesto its three layers

14) (27th-37th days): major cardiacsepta are formed

11) (beginning): paraxial mesodermbegins forming somitomeres

15) (end): septum primum appears

12) (20th day): first pair of somitesappear

16) (end): two primitive ventriclesbegin to expand

13) (at): first blood islands appear in

the wall of the yolk sac

17) (27th day): most of the 1st aortic

arch has disappeared

14) (end of): intraembryonic mesodermdifferentiates into paraxial, lateral plate,and intermediate mesoderm

18) (during): umbilical arteriesacquire secondary connection withcommon iliac arteries

15) (middle of): vascular systemappears

19) (during): the cardinal veins forma symmetrical system

16) (late in presomite stage - < 20days): splanchnic mesoderm is inducedby underlying pharyngeal endoderm toform cardiac myoblasts

20) (when 4 weeks old): lung budforms

17) (middle): liver primordium appears 21) (by 22nd day) the two heart tubesjoin to form a single slightly bentheart tube18) (in) cloacal folds form

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5th Week 6th Week 8th Week

1) (beginning): germ cells arrive atgonads (cf. 6thWeek: 9)

1) (in): hand-plates and foot-platesform

1) (end of 2nd month): major featuresof external body form are recognizable

2) (during): lens placodes form eyelenses

2) (by the): first hyaline cartilagemodels formed

2) (end of embryonic period):endochondral ossification of bones ofextremities begins

3) (end): 42-44 pairs of somites arepresent

3) (by the): diaphragm is at the levelof thoracic somites (cf. 4thWeek: 10)

3) (end of 2nd month): majority ofmesonephros has disappeared

4) (by the): yolk sac duct, allantois,and umbilical vessels are restricted toumbilical ring

4) (27th-37th days): major cardiacsepta formed

4) (by the): testosterone productionbegins

5) (end of): muscle cells collected intoepimere and hypomere

5) (during): physiological umbilicalherniation

5) (end of 2nd month): urogenitalmesentery attaches testis andmesonephros to posterior abdominalwall

6) (4th and 5th weeks): left-rightshunting of blood occurs

6) (at): cecal bud appears

7) (during): obliteration of rightumbilical and left vitelline veins

7) (middle of 2nd month):mesonephros forms a large ovoidorgan on each side of the midline 9th Week

8) (end): AV canal separated into rightand left canals

8) (until): germ cells don't appear ingonads

1) (by the 3rd month): chorion laeve isformed

9) (near end of): posterior extremityof bulboventricular flange terminates

9) (in): germ cells invade the gonads(genital ridges)

2) (beginning of 3rd month): somedorsal bands of diaphragm originate atthe level of 1st lumbar vertebra

10) (during): truncus swellings appear 10) (42 days): 2 mandibular, 2maxillary, and 1 frontonasalprominences can be recognized

3) (during): caudalmost part of analcolumn recanalizes from ectodermproliferation

11) (29th day): 1st and 2nd aortic archeshave disappeared

12) (in): three major vein pairs(vitelline, umbilical, and commoncardinal) can be distinguished

7

th

Week 10

th

Week13) (5th-7th weeks): subcardinal,supracardinal, and sacrocardinal veinsare formed

1) (during): limbs rotate in oppositedirections

1) (at): left common cardinal vein isobliterated

2) (in): 1st indication of limbmusculature appears

2) (in): liver weights about 10% oftotal body weight

14) (until): lymphatic system doesn'tappear

3) (by the): pleuroperitoneal folds fusewith the mesentery of esophagus andwith septum transversum to closeconnection between pleural andperitoneal cavities

3) (during): herniated loops begin toreturn to the the abdominal cavity

15) (beginning): bronchial budsenlarge to right and left main bronchi

4) (by the): differentiation of theglomerular capillaries begins followedby production of urine

16) (by the): dorsal mesentery isestablished

4) (5th-7th weeks): subcardinal,supracardinal, and sacrocardinal veinsare formed

17) (in): spleen primordium appears 5) (end): cloacal membrane ruptures(cf. 4thWeek: 5)

18) (in): midgut is suspended bydorsal mesentery and connected toyolk sac

6) (until): goands do not aquire maleand female characteristics

19) (in): metanephros appears 7) (in): cortical cords develop in theovary

20) (4th-5th weeks): pharyngeal archesappear

21) (in): inferior parathyroid andthymus glands are formed

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12th Week Unique Months

1) (end of 3rd month): oogonium issurrounded by flat epithelial cells

1) (5th month): maximum 7 milliongerm cells are found in ovary

17) (during 7th month): sufficientcapillaries are present for survival

2) (end of 3rd month): corpus luteumgraviditatis is about one-third to one-half the total size of the ovary

2) (7th month): majority of oogoniahave degenerated

18) (last 2 months several years after birth): terminal sacs increasesteadily in number

3) (end of 4th month): corpus luteum

continues to secrete progesteroneuntil this time

3) (end of 3rd month): head size = CRL

19) (6th month): at end, type IIalveolar cells appear

4) (by the): external genitalia aredeveloped enough for recognition byultrasound

4) (beginning of 5th month): head size= 1/3 CHL

20) (last 2 months): hepatichemopoietic activity subsides

5) (end of 5th month): weight of fetus< 500 g

21) (until the end of the 5 th month):more tubules form at the periphery ofmetanephros

5) (end of 3rd month): amniotic cavitycomes into contact with chorionobliterating chorionic cavity

6) (during 5th month): movements ofthe fetus can be felt by the mother

6) (by the): primary centers ofossification are present in all the longbones of the limbs and skull

7) (beginning of 4th month):cytotrophoblast cells and someconnective tissue of villi disappear

22) (4th month): testis cords becomehorseshoe-shaped and cortical ovariancords are split to isolated clusters

7) (end of 3rd

month): cross-striationsof skeletal muscles appear

8) (4th

month): placenta has fetal andmaternal components

23) (5th

month): vaginal outgrowth isentirely canalized

8) (at): bile is formed by hepatic cells 9) (4th and 5th months): decidual septaare formed

24) (4th month): solid cord form ofexternal urethral meatus forms

9) (near): kidneys become functional

10) (end of 3rd month): urethra givesoutgrowth of male glands

10) (14 weeks): IgG is transportedthrough placenta

25) (28 weeks): testes migratethrough the inguinal canal

11) (by approximately): testis reachesinguinal region

11) (after 3rd month gradually): yolksac disappears

26) (33 weeks): testes reach scrotum

27) (in 2nd month): yolk sac lies inchorionic cavity12) (3rd-5th months): palatine tonsils

are infiltrated by lymphatic tissue12) (5th month): fetus beginsswallowing amniotic fluid

13) (14 weeks): liver begins forming-fetoprotein which leaks intomaternal circulation

28) (during 2nd month): solid phase ofduodenum

14) (4th month): ossification of ossiclesbegins

29) (2nd month): counting somitesbecomes difficult and CRL is used

15) (6th month): 17 generations ofbronchiolar subdivisions are formed

30) (2nd month): increase in head size,and formation of limbs, face, ears,nose and eyes

16) (7th month): up to this timebronchioles divide continuously tomore and smaller canals

31) (in 3rd month): pancreatic isletsdevelop

32) (during 3rd month): chorion laeveis formed

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