Third Week of Development

(Trilaminar Germ Disk)

The main events during this period:

• Gastrulatin.

• Formation of the notochord.

• Establishment of ^ body axes.

• Growth of ^ embryonic disc(cephalocaudally).

• Further development of ^ trophoblast.

Gastrulation

• The process that establishes all three germ layers ( ectoderm, mesoderm, & endoderm), which begins with ^ formation of ^ primitive streak.

• ^ Primitive streak: a narrow groove formed on ^ surface of ^ epiblast, which is clearly visible at 15- 16 days embryo.

• ^ cephalic end of ^ streak, ^ primitive node, consists of a slightly elevated area surrounding ^ small primitive pit .

• Cells of ^ epiblast migrate toward ^ primitive streak, upon their arrival, they detach from ^ epiblast & slip beneath it, this inward movement is known as invagination .

• Once ^ cells have invaginated ,some displace ^ hypoblast, creating ^ embryonic endoderm.

• Some cells come to lie between ^ epiblast & newly created endoderm to form mesoderm.

• Cells remaining in ^ epiblast then form ectoderm.

• Epiblast layer is ^ source of all ^ germ layers.

• Oropharyngeal membrane at ^ cranial end of ^ disc consists of small region of tightly adherent ectoderm & endoderm without mesoderm.

• Cloacal membrane at ^ caudal end of ^ disc similar to ^ orophangeal memb.

• ^ Prechordal plate forms between ^ tip of ^ notochord & ^ oropharyngeal membrane & is derived from ^ 1st cells that migrate from ^ node in cephalic direction.

• As more & more cells move bet. ^ epiblast & hypoblast layers, they begin to spread laterally & cranially.

• Gradually, they migrate beyond ^ margin of ^ disc & establish contact with ^ extraembryonic mesoderm covering ^ yolk sac & amnion.

Formation of the Notochord

• Prenotochordal cells invaginating in ^ primitive node move forward cranially in ^ midline until they reach ^ prechordalplate, these cells become intercalated in ^ hypoblast for a short time at ^ midline then form ^ notochord plate.

• Cells of ^ notochordal plate proliferate & detach from ^ endoderm & form a solid cord of cells , ^ definitive notochord.

• ^ Cranial end forms first, & caudal regions are added as ^ primitive streak assumes a more caudal position.

• ^ notochord & prenotochordal cells extend cranially to ^ prechordal plate & caudally to ^ primitive pit.

• The primitive pit forms an indentation in ^ epiblast, ^ neurenteric canal temporarily connects ^ amniotic & yolk sac cavities.

• When ^ cloacal memb. appears, ^ posterior wall of ^ yolk sac forms a small diverticulum that extends into ^ connecting stalk called ^ allantoenteric diverticulum, or allantois, appears around ^ 16th day of development.

Establishment of the body axes

• Establishment of ^ body axes, anteroposterior, dorsoventral, & left-right, takes place before & during ^ period of gastrulation.

• Cephalic & caudal ends of ^ embryo are established before ^ primitive streak is formed.

• There are groups of genes which control ^ process of gastrulation & establishment of ^ body axes.

Fate map established during gastrulation

• Regions of ^ epiblast that migrate through ^ primitive streak have been mapped, & their ultimate fates have been

determined.• For ex. cells that ingress through ^ cranial region of ^ node

become prechordal plate & notochord; those migrating at ^ lateral edges of ^ node& from ^ cranial end of ^ streak become paraxial mesoderm; cells migrating through ^ midstreak region become intermediate mesoderm; & those migrating through ^ more caudal part of ^ streak form lateral plate mesoderm.

Growth of the embryonic disc

• The embryonic disc, initially flat almost round, gradually becomes elongated, with a broad cephalic & narrow caudal end.

• Continuous migration of cells from ^ primitive streak in cephalic direction leads to growth & elongation of ^ cephalic region.

• The migration of cells from ^ primitive streak forward & laterally continues until ^ end of 4th week.

• In ^ cephalic region, germ layers begin their differentiation by ^ middle of ^ 3rd week, whereas in ^ caudal part, differentiation begins by ^ end of ^ 4th week.

Thus gastrulation (formation of germ layers) continue in caudal segments while cranial structures are differentiating, causing ^ embryo to develop cephalocaudally.

Further development of ^ trophoblast

• By ^ beginning of ^ 3rd week, ^ trophoblast is characterized by primary villi that consist of a cytotrophoblastic core covered by a syncytial layer.

• During further development, mesodermal cells penetrate ^ core of primary villi & grow toward ^ decidua, here are called secondary villi.

• By ^ end of ^ 3rd week, mesodermal cells in ^ core of ^ villus begin to differentiate into blood cells & small blood cells forming villus capillary system, ^ villi are called tertiary or definitive placental villus.

• Capillaries in tertiary villi make contact with capillaries developing in ^ mesoderm of ^ chorionic plate & in ^ connecting stalk, these vessels , in turn, establish contact with intraembryonic circulatory system, connecting ^ placenta & ^ embryo.

• Hence, when ^ heart begins to beat in ^ 4th week of development, ^ villus system is ready to supply ^ embryo with nutrients & oxygen.

• Meanwhile, cytotrophoblast cells in ^ villi penetrate into overlying syncytium until they reach ^ maternal endometrium, here they contact with similar extensions from ^ neighboring villus stems forming a thin outer cytotrophoblast shell.

• Villi that extend from ^ chorionic plate to ^ decidua basaliscalled stem or anchoring villi, those that branch from ^ sides of stem villi are free (terminal) villi, through which exchange of nutrients & other factors will occur.

• By ^ 19th-20th day ^ embryo is attached to its trophoblastic shell by a narrow connecting stalk.