Embryology Stages of development Preembryonic period Embryonic period

Embryology Stages of development

• Pre-embryonic period • Embryonic period • Fetal period

Stages of development before birth • Pre-embryonic period---conception to week two-----fertilized ovum or zygote undergoes mitosis. Formation of morula, appearance of blastocyst, blastocyst implanted and two layer germ disc is formed. teratogen usually cause loss of conceptus during this period.

• Two cell stage----approximalely after 30 hours • Four cell stage----approximalely after 40 hours • Twelve to sixteen cell stage--- morula--approximalely after 3 days • Blastocyst stage-------approximalely after 4 days • Implantation starts ---5 to 6 days of development.

Formation of morula

Formation of blastocyst

Second week of development • At eigth day of development blastocyst patly embeded in endometerial stroma. • Area over embryobast i. e. trophoblast has differentiated into inner layer of mononucleated cell---cytotrophoblast outer layer of multinucleated cell---syncytiotrophoblast

Embryoblst or inner cell mass • It is differentiated in two layers • A Layer adjacent to blastocyst cavity is known as hypoblast layer • A layer of high columnar cell adjacent to hypoblastlayer is known as the epiblast layer • Thes two layer are in contact and form bilaminar embryonic disc.

• At the same time a cavity develops in epiblast layer which is known as amniotic cavity • Those epiblast cells adjacent to cytotrophoblast are known as amnioblast. • Hypoblast cell proliferate and cover cytotrophoblast and form a membrane known as heuser’s membrane.

• A cavity is formed between hypoblast cells and heuser’s membrane-----primary yolk sac • Yolk cells proliferate and comes to lie between cytotrophoblast and amniotic membrane and heuser’s membrane and form extraembryonic mesoderm.

• Cavities are formed in extra-embryonc mesoderm known as extra-embryonic cavity. • These cavities unites to form extraembryonic coelom or chorionic cavity. • This space surrounds primitive yolk sac and amniotic cavity except where germ disc is connected to trophblast by connecting stalk.

• Formation of extraembryonic coelom divides extraembyonic mesderm in two layers. • Layer covering amnion and cyotrophoblast is known as somatotrophic mesoderm. • Layer covering yolk sac is known as splanchnopleuric mesoderm. • Cells of endometrium become polyhederal loaded with glycogen and lipids , intercellular spaces are filled with extravasate-------decidual reaction

• By 13 th day uterine epithelium is reformed • Cytotrophoblast starts proliferating and produce finger like projection (villous structure) into syncytiotrophoblast-----primary villi. • In the mean time , the hypoblast produce additional cells that migrate along inside of exocoelomic membrane and form a new cavity inside exocoelomic cavity.

• This new cavity is known as secondary or definitive yolk sac. • At the end of second week------ syntytiotrophblast starts secreting Human chorionic gonadotropin(HCG). ----localized thickening of hypoblastic cells known as prechordal plate(future cephalic region)

Stages of development before birth • Embryonic period---second to eight weeks -----three germ layer and placenta develop. Main body system form. because this is most active period of development and differentiation this period is most susceptable to major birth defect.

Applied anatomy • Ectopic pregnancy • abortions • Neural tube defect Among one of the most common anomalies. Meroencephaly or anencephaly is most serious neural tube defect. • Sacrococcygeal teratoma • Sirenomelia (caudal dysgenesis)