THIRD WEEK OF DEVELOPMENT Dr Samina Anjum

GASTRULATION It is the formative process by which the three germ layers and axial orientation are established in embryo.

Gastrulation is the beginning of body form (morphogenesis) Bilaminar germ disc converts into trilaminar germ disc. Embryo is referred as gastrula.

HUMAN GASTRULATION Begins with the formation of primitive streak- thickened linear band/opacity on the surface of epiblast due to proliferation of epiblast cells. Initially vague and then clearly defined by15th -16th day as the primitive groove forms.

The movement of the cells through the primitive streak results in the formation of a primitive groove

Streak elongates by addition of cells at its caudal end Cranial end proliferates to form a slightly elevated area- Primitive node, that surrounds a small depression the primitive pit. Concurrently the groove become continuous with the primitive pit

Upon arrival in the region of streak, epiblast cells become flask shaped, detach from epiblast, slip beneath it. This inward movement is called invagination.

Once the cells invaginate through the groove some displace the hypoblast and forms embryonic endoderm. Others lie between epiblast and newly formed endoderm and form mesoderm. Cells remaining in the epiblast form the embryonic ectoderm.

As more and more cells move between epiblast and hypoblast they begin to spread laterally & cranially. Gradually migrate beyond the margins of the disc and establish contact with EEM. In cephalic direction they pass on each side of Prechordal plate.

PRECHORDAL PLATE Lies between the tip of notochord and oropharyngeal membrane This is a small aggregation of mesodermal cells closely apposed to the endoderm Is derived from some of the first cells that migrate from the node. Prechordal plate is important for the induction of forebrain.

Buccopharyngeal membrane (prochordal plate): lies at the cranial end of disc & consist of tightly adherent ectoderm & endoderm cells. Ruptures in 4th week to join oral cavity with pharynx. Cloacal membrane: lies at the caudal end of disc & consist of tightly adherent ectoderm & endoderm cells.

FATE OF PRIMITIVE STREAK Actually forms mesoderm until the end of 4th week and then regress & degenerate and soon disappears.

REMNANT OF PRIMITIVE STREAK Remanants of primitive streak persist in the sacrococcygeal region and form tumors -Sacrococcygeal teratomas These are clusters of pluripotent cells; contain tissues derived from all the three germ layers. Most common tumor in newborn : 1 in 37000

Thus the epiblast through the process of gastrulation is the source of all the germ layers. Cells in these layers will give rise to all of the tissues and organs in the embryo

FORMATION OF NOTOCHORD Prenotochordal cells invaginating in the primitive pit, move forward cranially until they reach the prechordal plate forming a median cellular cord, the notochordal process. Formation of notochordal canal Extent of notochordal process

The floor of the notochordal process fuses with the underlying embryonic endoderm. The fused layers gradually undergo degeneration, resulting in the formation of openings in the floor of the notochordal process, which brings the notochordal canal into communication with the yolksac.

The openings rapidly become confluent and the floor of the notochordal canal disappears; the remains of the notochordal process form a flattened, grooved notochordal plate Beginning at the cranial end of the embryo, the notochordal cells proliferate and the notochordal plate infolds to form the definitive notochord.

The proximal part of the notochordal canal persists temporarily as the neurenteric canal , which forms a transitory communication between the amniotic and yolk sac cavities. When development of the notochord is complete, the neurenteric canal normally obliterates. The notochord becomes detached from the endoderm of the yolk sac, which again becomes a continuous layer and itself forms the definitive notochord.

THE NOTOCHORD Defines the primordial longitudinal axis of the embryo and gives it some rigidity Provides signals that are necessary for the development of axial musculoskeletal structures and the central nervous system Contributes to the formation of intervertebral discs

ALLANTOIS Sausage shaped diverticulum appears on the16th day and grows into the mesoderm of the connecting stalk. In humans allantoic sac remains small. It has a respiratory or excretory function in embryos of reptiles, birds and some mammals.

The Allantois is involved in the abnormalities of bladder development The Allantois is involved in the abnormalities of bladder development. As the bladder enlarges, the Allantois becomes the urachus connecting the apex of bladder with the umbilicus.  In adults represented by median umbilical ligament

ESTABLISHMENT OF BODY AXIS ANTEROPOSTERIOR AXIS DORSOVENTRAL AXIS LEFT – RIGHT SIDEDNESS

AVM

FATE MAP ESTABLISHED DURING GASTRULATION Regions of epiblast that migrate and ingress through primitive streak have been mapped and their ultimate fates determined.

CLINICAL CORRELATES Holoprosencephly: High doses of alcohol kills cells in anterior midline of germ disc, results in deficiency of midline in craniofacial structures. Caudal dysgenesis (sirenomelia) – Insufficient mesoderm in caudal most region of embryo Situs inversus: Transposition of viscera 20% of these cases develop Kartagener syndrome(abnormal cilia in air passages, nasal cavity)

GROWTH OF THE EMBRYONIC DISC Initially flat & round, gradually becomes elongated with broad cephalic and narrow caudal end. In cephalic part germ cells begin their differentiation while the caudal part differentiates by the end 4th week. Thus while gastrulation is occurring in caudal segments cranial structures are differentiating------ Cephalocaudally

FURTHER DEVELOPMENT OF TROPHOBLAST

Cont… Capillaries of tertiary villi contact with capillaries in mesoderm of chorionic plate and connecting stalk, which inturn contact with intraembryonic circulation

TROPHOBLAST AT THE END OF THIRD WEEK OF DEVELOPMENT Radial appearance Hence when Heart begins to beat in the 4th week of development the villous system is ready. Intervillous spaces: lined with syncytiotrophoblast

OUTER CYTOTROPHOBLAST SHELL, STEM VILLI & FREE VILLI

Cont…. Embryo is attached to trophoblastic shell by Connecting stalk Umbilical cord

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