Introduction

Week 3 embryonic disc

Gastrulation

Key events of human development during the third week (week 3) following fertilization or clinical gestational age week 5, based on last menstrual period.. Note that during this time the conceptus cells not contributing to the embryo are contributing to placental membranes and the early placenta.

During the third week of development conceptus implantion in the uterus wall is complete and trophoblast cells continue to invade uterine wall in the process of early placentation (villi formation). Within the conceptus, gastrulation converts the bilaminar embryo into the trilaminar embryo (ectoderm, mesoderm, endoderm). Morphological changes include an epithelial to mesenchymal cell transition and folding of the embryonic disc. Carnegie stages 7 to 9 covers this period of human embryo development.

Outside of the embryo, the extraembryonic spaces (chorionic, amniotic, yolk) and intraembryonic spaces (coeloms) continue to develop. Trophoblast cells continue to invade uterine wall and the emdometrium is being converted into the decidua, the process of villi formation and early placentation has begun

Term

Major Events of 3rd Week

Definition

1.Craniocaudal folding caused by growth of CNS

2. Lateral folding caused by growth of somites,

amnion, and other components of lateral body

wall. 3. craniocaudal folding and lateral folding

change shape of embryo from a two-dimensional

disk to a 3-dimensional cylinder.

Term

Gastrulation

Definition

process establishes three definitive germ layers of

embryo, thereby forming a trilaminar embryonic

disk by day 21 of development. first indication

primitive streak

Term

Ectoderm

Definition

gives rise to epidermis, the central and peripheral

nervous systems, the retina of eye.

Term

Endoderm

Definition

source of epithelial linings of respiratory passages

and GI tract, glands into GI tract, glandular cells

of associated organs liver, pancreas.

Term

Mesoderm

Definition

gives rise to smooth muscular coats, connective

tissues, vessels associated with tissues and organs,

forms most of cardiovascular system. blood cells

bone marrow, skeleton, striated muscle

Term

Primitive Streak

Definition

first sign of gastrulation consists of: primitive

groove, node, pit. composed of epiblast.

Term

Mesenchyme

Definition

cells leave the deep surface of Primitive groove

and form loose network of embryonic tissue

called mesenchyme or mesoblast, which forms the

intraembryonic mesoderm

Term

Embryonic endoderm

Definition

some cells of epiblast displace hypoblast forming

embryonic endoderm in roof of yolk sac.

Term

Cloacal membrane

Definition

caudal to the primitive streak and future site of

anus, where epiblast and hypoblast cells are

fused.

Term

Epiblast products

Definition

ectoderm, intraembryonic mesoderm, and

endoderm of trilaminar embryonic disk.

Term

Intraembryonic Mesoderm

Definition

germ layer that orms during week 3 in contrast to

extraembryonic mesoderm, which formed during

week 2.

Term

Intermediate Mesoderm

Definition

connects paraxial mesoderm and lateral

mesoderm. forms longitudinal dorsal ridge

known as urogenital ridge, involved in future

kidney and gonad formation.

Term

Lateral mesoderm

Definition

thin plate mesoderm in which intraembryonic

coelem forms, dividing lateral mesoderm into 2

layers: a. intraembryonic somatic mesoderm

b.intraembryonic visceral mesoderm

Term

Notochod functions

Definition

induces overlying embryonic ectoderm to thicken

and from neural plate. induces formation of

vertebral bodies. forms nucleus pulposus of each

intervertebral disk in a newborn.

Term

Cardiogenic mesoderm

Definition

where primordium of the heart begins to develop

Term

Neural Plate

Definition

Primordium of CNS. developing notochord

induces overlying embryonic ectoderm to thicken

and form neural plate

Term

Neural plate

Definition

as notochord develops embryonic ectoderm

thickens to form an elongated plate of thickened

epithelial cells.

Term

Neuroectodermal cells

Definition

lie along crest of each neural fold and form neural

crest which give rise to spinal ganglia and the

ganglia of autonomic nervous system

Term

Neural Crest derivatives

Definition

CN ganglia V,VII,IX,X; meningial layers,

schwann cells, cells of suprarenal medulla, several

skeletal and muscular components in head.

Term

Somites

Definition

arise from division of paraxial mesoderm that

pair into cuboidal bodies.

Term

Angiogenesis

Definition

blood vessel formation begins in extraembryonic

mesoderm of yolk sac connecting stalk and

chorion.

Term

Blood Vessel development

Definition

develop from endothelial cells as the vessels

develop in walls of yolk sac and allantois at end of

third week.

Term

Blood Formation

Definition

Blood formation does not begin until the fifth

week and occurs in embryonic mesenchyme, the

liver, spleen, bone marrow, and lymph nodes.

Term

Primordial Cardiovascular System

Definition

forms from mesenchymal cells in cardiogenic

area, by formaing a primitive heart tube. form

primordial cardiovascular system. cardiovascular

system first functional system.

Term Definition

Cardiogenic mesoderm

two endocardial heart tubes arise from

cardiogenic mesoderm. these fuse to form

primitive heart tube, which develops into

endocardium.

Term

Chorionic Sac

Definition

Extraembryonic somatic mesoderm and two

layers of trophoblast constitute the chorion.

Term

Chorionic Cavity

Definition

chorion forms the wall of chorionic sac which is

area surrounding yolk-sac and the amniotic

cavity.

Term

Transvaginal ultrasound

Definition

measures chorionic sac diameter. measurement is

valuable for evaluating very early embryonic

development and pregnancy outcome.

Term

Chorionic villi

Definition

grow from cytotrophoblast that grow into the

syncytiotrophoblast. Thought to be induced by

the underlying extraembryonic somatic

mesoderm. develop into chorionic villi of

placenta.

Term

Beginning of blood flow

Definition

3rd week embyonic blood flows slowly through

capillaries in chorionic wall. oxygen & nutrients

of mom in intervillous space diffuse through walls

of villi and enter embryo's blood. CO2 & waste

diffuse from blood in fetal capillaries through

wall of villi into maternal blood.

Video

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