Embryology: Events of the Pre-Embryonic period

Embryology

Developmental events that occur prior to birth

Mitosis

Cell division, increase cell number

Cell differentiation

transformation into specific cell types

Cell development

Process by which cells become fully functional

Pre-Embryonic Period

Week 1, implantation, formation of the bilaminar germinal disc, formation of the extraembryonic membranes, development of the placenta

Prenatal period

The first 38 weeks of human development. Sequence of events occurring from fertilization until birth

The three developmental stages

Pre-embryonic, embryonic, fetal period

Pre embryonic period

First two weeks after fertilization of egg, formation of a blastocyst, ends with implantation in lining of the uterus

Embryonic period

3 through 8 weeks, all major organ systems begin to develop

Fetal period

9 through 38 weeks, growth development continues, ends at birth.

Pre embryonic period week 1

Sperm fertilize ovum, after zygote is formed it undergoes a series of mitotic divisions called cleavage. At 16-cell stage the structure is called a morula (very dense). Soon after morula enters uterine cavity it becomes a blastocyst (has a cavity) fluid filled blastocyst cavity in it’s center

Stages of week 1 (pre embryonic period)

Zygote, cleavage, blastocyst

Components of blastocyst

Trophoblast (chorion (placenta)) and embryoblast (develops into embryo)

Trophoblast

Outer ring of cells that will develop into chorion (placenta)

Embryoblast

Cluster of tightly packed cells inside one side of the blastocyst that will form the embryo. Inner cell mass. Pluripotent (can differentiate into any cell type in the human body)

Implantation

Within a week of fertilization, the blastocyst enter the uterus. Invades superficial, functional layer of endometrium (lining of the uterus) by day 9 the blastocyst has completely burrowed into the uterine wall

Formation of the bilaminar germinal disc

By day 8, cells of the embryoblast differentiate into two distinct types Hypoblast and epiblast. Together these two layers form this.

Hypoblast

The layer of small cubodial cells facing the blastocyst cavity

Epiblast

Layer of columnar cells deep to the hypoblast

Formation of the extraembryonic membranes

The bilaminar germinal disc and trophoblast produce three extaembryonic membranes. Yolk sac, amnion, and chorion (placenta)

Yolk sac

Serves as a site for early blood cell and vessel formation. (only touches the hypoblast)

Amnion

Thin layer of cells that forms above and is derived from the epiblast. A fluid filed amniotic cavity (repture in this means water breaking) appears between the amnion and epiblast. (The cavity’s don’t touch each other)

Chorion

Outermost membrane responsible for formation of the placenta. Blood supply (supplies blood to the developing embryo).

Placenta

Is a highly vascularized organ that serves as an interface between embryo or fetus and mother

Development of the placenta

Embryonic portion of the placenta is chorion, maternal portion is from the functional layer of the endometrium

Main functions of the placenta

Exchange of nutrients, waste products, and respiratory gages between embryo and mother, transmission of maternal antibodies to embryo or fetus, production of estrogen and progesterone to maintain uterine lining.