Lecture 6 Embryology

embryonic development phases

-cleavage
-implantation
-primitive streak formation
-gastrulation
-neurulation

cleavage

-rapid miotic divisions

-immediately follows fertilization

-cells are called blastomeres

-end of phase results in a blastula

patterns of cleavage

determined by amount and distribution of yolk protein and factors in the cytoplasm that influence the angle and formation of the mitotic spindle

yolk rich pole

vegetal pole

low yolk

animal pole

rotational holoblastic cleavage

-mammals

-very small -- human is 100um in diameter

-first cleavage begins about 24 hours after sperm entry and completion of meiosis

-in mice, miRNA from sperm initiates first cell division

-slow, 12-24 hours apart

-blastomeres divide asynchronously

-zygotic gene expression is required for early cleavage

compaction in mammalian cleavage

-up to 8 cell stage blastomeres are loosely arranged

-at third cleavage, blastomeres form a compact ball of cells

-Due to increase in adhesion proteins

-Tight junction and gap junctions add stability and movement of ions between cells

-At 16 cells - mammalian embryo is called a morula

outer cells of morula become

trophoblast, inner cells become inner cell mass

implantaion

trophoblast divide to form the cytotrophoblast (forms vili) and the syncytiotrophoblast (embeds into uterine wall)

migration through the primitive streak

as cells migrate through the primitive groove, two layers emerge, epiblast and hypoblast (endoderm)

gastrulation

-Bilaminar germ disc- formed from the epiblast layer and endoderm
-Epiblast cells split - one group of cells form the amnion, creating the amniotic sac

amnion

closest to the embryo

chorion

superficial to amnion, contributes to placenta

placenta

A structure that allows an embryo to be nourished with the mother's blood supply
-produces hormones to retain the pregnancy

yolk sac and allantois

provides nutrients to embryo
allantois becomes umbilical cord

chorionic villi

allow diffusion of substances between maternal and fetal blood supply

development of 3 layer germ disc

-happens during week 3
-part of the establishment of body axes in humans
-anteroposterior, dorsoventral, and left-right axis formation takes place before and during the period of gastrulation

development of the notochord

invagination at the primitive pit
-required for development of nervous system

twins

defined by what extraembryonic tissues they share
-2 chorions, 2 amnions
-1 chorion, 2 amnions
-1 chorion, 1 amnion (most rare, can lead to conjoined)