Cleavage and formation of blastocyst
Cleavage of zygote, rapid mitotic divisions.
Zygote cleaves to 2-cell stage then cleaves again to 4-cell stage
Forms Morula (solid sphere of cells)
Forms blastocyst (stimulated by special secretions from uterine wall)
Blastocyst has inner cell mass, blastocyst cavity and trophoblast
Inner cell mass is animal pole as becomes animal,
Opposite pole referred to as 'vegetal pole'
Trophoblast is the outer superficial layer.
Wall of blastocyst, forms foetal (opposed to maternal) portion of placenta
Blastocyst loosely attaches to endometrium
Inner cell mass faces towards the endometrium.
Forms bilaminar disc:
Epiblast and hypoblast
Inner cell mass differentiates into two layers, hypoblast is a lower layer of cuboidal/columnar epithelial cells, epiblast is upper layer of cuboidal/columnar cells.
Small cavity appears in epiblast, enlarging forming amniotic cavity
Epithelial cells in roof of this cavity form amnion (foetal membrane)
Formation of exocoelomic membrane
Formed by migration of cells at edge of hypoblast moving to cover inner surface of blastocyst wall.
With hypoblast this membrane forms wall of yolk sac
Layer of cells derived from yolk sac begin to proliferate between it, the amnion and trophoblast. Form loose connective tissue (mesenchyme) and called
several cavities develop within mesoderm and fuse forming
The extraembryonic mesoderm fused to chorion and amnion so coelom forms choronic cavity. Band of extraembryonic mesoderm connects the embryonic disc to chorion : connecting stalk
Rearrangement and migration of epiblast cells of bilaminar disc
Establishes body plan of embryo
Bilaminar -> Trilaminar
Formation of primitive streak (groove on dorsal surface of epiblast). Formed by invagination of epiblast, elongates from caudal to cranial embryo establishing head tail and head ends
Primitive node formed at head end of primitive streak (node is rounded structure of epiblastic cells)
Epiblast cells remaining on dorsal surface are ectoderm
Faint depression forms on dorsal surface of ectoderm. The
Hypoblast cells now called endoderm
Mesenchyme displaces hypoblast, this layer called mesoderm
Occurs simultaneously as gastrulation
Stages of development
Fertilized ovum undergoes mitosis
Morula is formed
Blastocyst implants into uterine wall
Germ layers and placenta develop
Main body systems form
Organs grow and develop further
Locomotor system becomes functional