Embryology

LO: Describe the key stages and timelines of embryological, foetal and neonatal development

Branch of biology - development of organisms
Embryonic development in humans covers the first 8 weeks of development after fertilisation; at the start of the 9th week the embryo is termed a foetus
Normal period of gestation (pregnancy) is ~9 months or 40 weeks
Assists women's reproductive health

Week	Stage
1	Fertilisation to implantation
2	Bilaminar germ disc, Amniotic cavity and primary yolk sac
3	Gastrulation: Trilaminar germ disc, primitive streak, notochord, neural tube and body axes/growth of embryonic disc
3-8	Organogenesis
8-birth	Foetal period: Maturation tissues and organs and rapid growth of the foetal body

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Contact of the spermatozoon and ovum, normally in the ampulla (12-24 hrs after ovulation)
Fusion of female and male pronucleus (2 haploid to form a diploid zygote)
Two-cell zygote - cell cleavage into morula
Ingress of fluid forms a blastocyst cavity (~4 days after fertilisation)
Blastocyst consists of 2 cell layers: inner cell mass - embryoblast, outer cell mass - trophoblast
Uterine secretory phase: Endometrium wall forms nutrient rich bed that receives and nourishes the blastocyst via diffusion

Blastocyst splits into two layers to form the bilaminar germ disc
Epiblast - high columnar cells, Hypoblast - cuboidal cells
Epiblast cells is the source of all embryonic tissues
Formation of amniotic cavity and primary yolk sac
Cytotrophoblast and syncyntiotrophoblast form
Formation of chorionic cavity and plate
Development of uteroplacental circulation
Connecting stalk (primitive umbilical cord)
Formation of the secondary yolk sac

Diffusion can no longer supply nutrients needed to develop embryo
Formation of primitive blood vessels connected with the placenta
Formation of the primitive streak, pit and node - linear groove in the epiblast cells of bilaminar layer
Proliferation and migration of epiblast cells into primitive streak give rise to the trilaminar germ disc through a process of invagination or ingression
Formation of three cell germ layers: ectoderm, mesoderm, endoderm
Process of invagination of some epiblast mesenchyme cells
Replace hypoblast to become endoderm
Mesoderm formed in between
Epiblast cells remaining on surface are the ectoderm

Formation of the notochord
Mesenchyme cells migrate from primitive pit cranially under ectoderm to the oropharyngeal membrane
Induces the overlying ectoderm to form of neural plate
Sidedness determination
Establishment of the body axes
Differentiation of mesodermal elements
Paraxial mesoderm
Intermediate mesoderm
Lateral mesoderm

Ectoderm: Nervous system, epidermis, some glands, sensory epithelium
Mesoderm: Lining of body cavity and surround organs, muscle, connective tissue, bone, blood and blood vessels
Endoderm: Internal lining of body systems
Ecto - outside, Meso - middle, Endo - inside, Derm - skin

All three germ layers originate from pluripotent epiblast stem cells of the bilaminar germ disc

Found in mesenchymal cells and tissues (bone marrow, blood cells, adipose)
Multipotent
Potential for regenerative medicine but concerns about the potential to contribute to pathological conditions - mesenchymal tumours such as sarcomas and metastasis may arise

Ectoderm: dorsal (posterior) folding of ectoderm to form the neural tube - neurulation
Endoderm: ventral (anterior) folding to form the inner lining of gut tube
Mesoderm: Between the 2 tubes forming the body cavity and other tissues

Expansion of the amniotic cavity into extraembryonic (chorionic) cavity
Formation of the intraembryonic coelom (primitive body cavity)
Folds meet and pinch the yolk sac with some getting enclosed within the embryo forming the embryonic gut tube

Head and tail move into foetal position closing gut tube around yolk sac and connecting stalk
Midgut remains temporarily attached to the yolk sac via vitelline duct
After folding, the gut tube is suspended by the dorsal mesentery

Intraembryonic coelom is formed by the lateral palte mesoderm splitting into 2 layers
Visceral (sphlanchnic) - continuous with extraembryonic mesoderm covering the yolk sac, forms the wall of the primitive gut
Parietal (somatic) - continuous with extraembryonic mesoderm covering amnion, forms the body cavity wall

As notochord and neural tube form the mesoderm form bilateral longitudinal columns - paraxial mesoderm
Divide paired somites in a craniocaudal sequence
Mesenchyme cells from the somites give rise to the axial skeleton and all muscles and dermis of the body
Somites form bilaterally either side of the developing neural tube

Paraxial mesoderm forms somite which give raise to sclerotome and dermomyotome cells

As folding occurs ther 3 germ layers of the trilaminar disc differentiate into specific tissues and organs
Neurulation occurs early in folding process
Closely followed by the formation of other organ systems, cardiovascular being one of the early ones and first to function
End of organogenesis: systems are forced but most will need to mature and develop further to become fully functional
Maturation and development continues throughout the foetal period and into the postnatal period to adulthood

Embryonic period
Trilaminar germ disc differentiate into specific tissues and organs
All major organ systems
Neurulation - nervous system
Primitive cardiovascular system
Somites in MSK
Intraembryonic coelom - body cavities
At the end of embryonic period the main organ systems have been established but functionality of most is minimal

Developing notochord and surrounding mesenchyme induce formation of the neural plate
Neural plate: Neuroectoderm
Formation of the neural groove and folds fuse to form neural tube
Primitive central nervous system
Some neuroectoderm along the fusion crest migrate - neural crest cells