Embryology Flashcards
Embryology terminology
Embryo=
To grow within
Embryology terminology
Embryology=
Study of the embryo process. The first 8 weeks of development from fertilisation in human (total 266 days)
Terminology:
Embryonic period=
First 8 weeks- basic elements of tissues and organs formed
Terminology
teratogen=
(Te r a t o = Monster + gen = to form) Agents which upset embryological development Eg. Some chemical agents, ionizing radiation, rubella virus
What are the mechanisms of embryogenesis?
- Cell division - mitosis
- Apoptosis – programmed cell death
- Induction mechanisms - signals from adjacent tissues that induce change
- Cavity formation - pumping of fluid
- Cell migration
- Folding to rearrange flat layers into tubes within tubes
Describe the process of fertilization:
In ovulation, the secondary oocyte is in Metaphase 2. The site of fertilization is the ampula of uterine tube. The egg is fertilised through an acrosome reaction, in which the sperm penetrates zona pellucida (the outer layer of the oocyte) via the action of proteolytic enzymes (esp. acrosin). This results in the binding and then fusion of the surface membrane of sperm and oocyte. This then activates calcium waves within the oocyte. This is critical in:
- Blocking polyspermy: Cortical granule reaction: increase in calcium results in the fusion of cortical granules with the oocyte membrane, thus real easing their enzymes into the sub zonal space. These enzymes act to impair further penetration of zona by sperm by cleaving the binding sites on zona, making the zona resistant to proteolytic enzymes of the sperm and reducing sperm binding properties of oocyte membrane
- Resumption of meiosis by the oocyte (completion of Metaphase 2 and extrusion of 2nd polar body)
The results of fertilization are: the nuclei DNA decondenses and forms nuclei, then DNA replicates in pronuclei. The genetic material from male and female fuse and therefore diploidy is restored (46 chromosomes). Results in a new individual with their chromosomal sex determined (XY or XX). Completion of fertlization initiates cleavage ( formation of 2-cell- each cell called a blastomère- embryo)
Outline the stages of early embryological development from zygote to blastocyst.
- Secondary oocyte with first polar body
- Fertlization occurs (0 hours)
- Zygote
- 2 -celled stage (30 hours)
- 4 celled stage
- 8 celled stage
• cells loosely arranged until 8 cell stage - Morula (72 hours)
• blastomeres maximise contact between each other to form a compact ball called a morula. -
• (Morula = mulberry) - Blastocyst
• cells polarized -
• basal nucleus other organelles are apical.
• Polarised phenotype important for differentiation of cells in the blastocyst.
• (Blast = bud precursor, cyst=bladder, fluid filled cavity).
• The fluid- filled cavity in a called a blastocoele (coule =cavity).
• There are two identifiable regions. Outer trophoblast - troph-nourish- (forms embryonic placenta). Inner cell mass (forms embryo) - Implanted blastocyst ( 6 days )- must hatch from zona pellucida to implant in endometrium of uterus
The synchotiotrophoblast:
1. Erodes endometrial connective tissue
2. Secretes hCG (‘rescues” corpus leteum— maintains progesterone secretion—maintains pregnancy)
-Note: hCG is hormonal basis for preganancy test
Embryo is fully implanted with endometrium 10 day post-fertilization
•4. Describe the process of implantation and formation of the embryonic disc (bilaminar embryo)
1.As the trophoblast differentiates into two layers during implantation, so does the inner cell mass.
- Embryonic disc = bilaminar embryo
-Epibalst
-Hypoblast
Dorsal- Ventral axis established
Amniotic cavity forms as amnioblasts separate from the epiblast
3.Hypoblast migrates to form 1o umbilical vesicle (yolk sac) and later 2o or ‘definitive’ umbilical vesicle (yolk sac)
Extraembryonic mesoderm from hypoblast
4.Spaces form within extraembryonic mesoderm and coalesce to form extraembryonic coelom = chorionic cavity
(Embryonic disc is suspended in chorionic cavity by a connecting stalk of extraembryonic mesoderm
The bilaminar embryonic disc is formed when the inner cell mass forms two layers of cells, separated by an extracellular basement membrane. The external layer is called the epiblast and the internal layer is called the hypoblast. Together, they compose the bilaminar embryonic disc.)
NOTE:
The two layers are distinct in human embryos from day 8.
Describe the process of gastrulation
Gastrulation (bilaminar embryo becomes trilaminar)
- Epiblast cells in the region of primitive streak start to proliferate + migrate and differentiate. They migrate through the primitive groove and replace the hypoblast with a layer known as the endoderm — pushes hypoblast laterally continues to line umbilical vesicle to form endoderm.
- Another wave of migration of epiblast cells to form a third layer between newly formed endoderm and epiblast known as mesoderm.
- Then epiblast cells lose their ability to become migratory— and differentiate into ectoderm.
- 3 primary germ layers give rise to — 4 primary tissue types that make up the body —connective +muscle +epithelial+nervous tissue (for all organisms)
4.Migration of mesoderm:
-Mesoderm migrates laterally and cranially
-Cells that migrate through primitive pit form Notochord
-No mesoderm at oropharyngeal membrane nor cloacal membrane
When the embryo folds- you will have a gut tube that is lined by endoderm that starts at mouth and ends at anus- surrounded by layers of mesoderm and ectoderm
Notochord- what is it and what does it do?
Mesodermal condensation of cells derived from primitive
node
- Develops during days 16-22
- Lies in the midline
- Functions mostly in the first 6 weeks (bit of an orchestrator)
of the intervertebral disc (cells of notochord origin replaced during childhood)
- Induction of nervous tissue (from ectoderm)
- Median hinge point during formation of neural tube
- Induction of vertebral bodies (from mesoderm)
- Failure of induction mechanism causes vertebral column abnormalities including spina bifida
• May have remnants into early childhood i.e. nucleus pulposus
Summary of progress so far
The embryo is nearing the late 3rd week since fertilization.
The basic features are:
- Three primary germ cell layers are present –
- Ectoderm – forms outer skin (epidermis) & nervous system
- Mesoderm – forms muscle, bone, cartilage etc
- Endoderm – forms epithelium of gut & respiratory tracts, epithelium of accessory digestive organs - Embryo is orientated – head (cranial) to tail (caudal) & left to right, medial to lateral & dorsal to ventral.
- The embryo is now going to fold
Summary of Week 1 Milestones:
Week 1
Product of fertilization is zygote which undergoes cleavage divisions — morula —blastocyst
Blastocyst:
inner cell mass — embryo — fetus trophoblast — fetal side of placenta
Blastocyst hatches from the zona pellucida before commencing implantation into the endometrium about day 6.
Summary of Week 2 Milestones
Week 2
Implantation completed by about day 10
Differentiation of inner cell mass into:
-epiblast And hypoblast (embryonic disc bilaminar embryo)
Epiblast is continuous with amnion
- amniotic cavity dorsal to bilaminar embryo
Hypoblast continuous with lining of umbilical vesicle (yolk sac)
- umbilical vesicle is ventral to bilaminar embryo
Dorsal – ventral axis established
Hypoblast extraembryonic mesoderm (EEM)
- spaces within EEM form chorionic cavity
What is are the features of bilaminar embryo?
Primitive streak defines major body axes
Cranial – caudal
Medial – lateral
Left - right
Embryonic disc
(revision)
Gastrulation
(revision)
Migration of mesoderm
(revision)
Mesoderm migrates laterally and cranially — different fates
Cells that migrate through primitive pit form Notochord
No mesoderm at oropharyngeal membrane nor cloacal membrane
Notochord induction of ectoderm:
Notochord induces overlying Ectoderm to proliferate & differentiate to form Neural plate
Describe neural tube formation:
- Neural folds fuse to become the Neural tube. Neural groove becomes neural canal Neural tube separates from surface ectoderm—epidermis of skin
- Neural folds close like a zipper forming the neural tube beginning in the middle of the embryo —cranially and caudally . Two openings remain – cranial & caudal neuropores . Failure of neuropores to close underlies neural tube defects
- Neural crest cells form at the tips of neural fold. Later migrate to form neurons of peripheral nervous system (and a whole bunch of other things….)
Neural tube formation (simplified)
- Neural folds fuse to become the Neural tube
- Neural groove becomes neural canal
- Neural tube separates from surface ectodermis— epidermis of skin
Neural tube formation: superior view
Neural folds close like a zipper forming the neural tube beginning in the middle of the embryo —cranially and caudally
Two openings remain – cranial & caudal neuropores
Failure of neuropores to close underlies neural tube defects
Neural tube formation: neural crest formation
Neural crest cells form at the tips of neural fold. Later migrate to form neurons of peripheral nervous system (and a whole bunch of other things….)
How does the ectoderm derivative of the neural tube
Walls of neural tube expand, fold and increase in size differentially to form brain and spinal cord
Overview of mesoderm development:
• Paraxial mesoderm
- Head mesoderm (unsegmented)
- voluntary muscles of face, jaw, throat
Somites (segmented)
- axial skeleton,
- voluntary muscles,
- parts of dermis
• Intermediate mesoderm
- urinary system
- genital system (parts)
• Lateral plate mesoderm
- most smooth muscle
- most CT (bone, cartilage)
- cardiovascular system
(Soma=body=somites=body parts)
Paraxial mesoderm - Somites
The embryo becomes SEGMENTED This is a key feature of most animals & humans
Describe somite differentiation:
Each somite differentiates into three regions
• Sclerotome
- bones of axial skeleton
• Dermatome
- CT of skin (dermis)
• Myotome
- skeletal muscles of body, head and limbs
(Sclero = hard Myo = muscle Dermato = skin Tome = segment)
Where can the intraembryonic coelom be found
Intraembryonic coelom forms within lateral plate mesoderm
Intraembryonic coelom form body cavities. How are these cavities formed?
By splitting of the embryonic body walls
Formation of body cavities: Lateral mesoderm is divided into two layers by the intraembryonic coloem:
- Visceral
- Parietal
As embryo folds cavity is encompassed into the body
Embryonic folding overview
Embryo folds longitudinally and laterally towards the ventral surface
Describe the longitudinal (Head-tail) folding of the embryo
- Viewed in sagittal section
- Folding initiated by rapid growth of neuroectoderm
- Heart bulge (from cardiogenic mesoderm) initially cranial to oral membrane
- Result in primitive gut lined by endoderm (‘inside’ lining of embryo)
- Ectoderm now covers ‘outside’ of embryo
- Remainder comprised of mesoderm & cavities
Describe the lateral folding of the embryo
Gut forms as umbilical vesicle is pinched off by folding
What is the fate of the germ layers?
Ectoderm:
- nervous system
- epidermis (not entire skin!)
Mesoderm:
- muscle, connective tissue – i.e. “form” of body some epithelial structures
- parts of urogenital system
- endothelium of heart and blood vessels
Endoderm:
epithelial lining of gut, respiratory tract, urinary bladder and urethra functional components of organs associated with the gut (eg, liver, pancreas)
