Embryology Flashcards
Fate of ectoderm
• CNS- central nervous system
• PNS- peripheral nervous system
• epidermis of skin, hair and nails
• Mammary, sweet and sebaceous glands
• Pituitary gland
• Enamel of teeth
Fate of mesoderm
• the musculoskeletal system
• Skin- deep layers
• Abdominal and chest walls and lining
• Bowel wall (not epithelium)
• Urogential system
Fate of endoderm
• epithelial lining of gastrointestinal tract, respiratory tract and bladder
• Parenchyma of thyroid gland, parathyroids, liver and pancreas
3 germ layers
Ectoderm
Mesoderm
Endoderm
Embryology: day 1
Fertilisation
Embryology: day 2
Formation of the zygote- cell formed by fusion of egg and sperm
Embryology: day 3
Cell division - formation of the morula
Embryology: day 5
Late blastocyst - cavity formed within the morula
Embryology: day 6-12
Implantation into uterine endometrium
Embryology: day 8-9
Formation of bilaminar disc (will become the embryo)
Embryology: week 3
Gastrulation- bilaminar disc becomes trilaminar disc: 3 germ layers form)
Embryology: week 4
Neurulation (neural tube forms) and embryonic folding (flat disc into a cylindrical form)
Fertilisation
Fusion of gametes = genetically unique individual
•Millions sperm enter the female reproductive tract
•Can survive for several days
•Travel through the uterus by muscular contractions
Relatively few sperm reach the uterine tube
•Sperm ‘conditioned’ in the tube (capacitation) before they are capable of fertilisation
•Capacitated sperm penetrate the corona radiata and bind to the zona pellucida
•After binding, the acrosome reaction occurs - sperm can penetrate the ZP
•The ZP changes so no more sperm can enter the oocyte - prevents polyspermy
2 required process for fertilisation
Capacitation
Acrosomal reaction
Capacitation
epithelial interactions between sperm, uterine wall
Glycoprotein coat and seminal plasma proteins covering acrosomal region removed → easier enzyme release → acrosomal reaction
Acrosomal reaction
after binding to zona pellucida
Release of enzymes (eg acrosin, hyaluronidase) needed to penetrate zona pellucida
How long are oocytes viable after ovulation
12-24 hours
How long are sperm viable after ejaculation
Up to 6 days
1st phase of fertilisation
Penetration of corona radiata: capacitated spermatozoa allowed to pass through corona radiata
2nd phase of fertilisation
Penetration of zona pellucida and sperm binding
• zona pellucida- glycoprotein layer surrounding oocyte
• approx. 500 spermatozoa arrive at this layer
• sperm binding initiates release of acrosin (hydrolysis enzyme) → sperm cell penetrates zona pellucida → sperm makes contact with oocyte → cortical reaction (release of lysosomal enzymes from cortical granules of oocyte) → cortical granules initiate zona reaction, prevent further sperm penetration (polyspermy) by forming protective hyaline layer, inactive receptor sites on zona pellucida
• also activates oocyte to prepare for 2nd meiotic division
3rd phase of fertilisation
Fusion of oocyte and sperm cell- forms zygote
• Interactions between integrins and ligands → adhesion of sperm, oocyte (fusion of sperm, egg plasma membranes)
• secondary oocyte completes meiosis II → forms female pronucleus, second polar body
• head, tail of spermatozoa enters oocyte → travels to female pronucleus (containing 23 chromosomes) using tail, energy generated by mitochondria
• tail, mitochondria detach → sperm nucleus becomes male pronucleus
• male, female pronuclei merge into single nucleus → cell becomes diploid
• prepared for mitotic division
Molar pregnancy
• compete mole- sperm fertilises an egg that doesn’t contain any genetic material. Abnormal trophoblast (forms placenta) develops but not an embryo
• Partial mole- normal egg fertilised by 2 sperm. An embryo starts to develop but is not viable
Complications of molar pregnancy:
• retained molar tissue can lead to form a cancer (choriocarcinoma)- treatment is very successful
Scientific relevance of molar pregnancy
fact that trophoblast can develop in the absence of maternal DNA suggests paternal DNA is important for trophoblast development
Cleavage
series of fast mitotic divisions of zygote → increase number of cells, decrease size
• 36 hours after fertilisation → first cleavage division → 2 cells (blastomeres)
• after 3rd cleavage, blastomeres (8) form compact ball of cells connected by tight junctions (compaction)
• 3 days after fertilisation, cells ot compacted embryo divide again → mulberry-shaped 16-cell morula (composed of 2 zones: inner cell mass and outer cell mass)
• 4-5 days after fertilisation, embryo consists of approx. 100 cells
• fluid accumulates within internal cavity (blastocoel) → blastocyst
Morula
composed of 2 zones: inner cell mass and outer cell mass
Still surrounded by zona pellucida (glycoprotein coat)
2 zones of blastocyst
Trophoblast
Embryoblast
Trophoblast
single layer of large flattered cells, stemming from morula’s outer cell mass; gives rise to placenta
Embryoblast
20-30 pluripotent cells located on one side, stemming from inner cell mass: gives rise to embryo
Inner cell mass
lies at embryonic pole of blastocyst- develops into embryo and may split into identical twins
Outer cell mass
develops into the placenta
Blastomeres
Cells in morula
IVF
In IVF, a blastomere can be removed for genetic testing prior to transfer into the uterus
Blastocyst hatching
Day 6:
• blastocyst hatches from the zona pellucida before uterine implantation
Implantation
- Capture of blastocyst on endometrium- Endometrium must be ready- window of implantation
- Blastocyst attaches more firmly to endometrium and trophoblast invade endometrium
- complex signalling between endometrium and trophoblast
Blastocyst must implant deeply enough but not too deep- endometrium closes by a fibrin plug
Trophoblast differentiation
Trophoblast:
• proliferates, forms 2 layers
1. Cytotrophoblast (cellular trophoblast): inner layer of moronucleated cells
Produces primary chorionic villi, protrudes into syncytiotrophoblast
2. Syncytiotrophoblast: outer multinucleated mass of cells (without distinct cell boundaries)
Invades decidua basalis with finger-like processes; makes enzymes that erode uterine cells; blastocyst burrows into decidua basalis surrounded by pool of blood leaked from degraded blood vessels
Cytotrophoblast
at the embryonic pole proliferates
Syncytiotrophoblast
invades the endometrium
Maternal vessels and lacunae
Trophoblast contacts maternal vessels which form blood filled spaces called lacunae - these will bathe the forming placental villi and allow gas exchange
Embryoblast
differentiates into 2 layers, forms flat bilaminar disc
1. Hypoblast - small cuboidal cells adjacent to blastocyst → yolk sac
2. Epiblast - columnar cells. Cavity forms inside (amniotic cavity). Lined with amnioblasts