Intro to Embryology Flashcards
Oocyte (ovum,egg)
female germ cell
sperm
male germ cell
zygote
cell resulting from union of oocyte and sperm at fertilistaion
Blastomeres
early embryonic cells
Morula
Solid mass of 12-32 blastomeres
Blastocyst
stage where morula has entered uterine cavity and blastocytic cavity develops within
Gastrula
stage after trilaminar embyronic disc formation
Neurula
Stage after gastrula when neural tube forms
Embryo
Developing human during embryonic stage (to end of 8th week)
Fetus
Period form 9th week to birth
Stages 1-3 (Fertilisation and Early cell division)
Day 1: Fertilisation
Day 2: Zygote
Zygote is a diploid cell produced from fusion of two haploid gametes (fertilised ovum)
Day 3: Morula
Dividing mass of 20-30 cells
Stages 3-4: Blastogenesis
Day 4: Stage 3
Inner cell mass (made of blastomeres)- becomes embryo
Tropoblast- becomes embryonic part of placenta
Day 5: Hatching of blastocyte/ shedding of the zona pellucida
Zona pellucida is a membrane which surrounds the ocum before implantation to then produce the late blastocyst
Day 5: Endometrial epithelium- undergoes structural and histological changes throughout the menstraul cycle in preperation for implantation
Endometrial glands- provide substances for survival, development and implantation for the cunceptus (fertizilised egg)
Day 6: Stage 4
Syncytiotrophoblasts- special layer of epitheliul cells through which substances cross the placenta
Stages 4-6: Formation of the Bilaminar Disc and Implantation
Day 8: Stage 5
Bilaminer disc- here the inner cell mass differenciates into two distinct layers. They are seperated here by an extracellular basement membrane;
External layer is called Epiblast
Internal layer is called the Hyperblast
Together both these layers form the bilaminar disc
Day 9: Formation of yolk sac which provides nutrition and gas exchange between the mother and cunceptus before the placenta is formed
Amnion is a membrane that covers the blastocyste and will eventually fill with fluid to form the embryonic sac
Day 10- Lucunae appear
These are spaces filled with maternal blood
Day 8-9: Formation of the Placenta and Embryo
Day 8: Cytotrophoblast- inner layer of the trophoblast
Becomes embryonic part of placenta
Day 9: Becomes embryo
Endoderm (inner)- constructs the digestive tube, primitive gut, respiratory tube. These are cuboidal cells
Mesoderm (middle)
Ectoderm (outer)- becomes the central nervous system, skin, hair and nails. These are column cells.
Day 11-13: Formation of the Bilaminar Disc and Extraembryonic Cavity
Day 11-12:
Exocoelomic cavity- inside the extraembryonic mesoderm
Important transfer interphase and a transfer of nutrients for the embryo
Day 13:
Exocoelomic cyst- reminisence of the primary yolk sac
Cavities become confluent to form new space called extraembryonic coelom or chorionic cavity
Day 13: Formation of the umbilical cord
Maternal capillaries nearby expand to form the maternal sinusoids
1) Chorionic vesicle hangs in the chorionic cavity
2) Chorionic vesicle joined to placenta cells by connecting stalk
3) Becomes umbilical cord
Week 3: GastrulationBilaminar Disc ————> Trilaminar Disc
Caudal: Tall end
Cranial: Head end
As the ectoderm grows a folding occurs along the caudal midline which creates the Primitive Streak
Primitve streak is a transient strcuture and its formation marks the start of gastrulation and establishes the anterior posterior body axis
Define Gastrulation
This is where the embryo transforms from a 1D layer of epithelial cells, the blastula, and reorganises into a multilayered multidimensional strcuture called the castella
Week 3: Formation of Germ layers
Cells from the base of the primitive streak detach and migrate to lie between the layers of the ectoderm and endoderm
The invagination of these cells creates a new layer called the mesoderm (middle)
The disc is now known as the trilaminar disc
-At the caudial end there is the Primitive streak
-At the cranial end theres the oropharyngeal membrane (aka buccal pharyngael membrane)- This forms a septum between the primitive mouth and the throat pharynx
Differentiation of Germ layers
Each of the three germ layers differentiate into specific tissues and organ systems
Ectoderm- outer layer
Epidermis at the skin, hair nails,
Various glands (mammary, sweat, sebaceous glands)
Central nervous system
Peripheral nervous system
Pituitary gland
Enamel of the teeth
Lens of the eye and parts of the inner ear
Sensory epithelium of nose, ear and eye
Endoderm- inner layer
Differenciates into specific tissues and organ systems
-Epithelial lining of the gastrointestinal tract, respiratory tract and urinary bladder
-Parenchyma of the thyroid gland, parathyroid gland, liver and pancreas
-Epithelial lining of the tympanic cavity and auditory tube
-Plays a part in the development of the notochord
Mesoderm- middle layer
All of the musculoskeletal system
Deep layers of the skin
Abdominal and chest walls and lining
The walls of the bowel (but not the lining of the bowel)
The urogenital system
Formation of the Notochord
1) Tube develops from end of primitive streak extending towards cranial end
2) Tube fuses with endoderm to become a groove
3) Plate fold to become a tube once again – the notochord
4) Notochord has a central role in further midline development. This is because it provides both mechanical and signalling keys to the developing embryo
Weel 4: Neurulation
What is Neurulation?
This is a process in which the neural plate, a thickened portion of the ectoderm along the midline of the embryo, bends up and fuses to form a hollow tube that will eventually differenciate into the brain, spinal cord and central nervous system
Process of Neurulation
- Neuroectodermal tissues differentiate from the ectoderm and thicken into the neural plate, separated by the neural plate border
- Neural plate bends dorsally until the two plates convergence to form the neural crest
- The neural crest is disconnected from the epidermis by the closure of the neural tube.
The cells of the neural crest differentiate to form most of the peripheral nervous system - The notochord degenerates, only persisting as the nucleus pulposus of the intervertebral discs.
Mesoderm cells differentiate into somites, precursors of the axial skeleton and skeletal muscle
Clinical revelance
Caudal end:
If fusion does not extend all the way to the caudal end of the embryo the child is born with spina bifida
Cranial end:
If fusion does not extend all the way to the cranial end of the embryo the child is born without the cerebral cortex (anencephaly)
Spina Bifida
Occulta: Mildest type of spina bifida (aka hidden spina bifida)-
With this there’s a small gap in the spine but there’s no opening or sack on the back so the spinal cord and nerves are usally fairly normal
Meningocele: Next more severe spina bifida
Here there’s a visible sack protruding from the spinal column
Myelomeningocele: Most severe and most serious type of spina bifida
Here a sack of fluid comes through an opening in the baby’s back and part of the spinal cords and nerves in the sack are damaged
Differenciation of Mesoderm
Intermediate mesoderm- generates the urogenital system
Including; kidneys, the gonads, reproductive tract
Paraxial mesoderm-Differentiates and divides into paired cuboidal bodies known as somites
Lateral mesoderm- Intercellular cavities in lateral plate (mesoderm). This forms progeny cells that constitue the heart, cardiovascular system, blood, kidneys, smooth muscle, lineage and limb skeleton in the developing vertebrate embryo
Somites
First appear in future occiptal the smallest of the four lobes in the cerebral hemisphere
Somites are the precursor populations of cells that give rise to important structures associated with vertebrate body, so associated with the body plan
This will eventually differenciate into the dermis, skeletal muscle, cartilage, tendons and vertebrae
The development is craniocaudally here
Intermediate mesoderm
Gives rise urogenital system
Kidneys
Gonads
Respective duct system
Lateral mesoderm
This splits into two layers:
Somatic (parietal) layer mesoderm – Outer layer
Covers inside of the chest and abdominal walls
Splanchnic (visceral) layer mesoderm – Second layer
Covers organs in the thorax and abdomen
Week 4: Embryo folding
Two types: lateral and head to tail caudial folding
Both these types of folding in both planes is actually occuring simultaneously
Lateral folding
Starting with the developing trilaminar disc
Axial or transverse slice (across embryo)
1) Flaps of lateral plate mesoderm fold towards ventral midline and merge everywhere except at umbilical cord
2) As the flaps fold, space is maintained which becomes intraembryonic coelom
3) Amnion and ectoderm continue to undergo rapid growth
4) Amnion now surrounds entire embryo
5) Endodermal layer meets at midline and fuses to form gut tube
6) Fusion is complete except at midgut level
7) Yolk sac and midgut have a wide connection until lateral folding is complete
Lateral folding summary
A portion of the yolk sac (which is lined with endoderm) is incorporated into the embryo to form the primitive gut
The remaining part of the yolk sac and allantois remain outside the embryo, connected through the umbilicus
Formation of Body Cavities
Somatic layer mesoderm- outer layer
Superficial layers of the skin formed from ectoderm
Intra-embryonic coelom forms body cavities - abdomen and chest
Cephalo – Caudal Folding
Starting with the bilaminar disc
There’s a Sagittal slice (along length of embryo)- a vertical plane running from front to back, dividing the
body or any of its parts into the right and left sides
Day 22- Part 1 of Caudal Folding
1) Cranial area contains oropharyngeal (or bucchopharyngeal) membrane, cardiogenic area (the heart) and the septum transversum
2) Cranial flexion brings these areas ventrally
3) These areas form the surface of the future face, neck and chest
4) The heart is brought to the thoracic position
5) The septum transversum is brought to the diaphragm
6) New position under heart separates coelom into thoracic and abdominal cavities
Day 23- Part 1 of Caudal Folding
1) Caudal flexion brings the cloacal membrane to the ventral surface of the embryo
2) Connecting stalk is moved ventrally so it merges with the neck of the yolk sac and attaches to the ventral surface of the embryo
3) Parts of the endodermal germ layer are incorporated into the embryo as the hindgut
4) The primitive streak moves from lying cranially to caudally in relation to the cloacal membrane
5) Allantois is partially incorporated into the embryo
Summary
Week 1 —Fertilisation, formation of the morula and blastocyst
Week 2—Implantation of blastocyst and formation of bilaminar embryonic discand early placenta
Week 3—Differentiation of cell layers to form the trilaminar embryonic disc
Week 4—Folding of the embryoand continuing development of the three germ layers (ectoderm, endoderm and mesoderm)
Week 5 – Week 8— Development of all external and internal structures (organogenetic period)
End of Week 8— Embryo looks like an ‘adult’ and is called the fetus. Growth becomes the predominant feature from this point onwards
