Embryology Flashcards
Define embryology
The study of progression from a single cell to a baby in 9 months is
called embryology. It includes molecular, cellular and structural
factors
What is an oogonium?
An immature female reproductive cell that gives rise to primary oocytes by mitosis.
What is the chromosome number of an oogonium?
2n
What is the chromosome number of a primary oocyte?
2n x 2
When doe mitosis and differentiation occur in oogenesis?
Fetal period
When does meiosis 1 occur in oogenesis?
Prior to ovulation (puberty)
What is the chromosome number of a secondary oocyte?
nx2
When does meiosis 2 occur in oogenesis?
After fertilisation
What is the chromosome number of an ovum?
2n: n maternal chromosomes and n paternal from sperm cell
Where does fertilisation occur?
Ampullary region of Fallopian tube
Why are spermatozoa not rejected by the female body?
A
What is embryonic age?
Time since fertilisation
What is gestational age
Time since last menstruation, i.e. Embryonic age plus 2 weeks
What is the germinal stage?
The time since fertilisation to the end of the second week
What is the embryonic period?
The time from the start of the third to the end of the eighth week
What is the foetal period?
The time from the stat of the ninth week to birth at 38 weeks
Define blastocyst
Blastocyst: A thin-walled hollow structure in early embryonic development that contains a cluster of cells called the inner cell mass from which the embryo arises.
What must the blastocyst lose before it can implant?
Before the blastocyst can implant it has to loose the zona pellucida. The zona pellucida is thought to prevent the morula/blastocyst adhering to the oviduct. The process of losing the zona pellucida is called hatching
What is the trophoblast?
A layer of tissue on the outside of a blastula, supplying the metro with nourishment and later forming the major part of the placenta
What does the trophoblast form?
Placenta and yolk sac
What does the zone pellucida do?
Stop the morula enlarging
Define histiotrophic nutrition
Nutrition provided to the embryo not from the
maternal blood. Important in humans up to the 12th week
Define haemotrophic nutrition
Nutrition provided by the mother’s blood.
Important in humans from 12th week on
Where does implantation occur?
Normally in the upper part of the body of the uterus, more often on the posterior wall
(Superior and posterior wall of uterine cavity)
When does implantation occur?
6th day in the endometrium
Define ectopic pregnancy
A pregnancy in which the foetus develops outside the uterus
Define placenta previa
A condition in which the placenta partially or wholly blocks the neck of the uterus, so interfering with the normal delivery of a baby
Define cytotrophoblast
Cells in the trophoblast
Define Syncitiotrophoblast
Cells that merge together in the trophoblast
What happens in the second week of development?
The trophoblast differentiates into 2 layers - the cytotrophoblast and
syncytiotrophoblast
The embryoblast forms 2 layers - the epiblast and hypoblast - the
bilaminar embryonic disc
The hypoblast contributes to the formation of two cavities – the yolk sac and chorionic cavity
Which period of development are all major structures and systems formed in?
Embryonic
Which period is there the greatest risk of congenital malformation due to environmental exposure or drug therapy?
Embryonic
When does the risk of structural defects decrease?
Fetal period - less sensitivity - risk of structural defects low except CNS
What are the key events in early development?
Fertilisation & implantation Gastrulation Neurulation Segmentation Folding
What is the epiblast?
The outermost layer of an embryo before it differentiates
How does the epiblast look at the end of the second week?
Uniform disk
What develops on the epiblast in the third week, and on which surface does it develop?
Primitive streak and node, dorsal surface
What is the function of the primitive node?
Contains cell which coordinate the whole process
At which point can a human embryo no longer be kept in vitro for ethical reasons?
When primitive streak formed as this is seen as the beginning of an individual since 2 streaks = twins
What happens to the epiblast and hypoblast in migration and invagination?
Epiblast cells drawn by haemotactic factors, push through layer, spread out throughout 2 layers of embryonic disk. Hypoblast disappears, new layers form underneath.
What are the 3 layers of the trilaminar disk?
Ectoderm, mesoderm, endoderm
Cells differentiate through the gastrulation process
Which layer has gaps/holes, and what do these later become?
Mesoderm, mouth and anus
What structures are derived from the ectoderm?
Organs and structures that maintain contact with the outside world, e.g. Nervous system, epidermis
What is derived from the mesoderm?
Supporting tissue, e.g. Muscle, cartilage, bone, vascular system
What is derived from the endoderm?
Internal structures e.g. Epithelial lining of GI tract respiratory tract; parenchyma of glands
What is situs inversus?
Complete mirror image viscera
Commonly results from immobile cilia
No problems associated with morbidity (usually)
Problems arise if there is both normal and mirror-image disposition
Prior to gastrulation the embryonic disk is bilaterally symmetrical but in the body there are clear left-right differences. What causes this?
Cells make signals for right sidedness and left sidedness
Action of cilia cells at the node results in leftward flow of left signals and rightward flow of right signals
Cilia beat in one direction
Side specifics signalling cascades initiated
Wrong way = situs inversus
What is gastrulation?
Bilaminar disk converted to trilaminar disk comprising the ectoderm, mesoderm and endoderm
What is neurulation?
Refers to the transformation of the neural plate into the neural tube. Embryo at this strange is called the neuralgia
What is the notochord?
Solid rod of cells running in the midline with important signalling role
What is the function of the notochord?
Directs conversion of overlying ectoderm to neurerctoderm - signals overlying ectoderm to thicken to become the neural plate
What does the notochord produce?
Signalling molecules which can only communicate with neuroectoderm cells as they have the receptors for these signals
What is the fate of the neural plate?
Edges elevate out of the plane of the disk and curl towards each other to create the neural tube
What is the paraxial mesoderm?
Aka persomitic or somitic mesoderm; area of mesoderm in the neurulating embryo that flanks and forms simultaneously with neural tube
Later becomes somites
What is the intermediate mesoderm and what is derived from it?
A region of mesoderm extending anterior-posterior between the paraxial mesoderm and lateral plate mesoderm.
Generates the urogenital system
What is the somatic mesoderm and what is derived from it?
The mesoderm adjacent to the ectoderm in early embryo after formation of the intraembryonic coelom
Limbs and body wall derived from it
What is the splanchnic mesoderm and what is derived from it?
Splanchnic (visceral) mesoderm overlies the endoderm and is a layer of continuous mesoderm covering yolk sac
Gives rise to the mesothelial covering of the visceral organs
What is the intraembryonic coelom?
Initially as single space located in the lateral plate of the mesoderm that will later form the 3 major body cavities: pleural, pericardial and peritoneal
What is segmentation?
Organisation of the paraxial mesoderm into segments known as somites?
What are somites?
One of the paired segments along the neural tube of a vertebrate embryo
What are somites formed from?
Formed by the transverse subdivision of the thickened mesoderm next to the mid plane
What do somites develop into?
The vertebral column and the muscles of the body
How many somites are there?
1st pair appear at day 20 in the occipital region
More appear in a craniocaudal sequence @ 3 pairs/day until 42 to 44 pairs are present by end week 5
Some disappear, leaving 31 pairs in total
Describe the appearance of somites
A regular block of mesoderm cells arranged around a small cavity
Give a brief overview of organised degradation
Somite walls break down:
Ventral wall breaks down leading to formation of the sclerotome
Further organisation of dorsal portion forms combined dermomyotome
Myotome proliferates and migrates dermatologist disperses
What is the sclerotome?
“Hard tissue section”
The ventral and medial portion of a somites that proliferates mesenchymal which migrates about the notochord to form the axial skeleton and ribs
What is the myotome?
“Muscle section”
The dorsal segment of a somite that differentiates into skeletal muscle
What is the dermatome?
“Skin section”
The portion of a somites which gives rise to the dermis
What does the organisation of the mesoderm into somites give rise to?
The repeating structures such as vertebrae, ribs intercostal muscles, spinal cord segments
Also guides innervation
How many pairs of spinal nerve are there?
31 (31 somites)
What is the buccopharyngeal membrane?
Portion which forms future mouth?
What is the cardiogenic area?
Portion which forms future heart
What is folding?
The folding of the flat trilaminar disk into a cylindrical embryo
What is cephalocaudal folding and what is lateral folding?
Cephalocaudal - head and tail fold
Lateral - sides fold
Result is the endoderm is tucked away and ectoderm is presented
What is the purpose of folding?
Draws together the margins of the disk
Creates a ventral body wall
Pulls amniotic membrane around disk
Embryo becomes suspended within amniotic sac
Pulls connecting stalk centrally
Also creates the primordium of the gut, puts the heart ad the primordium of the diaphragm in the right place and creates a new cavity within int embryo
When does the nervous system start to form?
End of 4th week
When has embryo folded by?
End of 4th week
