embryology Flashcards
define congenital
present at birth
define congenital disorders
birth defects
congenital disorders can be what?
- structural or functional
- condition present at or before birth
- genetic, infectious, nutritional and/or environmental in orgin
what are the periods in embryology?
- conceptus
- embryo
- fetus
What is the conceptus stage, and how long does it last?
The conceptus stage is from fertilization to the end of the 2nd week.
When does the embryo stage begin and end?
The embryo stage begins at the start of the 3rd week and ends at the end of the 8th week.
What is the time frame of the fetus stage in development?
The fetus stage begins in the 3rd month and continues until birth.
At what stage does organogenesis primarily occur?
Organogenesis primarily occurs during the embryo stage (3rd to 8th week).
Which stage marks the beginning of visible human form and growth?
The fetus stage (3rd month to birth).
What happens during Step 1: Ovulation?
A secondary oocyte is released from the ovary and swept into the oviduct
What occurs during Step 2: Fertilization?
A single sperm penetrates the secondary oocyte, and the sperm and egg nuclei fuse to form a zygote
What is the significance of Step 3: Cleavage?
The zygote undergoes rapid mitotic divisions, forming a pre-embryo that progresses through 2-cell, 4-cell, and 8-cell stages
What is formed in Step 4: Morula?
By day 4, successive divisions produce a morula, a solid ball of cells that enters the uterus.
What transformation occurs in Step 5: Blastocyst?
By day 6, the pre-embryo forms a blastocyst, a hollow ball of cells with a fluid-filled cavity. It frees itself from the zona pellucida and can increase in size.
What happens during Step 6: Implantation?
The blastocyst attaches to the uterine lining (endometrium) and begins to digest its way inward. Inner cell mass cells start forming the primary germ layers.
What is the role of the trophoblast in Step 5?
The trophoblast is the outer layer of the blastocyst that aids in implantation and interaction with the endometrium.
Where does fertilization typically occur?
Fertilization occurs in the oviduct.
What forms the inner cell mass, and what is its significance?
The inner cell mass is formed within the blastocyst and will develop into the embryo.
What is the duration of meiosis in the oocyte?
Meiosis in the oocyte is completed just before fertilization.
what is cavity formation?
facilitates cellular differentiation and organises embryonic structures
what is differentiation?
process where unspecialised to become specific
What are the two types of cells in a blastocyst?
The outer epithelial layer (trophoblast) and the inner cell mass
What is the role of the trophoblast in the blastocyst?
The trophoblast forms extraembryonic structures, including part of the placenta
What is the function of the inner cell mass in the blastocyst?
The inner cell mass gives rise to the embryo
When does the blastocyst implant into the uterine wall?
Between 5 and 10 days after fertilization
Which part of the blastocyst contributes to the formation of the placenta?
The trophoblast
What is the significance of blastocyst implantation?
It allows the blastocyst to attach to the uterine wall, facilitating further development and nutrient exchange
What is implantation?
Implantation is the process where the blastocyst attaches to and embeds itself in the uterine wall to establish a connection with the maternal blood supply
When does implantation occur?
Implantation occurs between days 5 and 10 after fertilization.
What are the three main steps of implantation?
Apposition: The blastocyst aligns with the uterine lining (endometrium).
Adhesion: The trophoblast cells of the blastocyst adhere to the endometrial cells.
Invasion: The trophoblast differentiates into two layers and invades the endometrium.
What happens during the apposition phase of implantation?
The blastocyst aligns with the uterine lining (endometrium).
What happens during the adhesion phase of implantation?
The trophoblast cells of the blastocyst adhere to the endometrial cells.
What happens during the invasion phase of implantation?
The trophoblast differentiates into two layers:
- Cytotrophoblast: Inner layer.
- Syncytiotrophoblast: Outer layer that invades the endometrium, securing the blastocyst.
What is the role of the trophoblast during implantation?
The trophoblast cells invade the uterine lining, securing the blastocyst and forming the foundation for the placenta.
Why is implantation important for the embryo?
Implantation ensures the embryo receives nutrients and oxygen for further development.
What are the two layers that the inner cell mass differentiates into during the second week of embryogenesis?
Epiblast and hypoblast.
What type of cells form the epiblast, and what is its significance in embryogenesis?
Columnar cells form the epiblast, which gives rise to all three germ layers (ectoderm, mesoderm, and endoderm) and contributes to the embryo.
What type of cells form the hypoblast, and what role does it play in development?
Cuboidal cells form the hypoblast, which contributes to extraembryonic structures like the yolk sac.
What structure is created by the epiblast and hypoblast together?
The bilaminar embryonic disc.
What is gastrulation, and why is it important in embryogenesis?
Gastrulation is the process during the third week of development where the bilaminar disc transforms into a trilaminar disc, forming the three germ layers: ectoderm, mesoderm, and endoderm.
Which germ layer forms first during gastrulation, and how does it form?
The mesoderm forms first as epiblast cells migrate through the primitive streak.
What structure marks the beginning of gastrulation?
The primitive streak.
What are the three germ layers formed during gastrulation, and what does each layer contribute to?
Ectoderm: Forms the skin, nervous system, and sensory organs.
Mesoderm: Forms muscles, bones, blood vessels, and connective tissues.
Endoderm: Forms the lining of the gut and respiratory systems.*
What role does the epiblast play in gastrulation?
The epiblast is the source of all three germ layers as its cells migrate through the primitive streak.
What happens to the hypoblast during gastrulation?
The hypoblast is displaced and contributes to extraembryonic tissues.
What is the primitive node, and what is its role in gastrulation?
The primitive node is a circular area at the cranial end of the primitive streak that guides the migration of epiblast cells to form the notochord and mesoderm.
What happens to the cells of the epiblast during gastrulation?
The cells of the epiblast migrate medially and into the primitive groove.
What is the fate of the first epiblast cells that migrate through the primitive groove?
The first cells move into the hypoblast and replace it to form the embryonic endoderm.
What do the later migrating epiblast cells become?
Later cells move into the space between the epiblast and endoderm, becoming the embryonic mesoderm.
What happens to the epiblast cells that remain in their original position after migration?
The cells left in the epiblast form the embryonic ectoderm.
What is the result of the epiblast cell migration and differentiation process?
The process results in the formation of the three germ layers: ectoderm, mesoderm, and endoderm, marking the three-layer stage.
What is the notochord, and how long does it persist?
The notochord is a cartilage-like, transient structure that disappears after performing its role.
What is the significance of the notochord in embryogenesis?
It is important for the induction of structures like the neural tube.
Where does the notochord extend during its formation?
It extends cranially along the midline from the primitive node to form a hollow tube.
How does the notochord grow in length?
It grows in length as cells are added from the primitive node.
What happens to the primitive streak during the formation of the notochord?
The primitive streak regresses as the notochord forms.
What is the primitive streak?
A thickened line of cells on the epiblast that defines the body’s major axes.
Where does the primitive streak form?
On the upper surface of the bilaminar disc, specifically on the epiblast.
What happens to the primitive streak during gastrulation?
It invaginates to form the primitive groove, allowing cell migration.
What are the three germ layers formed during gastrulation?
Ectoderm, mesoderm, and endoderm.
What does the mesoderm give rise to?
Muscles, bones, and connective tissues.
What does the ectoderm give rise to?
The skin and nervous system.
What does the endoderm give rise to?
Internal linings and organs like the gut and lungs.
Why is gastrulation important?
It establishes the body’s axes and forms the germ layers that develop into all tissues and organs.
Why is the node important for left-right symmetry?
Its cilia create nodal flow, directing signals that establish organ placement.
How does the node contribute to left-right asymmetry?
Cilia in the node rotate, creating leftward fluid flow, which bends cilia on the left, acting as mechanosensors to increase Ca²⁺ signaling in those cells.
What does the blastocyst give rise to?
The trophoblast and the inner cell mass.
What does the trophoblast differentiate into?
Cytotrophoblast and syncytiotrophoblast.
What does the inner cell mass give rise to?
The primitive endoderm (hypoblast) and the epiblast.
What does the primitive endoderm (hypoblast) form?
The extraembryonic endoderm, which develops into the yolk sac.
What does the epiblast give rise to?
The amniotic ectoderm and the embryonic epiblast.
What does the embryonic epiblast form?
The primitive streak, which gives rise to the embryonic mesoderm, embryonic endoderm, and embryonic ectoderm.
What tissues arise from the extraembryonic tissues pathway?
The yolk sac, cytotrophoblast, and syncytiotrophoblast.
What tissues arise from the embryonic tissues pathway?
The embryonic mesoderm, embryonic endoderm, and embryonic ectoderm.
What happens to ectodermal cells above the notochord during neural plate formation?
They thicken and differentiate to form the neural plate.
What happens to the neural plate after it forms?
It extends and folds to form the neural groove.
What is the first visible structure of neurulation?
The neural plate.
What is the role of neural folds in neural tube formation?
What is the role of neural folds in neural tube formation?
When does the caudal neuropore close during development?
By day 26.
When does the cranial neuropore close during development?
By day 24.
What is the role of the notochord in neurulation?
It induces the overlying ectoderm to form the neural plate.
What marks the completion of neural tube formation?
The closure of both cranial and caudal neuropores.
Where does the neural plate form in relation to the primitive node?
Cranial to the primitive node.
What do ectodermal cells differentiate into during neural plate formation?
A thick plate of pseudostratified, columnar neuroepithelial cells called the neuroectoderm.
What structure is formed by the invagination of the neural plate?
The neural groove.
How is the neural tube completed?
By the closure of the neural groove.
Neural crest cells give rise to what structures?
-Dorsal root ganglia
- Enteric ganglia
- Schwann cells
- Melanocytes
- Sympathetic and parasympathetic ganglia
- Dentine
how do neural crest cells form?
when the neural plate transitions to the neural tube and then separates from the surface of the ectoderm, neural crest cells are then released
What is the initial structure of the neural tube?
The neural tube is initially one cell layer thick and hollow.
What happens at the cranial end of the neural tube during development?
The cranial end starts to swell, forming vesicles.
What do the vesicles formed at the cranial end of the neural tube give rise to?
The vesicles give rise to the brain.
What does the remainder of the neural tube develop into?
The remainder of the neural tube develops into the spinal cord.
What happens to the embryonic disc due to the differential growth of various tissues?
- Embryonic disc and amnion: high growth rate
- Yolk sac: almost no growth
What is the shape and structure of the embryo at the end of the third week?
At the end of the third week, the embryo is flat, ovoid, and a trilaminar disc.
What happens to the embryo during the fourth week of development?
During the fourth week, the embryo grows rapidly, especially in length, and begins the process of folding to generate the body form.
What tissues exhibit a high growth rate during the body folding process?
The embryonic disc and amnion exhibit a high growth rate.
What is the main force driving the folding process of the embryo?
The main force driving the folding process is the differential growth of various tissues.
What happens to the yolk sac during the body folding process?
The yolk sac experiences almost no growth during the body folding process.
What structures stiffen the dorsal axis of the developing embryo?
The developing notochord, neural tube, and somites stiffen the dorsal axis.
what is the mesoderm divided into?
- Paraxial mesoderm
- Intermediate mesoderm
- Lateral mesoderm
what occurs in the paraxial mesoderm?
condenses becoming somitomeres and organise cranial to caudally. 37 pairs, 30 of which condense to become somites
how do somite pairs segregate?
ventro-medially
what is a somite?
mesodermal segments of vertebrate embryos that become vertebral column, skeletal muscle and dermis
in which direction does somitogenesis take place in the maturation of the presomatic mesoderm?
cranial to caudal
how is somitogenesis controlled?
controlled by the segmentation clock which act as signalling pathways
what is the importance of the intermediate mesoderm?
Th e urogenital system:
- Kidneys
- Gonads
- Respective duct systems
From which embryonic structure does the urogenital system develop?
The intermediate mesoderm.
What are the stages of kidney development?
Pronephros, mesonephros, and metanephros.
What is the role of the mesonephric (Wolffian) ducts?
They form the male reproductive system.
How do gonads develop?
From the genital ridges, influenced by sex chromosomes.
What determines male or female reproductive organs?
Males develop testes with testosterone; females develop ovaries in the absence of testosterone.
What happens to the Wolffian and Müllerian ducts?
Wolffian ducts form male structures; Müllerian ducts form female structures.
What structures form as the urogenital system matures?
Kidneys, ureters, bladder, and reproductive organs.
What is the lateral mesoderm form?
Divided into somatic (parietal - wall of organ)
and splanchnic (visceral - pertains)
* Ventro-lateral body wall
(connective tissue, not muscle) * Bones of the limbs
* Heart and vasculature
* Wall of gut
What is required for vasculogenesis to occur?
It requires the recruitment of mesodermal progenitors to the endothelial cell lineage ( sprouting + migration) and their organization into microvessels.
What is vasculogenesis?
Vasculogenesis is the de novo assembly of blood vessels from mesodermally derived cells.
What is angiogenesis?
Angiogenesis is the formation of blood vessels from pre-existing vasculature.
Fate of the intermediate mesoderm?
- Urogenital system
Fate of the paraxial mesoderm?
- Dermis of skin
- Axial skeleton
- Axial and limb muscles
Fate of the lateral mesoderm?
- Divided into somatic (parietal) and splanchnic (visceral)
- Ventro-lateral body wall (connective tissue, not muscle)
- Heart and vasculature
- Wall of gut
- Bones of the limbs
What is the gut tube?
Primitive digesting tube - originates from the endoderm
What does lateral folding of the embryo complete?
Lateral folding of the embryo completes the gut tube.
What happens to the mesoderm during lateral folding?
Mesoderm is recruited to the gut wall during lateral folding.
From which mesodermal layer does the gut tube’s mesodermal layer arise?
The mesodermal layer of the gut tube arises from the splanchnic mesoderm.
What is the role of somatic mesoderm in the body cavity?
The somatic mesoderm lines the body cavity.
What is the other name for somatic mesoderm?
Somatic mesoderm is also called parietal mesoderm.
What is the other name for splanchnic mesoderm?
Splanchnic mesoderm is also called visceral mesoderm.
Where does the stomach develop in the embryo?
The stomach develops in the distal part of the foregut.
What happens to the stomach around the middle of the fourth week?
There is a slight dilatation of the stomach.
How does the stomach grow during development?
The stomach enlarges and broadens ventro-dorsally.
How does the dorsal part of the stomach grow in relation to the ventral part?
The dorsal part grows faster than the ventral part, leading to the greater curvature of the stomach.
What rotation occurs as the stomach grows?
The stomach rotates 90 degrees clockwise.
What happens to the borders of the stomach during rotation?
The ventral border (smaller curvature) moves to the right, and the dorsal border (greater curvature) moves to the left.
How does the rotation of the stomach affect the duodenum?
The rotation bends the duodenum into a C-shape, moving it superiorly.
What is the first step in the development of endodermal organs?
The first step is endodermal thickening.
What happens after endodermal thickening?
Cells proliferate to form a bud.
What occurs during the further development of the bud?
The bud continuously lengthens and undergoes bifurcation/branching.
What forms from the ventral out-pocketing of the endoderm?
The respiratory diverticulum forms, which will become the trachea.
How does the respiratory diverticulum grow during development?
It grows ventro-caudally.
What happens after the initial growth of the respiratory diverticulum?
The respiratory diverticulum bifurcates into right and left primary tracheal buds, which will form the bronchi.
What occurs during the second bifurcation of the tracheal buds?
The secondary bronchial buds form (3 on the right, 2 on the left), which will develop into lung lobes.
What happens during the third bifurcation of the bronchial buds?
The tertiary bronchial buds form, which will develop into bronchopulmonary segments.
What occurs after the formation of tertiary bronchial buds?
There are 14 more branchings, leading to the formation of terminal bronchioles.
What structures are derived from the foregut?
The foregut gives rise to the pharynx, esophagus, stomach, and proximal duodenum.
What structures are derived from the midgut?
The midgut gives rise to the distal duodenum and half of the colon.
What structures are derived from the hindgut?
The hindgut gives rise to the other half of the colon to the anus.
What are some derivatives of the gut tube?
Derivatives include the thyroid, parathyroid, thymus, lungs, liver, gall bladder, pancreas, and urinary bladder.
What is the first step in vasculogenesis?
Haemangioblast migration is the first step.
What forms after haemangioblast migration?
Blood islands form.
What occurs during the process of vasculogenesis after blood island formation?
Cell differentiation occurs.
What role do pericytes play in vasculogenesis?
Pericytes are recruited during the process.
What tissues contribute to the formation of vasculature during vasculogenesis?
The mesoderm and endoderm contribute.
What cells are involved in the process of vasculogenesis?
Angioblasts, hematopoietic cells, endothelial cells, blood cells, and pericytes are involved.
From which germ layer does the heart develop?
The heart develops from the mesoderm, specifically the lateral plate mesoderm.
What are the primary heart fields, and what do they form?
The primary heart fields are clusters of cardiac progenitor cells that fuse to form the heart tube.
When does the heart tube form, and what does it consist of?
The heart tube forms by the end of the third week and consists of the sinus venosus, atrium, ventricle, and bulbus cordis.
What is heart tube looping, and why is it important?
Looping is the bending and folding of the heart tube to establish right and left sides and position the heart chambers.
What structures arise from the sinus venosus, atrium, ventricle, and bulbus cordis?
The sinus venosus forms the right atrium and coronary sinus, the atrium forms the left and right atria, the ventricle forms the left ventricle, and the bulbus cordis forms the right ventricle and part of the outflow tract.
How are the atria separated?
The septum primum and septum secundum form, creating the left and right atria with the foramen ovale for fetal blood flow.
How are the ventricles separated?
The ventricular septum forms to divide the left and right ventricles.
How is the outflow tract divided?
The aorticopulmonary septum divides the bulbus cordis and truncus arteriosus into the pulmonary trunk and aorta.
How do the atrioventricular (AV) valves develop?
The mitral and tricuspid valves form from endocardial cushions.
How do the semilunar valves develop?
The aortic and pulmonary valves develop from swellings in the outflow tract.
How is the heart vascularized?
Coronary vessels form from mesoderm surrounding the heart tube.
What fetal shunts bypass the lungs during development?
The foramen ovale connects the atria, and the ductus arteriosus connects the pulmonary artery and aorta.
When is the heart structurally complete?
By the eighth week, the heart is a fully formed four-chambered structure with separated circulatory routes.
What is unusual about many skeletal structures in the head?
They are formed from neural crest cells instead of mesoderm.
How many pairs of pharyngeal arches does the human embryo have?
The human embryo has four pairs of well-defined pharyngeal arches.
What are pharyngeal arches also called?
Pharyngeal arches are also called branchial arches.
What covers the outer layer of pharyngeal arches?
They are covered by ectoderm.
What is the core of the pharyngeal arches made of?
The core consists of mesenchyme derived from mesoderm and neural crest cells.
What lines the inside of the pharyngeal arches?
They are lined with endoderm.
What is the central skeletal element of each pharyngeal arch derived from?
It is a cartilaginous element derived from neural crest cells.
What nerve is associated with each pharyngeal arch?
Each arch has an arch-specific cranial nerve.
What type of muscle rudiment is found in each pharyngeal arch, and where is it derived from?
Each arch contains a striated muscle rudiment derived from head mesoderm.
What type of artery is present in each pharyngeal arch, and what is its origin?
An aortic arch artery is present, with endothelial cells derived from mesoderm.
