Embryology Flashcards
(135 cards)
1
Q
What is embryogenesis driven by?
A
Evolutionary, genetic, epigenetic and environmental factors.
There are structural similarities between all species (same number of pharyngeal arches), these drivers allow for species differences.
2
Q
Why are the processes of embryogenesis important for our understanding?
A
allows us to better understand disease, regeneration, and repair during the post natal period (juvenille to adult).
3
Q
Where do embryo’s start from?
A
Fertilised zygote.
4
Q
What are the two pronuclei?
A
Genetic material, one of maternal, and one of paternal origin, within the fertilised zygote.
5
Q
What can errors in embryogenesis result in?
A
Embryonic loss Fetal death Fetal mummification Abortion Stillbirth Birth of nonviable neonates Birth of viable offspring with defects.
6
Q
What factors influence embryogenesis?
A
Multifactorial process.
Congenital (with genes) - development disruption results in deviation from normal that is present or apparent at birth.
Genetic, environmental (nutritional), physical and infectious agents can all be etiologic determinants.
7
Q
What biological molecules can affect growth and development in the adult and embryo?
A
Carcinogens - initiate or induce neoplasia/carcinogenesis.
Mutagens - produce a change in the genetic code.
Tetratogens - cause the development of physical defects in the embryo/foetus.
Some chemical agents can be one, two or all of the above (eg. radiation).
8
Q
Describe, specifically, the effect of Tetratogens.
A
Early embryogenesis; cause mutations in DNA at genomic or chromosomal level.
Mid embryogenesis/early foetal; effects on cell proliferation, differentiation or cell death.
Late foetal; most tissues relatively protected, only highly proliferating tissues still susceptible (eg. palate, eye, cerebellum).
Example - Veratum and cyclopia; cattle or sheep ingesting this plant in early embryogenesis will have young with cyclopia. Inhibits sonic hedgehog signalling pathway which gives rise to midline of head.
9
Q
What is the most critical period of sensitivity to factors affecting development?
A
Embryonic development - weeks 3-8.
Systems that take the longest to develop, or are the most complex, generally exhibit the most severe effects.
10
Q
What are some physical causes of embryonic/foetal abnormalities?
A
Congenital joint contracture can be caused by in-utero crowding.
Spinal deformities and limb abnormalities can present in foals following a transverse or caudal presentation.
Aggressive palpation for pregnancy diagnosis can result in limb deformities or disruption of the delicate vascular supply to the intestical tract (artesia coli).
11
Q
Describe the process of fertilisation.
A
- Sperm attracted to egg by secretion of soluble molecules from the egg (chemo-attraction).
- Exocytosis occurs from sperm acrosomal vesicle to release degrading enzymes.
- Sperm binds to the extracellular matrix (mammals - zona pelucida) surrounding the egg.
- Sperm passages through extracellular matrix (acrosome reaction).
- Cell membranes of the egg and sperm fuse - fertilisation.
12
Q
What is the acrosome reaction?
A
Sperm releases proteolytic enzymes that digest a hole in the zona pelucida for the sperm to travel through.
13
Q
What is ‘activation’ of the fertilised egg?
A
Occurs after fusion of the sperm head to the cell membrane of the ova. Results in series of Ca ‘waves’ that result in;
reactivation of the genetic material of the ova,
resumption of proliferation
release of inhibition of the maternal genome
exocytosis of the male and female pronuclei
14
Q
What are the common causes of infertility in all species?
A
Failure of fertilisation/implantation
Early embryonic loss
15
Q
What is ectopic pregnancy?
A
Implantation in an inappropriate position (uterine/fallopian tube, ovary, peritoneal cavity).
Much more common in humans than domestic species.
16
Q
What is the normal rate of embryonic loss?
A
Normal process that ranges between 25-50% in domestic species.
Caused by multiple factors.
17
Q
What is late embryonic loss?
A
Mummification
Abortion
Many causes including viral, nutritional, toxicosis.
18
Q
What does placental insufficiency/failure result in?
A
Poor in utero foetal growth
Abortion
Embryonic loss
19
Q
What are the stages from fertilisation to implantation?
A
Fertilisation (ampulla region) - zona pellucida (first cleavage) - 2-cell stage - morula (fallopian tube) - blastocyst (uterus) - early stage of implantation.
20
Q
What is cleavage?
A
Series of rapid (mitotic) divisions following fertilisation.
Cell size DIMINISHES progressively (go from one really big cell to bunch of smaller cells).
Absence of cell growth phase between each division.
Result is blastocyst.
21
Q
Describe cleavage in mammals compared to other animals?
A
Relatively slow - 12-24hrs between divisions.
Asynchronous - all blastomeres do not divide at the same time.
Produce compact ball cells encircled by the zona pellucida - outer surface is extraembryonic tissue, cecntral surface is foetus and extraembryonic tissue.
22
Q
What are blastomeres?
A
Cells resulting from the cleavage process.
Cleavage occurs within isthmus.
Contractions propel embryo forward, 4-5d to reach uterus, secretions from epithelial lining provide nutrients, and specific proteins that contribute to development.
This is where embryonic stem cells come from!
23
Q
What is a morula?
A
Solid ball of cells.
16-64 blastomeres.
Formed near the end of cleavage and surrounded by ZP.
24
Q
Compare mammalian and non mammalian ovulation/fertilisation/early embryogenesis.
A
Non-mammalian; large number of oocytes, large in size, large number of fertilisation events. Fertilisation occurs outside organism. Primary cleavage events are controlled by the maternal genome, synchronous cleavage and no compaction (never appear like football).
Mammalian; small number of oocytes, small in size, small number of fertilisation events. Fertilisation occurs inside the organism. Primary cleavage events controlled by embryonic genome. Asynchronous cleavage and compaction.
25
Describe implantation.
For this to occur, the blastocyst must shed the zona pellucida (stops embryo attaching to other cells while moving down to uterus). This process is called hatching.
Implantation now begins, polytocous species have multiple blastocysts evenly distributed. Monotocous species have single blastocyst, some have preferred implantation sites (eg. horse).
Process of apposition - adhesion and attachment of trophoblast to uterine lining.
Timing from ovulation to implantation varies between species from 6d to 56d - this is due to the different types of implantation and their mechanisms for embedding the embryo into the uterine lining.
26
How is implantation classified?
According to the relationship between the blastocyst and uterine lumen.
Central - blastocyst remains within uterine lumen (ungulates, carnivores, lower primates). Varied timing
Eccentric - blastocyst lies within uterine crypt or recess (mouse, rate, hamster, rabbit, some bats). Fast.
Interstitial - conceptus invades the uterine wall (guinea pig, chimp, man). Very fast.
27
What is the purpose of foetal membranes/extraembryonic membranes?
Provide protection, nutritional and excretory requirements for the developing embryo.
28
What are the 4 major extraembryonic membranes?
Yolk sac
Amnion
Chorion
Allantois
29
Discuss the Yolk Sac.
First membrane formed, critical for vascular supply.
Involved in early haematapoiesis and angiogenesis.
Vitelline blood vessels - gut vessels (cranial mesenteric, hepatic portal vein).
30
Discuss the amnion.
Surrounds the foetus, covers umbilical cord and continues with body surface at umbilicus.
Fluid filled, filtration for superficial blood vessels.
Secretions from alimentary and respiratory tracts.
Filters urine from kidneys and may contain allantoic fluid which may be transported across the allantoamnion.
31
Discuss amniotic fluid.
Cushions foetus.
Allows unrestricted movement - important in later stages of development, prevent pressure related growth abnormalities.
At birth when it ruptures it acts as lubricant.
32
Describe the changes in the amnion at birth.
Horse, dog, cat - raphe degenerates, may be born in amniotic sac.
Ruminants, pigs - raphe retained, amniochorion formed above dorsal aspect of embryo, ruptures at birth, foetus born without surrounding amniotic sac.
33
Discuss the chorion.
Established at the same time as the amnion. Chorionic sac surrounds the embryonic membranes and foetus.
Composed of trophoblast (outer layer) and somatic mesoderm (inner layer).
Lies in apposition to uterine lining; participates in formation of foetal component of placenta.
34
Discuss the allantois.
Develops as diverticulum of the hindgut.
Varies considerably in size between species.
Humans/primates this is a residual structure, pigs this is very large, birds and reptiles this is a very large waste sac.
Stores urinary waste.
In birds this also mediates gas exchange and Ca transport from shell to embryo by fusion with the chorion.
35
What is placentation?
Structural organisation and mode of attachment of the placenta.
36
What is the placenta?
Organ of metabolic interchange between mother and foetus.
Large surface area for nutritional, excretory, immunological and endocrine functions.
There are two components that undergo modification; maternal (endometrial lining of uterus) and foetal (chorion and yolk sac are transitory placenta, chorion and allantois are definitive placenta).
Keeps two blood supplies completely separate to prevent maternal antibodies being produced against foetus.
37
How is the embryo provided with its nutritional requirements?
Haemotropic source - maternal blood stream is primary source.
Histotropic source - coiled endometrial glands produce histotrophe (uterine milk - fat and glycogen).
38
How are placentae classified?
1. Choriovitelline placenta - first placenta; fusion of chorion and yolk sac, vitalline vessels formed, transitory in most mammals. Preceeds development of chorioallantoic placenta.
2. Chorioallantoic placenta - definitive placenta; comprises chorion and allantois, allantoic vessels form and become umbilical vessels, definitive (final and functional until birth).
39
What are the different types of placenta?
Diffuse - plaental zone covers almost entire surface of chorionic sac. Horse and pig.
Cotyledonary - placental zone restricted to specialised cotyledons. Cotyledons develop in response to caruncles (place of chorionic contact). Caruncles are permanent and arranged in rows.
Zonary - placental zone is band around central region. Complete in dogs, cats, incomplete in bears and mustelids. Trophoblast is modified and invades endometrium.
Discoid - placental area has 1 or 2 disc shaped areas. Man/rodent - 1 disc, monkey - 2 discs.
40
Describe what occurs to the placenta at birth.
Loss of maternal tissue at birth.
Deciduate - invasion and destruction of maternal tissue results in shedding of maternal tissue. Carnivores, primates and rodents.
Non-deciduate - virtually no loss of maternal tissue at parturition. Ruminants, horses, pigs.
41
What is a normal cause of dystocia?
Failure of normal twin presentation.
42
What can dystocia result in?
Fetal mortality
Maternal mortality
Both
43
What is the leading cause of death in calves, post partum infection, and death in heifers?
Dystocia
| It is a major economic loss to the beef industry.
44
Discuss the formation of the embryonic disc.
Flattened pear shaped disc develops from the ICM, initially it is bilaminar but it then becomes trilaminar (ecto, meso, endoderms).
Head fold is the fastest to develop.
Somites form and act as building blocks for axial skeleton.
45
What is gastrulation?
The transition from bilaminar to trilaminar embryonic disc (formation of 3 primary germ cell layers-go on to form all tissues of the embryo). Endodermal and mesodermal (migratory) cells migrate through the primitive streak.
Can occur as early as 24-28hrs post fertilisation (chick).
Hypoblast cells become part of he yolk sac, epiblasts contribute to embryo proper (present on caudal midline and invaginate).
INVOLVES ELONGATION OF THE PRIMITIVE STREAK AND MIGRATION OF CELLS.
46
What is the primitive streak?
First and most obvious structure to form in the early embryo.
47
What is the AVE?
Anterior visceral endoderm
48
What is one of the first patterning events?
Creation of the head from the tail. Occurs during the gastrulation/primitive streak stage.
49
What prevents the head cells consuming the whole body?
Node
50
What distinguishes the head end from the rest of the early embryo?
```
The AVE (head organiser) and the node (rest of body organiser).
No AVE = no head.
```
51
Which surface of the embryo is in contact with the amniotic cavity?
Embryonic ectoderm is in contact with amniotic cavity.
| Allows passive transfer of external proteins and other molecules.
52
How are the genes expressing the posterior mesoderm formation activated?
Epiblast expresses the posteriorising nodal protein.
53
What does the AVE express?
Dorsal factors Oxt2
54
What does the anterior visceral endoderm secrete?
Two antagonist of the nodal protein;
Lefty1 - binds to the nodal receptor
Cerberus - binds directly to nodal - overexpression of this results in 2 heads.
55
Discuss anterior/posterior axis formation in the chick embryo.
Axis formation is determined by gravity.
Rotation in the shell results in the light components of the yolk pushing up one side of the blastoderm.
Higher side becomes posterior region of embryo.
56
How is asymmetry established (left and right) in the chick embryo?
1. To the left hand side of Hensen's node (2/3 up primitive streak) sonic hedgehog activates cerberus which causes expression of nodal.
2. Nodal activates expression of Pitx2 (gene that confers 'leftness'-heart location).
57
What is the epiblast?
Outermost layer of embryo before it differs into ectoderm and mesoderm.
58
Why is patterning so important?
Asymmetry occurs from the earliest stages of embryo development. This contributes to normal development of the blastocoel cavity.
As well as implantation of more than one embryo, loss of asymmetry in the early embryo can give rise to twins (cleavage, morula, early blastocyst).
59
What are dizygotic twins?
Twins arising from two ova from two ovarian follicles fertilised by seperate spermatazoa during a single breeding cycle.
Non-identical/fraternal/littermates.
60
How does the rate of twinning differentiate between domestic animals?
Varies between species and breed.
Cattle - similar to humans, natural twinning (dizygotic) 2-3%, monozygotic 0.1%.
Sheep - natural twinning 2-5% (lowland > highland)
Horses - multiple ovulations <30%, twins <2% (innate physiological mechanisms inhibiting implantation of twins).
61
What is the incidence of monozygotic twins arising from the primitive streak?
Observed in humans, sheep and pigs.
| Incidence probably <1%.
62
What is a chimera?
Two morulas merge and produce one individual containing cells with two distinct genotypes.
63
Describe the timing of separation between monozygotic twins?
Pre-blastocyst - split occurs prior to formation of blastocyst (morulla stage). Two blastocoels, two embryos with 2 ZP, two chorions and amnions.
Early blastocyst - one blastocoel with two ICM, one ZP with two embryos inside, one chorion but 2 amnions.
Late blastocyst - one blastocoel with one ICM, one ZP and embryo, 2 foetus's with one chorion and amnion (increased incidence of conjoined cases). Split occurs late in blastocyst formation.
64
What are stem cells?
Building blocks of the embryo and its tissues.
It is a cell that can produce identical copies of itself indefinitely.
Stem-like qualities are associated with metastatic (cancer) states and therefore are part of the diagnostic and prognostic criteria for cancerous biopsy material.
Understanding how they work tells us how tissues are made and therefore how to repair them.
65
What are neural crest cells?
Cells that create skin pigmentation patterns and contribute specialised cell types to other organ systems.
Defect leads to lethal white foal syndrome.
They are 'committed' stem cells.
66
Describe the stages of specialisation of a stem cell?
Pluripotent stem cell undergoes lineage commitment, becomes committed stem cell. This undergoes specification to become progenitor cell. Differentiation then occurs and the cells become differentiated cells.
67
What is the foundation cell of the embryo?
Embryonic stem cells.
Become mesoderm, endoderm and ectoderm.
They are derived from 'committed' stem cells that contain progenator cells.
68
Is lineage commitment a reversible process?
No
69
Discuss stem cell niches.
Create an inductive microenvironment which allows cells to be renewed and directed to a particular fate in states of normal growth, repair and disease.
Eg. bone marrow, brain, intestinal crypt, liver, skin/hair, teeth.
70
What are neural stem cells?
Adult neural stem cells are present in the dentate gyrus of the hippocampus.
They express markers of embryonic neural stem cells (Pax6, Sox2).
They are required for cell renewal for learning and memory.
71
What are NCC's?
Neural crest cells. They are a migratory cell population that detaches from the embryonic neural epithelium (neuroectodermal origin).
Early NCC's are pluripotent.
72
What are the stages of formation of the neural tube?
1. Cells of the neural plate identifiable in dorsal ectoderm.
2. Cells of midline hinge point (MHP) anchor to notochord and cells of presumptive epidermis are pushed towards midline.
3. The two sides of the neural elevate, epidermal cells become opposed.
4. Cells of the dorsolateral hingepoint (DLHP) anchor to overlying epidermis and become wedge shaped.
5. Two sides of the neural tube contact with neural crest cells linking the neural tube to the epidermis.
6. The NCC's migrate away, leaving the NT and epidermis separate.
73
What is the difference in mammals when it comes to neural tube formation?
NCC's emigrate from the tip or 'crest' of the still open neural folds.
74
What is the difference in birds when it comes to neural tube formation?
Arise only after neural tube closure.
75
What are the 4 distinct populations that NCC's arise in?
Diencephalon
Midbrain
Hindbrain
Trunk
76
Discuss the Trunk neural crest.
Early migration of cells occurs through anterior portion of the somites, forming the DRG (sensory neurons), sympathetic ganglia, adrenal medulla, and nerve clusters surrounding the aorta. Ventral pathway - coordinated by both inhibitory and permissive factors.
Later migration leads to pigmented melanocytes of the skin via the dorsal pathway - NCC's follow attractive cues expressed by the dermis.
Also give rise to Vagal NCC (somites 1-3; cardiac neural crest), sacral NCC, parasympathetic (enteric) ganglia of the gut (required for peristaltic bowel movement), cardiac NCC (provides muscular-connective tissue wall of large arteries) and the septum that separates pulmonary circulation from the aorta.
77
Discuss the cardiac neural crest.
Cardiac NCC's migrate from arches 4 and 6.
Form the septum that separates the truncus arteriosus into pulmonary artery and aorta.
Cells express paired box-containing transcription factor Pax3.
Mutation in Pax3 gives rise to loss of cardiac NCC's - persistent truncus arteriosus.
78
Discuss the cranial neural crest cells. What are the 3 populations?
Diencephalon, Midbrain, Hindbrain.
Can differentiate into bone, cartilage and connective tissue (unlike trunk NCC).
Rostral cranial NCC's give rise to frontonasal skeleton.
More posterior cranial NCC's fill the pharyngeal arches (gills) where they form the cartilage and bone of the jaw, middle ear, neck, thymic cells, and odontoblasts.
Also important for membranous bones of the skull vault.
79
How are the pharyngeal arches formed?
Largely from cranial NCC's.
| NCC's migrate into the pharyngeal arches and frontonasal processes.
80
What is the consequence of failure of mandibular arch neural crest cell migration?
Mandibular hypoplasia
81
What are somites?
Building blocks that give rise to a number of organs and systems in the body (including the musculoskeletal).
They are the earliest visible structures in the embryo.
82
What is somitogenesis?
Process by which somites form.
| Abnormal somitogenesis causes skeletal abnormalities or embryonic lethality.
83
What are the 4 types of specified mesodermal cells present in the trunk of the embryo at neural tube stage?
Chordamesoderm (notochord).
Paraxial (somitic) mesoderm (connective tissue, bone, muscle, cartilage).
Intermediate mesoderm (urogenital system and adrenals).
Lateral plate mesoderm (heart, blood vessels, blood cells, body cavities, non-muscular components of the limbs).
84
What are the 4 cell types of mature somites?
Sclerotome - vertebrae and rib cartilage.
Myotome - musculature of back, ribs, and limbs.
Dermatome - dermis of the back.
Syndetome - tendons and blood vessels.
85
Describe the locations of the specified mesodermal cells in relation to the neural tube.
Neural tube is central, either side is paraxial mesoderm. Below neural tube is small chord of chordamesoderm. Outside of the two paraxial mesoderm masses is the intermediate mesoderm then the lateral plate mesoderm.
86
What determines the migration pathways of neural crest cells and spinal axons?
Somites.
87
In what direction do somites form?
Anterior to posterior direction.
88
How often is a new somite formed in the chick embyro? Is it singular or paired?
Somites always form in identical pairs.
| Formed every 90 minutes.
89
What is critical for normal somitogenesis?
Boundary formation - notch signaling acts to establish somite boundaries.
If this is disrupted, alteration in skeletal development, failure of separationg of thoracic vertebrae and associated structures fail to form.
90
How do somites start?
As condensations of mesenchymal cells.
91
What are muscle precursor cells? What are the 3 types?
Myoblasts.
3 types;
1) primaxial myoblasts - closest to neural tube, form primaxial muscles (intercostals, deep muscles of back).
2) Abaxial myoblasts - further from neural tube, form abaxial muscles (body wall, limbs, tongue).
3) future satellite cells - arise from central myotome, remain undifferentiated, postnatal growth and repair.
92
What determines what muscles/skeletal components will be produced?
On global scale; Hox gene code and AP axis specification.
Within the somite; ventromedial aspect of the somite is specified to become sclerotome by paracrine factors emanating from the floorplate (Shh).
Express Pax1 - required for cartilage differentiation.
Inhibit myogenic genes (Pax3, MyoD, Myf5).
93
Describe the stages of generation of muscle tissue from myoblasts.
1) Determination; Myotome cells are specified to become myoblasts by induction of MyoD (primaxial myotome Pax3-->MyoD, medial myotome Myf5-->MyoD).
2) Dividing myoblasts; Myoblast numbers are expanded under the influence of FGFs (fibroblast growth factors).
3) Cell alignment; cell adhesion molecules control muscle cell alignment.
4) Initial myotube formation; myoblasts begin to fuse together to create myotubes, this occurs as myoblasts exit the cell cycle due to depletion of FGFs.
5) Myotube maturation; myotubes complete fusion and co-ordinated contraction can be initiated.
6) Complete muscle fibre and stem cell.
94
How is the skeleton generated?
```
Osteogenesis
3 distinct lineages;
Somites --> axial (vertebral) skeleton
Lateral plate mesoderm --> limb skeleton
Cranial neural crest --> craniofacial bones and cartilage
```
95
What are the two major modes of bone formation?
Direct conversion of mesenchyme to bone is intramembranous ossification.
Indirect conversion via cartilage to bone is endochondral ossification.
96
What is intramembranous ossification?
Mesenchymal cells condense to form osteoblasts which lay down an (osteoid) collagen proteoglycan matrix that is able to bind calcium.
Osteoblasts then become embedded in the matrix and become osteocytes.
Process involves BMP's (bone morphogenetic proteins) and a transcription factor (CBFA1).
97
What is endochondral ossification?
Occurs primarily in the vertebral column, ribs, pelvis and limbs (somitic and lateral plate mesoderm-derived bones).
Invovles formation of cartilage from aggregations of mesenchymal cells.
Replacement of cartilage with bone.
Occurs in 5 phases.
98
What are the 5 phases of endochondral ossification?
| Actually 7
1) Mesenchymal cells commit to becoming cartilage (increased Pax1 and Shh).
2) Committed mesenchymal cells condense into nodules and differentiate into chondrocytes (increased N-Cam, Sox9 and BMP).
3) Chondrocytes proliferate and create cartilage model of the bone (secretion of cartilage specific ECM).
4) Chondrocytes stop proliferating and increase their cellular volume.
5) Enlarged chondrocytes alter secretion of ECM to enable calcification by calcium phosphate. Induction of collagen X and fibronectin induces angiogenesis (increased VEGF).
6) Hypertrophic chondrocytes die, surrounding cells differentiate into osteoblasts. Blood vessels invading the bone bring in osteoblasts and chondrocytes which remove dead cells (chondrocyte) debris (osteoblasts and chondroclasts keep inducing angiogenesis and VEGF).
7) Completely developed bone.
99
True or false, the limb has polarity?
True.
| Proximal/distal and anterior/posterior.
100
What are the 3 limb segments?
1) Proximal stylopod (humerus/femur).
2) Zeugopod (radius, ulna/tibia, fibula).
3) Distal autopod (carpals/tarsals).
101
How does the limb develop?
From the limb bud - a mesenchymal condensation of the lateral paraxial mesoderm on the dorsolateral body wall.
Forms at 2 levels; forelimb C5-8, hindlimb L3-5.
Sheep/pigs/cats - limb bud development commences at end of 3rd week of gestation.
Humans, cattle, dogs - occurs in 4th week.
102
What are the limb inducing signals?
Lateral plate mesoderm destined to become limb skeleton.
Secretion of paracrine factor FGF10 initiates limb bud formation - interaction between mesoderm and overlying ectoderm.
Formation of the apical ectodermal ridge (AER).
103
Does the fore, or hindlimb develop fist?
Forelimb. 1-2 days before the hindlimb.
104
What is the AER?
Apical Ectodermal Ridge.
Keeps the underlying mesenchyme in a 'plastic' proliferative state to ensure limb outgrowth - the progress zone (PZ).
Maintains expression of molecules that specify the A/P (thumb-pinkie) axis.
Interacts with proteins that specify the D/V (knuckle-palm) axis of the distal limb.
If the AER is removed, limb development ceases.
If there is extra AER, limb is duplicated.
105
What is FGF and why is it important?
Fibroblast growth factors.
Involved in initiation and maintenance of the AER.
If it is disrupted limb outgrowth is affected.
Regulate mesechymal proliferation.
106
How are digits formed?
Increased mesenchymal density in distal extremity.
Tissue between rays is programmed to degenerate, if there is no mesenchymal cell death then a soft tissue web is left between digits.
Digits begin to form 4-10d after limb bud appears.
107
How do FGF's regulate mesenchymal proliferation?
N-Myc (oncogene, between digits) in limb bud acts downstream of AER derived FGF's in regulating the pool of undifferentiated mesenchyme.
Undifferentiated mesenchyme gives rise to precartilagenous condensations that are pre-specified to generate proximal (stylopod), medial (zeugopod) and distal (autopod) skeletal elements.
Loss of N-Myc results in reduced mesenchymal cells and reduced limb size.
108
What is the ZPA?
Zone of polarising activity.
Area where A/P axis specification occurs (very early).
Signal eminates from mesoderm cells located at posterior junction between limb bud and body wall.
Duplication of ZPA results in distal limb duplication.
109
What is the polarising signal expressed in the ZPA?
Shh
| Overexpression of Shh in the limb bud causes mirror duplication of the digits - Polydactyly (many digits).
110
How does distal limb modification occur?
The ancestral 5 digits are reduced in sequence; first the 1st, 5th, then 2nd are lost, then finally the 4th.
This loss is achieved by destruction or fusion of digital rays.
111
How are bones of the limb formed?
By endochondral ossification.
112
Describe the 5 steps of appendicular skeleton ossification.
1. Ossification commences in clavical (usually).
2. Midshaft ossifications - humerus, radius, ulna, femur, tibia, fibula.
3. Scapula
4. Metacarpals, followed shortly thereafter by the metatarsals and phalanges.
5. Pelvis.
113
Which bones are non-ossified at birth?
Carpals, most of the tarsals (except talus and calcaneus), and epiphyses of long bones.
114
What is dwarfism?
Achondroplasia.
Mutation in FGFR3 - initiates premature differentiation of epiphysial growth plates of the long bones.
Mutations in FGFR2 and FGFR1 also give rise to skeletal and craniofacial bone defects.
115
Discuss dwarfism in cattle.
Dexter cattle carry genetic defect causing a dwarf phenotype in the heterozygotes (Dx +/-).
Gene is BD1.
Homozygotes (Dx +/+) are stillborn with extreme shortening of limbs and gross craniofacial defects (bulldog calves).
Not caused by mutation in the FGFR3 gene.
116
What are some examples of dwarfism in the canine world?
Ateliotic pituitary dwarfism - very small dogs (Chihuahuas).
Micromelic achondroplasia - dogs with very short legs (corgi's, dachshunds).
Brachycephalic achondroplasia - dogs with very short noses (boxers, pitbulls).
117
What happens if high levels of growth hormone (GH) are produced prior to puberty?
Causes failure of growth plate closure - giantism.
118
What happens if there are high levels of GH after puberty?
Acromegaly - enlargement of the extremities.
119
What is the most common class of congenital defects?
Cardiovascular defects.
Frequently encountered in dogs and cattle.
Classified as either cyanotic (insufficient oxygen provided to peripheral vasculature) or acyanotic (suffiecient oxygen - but other issues).
120
What are some forms of acyanotic abnormalities?
1. Aortic stenosis - obstruction of the left ventricular outflow (frequent in large dog breeds).
2. Pulmonary stenosis - narrowing of the pulmonary outflow (most common canine cardiac defect).
3. Ventral septal defects.
4. Artial septal defects.
121
What are some forms of cyanotic abnormalities?
1. Large ventricular septal defects including a syndrome called Tetralogy of Fallot (multiple defects).
2. Transposition of the outflow vessels (reversal of pulmonary and systemic flows).
122
What are some common congenital malformations related to the cardiovascular system?
1. Patent ductus arteriosus.
2. Narrowing of the aorta.
3. Persistent right aortic arch.
4. Duplication of the cranial and caudal vena cava.
123
How does specification of the heart primordia occur?
Presumptive heart cells are born in the primitive streak, these form the posterior Hensen's node and extend caudally.
Future cardiogenic mesoderm migrates through the PS lateral to and at the level of Hensen's node.
This is initiated through the FGF and BMP pathways via adjacent endoderm.
124
How is the heart anlage (rudimentary organ) created?
Cardiac precursors migrate to the midline.
Cardiomyocytes migrate between the ectoderm and endoderm to the lateral walls of the gut tube.
The two cardiogenic cell populations conjoin at the level of the foregut endoderm.
125
What does failure of midline migration result in?
Cardia bifida.
| Mutation of Foxp4 results in development of 2 hearts.
126
Describe heart 'looping'.
Initial formation of the heart is as a two chambered tube (atrium and ventricle).
Heart looping converts the AP polarity of the heart tube to the L/R specified structure of the adult heart.
Stages;
1. endocardial cushion forms to separate left and right ventricles (atrioventricular channels).
2. Primitive atrium separated by 2 septa which grow towards the endocardial cushion.
127
What are the stages of development of the conduction systems?
Pacemaker cells - first formed in caudal part of left cardiac tube, subsequent centre is formed in the right horn of the sinus venosus (becomes incorporated into right atrium-sinoatrial node). Prior to formation of the separate chambers, entire cardiac tube contracts as a single unit but with differences in the rate of contraction dependent on location (arterial section fastest - truncus slowest).
128
Discuss development of the vascular system.
Two temporarily separate processes;
1) Vasculogenesis - generation of the blood vessel network.
2) Angiogenesis - extension and pruning.
Primitive streak stage; precursors of both blood vessels and cells arise from lateral plate mesoderm and condense into blood islands which contain blood porgenitors and angioblasts (vessels).
129
What are the two sites of primary vasculogenesis?
1) Extraembryonic - formation of blood islands in the yolk sac provide early embryo with primitive red blood cell type. Early vascular system and critical step in amniote development (birsd-vitelline veins, mammals-umbilical veins).
2) Intraembryonic - arises from angioblasts in mesoderm surrounding individual developing organs. They are not sites of erythropoiesis, the networks develop individually and are joined to the central nervous system later (angiogenesis).
130
What are the aortic arches?
Aortic arches are 6 looped vessels branching from the truncus arteriosus, they loop over the developing foregut to reach the dorsal aorta.
Regression of the arches gives the adult arterial system.
131
Discuss vascular growth and pruning.
ECM critical for this process.
VEGF causes loosening of cell-cell contacts and degredation of ECM in target areas.
Exposed endothelial cells are stimulated to proliferate resulting in the production of new vessels.
132
Is vasculogenesis critical for embryonic survival?
Yes
133
What are the key molecules involved in Angiogenesis?
VEGF - degradation of ECM.
TFG-beta - stabilises ECM post development
PDGF - recruits pericytes to the capillary wall.
Inhibitors of ECM degrading metalloproteinases; Collagen XVIII - stabilises capillaries and protects against Mpase degradation.
134
How are arteries and veins differentiated?
Expression of Ephs and Ephrins in vascular endothelial cells.
Arteries - ephrin-B2
Veins - EphB4 tyrosine kinase
135
Discuss the transition from fetal to neonatal circulation.
Fetal circulation; involves the ductus arteriosus diverting blood from the pulmonary artery to the aorta to the placenta. No blood returns from the pulmonary vein so the left ventricle needs to be filled via another route (foramen ovale-atrial septum). Foramen ovale and ductus arteriosus are both open.
Neonate circulation; pressure in lH side of heart increases dramatically when first breaths are taken, this causes closure of the foramen ovale, separating pulmonary and systemic circulation. The ductus arteriosus closes, separating the connection between the aorta and the pulmonary artery.
