Embryology Flashcards
What is embryogenesis driven by?
Evolutionary, genetic, epigenetic and environmental factors.
There are structural similarities between all species (same number of pharyngeal arches), these drivers allow for species differences.
Why are the processes of embryogenesis important for our understanding?
allows us to better understand disease, regeneration, and repair during the post natal period (juvenille to adult).
Where do embryo’s start from?
Fertilised zygote.
What are the two pronuclei?
Genetic material, one of maternal, and one of paternal origin, within the fertilised zygote.
What can errors in embryogenesis result in?
Embryonic loss Fetal death Fetal mummification Abortion Stillbirth Birth of nonviable neonates Birth of viable offspring with defects.
What factors influence embryogenesis?
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.
What biological molecules can affect growth and development in the adult and embryo?
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).
Describe, specifically, the effect of Tetratogens.
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.
What is the most critical period of sensitivity to factors affecting development?
Embryonic development - weeks 3-8.
Systems that take the longest to develop, or are the most complex, generally exhibit the most severe effects.
What are some physical causes of embryonic/foetal abnormalities?
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).
Describe the process of fertilisation.
- 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.
What is the acrosome reaction?
Sperm releases proteolytic enzymes that digest a hole in the zona pelucida for the sperm to travel through.
What is ‘activation’ of the fertilised egg?
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
What are the common causes of infertility in all species?
Failure of fertilisation/implantation
Early embryonic loss
What is ectopic pregnancy?
Implantation in an inappropriate position (uterine/fallopian tube, ovary, peritoneal cavity).
Much more common in humans than domestic species.
What is the normal rate of embryonic loss?
Normal process that ranges between 25-50% in domestic species.
Caused by multiple factors.
What is late embryonic loss?
Mummification
Abortion
Many causes including viral, nutritional, toxicosis.
What does placental insufficiency/failure result in?
Poor in utero foetal growth
Abortion
Embryonic loss
What are the stages from fertilisation to implantation?
Fertilisation (ampulla region) - zona pellucida (first cleavage) - 2-cell stage - morula (fallopian tube) - blastocyst (uterus) - early stage of implantation.
What is cleavage?
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.
Describe cleavage in mammals compared to other animals?
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.
What are blastomeres?
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!
What is a morula?
Solid ball of cells.
16-64 blastomeres.
Formed near the end of cleavage and surrounded by ZP.
Compare mammalian and non mammalian ovulation/fertilisation/early embryogenesis.
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.
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.
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.
What is the purpose of foetal membranes/extraembryonic membranes?
Provide protection, nutritional and excretory requirements for the developing embryo.
What are the 4 major extraembryonic membranes?
Yolk sac
Amnion
Chorion
Allantois
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).
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.
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.
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.
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.
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.
What is placentation?
Structural organisation and mode of attachment of the placenta.
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.
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).
How are placentae classified?
- Choriovitelline placenta - first placenta; fusion of chorion and yolk sac, vitalline vessels formed, transitory in most mammals. Preceeds development of chorioallantoic placenta.
- Chorioallantoic placenta - definitive placenta; comprises chorion and allantois, allantoic vessels form and become umbilical vessels, definitive (final and functional until birth).
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.
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.
What is a normal cause of dystocia?
Failure of normal twin presentation.
What can dystocia result in?
Fetal mortality
Maternal mortality
Both
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.
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.
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.
What is the primitive streak?
First and most obvious structure to form in the early embryo.
What is the AVE?
Anterior visceral endoderm
What is one of the first patterning events?
Creation of the head from the tail. Occurs during the gastrulation/primitive streak stage.
What prevents the head cells consuming the whole body?
Node
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.
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.
How are the genes expressing the posterior mesoderm formation activated?
Epiblast expresses the posteriorising nodal protein.
What does the AVE express?
Dorsal factors Oxt2
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.
