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1

First embryological studies

Aristotle - function of the placenta
- Not until late 19th century epigenesis accepted

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Developmental Anatomy

studies of development of complex, muticellular organisms

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Two types of development

1. Ontogenetic = transformation of zygote into a new individual
2. Phylogenetic = transformation of a simple (group of cells) organism to complex organism (origin and evolution of species over time)

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Embryo

from time of fertilization to completion of organogenesis (*most critical stage)
in humans: 1st 8 weeks

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Organogenesis

establishment of major organ primordia

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Fetus

remainder of prenatal growth, differentiation and maturation of body systems

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3 phases of early development

1. Gametogenesis
2. Cleavage
3. Gastrulation

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Gametogenesis

differentiation and maturation of primordial germ cells into gametes (male and female reproductive cells - ovum and spermatozoa)

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Origin of germ cells

endoderm of yolk sac

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Oogenesis

= maturation of primordial germ cells into female gametes

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Oogonia

primordial germ cells within female embryonic gonad - continue mitotic divisions until birth and then stops

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some oogonia differentiate into...

primary oocytes
(many oogonia and primary oocytes undergo apoptosis)

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primary oocytes

enter meiosis I and remain in resting stage until sexual maturity (at birth - only resting present)

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Meiosis

reduction-division process --> haploid cells (loose 1/2 chromosome content)

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Meiosis I results in?

haploid secondary oocyte and polar body (degenerates)

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When does secondary oocyte enter meiosis II?

Ovulation

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When does oocyte complete meiosos II?

only if fertilization occurs

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Oogenic meisosis is different than?

spermatogenic meiosis b/c two gametes have special attributes

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in what species are primary oocytes released at ovulation?

Dog & Fox

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two types of ovulation

spontaneous or induced (cat, rabbit, camelid, alpaca)

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Spermatogenesis

= maturation of primordial germ cells into male gametes

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Spermatogonia continue mitosis and then enter resting phase until?

sexual maturity

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at sexual maturity spermatogonia differentiate into?

primary spermatocytes

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Meiosis I of spermatocytes can be completed when?

Any time

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Result of meiosis I of spermatocytes?

2 equal haploid secondary spermatocytes

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Secondary spermatocytes complete meiosis II and produce?

4 haploid spermatids

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spermatids --> spermatozoa

spermiogenesis (maturation) to become motile (loos cytoplasm and gain tail)

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Capacitation

physiological changes of spermatozoa in female genital tract necessary for fertilization

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Spermatogenesis is?

continuous process (all types of cells found)

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3 main points about oogenesis (difference from spermatogenesis)

1. only 1 daughter cell becomes ovum (polar bodies degenerate)
2. process begins before birth
3. one (maybe a few if multiparous) mature with each ovulation cycle

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3 main points about spermatogenesis (difference from oogenesis)

1. each daughter cell develops into a spermatozoon (4)
2. process begins at sexual maturity
3. millions produced at a time

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What is fertilization?

fusion of gametes to form a zygote

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Where does fertilization occur?

Ampulla of uterine tube

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3 steps in fertilization?

1. penetration of zona pellucida (glycoprotein layer)
2. fusion of gamete membranes
3. integration of genetic material (each bring haploid # to make diploid zygote)

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Results of fertilization:

- restoration of diploid chromosome #
- genetic sex determination (ova carry X and sperm carry X or Y)
- initiation of cleavage

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Sex determination

in birds, snakes and some insects- males are homogametic ZZ and females ZY
- in crocodiles and turtles - sex determined by incubation temperature of eggs (lower temperatures = males)

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what is cleavage?

mitotic division of zygote

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Morula

= spherical mass of cells (blastomeres) formed by rapid mitotic cell divisions

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Does the size of embryo change during cleavage?

NO - cell sizes decrease at # increases - still encased in zona pellucida

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Two main functions of zona pellucida after fertilization?

- prevent adherence in uterine tube
- prevent embryo rejection by mother

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Blastula

single layer of cells lining a central cavity called a blastocyst (cells begin to secrete fluid that displaces cells)

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Once cell becomes blastula what occurs?

zona pellucida degenerates and implantation follows

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Trophoblast

outer single-cell layer that forms the fetal membranes

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inner cell mass

(aka embryonic disc)
central core of cells that forms the embryo

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At blastocyst stage which 2 cell types have restricted developmental potential?

trophoblast cells and embryonic cells of inner cell mass

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Roles of trophoblast cells

#1 = form fetal membranes
OTHER:
- attachment, maternal recognition of pregnancy (INF production) & nourishment of embryo in beginning

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Gastrulation

= formation of germ layers

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Cells of inner mass delaminate to become?

hypoblast surrounding the trophoblast

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Hypoblast cells become?

extraembryonic endoderm (forms yolk sac)

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some inner cells also become?

epiblast (above hypoblast)

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Epiblast splits to form:

extra-embryonic epiblast (lining amniotic cavity) & embryonic epiblast - source of all 3 germ layers

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What is the source of all 3 germ layers?

embryonic epiblast

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Germinal layer: Ectoderm

epidermis and nervous system

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Mesoderm:

cardiovascular system, urinary and genital systems, blood cells, CT and muscle

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Endoderm

digestive and respiratory tracts, urinary bladder, tympanic cavity, parenchyma of thyroid and parathyroid glands

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4 fates of embryonic epiblast cells:

1. those on midline form the primitive streak
2. some displace hypoblast to form endoderm
3. some migrate between epiblast and endoderm to form mesoderm
4. the rest become ectoderm

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Roles of the primitive streak

- temporary embryonic structure
- establish longitudinal (cranial-caudal) axis of embryo
- master organizer of mesoderm as segregation of epiblast cells occurs according to area of streak they migrate to

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epiblast cells through the primitive node form

notocord

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epiblast cell caudal to the node

paraxial mesoderm

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epiblast cells through midstreak

intermediate mesoderm

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epiblast cells caudal to streak =

lateral plate mesoderm

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Neurulation

= establishment of the nervous system

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Notochord induces the formation of what structure?

Neural Plate

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Folds of the neural plate fuse to form ?

neural tube

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Roles of notochord

1. inductor of CNS
2. support structure for developing embryo
3. involved in formation of vertebral column (persists in adults as nucleus pulposus - intervertebral disk)

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Neuropores

temporary communication of neural tube with amniotic cavity

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What does the neural tube differentiate into in fetus?

brain vesicles and spinal cord

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Where do neural crest cells come from?

break free from crest of neural plate

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Derivatives of pluripotent neural crest cells

1. sensory and autonomic ganglia
2. adrenal medulla
3. melanocytes
4. CT structures of head and face

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Paraxial Mesoderm becomes what structure?

Somites - segmented and formed successively (unidirectionally) - number of pairs varies by species - used to estimate age of embryo

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Somites =

foundation of metameric (segmental) organization of body

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Intermediate mesodern becomes

urogenital structures

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Lateral plate mesoderm divides into which 2 layers?

Dorsal and ventral layer

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Dorsal layer of the lateral plate mesoderm

associates with ectoderm to form parietal pleura/peritoneum

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ventral layer of the lateral plate of mesoderm

associates with endoderm to form visceral pleura/peritoneum

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What is the space between the dorsal and ventral layer of the lateral plate mesoderm called?

intraembryonic body cavity = coelom

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3 body folds in establishment of cylindrical body form

1. head fold: brain extends into amniotic cavity and folds ventrally
2. tail fold proceeds caudally
3. lateral folding: proceeds bidirectionally - (where head and tail fold come together

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Result of the lateral folding

partial incorporation of coelom and yolk sac into body
Tube - within - a- Tube:
outer tube = body wall, inner tube = primitive gut and space = body cavity

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How does the fetus get nutrients until establishment with placenta and maternal connections?

middle part of tube remains temporarily connected to yolk sac in fetal stage until yolk sac replaced by umbilical cord

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Placodes:

ectodermal thickenings in head region - form sensory organs (ectoderm that maintains connections with environment - ex. eyes, ears etc.

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Pharyngeal arches

aggregations of mesoderm in neck region important in forming face

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Limb buds

ectodermal outpocketings on ventrolateral body wall - distal end becomes digits

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First two organs to have prominences

heart and liver - cardiovascular system is first to become functional - deliver O2 to other developing areas

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Growth =

most outstanding event in early development - differential growth can account for diversity in form

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growth is controlled and made possible by number of factors:

- constitutional (size by species - individuals grow at different rate but final size)
- nutritional
- hormones (thyroid/pituitary)
- vitamins
- growth -arresting factors with age

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Differentiation =

development of specialized cell types
- antagonistic to growth (factors promoting differentiation interfere with cell division)
- is determined by cells genome and position in embryo
- determined by # of interactions and effective communication b/w cells

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Induction =

group of cells directs differentiation of other cells (Ex. notochord induces formation of neural tube) - essential part of differentiation

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2 stages in cell differentiation:

1. uncommitted = totipotent - up until 8- cell morula (can become anything)
2. committed = pluripotent - cell with restricted developmental potential (Ex. inner cell mass vs trophoblasts

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2 Phases of commitment:

1. specification = cell can differentiate independently (not influence by other cells)
--> transitional commitment (reversible)
2. determination = stage of irreversible commitment
--> conditional commitment = regulative development (faith of cells by body conditions and environment of cell

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Morphogenesis

= a mass of cells becomes a structure

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morphogenic field =

subset of cells forming an organ (each field controlled by tissue master genes)

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Cell migration:

controlled and influenced by interaction with their environment
Ex. migration of embryonic germ cells to form gametes in embryonic gonads - only ones who make it to gonads will continue mitosis

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cell aggregation:

cells with matching tissue - receptors adhere to form compact masses

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cell splitting

(identical twins)
cells delaminate and re-aggregate to form new layers (ex. inner cell mass --> hypoblast)

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cell fusion

growing together of 2 localized areas Ex. formation of tubular structures, lateral body folds & neural plate to form neural tube

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cell death (apoptosis)

- essential for shaping complex organs - ex. digits - mesenchyme b/w digits degenerates & tail & male mammary tissue

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Folding

significant event in establishment of body form - outward (evagination/outpocketing),
- inward (invagination/inpocketing)
- linear (groove)

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Fetal membranes: Chorion

= surrounds the entire conceptus

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Amnion

fluid-filled cavity around embryo

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Allantois

forms placental vessels

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yolk sac

continuous with midgut

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Teratology

study of abnormal development

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Teratogenesis

study of abnormal mechanisms of development

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Teratogen

any agent that induces defects during development (radiation, infectious agents, drugs, environmental chemicals etc.)

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Congenital abnormality

aka anomaly, malformation, maldevelopment birth defect etc.
= any structural, functional, metabolic and behavioral disorder present at birth

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disruptions

= alteration of formed structures from destructive processes (ex. blood supply)

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deformations

= due to mechanical force
ex. twins in horses

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2 categories of what cause congenital abnormalities

genetic or environmental

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most congenital abnormalities present when?

at birth --- or become apparent shortly after or later as organism develops

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2 categories of congenital abnormalities

structural or functional (ex. absence of organ vs absence/defective enzymes)

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4 manifestations of congenital anomalies

different degrees of severity (mild to lethal)
-death
- malformation
- growth retardation
- functional defect

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Congenital abnormalities can affect any body structure (internal or external) but what common?

Highest: musculoskeletal
lowest: respiratory

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VACTERL association

Ex. with tracheoesophageal fistula - birth defects that are commonly in association with each other:
vertebral, anal, cardiac, tracheal, esophageal, renal and limb defects

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Viral teratogens can be ...-specific?

species or breed specific

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Syndrome

= multiple defects that have specific common name

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Most critical determining factor for effects of congenital abnormality?

in what stage of development it occurs

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Birth defects in the stage of pre-differentiation (1)

- before organogenesis
- "all or none rule"
--> either lethal (cytotoxic effect on embryonic cells) or none (cells unresponsive)

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Birth defects in the stage of differentiation (2)

stage of MAXIMUM susceptibility for STRUCTURAL abnormalities with effects relying on which organ

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Rule of Four

by 4 weeks of development, embryo should have a 4 chambered heart and 4 limb buds

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Birth defects in the stage of advanced morphogenesis (3)

- susceptibility declines as oganogenesis advances - mostly FUNCTIONAL (often hereditary)
- some structures like palate have more than one critical stage

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General rule for stages of development and birth defects

the earlier the insult, the more severe the consequences
Ex. notochord --> CNS

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Susceptibility to teratogens affected by:

1. embryonic genotype
2. maternal conditions (genotype, nutrition, toxins etc.)

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Abnormal genes

mutations

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3 types of mutations and effects:

- substitution, deletion or insertion
- lethal (completely non-functional product necessary for life), silent (does not change final product), gain (product better)

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Genetic mutations increase with:

inbreeding

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inbreeding negative effects:

- reduce genetic variation
- increase transmission of recessive deleterious genes

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Abnormal chromosomes and birth defects:

- sex & somatic chromosomes
- abnormal # or structure
- rare in domestic animals
- most due to non-disjunction of chromosomes (one cell gets extra and one gets less)
- in most cases, embryo spontaneously aborted
- phenotyping alterations usually expressed as syndrome

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Klinefelter syndrome caused by:

trisomy of sex chromosomes - XXY karyotype
- most common sex chromosome abnormality

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clinical signs of Klinefelters

seen only in males: detected at puberty because phenotypic males with gonadal dysgenesis and infertility

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example of animal with klinefelters

tortise shell (calico) male cat - calico hair pattern sex linked to females

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Turner's syndrome cause:

monsomy of sex chromosome in female (XO) karyotype (YO karyotype is lethal)

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Clinical signs of turners syndrome:

phenotypic female with gonadal dysgenesis and infertility

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turners most common in what species:

pig, horse and cat

134

Genetic testing to prevent undesirable traits

karyotyping, genotyping and pedigree analysis

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Environmental factor: Ionizing radiation

potent teratogen in all species - xrays cause free radicals which break chromosomes & response is dose dependent - avoid exposure if pregnant

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Chemical factors

therepeutic drugs: potential teratogens and carcinogens
effects depend on drug, dosage and chemical structure - avoid use of cytotoxic drugs in pregnant

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Biological factor: infectious agent

some diseases can cross placenta (rubella) and some cannot - same with zona pellucida
- produce mutliple defects especially early in development - caution with live vaccines although most use non-virulent strain

138

plants:

contain toxic and teratogenic compounds - most commonly affects structure = appendicular skeleton (affects cell migration)

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developmental failure

= agenesis (failure to develop) or aplasia (failure to grow)
cause: absence of primordial tissue or abnormal inductive tissue interactions
Ex. acardia, amelia (limbs) and kidney agenesis

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Developmental excess

Hyperplasia: excess in size
in #: supranumerary (toes)

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incomplete development

= hypoplasia- reduced cell proliferation

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failure to fuse

cleft palate, septal heart defects and spinal bifida

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hyperplasia vs hypertrophy

hyperplasia = increase size by #
hypertrophy = increase size by mass of cells

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failure of migration

at cell level (crest cells) or organ level (ectopia cordis - heart in neck & cryptorchidism)

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persistent embryonic strucutres

= lack of apoptosis and shaping of structures

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Atresia

congenital absence or closure of a natural body orifice or lack of patency or tubular structure (gut atresia)

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duplication

conjoined/parasitic twins

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organ malpositioning

- dextrocarida = heart on right
= situs inversus = mirror image

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agenesis

failure of development

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allantois

extension of hindgut into extraembryonic coelom that forms vessels of umbilical cord

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amelia

congenital absence of limbs

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amnion

= extraembryonic membrane surrounding embryo and containing amniotic fluid

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anencephaly

= congenital absence or reduction in size of cerebral hemispheres

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angioblast

precursor cell of capillary endothelium

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aplasia

incomplete or defective development of tissue or organ

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atresia

congenital absence or closure of normal body opening

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blastocyst

large, fluid filled cavity of blastula formed by cleavage of zygote

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blastomeres

cells produced by cleavage of zygote

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blastula

multicellular embryo formed as result of cleavage

160

blood islands

masses of splanchnic mesoderm that form first red blood cells and vessels

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Brachygnathia

abnormal shortness in jaw

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chimera

organism composed cells derived from genetically different zygotes

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chorion

fetal portion of placenta

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cleavage

cell division in zygote to form multicellular embryo

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cloaca

dilated caudal end of embryonic hindgut in mammals

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coelom

body cavity lines by mesoderm

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conceptus

developing organism with its extraembryonic membranes (from fertilization to birth

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cryptorchidism

failure of one or both testes to descend to scrotum

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congenital defect

abnormalities of structure or function present at birth

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cyclopia

complete or partial fusion of orbits of eye

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delamination

process by which cell layer splits into two seperate layers

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dysgenesis

defective development or malformation

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dysplasia

developmental alteration in shape or size of tissues/cells

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dizygotic twins

develop from individually fertilized ova

175

ectoderm

outermost germ layer of embryo

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ectopia

abnormal location of organ or tissue

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embryo

stage in developing organism after cleavage but before birth or hatching

178

endoderm

innermost germ cell layer of embryo that develops during gastrulation

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epiblast

outer layer of embryo that gives rise to all 3 germ layers

180

epispadias

congenital abnormal urethral opening (on dorsal side penis)

181

fertilization

formation of single celled zygote through the union of male and female gametes

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fistula

abnormal passageway

183

freemartin

a mammalian intersex sterile animal

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gamete

mature haploid germ cell from male or female

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gametogenesis

formation of male and female gametes - ova and spermatozoa from germ cells

186

gastrulation

early embryonic stage in which cell migration forms distinct primary germ layers

187

germ cell

a cell found within gonads giving rise to gametes

188

germinal layers

3 major embryonic layers (ectoderm, mesoderm and endoderm) - each layer is precursos to different tissues and organs

189

gubernaculum

cord of mesenchyme between testis and scrotum - assits in descent of testes

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haploid

cell with single set of unpaired chromosomes

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hydranencephaly

absence of cerebral hemispheres

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hydrocephalus

abnormal accumulation of CSF within cerebral ventricular system

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hyperplasia

increase in cell numbers

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hypertrophy

increase in size of a part of organ

195

hypoblast

inner embryonic layer that will become endoderm of extraembryonic membranes

196

hypoplasia

incomplete development or underdevelopment of organ or tissue

197

induction

developmental process in which the fate of a group of cell is determined by interaction with another group of cells

198

ingression

migration of cells from the surface of an embryo to the interior

199

inner cell mass

cluster of cells segregated from blastocyst from which entire embryo develops

200

karyotype

chromosomal makeup of individual

201

kyphosis

abnormal dorsal curvature of vertebral column in thoracic region

202

lordosis

abnormal ventral curvature of the vertebral column in thoracolumbar region

203

lumen

cavity or passageway within tubular structure

204

meiosis

two consecutive cell divisions whereby the sult daughter cells are haploid

205

meningocele

herniation of meninges through opening in skull of vertebrae

206

monozygotic twins

twins formed by split single ovum

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meromelia

congenital absence of part of limb

208

mesenchyme

middle germ layer formed in embryo during gastrulation

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mesoderm

middle germ layer formed in embryo during gastrulation

210

mitosis

nuclear cell division producing two cells with identical chromosome complement as parent cell

211

morphogenesis

development of form of organ of part of body

212

morphogens

signaling molecules that elicit different cellular responses

213

morula

early stage of blastular in which the solid mass of blastomeres resembles mulberry

214

myotome

portion of somite giving rise to striated muscle

215

neural crest

grop of cells migrating from neural plate to form ganglia, pigment cells etc.

216

neural plate

a median dorsal thickening of ectoderm from which the neural tube will form

217

neurpores

rostral and caudal openings of neural tube that close at later stage in development

218

neurulation

process of formation of meural plate and then neural tube

219

notochord

rod like mesodermal structure defining the longitudinal axis in developing embryo

220

oocyte

female germ cell that undergoes two meiotic divisions to become mature ovum

221

oogenesis

process of maturation of oocyte into an ovum

222

oogonium

primordium of oocyte in ovary

223

organogenesis

formation of organs in developing embryo

224

parthenogenesis

development without fertilization

225

phenotype

appearance of animal as determined by genotype

226

polyploidy

presence of more than 2 haploid sets of chomrosomes in cells

227

polyspermy

event in which two or more spermatozoa fertilize ovum

228

primitive streak

longitudinal cleft on dorsum of embryo that guides the migration of epiblast cells

229

pronucleus

haploid - reduced- nucleus of gamete

230

rachischisis

congenital fissure of vertebral column

231

somatopleure

membrane composed of ectoderm and somatic mesoderm (froms part of body wal)

232

somites

segmented mesodermal blocks on either side of developing spinal cord

233

spermatid

haploid cell in male testes that will become spermatozoon

234

spermatogenesis

process by which mature spermatozoa are formed

235

spermatozoon

mature haploid male gamete

236

stem cell

undifferentiated cell that is mitotically active in either replacing dying cells or producing gametes

237

syndactyly

incomplete seperation of digits

238

syndrome

group of abnormalities occurring together

239

teratogen

agent which can cause permanent structural or functional alterations of embryo or fetus

240

teratology

science that studies origin, causes and prevention of birth defects

241

teratoma

tumor containing derivatives from all three germ layers

242

trophoblast

outer layer of conceptus that develops into fetal portion of placenta

243

urachus

vetigial remnant of allntois

244

vestigial

any strucutre or organ that is reduced in size or function when compared to embryonic structure

245

vitelline

referring to yolk of egg or ovum

246

vitelline duct

connection between yolk sac and midgut

247

yolk

protein and fat stores found in cytoplasm of ovum

248

yolk sac

extraembryonic membrane enclosing and absorbing yolk of amniotic embryos

249

zygote

fertilized ovum before the first cleavage division