Animal growth and development / Developmental biology

Examples of consequences of errors in normal embryo/foetus

• embryonic loss > cannot be noticed or seen, no major signs, indication may be longer oestrus

• fetal death > any stage of pregnancy

• fetal mummification > dies inside the uterus, may be osmotic pressure, foetus loses liquid & becomes “liquid”

• abortion > at any stage of pregnancy

• stillbirth

• birth of nonviable neonates

• birth of viable offsrping with defects

Embryogensis is a?

Multifactorial process

Define congenital

A developmental disruption results in a deviation from normal that is present or apparent at birth. Genetic, environmental (nutritional), physical and infectious agents have all been defined as etiologic, determinants. Can be multi-factorial

Define Carcinogen

Agents or factors that initiate or induce neoplasia = excessive and abnormal growth of tissue: neoplasm / carcinogensis = initiates cancer

Define Mutagen

Agents or factors that produce a change in the genetic code of an organism

Define Tetragens

Agents or factors that cause the dev of physical defects in the embyro or foetus, has windows of susceptibility:

early embyrogenesis > primarily cause effects on DNA - mutations at a genomic or chrmosomal level

mid embryogensis / early foetal > effects on cell proliferation, differentiation or cell death / affects cell growth

late foetal > most tissues relatively protected, only highly proliferating tissues still susceptible (e.g. palate, eye, cerebellum)

What can affect the critical periods of sensitivty to external / internal factors affecting development

factors include: teratogens, envi, genetic, mechanical.

the longer the time of dev the higher the chance of being affected

Common congentical defects of domesticated animals, not a question

Physical causes of embryonic abnormalities?

• physical trauma during gestation / on mother

• congenital joint contracture by in-utero crowding > when animal gives birth to many offspring, crowding in uterus

• spinal & limb deformities in foals following transverese or caudal presentaton > position of the foetus in utero

• aggressive palpation cause either limb deformities or disruption of vascular supply to intestinal tract = no faecal matter exiting, atresia coli

Describe Fertilisation

The process whereby a spermatozoon and a ovum fuse to form a single celled zygote in uterine tube (exact location dependent on species)

The 2 accessory follicle cells surrounding the egg once it leaves the ovary

• Corona radiata - form barrier between sperm and egg

• Zona pellucida - “clear area”, lies between the CR & egg

How does the sperm fuse its genetic material with the egg & what are the consequences if there isn’t enough sperm?

• sperm must transit through the thick outer coating of the egg (ZP)

  • to acheive this each sperm releases enzymes from its acrosome

  • enzymes weaken both CR & ZP, allowing 1 sperm to wiggle through to egg

  • if there aren’t enough sperm, not enough enzyme is released & none of the sperm will reach the egg

  • whent he first sperm finally contatcs the egg’s surface, the plasma membrane of egg & sperm fuse, & sperm’s head is drawn into the egg cytoplasm> egg has vesicles that secrete a chemical to fuse the sperm to it

Detail fertilisation

once sperm enters, triggers 2 critical changes:

1 > vesicles near the surface of the egg release chemicals into the ZP that reinforce it & prevent additonal sperm from entering

2 > the egg has been in a dormant state until fertilisation ( no cell division or replication of genetic material has occured since the germ cell was laid down in the embryo)

• in order for embryogeneis to proceed the fertilised ovum must be “activated” > calcium waves

• occurs after fusion of sperm head to cell membrane of the ova

What do calcium waves result in?

• reactivation of the genetic material of the ova

• resumption of profilferation

• release of inhibition of the material genome

• exocytosis of M & F pronuclei (genetic material that gives rise to future organism)

Sumarise the 5 steps of fertilisation

1.      Sperm is attracted to the egg by secretion of soluble molecules from the egg itself

2.      Exocytosis occurs from the sperm acrosomal vesicle to release degrading enzymes

3.      Sperm binds to extracellular matrix  = ZP in mammals

4.      Sperm passages through the extracellular matrix

5.      Cell membranes of egg & sperm fuse – fertilisation & a zygote is produced

Embryos start from?

Fertilised zygote

What are pronuclei?

genetic material of the fertilised zygote, one of maternal & one of paternal origin > in humans each contain 23 chromosones that give rise to 46 total

What is a structural similarity between species and what drives them to be different?

> in early embryogenesis, all embryos have the same number of pharyngeal arches

> evolution, environment & genetics drive species specific differences in development during gestation & beyond

What are the events that occur from fertilisation to implantation?

Summary of stages involved in embryonic cleavage & implantation 1

Summary of stages involved in embryonic cleavage & implantation 2

What is cleavage?

• a series of rapid (mitotic) division following fertilisation

  • cell size progressively diminishes from that of the zygote

  • absence of cell growth phase between each division

How does cleavage in mammals compare to other animals?

• slower > 12-24 hrs between divisions

• asynchronous > all blastomeres don’t divide at the same time

• produces compact ball cells encircled by ZP

  • outermost > extraembryonic tissue only

  • central > foetus & extraembryonic tissue

  • = morula

What are the cells resulting from the cleavage process

Blastomeres > each individual cell is called a blastomere

How and where does cleavage occur?

• occurs within isthmus

• contractions propel embyro forward

• produces compact ball cells encircled by ZP

  • outermost > outerembryonic tissue only

  • central > foetus & extraembryonic tissue

• 4-5 days in most domestics to uterus

• secretions from epithelial lining

  • provide nutrients

  • specific proteins contribute to development

What is a morula?

• solid ball of cells

• 16-64 blastomeres

• formed near the end of cleavage processes

• surroounded by the ZP

What needs to happen for implantation to occur?

The blastocyst (comes after the blastula) must shed the ZP > hatching

What happens during implantation?

• commences once blastocyst is free of ZP - delayed in horse

• polytocous species (multiple) blastocyts evenly distributed

• some monotocous species have preferred implantation site e.g. horse base of uterine horn

• uterine endothelium catches the blastocyst on a sticky extracellular matrix

• blastocyst secretes proteases (enzymes that break down protein) > embed

Timing from ovulation to implantation in different species

• Sheep, cat > day 14-18

• Pig > day 12-16

• Dog > day 14-18

• Cattle > day 17-35

• Horse day > 17-56

• Human day > 6-7

• partly due to different types of implantation & their mechanisms for embedding of the embryo into or on the uterine lining

What are the different types of implantation?

• classified according to relationship between blastocyst & uterine lumen

• central

  • blastocyst remains within uterine wall

  • ungulates, carnivores & lower primates

  • wide variation in timing

• eccentric

  • blastocyst lies within uterine crypt or recess

  • mouse rat, hamster, rabbit and some bats

  • quick process

• interstitial

  • conceptus invades the uterine wall

  • guinea-pig, chimpanzee & man

  • quickest process

Explain Interstitial implantation

What are the foetal membranes, extraembryonic membranes and what is their purpose?

• provide protection & nutritional & excretory requirements for the dev embryo

• four major membranes

  • yolk sac

  • amnion

  • chorion

  • allantois

Yolk sac

• first membrane formed

• involved in early haematopoiesis > formation of blood cellular components & angiogenesis > formation of new blood vessels

• vitelline blood vessels - gut vessels

  • cranial mesenteric artery

  • hepatic portal vein > blood vessel that carries blood from the gastrointestinal tract, gallbladder, pancreas and spleen to the liver

Amnion

• surrounds foetus

• covers umbilical cord & continuous with body surface at umbilicus

• fluid filled

• filtration from superficial blood vessels of embryo

• secretions from alimentary & respiratory tracts

• urine from kidneys > from urethra until bladder sphincter is patent

• allantoic fluid which may be transported across the allantoamnion > fused membrane

Amniotic fluid

• cushions the foetus

• allow unrestricted mvt

  • important in later stages of dev

  • prevents pressure related growth abnormalities

• at birth when ruptures acts as lubricant for the birth canal

Amnion at birth

• horse, dog & cat raphe (> the line of fusion of the amnionic folds over the embryo in reptiles, birds, and certain mammals) 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

Chorion

• established at the same time as amnion

• chorionic sac surrounds other embryonic membranes & foetus

• composed of

  • trophoblast > outer layer

  • extraembryonic mesoderm > inner layer

• lies in apposition to uterine lining

  • participates in formation of foetal component of placenta

Allantois

• dev as diverticulum of the hindgut

• varies considerably in size between species

• humans / primates : residual structure

• pigs : v large

• reptiles & birds : v large waste sac

• stores urinary waste

• in birds also mediates gas exchange & calcium transport from shell to embryo by fusion with the chorion (called the chorioallantoic membrane)

Placenta

• placentation is the structural organisation & mode of attachement of the placenta

• organ of metabolic interchange between mother & foetus

• large surface area

  • nutritional

  • excretory

  • immunological

  • endocrine

What are the components that undergo modification for the placenta

• maternal

  • endometrial lining of uterus

• foetal

  • chorion

Provision of the embryos nutritional requirements

• number of sources provide the embryo/foetus with nutrition

  • haemotropic source

    • maternal blood stream > primary source

  • histotropic source

    • coiled endometrial galnds roduce histotrophe > uterine milk > fat & glycogen

The different classifications of placenta

• based on gross appearance of the definitive placental zone

• diffuse

  • placental zone covers almost entire surface of chorionic sac

  • horse & pig

• cotyledonary

  • placental zone restricted to specialised cotyledons

  • cotyledons develop in respponse to chorionic contact > caruncles

  • caruncles permanent arranged in rows

    • 70-140 in cows

    • 88-110 in sheep

    • 160-180 in goats

  • cotyledon & caruncle > placentome

• zonary

  • placental zone in band around central region

  • complete in dogs & cats incomplete bears & mustelids (ferrets, skunk, weasels)

  • trophoblast modified & invades endometrium

• discoid

  • placental area 1 or 2 disc shaped area

  • man, rodents 1 disc

  • monkeys 2 discs

What happens to the placenta at birth and how are the diff placentas classified

• loss of maternal tissue at parturition

• deciduate

  • invasion & destruction of maternal tissue resulats in shedding of maternal tissue

  • decidua > modified mucosal lining of the uterus (that is, modified endometrium) that forms every month, in preparation for pregnancy, forms the maternal part of the placenta and remains for the duration of the pregnancy. After birth the decidua is shed together with the placenta

  • maternal haemorrhage may occur > carnivores, primates & rodents

• non-deciduate

  • virutally no loss of maternal tissue at partiturition

  • ruminants, horses, pigs

Briefly describe the formation of the Inncer cell mass (ICM)

• cellular division creates morula

• cavitation occurs due to sodium being pumped actively into central area

• osmotic pressure (osmotic pressure imbalance) causes the cavity within the blastocyst to expand and form blastocoel > migration

What is the blastocoel?

the fluid filled cavity, or space, in the developmental stage known as the blastocyst

List the steps of animal embryonic dev

zygote > morula > blastocyst > gastrula > embryo

fertilisation > cleavage > blastulation > gastrulation > organogenesis

Embryonic disc formation

• further fluid & cellular migration creates disc from the ICM which is a flattened pear shape

• disc initally bilaminar but becomes trilaminar > GASTRULATION

What makes up the bilaminar disc

• epiblast

  • superficial layer > embryonic body

• hypoblast

  • deep cell layer form extraembryonic membranes

  • cells migrate - delaminate - from second layer under trophoblast - blastoceol now covered by two layers > the yolk sac

What is gastrulation

• also called germ layer formation is the stage of embryological dev where disc becomes trilaminar

• due to cellular migrations from surface to interior

• mvt creates depression known as primitive streak

• layers of the trilaminar disc gives rise to specific organ systems

What causes the primitive streak to form?

devs in the epiblast cell layer bc of the weight of the cellular migration

What is the node at the top of the primitve streak called

• Hensen’s node

not a question, need to know this

Detail the triminar layers & what they form in the adult tissues

What is patterning

the mechanism by which initially equivalent cells in a developing tissue in an embryo assume complex forms and functions

What is the first patterning event

• the creation of a “head” from a “tail” > polarisation

• anterior > cranial to posterior > caudal regions

What are the two major signalling centres that pattern the embryo and what are their functions during the gastrulation/primitive streak stage?

• anterior visceral endoderm AVE > head organiser

• node > rest of body organiser

What covers the dorsal surface during the gastrulation/primitive streak stage?

• the embryonic ectoderm > in contact with the amniotic cavity

How is the anterior posterior formation formed in avian species?

• gravity

• rotations in the shell results in the light components of the yolk pushing up one side of the blastoderm

• the higher side becomes the posterior region of the embryo

List the steps in establishing asymmetry in the early chick embryo - making a left and right

1. to the left hand side of the Hensen’s node, sonic hedgehog protein (encoded by the Shh gene) activates cerberus which causes expression of nodal

2. nodal in turn activates the expression of Pitx2 > gene that confers “leftness” to that side of the body (also expressed in the head region but plays a different role)

Left right asymmetry in the dev mammal, not a question

Why is patterning so important?

• assymetry in the morulae contributes to normal dev of the blastocoel cavity

• loss of assymetry in the early embryo can (cleavage, morula, early blastocyst) give rise to twins (also implantation of more than one embryo)

What are the two types of twins?

• dizygotic > twins arise from 2 ova from 2 ovarion follicles fertilised by separate spermatazoa during a single breeding cycle (fraternal twins / littermates)

• monozygotic > arise at the primitive streak stage, observed in humans, sheep & pigs

What is the rate of twinning

• cattle > dizygotic : 2-3% & monozygotic : 0.1%

• sheep > dizygotic 2-5% (lowland > highland)

• horses > multiple ovulation <30%, twins <2% → due to innate physiological mechanisms inhibiting twin implantation in mare

• monozygotic incidence <1%

Timing of separation of monozygotic twins, not a question

What is the definition of a stem cell?

• a cell that can produce identical copies of itself (self-renewal) indefinitely (true stem cell vs progenitor cell)

What do stem cells do in an embryo

• building blocks of the embryo and all its tissues

What is potency and how are the potency and the commitment of a stem cell linked

• potency is the ability of a stem cell to become any type of cell

• potency reduces once a stem cell becomes commited

• once it has committed it has a reduced ability to come back to a pluripotent stem cell

What are the earliest visible structures in the embryo

• somites > condensed mesodern cells

What is the trunk of the embryo at neural tube stage comprised of?

• 4 types of specific mesodermal cells

  • chordamesoderm (notochord)

  • paraxial (somitic) mesoderm (connective tissue, bone, muscle, cartilage) > make up the somites

  • intermediate mesoderm (urogenital system & adrenals)

  • lateral plate mesoderm (heart, blood vessels & blood cells, body cavities, non-muscular components of the limbs)

• mnemonic > cabbages partake in lesbianism

Where do somites form as the neural folds rise into apposition?

• pairs of somites form on either side of the neural tube

How are these somites derived / how do they start out?

• condensation of the paraxial mesoderm

What are the 4 cell types of mature somites

• sclerotome > vertebrae & rib cartilage

• myotome > musculature of back ribs & limbs

• dermatome > dermis of the back

• syndetome > tendons & blood vessels

• mnemotic => sexting my dermatologist sexy

What does “epithelialisation of somites” refer to?

• after separation, expression of fibronectin & N-cadhrein organise “inner” mesenchymal” & “epithelial” components

What are the steps in the generation of muscle tissues?

A. myotome cells are specified to become myoblasts by induction of MyoD (protein regulator of muscle differentiation)

B. myoblast numbers are expanded under the influence of FGFs > fibroblast growth factors

C. cell adhesion molecules control muscle cell alignement

D. myoblasts begin to fuse together to create myotubes, this occurs as myoblasts exit the cell cycle due to depletion of fibroblast growth factors (FGFs)

E. F. Myotubes complete fusion & coordinated contraction can be initiated

• myotome cells become the myoblast cells > when there are enough myoblast cells they line up and become the muscle cells

What does sclerotome need for cartilage differentiation?

• expression of Pax1 gene > required for cartilage differentiation

List the lineages for osteogenesis and the major modes of bone formation

• 3 distinct lineages generate the skeleton

  • somites > axial (vertebral skeleton)

  • lateral plate mesoderm > limb skeleton

  • cranial neural crest > craniofacial bones & cartilage

• 2 major modes of bone formation

  • direct conversion of mesenchyme to bone > intramembronous ossification

  • indirect conversion via cartilage to bone > endochondral ossification

Explain 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 & become osteocytes

• process involves BMP’s (bone morphogenetic proteins) & a transcription factor CBFA1 (core binding factor alpha 1)

not a question

What is endochondral ossification

• occurs primarily in the vertebral column, ribs, pelvis, & limbs (somitic & lateral plate mesoderm-derived bones)

• involves formation of cartilage from aggregations of mesenchymal cells

• replacement of cartilage with bone

How are limbs zones of polarising activity

• the limb has polarity > finger / toes at one end & humerus / femur at the other end

What segments are tetrapod limbs created as

• 3 segments

  • proximal stylopod (humerus / femur)

  • middle zeugopod (radius, ulna / tibia, fibula)

  • distal autopod (carpals / tarsals)

  • mnotic : see zebras autopilot

How does the limb develop?

• limb bud

  • in sheep, pigs & cats limb bud development commences at the end of the of the 3rd week of gestation

  • in humans, cattle & dogs, occurs in 4th week

  • limb buds form as mesenchymal condensation of the later plate mesoderm

What are limb buds and at what levels do they form?

• begin as small elevation on dorsolateral body wall

• aggregation of underlying somatic mesoderm

• form at 2 lvls

  • forelimb - C5-8

  • hindlimb - L3-5

What precedes, the forelimb or the hindlimb?

• the forelimb precedes the hindlimb

What are limb inducing signals?

• lateral plate mesoderm destined to become limb skeleton

• secrete paracrine factor FGF10

• FGF10 initiates limb bud formation - interaction between mesoderm & overlying ectoderm

• formation of the apical ectodermal ridge (AER)

What are the roles of the AER

• the AER keeps the mass of underlying mesenchyme (the progress zone (PZ)) in a “plastic” proliferative state to ensure limb outgrowth

• 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

What is the relation between FGF and AER

• FGF is involved in initiation & maintenance of the AER

• disrupt FGF signalling, affect limb outgrowth, digit formation but not bone differentiation

How do digits form?

• increase mesenchymal density in distal extremity

• tissue between rays programmed to degenerate

• digits 4-10 days after limb bud appears

• if no mesenchymal cell death - soft tissue web

What is the ZPA and briefly explain ZPA and AP axis specification

• A/P axis specified very early

• singal emanating from mesodermal cells located at posterior junction between limb bud and body wall

• called the zone of polarising activity => ZPA

What happens if there is a duplication of the ZPA?

distal limb duplication

What is the polarising signal expressed in the ZPA?

• Sonic hedgehog > SHH

• overexpression of SHH in the limb bud results in mirror-duplication of digits > polydactyly

List examples of postnatal limb growth and development

• achondroplasia (hereditary dwarfism) > initiates premature differentiation of epiphysial growth plates of the long bones

• if high levels of GH are produced prior to puberty > failure of grwoth plate closure - giantism

• if high levels of GH after puberty > acromegaly enlargement of the extremities

Why is it important to know the dev of the cardiovascular system

• cardiovas. defects represent the most common class of congenital defects presenting in animals

• frequently encountered in dogs and cattle

• classified as either cyanotic (insufficient oxygen provided to peripheral vasculature) or acyanotic (sufficient oxygen - but other issues)

List examples of acyanotic & cyanotic abnormalities

• acyanotic abnormalities

  • aortic stenosis (obstruction of the L ventricular outflow) > frequently detected in large breeds of dogs

  • pulmonary stenosis (narrowing of the pulmonary outflow)

  • ventricular septal defects

  • atrial septal defects

• cyanotic abnormalities

  • large ventircular septal defects including a syndrome called Tetralogy of Fallot (multiple defects)

  • transposition of the outflow vessels (reversal of pulmonary & systemic flows)

What rank of the embryo at neural tube stage is involved in cardiovascular dev

lateral plate mesoderm

What does specification of the heart primordia mean?

• heart is 1st functional organ to dev, evident at 18 days

• future cardiogenic mesoderm (near Hensen’s node) migrates through the primitive streak

• intiated through the FGF (fibroblast growth factor) & BMP (bone morphogenic protein) pathways via adjacent endoderm

Explain the formation of the heart tube

• develops from splanchnic mesoderm near the head of the embryo in a region known as cardiogenic area

• the cardiogenic area begins to form 2 strands called cardiogenic cord

• lumen develops rapdly in the cardiogenic cord & referred to as endocardial tubes

• the part of the intra-embryonic cavity (space between the somatic & splanchnic mesoderm) close to spetum transversum at the cranial end of the embryo reorganises to give a distinct cavity known as pericardial cavity

lateral folding, not a question