HUB2019F Intro to Embryology

embryonic development

process of development from a single fertilised egg to a fully formed organism

cellular processes of embryonic development

-cell division
-cell migration
-cell shape change
-cell differentiation
-cell death

why study embryology?

-understand how organs are formed
-gain insight into the causes of birth defects(genetic or environmental)
-how ensure the best pre-natal care for the developing embryo

fertilisation

merging of 2 gametes, the egg and sperm, to form an embryo

Where does gametogenesis occur?

gonads (testes and ovaries)

what are gametes formed from

primordial germ cells

main steps of gametogenesis

-increase in the number of germ cells by mitosis
-decrease in the chromosome number in the germ cells by meiosis
-structural and functional changes leading to the maturation of egg and sperm

mitosis vs meiosis

-start off diploid
-chromosomes replicated during interphase forming 2 sister chromatids joined by a centromere
-mitosis splits the sister chromatids into 2 diploid daughter cells
-meiosis produces 4 daughter cells that are haploid through 2 successive cell divisions: meiosis 1 and 2

meiosis 1: prophase 1

homologous chromosomes form tetrads by synapsis

Meiosis 1: Metaphase 1

homologous chromosomes line up on the metaphase plate(independent assortment)

meiosis 1: anaphase 1

homologous chromosomes separate as spindle fibers shorten(sister chromatids stay together)

meiosis 1: telophase 1

nuclear membranes form around separated chromosomes

homologous chromosomes

one from each parent. have the same complement and layout of genes

tetrad

group of 4 chromatids

synapsis

fusion or joining

crossing over (prophase I)

exchange of DNA between maternal and paternal chromosomes creates genetic variation in resulting gametes

Meiosis 2: Prophase 2

nuclear membrane dissolves, chromosomes condense and mitotic spindle forms

Meiosis 2: Metaphase 2

chromosomes line up on metaphase plate

meiosis 2: anaphase 2

sister chromatids separate as spindle fibers shorten

meiosis 2: telophase 2

nuclear membranes form around separated chromatids

Meiosis 2: Cytokinesis

results in 4 haploid cells

Spermatogenesis

-in seminiferous tubules of the testes
-stimulated by hormones at puberty and lasts until death
-dormant germ cells divide by mitosis to produce spermatogonia
-some spermatogonia divide twice by meiosis to ultimately form 4 haploid spermatids

spermiogenesis

-spermatids undergo a dramatic shape change to form the mature sperm
-acrosomal cap
-head
-midpiece
-long flagellum

acrosomal cap

hydrolytic enzymes

head

the cell nucleus

midpiece

mitochondria for energy

long flagellum

microtubules to propel the cell

oogenesis

-takes place in the ovary
-in the embryo, germ cells divide by mitosis to produce oogonia
-meiosis 1 at 5 months of embryonic development
-arrests at prophase 1 as a primordial follicle = oocyte surrounded by single layer of flattened follicle cells
-not certain when maturation process starts

first step of oogenesis

follicle cells plump up around the oocyte to form a primary follicle

oogenesis after puberty

many follicles will start maturing but usually only one ovulated each month

zona pellucida formation

follicle cells and oocyte secrete glycoproteins to form the zona pellucida

growing follicle

follicle cells proliferate to form a multilayered capsule around the oocyte

Graafian follicle

growing follicle swells due to stimulation by rising hormone levels at puberty of FSH from the pituitary gland

ovulation

follicle wall thins, ruptures, and oocyte slowly extruded onto the ovary

what facilitates the rupture of follicle wall

luteinising hormone

oocyte transport

ovulated oocyte is actively captured by the fimbriated oviduct where it is available for fertilisation

sperm transport

-passively transported from the seminiferous tubules to the epididymis
-ejaculation
-mixes with seminal fluid from seminal vesicle and prostate gland
-exit through urethra
-sperm deposited in the upper vagina
-pass through cervix

ejaculation

rapid transport through the ductus eferens

sperm capacitation

-occurs within the uterine tube(removal of cholesterol molecules from the cell membrane that were deposited from the semen
-increases permeability to calcium(increases motility and facilitates the release of enzymes)

Fertilization

attachment to and penetration of outer layers of the oocyte: corona radiata and zona pellucida

fertilization is facilitated by:

-glycoproteins on the ZP surface
-receptors on the sperm surface
-acrosome reaction: enzymes (acrosin) secreted from the sperm acrosome
-active swimming movements of the sperm

fusion of the oocyte and sperm plasma membrane:

mediated by membrane proteins on the oocyte (integrins) and sperm (fertlins and cyritestin)

penetration results in a wave of calcium spreading from the site of sperm entry resulting in:

-the completion of meiosis 2 in the oocyte
-cortical/zona reaction:
cortical granules fuse with the plasma membrane, releasing polysaccharides into the perivitelline space and ZP rises and hardens, impenetrable to additional sperm(polyspermy)

cleavage

mitotic divisions that divide the enormous volume of the zygote into blastomeres

holoblastic cleavage

complete cleavage of sea urchins, frogs and mammals with sparse yolk

meroblastic cleavage

incomplete cleavage fish, reptiles, birds with dense yolk

Blastocyst formation

-from 8 cell stage, blastomeres start to compact
-facilitated by cell-adhesion proteins on the cell membranes
-cells divide and change position:
1. small group of cells on inside become inner cell mass
2. larger group of external cells forms the trophoblast
3. fluid enters the embryo by osmosis to form the blastocyst cavity

first differentiation event starting at the morula stage continuing into blastocyst:

-inner cell mass gives rise to the embryo
-trophoblast gives rise to extra-embryonic tissues including the chorion and part of the placenta

cell potency

a cell's potential to differentiate into other cell types

3 types of monozygotic twins

-2-cell embryo divides into 2 leading to 2 separate embryos
-Inner cell mass divides in 2 and twins share extra-embryonic membranes
-inner cell mass does not completely divide in 2 leading to conjoined twins

Pre-implantation genetic diagnosis

between 8-cell and blastocyst stage a cell can be removed from an in vitro fertilised embryo for genetic testing without damaging the embryo: the remaining cells compensate for its loss

hatching

in the uterus, the embryo sheds the ZP to prepare for implantation. this is facilitated by enzymes secreted by the trophoblast

implantation

attachment of a fertilized egg to the endometrium. this is facilitated by an extracellular matrix of sugars, collagen etc. on the endometrium and receptors on the trophoblast

the trophoblast differentiates into 2 layers:

-a cellular trophoblast called the cytotrophoblast
-a layer of fused cells called the syncytiotrophoblast

the syncytiotrophoblast and cytotrophoblast as well as cells from the uterus will start to form the placenta:

projections from the 2 contact blood vessels in the uterine wall to allow the exchange of nutrients and gases

before and during implantation, the inner cell mass sorts to form 2 epithelial layers:

-epiblast: becomes the embryo and the amnion
-hypoblast: migrates along inside of blastocyst cavity to give rise to the primary yolk sac

extraembryonic mesoderm

populates space below the trophoblast. not certain if it is derived from yolk sac, epiblast or TB

epiblast:embryo and amnion

fluid filled sac which eventually encases the whole embryo

hypoblast: yolk stalk + sac

-connected to the developing gut
-source of nutrition for early develop
-stalk ultimately forms umbilical cord

syncytiotrophoblast and cytotrophoblast

chorion and placenta which facilitate nutrient and gas exchange between embryo and mother

Gastrulation

formation of the 3 primary germ layers: ectoderm, mesoderm, and endoderm

First sign of gastrulation

cells move towards the primitive streak, through it and down into the embryo

epiboly

spread of cells across the embryo surface (exterior)

ectoderm

skin including hair and nails, central nervous system including parts of the eyes and ears, neural crest

mesoderm

connective tissue including blood, bone and cartilage

endoderm

gastrointestinal tract, gastrointestinal glands, epithelial lining of the respiratory system

notochord

-cylinder of mesodermal cells
-forms from head to tail as primitive streak regresses
-important signalling centre for development
-contributes to the spinal column but then degrades

neural induction

diffusible molecular signals from the node and the notochord induce the overlying ectoderm above to form a thickened neural plate

neurulation A:

neural plate cells are elongated. neural groove initiates

neurulation B:

the neural plate bends at the median hinge point, anchored to the notochord below

neurulation C:

lateral hinge points form and neural folds begin to converge. facilitated by pushing forces from the non-neural ectoderm

neurulation D:

neural tube closes at the midline

what is the neural crest

-described as the 4th germ layer
-embryonic tissue that develops in association with the dorsal neural tube
-migrates extensively throughout the embryo
-multipotent

Multipotent

gives rise to multiple tissue types

nerual crest migration

-neural crest cells delaminate from the neural tube: epithelial to mesenchyme transition
-streams of cranial neural crest cells migrate around the embryo

how do we know that the neural crest is multipotent?

lineage tracing

Radioisotopic Labeling

follows migration of trunk neural crest cells deep into the embryo

two different pathways of nerual crest migration

dorso-lateral and ventral

dorso-lateral pathway

gives rise to melanocytes

ventral pathway

gives rise to dorsal root ganglia and sympathetic ganglia

disadvantages of radioactive labelling

-takes very long to image
-label gets diluted as the cells divide
-if cells die, label can get taken up by neighbours

who devised the quail-chick chimera method?

nicole le douarin in the 70's

quail-chick chimera

-develop at similar rates making it possible to stage match and graft tissue from one species to another
-quails have a condensed mass of heterochromatin in their nucleolus(histological feulgen stain) or quail-specific QCPN antibody

transgenic green fluorescent protein chickens

-used viral infection to incorporate the DNA coding for GFP into the chicken gametes
-the resulting embryos express GFP in all their cells

what advantage do green fluorescent proteins have over quail chick chimeras:

-eliminates species specific differences
-easy to visualize the bright grafted tissue in live and fixed embryos
-labels the entire cell allowing visualisation of neural processes

folding of the embryo

-trilaminar disc is rolled up to form a 3D body form
-flat embryonic endoderm is folded laterally to form the gut tube
-neural tube folded over in the cranial caudal direction

progression of human limb development

-limb stats as buds protruding from the body wall
-buds elongate to form upper limb, lower limb and autopod
-interdigital cell death frees the digits

Limb bud initiation

-after gastrulation, the mesoderm is divided into paraxial, intermediate and lateral plate mesoderm
-limb buds grow out from the lateral plate mesoderm

what do the T-box transcription factors do?

TBX4 and 5 in the lateral plate mesoderm induce hindlimb and forelimb bud outgrowth

what do the T-box transcription factors induce?

expression of the signalling protein fibroblast growth factor 10 in the mesoderm, which in turn activates fibroblast growth factor in the ectoderm

what does fibroblast growth factor signalling induce?

cell proliferation and limb bud outgrowth

2 singalling centres that patternt he axes of the developing limb:

-apical ectoderm ridge
-zone of polarising activity

fibroblast growth factor is ___ to induce supernumerary limb bud outgrowth

sufficient

what leads to programmed cell death(apoptosis)

activation of bone morphogenic expression in the interdigital tissue

interdigital cell death

freeing the digits by apoptosis

syndactyly

retention of interdigital tissue resulting in fusion of digits