HUB2019F Intro to Embryology

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

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

-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

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

gonads (testes and ovaries)

primordial germ cells

-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

-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

homologous chromosomes form tetrads by synapsis

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

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

nuclear membranes form around separated chromosomes

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

group of 4 chromatids

fusion or joining

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

nuclear membrane dissolves, chromosomes condense and mitotic spindle forms

chromosomes line up on metaphase plate

sister chromatids separate as spindle fibers shorten

nuclear membranes form around separated chromatids

results in 4 haploid cells

-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

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

hydrolytic enzymes

the cell nucleus

mitochondria for energy

microtubules to propel the cell

-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

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

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

follicle cells and oocyte secrete glycoproteins to form the zona pellucida

follicle cells proliferate to form a multilayered capsule around the oocyte

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

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

luteinising hormone

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

-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

rapid transport through the ductus eferens

-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)

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

-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

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

-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)

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

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

incomplete cleavage fish, reptiles, birds with dense yolk

-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

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

a cell's potential to differentiate into other cell types

-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

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

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

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

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

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

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

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

fluid filled sac which eventually encases the whole embryo

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

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

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

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

spread of cells across the embryo surface (exterior)

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

connective tissue including blood, bone and cartilage

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

-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

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

neural plate cells are elongated. neural groove initiates

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

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

neural tube closes at the midline

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

gives rise to multiple tissue types

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

lineage tracing

follows migration of trunk neural crest cells deep into the embryo

dorso-lateral and ventral

gives rise to melanocytes

gives rise to dorsal root ganglia and sympathetic ganglia

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

nicole le douarin in the 70's

-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

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

-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

-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

-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

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

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

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

cell proliferation and limb bud outgrowth

-apical ectoderm ridge
-zone of polarising activity

sufficient

activation of bone morphogenic expression in the interdigital tissue

freeing the digits by apoptosis

retention of interdigital tissue resulting in fusion of digits