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The process by which differing propertiesare conferred on parts of the embryo at different times and inspecific locations, such that specific
structures arise in the mature organismin a reliable fashion (the Why question).
•The studyof the mechanisms by which the shape of the embryo arises (the How question)
•Dynamic movements of tissues and cells
•The process by which different regionsof an embryo (or structure) become spatially organized to produce different structures
•Subsequent morphogenesis anddifferentiation complete the process.
•Primary tissue layers that ultimatelygenerate all of the major differentiated tissues of the body.
•Higher animal are triploblastic (3 germlayers)
•The three germ layers are ectoderm,mesoderm, and endoderm.
•The process by which one cell or tissue(the inducer) sends a signal to a second cell or tissue (the induced cells),resulting in a specific and reproducible change in their differentiation.
Sperm are among the most highly specialized cell types everdescribed.
Such specialization is designed to get the sperm to the egg and tofuse with it.The testes are very efficient "spermfactories," which produce vast numbers of these elaborate
Spermdevelop in association with specialized somatic cells.
arereleased into the lumen
Tightjunctionsare formed between adjacent Sertoli cells allowing the Sertoli cells toregulate the environment that bathes the germ cells.
The coordination of most developmental processes requires a systemof extracellular signals to control cellular survival andproliferation, specification of cell fate, patterning and promotion of cell differentiation.
a signalsource, a signal reception systemand an intracellular signal response system.
reciprocalparacrine interactions between these two cell types, and the overall coordination of spermatogenesisis orchestrated by endocrineinteractions between the pituitary gland and the somatic cells of thetestis.
extendspast the tip of the nucleus.
Itoften assumes aspecies-specific shape.
Inother species, it is small and inconspicuous (e.g., in humans).The acrosome contains hydrolytic enzymes, which arereleased from the sperm during the acrosome reaction. This occurs in the immediate vicinityof the egg and causes the sperm plasma membrane and the outer acrosomal membrane tovesiculate and be shed, thus releasing the enzymes.
containsthe motorapparatus of the sperm.
It consists of twocentral microtubules surrounded by 9 microtubule doublets (the 9+2 pattern). (This structure iscalled the axoneme)
theouter doublets are composed of dynein.
The essential role of dynein in motility is demonstrated by immotile cilia syndrome. Dynein arms can be eliminated fromdemembranated sperm.
Restorationof dynein re-establishes motility.
Modifieddynein molecules that lack ATPase activity are immotile.
Mitochondriaare located in the middlepiece.
Mitochondriain mammalian sperm are elongated and wrapped around the axoneme.
Whywould the sperm have such elaborate mitochondria for their small size
Sperm from marine andfreshwater invertebrates.
Thesesperm are usually characterized by roundor conical nuclei and small acrosomes.
Theacrosome reaction of marine invertebrate sperm involves eversion of a longslender acrosomal process, the tip of which contacts the egg duringfertilization.
Thesurface of the acrosomal process is coated with a protein called bindin,which binds the sperm to the egg vitelline envelope.
Whatis the role of intercellular signaling in spermatogenesis?
In mammals, somatic histones are removed by protease digestion.
In the first step, the histones are replaced by small, highlybasic transition proteins. This eliminates nucleosomes and stopstranscription.The replacement of the transition proteins with protamines stabilizes and further compacts the chromatin via the formation of disulfide cross-bridges.
This has important consequences for gene expression during spermiogenesis. It means that all events afterthis must rely upon post-transcriptional processes.
It is a generalized phenomenon that spermatids lack transcription.
Gametes are cells specialized in sexual reproduction. They contain half of the maximum number of chromosomes of the species and unite with another gamete giving birth to a zygote with double of the number of chromosomes of the gametic cells.
In humans gametes are formed by meiosis; the male gametes are the sperm cells and the female gametes are the egg cells.
The high concentration of mitochondria at the base of the flagellum of the sperm cell is necessary for the energetic supply of the flagellum (for it to beat and move the sperm cell).
During oogenesis dramatic changes take place within theoocyte.
These include changes in the size, yolk contentand distribution, meioticstate of the oocyte, as well asprogressive changes in the distribution of maternally supplied mRNAs andproteins that are required for development following fertilization.
The task of oogenesis is toconstruct a large cell containing a large and complex dowry of resources forconstruction of the embryo before it can either make them on its own or obtainthem from its environment.
are completed either before or shortly afterbirth. But, thereis no oocyte growth until puberty. Then,cohorts of oocytes resume development during each cycle.
- production of steroid hormones,
- transportation of essential components to theoocyte, and
- formationof accessory layers surrounding the oocyte (later, the egg).
folliclecells and nurse cells.
What is the major distinctionbetween these two categories of cells?
acellular zona pellucida, which is secreted by the oocyte.
The zona is penetrated by many short microvilli from thesurface of the oocyte and cytoplasmic processes from the follicle cells.
a cluster of granulosa cellssurround the accentrically-locatedoocyte. Theyare collectively called the cumulusoophorus.
The cumulus is shed along withthe egg at ovulation.
a cluster of granulosa cellssurround the accentrically-locatedoocyte. The cumulus is shed along withthe egg at ovulation.
The insects there is Incompletecytokinesis of the terminal oogonial divisions results in a cluster of 16cells interconnected by cytoplasmicbridges.
One ofthese cells differentiates into the oocyte, and the rest become nurse cells.The nurse cells, which become highly polyploid (1024N), provide macromolecules and even organelles to thedeveloping oocyte.
The entire nurse cell-oocytecomplex is surrounded by folliclecells, whichalso play significant roles in oogenesis.
Polarity is a salient feature ofoocytes.
In somatic cells, the ratio ofnuclear DNA:mitochondrial DNA is Inthe fully-grown oocyte, that ratio has reversed to
How is an amphibian egg classified?
Yolk is formed in one of two ways
in an asymmetrical pattern, with most platelets andthe largest ones located in the vegetalhemisphere. The other hemisphere is the animalhemisphere.
This polarity foreshadows the polarity of the embryo itself.
unfertilized egg with enlarged nucleus contatining an obvious nucleolus
meiosis has begun, but not to prophase 1 yet
germinal vesicle disappears right after fertilization
The cortex is a semi-rigid gel(as distinct from the endoplasm, whichis fluid). The cortex resists centrifugation.
Thecortex may contain specialized organelles, such as cortical granules and/or pigment granules
granules Corticalare membrane-enclosedorganelles that release their contents at fertilization.The cortical granules are formed in the endoplasm
A ribonuceoprotein matrix accumulates on the loops, which consists of proteins and nascent RNA (i.e.,RNA that was transcribed on the loop chromatin)
Meiosis I is suspended at the diplotene stage, and the homologs remain attached by chiasmata. Each homolog is comprised ofparallel strands of sister chromatids. At intervals, the chromatin is compacted intochromomeres, and chromatin loops extendlaterally from the chromomeres
Considering the duration of thelampbrush phase, a tremendous amount of RNA can be synthesized during oogenesisin these organisms.
What isunusual about the ribosome content of oocytes?
How hastranscription been demonstrated on lampbrush chromosomes?
that the sperm penetrate surface coats that surroundthe egg. This is facilitated by the acrosomereaction, in whichthe membranes enclosing the acrosome are shed,releasing the contents of the acrosome.
Mammalian sperm beforethey are able to undergo the acrosome reaction.
functionsas a sperm receptor.The sperm-binding activity of ZP3 is mediated bythe oligosaccharide side chains of ZP3. The role ofthe oligosaccharides is demonstrated by experiments in which either removal or modification of thesugars causes loss of sperm-bindingactivity
Bindingof the sperm to the zona triggers
the sperm centriole contributesto the first embryonic cleavage spindle
two majorconsequences of cleavage:
productionof a multicellular organism from a single-celled zygote
and acquisition by individual cells or groups of cells of differences thatwill facilitate their later development into different cell types.
Thus, blastomere volume is decreased duringcleavage to produce the small,individual cellular units that participate in the subsequent morphogenic eventsthat mold the embryo.
(mammals) first cell division is parallel to the animal- vegetal axis. 2nd two blasomteres divide at right angles to eachother, one is parellel to the axis the other is perpendicular
- slow and asychronous
- when zygote reaches 8 cells the blastomeres change shape to maximize contact with one another
a consequence is bilateral symmetry thatcharacterizes the embryo and adult that occursduring the first cell cycle .This reorganization (rotation of symmetrization) occurs bya 30°rotation of the egg cortex in relation to the vegetal mass
In thevegetal hemisphere, the clear cortical cytoplasm is shifted upward away fromthe sperm entry site and toward the presumptivedorsal side of the embryo, whereas in the animal hemisphere, the pigmentedcortex shifts downward toward the sperm entry site.This rotation produces the gray crescent
Although fertilization results inunion of maternal and paternal genomes,zygotic gene activity is notrequired until the blastula stage.
In fact, after fertilization andthrough cleavage, the maternal, paternal and zygotic genomes are completelydispensible. This is because in the egg,there exists a stockpile of maternallyderived mRNAs which govern embryogenesisthrough cleavage to the blastula stage.
found to affect dorsal-ventral and anterior-posteriorpolarity in the developing embryo.
the translational regulation of maternal mRNAs.
a)increased intracellular pH and Ca++
b) increased polyribosome content and translational efficiency.
Instead,the blastula consists of just one cell layer. Unlike mammals, other vertebratesdo not attach to the uterine wall and do not form a placenta, so there is noneed for the differentiation of trophoblast cells and the ICM.
- During early cell divisions, the genes encodedby the zygote's DNA are not expressed. Instead,the maternal mRNAs deposited within the egg control early cellular processes.
- The midblastula transition is the point atwhich the zygote's DNA beginsto be expressed, and control of transcription switches from motherto embryo.
- This transition occurs when thenuclear:cytoplasmic ratio is about 1:6.
Thepre-MBT stage in development has three characteristics:
- thecleavage furrow passesentirely through the egg to divide the cell in half.
- The seconddivision divides the original cell into fourequal-sized blastomeres.
- The third division is creates larger cells onone side of the equator due to the yolk distribution
- occursin yolk-rich eggs, inwhich the cleavage furrow cannot formcompletely through the cell.
- Cell divisions differ based on the organism: birdshave restricted cell divisions at the tip of the cell/yolk; drosophila cells divide within the yolk and migrate
Tight junctions form amongoutside cells that will give rise to trophectoderm. tight junctions prevent ion andwater movement between cells. Na/K pumps move Na into center ofembryo leads to an increase in osmoticpressure between cells in center of embryo. water diffuses through cells, ispump and secreted into center of embryo to dilute the increasing ionconcentration and return to iso-osmotic conditions leads to the formation of acavity called the blastocoel.
initially microvilli surroundmost of blastomere
as compaction procedes, microvilli move to one one side of blastomere allows gap and tight junctions toform between blastomeres where microvilli are not present. tightjunctions which form among blastomeres on the outside (polarized cells) ofthe embryo create a sealbetween blastomeres. the cells involved give rise tothe trophectoderm. gap junctions form among nner(non-polar) blastomeres and between inner and outer (polar) blastomeres. communication give rise to ICM
occurs at fixed time afterfertilization not cell number
loss of observable boundries in betweencells
a differentiational eventcontrolled by the embryonic genome
microfilaments and tubulesinteract with plasma membrane to cause this to occur
During gastrulation, several importance things are accomplished:
1. The three primary germ layers are established.
2. The basic body plan isestablished, including the physical construction of the rudimentary primary bodyaxes.
3. As a result of the movementsof gastrulation, cells are brought into new positions, allowing them tointeract with cells that were initially not near them. This paves the way for inductiveinteractions,4. which are the hallmark of neurulation and organogenesis
the ectoderm overgrows backwards on the endoderm; ultimately the entire embryo (except for the smallarea called the yolk plug) iscovered by the ectoderm.
In other words the pigmented micromeres (animal half) grow overthe megameres (vegetative half).
The reason for overgrowth is the rapid rate of division ofmicromeres.
During this processthe cells which have grown backward during emboli now roll inside at the marginof the blastopore.
The endoderm is the first to roll inside.
The cells of the notochord and mesoderm which were formedoutside now migrate over the lip of blastopore and become internal and arrangethemselves on the roof, sides and the floor of the archenteron.
The notochord cells are found on the roof along the midline.
At the end of gastrulation the yolk filled cells get rearranged.
This rearrangement shifts the centre of gravity of the gastrulaand results in a rotation of eighty to ninty degrees.The blastopore moves backwards and is finally located slightly above itsoriginal location at the beginning of the gastrula
First,there is shifting of the cells of the notochord and mesoderm towards the dorsallips.
Secondly,there is a dorsal convergence of notochord and mesodermal cells towards themiddorsal area of the blastopore.
Regular in shape, arranged in layer or sheet, often only one cell thick, called an epithelium.
Major feature is polarity.
Bounded on one side (basal surface) by a basal lamina, thin mat of ECM secreted by the cells
1) Tight junctions prevent leakagebetween cells.
2) Gap junctions allow passage ofsmall molecules (< 1 kD). 3)
Other structural junctions include transmembranelinker proteins, e.g. cadherins
Irregular in shape, mobile, non-polar, separated from each other by ECM.
Move by interacting with ECM components.
Epithelia can move as a sheet by invaginating to form a pocket (like making an indentation in a ballon), or involuting (flowing, as a sheet, "around a corner") to form one cell layer inside another.
allows a flat sheet of cells to form a column of cells by transiently releasing and re-intercalating cells in different positions (radial interacalation).
which works by intercalating new cells into a
preexistingsheet so that it spreads (medio-lateral intercalation: basically the oppositeof convergentextension).
on adhesion molecules at the cell surfacethat are connected to the cytoskeletonin the cell interior All these movements dependent on
Examples of gastrulation
1) conversion of a ball ofcells into an embryo with three layers of different kinds of cells (endoderm,mesoderm and ectoderm)
2) bringing previously separated celltypes into contact, allowing new inductive signals that affect later cell determinationand differentiation; and
3) establishing therudimentary body plan of the animal: “a tube within a tube,” (endoderm in theinterior, ectoderm on the surface, and mesoderm in the middle)
Blastoporeforms on future dorsal side of the embryo, in region of the “gray crescent”between animal and vegetal hemispheres just belowbottom of the blastocoel
Subsequent involution over dorsal lip of blastopore, displacing theblastocoel.
Bottle cells, at advancing tip ofthe archenteron, lead the way.
ectoderm, which has spread tocover the whole surface,
endoderm, which is now allinside, andmesoderm, derived from the marginal zone cells that have moved inside, between ectodermand endoderm.
involves cell rearrangements as well as celldivision.
futurehead ectoderm and mesoderm on the roof of the archenteron near the formeranimal pole,
spinal cordlying along dorsal side of the
archenteron toward original blastopore;
(reptiles & birds)
equivalent of amphibian dorsal lip
contains many signaling molecules
Hensen’s node iswhere cells begin to migrate into the embryo, primarily by ingression.
Gastrulation movements begin at the node, and progress towards the posterior of the embryo
theendoderm cells move in first, then mesoderm.
Cells that will form the dorsal mesoderm (notochord, somites, head) migrate infirst, followed by cells that will form the intermediate mesoderm of organs (kidney,gonads), and lastly the cells that will form the lateral plate mesoderm (heart,blood).
Prospective ectodermstays on the outside, as in all embryos.
a sphere of outer cellsconnected by tight junctions, which will give rise to supporting extraembryonic structures (amnion and parts of theplacenta).
Presumptiveendoderm cells form
movement + reorganization of cells. In contrast to the cleavage stage, cell
division is not the major determinative or morphogenetic influence, although it is occurring throughout gastrulation.
- endoderm, the inside layer lining the gut,
- ectoderm, the outside layer that will give rise to the skinand nervous system,
- and most importantly a new cell layer, the mesoderm,between them, which will form tissues in sea urchin such as the spicules andmuscle, and in vertebrates the muscles, blood vessels, and organs such asheart, kidney, and gonads.
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