Developmental Biology Exam 1

def: science of becoming

• 1. cellular diversity
• 2. order w/in indiv. organism

• 1. A single cell will give rise to hundreds of different cell types
• A. 210 different cell types
• B. more types if you include transient stages such as mesenchyme

• 1. divide
• 2. migrate
• 3. die
• IN COORDINATED FASHION

tightly regulated

germ cells

• 1. Some organisms can regenerate their entire body, or parts thereof
• 2. Although mammals are generally poor regenerators, there are some cells that retain the ability to form new structures in adults
• 3. How do stem cells retain this capacity, and can we harness it to cure debilitating diseases?

• 1. process of selection optimizes a genetic characteristic
• 2. also developmental history my constrain future development

• 1. dev. bio is influenced by cues from the environment that surrounds the embryo and larvae
• A. Temperature-Dependent Sex Determination
• B. The formation of the reproductive system in some insects depends on bacteria that are transmitted inside the egg
• C. Some chemicals can disrupt normal development (malformations

• 1. Def: fertilized egg
• 2. ONLY a single cell
• 3. divides mitotically thereafter

• 1. Def: the study of development
• 2. between fertilization and birth
• 3. however, development is not limited to the fertilization and birth stage
• a. ex/'s: regeneration of body parts, red blood cells, etc.
• 4. 3 major approaches
• A. anatomical 
• a. this is the basis of dev. biology
• B. experimental 
• C. genetic approach

• 1. all animals undergo a similar life cycle
• 2. undergo embryogenesis for majority of metazoans
• a. this embryogenesis can be categorized into 6 processes

All stages of development between fertilization and hatching (birth)

• 1. fusion of gamete cells
• A. results in a complete genome for zygote to begin development
• 2. stimulates egg to begin development by:
• A. activates protein synthesis,
• B. DNA synthesis, and the
• C. cell cycle – also mitosis-promoting factor, or MPF, which
• a. Mitosis promoting factor: regulates cell cycle of early blastomeres.
•     i. cyclin B: 
•     ii. Cyclin dependent kinase

• 1. Def: series of mitotic divisions immediately after fertilization
• 2. End of cleavage: blastula is formed (when the blastomeres form a sphere)
• 3. What is occuring:
• A. initially mRNA and proteins from mom controls cell division
• B. initially cytoplasm is not increasing but further divided into cells.
• C. once mRNA from mom are all used up, -> mid blastula transition. 
• a. "gap stages" are now in mitosis (not just M and S) 
• b. synchronicity of mitosis lost, becoming more specialized 
• 4. Initial stages are bi phasic- alternating between mitosis and synthesis (no interphase)
• 5. What is controlling cell division: proteins and mRNA packed by the mom-not from embryo.
• (Cleavage starts before the genome takes over and controls it.)
• 7. occurs rapidly in invertebrates
• 8.

1. Def: enormous volume of zygote cytoplasm divided further into separate cells called this

• 1. spherical structure of blastomeres 
• 2. (at end of cleavage)

• 1. Definition: Rearranging (and migration) the blastomeres so they form sheets of cells and form layers
• Therefore at the end:
• A. At the end of gastrulation, the ectoderm (precursor of epidermis, brain, and nerves), is on the outside of the embryo
• B. Endoderm (precursor of the gut and respiratory systems) is on the inside
• C. Mesoderm (precursor of connective tissue, blood, heart, skeleton, gonads, and kidneys) is between ectoderm and endoderm
• 2. Beginning of this stage: when blastopore is formed. 
• A. blastopore forms 180° from sperm entry. 
• B. the blastopore is the dorsal side of frog, or back side, and sperm is belly side.
• 3. What is occurring: so dimple, or blastopore, 
• 3. Product of gastrulation: this embryo called GASTRULA now has
• A. endoderm (inner most layer)
• B. mesoderm (middle)
• C. ectoderm (exterior)
• 4. 
• **also has axes form

• Gastrula has the three layers and the blastopore

• 1. Cells interact and rearrange themselves to produce tissues and organs
• a. (after gastrulation, the organization of cells into layers of e,m, and e)
• b. Involves chemical signals between cells of the germ layers
• c. Some cells undergo long migrations from place of origin to final location
• 2. in frogs: notochord forms (rod of mesodermal tissue in the most dorsal part of embryo) 
• A. signals to ectodermal tissue to stop and not become epidermis.
• B. this ectodermal tissue becomes the neural tube (notochord and neural tube DIFFERENT originating from dif. germ layers)
• (embryo at this stage is a neurula)
• A. communicate w/ neighboring mesodermal tissue to develop them into somites, or precursors for dermis, back muscles, and the vertebrae.

• –E.g., precursors of blood cells, lymph cells, pigment cells, and gametes
• 4. in frogs :when mouth and anus forms in embryo. 
• A. elongation
• B. neurons connecting w/ each other 
• C. gills form
• D. feeding for itself occurs when it hatches and yolk supply is exhausted.

• In many species, organism must undergo metamorphosis before becoming a sexually mature adult
• a. Larval stage: physically different from adult
•     i. In some species, larval stage lasts the longest and involves feeding and/or dispersal (adult stage is brief with sole purpose to reproduce)
• b. In frogs.
• A. Initiation:thyroid gland hormones, and almost every organ is modified
• B. Morphological:
• a. Hindlimbs and forelimbs differentiate as tail recedes
• b. Cartilaginous skull replaced by predominantly bony skull
• c. Horny teeth disappear and mouth and jaw takes new shape as tongue muscle develops
• d.Lengthy intestine shortens to suit carnivorous diet of adult frog
• e. Gills regress and lung enlarges

• 1. Def: development of gametes
• 2. Germ cells: the cells set aside to produce the next generation
• A. consist of gametes and precursor cells
• 3. **one of first embryogenesis steps is separation of somatic and germ cells
• 3. This is not completed until physical maturity (puberty)
• 4. Environment can trigger certain stages of life cycle such as gametogenesis
• a. ex/ male sperm formation in frogs. mating season sperm is made in the preceding spring
• b. ex/ female frogs: photoperiod and temp cues initiate the pituitary gland to produce estrogen which triggers liver to make yolk

• 1. pituitary gland: stimulates ovaries to produce estrogen
• 2. this estrogen triggers liver to make yolk.
• A. Yolk: deposited in one portion of the cytoplasm
• -called the vegetal hemisphere
•     a. effects cleavage
•     b. yolk is viscous: cleavage occurs  more slowly in this area.
•     c. animated , or animal hemisphere in non-yolk part. cleavage happens more rapidly here

• 1. Effect: egg to have an axis
• A. mobilizes cytoplasm to dif. parts of egg
• a. Important for determining 3 body axes of frog: anterior-posterior, dorsal-ventral, and right-left
• B. activates molecules to begin cell cleavage and gastrulation
• C. Fertilization allows haploid female pronucleus to merge with haploid male pronucleus to form a diploid zygote nucleus
• 2. external process for frogs
• A. "amplaxis": male rubs over her abdomen to stimulate her to release
• 3.

• ** remember def. is series of rapid mitotic divisions after fertilization
• 1. now , volume of frog egg stays the same, but now divided into tens of thousands of cells.
• A. remember* that vegetal cells are dividing more slowly, however:
• B. the vegetal hemisphere cells become increasing larger than animated ones
• 2. during cleavage, the blastocoel forms :  fluid filled cavity, from the animal hemisphere cells
• 3. inside is blastocoel, remaining is vegetal hemisphere

• 1. At the end of gastrulation, the ectoderm (precursor of epidermis, brain, and nerves), is on the outside of the embryo
• 2. (precursor of the gut and respiratory systems) is on the inside called endoderm
• 3. Mesoderm (precursor of connective tissue, blood, heart, skeleton, gonads, and kidneys) is between ectoderm and endoderm

• 1. Def: generation of organs
• 2. Begins w/: notochord formation
• A. notochord: fibrous rod in mesoderm
• 3. what above now causes:
• A. Signals ectodermal cells above (using a morphogen) that they will not become epidermis
• a. occurs in 2 weeks of human
• b. Rather these dorsal ectodermal cells elongate, stretch, and fold to form the neural tube and become the nervous system          i. (embryo at this stage is a neurula)
• c. Then  future epidermal cells of the back cover the neural tube
• B. after notochord and neural tube are both developed in respective order, somites are formed.
• a. this occurs when mesodermal tissue adjacent to neural tube and notochord becomes segmented into somites
•    i. somites: precursors of back muscles, spinal vertebrate, and dermis
• C. embryo also forms mouth , anus , and elongates into tadpole structure. this entails:
• a. Neurons make connections to other neurons and muscle cells, gills form
• b. Hatched tadpole will feed for itself as soon as yolk supply is exhausted

• precursors of back muscles, spinal vertebrate, and dermis
• b. mesodermal tissue
• c. created by neural tube and notochord

mouth part before a mouth.

• 1. Striking Transformation of fully aquatic tadpole larva into terrestrial adult frog
• 2. Initiated by hormones from the thyroid gland, and almost every organ is modifed
• A. Hindlimbs and forelimbs differentiate as tail recedes
• B.Cartilaginous skull replaced by predominantly bony skull
• C. Horny teeth disappear and mouth and jaw takes new shape as tongue muscle develops
• D. Lengthy intestine shortens to suit carnivorous diet of adult frog
• E.Gills regress and lung enlarges

2. transformation is also rather quick bc the half tad/ half frog is very vulnerable

• 1. As metamorphosis ends, development of germ cells begins (can take a long time; e.g. in R. pipiens, it takes 3 years for eggs to mature in ovaries)
• 2. Involves meiosis – genetics, development, and evolution in eukaryotes is predicated on this process
• A.Allows fertilization, and results in recombination of genes from two parents
• In meiosis:
• B.Chromosomes are duplicated (and sister chromatids remain attached at centromeres) prior to cell division
• C. Homologous chromosomes (duplicated) pair together
• D. First meiotic division separates homologous chromosomes (i.e., each attached pair of sister chromatids)
• E. Second meiotic division splits the centromere and separates sister chromatids, which now become independent chromosomes
• F. Environmental factors control life cycles (distinct breeding season triggered by photoperiod and temp cues inform pituitary gland)
• G. Pituitary secretes hormones that stimulate ovaries to produce estrogen
• H. Estrogen directs liver to make and secrete yolk proteins, which are transported through the blood into the enlarging eggs in ovary
• F. Yolk transported to vegetal hemisphere (food for embryo)
• G. Sperm formation also is seasonal (all sperm for a breeding season are produced in the preceding summer)
• Stored early . “arrested development” setting some aside.

Set aside “beginning”

• A. gamete is the name of the cell.
• Pronucleus: distinguish it from something that is fertilized. "chromosomal condition"has a pronucleus. its a part of it.
• B. these are haploid and fuse to make a zygote nucleus.

• 1. can be internal or external.
• 2. Fertilization also causes cytoplasmic migration to different parts of egg
• 3. Fertilization also causes cytoplasmic migration to different parts of egg
• A. makes 3 body axes in frogs, dorsal-ventral, right-left, anterior-posterior
• 4. activates molecules necessary for  cleavage and gastrulation

• 1. Def: changes in anatomy during development of different organisms
• 2. Major contributors:
• A. Aristotle
• a. The Generation of Animals (ca. 350 BC), (first known study of this discipline)
• b. first to document life cycle themes:
• i.Oviparity (born from eggs)
• ii.Viviparity (‘live’ birth) didn’t realize there was an egg in the female
• iii. Ovoviviparity (eggs that hatch inside the body): appear as eggs, eggs are in the oviducts, and the mother gives birth to live young. Ex/ sharks, some snakes. No placenta.
• Retaining the eggs in the oviducts. (doesn’t supply the nutrients, the egg does, also doesn’t excrete waste of embryo)
• c. first two distinguish two major cell divisions in which embryos come from:
• i. Holoblastic: entire egg cleaves mitotically to become embryo.
• ii. Meroblastic: part of egg becomes yolk and other part cleaves to become embryo.
• - for chickens: easily visible egg, easy to use as a model
• -Blastoderm. Doesn’t contain yolk. Cleavage occurs there. Part of a meroblastic process
• B. William Harvey

• 1. change in dev. that causes evolutionary change,
• 2. how does ancestry  can constrain possible future changes and evolution

study of birth defects (what organs are affected by mutations in particular genes)

• 1. contributed to comparative embryology.
• 2. concluded that all animals comes from eggs.
• 3." On the generation of living creatures," 
• –Precluded spontaneous generation of animals from mud or excrement
• 4. Was first to see blastoderm of the chick embryo,
• a. and noticed that “islands” of blood tissue form before the heart does
• (The first to see the heart form in an embryo)

• 2. oldest known illustration of chick embryo.
• 3. believed in preformation

• Do organs 
• 1. form de novo (epigenesis)
• A. Epi: above
• Genes: genes
• (something that happens above the genes)
• 2. or preexist in miniature forms in egg or sperm (preformation)
• A. avoided invoking a special force
• B. little man in sperm is called homonculus
• C. Malphigi believed in this.
• D. Had the backing of 18th century science, religion, and philosophy

• 1. growth of existing structures not production of new ones.
• 2. Encapsulation: ensures species remain the same over generations
• A. before darwin
• 3. No cell theory to provide lower limit to the size of preformed organisms,
• A. aligns with Descartes’ principle of infinite divisibility
• B.Charles Bonnet: “Nature works as small as it wishes”
• C. No limits of cell, absorbance/metabolism of surface area limits growth of cell.
• 4. A.could not explain the "blending" of genes from children of oppositely covered parents.
• B. scientist "Kolreuter" produced hybrid tobacco plants, this experiment disproves preformation

• 1. began w/ Kaspar Wolff
• A. embryonic parts develop w/out presence of adult counterparts.
• B. heart, blood tissue, and intestines arose anew
• 2. "essential force" produced organ development

• 1. improved microscopes, lab techniques, and German university reforms.
• scientists involved are Pander, von Baer, and Rathke

epidermis, brain, nervous system

epithelium of digestive tube and associated organs (including lungs)

blood, heart, kidney, gonads, bones, muscles, conn. tis

• 1. Pander: discovered the 3 germ layers in chick embryo
• 2. Von Baer:
• A. (vertebrates) same tissue resulted in the same organs.
• B. discovered notochord (which directs ectodermal tissue to be the nervous system)
• C. discovered the mammalian egg
• D. 4 principles of von Baer
• 3. Rathke: 
• A. developmental homology : Pharyngeal arch->
• a. gill supports in fish: gill arches and hyandomibular
• b. birds, reptiles, amphibians: quadrate and articular bones
• c. human middle ear bones: incus and malleus 
• B. Discovered tissue interactions called ‘induction’

• 1. general features that are shared among large groups of animals appear first before specialized features. 
• 2. More specialized features appear from the general features
• 3. the embryo does not pass thru rungs of lower animal life to higher animal life. The tissue becomes more and more specialized.
• 4.

• 1. ontogeny recapitulates phylogeny: discredited, alternative view to von baer's principles.
• 2. repeating forms of the ancestors: human embryos developed through stages of the forms of all the major groups of adult animals, literally manifesting a sequence of organisms in a linear chain of being

• 1. epithelial cells: 
• A. tightly connected to one another in sheets
• 2. mesenchymal:
• A. unconnected to one another and operate as separate units.
• B. loosely organized connective tissue consisting of fibroblast-like and sometimes migratory cells separated by extracellular matrix)

• 1. direction and # of cell changes:
• 2. cell shape changes: often critical; changing the shapes of epithelial cells often creates tubes out of sheets; change from epithelial to mesenchymal allows cells to migrate away from an epithelial sheet
• 3. cell migration: germ cells migrate into developing gonad
• 4. cell growth: cells change size, many undergo ‘asymmetric’ cell division that produces one big and one small cell with different fates
• 5. Cell death: apoptosis, or programmed cell death, is critical of certain cells at particular times and places
• 6. often critical; changing the shapes of epithelial cells often creates tubes out of sheets; change from epithelial to mesenchymal allows cells to migrate away from an epithelial sheet

• Mesenchymal
• 1. Condensation: mesenchyme -> epithelium
• 2. Cell division: mitosis produces more cells = hyperplasia
• 3. Cell death: apoptosis
• 4. Migration: cells move at particular times and places
• 5. Matrix secretion and degradation: synthesis or removal of extracellular layer
• 6. Growth: cells get larger = hyper trophy ex/ fat cells.
• Epithelial
• 1. Dispersal: epithelium becomes mesenchyme (entire structure)
• 2. Delamination: epithelium becomes mesenchyme (part of structure)
• 3. Shape change or growth: cells remain attached as morphology changes
• 4. cell migration (intercalation): epithelia cells merge to form fewer rows
• 5. cell division: mitosis w/in row or column
• 6. matrix secretion and degradation: synthesis or removal of extracellular matrix
• 7. Migration: formation of free edges?

• 1. a diagram that ‘maps’ larval or adult structures onto the region of the embryo from which they arose
• 2. one of most important programs of descriptive embryology

• 1. direct observation: 
• ex/ when remove b4.1 blastomere, no tail is produced.
• 2. Dye marking:
• A. Vogt first one to employ this.
• B. dilutes as cells divide, so start with strong fluorescent dye.
• 3. Genetic Labeling/ chimera effect:
• A. organismal chimera
• (distinguish between host and donor)
• B. transgenic chimera.
• C. qual has more distinguishable nuclei- condensed dna/heterochromatin
• D. Cell-specific antigens that are quail-specific can tag quail cells
• E. also Rawles proved : 
• Able to prove extensive migration from neural crest cells.
• Can also make chimeras between different types of chimera.
• Chimera-form of genetic labeling
• Con: only a finite # of organisms that can do this with
• Transgenic: fusing genes together
• Clever way this happens.

• 1. Green fluorescing protein- only seen in jelly fish. 
• 2. modified this so that virus can infect intended cell with this embedded gene.
• 3. Only donor cells that are tagged will emit green glow.
• 4. EX/ In this ex/ cut and paste neural crest cells into different embryo and then see the gut cells develop.

• 1. cytoplasm pertinent in determing patterns of cleavage, gastrulation, and cell specification
• 2.

• 1. Sponges
• 2. Diploblasts –only two germ layers form
• 3. Protostomes- all three germ layers
• 4. Deuterostomes-all three germ layers

• 1. sponges
• 2. diploblasts: assymetrical animals. cnidarians,
• 3. Triploblasts: deuterostomes and protostomes.
• A. deuterostomes: has chordates, which have a notochord
• B. protostomes:

• 1. have three types of somatic cells (archeocytes originate from this)
• 2. NO:
• A. mesoderm,
• B. no true organ systems
• 3. Share gene regulatory proteins and signaling cascades with other metazoans 
• 4. Mesohyl: not connective tissue but like connective tissue- amoeboid cells produce collagen

• 1. Two germ layers – ectoderm and endoderm
• 2. Cnidarians (jellyfish and hydras) and ctenophores (comb jellies)
• 3. Generally considered to have radial symmetry and no mesoderm
• 4. However, some may have little mesoderm and display bilateral symmetry at parts of the life cycle
• 5. Jellyfish possess striated muscle, but these muscles are not related either molecularly or developmentally to mesodermally derived muscles of vertebrates or insects (evolutionary convergence)

• 1. protostomes and deuterostomes
• 2. bilateralism
• 3. Mesoderm: circulatory system and muscles allow for greater mobility and larger bodies
• 4. have a TRUE body cavity
• 5.

• 1. Greek, “mouth first” 
• 2. Mouth is formed first, at or near the opening to the gut that is produced during gastrulation
• 3. Coelem: body cavity
• A. Schizocoelus- mesoderm splits to form the body cavity.
• (schizo-split...obvi)
• B. Trochophore-larval form that looks like a top. a dradal spinner.
• 4. 2 major branches:
• A. Ecdysozoans: molt their exterior skeletons (like arthropods)
• B. Lophotrochozoans:
• A. cleavage (spiral) and a common
• B. larval form (trochophore), a planktonic larval form with characteristic bands of locomotive cilia

• 1. Major lineages are chordates and echinderms
• A. 
• 2. Greek, “mouth second” – the oral opening is formed after the anal opening
• 3. Most have body cavity from mesodermal pouches extending from the gut (enterocoelous formation of body cavity)

cleavage stage cells

• A. embryonic stage composed of blastomeres;
• B. a mammalian blastula is called a blastocyst
• C. beginning of gastrulation (end of cleavage)
• D.

1. cavity within the blastula (a blastula that lacks a cavity is called a stereoblastula)

1. invagination where gastrulation begins

• 1. F(x): drives shift between M and S phase:
• (mitotic or active phase) and S phase, dna duplication.
• A.Brings about chromatin condensation,
• B. nuclear envelope depolymerization, and
• C. organization of the mitotic spindle (but requires Cyclin B to function 
• 2. two molecules:
• A. cyclin B:
• a. cyclical, accumulating in S phase and degrades in M
• b. encoded from mom's mRNA
• c. Cyclins push go in into gap phase. 
• B. CDK, cyclin dependent kinase:
• a. CDK activates mitosis by phosphorylating several target proteins

• 1. quality (distribution) and quantity of yolk 
• 2. factors in egg cytoplasm that affect mitotic spindle
• Types of cleavage:
• 1. Holoblastic : cleavage of entire egg
• A. Isolectithal:eggs have sparse, equally distributed yolk (e.g., mammals, sea urchins, snails
• 2. Meroblastic:  cleavage of only a portion of the cytoplasm
• A. cleavage cannot penetrate viscous, yolky portion.
• B. When yolk carries embryo all the way to development
• C. birds , reptiles fish.

• 1. Holoblastic : complete cleavage of entire egg.
• A. Isolecithal: sparse, equally distributed yolk.
• a. radial: echinoderms
• i. the meridianal than equatorial splits
• b. spiral: offset cleavage (molluscs, flatworms)
• c. bilateral: mirror image (tunicates)
• d. rotational: (mammals, nematodes) offset blastula
• B. Mesolecithical: moderate vegetal yolk
• a. displace radial cleavage. (looks exactly like radial except portions form a bigger circle instead of each an indiv. circle) 
• 2. Meroblastic: When yolk carries embryo all the way to development and is a significant portion of egg
• A. Centrolecithal: yolk in the center.
• a. superficial cleavage: div. occur around periphery, nuclei gather around the edge
• b. insects
• B. Telolecithal: only a small portion of egg free of yolk
• a. Discoidal: clear disc of cytoplasm develops into embryo
• reptiles, fish , birds
• b. bilateral cleavage (cephalopod molluscs)

• 1. Invagination: 
• 2. Involution: inward movement of an expanding outerlaye
• 3. Ingression: migration from external to internal, in -gression, like re-gression
• 4. Delamination: split one sheet to two. new sheet doesn't move to center like ingression, its now the second to last layer
• 5. Epiboly: epidermal sheets pulling down a layer of sheets. Morphing of cells causes push

• Two levels:
• 1. Specification: "the engagement"
• A. cell can differentiate autonomously when in "neutral environment"
• 2. Determination: 
• A. cell can differentiate autonomously when in non-neutral environment.

• 1. autonomous,
• 2. conditional, and
• 3. syncytial

• 1. type of specification, which is first step in differentiation.
• 2. ex/ Macho: an mRNA that encodes a TF that will make the cell a tail muscle. if isolated and placed into other cells, the cell will become a tail cell
• 3. a four blastomere separated tunicate will produce
• 4. most of the early embryo are determined this way
• 5. mosaic associated with this
• 6. aligned with "germ plasm theory"

• 1. type of specification
• 2. ability for cells to determine its fate w/ interaction of other cells.
• 3. easily influenced
• 4. influenced by "paracrine factors" and "morphogens"

• 1. Weismann: 
• 2. aligns with autonomous specification: each cell specifies autonomously.
• 3. germ cells receive whole chromosome while somatic cells received subsets , and those subsets led to the autonomous specification and caused the diversity of our cell types.

• 1. Germ plasm theory
• 2. frog study proves autonomous specification: left and right halves separated during cleavage, these will separate into separate deteminants that remain left only and right only. 
• A. Roux proved this by killing each L or R side and seeing only one side grow. 
• a. defects experiments: explained how blastomere develops akin to dead tissue, not if L /r side develop autonomously
• B. Dreisch finally proved conditional specificatoin

• A. proved conditional specification
• B. conducted isolation experiments (correct way) & recombination experiments
• C. Determined:
• a. 2 cell embryo resulted in complete larva
• b. 4 or 8 cell embryo resulted in pluteus larva
• c. -> interactions among cells resulted in embryo not autonomous specification
• d. If this was autonomous, each of the four cells would receive one fourth of the genome.
• D. Also determined (thru recombination)
• a. shuffling ventral and dorsal cells did not mean the animal will look disfigured with alternating dorsal/ventral cells. The original dorsal cells and ventral cells did not follow their respective fates accroding to its origin, but rather its placement within the embryo.

when the majority of the early blastomeres are conditionally specified

• 1: a signal that serves a local cell (doesn’t go thru blood stream)
• 2. also called secreted factor

• a biochemical molecule that that can determine fate of cell by concentration
• b. chemical that is associated with conditional specification.
• c. can be :
• TFs produced within cells, OR 
• paracrine factors,
• d. differs from morphogenetic determinant
• e. distance from secretion of paracrine factor determines fate
• f. these p.f.'s enter intercellularly

• 1. MD:
• A. not dependent of concentration. Are diffusable.
• B. Specify in a qualitative way. Presence or absence. , binary 0 or 1.
• any concentration triggers fate.What is this exactly?1.TF’s produced within cells.

• 1. hybrid of conditional and autonomous.
• 2. creates a synchism: a cell with a lot of nuclei and hasn't sep. into indiv. cells yet.
• 3. Morphogen gradients of TFs within a cell, rather than morphogens between cells
• 4. concentration of above still matters.