CELLULAR DEVELOPMENT OF THE ZYGOTE

HOW DO ZYGOTES FORM ORGANISMS When a zygote is undergoing early cleavage division, there must be a way for embryonic cells to differentiate into the cell types that make up the organism Strategy 1: Uneven distribution of cytoplasmic determinants in the unfertilized egg There are different mRNAs, proteins and other molecules in different regions of the egg cell As a result, different factors cause different cells to have different fates Additionally, the orientation of the egg gives rise to the orientation of the body axis

HOW DO ZYGOTES FORM ORGANISMS Strategy 2: Once the initial orientation of the organism begins to take form, interactions occur between the embryonic cells Cells release chemical signals that cause surrounding cells to “differentiate” in certain ways. This process of induction creates the specialized cell types that form different tissues.

FATE MAPS Researchers have discovered the fate of every early cell in the zygote for simple organisms Caenorhabditis elegans (type of worm) As a result of this research, we have two important conclusions: Specific tissue types are developed to early “founder cells” As the development of the blastula (developing cellular embro) proceeds, the range of cells that can be created becomes more limited This is attributed by the fact that as cells differentiate, they become “fixed” to that cellular fate

DETERMINING CELLULAR ASYMMETRIES The fact that the developing blastula does not rise uniformly gives rise to asymmetry (later forms the structures of the organism) To study this we will first examine how the basic axes of the body are formed Organisms that have bilateral symmetry (look the same on the left and right half... Humans) must have some basic orientations They must have an anterior-posterior axis (top to bottom – head to toe); Dorsal-ventral axis (front to back – stomach to back); And left and right sides (self explanatory) This gives the organism 3D development

METHODS OF DETERMINING BODY ORIENTATION In organisms that do not develop in amniotic fluid, much of the organisms orientation is already determined during oogenesis or fertilization Example: Frogs have the their axes determined by positioning of the yolk in the egg In organisms with amniotic fluid, body axes are not developed until later in development. Example: Chicks have their anterior-posterior axes developed by gravity. Later, pH levels establish dorsal-ventral axes.

METHODS OF DETERMINING BODY ORIENTATION In many vertebrates, the zygote starts out as totipotent (it can form all types of cells) The first cleavage (division) is asymmetrical. This causes each of the cells produced to have different cytoplasmic determinants (chemicals/enzymes that cause different cellular pathways for development) In some mammals, the cells in the embryo can remain totipotent until about the 16 cell stage In this case, the relative position of the cells determine their differentiation

CELL FATE Regardless, of how the zygote starts out, cells lose their ability to differentiate as time goes on The longer cell development continues, the more a cells “fate” is determined.

CELL FATE DETERMINATION After the general axes of an organism have been determined, cells begin to influence each other through a process called induction Release of chemicals that switch on different sets of genes This makes the receiving cell differentiate into a specific cell type Example: Dorsal cells in frog embryos release a series of growth factors (including Bone morphogenic proteins – BMP) These factors cause surrounding cells to differentiate in a specific path (BMP-4 goes down the dorsal side of the frog and induces the formation of the notochord and neural tube (equivalent of spinal chord)

CELL FATE DETERMINATION All of these induction pathways lead to a complicated array of signaling that leads to the formation of the organisms form Another example is the formation of a vertebral limb

LIMB FORMATION Limbs begin with a bump on the embryonic tissue called limb buds Each component of the limb (muscle, skin, bone) develops with a precise location in relation to 3D Inductive signaling plays a key role in this positional information (where the cells are located in relation to each other) In general, there are two “organizer” regions that determine limb development Apical ectodermal ridge (AER) Zone of polarizing activity (ZPA)

APICAL ECTODERMAL RIDGE The apical ectodermal ridge is responsible for the outgrowth of the limb An analogy is the branch growing out from the trunk of a tree This outgrowth is controlled to extend out from the proximal-distal axis (why your arm grows out laterally and not straight out of your chest)

ZONE OF POLARIZING ACTIVITY This is responsible for the anterior-posterior development of the limb Includes the orientation of the arm Elbows point down not up Your hands are oriented correctly so that you do not literally have 2 left thumbs Again cells send and receive signals from surrounding cells to orient themselves in relation to other cells