Agenda 3/24 Development Quick Lecture Development Documentary and Notes Homework: Developmental Biology Worksheet, Study for Unit Exam Turn in: Gene Expression Essay Warm Up: Summarize three major things that occur during the development of embryo to fetus.

Development and Genes

What you need to know How we go from one unspecialized cell to many specialized cells Determination Differentiation Morphogenesis Role of Mitosis and Gene Expression

Characteristics of Development for Multicellular Organisms Development is the process of timed genetic controlled changes that occurs in an organism’s life cycle. Mitosis Cell differentiation Pattern formation Morphogenesis All four processes are anchored by differentiation with regard to gene expression. Most cells in a multicellular organism have the same genome or DNA. Genes must be turned on and turned off during development. Zygote->Divides->Tissues Differentiate -> Organs -> Organ Systems-> Total Organism Mitosis will result in increased number of cells. Usually in the beginning this occurs without cell growth so the number of cells increases but decrease in size. This is called cleavage. Also the early cells do not appear to differentiate but in reality the cells will contain different materials in the cytoplasm. Differentiation is process of cells becoming specialized. Pattern formation involves the process of cells becoming oriented to the body plan. It involves the cell’s ability to detect positional information that impacts the fate of the cell Morphogenesis is the physical process of organizing specialized cells giving rise to its organs and organ systems.

Cleavage (mitosis) Cleavage is the time of rapid mitosis without significant growth of daughter cells. Cells become increasing smaller. Each cell is called a blastomere. G1 and G2 phases of cell cycle is shortened or eliminated. Mitosis during development is very rapid often once every several minutes compared to an average of once every 24 hours for normal cells. The pattern of cleavage varies from species to species. Also the size of the resulting daughter cells and what they contain may also vary.

Gastrulation Gastrulation is important in cell differentiation, pattern formation and morphogenesis. After cleavage, development in animals is often accompanied by mass movement of cells called gastrulation.

Cell, Genes and Development Cells have two general classes of genes. Housekeeping genes which are necessary to go about the “business” of life. For example genes that code for the enzymes for cellular respiration are housekeeping genes. Most cells have all of these activated Specialized genes that produce unique gene product important to the cells differentiation. For example the activation of the crystallin gene that produces product necessary for the development of the lens of the eye. Point out that most all cells are have the same DNA that the original fertilized egg or zygote had but that does not mean that the cells can give rise to a new organism as genes are selectively activated and deactivated. During development genes must be activated and other genes inactivated. Transcription factors are quite often responsible for this.

Genes and Development It would be wasteful for lens cells to produce albumin, and in the same way it would be wasteful for the liver cells to produce crystallin. These specialized genes must be regulated so that they are only activated when they are needed and timing is critical.

Determination comes before Differentiation. Determination are those things or processes necessary to commit a cell to a particular type of cell or fate. Differentiation is those changes that occur in a cell to make it a certain cell type. Totipotent cells can form all the cell types in a body, plus the extraembryonic, or placental, cells. Embryonic cells within the first couple of cell divisions after fertilization are the only cells that are totipotent. Pluripotent cells can give rise to all of the cell types that make up the body; embryonic stem cells are considered pluripotent. Multipotent cells can develop into more than one cell type, but are more limited than pluripotent cells; adult stem cells and cord blood stem cells are considered multipotent.

Determination Determination – Events that lead to the observable differentiation of a cell. Once determination has occurred then the final fate of the cell is sealed. If a determined cell is placed in another location in the organism, it will still differentiate into the cell that was its normal fate.

Determination In humans, after cleavage and gastrulation is complete, a structure called a blastocyst is forms. There is a outer layer of cells which will form embryonic membranes like the amniotic and chorionic membranes. There is a mass of cells inside the outer cells called the inner cell mass. These cells are pluripotent in that they can give rise to any tissue EXCEPT embryonic membrane.

How Determination Occurs There are two sources responsible for determining the fate or development of cells. Cytoplasmic Determinants Induction via signals secreted by neighboring cells . Cytoplasmic Determinants-These are signals that are in the cytoplasm. Once the egg is fertilized then changes begin. Induction via signals secreted by neighboring cells or hormones. The nuclei of the resulting daughter cells are exposed to different cytoplasmic determinants which influences the expression of certain genes. This will in turn determine the fate of the resulting daughter cells.

Initial Development Governed by Cytoplasmic Determinants Development can begin with fertilization of an egg and subsequent division of cytoplasmic determinants during cytokinesis. There is unequal distribution of the cytoplasmic determinants to the daughter cells as illustrated. The unfertilized egg has both proteins and mRNA needed for development. Those molecular molecules are not distributed equally in the cytoplasm of the egg. The nuclei of the resulting daughter cells are exposed to different cytoplasmic determinants which influences the expression of certain genes. This will in turn determine the fate of the resulting daughter cells. When mitosis occurs the resulting daughter cells are no longer identical in their cytoplasm and how the cells will differentiate.

Initial Development Governed by Inductive Signals Once there are a multitude of cells, neighboring cells may produce signal Initial Development Governed by Inductive Signals molecules that can interact with receptor sites and receiving cells. This causes the activation of a signal transduction pathway for the receiving cell. This can send the cell down a specific developmental pathway.

Initial Development Governed by Cytoplasmic Determinants Cell Differentiation Differentiation of activated genes and inactive genes Appearance of mRNA for cell specific proteins Changes in cellular structure The nuclei of the resulting daughter cells are exposed to different cytoplasmic determinants which influences the expression of certain genes. This will in turn determine the fate of the resulting daughter cells.

Morphogenesis How the organism gets it shape Hormones, cell signaling, and various other stimuli can all affect morphogenesis

Morphogenesis Can Involve Cell Death Occurs Apoptosis occurs in the development of the digits of a hand or paw. Shown is the development of a mouse paw. The genes that regulate apoptosis are similar in both vertebrates and invertebrates such as nematodes. Fungi including yeast also have genes that regulate apoptosis indicating these are ancient genes. The region that develops into feet or hands initially has a solid, platelike structure. Apoptosis eliminates the cells in the interdigital regions, thus forming the digits. The embryonic paws shown in the fluorescence light micrographs were stained so that the cells undergoing apoptosis appear bright green. The apoptosis of cells begins at the margins of each interdigital region and peaks as the tissue in these regions is reduced. Failure of apoptosis in humans results in either webbed fingers or toes.

During Development Cell Death Occurs Apoptosis- Is a programed cell death. Occurs in Infected cells Development Cells that are aged and no longer functional The cell below is a normal leukocyte and the cell above is a cell undergoing apoptosis Apoptosis occurs when cellular agents cleave the nuclear DNA, fragment the organelles and other cytoplasmic components. The cells package up the various parts into vesicles which form lobes with the cellular membrane. Theses vesicles are then shed. Quite often these vesicles are engulfed by phagocytes. The process protect neighboring cells from damage that may have occurred if the contents of the cell were just leaked out. Necrosis is the process of cell death occurring by damaging the cell membrane and the contents leaking out. This does occur by physical trauma, lytic viral infection, complement mediated lysis (immune response).

What you need to know How we go from one unspecialized cell to many specialized cells Determination Differentiation Morphogenesis Role of Mitosis and Gene Expression *Take notes on all the examples you see of these in the film!