Lecture 1: Developmental Biology

1. Lecture 1: Developmental Biology

2. Developmental BiologyEmbedded Assessment • Draw a four-day old human embryo • Note the approximate size or scale • Include as much detail as you can in 5 minutes

3. The animal cell Golgi complex Endoplasmic reticulum (ER) Mitochondrion Nucleus Plasma membrane Nuclear membrane Vacuole

4. Multicellular organisms have a variety of differentiated cell types Immature undifferentiated cells Mature differentiated cells (200 different cell types) Stem cell Heart muscle cell (Cardiomyocyte) Epidermal skin cells Progenitor cell Neuron Red and white blood cells

5. All cell types in a multicellular organism are generated from a single cell [Image taken from Gilbert’s “Developmental Biology”, 8th edition, Sinauer].

6. The cell cycle and mitosis The cell cycle Mitosis (parental cell) Prophase Mitosis (M) Prometaphase Interphase (daughter cell) Resting phase Metaphase Telophase Anaphase DNA synthesis (S) 5

7. Symmetric versus asymmetric cell division in stem cells Symmetric stem cell division Asymmetric stem cell division expansion maintenance Progenitor Stem cell Two stem cells 6

8. Meiosis First meiotic division (reduction division) Second meiotic division (mitosis with DNA replication) Paternal homolog Two parental cells (2n) Parental cell (2n) Maternal homolog Prophase 2 Prophase 1 (4n) Crossing over Metaphase 2 Metaphase 1 Anaphase 1 Anaphase 2 Telophase 2 Telophase 1 Four daughter cells (n)

9. Chromosomes, genes and DNA Nucleus Duplicated chromosome Duplicated chromosome DNAhelix gene • The nucleus contains genetic material in structures called chromosomes • Chromosomes are long strands of DNA wrapped around a protein core • DNA is made of four chemical bases: A, T, C and G • Sequences of chemical bases make up genes • Animals share common genes • Genes are the basic units of heredity • Humans have ~25,000 genes • The entirety of DNA in a cell is an organism’s genome

10. The Central Dogma represents the flow of genetic information Transcription Translation DNA RNA PROTEIN 9

11. Transcription: DNA makes RNA Transcription DNA RNA RNA polymerase Strand of DNA Forming strand of mRNA

12. Translation: RNA makes protein Translation RNA PROTEIN

13. Summary of gene expression • Begins with genes in the nucleus • Genes have a code consisting of A, T, C and G • The code is “transcribed” into RNA (a messenger) • Messenger RNA (mRNA) brings the code to the cytoplasm • The genetic code uses groups of three bases (CCG, GUU) to encode each amino acid of a protein chain • Groups of three bases specify unique amino acids • Amino acids are the building blocks of proteins • Proteins are long chains of amino acids

14. Proteins: the product of translation • Hemoglobin (carries oxygen in blood) • Insulin (regulates sugar breakdown/storage) • Enzymes (catalyze biochemical reactions) • Skin and hair color pigments • Signaling molecules • Control cell division • Coordinate development • Help ward off infection

15. Various differentiated cell types express different proteins Cell type Heart muscle cell (Cardiomyocyte) Motor neuron Red blood cells Unique protein Hemoglobin: transports oxygen from lungs and carbon dioxide from body Choline Acetyltransferase: enzyme that produces the chemical signal for neuron-muscle communication Myosin Light Chain 2: causes muscle contraction

16. Transcription factors regulate the flow of genetic information Transcription Translation DNA RNA PROTEINS Gene regulation • Some proteins termed “transcription factors” regulate the flow of genetic information. • These are nuclear proteins capable of binding DNA. • They regulate the process of gene transcription in immature and differentiated cells. • Transcription factors are essential for the processes of development and stem cell maintenance. 15

17. Signaling proteins are essential for cell-cell communication Secreted signaling molecules • Secreted proteins • Form gradients when secreted from cells • Function by binding proteins at the surface of plasma membrane known as receptors • Activate intracellular proteins that relay information from the surface to inside the cell

18. Differential gene expression underlies the presence of distinct proteins in various cells Heart muscle cell (Cardiomyocyte) Motor neuron Red blood cells Gene expression ON OFF OFF -globin gene OFF ON OFF ChAT gene ON OFF OFF Myosin light chain 2 gene

19. Differential gene expression underlies the process of differentiation • Every nucleus contains a complete genome established in the fertilized egg (with a few exceptions). • The mouse genome contains tens of thousands of genes but many are not expressed in all tissues. • Many genes are differentially expressed in various tissues or organs. • Unused genes in differentiated cells are not destroyed or mutated - they retain the potential to be expressed. • Only a small percentage of the genome is expressed in each cell.

20. Differential cell signaling contributes to the generation of cellular diversity Cell signaling pathways Shh Activin/TGF Erythropoietin Patched/ Smoothened BMPRI EPO receptor Progenitor cell Progenitor cell Progenitor cell Heart muscle cell (Cardiomyocyte) Motor neuron Red blood cells 19

21. The beginning of human development • Gametogenesis: formation of eggs and sperm • Oocytes and spermatocytes (23 chromosomes) • Chromosomes in gametes are reduced by half • The story of sperm • The story of eggs • Fertilization • One sperm + one egg, chromosome number restored • The genes from each are required for development • Embryogenesis: Formation of the embryo • The zygote is the earliest form of a human embryo

22. Spermatogenesis: generation of male gametes (sperm) 2N • Meiosis produces four sperm cells from one germ cell (spermatogonium). • First division is a reduction division (separates homologous chromosomes that have been duplicated prior to meiosis; DNA content reduced from 4n to 2n). • Second division is a mitosis without DNA replication, generating haploid cells (n chromosomes). • Spermatogenesis occurs throughout an adult male’s life.

23. Oogenesis: generation of female gametes (oocytes) • Oogenesis: meiosis that produces one egg and three polar bodies. • First meiotic division begins in the female embryo but stops before homologous chromosomes are separated. • First meiotic division resumes at puberty. • The second meiotic division occurs after fertilization, before sperm and egg nuclei fuse. • Females lose many germ cells over the course of their lifetime. Meiosis I Meiosis I Embryo Primary oocyte Secondary oocyte (2n) Polar body (2n) Meiosis II after fertilization Puberty Egg (n) Polar bodies (n)

24. Fertilization • Fusion of sperm and egg to create a new individual. • The diploid cell is called a zygote. • Restores the DNA content and combines genes from both parents (sexual reproduction). • Major events in fertilization: • Sperm and egg recognize and contact each other • Block of polyspermy • Second meiotic division of secondary oocyte (2n) to produce egg (n) • Fusion of female and male pronuclei • Stimulation of zygotic metabolism and cell cleavage A sperm cell attempts to penetrate the ovum’s coat in order to fertilize it

25. Cleavage (days 1-6) 2 cell stage 4 cell stage 8 cell stage Morula • Zygote divides into two cells • Day two: morula (Latin for mulberry) • Cell signaling begins • Embryo begins to organize • Blastocyst forms on days 4-6 • Two parts of blastocyst • Trophectoderm (placenta, amnion) • Inner cell mass (embryo) • Size is 0.1 mm Blastocyst Trophectoderm Inner cell mass

26. The origin of embryonic stem cells (ES cells) 1. ES cells can be derived from the morula 2. ES cells are normally derived from the inner cell mass of the blastocyst 3. ES cells can be derived from primordial germ cells 4. ES cells can be derived from adult somatic cells [Figure modified from Gilbert’s “Developmental biology”, 8th edition, Sinauer]

27. Embryogenesis (week 2): formation of germ layers Amnion Implantation Uterus Blastocyst Ectoderm Yolk sac Epithelial skin cells, inner ear, eye, mammary glands, nails, teeth, nervous system (spine and brain) Endoderm Stomach, gut, liver, pancreas, lungs, tonsils, pharynx, thyroid glands Mesoderm Blood, muscle, bones, heart, urinary system, spleen, fat

28. Lineage restriction: differentiation into specialized cells Pluripotent Multipotent Totipotent brain skin Ectodermal cell bone marrow Zygote ES cell Mesodermal cell heart gut Endodermal cell progenitor cells differentiated cells

29. The hematopoietic system as an example of lineage restriction Multipotent stem cell Progenitor cell Differentiated cell [Image taken from Gilbert’s “Developmental biology”, 8th edition, Sinauer].

30. Summary Immature (undifferentiated) cells Mature (differentiated) cells Differential gene expression Differential signaling pathways Fertilization Early embryogenesis Origin of ES cells Lineage restrictions 29

31. Intro to Developmental Bio: Concept Mapping Terms Create a concept map using the key concepts from today’s lecture. You should include (but are not limited to) the following terms/concepts. Due by ___date_____: 30