Devbiol-Le-1-Notes (1).pdf, Developmental Biology Notes

DEVBIOL: Developmental Biology Lecs 1-4: Intro, Gametogenesis, Fertilization, & Cleavage to Blastulation Dr. Gliceria Ramos Term 2 AY 2022-2023 Transcribed: Berana, Capistrano, Dalapo, Delos Angeles, Juachon, Narbonita, Paclibar, Sartorio, Silao, Soliman...

DEVBIOL: Developmental Biology Lecs 1-4: Intro, Gametogenesis, Fertilization, & Cleavage to Blastulation Dr. Gliceria Ramos Term 2 AY 2022-2023 Transcribed: Berana, Capistrano, Dalapo, Delos Angeles, Juachon, Narbonita, Paclibar, Sartorio, Silao, Soliman now more focused on processes Outline Embryo: early stage when the 1. Introduction developing animal does not yet 1.1. Concepts in resemble the adult of the species Developmental Biology 1.2. History and Persons DEVELOPMENTAL BIOLOGY Involved Embryology evolved into 1.3. Approaches in the Study developmental biology, the analysis of Embryology 2. Gametogenesis of biological development 2.1. Phases of The study of how a (simple) single cell Gametogenesis becomes a (complex) multicellular 3. Fertilization embryonic stage 3.1. Major Events in Study how (simple) individual cells Fertilization differentiate into specialized cells (in 4. Cleavage to Blastulation 4.1. Functions/ structure and function) Accomplishments of Cleavage Division 4.2. Factors influencing Cleavage Pattern LECTURE 1: INTRODUCTION TO DEVELOPMENTAL BIOLOGY DEVELOPMENTAL BIOLOGY AND EMBRYOLOGY Developmental Biology: came from embryology SIGNIFICANCE OF DEVELOPMENTAL BIOLOGY Embryology: study of embryos To understand the normal and ○ traditional definition: abnormal development (descriptive) study of structural changes in embryonic development (from a zygote to 2, 16, 32, etc. cell stages to a body form) To understand better the mechanisms of such development The points above serve as foundations for ○ contemporary definition: the ff.: study of developmental Development of new techniques for processes of integrated prenatal diagnoses and treatments complex phenomena; it’s Therapeutic procedures to circumvent problem of infertility  Interventions to prevent birth defects Gametogenesis: generation of and to address abnormalities specialized cells involved in Get involved with stem cell research! fertilization – gametes Oogenesis: generation and All the foundations above then leads to: maturation of oocytes Improvement of prenatal development Spermatogenesis: generation and Improved long-term postnatal effects maturation of spermatozoa DEVELOPMENTAL BIOLOGY SCOPE MAJOR ACCOMPLISHMENTS OF ONTOGENETIC DEVELOPMENT Generation of cell number (growth) Cellular diversity within generation (differentiation) ○ From embryonic stem cells (early cleavage cells) giving rise to different cell types in the body Cellular order within generation Usually, embryology and (morphogenesis) developmental biology starts with ○ Giving rise to the general fertilization and ends with birth body form of an organism (mammals), hatching (avian and specific to the species it reptilians), or metamorphosis belongs to (amphibians). The ends are Continuity in life convenient landmarks in the continuing process of development MODES OF ONTOGENIC Development: series of uninterrupted DEVELOPMENT correlated events MOSAIC DEVELOPMENT ○ Types: Where the fate of a cell depends ○ Ontogenic - development upon the specific cytoplasmic of a new individual determinants in the zygote galned during from a fertilized Cytoplasmic determinants are cell division oocyte in the sexual unequally or asymmetrically reproduction distributed or apportioned to from the budding off daughter cells during cell division from a parent Example in Molluscs (snails, organism in asexual gastropods, and bivalvia) reproduction ○ Phylogenetic: evolutionary development of a species development from a simple life form to more complex forms of life Usually, in embryology and developmental biology, we start with gametogenesis  MOSAIC DEVELOPMENT: If a part of the embryo is removed, HOW IT WORKS certain cell types would be lacking in later stage of development If a blastomere is isolated, it cannot develop REGULATIVE DEVELOPMENT Where the fate of a cell depends upon interactions with neighbor cells, not by what piece of cytoplasm it has acquired during cell division Cell to cell interactions - involve In vertebrates like molluscs, they signaling factors which have an use mosaic mode of development influence on the development of a Orange cells are the embryo. Cells cell, its developmental pathway 1, 2, 3, 4 vary in the cytoplasmic The fate of the cells are not limited determinants they gained during early. Cells can give rise to any cell cell division type to the body, they have Cell 1 was able to acquire totipotential or unlimited potential cytoplasmic determinants that will Characteristic feature of guide its development into a vertebrates, but in combination muscular foot with mosaic development Whereas, Cell 3 was able to REGULATIVE DEVELOPMENT: HOW IT WORKS acquire cytoplasmic determinants that will guide its development pathway into a visceral mass If all were intact (Cells 1-4), this will result in the formation of a normal larva But, if one of these cells were excised early on, so three were left. This will not grow into a normal In this, one of the blastomeres was larva, but into a defective larvae. excised. The excised blastomere It might lack the muscular foot became a normal larva, just the because the cell for it (Cell 1) is same with the three remaining. excised. Removal of one is compensated by Each cell was marked to follow a the remaining cells so it can still certain development pathway. form complete normal larvae or Cells have a limited development embryo potential. When a blastomere is isolated early in cleavage, it can form a Remember, mosaic development new complete individual is where the fate of a cell depends primarily on cytoplasmic determinants that are unequally distributed to the cleavage cells!  COMPARING AND CONTRASTING neurulation is the laying MOSAIC DEVELOPMENT AND down of the body axis or REGULATIVE DEVELOPMENT pattern formation Morphogenesis - body formation in creating the general body plan of the embryo characteristic by the species it belongs to Cell differentiation - pattern formation and morphogenesis During mosaic development, happens because of this cleavages produce cells that are ○ By this time, the embryonic limited in their developmental cells are already in their potential. This can be destined locations in the accomplished by asymmetrically embryo distributing molecules that regulate Growth - the result of cell development. Daughter cells then differentiation, pattern formation, inherit different sets of these and morphogenesis determinants during cell division. germ layers ○ Fate of daughter cells are SCOPE OF EMBRYOLOGY X micromeres limited macromeres ○ Relies on cytoplasmic determinants that are unequally distributed among embryonic cells During regulative development, cleavages produce daughter cells with equivalent and relatively Gametogenesis unlimited cell fates. Interactions ○ Spermatogenesis and between cells regulate oogenesis development. Fertilization ○ Fate of daughter cells are ○ Steps and prevention of unlimited polyspermy ○ Relies on cell to cell Cleavage interactions, involving cell ○ Patterns and influences to cell signals and its fuel for onset Blastulation bodyaxes or KEY PROCESSES OF DEVELOPMENT Gastrulation ~patternformation Cleavage division - precedes Neurulation/Organogenesis -not yet gastrula and neurula functional ○ Precursor cells of an organ Pattern formation (body axes Histogenesis (tissue formation) formation, starts actually and Differentiation 2 specific functions gastrula between gastrula and neurula) Pattern Formation and specialization stage ↓ ○ Anterior-posterior axis pattern formation Morphogenesis due to cell differentiation Ibody formation) ○ Dorsal-ventral axis Fetal Growth axes ○ Left-right axis ↓ neurula stage ○ The transition from gastrulation to  FUNDAMENTAL QUESTIONS structure of the organism ADDRESSED IN DEVELOPMENTAL and its component parts. BIOLOGY PROBLEM OF GROWTH (CELL How does the fertilized egg gives DIVISION) rise to an adult? How are cell division and growth How does that adult provides yet tightly regulated? another body? What dictates an embryonic THE QUESTIONS SUBDIVIDED INTO cell…? GENERAL PROBLEMS OF PROBLEM OF REPRODUCTION DEVELOPMENTAL BIOLOGY: How are reproductive cells set Problem of differentiation apart during embryonic Problem of morphogenesis development? Problem of growth Only the germ cells pass chicken Problem of reproduction characteristics on to the offspring before Problem of evolution ○ “Mother hein is only the egg Problem of environmental egg’s way of making integration another egg”... Samuel Butler PROBLEM OF DIFFERENTIATION: PROBLEM OF EVOLUTION How does the same genetic How do changes in development information result in different cell create new body forms? And what types? changes are possible? How can the fertilized egg Why is the distinction between generate different cell types? analogous and homologous * structures important? THE QUESTION OF ENVIRONMENTAL INTEGRATION How is the organism’s phenotype influenced by the environment? The differences in the color of butterfly wings is related to the Zygote becomes blastula which exposure, during their caterpillar has now micromeres and stage, to varying temperature and macromeres. Blastula becomes day length. More intense the gastrula, where germ layers temperature and exposure to day (ectoderm, mesoderm, endoderm) time, the more vivid colors they exist. become. PROBLEM OF MORPHOGENESIS: How do cells form ordered IMPORTANT BASIC CONCEPTS IN structures? EMBRYOLOGY/DEVELOPMENTAL How are cells positioned in the BIOLOGY right place? At the right time? The ff. was not explained. Just How form and pattern emerge from enumerated (for now): the simple beginnings of a fertilized 1 Concept of guidelines egg? Concept of 2 fate, 3 potency, 4 determination ○ Morphogenesis be investigates how this Concept of capacity and regulation of cell fates competence contributes to the form and  5 Concept of Embryonic induction 7 Concept of 8Regulation Concept of Inevitability 10 Concept9 of Differentiation The Hox (homeobox) genes I CONCEPT OF GUIDELINES Above are line drawings of Guidelines - directive influences amphibian/fish oocytes on embryonic development Maternal factors are the maternal -come-off as: mRNA. But these are silenced or ○ Preformed Guidelines not being expressed as proteins, present right at the waiting for their activation. start of ontogeny, Oocytes (amphibian and fish even before oocytes) have distinct poles: fertilization animal and vegetal refers to maternal ○ Vegetal pole is the one with factors as a result of more yolk activation of The maternal factors are maternal color-coded (like VegT mRNA is genes/maternal purple) effect genes The red boxes are the expression ○ Progressively-formed of proteins at the and signaling Guidelines factors appear gradually in The unequal distribution of vegetal every step of pole having more maternal factors ontogeny and the animal pole having more these pertains to proteins – these are preformed cellular products as guidelines a result of zygotic genes PERFORMED GUIDELINES: MATERNAL EFFECT GENES/FACTORS Zygotic Genes - fusion of IN AMPHIBIAN AND FISH OOCYTE maternal and paternal genes that resulted from the nuclear fusion during fertilization PREFORMED GUIDELINES Maternal genes/maternal effect genes Oocyte cytoarchitecture Balbiani body (at the vegetal pole) c reformedat I the 00cyte  ○ accumulation of Egg Cytoarchitecture - study of mitochondria and the cellular structure of the egg, cytoplasmic granules (germ like its yolk distribution granules) containing Vegetal pole - end with the highest silenced mRNAs concentration of yolk ○ Mitochondrial cloud + ○ All animals have different cytoplasmic granules (germ amounts of yolk though granules) ○ Yolk - affects the cleavage Maternal mRNAs are silent and are pattern of the oocyte organized in cytoplasmic granules together with several regulatory PROGRESSIVELY-FORMED proteins responsible for their GUIDELINES post-transcriptional processing and thus translational regulation Maternal mRNAs are silent until the oocyte is activated by the sperm, in other words it is activated by fertilization. Protein expression occurs immediately Guidelines that appear gradually in In the image above, sybu and wnt6 every step of ontogeny are now translocated from the At the cleavage stage, the pattern vegetal pole closer to the animal is under maternal control. The pole where it will influence the maternal effect genes and establishment of the future dorsal cytoarchitecture is more on side of the embryo preformed guidelines In the late cleavage stage, Remember… transitioning to the blastula stage, BB (Balbiani Body) - vehicle for maternal factors become depleted. transporting and localizing When it is depleted, it switches to maternal factors to the vegetal activation zygotic genes which cortex during oogenesis by means contains the paternal genes of microtubule network and motor New proteins are expressed proteins (yellow arrows) progressively in the gastrula stage At egg activation and fertilization, and in the next stages Sybu and Wnt8 (other maternal Guidelines come in the form newly factors are expressed and) are synthesized proteins and signaling translocated to the future dorsal factors axis through microtubule-mediated transport (blue arrows) PROGRESSIVELY-FORMED GUIDELINES: ILLUSTRATION Anterior-posterior axis is coupled EGG CYTOARCHITECTURE to gastrulation ○ Gradually in gastrulation, anterior-posterior axis is being laid down ○ This is guided by developmental potential  and inducing properties of ○ Where in zygote undergoes cells in the dorsal lip of the lineage decisions to form blastopore (DLB) change placenta, yolk sac, or the with time. fetus ○ Early cells in DLB becomes anterior mesoderm which then becomes neural tissue ○ Latter cells in DLB Dependent on: becomes posterior ○ Cell asymmetries mesoderm which induces ○ Unequal cytoplasmic posterior neural determinants structures towards the ○ Inductive information lower part of the spinal cord (signaling factors) Fibroblast growth factor ○ Morphogens (chemical Bone signators substances in the morphogenetic developing embryo that proteins guide body formation) - Below is an illustration that shows how cell asymmetries resulting to blastopore different formation of cell lineages in the embryo Interleukin and insulin-like growth factor The image above is in gastrula stage, cut sagittally The opening closed by a L yolk plug (where endoderm is at) is called as the blastopore DORSAL BLASTOPORE LIP Has inducing property but cells Cell with a black nucleus is a polar must have the ability to respond body ○ It synthesizes signaling Cells are color coded. Red factors that expressed at expresses Oct4 gene at a low different areas and at times level, orange expresses Oct4 at which denotes high level progressively-forme ○ Oct4 - gene required for guidelines maturation of inner cell ○ Wnt signal activity: high in mass (ICM) posterior, low in anterior ○ Cdx2 - required for the 2 maturation of FATE trophoectoderm (TE) “What cells would become” The range of cell types that a Four cells become eight cells then particular embryonic cell can give 16 cells then the blastula stage. In rise to mammalian development, blastula stage is called as the blastocyst  early cleavage In here, you can see two distinct Totipotent - has total potential to populations of cells ICM and TE give rise to any cell type (trophectoderm) ○ Blastomeres are totipotent, Red, which expresses Oct4 at a if one is excised it has the low level, divides asymmetrically. potential to any cell rise up While orange divide to a complete embryo asymmetrically (regulative mode of What happens with Red is that it development) has small and large cells which Pluripotent - inner cell mass (ICM) compose the ICM. Large ones on cells the outside, small cells inside. ○ ICM cells cannot create Cell asymmetries influence the fate yolk sac and amnion and of the cell! other extraembryonic 3 membranes as it is POTENCY pluripotent; however, is The ability of a cell to follow a capable of self-renewal developmental pathway ○ Can differentiate into any Embryonic stem cells - body tissue, but cannot unspecialized; can undergo support full development of unlimited self-renewal (totipotent) the entire organism/embryo Multipotent - cells that can differentiate into cell types within a given lineage ○ Hematopoietic stem cells in the bone marrow can give rise to blood cell lineages such as RBCs and WBCs Unipotent - fully specialized, can generate its own specific type ○ Examples: Gut cells’ epithelial layer, they renew itself because of stomach acid Stratum basale, inner layer of the epidermis, undergoes active mitosis to generate new skin cells but cannot be liver cells 4 DETERMINATION Determination - gradual commitment to a certain cell fate; geared to follow a certain developmental pathway  and on the other hand, the ectoderm has the competence to respond to the evocative influence ○ Illustrating primary induction Pathway 1 (and 2), primary germ layers are generated from inner cell mass. ICM cells are also embryonic stem cells as they are self-renewal During embryonic development, Pathway 3, primordial germ cells primary induction is not the only are embryonic stem cells as they type of interaction among cells; are also self-renewal. These cells primary is followed by secondary can become embryonic germ cells, type of induction, followed by so they are pluripotent reciprocal interaction, followed by 5 epithelial mesenchymal interaction EMBRYONIC INDUCTION – which will be discussed Evocative influence of cells, within eventually a cell or a cell with a neighboring cell, and between cells 7 6 CONCEPT OF REGULATION CAPACITY & COMPETENCE Regulative vs mosaic development L ○ Regulative development ectoderm = the potential of a cell is much greater than what is C chordamesoderm indicated in its normal fate; in other words, has One of the developmental unlimited potential (if landmarks, most distinguishable controlled exclusively by feature of a gastrula, is the inductive signals) formation of a dorsal lip of ○ Mosaic development = blastopore, which will have embryonic cells can eventually the chordamesoderm develop only according to (CM), which has very powerful their early fate; their fate is inducing properties determined very early (if controlled exclusively by If this were to be traced, this 8 cytoperminants( chordamesoderm (CM) can act on CONCEPT OF INEVITABILITY the overlying ectoderm to form the Phenomena that will take place precursor cells of the central and the embryo cannot evade from nervous system, starting out as the this neural plate stage Ex.: ○ This goes to say that the ○ Apoptosis (programmed chordamesoderm has the cell death) evocative influence on the ○ Biological clock ectoderm; or the mesoderm ○ Intracellular clock has the capacity to induce,  ○ Presence of normal (Muscles), MG homeobox genes (midgut), HG ○ Positional information (hindgut) (purple) ○ There are undifferentiated FUNCTIONS OF PROGRAMMED CELL embryonic cells collectively DEATH (PCD) DURING DEVELOPMENT termed as imaginal discs Novel structures raised from undifferentiated cells termed imaginal discs (various colors) ○ Development fates and locations are shown in the adult PCD also controls cell number by deleting cells which fail to partner ○ A mechanism that operates during embryonic development of the peripheral nervous system (neurons to target cells) PCD eliminates dangerous and One of the functions is to regulate abnormal cells such as sculpting of the structures autoreactive lymphocytes ○ Formation of sculpting the PROGRAMMED CELL DEATH (PCD): ILLUSTRATION heart, creating the auricles and ventricles ○ Sculpting the innear ear with the ossicles and the labyrinth structures inside PCD regulates proper structure sculpting by eliminating interdigital-webbings. ○ In adulthood, the PNS development: interdigital-webbings are ○ Neurons are overproduced already carved ○ Survival depends on ○ The cells in between the competition for limited digits were eliminated by amounts of programmed cell death survival-promoting factors mechanism produced in target tissues Drosophila melanogaster (2nd ○ Quantitative matching of example of the image) neurons with their targets ○ At metamorphosis: Apoptosis adjusts the number of Larval structures nerve cells to match the number of are destroyed: SG target cells that require innervation (salivary glands) M BIOLOGICAL CLOCK: THE SOMITE SEGREGATION CLOCK Somites - repeated structures in embryogenesis ○ Paired muscles on the dorsal side of the body ○ Ex. multifidus spinae, coccygocostalis(?), Oligodendrocytes - one of the iliocostalis group of neuroglial cells that form ○ Generated sequentially the myelin sheath of nerve cells First pair somite, outside the CNS second pair somite, ○ Components of its third pair somite cell-intrinsic timer: ○ Generated because of the Signaling factor expression of a specific (PDGF-platelet- group of genes that derived growth oscillates in a 2-hr cycle factor) serves as (works as the biological timer component clock) and measures 7 oscillates in a elapsed time 2-hr cycle Effector (TH-thyroid hormone) stops cell division and initiates differentiation at appropriate time; also controls spatial Notch signaling via a transcription temporal gene factor Hes7 is the central expression mechanism for generation of (expression of oscillatory gene expression genes at the right ○ Notch signaling is a location at the right signaling involved in time) fine-tuning formation of p27/Kip1 - a somites, activating specific cell-cycle inhibitor group of genes accumulates in the precursor cells as CELL-INTRINSIC TIMERS they proliferate Intracellular developmental It seems likely that similar timing programs that change precursor mechanisms operate in other cell cells over time lineages of the embryo Ex. those that stop mitotic division 9 HOMEOBOX GENES The master developmental control genes Act at the top of genetic hierarchies regulating aspects of  morphogenesis and cell Homeobox occur in clusters differentiation animals (clusters of genes; vary in number) The homeobox has a DNA Conserved from invertebrates to sequence containing about human 180-nucleotide sequence In invertebrates and vertebrates, (contained like a box, hence the there is a correlation between the term) position of Hox genes in the cluster Encodes DNA-binding proteins (60 and their expression pattern along amino acid homeodomain) the anterior-posterior axis of the ○ Homeodomain act as body transcription factors that 10 can control and regulate CONCEPT OF DIFFERENTIATION the activity of the genes underneath them ○ Regulate gene expression ○ Control aspects of morphogenesis and cell differentiation Hox genes in normal development 1) Patterning embryonic structures, with segmentations, such as the axial skeleton 2) Patterning of the limbs, the genital and digestive tracts 3) Craniofacial morphogenesis and the development of the nervous system – prosencephalon, mesencephalon, rhombencephalon might occur from posterior axis Homeobox genes are responsible for specifying cell identity and The single cell is a zygote which positioning during embryonic embarks into a series of mitotic development – craniocaudal division forming the cleavage or morphogenesis morula ○ Morula: solid ball of cell Since it is an ordinary mitotic division, the daughter cells are genetically equal (contains identical copies of DNA) Following the developmental pathway of the two daughter cells, the left daughter cell proceed to form neuron, and the other proceeds to form epithelial cell  DIFFERENTIAL GENE EXPRESSION development are DNA methylation and Histone acetylation Affects the differential expressions of Pluripotency-associated genes & Developmental genes ○ Pluripotency-associated genes are highly expressed during the pre-implantation stage Cells are pluripotent Pluripotency genes must be turned on and must be expressed at a Differential Gene Expression: They higher rate vary in the subset of gene they compared with express developmental ○ The expression of genes neuron-specific and ○ Developmental genes are epithelial specific gene highly expressed during ○ For a cell to become a later stage of development neuron cell, only a specific Genes are expressed differently in a subset (upper portion) of specific timeline the genome is expressed ○ In Pluripotency-associated (neuron-specific gene) genes’ timeline, cells are ○ For a cell to differentiate pluripotent into an epithelial cell, it Pluripotency genes needs to express the lower must be turned on subset of the gene and must be (epithelial specific gene) expressed at a higher rate compared with developmental genes ○ Since developmental genes are not needed during the Pluripotency-associated genes’ timeline, they are Regulated by mechanisms turned off ○ DNA methylation ○ In developmental genes, ○ Histone modifications: can since embryonic cells have be methylation or their determined fate, the acetylation pluripotency genes are H3K27 methylation turned off H3K4 methylation The turning on and off of genes rely on ○ Most common mechanisms the three mechanisms (DNA during embryonic  methylation, H3K27 methylation, and H3K4 methylation) EARLY HISTORY ARISTOTLE Harvey contributed to the idea “omne vivium ex ovo” promoting the existence of ova ○ “omne vivium ex ovo” = all life comes from an egg HAMM & LEUWENHOEK (1677) 1st systematic study of embryos ○ He was the first one to do the systematic study of embryos Recorded different stages in the development of the chick embryo Recognized that there are multiple ways that organisms reproduce HISTORICAL BACKGROUND Observed human sperm Important people in the history of embryology ANTONI VAN LEEUWENHOEK Reinierde (1632-1723) WILLIAM HARVEY & GRAAF (1672) Discovered animalcules in semen Argued that a tiny preformed human was already present in these Cat ovary animalcules 1st to describe the ovarian follicle His discovery supposedly led to the ○ “Graafian follicle” prereformanist period Coined the Theory of Preformation  LAZZARO SPALANZANI & CASPAR unfertilized chick embryo to the FRIEDRICHWOLFF (1733-1794) formation of the tiny entity with proliferating blood vessels Spalanzani: The presence of both male and female sex products, and TWO EARLY VIEWS HOW ANIMALS both of these products are necessary DEVELOPED FROM AN EGG for the initiation of ddevelopment Theory of Preformation ○ There is no profound entity Theory of Epigenesis inside the sex cells Wolff: Embryological development THEORY OF PREFORMATION occurs thru progressive growth and All parts of the future embryo were development imagined to be already in the egg ○ The organism (human) but these were transparent, folded, goes through progressive small and cannot be seen growth and development The embryo is preformed in the ○ Laid the Epigenetic concept egg Spalanzani and Wolff coined the Theory of Epigenesis LAZZARO SPALLANZANI (1729-1799) Successfully performed the first artificial insemination (using frog eggs) THEORY OF EPIGENESIS CASPAR FRIEDRICH WOLFF The egg does not contain a (1738-1794) preformed embryo but only the First person to demonstrate materials of which the embryo is morphogenesis formed Saw the development of structures ○ Stressing the importance of out of structureless materials sex products He made use of chick embryo, The embryo is formed by tracing the formation of structures progressive series of events out of structureless materials from  ○ Structureless, gradual acquisition of morphology ○ Then, formation of the embryo with the blood vessels Amnion- one of the extraembryonic membranes, enclosing, protecting the developing embryo Karl Ernst Von Baer (1828): "Father of Embryology" laid the Von Baer's Law Von Baer's Law: The more general features that are common to all members of a group of animals developed earlier than the more special features which distinguish the various members of the group. VON BAER’S LAW 1. Embryos look similar during early stages; all have gill slits, tail, somites, notochord which forms KARL ERNST VON BAER (1792-1876) the scaffolding for the formation of the vertebral column 2. Uninterrupted series of correlated events 3. Differences become gradually more distinct Made significant strides in descriptive embryology searching for the VITAL FORCE the first person to note the many similarities between the embryos of vertebrates particularly amniotes  ERNST HAECKEL (1834-1919) ○ If you hate being a chick, blame your mother for being a hen. Leading authority in embryology during the late 1800s Laid down the concepts of : "ONTOGENY RECAPITULATES PHYLOGENY" ○ Individual development follows the development of the ancestor species Haeckel with Johannes Muller re-interpreted Von Baer's Law in the light of evolutionary theory LAW OF BIOGENESIS There is only every living thing in Law of Biogenesis: Ontogeny is a its individual development passes shortened/ modified recapitulation through series of constructive of phylogeny stages like those of the evolutionary development of the "ONTOGENY RECAPITULATES race to which it belongs. PHYLOGENY" Individual development progresses CHRISTIAN PANDER through the adult stages of the Christian Pander: existence of organism's ancestors. germ layers ○ Meaning, there is only one ○ Known for identifying the way to upbuild a given primary germ layers: organism and that endoderm, ectoderm, organism will do it in mesoderm essentially the same way Nucleus of chicken egg = nucleus as his ancestors did. of Pander ○ An individual can do this more quickly, more economically than his ancestors by making use of something nature has provided for it. ○ There’s no other way.  Rathke's pouch HEINRICH RATHKE which future parts of a developing Affinity between embryos of higher organism are determined and lower vertebrates (because of the 1880 - He began a program which presence of pharyngeal pouches) experimented on frog eggs to ○ every species forms gill elucidate mitotic cell division slits at certain development stages INTEGRATION OF GENETICS INTO Difference between a pouch and a EMBRYONIC DEVELOPMENT cleft (pharyngeal) ○ Pouch - side lined with AUGUST WEISMANN endoderm; outpocketing of The Germplasm theory the oral/pharyngeal cavity ○ Self-reproducing ○ Cleft - side lined with determinants as guiding ectoderm force for morphogenesis ○ The offspring does not inherit its characteristics from the body (soma) of the parent but only from the ferm cells (egg and sperm) ○ The germ cells are not influenced by the body that bears them Chromosomes OSCAR & RICHARD HERTWIG Figure: Representation of organism with Oscar - 1st to observe sexual pharyngeal pouch. Endoderm (green), reproduction Ectoderm (blue), Mesoderm (lavender) ○ Demonstration of fertilization (sea urchin) SCHLEIDEN & SCHWANN (1839) ○ Existence of polar bodies Laid the foundation of Modern ○ Advances in the Embryology and Histology with the understanding of meiosis advent of the microscope Matthias Schleiden - 1838 ○ German Botanist, concluded that all plant parts are made of cells Theodor Schwann - 1839 ○ German Physiologist, close friend of Schleiden, stated that all animal tissues are composed of cells HANS SPEMANN & HILDE MANGOLD WILHELM ROUX (June 9, 1850 - Sept. 1935 Nobel Prize for Medicine 15, 1924) Concept of Embryonic Induction Founder of experimental embryology Organizer effect (dorsal lip of He believed that mitotic cell division of blastopore) the fertilized egg is the mechanism by MODEL ORGANISMS THAT WERE THE BASIS OF OUR KNOWLEDGE ON EMBRYOLOGY Wilhelm Roux - Frog & Sea Urchin Thomas Hunt Morgan - Fruitfly ICSI: intracytoplasmic sperm injection (1990) ○ The first ICSI baby was born in 1992. MILESTONES IN DEVELOPMENTAL ○ This is the technology of BIOLOGY choice in cases of extreme infertility. ○ Introducing directly the sperm into the oocyte cytoplasm. ○ They make use of a micromanipulator. APPROACHES IN THE STUDY OF EMBRYOLOGY Descriptive Embryology ○ Involves detailed study of structure and arrangement of minute internal organs. ○ Concerned with explanations of structural features ○ Investigates when and how a process is carried out. Comparative Embryology ○ Establishes relationships between developmental stages Experimental Embryology ERA OF ART (ASSISTED REPRODUCTION TECHNOLOGY) ○ Finds out why a process is carried out at a specific EDWARD & SLEPTOE (1978) time in a specific manner. In-Vitro Fertilization (IVF) ○ Ascertains which activate 1st test tube baby - Louise Brown or regulate the development process  Chemical Embryology WHERE DOES IT TAKE PLACE? ○ Involves biochemical Oogenesis and folliculogenesis: investigations of the generation and maturation of the embryo; ushered in oocytes which goes along with the molecular biology. formation of investments around Teratology the oocyte. The increase in the ○ Study of embryonic follicle layers. malformations ○ It takes place in the ovary. Reproductive Embryology ○ Techniques in fertilization, implantation of embryos ○ Endocrinology of reproduction and embryonic development ○ Concepts on conception and contraception. Developmental Biology ○ Broader approach from embryonic development to Spermatogenesis: takes place in postnatal development the seminiferous tubules of the Normal growth testis. Metamorphosis Regeneration Tissue repair LECTURE 2: GAMETOGENESIS SERIES OF CHANGES/PROCESSES Gametogenesis consists of a series of changes/processes that gets to transform the primordial germ cells into specialized sex cells. Primordial germ cells → primordial follicles → spermatogonia → Frontispiece of William Harvey’s spermatozoon → oocyte book on The Generation of Animals Primordial germ cells: All living beings come from an egg. specialized generative cells, (Omne vivum ex ovo) collectively called germplasm. ○ In the case of mammals, there is no obvious  readily identifiable Just one ovum from one germplasm. oogonium. ○ In the case of animals other In the case of one than mammals, spermatogonium that completes germplasm can be easily the process, there are four sex distinguished as a cells that are produced, the cell-dense association of spermatozoa. mRNAs and mRNA-binding * proteins such as VASA, gonial stage DAZL, and MILI homologues. This can be stained. primary sex cell This can be easily stages identified in other animals' oocytes. secondary sex cell stages GENERAL OVERVIEW OF GAMETOGENESIS Gonial Stage: Oogonium and spermatogonium undergo one more round of growth and differentiation and beget primary PHASES OF GAMETOGENESIS sex cells: primary spermatocyte 1. Generation of germ cells and migration and primary oocyte. to the gonads The primary sex cells stages are 2. Multiplication of germ cells in the the ones embarked in the first gonads (mitosis) meiotic division (meiosis I). 3. Reduction in chromosome number by Primary sex cell stages that half (meiosis) complete the first meiotic division 4. Maturation and differentiation beget the secondary sex cell stage: secondary spermatocyte 1.GENERATION OF GERM CELLS and secondary oocyte. These cells AND MIGRATION TO THE GONADS enter the second phase of meiosis Germ cells arise outside the (Meiosis 2). They get spermatids gonads for the secondary spermatocyte Recognizable at an early stage of and mature oocyte for the development in some of the secondary oocyte. vertebrates, like in the case of In the case of oogenesis, there are anuran amphibians (frogs) formations of polar bodies such that at the end of one round of oogenesis, there is only one functional sex cell produced.  ANURAN AMPHIBIANS EXTRAEMBRYONIC MEMBRANES Amnion - extraembyonic membrane most intimate with the developing embryo Yolk sac - color-coded yellow in the image below Allantois - yellow as well ○ In anuran amphibians, in its Chorion - pink in the image below unfertilized egg, it can easily be identified in the PGCs MIGRATION TO THE GONADS circumscribed region closer to the vegetal pole ○ If you traced it after fertilization to the cleavage stage to the blastula stage, it is still midway between the animal pole and vegetal pole ○ In gastrula stage, it Recall that a blastocyst has an becomes unidentifiable in ICM and the trophoectoderm for the endoderm layer mammals ICM forms the embryo proper. AMNIOTES: BIRDS, Embryo proper will split into REPTILES, MAMMALS epiblast and hypoblast Primordial germ cells (PGCs) Take note of the fore gut (farthest can be identified in the yolk sac away from the allantois), mid gut endoderm because of the (closes to the yolk sac’s center), presence of alkaline phosphatase and hind gut (closest to the PGCs are large sized, high content allantois) in the picture above of alkaline phosphatase Allantois is an outpocketing of ○ alkaline phosphatase - the hindgut marker for PCGs In humans, by 3rd week, 24-days In the case of humans, PGCs can post fertilization, the PGCs wander be recognized in the yolk sac in an amoeboid motion encoderm at 24-day post ○ From the primary ectoderm, fertilization they go down to the yolk ○ Roughly 3 weeks of sac wall, and exit to the embryonic development allantois HANNEL & EDDY (1986) In the mouse, PGCs originally reside in the epiblast of gastrula GINSBURG (1990) Found that the PGCs are localized in the extra embryonic mesoderm posterior to the primitive streak in a 7.25th day old embryo  Red dots represent the primordial PAANO ANG MOVEMENT germ cells (PGCs) NG PCGs? (SUMMARY) The image above shows that the From the yolk sac endoderm, PGcs are now extra embryonal in passing through the allantois, the yolk sac wall, they get moves through the hindgut wall, separated from the rest of the passing through the dorsal embryo. They are now outside the mesentery, and to the left and embryonic body right genital ridges From here, PCGs will migrate from Mema mnemonic: YAHDG (Yah the the extra embryonic mesoderm Dog) of the allantois back into the embryo proper Rodents (as mammalian model) Development of PGCs depend on signals. If the image above is going to be ○ These signals based from cut, by the blue line below. A the studies is BMP (bone cross-section passing through the morphogenetic protein) notochord , hindgut, mesentery signal factor. suspending the hind gut, and the Radical expression of pluripotency left and right genital ridges markers. ○ Oct-4+, NANOG, and SOX2 genes Studies strongly suggests: 2 ○ BMP (bone morphogenetic I protein) signal factor Then it will look like this: Z Here comes, the signals. It acts as on the 2 Precursor PGCs, located at the posterior of the epiblast. The precursor PGCs increasing number. And then, the big number they Notochord is the blue migrate by amoeboid motion into Hind gut is the orange the yolk sak, and at the allantois Dorsal mesentery suspending the they get specified. hindgut is the small bridge-like What is meant by specification? structure that connects hindgut to ○ They get specified, kung the genital ridges baga magkakaroon na sila Genital ridges are the future sex ng genetic markers (tattoo), organs which means sila lang  pwede maging germ cells; The Life Cycle of Murine Germ Cells they are marked. ○ They also start to express pluripotency gene markers, such as the Oct-4+, NANOG, and SOX2 genes, which are among the pluripotency gene markers of the primordial germ cells. If na sa Allantois na sila that means specified na sila. The next After fertilization: zygote → step is to determined a certain cleaving forming a 2-cell embryo developmental pathway, that is to → giving rise to the blastocyst become sex cells. (middle row). ○ Blastocyst consists of the Proposed Migration Pathway trophectoderm (green), and the inner cell mass (orange). After implantation: inner cell mass/ epiblast (orange) → cavitation and epithelializes At gastrulation: a new germ layer → the mesoderm (blue). ○ The primordial germ cells (PGCs) are specified at E7.25 (black arrow) at the base of the mesodermal allantois. Thereafter, the PGCs migrate So this is the proposed migration thrpugh the forming hindgut → pathway. From the primary dorsal mesenterium → genital ectoderm of the developing ridges. embryo, to the yolk sac endoderm, Upon sex determination: female to the allantois (passing to the (oogonia) and male hindgut wall), to the dorsal (spermatogonia) germ cells follow mesentery, and to the left and right different fates. genital ridges in the embryo proper. So in the yolk sac endoderm, they become extraembryonal. In the allantois, they become specified at the base, which means they start to get their markers, nakakatattoo na sila. They now have their genetic markers, so they are now destined to become the sex cells.  The primordial germ cells undergo migration, from the left and to the right genital ridges, but not all of them can reach their destinations (since some of them nawawala), and those who went astray can’t reach their destination, in result they develop a teratoma. It is a bizarre form of tumor, and it looks like a mixture of different types of differentiating cells. The different timing and duration of So this is oropharyngeal teratoma, meiosis in human males and and they are fatal. females. a.) in males, meiosis are They undergo mitotic division. initiated continuously from puberty on, and each meiosis is completed within a few weeks. b.) in females, all meiosis begin prior to birth, but normally only one oocyte completes meiosis during each reproductive cycle between puberty and menopause. In males, mitosis is continuous, during embryonic development, after birth, all throughout life; mitosis is continuous. In females, mitosis stops early, The 2nd phase, proliferation of mitosis can only proceed up to a germ cells, by ordinary mitotic fifth month of embryonic division. development, in a female embryo. So from their journey from the yolk By the time it is at its seventh sac and into the allantois, to the month embryonic development, all hindgut, to dorsal mesentery, and of the oogonial have already enter all through their journey the the first meiotic division, so primordial germ cells are oocytes na sila. undergoing continuous mitotic cell division. CHANGES IN HUMAN If it is a female embryo, the PGCs GERM CELL NUMBER will divide to form an oogonia, the PGCs will divide to form a spermatogonia in males. These are the mitotical active sex cells. Ordinary mitotic division are basically the same, they only differ in patterns. Mitotic patterns in the gonads differ widely between males and females. The peak of germ cells (in 7 million) is at 7 months of  pregnancy before birth. All the Third phase is where the oogonia already became the chromosome number was reduced primary oocytes at the 1st meiotic by half (meiosis) arrest at birth So at birth, all female mammals have in her ovary primary oocytes. GAMETOGENESIS This is why oogonia was not asked Random Trivia: All living beings for us to be located in the cats’ come from an egg (Frontispiece of ovaries. At time of birth, wala nang William Harvey’s book on The oogonia sa ovaries ng girls Generation of Animals) At birth, 2 million of those oocytes, only around 400,000 oocytes are GAMETOGENESIS: DEFINITION retained at puberty. Gametogenesis - generation and These 400,000 oocytes, only maturation of the sex cells 400-500 oocytes can complete ○ Oogenesis and maturation during the reproductive folliculogenesis - age, between the onset of puberty generation of oocytes and and the start of menopause formation of surrounding follicle cells MATHEMATICAL PART -takes place in ovary of The onset of puberty is 13 years females old ○ Spermatogenesis in the The onset of menopause is 48 seminiferous tubules - years generation and maturation Then, the reproductive years if 35 of the spermatozoa years (48-13) -takes place in the testis of 35x12 months, which is equal to males 420 months Cross sections of cells responsible 1 oocyte will mature every month, for gametogenesis: so 420 oocytes are matured (which is within the range) Summary: No. of Fertile Oocytes = (Puberty Age - Menopause Age in Years) * 12 REDUCTION IN CHROMOSOME NUMBER BY HALF (MEIOSIS) SERIES OF CHANGES UNDER GAMETOGENESIS Primordial germ cells passing through events to become specialized sex cells, spermatozoa or ovum (mature oocytes) Primordial germ cells - degenerative sex cells that collectively called as germplasm ○ Germplasm can be distinguished easily in  lower animals other than Then they (secondary stage sex cells) mammals embark in another meiotic phase ○ No obvious germplasm in (meiosis II) and beget four spermatids mammals before and one oocyte. fertilization Why only one oocyte? Because during ○ A cell-dense association of one of the phases of meiosis I, there is mRNAs and mRNA-binding an unequal division of the primary proteins such as VASA oocyte which resulted to the DAZL, and MILI production of a secondary oocyte and homologues (found in the a polar body cytoplasm), which can be After the formation of spermatids and identified by biological a mature oocyte, specialization staining of the oocytes through differentiation into spermatozoa and ovum. OVERVIEW One spermatogonium underwent spermatogenesis results to four sex cells/spermatozoa produced One oogonium underwent oogenesis resulted to one functional sex cell produced, one oocyte/ovum PHASES OF GAMETOGENESIS 1. Generation of germ cells and migration to the gonads 2. Multiplication of germ cells in the gonads (mitosis) while in transit/migration to gonads 3. Reduction in chromosome number by half (meiosis) 4. Maturation and differentiation 1.GENERATION OF GERM CELLS AND MIGRATION TO THE GONADS Germ cells arise outside the gonads In spermatogenesis, it starts with Recognizable at early stage of spermatogonium development In oogenesis, it starts with oogonium ○ For example, in anuran First event, they undergo growth and amphibians, in the differentiation and they reach the unfertilized oocytes, germ primary stage and become primary cells can be identified as a spermatocyte and primary oocyte circumscribed region in the respectively. cytoplasm. This is the Primary stage sex cells then undergo cell-dense region meiosis I, which begets the secondary containing mRNAs and stage: secondary spermatocyte and mRNA-binding proteins secondary oocyte. ○ Upon the onset of fertilization, from one cell  stage to cleavage stage, ORIGIN OF PGCS: HOW THREE the cell dense region can FINDINGS ARE HARMONIZED be traced and the cytoplasm closer to the animal pole ○ When traced up to the gastrula, they colonize the endodermal layer of the Blastocyst, it has an inner cell gastrula mass and the trophectoderm ○ They are generated and Inner cell mass - give rise to the can be recognized at an embryo proper early stage of development -embryo proper are the blue cells ○ Germplasm, primordial sex surrounded by the amnion in the cells, kasi ay nakikita bago picture pa fertilization -at first, it will split into epiblast and ○ Ang germplasm ay hindi hypoblast ganito kadaling makita sa ○ Epiblast - where the higher animals embryo will form as the primitive streak (groove AMNIOTES: BIRDS, REPTILES, that becomes distinct germ MAMMALS layers) Primordial germ cells (PGCs) are -primordial germ cells are easily identified in the yolk sac generated first in the endoderm epiblast then migrate out PGCs are large sized, high content the embryo to the of alkaline phosphatase endodermal layer of the Recognized here as early as, for yolk sac humans, at 24-day post-fertilization Trophectoderm - gives rise to the in the yolk sac endoderm extra embryonic membranes like ○ Roughly three weeks of the placenta embryonic development -this and other components of the This is the classical notion. embryo will expand from the embryonic area to the sides, which HANNEL & EDDY (1968): IN THE MOUSE gets to form the other extra Claimed that PGCs originally embryonic membranes like the reside in the epiblast of gastrula amnion, yolk sac, allantois, and in the mouse. chorion GINSBURG (1990) In humans, by the 3rd week, the Localized the region in the extra PGCs wander in an amoeboid embryonic mesoderm posterior manner to the primitive streak in a 7.25th Main flow of PGCs: primary day embryo ectoderm to yolk sac wall to near Extra embryonic mesoderm is exit of allantois found outside the embryo. More detailed flow of PGCs: From the epiblast, primary  ectoderm, going down here in the Hind Gut - suspended by yolk sac endoderm. From the yolk mesentery sac endoderm, they will exit Genital Ridges - forming gonads passing through the allantois and of the embryo go back to the embryo proper. PGCs become extra embryonic ADVANCED STAGE OF here, when they leave via the PGCs MOVEMENT allantois, and then go back into the embryo when it reaches the embryonic body after the hindgut WHY PGCs GO OUT? Rodents (as mammalian model) ○ Development of PGCs depend on signals ○ Radical expression of pluripotency markers Oct-4+, NANOG, and SOX2 genes ○ Studies suggest BMP (bone morphogenetic Developing embryo above, its protein) signal primordial germ cells are factor colonizing the yolk sac endoderm near the attachment of the When they go out into the allantois. Allantois is then endoderm and aggregate to the connected to the hind gut. base of the allantois, they become Allantois - outpocketing of the specified hind gut They start to express pluripotency gene markers and the most known If you cut it cross-sectionally like the pic of primordial germ cells are below… Oct-4+, NANOG, and SOX2 genes Precursor PGCs are activated by the BMP, increasing in number by mitotic division When they reach the threshold population, they start to migrate away from the embryonic area, becoming extra embryonal going to From the yolk-sac endoderm,