EMBRYOLOGY Introduction to Embryology PDF

DRA. FERRARIS | A.Y. 2024-2025 | Sem 1 August 08, 2024 MODULE 1: INTRODUCTION TO EMBRYOLOGY TABLE OF CONTENTS...

DRA. FERRARIS | A.Y. 2024-2025 | Sem 1 August 08, 2024 MODULE 1: INTRODUCTION TO EMBRYOLOGY TABLE OF CONTENTS ○ Vital dyes - used to stain living cells to follow their fate. I. Introduction to Embryology ○ Radioactive labels and autoradiographic techniques A. Clinical Relevance were employed in the late 1960s. B. Brief History of embryology ○ One of the first genetic markers also arose about this II. General Embryology time with the creation of Chick-Quail Chimeras. A. Gene Transcription - Quail cells, which have a unique pattern to their B. Other Regulators of Gene Transcription heterochromatin distribution around the nucleolus, C. Induction and Organ Formation were grafted into chick embryos at early stages of D. Cell-to-Cell Signaling development. E. Key signaling pathways for development - Later, host embryos were examined histologically, III. Gametogenesis: Conversion of germ cells into male and and the fates of the quail cells were determined. female gametes - Permutations of this approach included A. Primordial germ cells development of antibodies specific to quail cell B. Chromosome theory of inheritance antigens that greatly assisted in the identification of C. Causes of birth defects and spontaneous abortion these cells. D. Chromosomal abnormalities - Monitoring cell fates with these and other E. Gene mutations techniques provides valuable Information about the F. Diagnostic techniques for the identification of origins of different organs and tissues. genetic abnormalities Grafting Experiments IV. Morphological changes during maturation of gametes ○ Provided the first insights into signaling between A. Oogenesis tissues. B. Spermatogenesis ○ Examples: V. References - Grafting the primitive node from normal position on I. INTRODUCTION TO EMBRYOLOGY the body axis to another has induced a second body axis. A. CLINICAL RELEVANCE - Employing developing limb buds has shown that if Embryology – investigates the molecular, cellular, and a piece of tissue from the posterior axial border of structural factors contributing to the formation of an one limb was grafted to the anterior border of a organism. second limb, then digits on the host limb would be Importance of Embryology Studies: duplicated as the mirror image of each other. ○ They provide knowledge essential for creating health ○ Zone of Polarizing Activity (ZPA) care strategies for better reproductive outcomes. - Posterior signaling region ○ They have resulted in new techniques for prenatal - The signaling molecule is known as Sonic diagnoses and treatments; therapeutic procedures to Hedgehog (SHH) circumvent problems with infertility; and mechanisms to prevent birth defects, the leading cause of infant The Science of Teratology (1961) mortality. ○ study of the embryological origins and causes of birth defects. Our prenatal development produces many ramifications ○ Became prominent in 1961 due to the drug thalidomide affecting our health for both the short and long term, making the study of embryology and fetal development an important - Thalidomide topic for all health care professionals. ® Given as antinauseant and sedative to pregnant women, but unfortunately caused B. BRIEF HISTORY OF EMBRYOLOGY birth defects like abnormalities of the limbs Embryogenesis ⁃ Amelia – one or more limbs was absent. ○ aka Organogenesis ⁃ Phocomelia – lacking the long bones; ○ the process of progressing from a single cell through the only a hand or foot is attached to the period of establishing organ primordia (the first 8 weeks torso. of human development) ○ W. Lenz and W. McBride Fetal Period - Two clinicians who independently recognized the ○ The period from embryogenesis until birth. association between the drug and birth defects. ○ a time when differentiation continues while the fetus grows and gains weight. - Showed that the conceptus was vulnerable to maternal factors that crossed the placenta. Early Investigations In this Century Anatomical approaches Molecular approaches have been added to the list of ○ Dominated early investigations experimental paradigms used to study normal and abnormal ○ Became more sophisticated with advances in optical development. equipment and dissection techniques. ○ Reporter genes, fluorescent probes, and other marking Comparative and Evolutionary studies techniques have improved our ability to map cell fates. ○ Scientists made comparisons among species and so ○ Other techniques to alter gene expression, such as began to understand the progression of developmental knockout, knock-in, and antisense technologies, has phenomena. created new ways to produce abnormal development ○ Offsprings with birth defects were investigated and and allowed the study of a single gene’s function in compared to organisms with normal developmental specific tissues. patterns. ○ Teratology – the study of the embryological origins and II. GENERAL EMBRYOLOGY causes of birth defects. Genomes ○ Direct embryonic development 20th Century ○ Contain all the information required to make an Experimental Embryology individual. ○ Blossomed in the 20th century Genes ○ Numerous experiments were devised to trace cells ○ Sequences of information encoded in the DNA. during development to determine their cell lineages ○ Genes code for proteins, which in turn, regulate the (e.g., observations of transparent embryos from expression of other genes and act as signal molecules tunicates that contained pigmented cells which could be to orchestrate development. visualized through a microscope). Trans #1 Group 5: SagunTH, Bugayong, Bugayong, Dacquias, Fernandez, Tallon  MODULE #1 Introduction to Embryology ANA11 Levels of Regulation of Gene Expression - Contains a specific DNA-binding domain and a 1. Transcription: Different genes may be transcribed. transactivating domain that activates or inhibits 2. Post Transcriptional Modification: DNA transcribed from a transcription of the gene whose promoter or gene may be selectively processed to regulate which RNAs enhancer it has bound. reach the cytoplasm to become messenger RNAs (mRNAs). - Transcription factors activate gene expression with 3. Translation: mRNAs may be selectively translated. other proteins by: 4. Post Translational Modification: Proteins made from the ® causing the DNA nucleosome complex to mRNAs may be differentially modified. unwind; A. GENE TRANSCRIPTION ® releasing the polymerase so that it can Genes transcribe the DNA template; and ○ Basic unit of heredity ® preventing new nucleosomes from forming. ○ Sequences of information within the DNA strand Enhancers Chromatin – complex of DNA and proteins (mostly histones) ○ Regulatory elements of DNA that activates the utilization Nucleosome – basic unit of structure of chromatin of promoters to control their efficiency and rate of ○ Each nucleosome is composed of octamer of histone transcription from the promoter. proteins and approximately 140 base pairs of DNA. ○ Can reside anywhere along the DNA strand. ○ Nucleosomes are joined into clusters by linker DNA and ○ Also bind transcription factors (through the transcription other histone proteins. factor’s transactivating domain) and are used to ○ Keep the DNA tightly coiled, such that it cannot be regulate the timing of a gene’s expression and its cell- transcribed (inactive state). specific location. Heterochromatin (or Heterochrome) ○ Act by: (1) altering chromatin to expose the promoter or ○ Chromatin in coiled state à inactive state. (2) by facilitating binding of the RNA polymerase. ○ Appears as beads of nucleosomes on a string of DNA. Silencers ○ Transcription cannot occur. ○ are enhancers that can inhibit transcription. Euchromatin ○ allows a transcription factor to activate one gene while ○ Chromatin in uncoiled state à active state. silencing another by binding to different enhancers. ○ DNA must be uncoiled from the beads to allow transcription. Figure 2. A typical gene [Retrieved from Sadler, 2015] *Drawing of a "typical” gene showing the promoter region containing the TATA box; exons that contain DNA sequences that are translated into proteins; introns; the transcription initiation site; the translation initiation site that designates the code for the first amino acid In a protein; and the 3' untranslated region that includes the poly A addition site that participates in stabilizing the mRNA, allows it to exit the nucleus, and permits Its translation into a protein. DNA Methylation Represses Transcription Methylation of cytosine bases in the promoter regions of Figure 1. Nucleosome genes represses transcription à gene silencing. [Retrieved from Sadler, 2015] Methylation silences DNA by: Illustration showing nucleosomes that form the basic unit of chromatin. ○ Inhibiting the binding of transcription factors; or Each nucleosome consists of an octamer of histone proteins and ○ Altering histone binding resulting in stabilization of approximately 140 base pairs of DNA. Nucleosome are joined into clusters nucleosomes and tightly coiled DNA that cannot be by linker DNA and other histone proteins. transcribed. Genomic Imprinting A typical gene includes the following: ○ only a gene inherited from the father or the mother is Exons expressed, whereas the other gene is silenced. ○ Regions in a DNA strand that can be translated into ○ Caused by DNA methylation. proteins. ○ Approx. 40 to 60 human genes are imprinted, and their Introns methylation patterns are established during ○ Regions interspersed between exons spermatogenesis and oogenesis. ○ Not transcribed into proteins Promoter Region B. OTHER REGULATORS OF GENE EXPRESSION ○ Binds RNA polymerase to initiate transcription Recall: Transcription site v Nuclear RNA (nRNA) Transcription initiation site Ø initial transcript of a gene Ø aka premessenger RNA Translation initiation site Ø longer than mRNA because it contains introns that are ○ to designate the first amino acid in the protein removed (spliced out) as the nRNA moves from the Translation termination codon nucleus to the cytoplasm ○ End of translation 3’ untranslated region Alternative Splicing ○ includes poly A addition site which is a sequence that ○ Removal of different introns so that exons are spliced in assists with stabilizing the mRNA, allows it to exit the different patterns. nucleus, and permits it to be translated into protein. ○ Provides a means for cells to produce different proteins TATA Box from a single gene. ○ The promoter region where the RNA polymerase binds ○ Carried out by spliceosomes (complexes of small ○ Contains the sequence TATA. nuclear RNAs [snRNAs]) and proteins that recognize ○ In order to bind to this site, the polymerase requires specific splice sites at the 5’ or the 3’ ends of the nRNA. additional proteins called transcription factors. ○ Splicing isoforms ○ Transcription factors - Proteins derived from the same gene - Additional proteins needed for the polymerase to - Aka splice variants or alternative splice forms bind to the TATA box Trans #1 Group 5: SagunTH, Bugayong, Bugayong, Dacquias, Fernandez, Tallon  MODULE #1 Introduction to Embryology ANA11 -These afford the opportunity for different cells to MECHANISM OF ACTION use the same gene to make proteins specific for When a ligand binds its receptor, it induces a conformational that cell type. change in the receptor that activates its cytoplasmic domain. Post Translational Modifications Usually, the result of this activation is to confer enzymatic activity ○ Modification processes done to a protein after to the receptor, and most often, this activity is a kinase that translation to activate protein function. can phosphorylate other proteins using ATP as a substrate. ○ Example: cleavage, phosphorylation, combination with other proteins , release from sequestered sites, or be In turn, phosphorylation activates these proteins to targeted to specific cell regions phosphorylate additional proteins, and thus a cascade of protein interactions is established that ultimately acti-vates a C. INDUCTION AND ORGAN FORMATION transcription factor. This transcription factor then activates or Organs are formed by interactions between cells and tissues. inhibits gene expression. Induction Process where one group of cells or tissues causes another Juxtacrine Interactions set of cells or tissues to change their fate. Inducer – cell type/tissue that produces signal. do not involve diffusable proteins act by signal transduction pathways Responder – cell type/tissue that responds to signal. Competence Three Signal Transduction Pathways ○ The capacity to respond to such a signal. 1. A protein on one cell surface interacting with a receptor on ○ requires activation of the responding tissue by a an adjacent cell in a process analogous to paracrine competence factor. receptor. General Mechanism: Following an initial signal from one ○ Example: Notch pathway. tissue, a second tissue is induced to differentiate into a specific structure. The first tissue constitutes the inducer and the second is the responder. Once the induction process is initiated, signals [arrows] are transmitted in both directions to complete the differentiation process. Crosstalk between the two tissues or cell types is essential for differentiation to continue. (see Fig. 3, arrows) Examples ○ Epithelial Mesenchymal Interactions - gut endoderm and surrounding mesenchyme to produce gut-derived organs, including the liver and pancreas; - limb mesenchyme with overlying ectoderm (epithelium) to produce limb outgrowth and differentiation. - endoderm of the ureteric bud and mesenchyme from the metanephric blastema to produce nephrons in the kidney ○ Interactions between two epithelial tissues (e.g., Figure 4. Notch Pathway [Retrieved from Sadler, 2015] induction of the lens by epithelium of the optic cup). Notch receptors located on one cell bind a ligand from the DSL family that are located on an adjacent cell (juxtracrine signaling), and this receptor-ligand interaction activates a proteolytic enzyme that cleaves the Notch protein to produce the activated membrane anchored Notch extracellular turn cation (NEXT). NEXT is then cleaved by an intracellular secretase enzyme that results in the release of Notch intracellular domain [NICD] that represents the active signaling portion of the original Notch receptor. NICD translocate directly to the nucleus where it binds to transcription repressors and removes their inhibitory activity on downstream target genes of the Notch pathway. Figure 3. Epithelial-mesenchymal interaction. 2. Ligands in the extracellular matrix secreted by one cell [Retrieved from Sadler, 2015] interact with their receptors on neighboring cells. D. CELL-TO-CELL SIGNALING ○ The extracellular matrix is the milieu in which cells Cell-to-cell signaling is essential for induction, for reside. This milieu consists of large molecules secreted conference of competency to respond, and for crosstalk by cells, providing a substrate for cells on which they between inducing and responding cells can anchor or migrate. ○ Collagen. Type IV collagen is a component of the Paracrine Interactions basal lamina for epithelial cell attachment proteins synthesized by one cell diffuse over short distances ○ Proteoglycans (chondroítin sulfates, hyaluronic acid, to interact with other cells etc.), Glycoproteins. Laminin is a component of the act by signal transduction pathways by basal lamina for epithelial cell attachment. Fibronectin ○ a) activating a pathway directly molecules form scaffolds for cell migration. ○ b) blocking the activity of an inhibitor of a pathway ○ Integrins. Receptors that link extracellular molecules (inhibiting an inhibitor, as is the case with hedgehog such as fibronectin and laminin. These “integrate” signaling) matrix molecules with a cell’s cytoskeletal machinery, Signal transduction pathways include a signaling molecule thereby creating the ability to migrate along matrix (the ligand) and a receptor. scaffolding by using contractile proteins, such as The receptor spans the cell membrane and has three actin. They induce gene expression and regúlate domains differentiation as in the case of chondrocytes that ○ an extracellular domain, the ligand-binding region must be linked to the matrix to form cartilage. ○ a transmembrane domain ○ a cytoplasmic domain 3. There is direct transmission of signals from one cell to another by gap junction. Trans #1 Group 5: SagunTH, Bugayong, Bugayong, Dacquias, Fernandez, Tallon  MODULE #1 Introduction to Embryology ANA11 ○ These junctions occur as channels between cells E. KEY SIGNALING PATHWAYS FOR DEVELOPMENT though which small molecules and ions can pass. It is Sonic Hedgehog (SHH): Master Gene for Embryogenesis important in tightly connected cells like epithelia of the gut and neural tube because they allow these cells Almost a master gene and when this gene’s protein product binds to its receptor Patched (Ptc), it removes patched to act in concert. ○ The junctions are made up of connexion proteins that inhibition of Smoothened (Smo) form a channel, where adjacent cells are connected. Once activated, Smo causes upregulation of the GLI family NOTE: of transcription factors that control downstream signaling v The loss of function of a signaling protein through a gene by SHH. mutation does not necessarily result in abnormal SHH is a diffusible factor with a cholesterol molecule development or death. This is due to a great amount of bound to it, and it serves as a morphogen by establishing redundancy built into the process of signal transduction. concentration gradients that regulate cell responses. It is v For example: paracrine signaling molecules often have m the addition of cholesterol that links SHH to the plasma any family members such that other genes in the family membrane. Then, a palmitic acid moiety is added to the N- may compensate for the loss of one of their counterparts. terminus and SHH becomes fully functional. SHH signaling is involved in many developmental events, including Paracrine Signaling Factors establishing the midline and left-right asymmetry and in patterning many different organs. also called Growth Differentiation Factors (GDFs) used repeatedly to regulate development and differentiation of organ systems. 1. Fibroblast Growth Factor proteins produced by these genes activate a collection of Tyrosine receptor kinases called fibroblast growth factor receptors (FGFRs). They are particularly important for: ○ angiogenesis, axon growth, and mesoderm differentiation ○ development of the limbs and parts of the brain (FGF8). 2. Hedgehog Proteins Named because it was coded for a pattern of bristles on the leg of Drosophila that resembled the shape of a hedgehog. The genes, desert, Indian, and sonic hedgehog are found in mammals. The latter, being involved in a multitude of developmental events. 3. WNT Proteins There are at least 15 different WNT genes related to the segment polarity gene. Their receptors are members of the frizzled family of proteins. They are involved in regulatíng límb patterning, midbraín development, and some aspects of somite and urogenital differentiation among other actions. 4. The TGF-b Superfamily includes the TGF-bs such as the TGF-b1 of virally transformed cells, the bone morphogenetic proteins (BMPs), the activin family, the müllerian inhibiting factor (M IF, anti- müllerian hormone), and others. Figure 5. Sonic hedgehog pathway [Retrieved from Sadler, 2015] The protein binds to its receptor Patched (Ptc), a protein that normally TGF-b members inhibits the receptor-like protein Smoothened (Smo). Upon binding of ○ are important for extracellular matrix formation and SHH to Ptc, Ptc activity is eliminated, the inhibition of Smo is removed, epithelíal branching that occurs in lung, kidney, and and Smo is activated to, ultimately, upregulate activity of the GLI family salivary gland development. (1 to 3) of transcription factors that control expression of target genes. BMP family Planar Cell Polarity: Convergent Extension Pathway ○ induces bone formation and is involved in regulating regulates movements of cells and sheets of cells in the cell division, cell death (apoptosis), and cell migration plane of a tissue, such that the cells intercalated with other among other functions. cells in such a way that the tissue elongates, a process 5. Other Paracrine Signaling Molecules called convergent extension. These include serotonin, Y-amino butyric acid (GABA), responsible for lengthening the embryo during gastrulation epinephrine, and norepinephrine. and the neural tube during neurulation. They are not just transmitters for neurons; they also provide Several genes are involved in regulating this process important signals for embryological development Serotonin (5-HT) ○ acts as a ligand for a large number of receptors, most of which are G protein-coupled receptors. It regulates cell proliferation and migration and establishes laterality, gastrulation, heart development, and other early processes of development. Norepinephrine ○ acts through receptors and appears to play a role in apoptosis (programmed cell death) in the inter- digital spaces and in other cell types. Figure 6. Planar Cell Polarity Pathway [Retrieved from Sadler, 2015] Trans #1 Group 5: SagunTH, Bugayong, Bugayong, Dacquias, Fernandez, Tallon  MODULE #1 Introduction to Embryology ANA11 WNT and its receptor FRIZZLED, CELSR, and VANGL code for transmembrane proteins. Mutations in VANGL are linked to defects in humans. DISHEVELLED that codes for a protein that acts through Rho and Rae kinases to affect the cytoskeleton and other genes regulating cell movements. The Notch Pathway Notch transmembrane receptors bind to transmembrane ligands of the DSL (Delta/Serrate/ LAG -2) family, which requires cell-to-cell contact (juxtacrine signaling) for signaling to occur. Figure 7. Parts of a chromosome In mammals, there are four Notch family members and five (Source: National Human Genome Research Institute, 2023; Science transmembrane ligands (Jagged 1 and 2 and Delta 1 to 3). Photo Library, 2023 Binding of one of these proteins to a Notch receptor causes Karyotype a conformational change in the Notch protein such that part ○ complete set of chromosomes in a species; it is a process of it on the cytoplasmic side of the membrane is cleaved. of analyzing the number and shape of a chromosome; it The pathway is very straight forward in that there are no describes the number of chromosomes and what they look second messengers involved. Thus, the cleaved portion of like under the microscope. the protein enters the nucleus directly and binds to a DNA- binding protein that normally represses transcription of Notch target genes. Binding of Notch removes the inhibitory activity of the repressor and permits activation of downstream genes Notch signaling is involved in cell proliferation, apoptosis, and epithelial to mesenchymal transitions. It is especially important in neuronal differentiation, blood vessel formation and specification (angiogenesis), somite segmentation, pancreatic (3-cell development, B- and T-cell differentiation in the immune system, development of inner ear hair cells, and septation of the outflow tract of the heart. Mutations in JAG1 or NOTCH2 cause Alagille syndrome characterized by cardiac outflow tract defects as well as Figure 8. Karyotype skeletal, ocular, renal, and hepatic abnormalities. JAG (Source: National Human Genome Research Institute, 2023) mutations have also been linked to cases of tetralogy of Fallot (a cardiac outflow tract defect). SEX SEX CHROMOSOMES PRESENT III. GAMETOGENESIS: CONVERSION OF GERM CELLS MALE XY INTO MALE AND FEMALE GAMETES FEMALE XX A. PRIMORDIAL GERM CELLS Fertilization ○ Where development begins Cell cycle ○ an ordered set of events, culminating in cell growth and ○ Process where male gamete (sperm) and female gamete (oocyte) unite into a zygote division into two daughter cells. Non-dividing cells are not considered to be in the cell cycle. Primordial Germ Cells (PGCs) ○ Where gametes are derived from ○ Formed in the epiblast during the second week, move through the primitive streak during gastrulation, and migrate to the wall of the yolk sac. During the fourth week, these cells begin to migrate from the yolk sac toward the developing gonads, where they arrive by the end of the fifth week. Mitotic divisions increase their number during their migration and also when they arrive in the gonad. ○ In preparation for fertilization, germ cells undergo gametogenesis, which include: - Meiosis - to reduce the number of chromosomes; and - Cytodifferentiation - to complete their maturation Figure 9. Cell cycle B. CHROMOSOME THEORY OF INHERITANCE Mitosis Cell Division – process by which a parent cell divides into two Nuclear division plus cytokinesis, and produces two or more daughter cells; usually a small segment of a larger identical daughter cells cell cycle. Occurs in all somatic cells – diploid (2n) cells Chromosomes – transmit genetic information to next generation. Chromatid – two copies of the same chromosome attached together. Centromere – the primary constriction where the sister chromatids are attached; holds chromatid pair together. Kinetochore – protein structure that assembles on the centromere and attaches sister chromatids to mitotic spindle that move chromosomes during mitosis & meiosis. Trans #1 Group 5: SagunTH, Bugayong, Bugayong, Dacquias, Fernandez, Tallon  MODULE #1 Introduction to Embryology ANA11 Figure 11. Phases of mitosis Meiosis Cell division that takes place in germ cells only Requires two cell divisions Figure 10. Phases of mitosis Diploid germ cells give rise to haploid (n) gametes (Source: University of Lanceister) Composed of Meiosis I and Meiosis II Table 1. Stages of mitosis Interphase - DNA replicates (G2) - Centrioles, if present, replicate - Cell prepares for division Prophase - Nuclear membrane disintegrates, and nucleolus disappears - Chromosomes condense - Mitotic spindle begins to form and is complete at the end of prophase - Kinetochores begin to mature and attach to spindle Metaphase - Kinetochores attach chromosomes to the mitotic spindle and align them along the metaphase plate at the equator of the cell. Anaphase - Kinetochore microtubules shorten, separating chromosomes to opposite poles - Polar microtubules elongate, preparing cell for cytokinesis Telophase - Chromosomes reach poles of cell - Kinetochores disappear - Polar microtubules continue to elongate, preparing cell for cytokinesis - Nuclear membrane re-forms - Nucleolus reappears Figure 12. Summary of meiotic divisions I and II - Chromosomes decondense (Source: Choi, n.d.) Cytokinesis - Plant cells: cell plate forms, dividing daughter cells Phases of Meiosis I - Animal cells: cleavage furrow forms at equator of cell and pinches inward until cell divides in two Table 2. Phases of Meiosis I Prophase I Leptotene: duplicated chromosomes start to condense Zygotene: formation of synaptonemal complex; synapsis begins Pachytene: synapsis is complete; crossing over occurs Diplotene: disappearance of synaptonemal complex; chiasma is now visible Diakinesis: bivalent is now ready for metaphase I; fragmentation of the nuclear envelope Trans #1 Group 5: SagunTH, Bugayong, Bugayong, Dacquias, Fernandez, Tallon  MODULE #1 Introduction to Embryology ANA11 Relocates segments of maternal and paternal chromosomes Figure 7. Stages of prophase I (Source: Bioninja) by crossing over of chromosome segments, which “shuffles” the genes and produces a recombination of genetic material. Metaphase Paired homologous chromosomes align on the I metaphase plate C. CAUSES OF BIRTH DEFECTS AND SPONTANEOUS ABORTIONS Anaphase I Homologous chromosomes separate Chromosomal Abnormalities Telophase I Formation of two (2) daughter cells Disorders arising from structural or numerical alterations in one or more chromosomes Autosomes and sex chromosomes can be affected Involve errors in cell division (mitosis or meiosis) 50% of conceptions end in spontaneous abortion and 50% of these have major chromosomal aberrations. Therefore, 25% of conceptuses have major chromosomal abnormalities. Genetic Factors Disorders can arise from single or complex gene mutations, in which other factors such as diet, chemical exposure, and medication can also increase the risk of acquiring genetic disorders D. CHROMOSOMAL ABNORMALITIES D.1 Numerical Abnormalities Figure 13. Summary of meiosis I (Source: Biology Dictionary) Euploid refers to any exact multiple of n ○ e.g. diploid (2n), triploid (3n) Special events in Meiosis I Polyploid is the condition of possessing more than 2 Synapsis: pairing of homologous chromosomes lengthwise. complete sets of chromosomes Pairing is exact and point to point (except for X and Y Aneuploid refers to a state in which there is an addition or chromosomes). subtraction of one or more chromosomes but not a complete Crossovers or interchange of chromatid segments between set paired homologous chromosomes. ○ Monosomy (2n-1) is the loss of one chromosome Chiasma formation: the X like structure where points of interchange of homologous chromosomes are temporarily ○ Trisomy (2n+1) is the addition of a single chromosome united. Causes of Chromosomal Abnormalities PHASES OF MEIOSIS II 1. Meiotic nondisjunction Process is similar to mitosis ○ Occurs when the homologous pairs (first meiosis) or End result: production of 4 haploid (n) cells (23 sister chromatids (meiosis II) fail to separate chromosomes from 2 cells formed in meiosis I). ○ It can occur during either the first or the second meiotic Table 3. Phases of Meiosis II division Prophase II Spindle fibers reform and attach to the centromere ○ The incidence of chromosomal abnormalities in women Takes inversely proportional time compared to increases with age (>35 years old) telophase Disappearance of the nucleoli and nuclear envelope Metaphase II Chromosomes align at the centromere Anaphase II Chromatids divide at the centromeres and move toward the opposite poles Telophase II Four haploid cells are formed containing half the number of the original homologous pair Figure 15. Meiotic nondisjunction Retrieved from Klug et al. (2020) 2. Mitotic nondisjunction ○ Occurs during anaphase when sister chromatids fail to separate ○ Occurs during the earliest cell divisions ○ Produces a condition called mosaicism, wherein some cells have abnormal chromosome numbers while others are normal Figure 14. Summary of meiosis II ○ Affected individuals may exhibit syndromes (Source: Biology Dictionary) 3. Chromosomal translocations Significance of Meiosis ○ Creation of fusion gene, w/c resulted from the transfer Provides constancy of the chromosome number from of a portion of one chromosome to another generation to generation by reducing the chromosome nonhomologous chromosome number from diploid to haploid, thereby producing haploid ○ Can be: gametes. - Balanced – no missing or extra genetic information Allows random assortment of maternal and paternal after translocation chromosomes between gametes. - Unbalanced – unequal exchange, w/c results in lost or altered phenotype Trans #1 Group 5: SagunTH, Bugayong, Bugayong, Dacquias, Fernandez, Tallon  MODULE #1 Introduction to Embryology ANA11 and eye defects, such as microphthalmia. anophthalmia, and coloboma ○ Incidence is approx. 1 in 20,000 live births 4. Klinefelter syndrome (47, XXY or 48, XXXY) ○ (S/S) sterility, testicular atrophy, hyalinization of seminiferous tubules, gynecomastia ○ Nondisjunction of XX homolog is the most common cause ○ Have 47 chromosomes (XXY) and a sex chromatin Barr body Figure 16. Balanced and unbalanced translocation ○ Can also have more than 2 X chromosomes, in which Retrieved from Leech (2020), the more copies of X chromosome present, the higher https://almostadoctor.co.uk/encyclopedia/chromosomal-abnormalities degree of cognitive impairment Disorders 5. Turner’s syndrome (45, X) 1. Down syndrome (Trisomy 21) ○ (S/S) female appearance, gonadal dysgenesis, short ○ (S/S) Growth retardation, varying degrees of intellectual stature, webbed neck, lymphedema of the extremities, disability; craniofacial abnormalities, including upward skeletal deformities, and a broad chest slanting eyes, epicanthic folds (extra skin folds at the ○ Only monosomy compatible with life medial corners of the eyes], flat faces, and small ears; ○ 80% of cases are caused by nondisjunction in the male cardiac defects; and hypotonia gamete ○ Other causes can be structural abnormalities of the X chromosome or mitotic disjunction Figure 17. Down syndrome Retrieved from Sadler (2015) ○ 95% of cases are caused by trisomy 21 resulting from meiotic nondisjunction, mostly during oocyte formation ○ ~4% of cases result from unbalanced translocation between chromosome 21 and chromosome 13, 14, 15, or 21 Figure 19. Turner syndrome ○ 1% of cases are caused by mosaicism from mitotic Retrieved from Sadler (2015) nondisjunction D.2 Structural Abnormalities 2. Edward’s syndrome (Trisomy 18) ○ (S/S) mental retardation, congenital heart defects, low- Occur when the structure of the chromosome is altered (chromosome alteration). Alterations occur due to the set ears, flexion of fingers breakage of chromosome/s and incorrect rejoining of the ○ Incidence is approx. 1 in 5000 newborns chromosomal segments Chromosome breakage may be due to DNA damage caused by environmental factors (viruses, radiation, and drugs) or as a part of the mechanism of recombination. There are several forms of chromosome alteration: deletion, duplication, and translocation. ○ NOTE: Usually, structural abnormalities occur when information is missing or added to the chromosome (duplication and deletion of chromosomal segments). ○ NOTE: Translocation of chromosomal segments may not have phenotypic effects or result in disease as such chromosome alterations leave the chromosome intact. IMPORTANT: Chromosome alteration does not alter the total number of chromosomes. Chromosomal Deletions Occurs when a broken part of the chromosome is missing or deleted. Breakage may be due to environmental factors. The deleted segment contains many different genes that Figure 18. Edward’s syndrome (trisomy 18) contain instructions. The missing instructions result in errors Retrieved from https://drthindhomeopathy.com/disease/trisomy-18- in the development of the fetus resulting in structural edward-syndrome/ abnormalities. 3. Patau syndrome (Trisomy 13) ○ (S/S) intellectual disability, holoprosencephaly, Structural abnormalities due to chromosomal deletions: congenital heart defects, deafness, cleft lip and palate, 1. Cri-du-chat syndrome ○ Partial deletion of the short arm of chromosome 5 Trans #1 Group 5: SagunTH, Bugayong, Bugayong, Dacquias, Fernandez, Tallon  MODULE #1 Introduction to Embryology ANA11 ○ S/S: cat-like cry, microcephaly (small head), intellectual ○ Microdeletion at the q arm of chromosome 22 disability, and congenital heart disease ○ Poor development of several body systems ○ S/S: palatal defects, conotruncal heart defects (disruption to the outflow tract of the heart), speech delay, learning disorders, and schizophrenia-like disorder Figure 20. [A] Deletion in the p arm of chromosome 5; [B] A child with Cri-du-chat syndrome Figure 23. DiGeorge syndrome Retrieved from https://healthjade.net/cri-du-chat-syndrome/ Retrieved from (Isgandarova et al., 2021) 2. Microdeletion syndrome or contiguous gene syndrome Genomic Imprinting ○ Microdeletions of a few contiguous genes Refers to the characteristics that are differentially expressed ○ Contiguous gene complexes are the sites where depending on whether they are inherited from the father or microdeletions occur. the mother. ○ Fluorescence in situ hybridization (FISH) is used to Examples: Angelman syndrome (maternal), Prader-Willi identify contiguous gene complexes. syndrome (paternal) ○ Types of microdeletion syndrome: - Angelman syndrome Fragile X Syndrome ® Microdeletion in the q (long) arm of chromosome 15 Caused by a break or weakness in the q arm of the X chromosome (particularly, in the FMRI gene) ® Inherited on maternal chromosome Incidence: ® S/S: intellectual disability, cannot speak, poor ○ Occurs in 1 per 5000 individuals motor development, and unprovoked and ○ Predominantly occurs in males than females due to the prolonged periods of laughter syndrome being X-linked - Prader-Willi syndrome ○ The 2nd most common genetic abnormality that causes ® Microdeletion in the q (long) arm of intellectual disability (most common is Down syndrome) chromosome 15 S/S: intellectual disability, large ears, pale blue irises, ® Inherited on paternal chromosome prominent jaw, and large testes ® S/S: hypotonia (poor muscle tone), obesity, intellectual disability, and hypogonadism and undescended testes Figure 24. Fragile X syndrome Retrieved from (Garber, Visootsak, and Warren, 2008) Fragile sites are regions in the chromosomes that are prone Figure 21. [A] Angelman syndrome; [B] Prader- to separate or break under certain cell manipulations Willi syndrome ○ NOTE: most fragile sites consist of CGG repeats Retrieved from Langman’s Medical Embryology (2015) E. GENE MUTATIONS Single gene mutation occurs when there is a change in the 3. Miller-Dieker syndrome structure or function of a single gene ○ Microdeletion at the p arm of chromosome 17 ○ ~8% of all human malfunctions can be attributed to ○ S/S: lissencephaly (surface of the brain appears single gene mutations smooth), developmental delay, seizures, and cardiac Dominant mutation occurs when the abnormality is due to and facial abnormalities a mutation in one of the alleles of a gene. Recessive mutation occurs when the abnormality is caused by a mutation in both alleles of a gene or the mutation is found in the X chromosome in the male (X-linked) Modifying factors affect the phenotypic expression of mutant genes resulting in variations. Inborn errors of metabolism ○ Phenylketonuria - Caused by a missense mutation in the gene encoding phenylalanine hydroxylase (PAH) ® Missense mutation occurs when a single Figure 22. Miller-Dieker syndrome Retrieved from (Pilz & Quarrell, 1996) nucleotide base is replaced resulting in the translation of a different protein 4. 22q11 syndrome (DiGeorge syndrome) ® PAH catalyzes the hydroxylation of phenylalanine (Phe) to generate tyrosine (Tyr) Trans #1 Group 5: SagunTH, Bugayong, Bugayong, Dacquias, Fernandez, Tallon  MODULE #1 Introduction to Embryology ANA11 - Elevated Phe level and decreased Tyr levels in the The majority of oogonia continue to divide by mitosis, but blood some arrest their cell division in prophase I and form primary - S/S: intellectual disability, microcephaly, oocytes. hypopigmentation and eczema, During the next few months, oogonia increased rapidly in hyperactivity/behavioral problems, and seizures number. and tremors By the 5th month, the number of germ cells reached seven ® Musty odor to skin and urine million and cell death (atresia) began. ○ Homocystinuria is the inability to break down the By the 7th month, the majority of oogonia have amino acid methionine causing the build-up of degenerated. methionine and homocysteine in the blood and urine All surviving primary oocytes have entered prophase I of ○ Galactosemia is the inability to convert galactose to meiosis I wherein most of them are individually surrounded glucose due to mutations in the galactose-1-phosphate by flat follicular epithelial cells. uridylyl transferase (GALT) gene resulting in GALT A primary oocyte, together with its surrounding flat epithelial deficiency. cell, is called a Primordial follicle. F. DIAGNOSTIC TECHNIQUES FOR THE IDENTIFICATION Maturation of oocytes continue during puberty OF GENETIC ABNORMALITIES All primary oocytes that enter prophase of Meiosis I will enter Cytogenetic analysis is used to assess chromosome the Diplotene Stage where they are arrested until puberty. number and integrity by staining the chromosomes with ○ Diplotene stage is a resting stage during prophase I, Giemsa stain to reveal G bands. characterized by a lacy network of chromatin. ○ High-resolution metaphase banding techniques can ○ The arrested state is produced by oocyte maturation reveal a greater number of bands which represent inhibitor (OMI), a small peptide secreted by follicular smaller pieces of DNA. It is used to determine small cells. deletions. By puberty, only about 40,000 primary oocytes are present. Fluorescent in situ hybridization (FISH) uses fluorescent Fewer than 500 will be ovulated. probes that hybridize to chromosomes or genetic loci to identify ploidy and microdeletions. The results are visualized I. Pre-Antral Stage using fluorescence microscopy. ○ During puberty, as primordial follicles grow, the follicular Microarrays use specific DNA sequences as probes to epithelial cells change from flat to cuboidal in shape and hybridize to the target DNA or RNA sample. This technique proliferate to produce a stratified epithelium of is used to detect single nucleotide polymorphisms, granulosa cells forming the Primary Follicle. mutations, and changes in expression levels. ○ Granulosa Cells: Exome sequencing is the sequencing of exons (coding - Separate the surrounding connective tissue of the regions) in the genome to create an exome which is used to ovary which forms Theca Follica. identify mutations and polymorphism that alter proteins. - Together with the oocyte, they secrete a layer of glycoproteins on the surface, forming the Zona IV. MORPHOLOGICAL CHANGES DURING MATURATION OF Pellucida THE GAMETES ○ As follicles grow, Theca Folliculi organize into 2 layers, A. OOGENESIS the theca interna and theca externa Process whereby oogonia differentiate into mature oocytes II. Antral/Vesicular follicle The maturation of oocytes starts before birth and continues ○ Each month, 15 to 20 follicles selected from a pool of at puberty growing follicles begin to mature. ○ Some of the follicles die, whereas others begin to accumulate fluid in a space called the antrum entering the antral or vesicular stage. The follicle is termed a vesicular or an antral follicle. III. Mature Vesicular (Graafian) follicle ○ Can grow 25 mm or more in size ○ It is surrounded by the theca interna and theca externa ▪ Theca interna: inner layer of secretory cells, and composed of cells having characteristics of steroid secretion ▪ Theca externa: outer fibrous capsule, and gradually merges with the ovarian connective tissue ○ Fluid continúes to accumulate such that, immediately prior to ovulation, follicles are quite swollen and are called mature vesicular follicles or graafían follicles. ○ Granulosa cells surrounding the oocyte remains intact and form the cumulus oophorus ○ With each ovarian cycle, a number of follicles begin to develop, but usually, only one reaches full maturity ○ Surge in the luteinizing hormone (LH) induces the preovulatory growth phase and completes Meiosis I. ▪ This results in formation of two daughter cells, each with 23 double-structured chromosomes ○ The secondary oocyte receives most of the cytoplasm Figure 25. Phases of Oogenesis whereas the first polar body receives none. Maturation of oocytes before birth ▪ The first polar body lies between the zona pellucida After the arrival of the Primary Germ Cell (PGCs) in the ovary, and the cell membrane of the secondary oocyte in the they will begin to differentiate into oogonia. perivitelline space. Oogonia proliferates due to mitosis. ○ The cell will proceed to Meiosis II but will only be By the 3rd month of development, they will arrange in completed if the oocyte is fertilized. clusters and give rise to primary oocytes. ○ **Ovum is haploid and is to be fertilized by haploid The total number of primary oocytes at birth is estimated to sperm cell vary from 600,000 to 800,000. As oocytes form, epithelial cells surround them and form a single flattened cell layer called the follicular cell. Trans #1 Group 5: SagunTH, Bugayong, Bugayong, Dacquias, Fernandez, Tallon  MODULE #1 Introduction to Embryology ANA11 During the second meiotic division these cells immediately begin to form haploid spermatids Figure 26. Primordial follicle consisting of a primary oocyte surrounded by a layer of flattened epithelial cells; [B] Early primary or preantral stage follicle recruited from the pool of primordial follicles. As the follicle grows, follicular cells become cuboidal and begin to secrete the zona pellucida; [C] Mature primary (preantral) follicle with follicular cells forming a stratified layer of granulosa cells around the oocyte and the presence of a well-defined zona pellucida. (Source: Sadler, 2015) Retrieved From Batch Naraniag Trans Figure 27.[A] Vesicular and [B]Mature vesicular (graafian) follicles (Source: Sadler, 2015) Retrieved from Batch Naraniag Trans B. SPERMATOGENESIS Figure 29.Type A spermatogonia, derived from the spermatogonial Maturation of sperm begins at puberty. stem cell population, represent the first cells in the process of The process involves all of the events by which spermatogenesis until they become spermatozoa. Clones of cells are spermatogonia are transformed into spermatozoa. established, and cytoplasmic bridges join cells in each succeeding division until individual sperm are separated from residual bodies. (Source: Sadler, 2015) Retrieved from Batch __ Trans Spermatogenesis is regulated by Luteinizing Hormone (LH) production by the pituitary gland. LH binds to receptors on Leydig cells and stimulates testosterone production, which in turn binds to Sertoli cells to promote spermatogenesis. Follicle-stimulating hormone (FSH) is also essential because its binding to Sertoli cells stimulates testicular fluid production and synthesis of intracellular androgen receptor proteins. SPERMATOGENESIS Includes all the events by which spermatogonia are transformed into spermatozoa Figure 28. Phases of Spermatogenesis a. formation of the acrosome covers half of the nuclear At birth, germ cells in the male infant can be recognized in surface and contains enzymes to assist in penetration the sex cords of the testis as large, pale cells surrounded by of the egg and its surrounding layers during fertilization supporting cell. b. condensation of the nucleus; ○ Supporting cells are derived from the surface epithelium c. formation of neck, middle piece, and tail; and of the testis in the same manner as follicular cells, d. shedding of most of the cytoplasm as residual bodies become sustentacular cells, or Sertoli cells. that are phagocytized by Sertoli cells. Before puberty: the sex cords acquire a lumen and become the seminiferous tubules When fully formed, spermatozoa enter the lumen of PGCs give rise to spermatogonial stem cells. seminiferous tubules. Initiation: production of type A spermatogonia from the From there, they are pushed toward the epididymis by cells emerging from the stem cell population. contractile elements in the wall of the seminiferous tubules. ○ Type A cells undergo a limited number of mitotic Although initially only slightly motile, spermatozoa obtain full divisions to form clones of cells. motility in the epididymis. Last cell division: production of type B spermatogonia, which then divide to form primary spermatocytes In humans, the time required for a spermatogonium to Primary spermatocytes then enter a prolonged prophase (22 develop into a mature spermatozoon is approximately 74 days) followed by rapid completion of meiosis I and formation days, and approximately 300 million sperm cells are of secondary spermatocytes produced daily Trans #1 Group 5: SagunTH, Bugayong, Bugayong, Dacquias, Fernandez, Tallon  MODULE #1 Introduction to Embryology ANA11 Figure 30. Transformation of spermatid to spermatozoa (Source: Sadler, 2015) Retrieved from Batch Naraniag Trans ** Spermatids eventually mature into spermatozoon (sperm cells) which contains chromosomal information that can determine the probability of the offsprings sex V. REFERENCES Cri du Chat Syndrome—Symptoms, Causes, Treatment | NORD. (n.d.). Retrieved August 11, 2024, from https://rarediseases.org/rare-diseases/cri-du-chat- syndrome/ Garber, Kathryn & Visootsak, Jeannie & Warren, Stephen. (2008). Fragile X syndrome. European journal of human genetics : EJHG. 16. 666-72. 10.1038/ejhg.2008.61. Isgandarova, K., Molatta, S., & Sommer, P. (2021). Late diagnosed DiGeorge syndrome in a 44-year-old female: A rare cause for recurrent syncopes in adulthood—a case report. European Heart Journal - Case Reports, 5(5), ytab166. https://doi.org/10.1093/ehjcr/ytab166 Kattuoa, M. l, & Das, J. M. (2024). Lissencephaly. In StatPearls. StatPearls Publishing. http://www.ncbi.nlm.nih.gov/books/NBK560766/ Klug, W.S., Cummings, M., Spencer, C.A, Palladino, M.A., & Killian, D. (2020). Essentials of Genetics.Pearson. Madhok, S. S., & Shabbir, N. (2024). Hypotonia. In StatPearls. StatPearls Publishing. http://www.ncbi.nlm.nih.gov/books/NBK562209/ Milani, D. A. Q., & Tadi, P. (2023). Genetics, Chromosome Abnormalities. In StatPearls [Internet]. StatPearls Publishing. Pilz, Daniela & Quarrell, Oliver. (1996). Syndromes with lissencephaly. Journal of medical genetics. 33. 319-23. 10.1136/jmg.33.4.319. Sadler, T.W. (2015). Langman’s Medical Embryology. Wolters Kluwer Health. Stone, W. L., Basit, H., & Los, E. (2024). Phenylketonuria. In StatPearls. StatPearls Publishing. http://www.ncbi.nlm.nih.gov/books/NBK535378/ Team, H. J. (2019, March 6). Cri du chat syndrome causes, symptoms, life expectancy & treatment. Health Jade. https://healthjade.net/cri-du-chat-syndrome/ Trans #1 Group 5: SagunTH, Bugayong, Bugayong, Dacquias, Fernandez, Tallon