Embryology Student Summer 2024 PDF

Microanatomy Embryology: Origin of Tissues Don R. Bergfelt, MS, PhD Academic Hub: 35-205 [email protected] Objectives Terminology Relevance Teratogenic effects on embryonic development Stem cells & subclasses – totipotent, pluripotent, multipotent, oligopotent, unipotent Zygote formation (fus...

Microanatomy Embryology: Origin of Tissues Don R. Bergfelt, MS, PhD Academic Hub: 35-205 [email protected] Objectives Terminology Relevance Teratogenic effects on embryonic development Stem cells & subclasses – totipotent, pluripotent, multipotent, oligopotent, unipotent Zygote formation (fusion of male & female pronuclei), cleavage, morula, blastocyst, gastrulation Germ layers - ectoderm, mesoderm, endoderm Main tissue types – epithelium, connective tissue, muscle, nervous tissue Notochord, neuronal tube, & somite formation Divisions of mesoderm – paraxial, intermediate, lateral plate Differentiation of male & female gonads Relevance to Veterinary Profession: Basic & Applied Implications Basic knowledge of 3 germ layers that differentiate into tissues/organs Important for understanding teratogenic or development effects on general & reproductive health Structural Organization (ectoderm, mesoderm, endoderm) Embryogenesis Teratogens Teratogens (Gr. monster – producing) - any agent or factor that can cause congenital (present at birth) anomalies during embryo or fetal growth & development Genetic factors Radiation Chemical agents e.g., nicotine, drugs (thalidomide, tetracycline), alcohol Infectious agents e.g., pathogens such as parasitic, bacterial, viral Hormones e.g., anti-paramesonephric duct hormone in freemartins, steroids Congenital Anomalies of Teratogens Reproductive: Freemartin Cryptorchidism Alimentary: Cleft palate Atresia ani Urinary: Persistent urachus Neurological: Hydrocephalus Cerebellar hypoplasia Spina bifida Cardiovascular: Patent ductus arteriosus Ventricular septal defect Persistent right aortic arch Portocaval shunt Respiratory: Brachycephalic airway syndrome Body wall: Umbilical hernia Period of Susceptibility to Teratogens Effect of teratogens depends on timing of exposure Exposure during zygote formation unlikely to have an effect Most critical period is early embryonic development during organ and limb development Fetal brain develops through pregnancy and can be affected at any period Stem Cells Self renewing cells from which all other cells with specialized functions are generated Zygote (blastomeres) Gastrulation ectoderm mesoderm endoderm PGC (Some common features with pluripotent) Examples of Stem Cell Types Differentiate into any cell Differentiate into cells from 3 germ layers Differentiate into a single cell https://www.bing.com/videos/riverview/relatedvideo?q=can+unipot ent+cells+make+totipotent+cells&mid=C8487E945C4FA0DB9C43 C8487E945C4FA0DB9C43&FORM=VIRE Embryology - Gametogenesis Secondary oocyte Gametogenesis Uterine tube Spermatozoa Ovary Primary follicle Ovulation Developing follicles Tertiary (Dominant) follicle Zygote Embryology Morula Blastocyst Cleavage Uterus Embryology Fusion of Male & Female Pronuclei Secondary oocyte Fertilization & pronuclei Pronuclei fusion Cleavage – Blastulation - Gastrulation Cleavage Within the uterine tube, unicellular zygote divides by mitosis or CLEAVAGE to become multicellular EMBRYO Each daughter cell is identical and called a BLASTOMERE No increase in size since the early embryo is still encased by zona pellucida Zygote ➔ 2-blastomere embryo) ➔ 4-blastomere embryo *Note: the total size is equal between them all Morula Still within the uterine tube, BLASTOMERES continue to divide into a compact ball of 1632 cells within the zona pellucida Looks like a mulberry Zona pellucida Blastomeres Blastulation Hatching accommodates implantation Outer cells – TROPHOBLAST - will expand to form extraembryonic membranes (amnion, yolk sac, allantois, chorion) – related to placentation Inner cell mass – EMBRYOBLAST forms entire embryo Uterine lumen Endometrium Uterine wall Blastulation Differentiation of BLASTOMERES to Inner Cell Mass – EMBRYOBLAST surrounded by outer cells TROPHOBLAST The BLASTOCYST enters the uterus, “hatches” from the zona pellucida, & develops a cavity BLASTOCOELE, which will become the yolk sac FYI: Embryo transfer technology involves flushing the uterus around Days 5, 6, or 7 after the blastocyte(s) enter the uterus – those that are recovered are graded & transferred to recipient or surrogate animals Equine FYI: Capsule maintains sphericity of blastocyst after it hatches from zona pellucida Blastulation “Hatching” Hatching from the zona pellucida has to occur to accommodate implantation – hatching due to pressure of expanding blastocyst & enzymatic dissolution of the zona pellucida Fully hatched Gastrulation Embryoblast differentiates to form bilaminar disc of EPIBLAST (above, dorsal) & Hypoblast (below, ventral) FYI - Establishes dorsoventral axis Dorsal Amniotic cavity Ventral Gastrulation A process that establishes all 3 germ layers: ECTODERM (outermost), ENDODERM (inner most), & MESODERM (between other 2 layers) Uterine Endometrium Amnion Epiblast Hypoblast Hatching & Implantation Trophectoderm Blastocoele or yolk sac FYI: After blastulation, the trophoblast is contiguous with the ectoderm of the embryo and is referred to as the trophectoderm Gastrulation Cranial Primitive Streak: Epiblast thickens, cells ingress, streak lengthens along developing embryo establishing cranio-caudal axis Bilaminar: Bilaminar disc develops with cells of epiblast replacing cells of hypoblast to yield endoderm Trilaminar Disc: Trilaminar disc develops with cells of mesoderm emerging between other two germ layers Caudal Gastrulation as seen from dorsal view. Primitive streak, notochord, and 3 germ layers formed. Position of head and tail determined. Sweeney’s Embryology 1, 2, Migration path of caudal, lateral, and cranial Mesoderm 3: Formation of and direction of growth of Notochord Primitive Node Primitive Streak Caudal Disk Cranial Disk Gastrulation Summary General Germ Layer Derivatives All body tissues arise from these 3 germ layers: ECTODERM: Epidermal structures (of the skin), lining of oral, nasal cavity & anus; corneal epithelium, nervous system MESODERM: Connective tissues, muscle tissue, mesothelium, cardiovascular system, urogenital tract ENDODERM: Epithelial lining & glands of digestive & respiratory systems Structural Organization (ectoderm, mesoderm, endoderm) Germ Layer Derivatives: Summary Formation of Notochord Derived from mesoderm as a transient structure that provides direction Basis for vertebral column – provides direction & contributes to vertebra & intervertebral discs Induces ectoderm to differentiate into neuroectoderm providing basis for nervous system Ectoderm Endoderm Embryonic ectoderm Neural ectoderm Neurulation Neural tube formation: 1. Neural ectoderm thickens. 2. Neural folds dip down producing the neural groove. 3. Neural folds contact, neural ectoderm cells rearrange and are forming the neural tube with overlying ectoderm. Neurulation Derived from ectoderm (note, different levels of mesoderm) Basis for central nervous system (brain, spinal cord) Neural tube Neural tube Neurulation Cranial aspect = brain Caudal aspect = spinal cord Notochord = provides direction & contributes to formation of vertebral column Mesoderm: Intra-embryonic Mesoderm: Intra-embryonic Mesoderm: Paraxial Skeleton, Muscle, Skin Somitogenesis Differentiates into somitomeres on each side of neural tube & further to somites Major somites: ✓ Sclerotome – vertebrae, portions of skull, axial skeleton ✓ Myotome – striated muscles of head, trunk, limbs ✓ Dermotome – dermis of dorsal or back regions Caudal view Dorsal view Segmented Somites Cranial Dorsal Caudal Mesoderm: Paraxial Skeleton, Muscle, Skin SOMITES SOMITES: Sclerotome → Axial skeleton Myotome → Skeletal muscles Dermatome → Dermis All other cartilage and bones of the body and limbs are formed from lateral plate mesoderm Sweeny’s Embryology Mesoderm: Lateral Somatic (parietal) - dorsal & associates with ectoderm Contributes to serous membranes lining the peritoneal, pleural, & pericardial cavities Splanchnic (visceral) - ventral & associates with endoderm Contributes to cardiovascular system, blood, kidneys, smooth muscles Mesoderm: Extra-Embryonic Development of Placental Tissues: Derived from Epiblast Along with trophoblast (trophectoderm), contributes to formation of yolk sac, amnion, allantois, & chorion Vital functions in maternal-fetal protection, nutrient & waste exchange Extra-embryonic membranes: https://www.youtube.com/watch?v=clNxx DlmvKI Placenta Types Diffuse Cotyledonary Zonary rodent Discoid Mesoderm: Intermediate Gonadal Differentiation Differentiate into the urogenital system: Paired kidneys Paired adrenal gland cortex Paired gonads (ovaries, testes) Female & male reproductive ducts Gonadal Differentiation Undifferentiated Gonad Genital ridge: medial part of mesonephros (early kidney) & differentiate into capsule, stroma & connective tissue of gonad Coelomic epithelium: covers the genital ridge & differentiate into sex cords (sertoli or granulosa cells) of gonad Primordial germ cells (PGC): unipotent cells that originate in the epiblast (extraembryonic ectoderm) and migrate through the hindgut endoderm of yolk sac & differentiate into spermatogonia or oogonia between sex cords (i.e., sertoli or granulosa cells), respectively, in testes or ovaries Differentiated Gonad Paired testes: in presence of a Y chromosome (XY), there is an SRY gene that encodes for a testis determining factor & sertoli cells produce an anti-mullerian hormone (AMH) - Mullerian ducts degenerate & testes & Wolffian ducts (epididymis, vas deferens, ejaculatory duct, seminal vesicle) develop Paired ovaries: in absence of Y chromosome (XX), Wolffian ducts degenerate & ovaries & Mullerian ducts (oviducts, uterus, cervix, cranial vagina) develop Primordial Germ Cell Migration Primordial germ cells (PGC): migrate from yolk sac through hindgut to gonadal ridges & differentiate into spermatogonia or oogonia between sex cords (i.e., sertoli or granulosa cells), respectively, in testes or ovaries Gonadal Differentiation XY chromosomes SRY gene AMH Testes Mullerian ducts degenerate & Wolffian ducts develop Yolk sac XX chromosomes Genital ridge (intermediate mesoderm) Wolffian ducts degenerate & Mullerian ducts develop Ovaries Congenital Anomaly Freemartinism in cattle Infertile female with masculinized behavior and non-functioning ovaries. Genetically and phenotypically, the animal appears female, but development of various aspects of female reproductive tract are altered due to anti-Müllerian hormone from the male twin from shared placental vasculature. About 85% to 90% of the time a female to a male is a freemartin. Gametogenesis Diploid (2N) X & X chromosomes Ovaries Testes Follicles Diploid (2N) X & Y chromosomes Seminiferous tubules Haploid (1N) X chromosome Haploid (1N) Either X or Y chromosome Ovum Fertilization Sperm Embryogenesis Gametogenesis - Embryogenesis Secondary oocyte Gametogenesis Uterine tube Spermatozoa Primary follicle Ovulation Developing follicles Tertiary (Dominant) follicle Zygote Embryology Morula Blastocyst Cleavage Uterus