Chapter 29 - Development and Inheritance
Figure 29.13: Development of pharyngeal arches, pharyngeal clefts, pharyngeal pouches
The five pairs of pharyngeal pouches consist of - ectoderm - mesoderm - endoderm Contain - blood vessels - cranial nerves - cartilage - muscle tissue
Figure 29.8: Development of Notochordal Process
The notochordal process develops from the primitive node and later becomes the notochord
Figure 29.11: Placenta and Umbilical Cord
The placenta is formed by the - chorionic villi of embryo - the decidua basalis of the endometrium of the mother. Decidua Basalis of Uterus: - maternal portion of placenta Chorionic Villi of Chorion: - fetal portion of placenta Intervillous Space: - contains maternal blood
Table 29.1: Structures by the Three Primary Germ Layers - Endoderm
- Epithelial lining of GI tract (except oral cavity and anal canal) and epithelium of its glands - Epithelial lining of urinary bladder, gallbladder, liver - Epithelial lining of pharynx, auditory (eustachian) tubes, tonsils, tympanic (middle ear) cavity, larynx, trachea, bronchi, lungs - Epithelium of thyroid gland, parathyroid glands, pancreas, thymus - Epithelial lining of prostate and bulbourethral (Cowper's) glands, vagina, vestibule, urethra, and associated glands such as greater (Bartholin's) vestibular and lesser vestibular glands - Gametes (sperm and oocytes)
Table 29.1: Structures by the Three Primary Germ Layers - Ectoderm
- all nervous tissue - epidermis of skin - hair follicles, arrector pili muscles, nails, epithelium of skin glands (sebaceous and sudoriferous), mammary glands - lens, cornea, internal eye muscles - internal and external ear - neuroepithelium of sense organs - epithelium of oral cavity, nasal cavity, paranasal sinuses, salivary glands, anal canal - epithelium of pineal gland, pituitary gland, and adrenal medullae - melanocytes (pigment cells) - almost all skeletal and connective tissue components of head - arachnoid mater and pia mater
Table 29.1: Structures by the Three Primary Germ Layers - Mesoderm
- all skeletal and cardiac muscle tissue and most smooth muscle tissue - cartilage, bone, other connective tissues - blood, red bone marrow, lymphatic tissue - blood vessels and lymphatic vessels - dermis of skin - fibrous tunic and vascular tunic of eye - mesothelium of thoracic, abdominal and pelvic cavities - kidneys and ureters - adrenal cortex - gonads and genital ducts (except germ cells) - dura mater
Describe the third week of development
- begins 6 week period of rapid development and differentiation - three primary germ layers are established and are beginning of organ development in weeks 4-8
What is obstetrics?
- deals with management of pregnancy, labor, neonatal period
What is an ectopic pregnancy?
- development of embryo or fetus outside uterine cavity Reasons: - scar tissue in uterine tube due from previous ectopic pregnancy - decreased movement of uterine tube smooth muscle - abnormal tubal anatomy Location: - most often in uterine tube - ovary - abdominal cavity - uterine cervix Risks: - woman who smoke due to cilia damage in lining tube -scars from pelvic inflammatory disease - previous uterine tube surgery - previous ectopic pregnancy Signs and Symptoms: - 1-2 missed menstrual cycles - bleeding - acute abdominal and pelvic pain Treatment: - removal of embryo to avoid rupture of uterine tube - can result in death of mother - surgery - methotrexate - causes embryonic cells to stop dividing and eventually disappear
What is the neonatal period?
- first 28 days after birth
Describe the 5th - 8th Week of Development
- rapid brain development - growth of head End of 6th Week: - larger head relative to trunk - limbs develop more - neck and trunk straighten - heart has four chambers 7th Week: - regions of limbs become distinct - fingers appear 8th Week: - last week of embryonic period - digits of hands short and webbed - tail is shorter - eyes are open - auricles of ears visible End of 8th Week: - regions of limbs apparent - cells are removed via apoptosis on fingers - no longer webbed - eyelids come together and may fuse - tail disappears - external genitals begin differentiation - embryo has human characteristics
Describe pregnancy
- sequence of events that begins with fertilization - proceeds to implantation, embryonic development, fetal development - ideally ends with birth between 38-40 weeks after menstrual period
What is developmental biology?
- study of sequence of events - from fertilization of secondary oocyte by sperm cell - to formation of adult organism
Describe the trimesters of pregnancy.
1) First Trimester: - critical stage of development - rudiments of major organ systems appear - stage which developing organisms most vulnerable to drugs, radiation, microbes 2) Second Trimester: - nearly complete development of organ systems - fetus assumes distinct human features 3) Third Trimester: - period of rapid fetal growth - early stages, most organ systems become fully functional
Describe the Development of Extraembryonic Coelom in 2nd week of development
12th Day of Fertilization: - extraembryonic mesoderm develops Extraembryonic Mesoderm: - mesodermal cells - derived from yolk sac - form connective tissue layer (mesenchyme) around amnion and yolk sac Extraembryonic Coelom: - large cavity in extraembryonic mesoderm
Describe the development of somites
17th Day After Fertilization Paraxial Mesoderm: - paired longitudinal columns - formed by mesoderm (adjacent to notochord) and neural tube Intermediate Mesoderm: - paired cylindrical masses - formed by mesoderm lateral to the paraxial mesoderm Lateral Plate Mesoderm: - pair of flattened sheets - formed by mesoderm lateral to intermediate mesoderm Somites: - series of paired, cube shaped structures - formed by segmentation of paraxial mesoderm End of 5th Week of Fertilization - 42-44 pairs of somites present - number of somites can correlate with age of embryo Differentiation of Somites into Regions 1) Myotome: - develop into skeletal muscles of neck, trunk, limbs 2) Dermatomes: - form connective tissue - dermis of skin 3) Sclerotomes: - form vertebrae and ribs
Table 29.2: Summary of Changes during Embryonic and Fetal Development - Fetal Period (13-16 weeks)
18 cm (6.5-7 in) 100 g (4 oz) - head is relatively small than rest of body - eyes move medially to final positions - ears move to final positions on sides of head - lower limbs lengthen - fetus appears more humanlike - rapid development of body systems occur
Table 29.2: Summary of Changes during Embryonic and Fetal Development - Fetal Period (17-20 weeks)
25-30 cm (10-12 in) 200-450 g (0.5 - 1 lb) - head is more proportionate to rest of body
Describe the Development of Intraembryonic Coelom
3rd Week of Development Intraembryonic Coelom: - small spaces that form in lateral plate of mesoderm enlarge to the intraembryonic coelom Intraembryonic Coelom In Two Parts: 1) Splanchnic Mesoderm: - forms heart and visceral layer of serous pericardium - blood vessels - smooth muscle and connective tissues of respiratory and digestive organs - visceral layer of serous membrane of pleurae and peritoneum 2) Somatic Mesoderm: - bones, ligaments, blood vessels and connective tissue of limbs - parietal layer of serous membrane of pericardium, pleurae and peritoneum
Describe the fourth week of development
4th - 8th Week - all major organs develop in this time Organogenesis: - formation of body organs and systems End of 8th Week - all major body systems have begun development - functions are minimal - requires presence of blood vessels to supply developing organs with oxygen and nutrients 4th Week after Fertilization Embryonic Folding: - change in shape and size of the embryo - goes from two-dimensional trilaminar embryonic disc to three-dimensional cylinder - main force due to different rates of growth - rapid longitudinal growth of nervous system (neural tube) Cylinder: - endoderm in center - mesoderm in middle - ectoderm on outside Folds: - embryo curves into a C shape Folding in Median Plane - head fold and tail fold - brings heart and mouth into adult position Folding in Horizontal Plane - two lateral folds - brings developing anus into adult position Primitive Gut: - folds move to midline - lateral folds become part of dorsal part of yolk sac into embryo - becomes GI tract Primitive Gut Differentiation Foregut: - anterior Midgut: - intermediate Hindgut: - posterior Stomodeum: - oropharyngeal membrane in head of embryo - separation of oropharyngeal membrane - region of foregut - future oral cavity - oropharyngeal membrane rupture at 4 weeks - pharyngeal membrane makes contact with stomodeum Cloaca: - cavity which the hindgut expands into - divides into a ventral urogenital sinus and dorsal anorectal canal Proctodeum: - outside of embryo - in tail region - small cavity Cloacal Membrane: - separates the cloaca and proctodeum - moves downward due to tail folding - urogenital sinus, anorectal canal, proctodeum move closer to final positions 7th Week of Development - cloacal membrane ruptures - urogenital and anal openings are created 4th Week of Development Pharyngeal or Branchial Arches - five pairs - develop on each side of head and neck regions - begin development on 22nd day after fertilization - form swellings on surface of embryo Dermis: - outer covering of ectoderm - inner covering of endoderm - middle covering of mesoderm Contains: - an artery - cranial nerve - skeletal cartilaginous rods that support the arch - skeletal muscle tissue that attaches to and moves cartilage rods Units: - skeletal component - nerve - muscle - blood vessels 1st Pharyngeal Arch: - often called mandibular arch - develops into mandible Pharyngeal Cleft: - on ectodermal surface of pharyngeal region - groove that separates each pharyngeal arch Pharyngeal Pouches: - balloonlike outgrowths Optic Placode: - sign of developing ear - future internal ear - 22 days after fertilization Lens Placode: - thickened area of ectoderm - becomes the eye Middle of 4th Week Upper Limb Buds: - outgrowths of mesoderm - covered by ectoderm End of 4th Week Lower Limb Buds: - develop Heart Prominence: - distinct projection on ventral surface of embryo Tail: - distinct
Table 29.2: Summary of Changes during Embryonic and Fetal Development - Fetal Period
550-800 g (1.25 - 1.5 lb)
Table 29.2: Summary of Changes during Embryonic and Fetal Development - Fetal Period (9-12 Weeks)
7.5 cm (3 in) 30 g (1 oz) - head about half length of fetal body - fetal length nearly doubles - brain enlarges - face is broad - eyes fully developed, closed, widely separated - nose develops bridge - external ears develop, are set low - bone formation continues - upper limbs almost reach full length - lower limbs not fully developed - heartbeat detected - gender is distinguishable from external genitals - urine secreted by fetus added to amniotic fluid - red bone marrow, thymus, spleen participate in blood cell formation - fetus begins to move - movements not felt yet by mother - body systems continue to develop
Development of Bilaminar Embryonic Disc in 2nd week of development
8 Days of Fertilization: - cells of embryoblast differentiate into two layers 1) Hypoblast (primitive endoderm) 2) Epiblast (primitive ectoderm) - together form flat disc Bilaminar Embryonic Disc: - flat disc formed by hypoblast and epiblast Amniotic Cavity: - small cavity within epiblast - enlarges to form amniotic cavity
Describe the Development of the Yolk Sac during 2nd week of development
8th Day of Fertilization: - cells of hyoblast migrate and cover inner surface of blastocyst wall Exocoelomic Membrane: - thin membrane formed by migrating columnar cells become squamous cells Yolk Sac: - together with hyoblast and exocoelomic membrane - former blastocyst cavity during earlier development - relatively empty and small - decreases in size as development progresses Functions: - supplies nutrients during 2-3 week of development - source of blood cells from 3-6 weeks of development - contains first cells (primordial germ cells) that will migrate into developing gonads - differentiate into primitive germ cells and form gametes - forms part of gut (GI tract) - shock absorber - prevent drying of embryo Bilaminar Embryonic Disc: - now positioned between amniotic cavity and yolk sac
Development of Sinusoids in the 2nd week of development
9th Day of Fertilization: - blastocyst becomes completely embedded in endometrium Lacunae: - small space where syncytiotrophoblast expands 12th Day of Fertilization: Lacunar Networks: - interconnecting spaces of fused lacunae Maternal Sinusoids: - dilation of endometrial capillaries around developing embryo Maternal Blood: - syncytiotrophoblast erodes some of maternal sinusoids and endometrial glands - maternal blood and secretions from glands enter lacunar networks and flow through them - rich in materials for embryonic nutrition - disposes embryo wastes
Figure 29.6: Principal Events of Second Week of Development
About 8 days after fertilization, the trophoblast develops into a syncytiotrophoblast and cytotrophoblast; the inner cell mass develops into a hypoblast and epiblast (bilaminar embryonic disc)
Describe the development of the trophoblast during the 2nd week of development
After 8 Days of Fertilization: - trophoblast develops into two layers in region between blastocyst and endometrium - two layers become part of chorion 1) Syncytiotrophoblast: - contains no distinct cell boundaries 2) Cytotrophoblast: - between embryoblast and syncytiotrophoblast - composed of distinct cells During Implantation: - syncytiotrophoblast secretes enzymes that enable blastocyst to penetrate uterine lining by digesting and liquefying endometrial cells - blastocyst becomes buried in endometrium and inner 1/3 of myometrium - hCG is secreted by trophoblast Human Chorionic Gonadotropin Hormone (hCG): - rescues corpus luteum from degeneration - sustains progesterone and estrogen secretions - peak secretion in 9th week of pregnancy - presence in maternal blood or urine indicator of pregnancy - hormone detected by home pregnancy tests Progesterone and Estrogen Secretions: - maintain uterine lining in a secretory state - prevents menstruation 9th Week of Pregnancy: - peak hCG secretion - placenta fully developed - placenta produces estrogens and progesterone to sustain pregnancy
Describe the cleavage of the zygote during first week of development
After fertilization cleavage occurs Cleavage: - rapid mitotic cell division of zygote 24-30 hours - first division of zygote within 24 hours after fertilization - 6 hours later is completed 48 hours (2 days later) - second cleavage is completed - four cells 72 hours (3 days) - 16 cells Blastomeres: - progressively smaller cells produced by cleavage Morula: - solid sphere of progressively smaller cells - surrounded by zona pellucida - same size as original zygote
Development of Amnion in 2nd week of development
Amnion: - amniotic cavity enlarges - single layer of squamous cells form domelike roof above epiblast - overlies only bilaminar embryonic disc at first - embryonic disc increases in size, begins to fold - amnion eventually surrounds entire embryo - usually ruptures before birth Amniotic Cavity: Roof: - formed by amnion Floor: - formed by epiblast Amniotic Fluid: - derived from maternal blood - fetus contributes to fluid be excreting urine into amniotic cavity - shock absorber - regulates fetal body temperature - prevents fetus from drying out - prevents adhesions between skin of fetus and surrounding tissues - embryonic cells sloughed off into amniotic fluid Amniocentesis: - embryonic cells examined in the amniotic fluid - withdrawing amniotic fluid and analyzing fetal cells and dissolved substances
Describe the Development of Cardiovascular System
Beginning of 3rd Week Angiogenesis: - formation of blood vessels - begins in extraembryonic mesoderm of yolk sac, connecting stalk and chorion - necessary for sufficient nutrients for embryo (cannot be received from yolk in yolk sac and ovum) - initiated by mesodermal cells that differentiate into hemangioblasts Hemangioblasts: - initiate angiogenesis Angioblasts: - further development of hemangioblasts - aggregate to form isolated masses of cells called blood islands - arrange around each space that develops in blood islands - forms endothelium and tunics (layers) of developing blood vessels Blood Islands: - isolated masses of cells - form lumens of blood vessels - grow and fuse forming extensive blood vessel network throughout embryo 3 Weeks After Fertilization Hemangioblasts: - in blood vessel walls of yolk sac, allantois and chorion - develop into blood cells, blood plasma - hemangioblasts develop into pluripotent stem cells that form blood cells Blood Formation: - 5th week within embryo - 12th week in spleen, red bone marrow, thymus Day 18 and 19 - heart forms from splanchnic mesoderm in head of embryo Cardiogenic Area: - region of splanchnic mesodermal cells Endocardial Tubes: - pair of tubes formed by mesodermal cells in response to induction signals to underlying endoderm Primitive Heart Tube: - fusing of the endocardial tubes End of 3rd Week Primitive Heart Tube: - bends in on itself - becomes 's' shaped - begins to beat - joins blood vessels in other parts of embryo, connective stalk, chorion, yolk sac to form primitive cardiovascular system
Describe implantation in the first week of development
Blastocyst remains free within: - uterine cavity - for 2 days - before attaching to uterine wall Endometrium in: - secretory phase Implantation: - 6 days after fertilization - blastocyst loosely attaches to endometrium - implants in posterior portion of fundus or body of uterus - inner cell mass toward endometrium 7 days After Fertilization: - blastocyst attaches to endometrium more firmly - endometrial glands enlarge - endometrium becomes more vascularized (forms new blood vessels) - blastocyst eventually secretes enzymes and burrows into endometrium and becomes surrounded by it Decidua (endometrium) - endometrium called decidua after implantation - separates from endometrium after fetus delivered Decidua Basalis: - portion of endometrium between embryo and stratum basalis of uterus - supplies glycogen and lipids to embryo and fetus - later becomes maternal part of placenta Decidua Capsularis: - endometrium between embryo and uterine cavity - evolving fetus causes it to bulge into uterine cavity and fuses with decidua parietalis - obliterating the uterine cavity - 27 weeks decidua capsularis degenerates and disappears Decidua Parietalis: - remaining modified endometrium - lines non involved areas of rest of uters
Figure 29.10: Development of Chorionic Villi
Blood vessels in chorionic villi connect to the embryonic heart - via the umbilical arteries and umbilical vein
Describe the Development of the Chorion in the 2nd week of development
Chorion: - two layers of trophoblast and extraembryonic mesoderm - surrounds embryo, later the fetus - principal embryonic part of placenta - produces hCG - inner layer eventually fuses with amnion Protection of Embryo and Fetus from Immune Responses of Mother in Two Ways: 1) Secretes proteins that block antibody production by mother 2) Promotes production of T lymphocytes that suppress normal immune response in uterus Chorionic Cavity: - after development of chorion the extraembryonic coelom is now called chorionic cavity Connecting (body) Stalk: - future umbilical cord - extraembryonic mesoderm - connects the bilaminar embryonic disc to the trophoblast - end of second week of development
Figure 29.2: Cleavage and the Formation of the Morula and Blastocyst
Cleavage refers to the early, rapid mitotic divisions of a zygote a) Cleavage of zygote, two cell stage (day 1) (polar bodies, blastomeres, zona pellucida) b) Cleavage, four-cell stage (day 2) (nucleus, cytoplasm) c) Morula (day 4) d) Blastocyst, external view (day 5) e) Blastocyst, internal view (day 5) (embryoblast - inner cell mass, blastocyst cavity, trophoblast)
Figure 29.1: Selected structures and events in fertilization
During fertilization - genetic material from a sperm cell and a secondary oocyte - merge to form a single diploid nucleus Path of Sperm Cell: - Corona Radiata - Zona Pellucida - Plasma Membrane of Secondary Oocyte - Cytoplasm of Secondary Oocyte
Table 29.2: Summary of Changes during Embryonic and Fetal Development - Embryonic Period
Embryonic Period 1-4 Weeks: 0.6 cm (3/16 in) - primary germ layers and notochord develop - neurulation occurs - primary brain vesicles, somites and intraembryonic coelom develop - blood vessel formation begins - blood forms yolk sac, allantois, and chorion - heart forms and begins to beat - chorionic villi develop - placental formation begins - embryo folds - primitive gut, pharyngeal arches, limb buds develop - eyes and ears begin to develop - tail forms - body systems begin to form 5-8 Weeks: 3 cm (1.25 in) 1g (1/30 oz) - limbs become distinct - digits appear - heart becomes four chambered - eyes far apart - eyelids fused - nose develops and is flat - face more humanlike - bone formation begins - blood cells start to form in liver - external genitals begin to differentiate - tail disappears - major blood vessels form - many internal organs continue to develop
Figure 29.12: Embryonic Folding
Embryonic folding converts the two-dimensional trilaminar embryonic disc into a three-dimensional cylinder
Describe blastocyst formation in the first week of development
End of 4th Day: - number of cells in morula increase as it moves from uterine tubes to uterine cavity 4-5 Day - morula enters uterine cavity Glycogen-rich secretion: - from glands of endometrium of uterus - passes into uterine cavity - enters morula through zona pellucida Uterine Milk: - glycogen-rich secretion from glands of endometrium - penetrates morula through zona pellucida - provides nourishment along with nutrients stored in cytoplasm of blastomeres of morula 32 Cell Stage: - fluid enters morula - collects between blastomeres - reorganizes around large fluid-filled cavities Blastocyst Cavity (blastocoel): - large fluid-filled cavities Blastocyst: - developing mass - once blastocyst cavity is formed - has hundreds of cells - same size as original zygote Two Distinct Cell Populations 1) Embryoblast or Inner Cell Mass: - located internally - eventually develops into the embryo 2) Trophoblast: - outer, superficial layer of cells - forms spherelike wall of blastocyst - will develop into outer chorionic sac that will surround fetus and fetal portion of placenta - site of exchange of nutrients and wastes between mother and fetus 5th Day of Fertilization - blastocyst erupts from zona pellucida - by digesting hole in it with an enzyme - then squeezes through hole Shedding of zona pellucida: - necessary for implantation into - vascular, glandular endometrial lining of uterus
What is placenta previa?
Entire or part of placenta covers or implants on inferior portion of uterus - covers internal os of cervix or near it - can cause spontaneous abortion - occurs in approximately 1 in 250 live births - maternal mortality increase from hemorrhage or infection Symptoms: - sudden, painless, bright-red vaginal bleeding in third trimester - cesarean section is preferred
Figure 29.5: Summary of events associated with the first week of development
Fertilization usually occurs in the uterine tube 1) Fertilization: - occurs within uterine tube - 12-24 hours after ovulation 2) Cleavage: - first cleavage completed about 30 hours after fertilization 3) Morula: - 3-4 days after fertilization 4) Blastocyst: - 4.5-5 days after fertilization 5) Implantation: - occurs about 6 days after fertilization
Describe the fetal period
Fetal Period: 9th week to birth - tissues and organs grow and differentiate - increased body growth - fetus vulnerable to effects of radiation, drugs
Describe the embryonic period
From fertilization to 8th week 1st week characteristics: - fertilization - cleavage of zygote - blastocyst formation - implantation
Figure 29.7: Gastrulation
Gastrulation involves the arrangement and migration of cells from epiblast.
Describe Gastrulation in the 3rd week of development
Gastrulation: - occurs 15 days after fertilization - bilaminar (two-layered) embryonic disc transforms into trilaminer embryonic disc - rearrangement and migration of cells from epiblast Embryonic Disc: - consists of epiblasts and hypoblast Trilaminer Embryonic Disc: - three primary germ layers 1) Ectoderm 2) Mesoderm 3) Endoderm Primary Germ Layers: - major embryonic tissues - tissues and organs of body develop Progression of Gastrulation 1) Primitive Streak: - faint groove on dorsal surface of epiblast - elongates from posterior to anterior part of embryo - establishes head and tail ends of embryo - establishes right and left sides of embryo 2) Primitive Node: - end of primitive streak - rounded structure formed by epiblastic cells 3) Invagination: - after formation of primitive streak - cells of epiblast move inward below primitive streak and detach from epiblast 4) Endoderm: - after cells have invaginated - displaced hypoblast cells - epithelia - composed of tightly packed cells - becomes epithelial lining of GI tract, respiratory tract, several other organs 5) Mesoderm: - cells that remain between epiblast and newly formed endoderm cells - loosely organized connective tissue - becomes muscles, bones, connective tissue, peritoneum 6) Ectoderm: - cells that remain in epiblast - epithelia - composed of tightly packed cells - becomes epidermis of skin and nervous system 16 Days After Fertilization Notochordal Process: - hollow tube of cells in midline of embryo - caused by migration of mesodermal cells from primitive node toward end of embryo 22-24 Days After Fertilization Notochord: - solid cylinder of cells that was once the notochordal process - plays role in induction - induces mesodermal cells to develop into vertebral bodies - forms nucleus pulposus of intervertebral discs Induction Process: - one tissue (inducing tissue) stimulates development of adjacent unspecialized tissue (responding tissue) into a specialized one - inducing tissue produces chemical substances that influences the responding tissue Oropharyngeal Membrane: - structure closer to head end - breaks down during week 4 of development - connects mouth cavity to pharynx and remainder of GI tract Cloacal Membrane: - structure closer to the tail end - degenerates in week 7 - forms openings of anus and urinary and reproductive tracts Allantois: - small, vascularized outpouching of wall of yolk sac - early formation of blood vessels and blood - associated with development of urinary bladder
Describe fertilization
Genetic material from: - haploid sperm cell (spermatozoon) - and haploid secondary oocyte - merge into single diploid nucleus 200 million sperm enters: - vagina Only 2 million reach: - cervix of uterus 200 reach: - secondary oocyte Sperm viable for up to: - 48 hours - after deposition in vagina Secondary oocyte viable for: - 24 hours after ovulation Location of Fertilization: - usually in uterine (fallopian) tube - within 12-24 hours after ovulation Pregnancy most likely to Occur: - intercourse takes place during 3 day window - 2 days before ovulation - 1 day after ovulation Path of sperm: - from vagina - into cervical canal - use flagella (tails) - sperm travel through rest of uterus to fallopian tubes with help from contractions in walls Prostoglandins: - located in semen - aid motility of sperm through uterus into uterine tube Capacitation Occurs once sperm reach: - secondary oocyte Sperm unable to fertilize oocyte until: - capacitation is complete - 7 hours later Series of functional changes: - cause sperm's tail to beat faster and stronger - prepares plasma membrane to fuse with oocytes plasma membrane Female reproductive tract secretions remove the sperms: - cholesterol - glycoprotein - proteins - from plasma membrane around head of sperm cell Only capacitated sperm capable of responding to: - attraction by oocyte - the chemical factors produced by surrounding cells of oocyte Penetration of Two Layers: - enable for fertilization to occur - along with acrosomal enzymes and strong tail movements - sperm able to penetrate corona radiata and make contact with zona pellucida 1) Corona Radiata: - granulosa cells that surround the oocyte 2) Zona Pellucida: - clear glycoprotein layer - between corona radiata and oocytes plasma membrane - glycoprotein ZP3 - sperm receptor Acrosome: - helmet structure surrounding head of sperm - contains enzymes Sperm Receptor: - glycoprotein ZP3 - in zona pellucida - triggers acrosomal reaction after binding to sperm head Acrosomal Reaction: - release of contents of acrosome - acrosomal enzymes digest path through zona pellucida - lashing sperm tail pushes sperm cell through - sperm that penetrates zona pellucida and reach oocytes plasma membrane fuses with oocyte Blocking of Polyspermy (fusion of more than one sperm cell): - caused by fusion of sperm cell with secondary oocyte - prevents fusion of more than one sperm cell Fast block to Polyspermy: - cell membrane of oocyte depolarizes (fast block to polyspermy) - depolarized oocyte cannot fuse with another sperm - depolarization triggers release of intracellular calcium ions - stimulate exocytosis of secretory vesicles from oocyte Slow block to Polyspermy: - molecules released by exocytosis inactivate the ZP3 receptors - and harden the entire zona pellucida Completion of Meiosis II - once sperm cell enters secondary oocyte - divides into larger ovum (mature egg) - and smaller second polar body that fragments and disintegrates - male pronucleus develops from nucleus of head of sperm - female pronucleus develops from nucleus of fertilized ovum Syngamy: - occurs after female and male pronucleui form and fuse - produces single diploid nucleus Zygote: - fusion of haploid (n) pronuclei restores diploid number (2n) of 46 chromosomes - now a fertilized ovum of 46 chromosomes Dizygotic (fraternal) Twins: - produced from independent release of two secondary oocytes - fertilization of the oocytes by different sperm - genetically different - may or may not be the same sex Monozygotic (identical) Twins: - from single fertilized ovum - same genetic material - always the same sex - arise from separation of developing cells into two embryos - 99% of cases occur in 8 days - separation occurs later than 8 days - produces conjoined twins
Figure 29.3: Relation of blastocyst to the endometrium of the uterus at the time of implantation
Implantation, the attachment of a blastocyst to the endometrium - occurs about 6 days after fertilization
Describe the development of chorionic villi and placenta
Lacunae: - spaces that fills with maternal blood - after the erosion of maternal blood vessels - from the invasion of embryonic tissue within uterine wall End of 2nd Week of Development Chorionic Villi: - fingerlike projections - consist of chorion (syncytiotrophoblast surrounded by cytotrophoblast) - projects into endometrial wall of uterus End of 3rd Week of Development - blood vessels develop into chorionic villi - blood vessels in chorionic villi connect to embryonic heart through the umbilical arteries and umbilical vein through connecting stalk - connecting stalk eventually becomes umbilical cord Intervillous Spaces: - unity of fetal blood capillaries within chorionic villi that have projected into the lacunae - bathe chorionic villi with maternal blood - maternal blood bathes chorion-covered fetal blood vessels * maternal and fetal blood vessels do not join Exchange of Oxygen and Nutrients Between Fetus and Mother: - blood from mothers intervillous spaces (space between chorionic villi) - diffuse across cell membranes into capillaries of the villi - waste produces (CO2) diffuse in opposite direction Placentation: - process of forming the placenta Placenta - site of exchange of nutrients and wastes between mother and fetus - produces hormones needed to sustain pregnancy - develops from two separate individuals - fully developed - looks like a pancake Function of Placenta - diffusion of oxygen and nutrients from maternal blood into fetal blood - diffusion of CO2 and wastes from fetal blood into maternal blood - protective barrier from most microorganisms Those that can pass through are: - AIDS - German measles - chickenpox - measles - encephalitis - poliomyelitis - drugs - some medications can pass freely Stores nutrients: - carbohydrates - proteins - calcium - iron - released into fetal blood when needed Placenta at Beginning of 12th Week - two separate parts 1) fetal portion: - formed by chorionic villi of chorion 2) maternal portion: - formed by decidua basalis of endometrium Umbilical Cord - connection between placenta and embryo (later fetus) - develops from connecting stalk - 2 cm wide, 50-60 cm long Two umbilical arteries: - carry deoxygenated fetal blood - to placenta One umbilical vein: - carries oxygen and nutrients - from mothers intervillous spaces into fetus Amnion surrounds: - entire umbilical cord - gives shiny appearance Wharton's Jelly: - derived from allantois - supporting connective tissue After Birth of Baby Afterbirth: - placenta detaches from uterus - umbilical cord tied off then severed - small portion of cord still attached (withers off 12-15 days later) Umbilicus: - scar left from umbilical cord
Describe Neurolation
Neural Plate: - notochord prompts ectodermal cells to form neural plate - end of 3rd week lateral edges of neural plate become elevated and fold Neural Fold: - lateral edges of neural plate that has folded and become elevated Neural Groove: - depressed midregion Neural Tube: - conversion of neural plate into neural tube by the fusing of neural folds - occurs in middle of embryo - progresses toward head and tail ends - neural tube cells develop into brain and spinal cord Neurolation: - process by which - neural plate - neural folds - neural tube form Neural Crest: - several layers of ectodermal cells that have migrated from neural tube Neural Crest Cells Give Rise Too: - sensory neurons - postganglionic neurons of peripheral nerves - adrenal medulla - melanocytes (pigment cells) of skin - arachnoid mater and pia mater of brain and spinal cord - almost all skeletal and connective tissue components of head 4 Weeks After Fertilization - head of neural tube develops into three areas Primary Brain Vesicles 1) Prosencephalon: - forebrain 2) Mesencephalon: - midbrain 3) Rhombencephalon: - hindbrain 5 Weeks After Fertilization 1) Prosencephalon: - develops into secondary brain vesicles called telencephalon and diencephalon 2) Rhombencephalon: - develops into secondary brain vesicles called metencephalon and myelencephalon - areas of neural tube adjacent to myelencephalon develop into spinal cord
What is anencephaly
Neural Tube Defects - caused by abnormal close and development of neural tube Include: - spina bifida - anencephaly Anencephaly: - cranial bones fail to develop - certain parts of brain remain in contact with bone and begin to degenerate - breathing and heart rate usually affected - fetus dies or child dies within couple days - chance is 1 in 1000 births and 2-4 times higher in female infants
Figure 29.9: Neurulation and the Development of Somites
Neurolation is the process by which the neural plate, neural folds, and neural tube form a) 17 Days Head End: - neural plate Tail End: - cut edge of amnion - primitive streak b) 19 Days Head End: - neural plate - neural groove - neural fold Tail End: - primitive node - primitive streak c) 20 Days Head End: - neural plate - neural groove Tail End: - somite - primitive streak d) 22 Days Head End: - neural fold - somite Somite: - dermatome - myotome - sclerotome Lateral Plate Mesoderm: - somatic mesoderm - splanchnic mesoderm
What is stem cell research and therapeutic cloning?
Stem Cells: - unspecialized cells that have ability to divide for indefinite periods - give rise to specialized cells - zygote is stem cell - pluripotent stem cells can give rise to nearly (not all) different types of cells - multipotent stem cells have specific function (develop from pluripotent stem cells) Pluripotent Stem Cells in Research: - derive from: 1) Embryoblast of embryos in blastocyst stage - destined for infertility treatments but not used 2) Non living fetuses terminated during 3 months of pregnancy Therapeutic Cloning: - procedure using genetic material of patient with particular disease to create pluripotent stem cells to treat the disease - used to treat cancer, Alzheimer, spinal cord injury, diabetes, heart disease, stroke, burns, birth defects, osteoarthritis, rheumatoid arthritis - stem cells found in adult woman in unfertilized eggs - reimplanted after cancer treatment to restore fertility
Figure 29.4: Regions of decidua
The decidua is a modified portion of the endometrium that develops after implantation
