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Chapter 58: Vertebrate Development

Chapter Outline

Chapter 58: Vertebrate Development

58.0 Introduction
  1. Vertebrate Reproductive Process

    1. Two Haploid Gametes Fuse to Form Diploid Zygote

      1. Zygote grows into a multicellular individual fig 58.1

      2. Compose of different tissues and organs

    2. Development in All Vertebrates Occurs in Six Stages

58.1 Fertilization is the initial event in development
  1. Stages of Development

    1. Fertilization Is the Union of Male and Female Gametes

      1. External process in fish and amphibians

      2. Internal in other vertebrates

        1. Sperm: Male gametes are small, motile cells

        2. Eggs: Also called oocyte

        3. Sperm introduced into female reproductive tract

        4. Encounter oocyte in fallopian tube, location of fertilization

      3. Consists of three stages

        1. Penetration

        2. Activation

        3. Fusion

    2. Penetration

      1. Secondary oocyte released from fully developed Graafian follicle at ovulation

      2. Surrounded by several protective layers fig 58.2

        1. Outer layer is granulosa cells that surrounded it in follicle

        2. Zona pellucida glycoprotein layer

        3. Egg plasma membrane

      3. Sperm advances through these layers

        1. Acrosome contains glycoprotein-digesting enzymes to go through two outer layers

        2. Sperm meets egg plasma membrane

        3. Egg cytoplasm bulges, engulfs head of sperm, permits entry of its nucleus fig 58.3

    3. Activation

      1. Egg activation initiated by sperm penetration

        1. Frogs, reptiles, birds: Many sperm enter egg, first one fertilizes it

        2. Mammals: Penetration alters egg membrane, prevents further entry

      2. Sperm penetration has three other effects on egg

        1. Completion of meiosis: Produce haploid nucleus, second polar body

        2. Rearrangement of egg cytoplasm around point of sperm entry

          1. Movements establish bilateral symmetry of developing individual

          2. Example: Formation of gray crescent in frogs fig 58.4

        3. Characterized by increase in protein synthesis and metabolic activity

          1. Protein coded for by mRNA present in unfertilized egg

          2. mRNA already present in unfertilized egg

      3. Egg can be activated with physical stimulation

        1. Development may continue parthenogenetically

        2. Few fish, amphibians, reptiles rely on this form of reproduction

    4. Fusion

      1. Fusion of sperm nucleus with egg nucleus, forms diploid nucleus of zygote

      2. Triggered by activation of the egg

      3. No fusion of nuclei if sperm nucleus injected without activation

58.2 Cell cleavage and the formation of a blastula set the stage for later development
  1. Cell Cleavage Patterns

    1. Second Major Event in Vertebrate Reproduction

      1. Rapid division of zygote into a greater number of sequentially smaller cells tbl 58.1

        1. Divisional process called cleavage

        2. Not accompanied by an increase in the size of the zygote

      2. Results in formation of the morula, 32 tightly packed cells

        1. Each cell called a blastomere

        2. Blastomeres secrete fluid into center as they divide

      3. Further divisions result in the formation of a hollow blastula

      4. Cells surround a fluid-filled cavity called the blastocoel

      5. Cell cleavage pattern is influenced by presence, location of the yolk fig 58.5

    2. Primitive Chordates fig 58.6

      1. Egg has little or no yolk

        1. Holoblastic cleavage: Occurs through the entire egg

        2. Example: Lancelets and agnathans

      2. Forms a symmetrical blastula with equally sized cells

    3. Amphibians and Advanced Fish fig 58.7

      1. Egg has more yolk in one hemisphere, division of yolk-laden cells is slow

      2. Holoblastic cleavage forms asymmetrical blastula

        1. Large cells with yolk at vegetal pole

        2. Mass of small yolk-poor cells at animal pole

    4. Reptiles and Birds fig 58.8

      1. Egg almost entirely yolk, small amount of cytoplasm at one pole

      2. Meroblastic cleavage occurs in blastodisc of polar cytoplasm

      3. Blastodisc is not spherical, but like a thin cap upon the yolk

    5. Mammals fig 58.9

      1. Similar to reptilian eggs from which they evolved

      2. Egg contains little yolk, cleavage is holoblastic

        1. Form ball of cells surrounding blastocoel

        2. Inner cell mass localized at one pole, analogous to reptile blastodisc

        3. Forms developing embryo

      3. Trophoblast: Outer cell mass analogous to reptile egg covering

        1. Part enters endometrium of uterus, contributes to placenta

        2. Part of placenta formed from fetal tissue, the trophoblast

        3. Part composed of modified maternal endometrial tissue

    6. The Blastula

      1. Regardless of appearance, cells of blastula differ from one another

        1. Sizes are different which effects rate of division

        2. Contain different portions of cytoplasm

      2. Cells contain signal substances that affect development

        1. Substances clustered at specific sites

        2. Cytoplasm reoriented with respect to site of sperm entry

        3. Signals endow different daughter cells with developmental instructions

      3. Egg is prepatterned

        1. Pattern of cytoplasm determines developmental fate of embryonic cells

      4. Cells in contact with different sets of neighboring cells

        1. Major factor influencing developmental fate of each cell

        2. Positional information important in vertebrates

        3. Sets up three embryonic axes

          1. Anterior-posterior

          2. Dorsal-ventral

          3. Proximal distal

        4. Provides each cell with unique information about its developmental fate

58.3 Gastrulation forms the three germ layers of the embryo
  1. The Process of Gastrulation

    1. Gastrulation: First Visible Result of Prepatterning

      1. Certain groups of cells move inward from surface

        1. Cells invaginate (dent inward) and involute (roll inward)

        2. Events determine basic developmental pattern of vertebrate embryo

        3. Results in formation of primary germ cell layers tbl 58.2

          1. Ectoderm forms epidermis and neural tissue

          2. Mesoderm gives rise to connective tissue, muscle, vascular elements

          3. Endoderm forms lining of gut and its derivatives

      2. Mechanism of cell movement

        1. Cells creep over stationary cells by actin filament contraction

          1. Change shapes of migrating cells

          2. Affect invagination of blastula tissue

        2. Migrating cells possess genetic surface polysaccharides

          1. Cells with similar polysaccharides adhere

          2. Migrate together as single mass

      3. Pattern of gastrulation dependent in shape of blastula

    2. Gastrulation in Primitive Chordates

      1. Process in organisms that develop from symmetrical blastulas

        1. Surface of blastula invaginates into blastocoel

        2. Half of blastula's cells move into interior

          1. Looks like indenting tennis ball

          2. Process stops when cells push up against opposite side

        3. Forms two-layered, cup-shaped gastrula fig 58.10

          1. Hollow structure is the archenteron which becomes the gut

          2. Opening of archenteron is the blastopore which becomes the anus

        4. Gastrulation produces an outer ectoderm and an inner endoderm

          1. Mesoderm forms from pouches pinched off the endoderm

          2. Primary tissues determine destiny of subsequent tissues and organs

    3. Gastrulation in Most Aquatic Vertebrates

      1. Organisms have asymmetrical yolk distribution

        1. Fewer, larger yolk-laden cells in vegetal pole

        2. Gastrulation more complex

      2. Layer of surface cells invaginates, forms crescent-shaped slit, site of future blastopore

        1. Cells from animal pole involute over dorsal lip of blastopore fig 58.11

          1. Same location as gray crescent of fertilized egg

          2. Involuting layer presses against opposite wall, eliminates blastocoel

          3. New cavity called archenteron, opening is the blastopore

          4. Blastopore fills with yolk-rich cells, forms yolk plug

        2. Outer layer of cells is the ectoderm, inner layer is the endoderm

        3. Mesoderm forms by migration of dorsal and ventral lip cells

    4. Gastrulation in Reptiles, Birds and Mammals fig 58.12

      1. Developing embryo is cap of cells, not whole sphere

      2. Two sides of embryo not separated by yolk

      3. Cell layers differentiate without movement

        1. Lower layer becomes endoderm

        2. Upper layer becomes ectoderm

      4. Mesoderm arises from invagination and involution of cells of upper layer fig 58.13

        1. Site of involution appears as a furrow on surface

        2. Called the primitive streak, analogous to elongated blastopore

58.4 Body architecture is determined during the next stages of embryonic development
  1. Developmental Processes During Neurulation

    1. Neurulation: Formation of Notochord and Hollow Dorsal Nerve Cord

      1. Process occurs only in chordates

      2. Notochord forms from mesoderm along embryo dorsal midline

        1. Flexible rod located along dorsal midline in all chordate embryos

        2. Function replaced by vertebral column that develops from mesoderm

      3. Neural groove forms from ectoderm above the notochord fig 58.14

        1. Layer of ectoderm invaginates, forms long crease

        2. Edges of groove move together, fuse and form the neural tube

        3. Neural tube differentiates into spinal cord and brain

      4. Process of induction influences formation of each tissue

        1. One embryonic region influences development of adjacent region

        2. Dorsal lip of blastopore induces formation of notochord

        3. Presence of notochord induces ectoderm differentiation into neural tube

    2. Changes in Mesoderm Affect Whole Body Architecture

      1. Formation of blocks of tissue called somites

        1. Progressively more somites formed during development

        2. Give rise to muscles, vertebrae and connective tissue

      2. Some body organs develop within another strip of mesoderm bordering somites

      3. Remaining mesoderm moves around, surrounds inner endoderm layer

        1. Mesoderm separated into two layers

          1. Outer layer associated with body wall

          2. Inner layer associated with gut

          3. Space between the two is the coelom, becomes adult body cavity

    3. The Neural Crest

      1. Neurulation occurs in all chordates, neural crest forms only in vertebrates

      2. Edges of neural groove form strip called neural crest prior to closing into tube

        1. Neural crest cells become incorporated into roof of neural tube fig 58.14d,e

        2. Cells shift to sides of developing embryo

      3. Key event in evolution of vertebrates

        1. Neural crest cells eventually develop into characteristic vertebrate structures

        2. Differentiation of neural crest cells depends on location

          1. Anterior portions merge with forebrain

            1. Clusters of associated ectodermal cells thicken into placodes

            2. Develop into parts of the sense organs located on the head

            3. Sense organs occur in pairs as a result of the two lateral strips

          2. Neural crest cells in posterior portions gave different developmental fate

            1. Remaining cells migrate from nerve tube to locations in head and trunk

            2. Form connections between neural tube and surrounding tissues

            3. Dictate development of characteristic vertebrate structures

            4. Migration is not simply change in relative position of cells

            5. Neural crest cells actually pass through other tissues

    4. The Gill Chamber fig 58.14

      1. Primitive chordates are filter feeders

        1. Use rapid beating of cilia to draw water in through slits in pharynx

        2. Slits evolved into vertebrate gill chamber

        3. Greatly improved respiration

        4. Key event in transition from filter-feeding to active predation

      2. Development of gills involve cells from neural crest

        1. Form cartilaginous bars between embryonic pharyngeal slits

        2. Induce portions of mesoderm to form muscles along cartilage

        3. Form neurons to carry impulses between nerve cord and muscles

        4. Aortic arch passes through each of bars

        5. Lined with neural crest cells, bars and blood supply branch and form gills

      3. Gill chamber is efficient pump

        1. Stiff bars can be bent inward by muscles controlled by nerves

        2. Gills are highly efficient oxygen exchangers

        3. Increase respiratory capacity

    5. Elaboration of the Nervous System

      1. Neural crest cells migrate to notochord

      2. Some form sensory neurons in dorsal root ganglia

      3. Others become specialized as Schwann cells

        1. Insulate nerve fibers

        2. Permit rapid conduction of nerve impulses

      4. Others form autonomic ganglia and adrenal medulla

        1. Important to sympathetic nervous system

        2. Cells in adrenal medulla secrete epinephrine when stimulated

        3. Similarity of epinephrine and norepinephrine related to neural crest derivation

    6. Sensory Organs and Skull

      1. Variety of sense organs develop from placodes

        1. Include olfactory and lateral line organs

      2. Teeth and cranial bones develop from neural crest cells

  2. Embryonic Development and Vertebrate Evolution

    1. The Role of the Neural Crest in Vertebrate Evolution fig 58.15

      1. Adaptations of neural crest cells promoted predatorial activities

      2. Resultant increased metabolic rate allowed for greater activity

      3. Other derivatives improved sensory capabilities

        1. Better detection of prey

        2. Improved ability to orient spatially during prey capture

        3. Allowed for quicker response to sensory information

    2. Ontogeny Recapitulates Phylogeny

      1. Patterns of development reflect simpler patterns that occur in earlier forms fig 58.16

        1. Bird and mammal development elaborate upon reptile development

        2. Reptile development elaborates upon amphibian development

        3. Traces of relic structures may be visible in early stages of development

          1. Human embryo at certain stages possesses pharyngeal slits

          2. At later stages has tail

      2. Patterns of development are built upon one another in incremental steps

        1. Instructions are layered upon other instructions

        2. Haeckel's "biogenic law": Ontogeny recapitulates phylogeny

          1. The course of vertebrate development (ontogeny) involves the same changes that occurred over the course of vertebrate evolution (phylogeny)

          2. Not literally a true phrase

          3. Embryonic stages reflect embryonic ancestors not adult ancestors

  3. Extraembryonic Membranes

    1. Membranes Evolved as an Adaption to Terrestrial Life

      1. Reptiles, birds and mammals develop within amniotic membrane

      2. One of several membranes formed from embryonic cells

        1. Located outside the body of the embryo

        2. Therefore called extraembryonic membranes

        3. Later become fetal membranes

        4. Include amnion, chorion, yolk sac and allantois

      3. In birds, amnion and chorion completely surround embryo fig 58.17

        1. Amnion is innermost membrane, surrounds embryo directly

          1. Suspends embryo in amniotic fluid

          2. Mimics aquatic environment of fish and amphibian embryos

        2. Chorion located next to egg shell

          1. Separated from other membranes by extra-embryonic coelom

      4. Yolk sac is critical for bird and reptile nutrition

        1. Present in mammals, does not nourish embryo

      5. Allantois derives from outpouching of gut

        1. Functions to store uric acid excreted in urine of birds

        2. Expands to form sac that fuses with chorion just under egg shells

        3. Membranes fuse, are supported with blood vessels

          1. Blood vessels brought into close contact with porous egg shell

          2. Serves as functioning "lung" of bird embryo

      6. Embryonic cells in mammals form inner cell mass and surrounding cells

        1. Inner cell mass becomes body of embryo

        2. Surrounding cells called trophoblast

        3. Trophoblast implants into endometrial lining of maternal uterus

          1. Becomes chorionic membrane fig 58.18

          2. Part of chorion in contact with endometrium becomes placenta

        4. Allantois contributes blood vessels to future umbilical cord

        5. Umbilical cord delivers fetal blood to placenta for gas exchange

58.5 Human development is divided into trimesters
  1. First Trimester

    1. Human Development Shows Its Evolutionary Origins

      1. Closely resembles development in birds

        1. Flattened blastodisk in birds or inner mass in mammals forms a primitive streak

        2. Gives rise to three primary cell types

      2. Human development takes 266 days divided into three trimesters

    2. The First Month

      1. First cleavage division occurs within 30 hours after fertilization

      2. Embryo continues to travel down fallopian tube to uterus

        1. Embryo in uterus is a blastula, called blastocyst in mammals

        2. Inner mass of cells and surrounding layer of trophoblast cells fig 58.9

      3. Embryo divides and initiates formation of amnion and chorion

        1. Blastocyst implants into endometrial lining of uterus

      4. Processes of second week

        1. Chorion forms branched extensions, chorionic frondosum (fetal placenta)

          1. Protrude into endometrium fig 58.19

          2. Induce endometrial tissue to become decidua basalis (maternal placenta)

          3. Chorionic frondosum and decidua basalis form placenta fig 58.20

          4. Mother's and embryo's blood in close proximity, do not mix fig 58.21

          5. Placenta and vascularization provides for

            1. Exchange of gases, oxygen and carbon dioxide

            2. Nourishment for embryo

            3. Detoxifies certain molecules in embryo

            4. Secretes hormones

      5. Hormones released by the placenta

        1. Human chorionic gonadotropin (hCG)

          1. Secreted by trophoblast cells before becoming the chorion

          2. Hormone assayed in pregnancy tests

          3. Action almost identical to LH, maintains corpus luteum

          4. Corpus luteum continues to secrete estradiol and progesterone

          5. Prevents further menstruation and ovulations

      6. Gastrulation in second week, primitive streak visible, primary tissues differentiated

      7. Neurulation occurs in the third week

        1. Formation of neural tube along dorsal axis of embryo

        2. First somites visible, give rise to muscles, vertebrae, connective tissue

        3. Blood vessels and gut evident by end of third week

      8. Embryo about 2 mm long

      9. Organogenesis occurs in the fourth week fig 58.22a

        1. Eyes and heart form, heat begins to beat

        2. 30 pairs of somites visible, arms and legs begin to form

        3. Embryo now 5 mm long

      10. Pregnancy may not yet be detected

        1. Importance of early detection of pregnancy

          1. 1960 use of tranquilizer thalidomide caused deformed babies

          2. Organogenesis may be disrupted by contraction of German measles

        2. Most spontaneous abortions occur during first month

    3. The Second Month

      1. Morphogenesis occurs, body begins to take shape fig 58.22b

        1. Limbs and body organs are recognizable

        2. Tail present, bones later fuse to form coccyx

      2. Major organs present, liver, pancreas, gallbladder

      3. Embryo now 25 mm, weighs one gram

    4. The Third Month

      1. Nervous system and sense organs develop fig 58.22c

      2. Limbs show movement, muscle activities begin

      3. Developing individual called a fetus

      4. All major organs established

      5. Hormonal events

        1. Placental secretion of hCG declines, corpus luteum regresses

        2. No menstruation since placenta secretes estradiol and progesterone fig 58.23

        3. Inhibit release of FSH and LH

          1. Prevent ovulation

          2. Maintain uterus, prepare for labor and delivery

          3. Stimulate development of mammary glands for lactation

  2. Second and Third Trimesters

    1. Second Trimester

      1. Bone growth occurs during the fourth month fig 58.22d

      2. In fifth month body covered with fine hair called lanugo

      3. Kicking and heartbeat detected, size is 175 mm and 225 grams

      4. Significant growth occurs in sixth month, 1 foot in size, 1.3 pounds

      5. Fetus cannot survive outside uterus without special medical support

    2. Third Trimester

      1. Predominantly a growth period, weight doubles several times

      2. Major nerve tracts of brain formed

      3. Neurological development continued after birth

        1. Birth must occur early to ensure passage through pelvis fig 58.24

        2. Birth occurs as soon as survivability is high

      4. Nutrients supplied via placenta

  3. Birth and Postnatal Development

    1. Birth

      1. In some mammals, changing hormone levels in the fetus initiate birth

        1. Have extra cell layer in adrenal cortex, called fetal zone

        2. Fetal pituitary secretes corticotropin

        3. Stimulates fetal zone to release steroid hormones

        4. Steroids cause maternal placenta to produce prostaglandins, induces contractions

      2. Human birth not initiated by this mechanism

        1. Uterus releases prostaglandins as a result of high levels of placental estradiol

        2. Estradiol stimulates uterus to produce more oxytocin receptors

        3. Uterus becomes more sensitive to oxytocin

        4. Prostaglandins begin contractions

        5. Sensory feedback stimulates release of oxytocin

      3. After birth, continued contractions expel the placenta and membranes

      4. Umbilical cord is tied and cut

    2. Nursing

      1. Milk production called lactation

        1. Occurs in alveoli of mammary glands fig 58.25

        2. Milk secreted into alveolar ducts

          1. Progesterone stimulates development of mammary alveoli

          2. Estradiol stimulates development of alveolar ducts

          3. Estradiol blocks actions of prolactin, inhibits prolactin secretion

          4. During pregnancy, mammary glands prepared for lactation, prevented from lactating

        3. Decline in progesterone and estradiol when placenta discharged after birth

          1. Anterior pituitary secretes prolactin, mammary alveoli produce milk

          2. Sustained by suckling stimuli, oxytocin produces milk-ejection reflex

      2. First milk produced is colostrum, nutritious, rich in antibodies

      3. Important pair bonding occurs during nursing fig 58.26

      4. Nursing may occur for one year or more

        1. At end of nursing, milk accumulation causes prolactin secretion to stop

        2. Accumulation of milk inhibits production of milk

    3. Postnatal Development

      1. Rapid growth continues after birth

        1. Body proportions are different as organs grow at different rates

        2. Head is disproportionately large, grows more slowly

        3. Allometric pattern of growth fig 58.27a

      2. Differential growth of head

        1. Most mammals exhibit extensive brain growth as fetuses

          1. Vast differences in appearance between infant and adult fig 58.27b

          2. Growth of cerebral portions decelerates, jaw continues to grow

          3. Humans are an exception

            1. Little difference in appearance between infant and adult humans

            2. Both brain and jaw grow after birth, proportions do not change

      3. Human brain growth after birth requires adequate nutrition and care

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