Comparative Anatomy Lecture 3,4,5

Telolecithal

Telolecithal

end + yolk ; yolk is asymmetrically distributed within the ovum

Phylogeny

evolutionary history of a lineage

Ontology

development of an individual organism from fertilized egg to adult

How are phylogeny and ontology related?

Closely related, ontogenetic development seems to repeat or recapitulate phylogenetic change

Recapitulation

exhibition (repetition) of ancestral adult features in the embryonic and juvenile stages of descendants ; common and is one outcome of heterochrony

Heterochrony

different + time ; changes in the timing or rate of developmental processes between ancestors and descendants

The external gills of adult Necturus salamanders is an example of what?

Paedomorphosis ; a result of heterochrony

Paedomorphosis

child + form ; the retention of ancestral juvenile characteristics by adult descendants

Developmental constraints

evolutionary changes result from modifications to the developmental process

Yolk is concentrated towards what pole?

Vegetal pole

Nucleus is at what pole?

Animal pole

Zygote

a single cell formed through fertilization of an ovum by a sperm

Cleavage

cell division

Blastula

stage at which the embryo contains only one tissue layer (or type) ; consists of several hundred cells

What is the nutritional source for developing embryos?

yolk

Gastrula

stage at which the embryo acquires multiple tissue or germ layers , first two then three

Layers of Gastrula

ectoderm - outer layer

endoderm - inner layer

mesoderm - middle layer

ectoderm

outer layer

endoderm

inner layer

mesoderm

middle layer

Neurula

-stage at which embryo has three germ layers and a neural tube

-Chordamesoderm extends forward along the midline from the blastopore or the primitive streak, and induces the overlaying ectoderm to thicken into a neural plate

Microlecithal -the ancestral chordate pattern

-cleavage is total ; cleavage furrows penetrate the entire cell, including the yolk

-cleavage is equal ; all blastomeres are of about the same size at any given time

-blastula is a hollow ball of cells with a cavity, called the blastocoel

Mesolecithal - lampreys, chondrostei, amphibia

- cleavage is total, but unequal

-concentration of yolk near the vegetal pole impedes cell division, so blastomeres in the vegetal hemisphere do not divide as quickly. blastomere are larger near the vegetal pole, and the blastocoel is displaced into the animal hemisphere

Macrolecithal -fishes, reptilia, aves, monotremata

-cleavage is partial ; mass of yolk is too large for cleavage furrows to penetrate, so it is limited to the small yolk-free region at the animal pole

- result is a flat, disk-shaped blastoderm that is separated from the yolk by a narrow blastocoel

Microlecithal - derived condition of Eutheria

-cleavage is total and equal

- ball-shaped blastula has an outer layer of cells, the trophoblast, and an inner cell mass. the inner cell mass displaces the blastocoel toward the vegetal pole

-blastula of all vertebrates consists of a single tissue layer (type) made up of several hundred cells

- polarity (animal-vegetal) relates to the axes of the future body

Gastrulation in Amphioxus(Cephalochordata) -represents a simple form of development that is ancestral to vertebrates:

-Vegetal hemisphere invaginates inward and extends to underlie the tissue layer of the animal hemisphere. -Rim of the double-walled cup or bowl shape closes up, leaving only a small opening, the blastopore.

-Embryo begins to elongate along the animal-vegetal axis, and rotates, developing anterior-posterior and dorsal-ventral polarity.

-Outer layer of the gastrula is the ectoderm.

-Inner tissue layer forms into a lengthwise tube, the archenteron, or primitive gut -gives rise to endoderm and mesoderm

Gastrulation in a mesolecithalvertebrateembryo (amphibian):

-Invagination will not work here, because the blastocoel is too small.

-Instead, cells roll inward (involution) creating the blastopore.

-Surface cells migrate toward the blastopore and involution extends into the blastocoel to form a second tissue layer.

-Gastrula first forms two layers: ectoderm and archenteron, and the ventral part of the archenteron is swollen with yolk.

-Archenteron will again form the endoderm and mesoderm, but the process is different in important ways.

Archenteron

-enclosed tube that runs along the length of the embryo.

-Dorsal part of the archenteron forms chordamesoderm, which forms the notochord and mesoderm.

-Notochord forms, as a lengthwise rod, from the dorsal wall of the archenteron.

Mesoderm

-forms from dorsolateral outpocketings of the archenteron.

-Hollow pockets of mesoderm "pinch off" in pairs, on each side of the notochord, forming somites.

-Cavities within these somites become the coelom.

-Ventral part of the archenteron becomes endoderm.

Enterocoely

mesoderm and coelom form from hollow outpocketings of the archenteron (primitive gut) ; Primitive type of mesoderm and coelom formation that is not found in vertebrates.

Gastrulation in a mesolecithal vertebrate embryo

-Dorsal part of the archenteron forms chordamesoderm, which forms the notochord (along midline) and paired left and right blocks of mesoderm, somites, dorsolaterally.

-Ventral part of the archenteron forms endoderm, which grows up under the chordamesoderm and encloses the gut tube.

Schizocoely

-split + hollow ; coelom forms by splitting or cavity formation within the mesoderm

-Process of mesoderm and coelom formation that is exhibited by all vertebrates.

-There is no outpocketingof the gut tube.

-Mesoderm (somites) that pinches off from the chordamesoderm is solid, not hollow, blocks.

-Coelom forms from cavities that open up in the somites, after they are formed.

Gastrulation in a macrolecithal vertebrate embryo (chick):

-Partial cleavage -cell division is restricted to a thin germinal disk at the animal pole, lying atop a large mass of yolk.

-Blastoderm delaminates, splits into two thin layers that are stacked on one another, the ectoderm and endoderm.

-Primitive streak, a lengthwise thickening, forms on the posterior part of the germinal disc.

-Surface cells move toward the primitive streak, where they involute and spread between the ectoderm and endoderm to form the mesoderm.

-Anterior to the primitive streak is the embryonic area, where the embryo develops.

-Involuted cells that move anteriorly from the primitive streak form the notochord(along the midline) and somites(mesoderm) to either side.

-Mesoderm that is lateral and posterior to the primitive streak will contribute to the extraembryonic, or fetal, membranes of the amniotic egg.

Gastrulation in mammalian microlecithal embryos -condition is derived from the macrolecithal(amniote) process:

-Blastocoel fills with maternal fluid and enlarges greatly.

-Trophoblast(outer layer of cells) forms the ectoderm.

-Endoderm forms by delamination of cells from the inner cell mass, which spread to line the trophoblast.

-Remaining inner cell mass (blastoderm) forms a primitive streak which produces the notochord and much of the mesoderm.

-Much mesoderm is also produced by mesenchyme(individual cells that detach from tissue layers and migrate) especially in the head and limbs.

Neurulation

establishes the foundation of the central nervous system

Neural folds

form along the margins of the plate and arch inward to form the neural tube, establishing the dorsal hollow nerve cord

Neural crests

some neural plate cells delaminate and aggregate along the corners of the neural folds ; unique to vertebrates

Neural crest cells

migrate individually over much of the body and induce other tissues to form many body structures

Ectodermal placodes

thickenings in the dorsolateral ectoderm of the head. interact with crest cells to form many important parts of the sensory and nervous system ; unique to vertebrates

Somitomeres

blocks of mesoderm aligned with and anterior to the notochord (in the head). Like somites, but not along notochord

Epimere

dorsal and segmented

mesomere

partially segmented

hypomere

unsegmented and sheet-like, extends to separate the endoderm and ectoderm. coelom expands only in the hypomere, where it separates the somatic layer (outer) from the splanchnic layer (inner)

Formation of basic vertebrate body form

- As neurulation progresses, the embryo starts to take on the recognizable shape and form of a vertebrate body, internally and externally

-stage is set for the differentiation of all of the vertebrate body organs and tissues, from these germ tissues

Derivatives of embryonic germ tissues

ectoderm

endoderm

mesoderm

ectoderm derivative

forms the epidermis of the skin and its derivatives (glands, hair, feathers, claws), lens of the eye, olfactory organs, inner ear, mouth lining

sensory ectoderm (neural tube)

forms the spinal cord and brain, motor nerves, retina of the eye

endoderm

forms the lining of the gut and of its derivative organs, including the lungs, liver, pancreas, and urinary bladder

mesoderm derivative

forms almost all of the muscular, skeletal, circulatory, and urogenital systems

extraembryonic (fetal) membranes

mesolecithal - in water

macrolecithal - in water

macrolecithal - in air

mesolecithal - in water

- cleavage is total

- yolk is contained within cells of embryo

- yolk supply is limited, so hatching is at small body size, and larvae must feed

- no extraembryonic membranes needed

nutrient intake- supply is internal

respiration- direct diffusion between embryo and surrounding water

excretion of waste- direct diffusion between embryo and surrounding water

macrolecithal - in water

-cleavage is only partial

-large mass of yolk is external to the embryo

- one extraembryonic membrane is needed

nutrient intake- yolk sac membrane

all 3 germ layers extend over the surface of yolk from the body stalk

vascularized membrane absorbs yolk and carries it to embryo

respiration- direct diffusion between embryo and surrounding water

excretion of waste- direct diffusion between embryo and surrounding water

macrolecithal - in air

-cleavage is only partial

-large mass of yolk is external to the embryo

-embryo is susceptible to desiccation in air

-direct diffusion between embryo and surrounding air is insufficient for respiration and excretion

- eggshell prevents desiccation

-more extraembryonic membranes needed

nutrient intake- yolk sac membrane

respiration- allantois membrane

excretion of waste- allantoic cavity

extraembryonic membranes of the amniotic egg

amniotic eggs possess four extraembryonic membranes : yolk sac, chorion, amnion, allantois, and usually an eggshell

mesoderm and endoderm extends over the mass of yolk to form :

yolk sac

mesoderm and ectoderm form a fold that extends around and fuses over the embryo forming :

outer chorion and inner amnion

enclosed amniotic cavity is filled with amniotic fluid which bathes and protects the embryo-

still in aqueous environment

Allantois

-forms as a pouch that extends from the gut into the extraembryonic coelom ( between chorion, amnion, and yolk sac)

-grows out to underly the eggshell, becomes highly vascularized, and functions for respiration

allantoic cavity

serves as a repository for excretory waste

Only vertebrates that do NOT require a wet environment to reproduce

Amniotes (includes Reptilia, Aves, Mammalia)

eggs of eutherian mammals are derived from:

macrolecithal, amniotic eggs of reptiles, but the eggs have very little yolk and the embryos complete development within the mother's reproductive tract

- embryo relies on placental transfer for nutrient intake, respiration, and waste excretion

placenta

structure that facilitates physiological exchange between maternal and fetal bloodstreams

- yolk sac, allantois, or both form the fetal portion of the placenta

chorio-allantoic placenta

in most mammals, the allantois is the primary contributor, and the location of the chorion results in it being incorporated into the placenta

umbilical cord

the body stalk

Integument

the outer covering of the body, including the skin and many derived structures.

-performs more functions than any other organ system in vertebrates

-often the largest (by weight) organ system in vertebrate bodies

functions of integument

1. physical protection of delicate tissues

2. water balance

3. thermoregulation

4.provides coloration

5. aids locomotion

6. respiration

7. secretion

8. houses many sense organs

two principal layers of skin

epidermis- from ectoderm on outer surface of the embryo

dermis- from dermatome, plus mesenchyme from the hypomere

some neural crest cells also invade the dermis

epidermis

stratified into two (or more) layers: stratum germinativum and stratum corneum

stratum germinativum

produces the rest of the epidermis ; cells divide and daughter cells migrate outward, transforming into many different kinds

stratum corneum

the dead outermost layer of the skin of tetrapods ; types and arrangements of epidermal cells is highly variable among taxa

- most cells are secretory in function. primarily mucous cells and proteinaceous cells

-produce a wide variety of secretions: mucus, slime, poisons, chemical signals, and light producing chemicals (in photophores)

-proteinaceous cells produce keratins and other proteinaceous products

keratins

water-insoluble proteins that form the horny material of feathers, hair, claws, reptile scales, and the stratum corneum

dermis

-usually thicker than the epidermis

-has fewer kinds of cells than epidermis

-contains a network of fibers

-most of the fibers are collagenous fibers; straight rope like strands

-elastic fibers are less numerous but they are what give skin its elasticity

-these fibers are arranged in different patterns and combinations to give skin added toughness, stiffness, or elastic recoil

layers of the dermis

stratum spongiosum - outer, vascular layer

stratum compactum - inner layer, usually thicker

smooth muscle fibers, fat deposits, and epidermal glands are often present within the dermis

development of skin derivatives

skin derivatives tend to form at the basal membrane, which separates dermis and epidermis ; mesenchyme along this boundary induces dermal, epidermal, or both tissues to form other derived structures

chromatophores

pigment cells that are similarly derived from neural crest cells and are concentrated along the epidermal-dermal boundary

integument of fishes- soft tissues

- all fishes have thin glandular skins

- skin contains many mucous glands, which secrete slimy mucus, forming a protective layer

-keratin is entirely absent (in most fishes)

integument of fishes- hard tissues

scales, denticles, armor plates, and teeth are all hard, dermal derivatives

enamel

- the hardest tissue in the vertebrate body

- composed entirely of crystalline calcium salts

-lies external to other hard tissues

dentinous tissue

-harder than bone but softer than enamel

-composed of calcium salts plus about 25% organic fibers

-lies internal to enamel and external to bone

-are many types, but that in tetrapods and some fishes is called dentine

bone - integument

-softer than dentine

- composed of calcium salts plus organic content

-lies internal to dentinous tissue (when both are present)

-usually has internal cells, spaces and canals, so it is less dense than other hard tissues

- is highly variable in structure

phylogeny of bony scales and derivatives

many hard structures of fishes and tetrapods are derived from the bony armor plates and scales of ostracoderms (cephalaspid and pteraspid agnathans) and placoderms

three primary layers of primitive armor plates and scales

denticles- outer surface; projections of dentine overlain by enamel

vascular bone- middle layer

lamellar (sheet-like) bone- basal layer

cosmoid scales

-most primitive type of fish scale

-smaller, but have the same layers as the ancestral armor

-present in placoderms and primitive sarcopterygians

ganoid scales

-lost the middle layer of vascular bone

-denticle layer is composed of ganoine, a lamellar enameloid

-present in acanthodii and some primitive actinopterygians, including gars

elasmoid scales

- evolved from ganoid

- lost the ganoine layer, leaving only lamellar bone

- thin layer of bone is interlaced with collagenous fibers, making the scale very thin and flexible

-occur in teleosts

osteoderms

-plates of bone that underlay the skin

-present in some reptiles and some extinct amphibians

membrane bones

-in head skeleton and pectoral girdle

- lost the enamel and dentine

-sank beneath the skin to join the internal skeleton

dermal denticles

-evolved independently from true scales

-bony basal layers lost, leaving only dentine and enamel

-small, tooth-like structures

-present in elasmobrachii

teeth

-also lost bony basal layer

-derived from the ancient integumentary armor of fishes

- as ossification of the integument was gradually reduced, denticles near the margin of the mouth specialized to aid in feeding and defense

integument of tetrapods- emphasis on epidermal derivatives

glands and keratinous strcutures

integument of amphibia

-skin of extant amphibians has a very thin epidermis and very thin stratum corneum

- very little keratin makes it susceptible to desiccation and easily abraded

-two type of glands in their skin- both of epidermal origin, but protruding into the dermis

mucous glands

secrete mucus to keep the skin moist (for respiration)

granular glands

produce distasteful or toxic substances (to deter predators)

integument of reptilia

-skin covered by horny scales

-keratin and secreted lipids form an airtight and watertight barrier

-epidermis forms a complete, heavily-keratinized, body covering of horny scales

-folds in the epidermis form what we call "scales"

-reptilian "scales" are NOT homologous with fish scales

-osteoderms underlie scales in some reptiles

-bony plates derived from dermal scales of fishes

-dermis is thin

-mucous glands are absent but SCENT GLANDS are common

-keratinaceous claws tip digits

what makes lepidosauria unique

lepidosauria shed the entire epidermis as a single unit

integument of aves

-thin skin with feather

-thin, weakly keratinized skin

-skin only loosely connected to underlying tissues

feather

-keratinaceous appendages of the skin

-are unique to birds

-cover most of the body

-epidermal in origin but the follicle protrudes into and is nourished by the dermis

-lower hindlimbs and feet are covered with horny, reptilelike scales

-keratinaceous claws tip digits

-very few glands in skin-only one in most

functions of feathers

flight

insulation

protection

uropygial gland

secretes oil to waterproof and condition feathers

integument of mammalia

-hair, scales, claws, and integumentary glands

-skin is thick, especially the dermis

- thick, fibrous dermis makes good leather

- epidermis thickens where hair is sparse and/or pressure and abrasion are common

-keratin forms: claws and hooves, armadillo armor, scales on opossum, and rat tails, horns, and hair

-hair is epidermal but the hair follicle sinks into the dermis

-many epidermal glands, most sink into dermis

-sweat, sebaceous, and mammary glands are unique to mammals

-also have scent glands