Chapter 44 Principles of Reproduction and Development

I. The Beginning: Reproductive Modes
A. There are two basic types of animal reproduction:
1 Asexual reproduction by budding (example: sponge) or fission (example: flatworm) results in
offspring identical to the parents; this is a useful strategy in stable environments.
2 Sexual reproduction permits adaption through variation but is biologically costly because the
sexes are separate; animals must produce gametes and must find each other (usually) for
fertilization to occur.
B. Difficulties must be overcome by animals that achieve reproductive success with separate sexes. 1 Reproductive timing must allow for male and female gametes to be available at nearly the same
time.
a. Sensory structures and hormonal controls must be precise in both parents.
b. Seasonal cues and behavioral patterns must evoke a suitable response in both sexes.
2 Fertilization also comes at a cost with separate sexes.
a. External fertilization in water requires the production of large numbers of gametes.
b. Internal fertilization requires an investment in elaborate reproductive organs, including the
penis, to transfer sperm to the female.
3 Energy is set aside for nourishing some number of offspring.
a. Those eggs with little yolk must develop larval stages quickly.
b. Others, such as birds, have adequate food reserves for a more lengthy development within
the shell.
c. Some eggs, such as those of humans, have no yolk; the embryo must be nourished with energy
molecules drawn from the mother.

II. Stages of Development
A. Gamete formation: eggs or sperm form and mature within the parents.
B. Fertilization begins when a sperm penetrates an egg and is completed when the sperm nucleus fuses
with the egg nucleus, resulting in formation of the zygote.
C. Repeated mitotic divisions—cleavage—convert the zygote to a blastula; cell numbers increase but not
cell size.
D. Gastrulation results in three germ layers, or tissues:
1 Endoderm is the inner layer; it gives rise to the gut and organs derived from it.
2 Mesoderm is the middle layer; muscle, organs of circulation, reproduction, excretion, and
skeleton are derived from it.
3 Ectoderm is the outer layer; it gives rise to the nervous system and the outer layers of the
integument.
E. Organ formation begins as germ layers subdivide into populations of cells destined to become unique in
structure and function.
F. During growth and tissue differentiation, organs acquire specialized chemical and physical properties.
III. Patterns of Development
A. Key Mechanisms: An Overview
1 Cell differentiation is the specialization of various cells in different locations within the embryo;
morphogenesis is the organization of these cells into tissues and organs.
2 Cytoplasmic Localization
a. Cytoplasmic determinants, such as proteins and RNA, in the oocyte help determine the future
shape and arrangement of body parts.
b. Simply by their location, daughter cells produced during cleavage end up at certain places in
the cytoplasm near these determinants.
3 Embryonic Induction
a. During embryonic development, groups of cells interact to affect other groups.
b. This interaction may be by means of hormones, growth factors, and so on that trigger actions
such as protein synthesis.
B. Developmental Information in the Egg
1 The oocyte contains the majority of materials that will affect early development.
a. RNA transcripts will be translated into proteins, such as histones, that are used in
chromosome replication.
b. Ribosomal subunits necessary for protein synthesis are stockpiled.
c. Microtubules will influence the division orientation during cleavage.
d. The position of the nucleus will identify the animal pole (closest) and vegetal pole (opposite),
where yolk will accumulate.
e. The amount and distribution of yolk dictate the cleavage planes and resulting spatial positions
of the resulting cells.
2 The sperm contributes little more than the paternal DNA.
C. Fertilization
1 Penetration of the egg by the sperm triggers a structural reorganization in the egg cytoplasm. 2 Microtubules move granules from the animal pole to form a gray crescent near the equator
opposite the penetration site.
3 Near the crescent, the body axis of the frog embryo will become established and gastrulation will
begin.
D. Cleavage
1 Repeated mitotic divisions of the zygote produce many cells that are not necessarily larger but
differ in size, shape, and activity.
2 One of the earliest recognizable stages is the blastula.
a. In sea urchins, this is a hollow single-layered sphere enclosing a space, the blastocoel.
b. In amphibians, the blastocoel is restricted to the animal pole because the yolk at the vegetal
pole impedes cleavage.
c. In reptiles and birds, the cleavage is restricted to a tiny, caplike region at the animal pole,
resulting in a blastodisk.
d. In mammals, an inner cell mass (future embryo) forms on the inside of a hollow sphere; the
early embryo is called the blastocyst and the blastocyst wall will contribute tissues to
the extraembryonic membranes.
E. Gastrulation
1 Gastrulation results in very little increase in size but does involve dramatic rearrangements of
cells.
a. In sea urchins, there is an inward migration resulting in the archenteron, the forerunner of
the gut.
b. In vertebrate embryos, rearrangements result in formation of the neural tube, which defines
the long axis of the body and is the forerunner of the brain and spinal cord.
2 The greatest significance of gastrulation is the establishment of the internal (gut), intermediate
(movement, support, blood circulation), and surface (integument) regions of the body.

IV. Cell Differentiation
A. In cell differentiation, a single fertilized egg gives rise to diverse types of specialized cells. 1 All differentiated cells have the same number and kind of genes, but through controls on the
expression of those genes some cells produce proteins not found in other cells.
2 Experiments have shown that the differentiated cell has not lost any of its original genetic
information because it can still direct differentiation if placed in the necessary environment. B. The production of identical twins from the two cells produced by the first cleavage is elegant
proof of the retention of full genetic composition after mitosis.

V. Morphogenesis
A. Morphogenesis is the organization of differentiated cells into tissues and organs; it is the result of
several events.
B. Cell Migrations
1 In active cell migration, cells move by pseudopods projecting from the cell body; this is seen in
the establishment of neural networks.
2 Cells are guided in their movement by following chemical gradients, a behavior called chemotaxis. 3 Cells also respond to adhesive cues provided by recognition proteins at the surface of other cells. C. Changes in Cell Size and Shape
1 In the formation of the neural plate, microtubules in the ectodermal cells elongate and other cells
become wedge-shaped by constriction of microfilaments.
2 The collective action of these cells causes the neural plate to fold over and meet at the embryo’s
midline to form the neural tube.
D. Localized Growth and Cell Death
1 Localized growth, which contributes to changes in sizes, shapes, and proportions of body parts, is
probably the result of regulatory genes.
2 Controlled cell death eliminates tissues and cells that are used for only short periods in the
embryo or adult; for example, humans develop with webs between the toes and fingers, but
they are not born that way!
E. Pattern Formation
1 Pattern formation refers to such mechanisms as cytoplasmic induction and embryonic induction,
which are responsible for the specialization of tissues and for positioning them in space. 2 Vertebrate Eye Formation
a. The retina of the eye originates from the forebrain, but the lens originates from epidermis.
b. When optic cups from a salamander embryo were placed under ectoderm of the belly region,
something from the optic cups caused the ectoderm to form a perfectly formed lens.
3 Chick Wing Formation
a. When certain groups of ectodermal cells are removed from the tip of a half-grown wing bud,
terminal wing buds never develop.
b. The removal of the ectodermal ridge somehow affects the mesodermal tissue, which no longer
produces the next bones in line.
4 Pattern Formation in Drosophila
a. Imaginal disks, which are clusters of cells in Drosophila larvae, give rise to specific body
parts.
b. Homeotic mutations apparently affect regulatory genes of the imaginal disks, leading to such
events as cells of the antenna disk differentiating into a leg.
5 Inducer Signals
a. Experiments clearly point to the inducer signals as chemicals.
b. Characterization of the chemicals has not been accomplished.

VI. Post-Embryonic Development
A. Animals as varied as nematodes and mammals have rather direct development whereby the adult is a
larger and more sexually developed version of the immature.
B. For insects and amphibians, the development is more indirect because of larval stages between the
embryo and adult.
1 The larvae of insects are sexually immature, free-living, and free-feeding.
2 The transformation to the adult (reproductive) form may be gradual or involve drastic
reorganization of body tissues.
3 The transformation of a larva to an adult is termed metamorphosis.
C. Reactivation of growth can occur between molts of an insect; it can also occur in regeneration of body
parts lost by accident or predator.

VII. Aging and Death
A. Aging is the progressive cellular and bodily deterioration built into the life cycle of all organisms. 1 Aging is seen in loss of hair and teeth, increased wrinkling, decreased metabolism, and changes in
collagen.
2 Some experiments indicate that cells have a limited division potential (is this the cause or the
result of aging?).
B. Perhaps aging is loss of the capacity for DNA self-repair, or perhaps autoimmune responses intensify
over time, producing increased vulnerability to disease and stress.