Reproduction
Early Developmental Stages
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Fertilization- is the union of a sperm and an egg from a zygote
When sperm enters the female reproductive tract biochemical changes to the sperm occur in the
final part of its maturation. The sperm is attracted by a chemical signal from the secondary
oocyte
Then the sperm swims up into the oviduct to reach the egg membrane where it has to penetrate
the protective jelly coat (zona pellucida) surrounding the egg by releasing enzymes
The membrane of the egg and sperm then fuse and the sperm nucleus (and centriole) enters the
egg
When the first sperm binds to the oocyte plasma membrane polyspermy is prevented by a “fast
block” to repel more sperm for a few seconds
An impenetrable fertilization membrane is then made known as the “slow block” when the
vesicles in the oocyte (cortical granules) secrete enzymes into the zona pellucida
Now fertilized, the secondary oocyte completes meiosis and then the sperm and egg pronuclei
are enclosed in a nuclear envelope
Embryonic Development
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The length of time from conception to birth is approximately nine months
During the first stages of development an organism is called an embryo
Cleavage is cell division without growth or in other words the number of cells are increased, not
the volume
Cellular stages of development : the cleavage makes a multicellular embryo and then the
formation of the blastula
Cleavage results in uniform cells which form the morula (ball of cells). A blastula is then
formed after the morula continues to divide.
Blastula- is a hollow ball of cells which have fluid filled cavity called a blastocoel
HCG or the human chorionic gonadotropin is produced by a developing embryo during
pregnancy and maintains the corpus luteum as well as the uterine lining needed to sustain a
pregnancy
Tissue Stages of Development
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The tissue stages of development are: the early gastrula and the late gastrula
The early gastrula stage starts when some cells start to push into the blastocoel making a
double layer of cells
This stage has two layers of cells: the outer layer is the ectoderm and the inner is the
endoderm (borders the gut)
The blastopore is the hole or pore made by invagination ( inward fold to form a pouch)
Gastrulation is not complete until three layers of cells that will become adult organs are made
In combination with the ectoderm and endoderm the late gastrula or middle layer of cells is
called the mesoderm
Ectoderm, Mesoderm, and Endoderm are called the embryonic germ layers ( the group of cells
in an embryo that interact to help form the organs and tissues)
Organ Stages of Development
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The organs of animals develop from the three germ layers. The newly formed cells lie on a
supporting rod called the notochord which was derived from the mesoderm cells
The nervous system develops just above the notochord, and at first there is a thickening of cells
called the neural plate
The neural plate then folds making the neural tube, which then makes the brain and spinal nerve
cord (after the notochord is replaced by the vertebral column)
The heart too, begins to form
Developmental Processes
•Development requires:
1) Growth
2) Cellular differentiation
3) Morphogenesis
•Cellular Differentiation- When cells become specialized in structure and
function
•Morphogenesis- Produces the shape and form of the body
Cellular Differentiation
•The zygote is totipotent; it has the ability to generate the entire organism.
The zygote also contains all instructions needed by any specialized cell.
•All embryonic cells are totipotent when cell division starts. Begin to
specialize after a few days.
•During cellular differentiation two mechanisms are used to lead to
differential gene expression:
-Cytoplasmic segregation
-Induction
Cellular Differentiation
Morphogenesis
•How animals achieve their ordered and complex body form
•Enabled by pattern formation
•In pattern formation cells of the embryo divide and differentiate, taking
orderly positions in tissues and organs
•Differences that occur are due to differences in gene expression
•Various genes become activated in different regions of the embryo
•Apoptosis (programmed cell death) is used in morphogenesis to shape
hands and feet
Human Embryonic and Fetal
Development
•Length of time from conception to birth is approximately nine months
•In humans pregnancy is divided into embryonic development (months 1 and 2) and
fetal development (months 3-9)
•During the embryonic period the major organs are formed and during fetal
development the structures are refined.
•Development can also be divided into trimesters.
•In the first trimester embryonic and early fetal development occur. In the second
trimester organs and organ systems develop, as well as the fetus begins to look
human. During the third trimester the fetus grows rapidly.
Extraembryonic Membranes
Embryonic Development
Embryonic Development
Embryonic Development
Embryonic Development
Embryonic Development
Embryonic Development
The Structure and Function of the
Placenta
The Structure and Function of the
Placenta
Fetal Development and Birth
The Stages of Birth
Standards
The Aging Process
-human population is older on average than ever before
-aging and death are important in terms of resources and
population
-animals are continuously replaced by individuals with new
genetic combinations that can adapt to environmental
changes(evolution)
Reproductive System
-low testosterone has been linked to decreased sex drive, excessive weight
gain, loss of muscle mass, osteoporosis, general fatigue, and depression
-menopause is the period in a woman’s life during which the ovarian and
uterine cycles cease usually between ages 45-55
-the ovaries become unresponsive to the gonadotropic hormones produced by
the anterior pituitary and they no longer secrete estrogen or progesterone
-hormonal changes during menopause produce physical symptoms such as
“hot flashes”, dizziness, headaches, insomnia, sleepiness, and depression
Integumentary System
-skin becomes thinner and less elastic(dec. in elastic
fibers,collagen fibers become cross-linked
-less adipose tissue in the subcutaneous layer cause older
people to feel colder
-these changes result in sagging and wrinkling of the skin
-sweat glands become less active resulting in decreased
tolerance to higher temperatures
-fewer hair follicles causes thinning
-decrease in melanocytes make the hair gray and skin pale
Cardiovascular System
-heart muscle weakens and increases in size
-max heart rate decreases
-heart rate and blood pressure to longer than normal to return to normal
resting levels after stress
-blood leaks back through heart valves that are less flexible
-arteries contain elastic fibers that become cross-linked and rigid
-diameter of arteries decreases due to atherosclerosis contributes to
increasing blood pressure
-diet low in cholesterol and saturated fatty acids along with a sensible
exercise program may help prevent age-related cardiovascular disease
Immune System
-thymus gradually decreases in size and eventually is replaced by fat and
connective tissue
-older people’s ability to generate T-cell responses to new antigens decreases
-evolutionary rationale for the decreased size of the thymus: it is energetically
expensive for an organism to maintain the thymus. Compared to younger
animals that must respond to a high number of new infections and other
antigens, older animals have already responded to most of the antigens they
will be exposed to
-antibody responses begin to decline, elderly don’t respond as well to vaccines
-activity of natural killer cells seem to change little with age, investigations are
being done to find out how these cells remain so active
Digestive System
-perhaps less affected system
-secretion of saliva decreases and more bacteria adhere to the
teeth, causing more decay and periodontal disease
-blood flow to the liver is reduced, resulting in less efficient
metabolism of drugs or toxins
-as a person gets older less medication is needed to maintain
the same level in the bloodstream
Respiratory System
-decreasing elasticity of lung tissues means that ventilation is
reduced
-because of rare use of entire vital capacity, these effects may
not be noticed unless the demand for oxygen increases, such as
during exercise
-cardiovascular problems are often accompanied with respiratory
disorders and vice versa
Excretory System
-blood supply to the kidneys is reduced, they become smaller
and less efficient at filtering wastes
-salt and water balance are difficult to maintain and the elderly
dehydrate faster than younger people
-urinary incontinence(lack of bladder control) increases with age
especially in women
-in men an enlarged prostate gland may reduce the diameter of
the urethra, causing frequent or difficult urination
Nervous System
-neurons are extremely sensitive to oxygen deficiency
-neurons die because of reduced blood flow in narrowed blood
vessels
-age-related loss in brain function is not due to neuron loss,
instead it is because of alterations in complex chemical reactions
or increased inflammation in the brain
-lifestyle factors also affect the functionality of the brain,
sufficient sleep and exercise are ways to maintain a healthy
brain
Sensory Systems
-more stimulation is needed for taste,smell,and hearing
receptors
-people over 80 notice a significant decline in the ability to smell
and some suffer from anosmia, the total inability to smell(health
hazard: smoke, gas leaks, spoiled food)
-harder to hear higher frequencies, identify voices, and
understand conversation in a group
-presbyopia(difficulty focusing on near objects) requires reading
glasses as you reach middle age
-cataracts and other eye disorders become more common
Musculoskeletal System
-muscle mass peaks: females age 16-19, males 18-24
-beginning in the 20s or 30s muscle mass decreases due to decreases
in size and number of muscle fibers
-regular exercise can slow the decline
-bones tend to shrink in size and density with age
-height loss occurs because of compression of the vertebrae
-loss of bone mass can be slowed be proper diet and exercise
Endocrine System
-thyroid gland activity declines resulting a lower basal metabolic
rate
-cells become less sensitive to the insulin produced by the
pancreas resulting in a rise in glucose levels
-human growth hormone declines with age
-HGH does not “cure” aging in fact one study found people with
lower levels of HGH in their system lived longer
Hypotheses About Aging
1.aging is due to preprogrammed genetic events
2.theories that aging is due to the accumulation of cellular
damage