Chapter 47 ~ Animal

Development

Menstrual Cycle

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Positive – Enhances a process until it is complete.

– During follicular phase – Estrogen stimulates release of LH. Increased LH stimulates follicle to release more estrogen until follicle matures

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Negative – stops a process one homeostasis has been reached

– During luteal phase, LH stimulates corpus luteum to secrete estrogen and progesterone. Once they reach sufficient levels, trigger hypothalamus and pituitary to shut off and inhibit secretion of LH and FSH

Embryonic development/fertilization Preformation~ until 18th century; miniature infant in sperm or egg At fertilization/conception: Acrosomal reaction~ hydrolytic enzyme action on egg jelly coat…. Fast block to polyspermy~ membrane depolarization prevents multiple fertilizations…. Cortical reaction~ release of calcium causes hardening of egg outer layer and creates a... Slow block to polyspermy and... Egg activation~ increases metabolic activity; protein synthesis

3 stages of embryonic develop:

1. Cleavage 2. Gastrulation 3. Organogenesis

Cleavage

Depends on the amount of yolk

– Sea urchin: very little yolk, equal cleavage

– Frog: more yolk, unequal cell division, little division in yolk

– Birds: most yolk, cell division limited to nonyolky disc at top of egg

The Fertilized Egg & Cleavage Blastomeres~ resultant cells of

cleavage/mitosis in blastula

Yolk~ nutrients stored in the egg

Vegetal pole~ side of egg with high yolk concentration

Animal pole ~ side of egg with low yolk concentration

Morula~solid ball of cells

Blastocoel~fluid-filled cavity in blastula

Blastula~hollow ball stage of development

Gastrulation Gastrula~ 3 layered, cup-shaped embryonic

stage, rearrangement of blastula, begins with forming blastopore

3 Embryonic germ layers: Ectoderm~ outer layer; epidermis; nervous

system, etc. Endoderm~ viscera, inner layer; digestive

tract and associated organs; respiratory, etc. Mesoderm~skeletal; muscular;, blood,

excretory, etc. Invagination~ gastrula buckling process to

create the... Archenteron~ primitive gut Blastopore~ open end of archenteron, could

become mouth or anus

Organogenesis: organ formation

Blastodisc~ cap of cells on top of yolk

Primitive streak~ invagination of blastodisc

Neural tube~ beginning of spinal cord

Somites~ vertebrae and skeletal muscles

Neural crest~ bones and muscles of skull

Amniote embryos

Extraembryonic membranes: •yolk sac (support; circulatory function) •amnion (fluid-filled sac; protection)

•chorion (placenta formation)

•allantois (nitrogenous waste)

Fetus and Placenta

Placenta begins to form when embryo is fully developed

Chorionic villi – extensions of chorion

Eventually disappear accept where placenta develops

Maintain lining of uterus Exchange of materials and

wastes through placenta and umbilical cord

http://www.visembryo.com/baby/index.html

Week 3

beginning development of the brain, spinal cord, and heart

beginning development of the gastrointestinal tract

Weeks 4 to 5

formation of tissue that develops into the vertebra and some other bones

further development of the heart which now beats at a regular rhythm

movement of rudimentary blood through the main vessels

beginning of the structures of the eye and ears

the brain develops into five areas and some cranial nerves are visible

arm and leg buds are visible

Week 6

beginning of formation of the lungs

further development of the brain arms and legs have lengthened

with foot and hand areas distinguishable

hands and feet have digits, but may still be webbed

Week 7

nipples and hair follicles form

elbows and toes visible

all essential organs have at least begun to form

Week 8

rotation of intestines facial features continue to

develop the eyelids are more developed the external features of the ear

begin to take their final shape Marks end of embryonic stage and start of

fetal stage

Weeks 9 to 12 the fetus reaches a length of

3.2 inches the head comprises nearly

half of the fetus' size the face is well formed eyelids close and will not

reopen until about the 28th week

tooth buds appear for the baby teeth

limbs are long and thin the fetus can make a fist

with its fingers genitals appear well

differentiated red blood cells are produced

in the liver

Weeks 13 to 16 the fetus reaches a length of

about 6 inches a fine hair develops on the

head called lanugo fetal skin is almost transparent more muscle tissue and bones

have developed, and the bones become harder

the fetus makes active movements

sucking motions are made with the mouth

meconium is made in the intestinal tract

the liver and pancreas produce their appropriate fluid secretions

Week 24 the fetus reaches a

length of 11.2 inches the fetus weighs about

1 lb. 10 oz. eyebrows and eyelashes

are well formed all the eye components

are developed the fetus has a hand

and startle reflex footprints and

fingerprints forming alveoli (air sacs)

forming in lungs

Weeks 25 to 28 the fetus reaches a length of

15 inches the fetus weighs about 2 lbs.

11 oz. rapid brain development nervous system developed

enough to control some body functions

eyelids open and close respiratory system, while

immature, has developed to the point where gas exchange is possible

a baby born at this time may survive, but the possibilities for complications and death remain high

Weeks 29 to 32 the fetus reaches a length

of about 15-17 inches the fetus weighs about 4

lbs. 6 oz. rapid increase in the

amount of body fat rhythmic breathing

movements occur, but lungs are not fully mature

bones are fully developed, but still soft and pliable

fetus begins storing iron, calcium, and phosphorus

Week 36 the fetus reaches a

length of about 16-19 inches

the fetus weighs about 5 lbs. 12 oz. to 6 lbs. 12 oz.

lanugo begins to disappear

increase in body fat fingernails reach the end

of the fingertips a baby born at 36 weeks

has a high chance of survival, but may require some medical interventions

Weeks 37 to 40

considered full-term at 37 weeks

may be 19 to 21 inches in length

lanugo is gone except for on the upper arms and shoulders

fingernails extend beyond fingertips

small breast buds are present on both sexes

head hair is now coarse and thicker

Birth When fetal brain matures, hypothalamus causes pituitary

to stimulate adrenal cortex so androgens are released Placenta uses androgens as a precursor for estrogens

Prostaglandin and Oxytocin* Estrogen, Prostaglandin and Oxytocin all cause uterus to

contract and expel fetus Birth

– Dilation of cervix– Birth of baby– Expulsion of afterbirth (placenta and extraembryonic

membranes)