u Chapter 47 ~ Animal Development

Menstrual Cycle

Feedback u 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

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

3 stages of embryonic develop: u 1. Cleavage u 2. Gastrulation u 3. Organogenesis

Cleavage u 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 u Blastomeres~ resultant cells of cleavage/mitosis in blastula u Yolk ~ nutrients stored in the egg u Vegetal pole ~ side of egg with high yolk concentration u Animal pole ~ side of egg with low yolk concentration u Morula~ solid ball of cells u Blastocoel ~fluid-filled cavity in blastula u Blastula ~hollow ball stage of development

Gastrulation u Gastrula ~ 3 layered, cup-shaped embryonic stage, rearrangement of blastula, begins with forming blastopore u 3 Embryonic germ layers: u Ectoderm~ outer layer; epidermis; nervous system, etc. u Endoderm ~ viscera, inner layer; digestive tract and associated organs; respiratory, etc. u Mesoderm ~skeletal; muscular;, blood, excretory, etc. u Invagination~ gastrula buckling process to create the... u Archenteron~ primitive gut u Blastopore~ open end of archenteron, could become mouth or anus

Organogenesis: organ formation u Blastodisc ~ cap of cells on top of yolk u Primitive streak ~ invagination of blastodisc u Neural tube ~ beginning of spinal cord u Somites~ vertebrae and skeletal muscles u Neural crest ~ bones and muscles of skull

Amniote embryos u Extraembryonic membranes : yolk sac (support; circulatory function) amnion (fluid-filled sac; protection) chorion (placenta formation) allantois (nitrogenous waste)

Fetus and Placenta u Placenta begins to form when embryo is fully developed u Chorionic villi – extensions of chorion u Eventually disappear accept where placenta develops u Maintain lining of uterus u Exchange of materials and wastes through placenta and umbilical cord u m/baby/index.html m/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 u 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 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 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 u When fetal brain matures, hypothalamus causes pituitary to stimulate adrenal cortex so androgens are released u Placenta uses androgens as a precursor for estrogens  Prostaglandin and Oxytocin* u Estrogen, Prostaglandin and Oxytocin all cause uterus to contract and expel fetus u Birth –Dilation of cervix –Birth of baby –Expulsion of afterbirth (placenta and extraembryonic membranes)