Chapter 42 Biology: Animal Development and Aging

where does fertilization occur?

the fallopian tube (oviduct)

embryonic development

the first two months of development in humans

cleavage

mitosis-like cellular division without the overall size of the embryo increasing

morula

• precursor to the blastocyst

• first stage embryo

• cluster of tightly packed cells

where does cleavage occur?

• occurs in the fallopian tube

• moves to the uterus

blastula

• the second embryonic stage formed from morula cell divisions

• hollow, fluid filled spheres

  • general term used for most animals

blastocyst

• mammal specific term for a blastula

  • composed of the trophoblast, inner cell mass, and the blastocoel

what does the inner cell mass in a blastocyst do?

• the ICM concentrates at one pole in a blastocyst and

• implants into the uterine wall first

• forms an embryo

chorion

a membrane that develops around the embryo of a fertilized egg

trophoblast

the outer sphere of cells in a blastocyst

what does the trophoblast become?

the chorion

what does the chorion contribute to?

making the fetal side of the placenta

how does the placenta exchange between maternal and fetal blood?

a thin tissue barrier separates the two blood supplies, making exchange possible through diffusion and transport

what does the placenta exchange between the mother and fetus?

• gas (O₂ into fetus, CO₂ out)

• nutrition

• waste exchange

blastocoel

the fluid filled cavity in a blastula

what happens at the end of the first week?

the blastocyst:

• reaches the uterus

• attaches to the endometrium

• implants (penetrates) into the tissue of the endometrium

amnion

sac in which the embryo develops in

what occurs during the second week?

gastrulation, forming the gastrula

gastrula

embryo transformation into a three primary germ layers

• embryonic tissues

what does the upper cell layer of a gastrula differentiate into?

ectoderm

what does the ectoderm eventually form?

the epidermis and neural tissue

what does the lower layer of the gastrula eventually differentiate into?

endodermw

what does the endoderm eventually form?

the lining of the gut, digestive system, and the respiratory system

what does the middle layer of the gastrula differentiate into?

the mesoderm

what does the mesoderm eventually form?

connective and cardiovascular tissue, muscle, skeletal system, and reproductive system

what happens in the third week of development

neurulation

neurulation

the formation of the nervous syetem

organogenesis

process in which the embryonic tissues develop into tissues and the organs of the body

when does the heart begin to develop?

the third week into the fourth week

what happens in the fourth and fifth weeks of development?

the umbilical cord is fully formed

when will the placenta be fully formed?

by the tenth week

what occurs in the sixth to eighth weeks?

• embryo becomes easily recognizable as human

  • organ systems are established

when is the foundation of fetal development formed?

the end of 3 months

what happens in the third through nine months?

• development of the embryo completes

• the developing human is referred to a fetus

  • extreme increase in size and growth

what are the two protective layers around the egg?

the corona radiata (outer cell layer) and the zona pellucida (protein shell beneath it)

what does the acrosome do?

the tip of the sperm head that releases enzymes to dissolve through the zona pellucida

what is the goal of the sperm?

to deliver its 23 chromosomes to the egg

what is polyploidy?

when too many sperm enter the egg, resulting in too many chromosomes

what are pronuclei?

the separate sperm and egg nuclei inside the egg before they fuse together

what happens when the two pronuclei combine?

46 chromosomes total and a zygote is formed

what is step one of fertilization

sperm pushes through the corona radiata

what is step 2 of fertilization

sperm releases enzymes from its acrosome to dissolve through the zona pellucida

what is step 3 of fertilization

sperm binds to and fuses with the egg’s plasma membrane

what is step 4 of fertilization

the sperm nucleus enters the cytoplasm of the egg

what is step 5 of fertilization

the egg releases cortical granules which harden the zona pellucida into a fertilization membrane, blocking any other sperm from entering

what is step 6 of fertilization?

the sperm and egg pronuclei are enclosed together in a nuclear envelope to combine chromosomes

how does ovulation begin?

the egg is released from the ovary and swept into the uterine tube by finger-like projections called fimbriae

how does the trophoblast start forming the placenta?

after implantation it splits into two layers— an inner layer of individual cells and an outer invasive layer that burrows into the uterine wall and erodes maternal blood vessels

histogenesis

the formation and differentiation of cells into specializes tissues and organs during growth

what structure produces blood cells for the fetus?

yolk sac

what are the four extraembryonic membranes?

amnion, chorion, yolk sac, and allantois

what does the amnion do?

• cushions embryo from physical shock

• allows free movement for normal development

• maintains stable temperature

  • prevents embryo from sticking to surrounding tissue

what is the yolk sac?

a membrane that forms beneath the embryo early in development

what does the yolk sac do?

• provides early nutrition to the embryo

• produces first blood cells

• produces earliest germ cells

what is the allantois?

a small membrane that develops as an outgrowth from the early gut of the embryo

what does the allantois do?

• helps w early waste removal

• gives rise to the blood vessels of the umbilical cord

what are chorionic villi?

finger like projections that grow out from the chorion into the pools of maternal blood

• where exchange between mom and the baby actually happen

amniocentesis

• a prenatal test in which a small amount of amniotic fluid, which contains fetal cells, is removed from the amniotic cavity

  • fetal cells are tested for genetic defects

chorionic villi sampling

• a prenatal test where chorionic villi are taken from the placenta where it attaches to the wall of the uterus

  • tested for birth defects and genetic diseases

  • can be done as early as the first trimester