Embryonic Development & Fertilization

• Embryonic development is when the embryo forms and develops.
• It starts with the fertilization of an egg, which then becomes a zygote.
• The zygote then undergoes rapid cell division.
• The scientific name for embryonic development is embryogenesis.
• Stages of pregnancy and development include: fertilization, embryonic development (cleavage), fetal development (differentiation), growth and development, and childbirth.

Fertilization

• Fertilization is the first stage of pregnancy and development.
• Fertilization is when the sperm and egg join to form a zygote.
• The zygote is the first cell of a new individual.
• Fertilization occurs in the Fallopian tubes.
• The zygote begins rapid mitotic cell division.
• Beginning of human development.
• The egg is viable for 12 to 24 hours after ovulation.
• Sperm are viable for 12 to 48 hours after ejaculation.
• Sperm cells must travel to the uterine tube for fertilization to be possible.
• Only thirty hours after conception, the zygote is no larger than the head of a pin.
• The zygote is still rapidly dividing.
• The zygote floats down from the fallopian tube towards the uterus.

Cleavage

• Cleavage is mitotic division of the cells in the early embryo.
• About 96 hours after fertilization, the morula forms.
• The morula is a solid ball of 32 cells resembling a raspberry.
• Five days after fertilization, the blastula forms.
• The blastula is a ball of more than 64 cells surrounding a fluid-filled cavity (the blastocele).
• Blastula is produced by the repeated mitotic division of a zygote.

Implantation

• Implantation is the fastening of the embryo to the wall of the uterus.
• Implantation occurs seven days after fertilization.
• Implantation takes place in the uterus.

The Embryo

• The embryo is the developmental stage from the start of cleavage until the ninth week.
• The embryo first undergoes division without growth.
• The embryo enters the uterus at the 16-cell state.
• The embryo floats freely in the uterus temporarily.
• Uterine secretions are used for nourishment.

Differentiation and Gastrulation

• Gastrulation involves cell migrations to form the three primary cell layers.
• Ectoderm forms the outer layer and develops into skin, hair, sweat glands, epithelium, brain, and nervous system.
• Endoderm forms the inner layer and develops into digestive, respiratory systems, liver, pancreas, bladder, and endocrine glands like thyroid and parathyroid.
• Mesoderm forms the middle layer and develops into body muscles, cartilage, bone, blood, reproductive system organs, and kidneys.
• The embryo begins to form organs during the third week, about 20 days after fertilization, and reaches approximately 0.5 cm in size.
• It's difficult to determine if it's a human or another vertebrate at this stage but is tall and visible.
• After one month, the embryo is 0.6 cm long, with ears, nose, and eyes not visible yet
• Small arm and leg buds and backbone are seen, and the heart beats.

The Fetus

• During the second month, major organ systems form, and limb buds develop in the fetus, which is about 3 cm long.
• Limbs become distinct with fingers and toes forming, and eyes begin to separate.
• The embryo becomes a fetus by the seventh week.
• All organ systems are formed by the end of the eighth week.
• Activities of the fetus are growth and organ specialization.
• A stage of tremendous growth and change in appearance.
• Beginning the eighth week, the sexually neutral fetus activates gene pathways for sex determination, forming testes in XY fetuses and ovaries in XX fetuses.
• External genitalia develop.
• By four months, the fetus is 18 cm long with an enlarged head and human facial features.
• The fetus increases in size during this trimester, and bony parts of the skeleton begin to form.
• Fetal movements can be felt by the mother.
• During this trimester, the fetus increases in size
• Circulatory and respiratory systems mature in preparation for air breathing.
• Fetal growth during this time uses large parts of its mother's protein and calcium intake.
• Maternal antibodies pass to the fetus during the last month, conferring temporary immunity.
• Fat develops under the skin, smoothing out wrinkles.
• Nails on fingers and toes are completely formed.
• Baby is at term.

Childbirth (Partition)

• Labor is a series of events that expel the infant from the uterus.
• Initiation of labor includes: rise in estrogen levels, the beginning of uterine contractions, the placenta releasing prostaglandins, the pituitary releasing oxytocin, and the combination of these hormones produces contractions.

Developmental Aspects of the Reproductive System

• Gender is determined at fertilization.
• Males have XY sex chromosomes.
• Females have XX sex chromosomes.
• Gonads do not begin to form until the eighth week.

The First Trimester

• The first trimester is when the cell starts to divide.
• The zygote begins to develop.
• At the end of this trimester, the head is round, eyes are closed, taste buds are forming, internal organs move into the body cavity, and upper and lower limbs are formed.

The Second Trimester

• In the second trimester, the baby is becoming more developed.
• At the end of the trimester, the baby's eyes are open, the sex is determined, and the baby weighs 12.5 ounces with growing bones and muscles.

The Third Trimester

• In the third trimester, the baby is developing.
• Footprints and fingerprints are formed, eyelids open and close, brain development grows rapidly, and the baby is born.