Embryonic Development and Fetal Development

• Embryonic development is when an embryo forms and develops.
• The process starts with the fertilization of an egg, which then becomes a zygote.
• The zygote undergoes rapid division.
• The scientific name for embryonic development is embryogenesis.

Stages of Pregnancy and Development

• Fertilization
• Embryonic development involving cleavage
• Fetal differentiation
• Growth and overall development
• Childbirth

Fertilization

• The egg is viable for 12 to 24 hours after ovulation.
• Sperm are viable for 12 to 48 hours after ejaculation.
• Sperm cells must reach the uterine tube for fertilization
• Sperm and egg join to form a zygote, which is the first cell of a new individual.
• The zygote results from the fusion of DNA from sperm and egg.
• Fertilization occurs in the Fallopian tubes
• The zygote begins rapid mitotic cell division.
• This marks the beginning of human development.
• Thirty hours after conception, the zygote, no larger than the head of a pin, is still rapidly dividing.
• It floats down from the fallopian tube towards the uterus.

Cleavage

• Cleavage is the mitotic division of cells in the early embryo.
• About 96 hours post-fertilization, the morula is a solid ball of 32 cells, resembling a raspberry.
• Five days after fertilization, the blastula is a ball of over 64 cells surrounding a fluid-filled cavity called the blastocele.
• The blastula is a product of repeated mitotic division of a zygote.

Implantation

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

The Embryo

• The embryo represents the developmental stage from the start of cleavage until the ninth week.
• The embryo initially undergoes division without growth.
• It enters the uterus at the 16-cell state.
• The embryo floats free in the uterus temporarily and is nourished by uterine secretions.

Differentiation and Gastrulation

• Gastrulation is a sequence of cell migrations where cells relocate to form the three primary cell layers.
• Ectoderm forms the outer layer and gives rise to skin, hair, sweat glands, epithelium, brain, and the nervous system.
• Endoderm forms the inner layer, developing into the digestive and respiratory systems, liver, pancreas, bladder, endocrine glands like the thyroid and parathyroid.
• Mesoderm forms the middle layer, it forms body muscles, cartilage, bones, blood, reproductive system organs, and kidneys.
• 20 days after fertilization the embryo begins to form organs during the third week and is 0.5 cm.
• One can’t tell between a human embryo and an animal embryo, the embryo is tall and visible.
• After 1 month the embryo is 0.6 cms, ears, nose, and eyes are not visible.
• Small arm and leg buds and the backbone can be seen, the heart beats.

Fetus

• During the second month, major organ systems and limb buds develop, the fetus being 3 cms.
• Limbs become distinct, fingers and toes begin to form, and eyes are farther apart.
• The embryo becomes a fetus by the seventh week.
• By the end of the eighth week, all organ systems are formed.
• Fetal activities primarily consist of growth and organ specialization.
• This stage is marked by tremendous growth and changes in appearance.
• Beginning the eighth week, the sexually neutral fetus activates gene pathways for sex determination.
• This process forms testes in XY fetuses and ovaries in XX fetuses with subsequent development of external genitalia.
• The fetus increases in size during the second trimester (4 months, 18 cms), with the head enlarged and face having human features.
• Bony parts of the skeleton start to form and fetal movements are felt by the mother.
• During the third trimester (7 months, 40 cms), the fetus continues to grow in size.
• The circulatory and respiratory systems mature for air breathing.
• Rapid fetal growth uses significant amounts of the mother's protein and calcium intake, as such, the fetus grows during this time.
• Maternal antibodies are passed to the fetus during the last month of pregnancy to provide temporary immunity.
• Around nine months (50 cms), The fetus has fat under the skin that contributes to smoother wrinkles.
• Nails on fingers and toes are completely formed and the baby is at term.

Key Stages Table

• Fertilization leads to a zygote within 1 day through fertilization and mitosis.
• Cleavage produces a morula by day 4 and a blastocyst around day 6-7 via mitosis and implantation of the blastocyst to wall of the uterus, three germ layers also form.
• Neurulation begins upon implantation, leading to differentiation and embryo development by day 10 via three germ layers such as endoderm, mesoderm and ectoderm which form.
• Growth occurs over months, where the embryo grows into a fetus.
• Childbirth depends upon the fetus being completely formed.
• Fertilization occurs in the Fallopian Tubes and cleavage occurs until 64 cells are achieved.
• Upon implantation, uterine walls sustain the developing embryo.

Childbirth (Partition)

• Labor involves a series of events that expel the infant.
• Initiation of labor involves rising estrogen levels, uterine contractions, release of prostaglandins by the placenta, and oxytocin by the pituitary gland.
• The combination of these hormones produces contractions.

Developmental Aspects

• Gender determination happens at fertilization.
• Males have XY sex chromosomes, and females have XX sex chromosomes.
• Gonads start to form around the eighth week.

First Trimester

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

Second Trimester

• In the second trimester, the baby becomes more developed.
• The baby's eyes are open at the end of the trimester.
• The sex is determined.
• The baby weighs 12.5 ounces.
• Bones and muscles are steadily growing.

Third Trimester

• In the third trimester, the baby is still developing.
• Footprints and fingerprints form.
• Eyelids open and close.
• Brain development grows fast and rapidly.
• The baby is born at the end of this stage.