The Development of a Pig Embryo
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Conception
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The pig embryo begins to develop from the moment of conception--the time when the boar's sperm penetrates the cell walls of the sow's ova. Within 14 to 16 hours, the single fertilized cell has started to split (or cleave) into smaller cells (blastomeres) by mitosis. By the second or third day of gestation, this primitive blastocyst has passed from the oviduct into one of the sow's two uterine horns where it will continue to grow until it begins to attach to the uterine lining on day 11 or 12.
Differentiation
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During this phase, the cells begin to differentiate and reorganize to form layers that will become the body organs. The ectoderm layer forms the central nervous system, the intestinal lining and the tissues of the integumentary system--the hooves, the skin, tooth enamel, the hair, the sweat glands and the sensory organs. The mesoderm layer will become the circulatory, skeletal and muscular systems. The renal system and male and female reproductive organs also begin forming from the mesoderm. The digestive system, liver and lungs and all of the pig's glandular organs will arise from the endoderm layer.
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Organ Formation
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The organs form while the fertilized egg rests in the sow's uterus, and by day 16 of gestation the heartbeat has become recognizable. The esophagus, stomach and intestines form around the sixteenth to eighteenth day.
Placentation
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By day 18, the extra embryonic membranes of the placenta have formed around the embryo and the embryo achieves final placentation (attachment) to the uterine wall. The placental membranes and fluids provide nutrients for the early embryo and protect the growing fetus from injury. The placenta also absorbs nutrients from the uterus, serves as a reservoir for fetal urine and produces protein hormones that regulate the sow's metabolism to further fetal growth. These hormones, called lactogens, reach their highest levels in the last 35 to 40 days of gestation and facilitate milk production in the mother.
Growth
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The adrenal glands begin to form around day 20 and striated skeletal muscles can be seen by day 22. By day 28, male testes or female ovaries are differentiated and the fetus begins secreting sexual hormones. The brain forms around day 34 to 35 of gestation, and the major bones of the body--the humerus, femur, and ribs--begin to harden. The fetal reproductive organs are fully intact by day 51.
Birth
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The fetal lungs and brain continue to grow and develop in-utero and the fetus will form skin, hair and hooves during the last 40 days of gestation. By parturition (birth) on day 114, the fetus has fully differentiated organs and can live outside the mother's body.
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