Prenatal Development Period

• At the beginning of the first week, conception takes place where a woman’s ovum is penetrated by and united with a man’s sperm during fertilization
• This union of the ovum and sperm subsequently forms a fertilized egg, or zygote
• After fertilization, the zygote then undergoes mitosis, or individual cell division, that splits it into more and more cells due to cleavage.
• The zygote becomes a blastocyst
• By the end of the first week, the blastocyst stops traveling and undergoes implantation and thus become embedded in the prepared endometrium, the innermost lining of the uterus on its back wall

Development from the 2nd to 8th week

• The implanted blastocyst grows by increased proliferation of the embryonic cells, with differentiation also occurring resulting in changes in cellular morphogenesis
• A bilaminar embryonic disc is eventually developed from the blastocyst

• The primitive steak forms within the bilaminar disc
• The primitive streak causes the disc to have bilateral symmetry, with a right half and left half.
• Some cells from the epiblast layer move or migrate toward the hypoblast layer only in the area of the primitive streak and become mesoderm, an embryonic connective tissue, as well as embryonic endoderm.
• With three layers present, the bilaminar disc has thickened into a trilaminar embryonic disc.
• With the creation of the new embryonic cell layers of mesoderm and embryonic endoderm, the epiblast layer is now considered ectoderm.
• The ectoderm gives rise to skin epidermis, the central nervous system (CNS), and other structure
• The mesoderm gives the rise to connective tissue, such as skin dermis, cartilage, bone, blood, muscle, and other associated structure
• The endoderm gives the rise to the respiratory epithelium and cells of glands

• The trilaminar embryonic disc undergoes anterior (cephalic) and lateral embryonic folding, which places forming tissue types into their proper positions for further embryonic development, as well as producing a somewhat tubular embryo
• Disc has undergone embryonic folding as a result of extensive growth of the ectoderm, and there is development of the brain with spinal cord, heart, and digestive tract.
• Three parts of digestive tract:
• Foregut—upper part, forms digestive tube from throat to duodenum

• Midgut—middle part, forms rest of small intestines, cecum, ascending colon and most of transverse colon
• Hindgut—lower part, forms descending colon, sigmoid colon and rectum of the large intestines
• The developing brain, face, and heart are also noted during the fourth week of prenatal development

• Primitive muscle cells from the mesoderm in the mandibular arch begin to differentiate
• These primitive muscle cells become oriented to the site of origin and insertion of the masticatory muscles that they will form
• Upper and lower lip formation start at 6th week
• The maxillary process fuses with each medial nasal process by the proliferation of the NCCs mesenchyme.
• Contributes to the side of upper lip and philtrum

• The mandibular muscle mass has enlarged, and the cells have begun to migrate into the areas where they will begin to differentiate into four muscles of mastication
• Muscle cell migration occurs before bone formation in the facial area

• During this final week of the embryonic period, the embryo exhibits definite human characteristics
• The cerebral hemispheres have grown so rapidly that the head now makes up 50% of the mass of the embryo
• The face occupies the lower half of the head, and the eyes continue to move to the more frontal plane
• Eyelid folds develop.
• These will become fused during the ninth week and remain so until the seventh month