The nail anlage appears in fingers of embryos 9 weeks old.
Externally the earliest recognizable nail structure is the primary nail field which is present in embryos at 10 weeks.
The first nail structure to keratinize is the distal ridge and is seen in embryos as early as 11 weeks.
The earliest matrix was found in the area of the lunula in embryos of 13 weeks.
Differentiation of the matrix primordium cells into matrix cells occurs in a distal to proximal direction beginning from the lunula. This process is manifested by a vacuolation of the matrix primordium cells, which is seen first at the lunula and later moves proximally as the embryo matures. By 20 weeks all the matrix primordium cells have matured into matrix. Possibly a small cluster of primordial cells remains in adult life at the most proximal tip of the matrix.
The matrix produces nail plate by a parakeratosis-like process without keratohyaline granules.
In embryos 13 weeks old the keratinization of the nail bed involves the production of keratohyaline granules and a stratum corneum. The adult nail bed produces a horny layer by a process which does not involve keratohyaline granules and a stratum corneum but is parakeratotic-like. The change in the process of keratinization of the nail bed is seen as the nail plate grows over the nail bed in embryos 20 weeks and older. Sulfhydryl stains are strongly positive in the nail plate of embryos early in life. This reaction decreases in intensity as the fetus becomes older, and in the adult nail it is mildly to variably positive.
Disulfide stains are mildly positive in the nail plate of embryos early in life. This reaction becomes more intense as the embryo becomes older and in the adult nail is strongly positive.
NARDO ZAIAS. Embryology of the Human Nail. Arch Dermatol. 1963;87(1):37–53. doi:10.1001/archderm.1963.01590130043010