Embryo

Embryo is the early developmental stage of all multicellular organisms. In general, embryonic development is the part of the life cycle that begins just after fertilization. The fertilized egg undergoes a series of cellular divisions known as cleavage without increasing cell mass up until 6 or 7 days and moving downward gradually in the fallopian tube. Only nuclear material (chromatin) is synthesized between divisions at the expense of the cytoplasmic substance of the cell. The cleavage pattern is quite similar among the individuals of a species but varies mainly among the animal species. A hollow ball-shaped cellular structure called blastula is formed after the success of several cleavages. In farm animals, the hatched blastocyst completes the implantation process in the uterine wall within 14-16 days of fertilization. The blastocyst is considered an embryo when surrounded by an amniotic sac, a fluid-filled membrane that protects the embryo. The maternal recognition of the embryo in the uterus stimulates placental development, which transfers nutrients to the growing embryo from the mother through the umbilical cord. After fertilization, placentation starts on day 21 in cattle. However, the embryo is nourished by the yolk in birds and amphibians. During the third and fourth week of gastrulation in cattle, the embryo differentiates into three types of tissue ectoderm, mesoderm, and endoderm. The ectoderm gives rise to skin, nervous system, and tooth enamel development; the mesoderm forms the circulatory system, connective tissues, bones, and muscles; and the digestive, lungs and urinary systems are developed from the endoderm. The embryo becomes a fetus after the 9th week of pregnancy and the fetal period continues until birth. The fetal period is characterized by rapid body growth and maturation of organs and systems. Fetus size depends mainly on genetic factors such as breed, genotype, sex of the fetus, and other environmental factors like age of the mother, nutrition, and management. [Mohammad Shamsul Alam Bhuiyan]