The Mystery of Embryonic Development and Sex: A Male Health Encyclopedia in the Origin of Life

The Formation of the Human Body and Sex Determination

The human body has nine major systems with complex physiological functions. The intricately structured and functionally complex human body originates from the union of sperm and egg cells: the fertilization and fusion of an egg cell to form a zygote, which then undergoes a series of cell divisions and differentiations to form a complete human body. The zygote is formed by the union of the father's sperm and the mother's egg cell. Human embryonic development can be divided into two stages: the embryonic period (weeks 1-8) and the fetal period (months 3-10). The embryonic period is mainly the early stage of human development, including cleavage, blastocyst formation, formation of the three germ layers, cell and tissue differentiation, and the establishment of the rudimentary structures of various organs. The embryonic period is a crucial period for rapid embryonic growth and the establishment of organ functions.

Under normal circumstances, the zygote begins to divide after 30 hours, from one cell dividing into two, and after about 3 days, it forms a solid body with 16 cells, called a morula. The morula moves into the uterine cavity via the fallopian tube. One week later, the blastocyst implants into the uterine wall. Two weeks later, the blastocyst is fully implanted and begins to grow and develop. The main change in the third week is the transformation of the embryonic disc from two layers to three layers: ectoderm, mesoderm, and endoderm. The ectoderm forms the nervous system, skin, hair, nails, and sensory organs. The mesoderm forms muscles, bones, blood, connective tissue, the circulatory system, and the urinary and reproductive systems. The endoderm forms the epithelium of the digestive and respiratory systems and related glands. Weeks 4 to 8 are the critical period for human embryonic development. All major external and internal structures begin to develop during this time, and the outline of the fetus becomes visible. During this period, various factors that interfere with development can cause abnormalities, deformities, or even developmental arrest, leading to spontaneous abortion. For example, rubella virus can affect fetal facial development, and malnutrition can cause malnourished fetal abortion. Under normal circumstances, a fetus grows and develops in the uterus for 40 weeks before being born at full term, beginning its long journey through life.

The concept of gender refers to the distinction between male and female humans. Throughout history and across cultures, various legends have existed regarding why boys and girls are born. In ancient Greece, it was believed that the union of an egg released from the mother's right ovary and a sperm from the father's right testicle resulted in a girl; conversely, the union of an egg released from the mother's left ovary and a sperm from the father's left testicle resulted in a boy. It was also believed that a father with a strong physique and high libido would have a male child, and vice versa. Ancient Chinese people believed that the sex of a child was greatly influenced by the environment. The *Huainanzi* states, "Mountain air produces more boys, marsh air produces more girls." Some believed that the sex of the fetus was related to the parents' nutritional conditions; abundant nutrition often resulted in girls, while malnutrition often resulted in boys. Therefore, after famines and wars, society often saw an increase in the number of boys. These are merely folk tales without any scientific basis and have not been scientifically verified.

The mystery of sex was only solved in the early 20th century with the discovery of chromosomes. Human somatic cells contain 23 pairs of chromosomes, one pair of which are sex chromosomes. In female somatic cells, this pair consists of two identical X/X chromosomes, while in male somatic cells, it consists of one X chromosome and a smaller Y chromosome. During the formation of reproductive cells—sperm and egg—the cells undergo meiosis, halving the number of chromosomes. The paired chromosomes separate and enter different daughter cells. Thus, all egg cells contain only one X chromosome, while sperm cells have two types: half with X chromosomes and the other half with Y chromosomes. During fertilization, the combination of either type of sperm with the egg is random, meaning the chance of combination is equal. Generally speaking, human sex differentiation mainly depends on the composition of sex chromosomes in the fertilized egg. Fertilized eggs that combine with Y-type sperm to form XY zygotes will develop into males, while those that combine with X-type sperm to form XX zygotes will develop into females. Although the sex of the fetus is initially determined at fertilization, laying the foundation for sex inheritance, it cannot be definitively determined before the sixth week of embryonic development. Before the sixth week, the sex of the embryo can be influenced by various factors in the maternal and external environments, developing in the opposite direction. This is because the growth and development of male embryos depends on androgens. If androgen is lacking or estrogen secretion is excessive, the embryo, which should develop into a male, may turn and become an individual with female characteristics. The development of female embryos does not depend on estrogen. Experiments have shown that removing the ovaries of female embryos, thus reducing estrogen secretion, does not affect the embryo's development into a female individual. Only under the influence of large amounts of androgens can a female embryo potentially develop into a male individual, while removing the gonads of a male embryo will cause it to develop into a female individual. Therefore, in rare cases, XX-type males and XY-type females can also occur. The gender commonly referred to is the socially accepted gender, which depends on the upbringing and lifestyle after birth. However, from a medical perspective, gender can be categorized into four aspects: chromosomal sex, gonadal sex, genital sex, and social sex. Under normal circumstances, a person's gender is consistent in all four aspects. That is, a male individual, whose social sex is male, should simultaneously possess XY sex chromosomes, male gonads (testes), and male external genitalia. If there are differences in these four aspects, then there may be gender abnormalities or malformations.

(1) Chromosomal sex: Also known as genetic sex, it is determined by the sex chromosomes of the fertilized egg at fertilization. Male individuals have an XY chromosome type, while female individuals have an XX chromosome type.

(2) Gonadal sex: The sex determined by the gonads is called gonadal sex. The male gonad is the testes, and the female gonad is the ovaries.

(3) Genital sex: This is determined by the type of genitalia. For males, it is the penis, scrotum, prostate, seminal vesicles, and vas deferens. For females, it is the labia, vagina, uterus, and fallopian tubes.

(4) Social gender: This refers to the socially recognized gender, determined by upbringing and lifestyle habits after birth. Some parents, driven by personal needs, often raise boys as girls or girls as boys, which can cause serious psychological problems and unhealthy habits in the child, negatively impacting their physical and mental health. This type of upbringing is unacceptable.