What is the Difference Between Male and Female Fetus

The main difference between male and female fetus is that male fetus has an X and a Y chromosome, whereas female fetus has two X chromosomes. Furthermore, the expression of genes in the Y chromosome results in the development of the testes in the male fetus, after 6 to 7 weeks of gestation, while during the early development, the gonads of the fetus, which are phenotypically female remain as undifferentiated fetal genitalia. Besides, the SRY (sex-determining region Y) is the sex-determining gene which serves as the primary sex determinant in the male fetus while X-chromosome-linked genes play a critical role in the testis- and ovary-determining cascades in both male and the female sex determination. 

Male and female fetus are the two genders of a fetus that occur in higher animals, including humans. Generally, the fetal stage in humans starts after the nine weeks of fertilization. 

Key Areas Covered 

1. What is Male Fetus
     – Definition, Genetics, Morphology, Development
2. What is Female Fetus
     – Definition, Genetics, Morphology, Development
3. What are the Similarities Between Male and Female Fetus
     – Outline of Common Features
4. What is the Difference Between Male and Female Fetus
     – Comparison of Key Differences

Key Terms 

External Genitalia Differentiation, Female Fetus, Gonads, Internal Genitalia Differentiation, Male Fetus

What is Male Fetus 

The male fetus is the early stage of the development of males characterized by the presence of an X and Y chromosome in the nucleus. However, in humans, the fetus is sexually indifferent until seven weeks after fertilization. Over the next five weeks, fetus undergoes the differentiation of sex as a male by developing sex organs and the secretion of sex hormones by the genes on the Y chromosome. The three structures of sex organs are the gonads, internal genitalia, and external genitalia. Especially, masculinization refers to the biological development of sex differences in males, differentiating them from females.

Figure 1: Y Chromosome

Gonads 

Furthermore, the SRY gene is the primary sex determinant whose gene product regulates the expression of genes, which triggers the development of testis as well. Testes are the gonads of the male fetus. Moreover, in males, Leydig cells of the testis secrete testosterone, which is the main sex hormone responsible for the sex differentiation in males at nine weeks after gestation. 

Figure 2: Male Sex Differentiation

Internal and External Genitalia 

Moreover, due to the presence of the SRY gene, the mesonephric system proceeds its development while regressing the paramesonephric system. Therefore, in the male fetus, mesonephric ducts develop into the ducts of the epididymis, ductus deferentia, ejaculatory ducts, and seminal vesicles. In addition to this, between 8-12 weeks after gestation, the male fetus undergoes external genitalia differentiation. Here, dihydrotestosterone is the enzyme responsible for the development of the characteristics of the external genitalia including the unambiguous penis with a phallic urethra, and a thinned, rugated scrotum. 

What is Female Fetus 

The female fetus is the early development stage of females characterized by the presence of two X chromosomes. Although the SRY gene serves as the basic sex-determining factor in the male fetus, the process of sex differentiation in the female fetus does not require additional factors.  

Gonads 

Generally, the gonadal ridge is the precursor of the gonads. Moreover, during the embryonic development, the gonadal ridge pinches off from the mesonephros. But, it still remains connected to each other by a fold of peritoneum. In the female fetus, ovaries develop from the gonadal ridge and a part of the mesonephros.

Figure 3: External Genitalia Differentiation

Internal Genitalia Differentiation 

Furthermore, the paramesonephric ducts do not undergo regression in the female fetus. However, they develop into the Fallopian tubes, uterus, cervix, and the upper two-thirds of the vagina.  

External Genitalia Differentiation 

Moreover, at the seventh week after fertilization, the genital tubercle, urogenital groove and sinus, and labioscrotal folds of the fetus develop into the clitoris, urethra and vagina, and labia. This occurs without the effect of any androgens as in the male fetus. 

Similarities Between Male and Female Fetus  

• Male and female fetus are the two fetuses that occur with different genders in animals such as humans.  
• Furthermore, they are the two normal types of a fetus, containing different combinations of only two sex chromosomes. Meanwhile, abnormal fetus contains an altered number of chromosomes.  
• Genetically, the two types of fetus show around 140 differences in gene expression.  
• In the early development of the embryo, both sexes possess equivalent internal structures, which include the mesonephric ducts and paramesonephric ducts.  However, these structures undergo regression under the influence of sex hormone production in the fetus, leading to internal genitalia differentiation.   
• Anatomy of the fetus other than the sex organs is similar in both male and female fetus.    
• Moreover, high levels of maternal blood estrogen levels occur throughout the pregnancy with both types of the fetus.  
• Also, the sex of the baby can be found out with an ultrasound between 18-20 weeks of gestation. The other methods of sex determination include amniocentesis (in 15 weeks) and chorionic villus sampling (in 11 weeks). 

Difference Between Male and Female Fetus 

Definition 

Male fetus refers to the fetus of a male, containing cells with a combination of XY chromosomes while female fetus refers to the fetus of a female, containing cells with a combination of XX chromosomes. 

Morphological Basis of the Development of Gonads  

The expression of genes in the Y chromosome results in the development of the testes in the male fetus after 6 to 7 weeks of gestation while, during the early development, the gonads of the fetus remain as undifferentiated fetal genitalia and are phenotypically female. 

Genetic Basis of the development of Gonads 

The SRY (sex-determining region Y) is the sex-determining gene, which serves as the primary sex determinant in the male fetus while X-chromosome-linked genes play a critical role in the testis- and ovary-determining cascades in both male and the female sex determination. 

Hormonal Basis of the Development 

Furthermore, male fetus tends to produce lower amounts of hCG while the female fetus tends to produce a higher amount of hCG. 

Pregnancy Symptoms 

Male fetus causes less desirable symptoms of pregnancy such as nausea and sickness, while pregnancy symptoms are slightly more common with a female fetus such as severe morning sickness. 

Extra-Embryonic Tissue development 

Moreover, male fetus invests less in the extra-embryonic tissue development while female fetus invests more in the extra-embryonic tissue development. 

Embryonic Tissue development 

Male fetus invests more in the embryonic tissue (body growth and tissue) development while female fetus invests less in the embryonic tissue development. 

Total Development and Survival 

The male fetus is inclined towards the development and survival while the female fetus adopts a risk-averse strategy towards development and survival.  

Sex Hormone Levels in the Amniotic Fluid 

A high concentration of testosterone occurs in the amniotic fluid of male fetus early in the second trimester while a high concentration of estrogen occurs in the amniotic fluid of female fetus early in the second trimester.  

Gonads 

The gonads of the male fetus are the testes while the gonads of the female fetus are the ovaries.  

Internal Sex Differentiation  

Mesonephric system is the precursor for the male internal genitalia while the paramesonephric system is the precursor for the female internal genitalia. 

Internal Genitalia 

The ducts of the epididymis, ductus deferentia, ejaculatory ducts, and seminal vesicles are the internal genitalia of the male fetus while Fallopian tubes, uterus, cervix, and the upper two-thirds of the vagina are the internal genitalia of the female fetus. 

External Genitalia Differentiation 

Dihydrotestosterone is responsible for the external genitalia differentiation in the male fetus while in the female fetus, external genitalia differentiation occurs without excess androgens. 

External Genitalia 

Moreover, the unambiguous penis with a phallic urethra and a thinned, rugated scrotum are the external genitalia in the male fetus while the clitoris, urethra and vagina, and labia are the external genitalia of the female fetus. 

Signs of the Ultrasound 

The three signs to identify a male fetus are the sagittal sign, the flow of urine, and the male genitalia while the two signs to identify a female fetus are the hamburger sign and the sagittal sign.  

Conclusion 

Basically, the male fetus is the early development stage of males characterized by the presence of a combination of X and Y chromosome. Although it appears phenotypically similar to the female fetus during the early stages of development, the male fetus starts the differentiation of sex due to the expression of the SRY gene. Moreover, it leads to the production of testosterone and the development of testis, the male gonads. Furthermore, this is followed by the internal genitalia differentiation and under the influence of dihydrotestosterone, external genitalia differentiation. In contrast, the female fetus, which is characterized by the presence of two X chromosomes, undergo sex differentiation without any influence of an external factor. Therefore, the main difference between male and female fetus is their genetics, morphology, and development. 

References:

1. Institute of Medicine (US) Committee on Understanding the Biology of Sex and Gender Differences; Wizemann TM, Pardue ML, editors. Exploring the Biological Contributions to Human Health: Does Sex Matter? Washington (DC): National Academies Press (US); 2001. 3, Sex Begins in the Womb. Available Here.
2. “Sexual Differentiation in Humans.” Wikipedia, Wikimedia Foundation, 26 Sept. 2019, Available Here.
3. McNamee, David. “Placenta Genes ‘Underpin Health Differences between Girl and Boy Babies’.” Medical News Today, MediLexicon International, 28 May 2014, Available Here.
4. Weiss, Robin Elise. “Signs Sonographers Look for to Determine the Sex of a Baby.” Verywell Family, Verywell Family, 18 July 2019, Available Here.

Image Courtesy:

1. “YChromShowingSRY2” By Je_at_uwo (Public Domain) via Commons Wikimedia   
2. “SRY Gene Pathway” By Silverthorn, Dee – Human Physiology: an integrated approach (Public Domain) via Commons Wikimedia   
3. “Anterior view of human female and male, with labels 2” By File:Anterior view of human female and male, with labels.jpg: Mikael HäggströmDerivative work: nagualdesign (CC0) via Commons Wikimedia