Medical Physiology, 3rd Edition

CHAPTER 53. Sexual Differentiation

Sam Mesiano, Ervin E. Jones

Reproduction is a fundamental process of life. All living organisms must reproduce either asexually (e.g., bacteria) or sexually (e.g., mammals). Asexual reproduction is highly efficient and produces large numbers of genetically identical offspring in a relative short amount of time. This strategy, however, is vulnerable to environmental changes because genetic and phenotypic variation between individual progeny is minimal and consequently the probability of producing progeny that have beneficial traits in a hostile environment is relatively low. In contrast, sexual reproduction is less efficient but produces progeny with markedly increased genetic and phenotypic variation, which increases the probability of producing individuals with characteristics that may be adaptive to environmental changes. In this context, natural selection favors sexual reproduction, and consequently most extant animals and flowering plants reproduce sexually.

In sexual reproduction a new individual is created by combining the genetic material of two individuals. Sexual reproduction involves the evolution of two sexually dissimilar individuals belonging to the same species, one male and one female, and each equipped with its own specific attributes necessary for its particular contribution to the process of procreation. Each sex produces its own type of sex cell or gamete, and the union of male and female gametes generates species-specific progeny. In addition, mechanisms—some simple, some complex—have evolved to ensure the proximity and union of the sex cells, known as syngamy. Thus, within each species, the relevant sexual characteristics of each partner have adapted differently to achieve the most efficient union of these progenitor cells. These differences between the sexes of one species are called sexual dimorphism. For example, oviparous species such as frogs release their eggs into a liquid medium only when they are in relative proximity to sperm. As effective as this approach is, it also typifies the wastefulness of reproduction among higher species inasmuch as most gametes go unfertilized.

Even among species that normally reproduce sexually, sexual dimorphism is not universal. For example, monoecious (i.e., hermaphroditic) species, such as cestodes and nematodes, have the capacity to produce both sperm and eggs. By definition, the ability to produce just one kind of gamete depends on sexually dimorphic differentiation.

Organisms that reproduce sexually normally have a single pair of sex chromosomes that are morphologically distinguishable from other chromosomes, the autosomes. Each of the sex chromosomes carries genetic information that determines the primary and secondary sexual characteristics of an individual; that is, whether the individual functions and appears as male or female. It has become abundantly clear that genes determine sexual differentiation and sexual expression and, as a result, mechanisms and patterns of reproduction.

The sex of the gonad is genetically programmed: Will a female gonad (ovary) or a male gonad (testis) develop? Although germ cells of the early embryonic gonad are totipotent, these cells develop into female gametes or ova if the gonad becomes an ovary, but they develop into male gametes or sperm if the gonad becomes a testis. These two anatomically and functionally distinct gonads determine either “maleness” or “femaleness” and dictate the development of both primary and secondary sexual characteristics. Endocrine and paracrine modulators that are specific for either the ovary or the testis are primarily responsible for female or male sexual differentiation and behavior and therefore the individual's role in procreation. imageN53-1


Definitions of Sex and Gender

Contributed by Ervin Jones

Gender—or, more accurately, gender identification—refers to the concept held by the individual (or by those raising the individual) that the individual is male, female, or ambivalent.

Sex refers to biological characteristics that distinguish female from male. The distinction may be made on the basis of chromosomes, gonads, internal and external morphology, and hormonal status.

Genetic Aspects of Sexual Differentiation

Differentiation of the Gonads

Development of the Accessory Sex Organs

Differentiation of the External Genitalia

Endocrine and Paracrine Control of Sexual Differentiation