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«1. Introduction 2. Ontogenetic Transitions and Adaptations 3. Hormones and Sexual Development A. Fetal Hormones and the Development of Reproductive ...»

Behavioral Embryology: Hormones and Sexual

Development

1. Introduction

2. Ontogenetic Transitions and Adaptations

3. Hormones and Sexual Development

A. Fetal Hormones and the Development of Reproductive Organs

B. Development of Sex Differences in the Brain

C. Perinatal Hormones and Behavioral Development

D. Gender and Sex

E. Is There a Difference in the Brains of Homosexuals and Heterosexuals?

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1. Introduction

Ontogenetically, all vertebrates must develop from a single celled zygote to an adult form. But, the different vertebrate classes accomplish this developmental change in different ways.

Amphibians go through a free living larval stage, which is morphologically and behaviorally very different from the adult stage.

Fish and Reptiles are typically much different, emerging from their embryonic stages very similar to adult forms in both behavior and morphology. Growth primarily characterizes development from hatching to sexual maturity.

Mammals and Birds undergo much more gradual and developmental transitions from a neonate (just born mammal or hatched bird) to adult form.

The degree of immaturity in neonatal mammals and birds is distinguished in terms of how altricial or precocial they are, which is their degree of maturity.

For example, humans, rats, mice, many nesting birds all have relatively altricial young requiring considerable parental care to make the transition from relatively immature to mature stages of development. Ducks, deer, cattle all have relatively precocial young at birth. They still require parental care, but less of it.

Behavioral Embryology focuses on the development of behavior in young animals, and how these changes are related to morphology, neuoanatomical changes, and hormonal changes.

2. Ontogenetic Transitions and Adaptations Earlier, we talked a lot about the Umwelten of different organisms, which is their sensorimotor environment that they perceive and react to.

Mammals and Birds can go through a succession of Umwelten during development.

Consider the difference in Umwelt between a newly born mouse or rat and it more mature stages (blind and deaf, etc.).

During each stage of development, an organism must be able to perform behaviors adaptive to its Umwelt. For example, for mammals they must be able to nurse, and how they do this can be quite different from one mammalian species to the next.

In behavioral embryology we should consider the behaviors exhibited by developing

organisms in two ways:

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1. Ontogenetic Transitions: Mammals and birds especially, must make behavioral transitions from neonatal behaviors to adult behaviors. These behaviors may be in preparation for later stages of development.

2. Ontogenetic Adaptations: Organisms must also behave in ways that facilitate immediate biological functions. Forexample, nursing in young mammals or pecking on a parent’s beak or opening the mouth in response to a parent with food.

This distinction is far from clear cut or mutually exclusive. For example, in the case of

fetal activity, three hypotheses have been put forward:

1. Fetal Activity as an Epiphenomenon: For example, mammals including humans exhibit activity spontaneously prenatally: movement of developing limbs, spontaneous breathing in and out of amniotic fluids. The epiphenomenal view is that all these forms of spontaneous activity are merely the consequences of the developing nervous systems, and are neither adaptive or preparations for postnatal behavior.

2. Fetal Activity as Preparation for postnatal Behavior: On this view, these behaviors prepare the way for the further development of behavior. For example, spontaneous breathing may be important for the differentiation of lung tissue and for correcting “bad” intercostal nerve connections with muscle segments (i.e.

muscles surrounding the ribs).

3. Fetal Activity as Ontogenetic Adaptation: On this view, fetal activity may be adaptations to specific environments. For example, fetal rats swallow amniotic fluids. The flow of these fluids over olfactory and taste receptors may help the neonatal rat identify the mother’s body as a food source, e.g., injecting citral into the amniotic fluid of surrounding prenatal rats. (ultrasounds in rat pups?) But, it must be kept in mind that (i) each of these hypotheses would be difficult to establish and (ii) it most likely true that many prenatal behaviors are both ontogenetic adaptations and ontogenetic transitional behaviors.

–  –  –

In talking about hormones and sexual development today, I’m going to be focusing primarily on ontogenetic transitions. Next lecture will focus more on ontogenetic adaptations.

–  –  –

A. Fetal Hormones and the Development of Reproductive Organs Sexual development in mammals begins at the time of fertilization with the production of typically two kinds of zygotes XX (female) and XY (male), and is an excellent example of how sex is not programmed by the genes.

–  –  –

After six weeks, H-Y antigen (a protein hormone) causes the medulla to develop into a testis. (The production of this hormone may be the major function of the Y chromosome).





In the absence of the H-Y antigen, the primordial gonads automatically develop into ovaries. If H-Y antigen is injected into XX (mammalian) fetuses, then testes develop.

After this point, the Y chromosome has no more influence on the development of sexual differences.

Both males and females have two compete sets of reproductive ducts: The Wolffian system (which has the capacity to develop into the male reproductive ducts, e.g., seminal vesicles, vas deferens) and the Müllerian system (which has the capacity to develop into the female ducts (e.g., uterus, vagina, fallopian tubes).

In the third month of male fetal development, the testes secret testosterone and Müllarian-inhibiting substance.

6 The development of the Müllarian system occurs in any fetus that is not exposed to testosterone during early development. If male or female gonads are removed, the Müllarian system develops.

TABLE 1. Timing of sexual differentiation in the human fetus.

Fetal age* Crown-rump length (weeks) (mm) Sex differentiating events

–  –  –

Androgen insensitivity syndrome (AIS): A developmental disorder of genetic males in which an insensitivity to androgens causes them to develop female bodies, but not internal female anatomy. Testes develop and release Mülarianinhibiting substance. The testes release lots of testosterone and enough estrogens to feminize the body in the absence of any sensitivity to androgens.

AIS comes in degrees and is caused by is a set of disorders of sex development caused by mutations of the gene encoding the androgen receptor. In complete AIS, people generally women with internal testes, genetically male, and normal female bodies by external appearance with some exceptions.

–  –  –

There can also be partial AIS (PAIS): male or female body, with slightly virilized genitalia or micropenis; testes in the abdomen; sparse to normal androgenic hair.

Adrenogenital syndrome. A disorder characterized by a decrease in the release of the hormone cortisol from the adrenal cortex, which results in the production of high levels of adrenal androgens and masculinizes the bodies of genetic females.

It is any of several autosomal recessive diseases resulting from mutations of genes for enzymes mediating the biochemical steps of production of cortisol from cholesterol by the adrenal glands Usually, the onset of this disorder is relatively late in development, and thus does not stimulate the development of the Wolffian system. Thus, there is often

only partial development of secondary sex characteristics:

–  –  –

Most the experimental research on sex differences in the brain is with rats.

Early research focused on the cyclic release of gonadatropins (LH, FSH) Early research by Pfeiffer (1936, Sexual differences of the hypophyses and their determination by the gonads. American Journal of Anatomy, 5, 195 - 225) found that gonadectomizing neonatal rats of either sex caused them to develop female cyclic patterns of gonadotropin release, whereas testes transplanted into gonadectomized females or intact females caused them to develop the less cyclic male pattern of gonadotropin release.

The control of the release of gonadatropins is by the hypothalamus, thus Pfeiffer’s research provided the first evidence of the sexual differentiation of the hypothalamus.

C. Perinatal Hormones and Behavioral Development Most of the research on hormones and behavioral development has focused on the role of hormones in copulatory behaviors in laboratory animals.

Early research (Phoenix CH, Goy RW, Gerall AA, Young WC 1959 Organizing action of prenatally administered testosterone propionate on the tissues mediating mating behavior in the female guinea pig. Endocrinology 65:369–

382) showed that the prenatal injection of testosterone both maculinizes and defeminizes a genetic female’s copulatory behavior. In that study they

1. They injected pregnant guinea pigs with testosterone.

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2. When the litters were born, they ovariectomized the female offspring.

3. When these ovariectomized female guinea pigs reached maturity, they injected them with testosterone and assessed their copulatory behavior.

4. They found that the females exposed to prenatal testosterone displayed more male like mounting behavior in adulthood than those who were injected with testosterone but not exposed to testosterone prenatally.

5. And as adults, when they were injected with estrogen and progesterone (explain spontaneous ovulation), females prenatally exposed to testosterone displayed less lordosis.

In a complementary study, they also found that the lack of exposure to testosterone feminizes and demasculinizes copulatory behavior in adults.

1. Male rats castrated shortly after birth failed to display the normal male copulatory pattern of mounting, intromission, ejaculation, when they were treated with testosterone and given access to sexually receptive females.

2. And when they were injected with estrogen and progesterone as adults, they displayed more lordosis than did castrated controls.

D. Gender and Sex

1. We have seen so far that the sexuality of a person or other mammal is not simply a matter of the individual’s genotype (i.e., the presence of X and/or Y chromosomes).

2. The hormones one is exposed to pre and postnatally determine to what degree male or female secondary sexual characteristics develop.

3. In addition to biological sex and sex characteristics, there is also the notion of gender.

4. Roughly, gender refers to social or cultural distinctions of being masculine or feminine, refers to the more biological aspects we have talked about.

5. Are the two distinct? For example, can the social environment of a person be arranged early in postnatal development so that an individual that an individual that is male or female sexually (e.g., in the genetic or secondary sexual characteristic sense) will adopt the opposite gender?

6. The theory that this can be done is the theory of gender neutrality, which states that gender can be socially constructed (with the help of surgery) independent of the biological sex of the individual.--John Money

–  –  –

http://www.bbc.co.uk/science/horizon/2000/boyturnedgirl.shtml http://infocirc.org/rollston.htm Video: part 1, part 2, part 3, part 4, part 5 E. Is There a Difference in the Brains of Homosexuals and Heterosexuals?

Actually, this question implies more than just a difference. Is there a difference that causes a difference in behavior?

Simon LeVay (Science, 1991), published the results of a postmortem study that compared the neuroanatomy of three groups of subjects: heterosexual men, homosexual men, and women (who were assumed to be heterosexual).

Previous research reporting sex differences in the anterior and preoptic areas of the hypothalamus of various species, focused LeVay’s research on these areas.

And as we have seen, the hypothalamus is an important area for sex differences in the brain.

Levay found a difference in the volume of the third interstitial nucleus of the anterior hypothalamus (INAH 3). There are two kinds of questions to ask about this study?

–  –  –

2. What does it really mean? What is the direction of causation? Sexuality in humans has a cognitive component, so the hypothalamus does not hold all the answers.

3. Nevertheless, because we do know that hormones effect behavior, changes in hormones can have a variety of causes, hormones can restructure the brain at any developmental stage of the organism, it would not be surprising if these or results like this turn out to be correct. How we are to interpret them is another matter.

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