Female Reproduction in Animals

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Female Reproduction in Animals

ScienceAide
23 Mar 2024
Biology

Reproductive Cycles and Hormones in Animals: And Overview

Most animals require sexual behavior in order to reproduce. Organisms need to make more of themselves in order for their species to keep plugging along, and for animals this involves the coordination of male and female gametes, or sex cells, as well as anatomical reproductive structures and hormones.

Gametogenesis is the process of making gametes from germ cells. Meiosis is done differently in plants and animals. In animals, meiosis happens within reproductive organs and the change from diploid cells to haploid gametes is a continuous process from diploid cell to gamete — sperm and egg. In male animals, the gamete-making process is called spermatogenesis, and in females it’s called oogenesis. In plants there might be extra mitotic divisions after meiosis and before gametogenesis. Plant gametogenesis is slower because it also includes a separate sporogenesis, or production of spores, the haploid cells, from diploid sporophytes.

There’s incredible diversity within the Animal kingdom when it comes to reproductive strategy, but the inner workings of the animal reproductive system is strikingly similar in raccoons, salamanders and bald eagles. In order to reproduce, these animals must find another member of their species that possesses the complementary gamete, and then engage in whatever behavior is required to put those gametes together. This behavior and the release of the gametes are mostly regulated by a complex interplay of hormones, chemical messengers that travel through the body via the blood to specific organs and tissues. These hormones cycle based on periodic rhythms determined by time of the month or even season.

Hormones are crucial for the regulation of gametogenesis in both male and female reproductive cycles, and some of the same hormones are used in both sexes. For instance, follicle stimulating hormone (FSH) and luteinizing hormone (LH) are named for the part they play in oogenesis, but it is also important in sperm production in males.

Reproductive Development During Gestation

Because reproductive structures are so alike in animals, we can generalize what happens in reproduction, even if we don’t know the specifics of hormone cycles, reproductive strategy, etc. For instance, meiosis in reproductive structures produce female and male gametes, called eggs and sperm, and after the egg is fertilized by the sperm, a zygote, or fertilized egg, is the result. From there, a new animal is made, whether it’s a dragonfly or a box turtle.

Unsurprisingly, humans do things in a specific way, and the following study guide will explore how the human female reproductive system works, from the hormones that are produced and secreted to the timing that controls reproductive cycles.

Human embryos don’t start out their fetal development as male or female — the genetic instructions for sex take their time determining what kind of gonads the baby will have — ovaries in females or testes in males. This decision is made by the sex-determining chromosome, called “XX” (with two of the same sex chromosomes) in females, and, XY, (two different sex chromosomes) in males. Some features, like nipples, exist in the growing embryo before the sex chromosome gives its instructions, which is why male humans have nipples even though they don’t have a function.

The default setting in humans is always to make a female, until about two months after fertilization, when a signal from a Y chromosome is registered as present or absent, at which point specialized male or female structures begin to develop, driven by hormones produced by the growing ovaries and testes, alongside the development of hormone receptors in the brain, which are organized differently depending on the sex of the baby, and therefore how estrogen and testosterone will be used in the body.

Reproductive Control in the Human Female

Once a baby is born, there is more sexual development to come, mediated by some of the same hormones that were at work in the developing fetus. At the time of her birth, a female baby has a partially-formed version of every egg she will have for her entire life. At the onset of puberty, or the the period during which adolescents of both sexes reach sexual maturity and become capable of reproduction, one of these eggs will complete its formation and be released through menstruation each month for many years until she reaches menopause, after which time she will have spent her egg supply and will no longer be able to reproduce.

Sexual development remains relatively dormant between birth and when a child enters puberty, sometime between the ages of 8 and 13. Puberty generally lasts between two and four years, and is characterized by thelarche, or breast budding, and the onset of monthly menstruation and changed appearance of the vulva. Additionally, we see hair growth in different areas of the body like the pubic area and underarms, as well as more sweat, acne, and other obvious
physical changes.

Hormones

The female reproductive system is regulated by an interaction between hormones produced by the brain’s hypothalamus and anterior pituitary gland, as well as the ovaries, the small, ovular glands attached by the fallopian tubes to the uterus, which produce eggs and hormones. The hypothalamus secretes gonadotropin-releasing hormone (GnRH), which triggers the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which are are both made in cells called gonadotropes found in the anterior pituitary gland, and released in small bursts every 1 to 4 hours. In response, both hormones (but primarily FSH) trigger ovarian structures called follicles to develop an oocyte, or potential egg, which leads to ovulation, the release of an egg from an ovary. The release of LH and FSH also stimulate the ovaries to secrete two sex hormones: progesterone and a type of estrogen called estradiol. These hormones are the ones primarily responsible for an adolescent girl’s changing body, the purpose of which is to prepare for sexual intercourse that will result in successful fertilization of an egg, and then pregnancy.

Most estrogen and progesterone in the bloodstream bind to plasma proteins, which renders them mostly inactive. However, it’s the hormones that remain unbound that stimulate the reproductive organs and breasts during puberty and beyond. Although the presence of estrogen and progesterone in the bloodstream generally prevent the pituitary gland from secreting LH and FSH, during some times of the month — like during ovulation — gonadotropin secretion is stimulated.

Ovarian cycle

The purpose of the Graffian follicle, also called the ovarian follicle, is to mature the oocytes that have been with a female since gestation. Every month the follicle releases an ovum, or mature egg. To do this, each month one of the two ovaries in the reproductive system undergoes a series of steps: the follicle matures, the ovum is shed, and the corpus luteum is developed. This temporary cellular structure forms on the ovary immediately following the release of the egg, its purpose being to release progesterone and other hormones in order to ready the uterus for the implantation of an embryo. If implantation does not occur, the corpus luteum breaks down, and menstruation occurs, but it’s maintained if implantation does occur.

During the follicular phase, the Graffian follicle produces estradiol, while FSH, and LH to a lesser extent, are secreted by the anterior pituitary gland. FSH levels rise at the tail end of the last menstrual cycle, and peak when the follicular phase begins, at which point the FSH levels cause between five and seven Graafian follicles to mature. A surge in LH then triggers ovulation, during which the follicle bursts at the surface of the ovary and releases an oocyte covered in follicular tissues, which will later become the barrier through which sperm must pass to contact the egg.

During the luteal phase, or second half of the menstrual cycle in which the uterus is made ready for implantation, what’s left of the Graafian follicle now functions as a corpus luteum during the luteal phase. It releases estrogen and progesterone to prevent any other follicles from developing and releasing oocytes. This is the same hormonal strategy used in birth control pills.

Uterine and menstrual cycles

Though length can vary widely, the average menstrual cycle lasts 28 days, beginning with the first day of the last period and ending with the first day of the following menstrual “period.”

As the uterus prepares to host a potential embryo, a boost of estrogen and progesterone from the ovaries thickens the endometrium, or internal lining of the uterus. If fertilization and implantation do occur, estrogen from growing follicles stimulates a thickening of the endometrium during the proliferative phase. After ovulation, progesterone levels increase and the endometrium becomes highly vascularized, secreting nutrients for embryo development in the secretory phase. As the corpus luteum disintegrates, and if no fertilization and implantation have occurred, estrogen and progesterone levels plummet, which constricts the endometrial blood vessels, causing them to shen blood and tissue.

If fertilization and implantation do occur after ovulation, the implanted embryo releases of hormone called human chorionic gonadotropin (hCG) into the mother’s circulatory system that maintains the corpus luteum so it can continue secreting estrogen and progesterone to build up the endometrium and create the placenta for the embryo. After several months, the placenta takes over the estrogen and progesterone production for the duration of the pregnancy.

Menopause

Human females ovulate for a finite period, after which their menstrual stops permanently, and they can no longer get pregnant. It’s marked by a 12-month period without any bleeding, including spotting.

During the menopausal transition, the body’s production of estrogen and progesterone declines, which causes physical and cognitive symptoms that vary between individuals. These symptoms can include things like hot flashes, trouble sleeping, joint and muscle pain, decreased libido and pain during sex, moodiness and irritability, forgetfulness, etc.

Estrous cycle

Unlike humans and other primates, the majority of mammals experience estrous, which involves the reabsorption of the thickened endometrial lining, rather than the shedding of it, if fertilization and implantation don’t occur. Behaviorally, most female mammals are only receptive to males for reproduction during estrous, which coincides with their uterus being ready for implantation. Some mammals experience estrous once yearly while others do it much more often.

Let’s help you remember the key topics with these memory tricks or mnemonic.

GEMS of Animal Reproduction
Think of the acronym GEMS to recall key aspects of animal reproduction.

G – Gametogenesis: The process of forming gametes (sperm and eggs) through meiosis.

E – Endocrine Regulation: Hormones like FSH and LH regulate reproductive cycles in both males and females.

M – Meiosis: A type of cell division that reduces chromosome numbers by half, producing haploid gametes.

S – Sex Determination: In humans, the presence of XX or XY chromosomes determines the development of ovaries or testes.

By remembering GEMS, you can recall the fundamental components of animal reproductive processes.

Test your understanding

1. What is the role of follicle-stimulating hormone (FSH) in animal reproduction?
a) Stimulates muscle growth
b) Regulates blood sugar levels
c) Initiates gamete production in both males and females
d) Controls heartbeat

2. During which process are haploid gametes produced from diploid cells?
a) Mitosis
b) Fertilization
c) Meiosis
d) Binary fission

3. In human embryonic development, what determines the differentiation into male or female reproductive structures?
a) Environmental temperature
b) Presence of specific hormones in the mother’s blood
c) The combination of sex chromosomes (XX or XY)
d) Nutritional intake during pregnancy

4. What is the default developmental pathway for human embryos before sex differentiation occurs?
a) Male
b) Female
c) Hermaphroditic
d) Neutral

5. Which statement best describes gametogenesis in animals?
a) It occurs exclusively in females.
b) It involves the production of gametes through mitosis.
c) It is the process of forming gametes via meiosis in reproductive organs.
d) It refers to the hormonal regulation of the menstrual cycle.

These questions cover the key concepts in the study guide. If you’d like to get the answers along with a step by step explanation of how to get to the correct choice.

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References:

Gametogenesis: https://www.sciencedirect.com/topics/immunology-and-microbiology/gametogenesis

Reproductive structure and function:
https://organismalbio.biosci.gatech.edu/growth-and-reproduction/animal-reproduction-ii-reproductive-structure-and-function/

Hormones:
https://www.merckmanuals.com/professional/gynecology-and-obstetrics/female-reproductive-endocrinology/female-reproductive-endocrinology#

https://med.uc.edu/landing-pages/reproductivephysiology/lecture-3/hormone-concentrations-during-puberty

Ovarian cycle:
https://pubmed.ncbi.nlm.nih.gov/11223365/

https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/graafian-follicles#:~:text=The%20Graafian%20follicle%20grows%20under,The%20follicle%20becomes%20preovulatory.

https://my.clevelandclinic.org/health/body/21849-corpus-luteum

https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/graafian-follicles#:~:text=The%20Graafian%20follicle%20grows%20under,The%20follicle%20becomes%20preovulatory.

https://my.clevelandclinic.org/health/body/23953-follicular-phase