Physiology of the Hypothalamus-Pituitary-Gonad Axis PDF

**Physiology of the Axis \" Hypothalamus - Pituitary - Gonad\" ( HPG Axis) and the Hormonal Control of the Reproductive System** What is hormonal control of the reproductive system? The hormonal control of the reproductive system is governed by a complex interplay of hormones, primarily regulated...

**Physiology of the Axis \" Hypothalamus - Pituitary - Gonad\" ( HPG Axis) and the Hormonal Control of the Reproductive System** What is hormonal control of the reproductive system? The hormonal control of the reproductive system is governed by a complex interplay of hormones, primarily regulated by the **Hypothalamus-Pituitary-Gonad (HPG) axis.** This system controls the development, maturation, and functioning of the reproductive organs in both males and females **The \"Hypothalamus-Pituitary-Gonad (HPG) Axis\"** is a critical part of the hormonal control system that regulates the reproductive system in both males and females. This axis involves the coordinated action of the hypothalamus, pituitary gland, and gonads (testes in males, ovaries in females) to control the production and release of hormones essential for reproduction, regulating reproductive function and maintaining hormonal balance called \"Homeostasis\". **HYPOTHALAMUS** The hypothalamus, located in the brain, has both neural and endocrine functions, producing and secreting hormones of **the gonadotropin-releasing hormone (GnRH)** in a pulsatile manner. The pulsatile secretion of GnRH is crucial for stimulating the pituitary gland. It has both neural and endocrine functions, producing and secreting many hormones. **PITUITARY GLAND** Is a bean-sized organ suspended from it by a stem called the **infundibulum** (pituitary stalk). The pituitary gland consists of an anterior and posterior lobe, with each lobe secreting different hormones in response to signals from the hypothalamus. **2 LOBES OF THE PITUITARY GLAND** **Posterior Pituitary (neurohypophysis)** \- An extension of the neurons of the paraventricular and supraoptic nuclei of the hypothalamus. -The posterior pituitary gland does not produce hormones, but rather stores and secretes hormones produced by the hypothalamus. Associated hormones in the Posterior lobe: Antidiuretic hormone (ADH ) - Stimulates water reabsorption by kidneys Oxytocin- Stimulates uterine contractions during childbirth Melanocyte- Stimulates melanin formation in melanocytes **Anterior Pituitary (adenohypophysis)** Responsible for creating and releasing hormones that affects many bodily processes Associated hormones in the Anterior lobe: Growth hormone (GH) - also called "somatotropin" that promotes growth of body tissues. Prolactin (PRL ) - Promotes milk production from mammary glands. Thyroid-stimulating hormone (TSH) - Stimulates thyroid hormone release from thyroid. Adrenocorticotropic hormone (ACTH) - Stimulates hormone release by adrenal cortex. Follicle-stimulating hormone (FSH)- Stimulates gamete production in gonads. Luteinizing hormone (LH) - Stimulates androgen production by gonads. In response to the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus, the anterior pituitary gland releases two hormones: Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH). These two hormones are collectively known as gonadotropins and they target the gonads (testes in males, ovaries in females). **GONADS** The gonadotropins (LH and FSH) travel through the bloodstream to the gonads. **Testosterone** - is the primary male sex hormone (androgen. Small amounts are also produced by the ovaries in females and the adrenal glands in both sexes. **Functions:** It is crucial for the development of male reproductive tissues, including the testes and prostate. Stimulates spermatogenesis (sperm production) in the testes. Responsible for secondary sexual characteristics such as increased muscle mass, deeper voice, facial and body hair. Maintains libido (sex drive) and contributes to normal erectile function. Present in females, but in smaller amounts and contributes to libido, mood regulation, abundance of body hair ( if there is an excess testosterone) , and bone health. **Estrogen** - is the primary female sex hormone produced mainly by the ovaries and, during pregnancy, by the placenta. **Functions**: Regulates the menstrual cycle by stimulating the growth and maturation of the ovarian follicles. Prepares the uterus lining (endometrium) for potential implantation of a fertilized egg. Promotes the development of secondary sexual characteristics such as breast development, body fat distribution, and widening of the hips. During puberty, estrogen helps regulate the growth of the reproductive organs (uterus, fallopian tubes, and vagina). Contributes to maintaining vaginal health by keeping the vaginal lining thick and lubricated. **Progesterone** - is a female sex hormone primarily produced by the corpus luteum in the ovaries after ovulation, and by the placenta during pregnancy. **Functions**: After ovulation, progesterone helps maintain the uterine lining (endometrium), making it receptive for implantation of a fertilized egg. If pregnancy occurs, progesterone supports the early development of the embryo and prevents uterine contractions that could expel the embryo. During pregnancy, progesterone is produced by the placenta and helps maintain a healthy pregnancy by suppressing the maternal immune response to the fetus. Prepares the mammary glands for milk production after childbirth. **In the male Gonad (testes)** LH stimulates the Leydig cells in the testes to produce testosterone, the primary male sex hormone. FSH, together with testosterone, acts on the Sertoli cells to promote spermatogenesis (sperm production). **In the female Gonad (ovaries)** The LH and FSH play key roles in the menstrual cycle: FSH stimulates the growth and maturation of ovarian follicles (which contain the eggs). Rising estrogen levels, produced by growing follicles, lead to an LH surge, which triggers ovulation (the release of an egg). After ovulation, the ruptured follicle forms the corpus luteum, which produces progesterone and estrogen to prepare the uterus for a potential pregnancy. **FEEDBACK REGULATION** The sex hormones produced by the gonads (testosterone, estrogen, and progesterone) feed back to the hypothalamus and pituitary gland to regulate the production of GnRH, LH, and FSH. **Types of feedback mechanisms:** 1. Negative Feedback - occurs when a product feeds back to decrease its own production. This type of feedback brings things back to normal whenever they start to become too extreme as it keeps the concentration of a hormone within a narrow range. Example: High levels of testosterone in males or estrogen and progesterone in females inhibit the release of GnRH, LH, and FSH, preventing overproduction. 2. Positive Feedback - occurs when a product feeds back to increase its own production. This causes conditions to become increasingly extreme. Example in females: In the middle of the menstrual cycle, increasing estrogen levels can briefly cause a positive feedback loop, leading to a surge in LH that causes ovulation. **UNIT V : SEXUAL DIFFERENTIATION/ PUBERTY** **SEXUAL DIFFERENTIATION-** Is the process by which the bodies of males and females develop distinct characteristics. This process begins in the womb and continues through early childhood. It involves the development of reproductive organs, secondary sexual characteristics (like body hair and voice changes), and other gender-specific traits. **PUBERTY-** is the stage of development during which a child\'s body matures into an adult body capable of sexual reproduction. It involves a series of physical and hormonal changes that lead to sexual maturity. **MALE PUBESCENT DEVELOPMENTAL STAGES:** **PREPUBERTAL AGE (AGES 8- 9)-** This stage is characterized by the absence of puberty-related changes. Boys have small testicles and penis, and there is no visible pubic hair or significant height increase. Boys may not show any outward signs of hormonal changes. Their bodies are generally consistent with prepubescent childhood. **EARLY PUBERTY (AGES 9-11)** The first signs of puberty appear with testicular enlargement and initial pubic hair growth. The penis begins to grow in length and girth. Boys may experience an increase in height and weight, and there may be some early signs of body odor due to increased sweat gland activity. Emotional changes may also begin, including increased sensitivity or mood swings. **MID-PUBERTY (AGES 11-14)** Boys undergo a deepening of the voice, growth of facial and underarm hair, and further height increases. Muscle mass begins to develop, and there is a noticeable growth spurt. The growth spurt is often accompanied by a temporary increase in appetite. Boys may also experience acne due to increased oil production in the skin. Mood swings and increased interest in sexual matters are common **LATE PUBERTY (AGES 14-16)** Boys reach their peak growth and achieve adult height. Muscle development continues, and facial hair becomes more prominent. The penis and testicles are fully developed, and nocturnal emissions (wet dreams). Boys may see increased definition in their muscle structure, and their voice stabilizes at its adult pitch. Social and emotional maturity continues to develop, and they may take on more adult like responsibilities. **POST PUBERTY (AGES 16-18)** Sexual and physical development stabilizes, and boys reach full sexual maturity. Their body shape becomes more defined and adult-like. By this stage, boys have reached their final height and body composition. They may also develop a more stable emotional and psychological profile as they approach adulthood. **FEMALE PUBESCENT** **DEVELOPMENTAL STAGES:** **PRE-PUBERTAL AGE (AGES 6-8)** Girls have not yet started puberty, so there are no signs of breast development or pubic hair. Their bodies remain childlike. Girls may experience growth in height and weight at a rate consistent with early childhood. Hormonal levels are stable, with no significant changes affecting physical appearance. **EARLY PUBERTY (8-11)** Breast buds begin to form, and pubic hair starts to grow. Girls experience an initial growth spurt. Girls may have changes in body odor and increased oil production leading to acne. Emotional changes such as mood swings and sensitivity can occur due to fluctuating hormones. There may also be a noticeable increase in appetite and weight gain. **MID-PUBERTY (AGES 11-14)** Breast development continues, menstruation begins, and girls experience a significant growth spurt. Body shape becomes more rounded with increased fat distribution in the hips and thighs. Menstrual cycles may be irregular at first, and girls may experience discomfort or cramps. Acne and other skin changes may persist. Emotional fluctuations are common, and social pressures related to body image and menstrual cycles can emerge. **LATE PUBERTY ( AGES 14-16)** Girls reach their adult height, and their breasts are fully developed. Menstrual cycles become regular, and the body shape stabilizes. The body continues to mature, with a more defined waist and rounded hips. Acne may improve, and overall physical growth slows down. Emotional and psychological development tends to stabilize, with a clearer sense of identity and self-image. **POST PUBERTY ( AGES 16-18 )** Physical and sexual development is complete. Girls achieve full sexual maturity and their body shape is fully established. By this stage, menstrual cycles are regular, and girls have achieved their final height and body shape. Emotional maturity continues to develop as they transition into adulthood. Girls may also experience increased self confidence and a more stable emotional state. **REPRODUCTIVE ANATMOY AND PHYSIOLOGY** **MALE REPRODUCTIVE ANATOMY AND PHYSIOLOGY** The male reproductive system comprises organs that form the reproductive and urinary system in men and individuals assigned male at birth, consisting of internal and external components, with internal parts located within the body and external parts outside. **The organs that make up the male reproductive system perform the following :** - Produce, maintain and transport sperm cells and semen. Sperm cells are male reproductive cells. Semen is the protective fluid around sperm. - Discharge sperm. - Produce and secrete male sex hormones. - The entire male reproductive system depends on hormones. Hormones are chemicals that stimulate or regulate activity in your cells or organs. The primary hormones that help the male reproductive system function include: - Follicle-stimulating hormone (FSH) - Your pituitary gland makes this and is necessary to produce sperm (spermatogenesis). - Luteinizing hormone (LH) - Your pituitary gland also makes this hormone. This is necessary to continue the process of spermatogenesis. - Testosterone - is the main sex hormone in people AMAB. It helps you develop certain characteristics, including muscle mass and strength, fat distribution, bone mass and sex drive (libido). **EXTERNAL-** Most of the male reproductive system is on the outside of your abdominal cavity or pelvis. The external body parts of the male reproductive system include the penis, scrotum and testicles. Another name for these parts is genitals or genitalia. - PENIS- A male organ used for sexual intercourse. It contains many sensitive nerve endings, and it has three parts: Root - base of the penis. It attaches to the wall of the abdomen. - SCROTUM - TESTICLES - EPIDIDYMIS They are long, coiled tube that rests on the back of each testicle. It carries and stores the sperm cells that the testicles create. The epididymis also brings the sperm to maturity --- the sperm that emerge from the testicles are immature and incapable of fertilization. During sexual arousal, muscle contractions force the sperm into the vas deferens. **INTERNAL PARTS OF THE MALE REPRODUCTIVE ORGAN/SYSTEM** - VAS DEFERENS- They are long, muscular tube that travels from the epididymis into the pelvic cavity, just behind the urinary bladder. The vas deferens transports mature sperm to the urethra in preparation for ejaculation. - EJACULATORY DUCTS Each testicle has a vas deferens that joins with seminal vesicle ducts to form ejaculatory ducts. The ejaculatory ducts move through the prostate, where they collect fluid to add to semen. They empty into the urethra. - URETHRA It is a tube that carries pee from the bladder outside of the body. It also ejaculates semen when the male has reached orgasm. - SEMINAL VESICLE They are sac-like pouches that attach to the vas deferens near the base of the bladder. Seminal vesicles make up to 80% of the ejaculatory fluid, including fructose. Fructose is an energy source for sperm and helps them move (motility). - PROSTATE GLANDS They are walnut-sized gland that rests below the bladder, in front of the rectum. The prostate adds additional fluid to ejaculate, which helps nourish sperm. The urethra runs through the center of the prostate gland. - BULBOURETHRAL (COWPER) GLANDS They are pea-sized structures on the sides of the urethra, just below your prostate. They create a clear, slippery fluid that empties directly into the urethra. This fluid lubricates the urethra and neutralizes any acids that may remain from your pee. **FEMALE REPRODUCTIVE ANATOMY AND PHYSIOLOGY** - The female reproductive system consists of internal and external organs. It creates hormones and is responsible for fertility, menstruation and sexual activity. It consists of body parts that help women or people assigned female at birth (AFAB): - Have sexual intercourse - Reproduce - Menstruate **EXTERNAL** The function of the external genitals is to protect the internal parts from infection and allow sperm to enter the vagina. The "vulva" is the collective name for all the external genitals. The main parts of the vulva or external genitals are: - LABIA MAJORA (LARGE LIPS) Enclose and protect the other external reproductive organs. During puberty, hair growth occurs on the skin of the labia majora, which also contain sweat and oil-secreting glands. - LABIA MINORA (SMALL LIPS) Can have a variety of sizes and shapes. They lie just inside the labia majora, and surround the opening to the vagina (the canal that joins the lower part of the uterus to the outside of the body) and urethra (the tube that carries pee from the bladder to the outside of the body). This skin is very delicate and can become easily irritated and swollen. - CLITORIS The two labia minora meet at the clitoris, a small, sensitive protrusion that's comparable to a penis in men or people assigned male at birth (AMAB). The clitoris is covered by a fold of skin called the prepuce and is very sensitive to stimulation. - VAGINAL OPENING The vaginal opening allows menstrual blood and babies to exit your body. - HYMEN The hymen is a piece of tissue covering or surrounding part of the vaginal opening. It's formed during development and present during birth. - OPENING TO YOUR URETHTRA The opening to the urethra is the hole you pee from. **INTERNAL** - VAGINA Muscular canal that joins the cervix (the lower part of uterus) to the outside of the body. It can widen to accommodate a baby during delivery and then shrink back to hold something narrow like a tampon. It's lined with mucous membranes that help keep it moist. - CERVIX Lowest part of the uterus. A hole in the middle allows sperm to enter and menstrual blood to exit. The cervix opens (dilates) to allow a baby to come out during a vaginal childbirth. The cervix is what prevents things like tampons from getting lost inside your body. - UTERUS Hollow, pear-shaped organ that holds a fetus during pregnancy. The uterus is divided into two parts: the cervix and the corpus. The corpus is the larger part of your uterus that expands during pregnancy. - OVARIES Small, oval-shaped glands that are located on either side of the uterus. The ovaries produce eggs and hormones. - FALLOPIAN TUBES Narrow tubes that are attached to the upper part of the uterus and serve as pathways for the egg (ovum) to travel from the ovaries to the uterus. Fertilization of an egg by sperm normally occurs in the fallopian tubes. The fertilized egg then moves to the uterus, where it implants into the uterine lining. **BREAST AND REPRODUCTION** Breasts are part of the female and male sexual anatomy. For women and people assigned female at birth (AFAB), breasts are both functional (for breastfeeding or chestfeeding) and sexual (bringing pleasure). For men and people assigned male at birth (AMAB), there isn't a physiological function. However, a man's breasts can still provide sexual pleasure. - The visible parts of your breast anatomy include your nipples and areolas. Most people are born with two breasts. Several kinds of tissue form the breasts of women and people AFAB. Muscles connect your breasts to your ribs. The three different types of breast tissue in women include: - Glandular - Also called lobules, glandular tissue produces milk for lactation. - Connective or fibrous - This tissue holds glandular and fatty breast tissue in place. - Fatty - This tissue fills in the areas between glandular and connective tissue and determines your breast size. **ANATOMY OF THE BREASTS** - There are many different parts to female breast anatomy, including: Adipose tissue - The breast is mainly made up of fatty tissue (adipose tissue). It extends from the collarbone to the armpit and across the ribcage. Lobes - Each breast has between 15 to 20 lobes, or sections. These lobes surround the nipple like spokes on a wheel. Glandular tissue (lobules) - Small sections of tissue found inside lobes have tiny bulblike glands at the end that produce milk. - Milk (mammary) ducts - Small tubes, or ducts, carry milk from glandular tissue (lobules) to the nipples. - Nipples - In the center of the areola. Each nipple has about nine milk ducts, as well as hundreds of nerves. - Areolae - Circular darker-colored area of skin surrounding the nipple. Areolae have glands called Montgomery's glands that secrete a lubricating oil. This oil protects the nipple and skin from chafing during breastfeeding. - Blood vessels - Circulate blood throughout the breasts, chest and body. - Lymph vessels - Transport lymph, a fluid that helps the  body's immune system fight infection. Lymph vessels connect to lymph nodes, which are found under the armpits, in the chest and in other places. - Lymph nodes - Small organs that help fight infection. - Nerves - Nipples have hundreds of nerve endings, which makes them extremely sensitive to touch. **Physiological Functions of the Breast Female** Hormones namely, estrogen, progesterone and prolactin play a key role in breast development and function. The main role of the breasts is to produce human milk. - Estrogen - stretches milk ducts and helps them create side branches to carry more milk. - Prolactin - promotes the production of progesterone and prepares glands for milk production. - Progesterone - increases the number and size of lobules in preparation for breastfeeding. This hormone also enlarges blood vessels and breast cells after ovulation. - Oxytocin - helps release (or eject) breast milk.