The Skeletal System

The skeletal system includes all the bones and joints of the body. Each bone makes up of different cells, protein fibers, and minerals. The skeletal system allows the muscles to move and provides attachment points to allow the joints’ motion and movements (1). 

 The bones are the body’s structural framework, and the red bone marrow produces blood cells that fuel the body with its necessary components for life (1). 

 There are 206 bones in the human body.

 There are the axial skeleton and the appendicular skeleton system. The axial skeleton has 80 bones regarding the skull, hyoid, auditory ossicles, ribs, sternum, and vertebral column (2).

 The appendicular skeleton has 126 bones. These bones are in the upper limbs, lower limbs, pelvic girdle, and pectoral shoulder girdle (2).

The skull has 22 bones, of which the mandible, which moves the jaw, and the temporal bone are the main frameworks of the skull (2).

 The mandible helps hold the trachea or the tongue muscles. The auditory ossicles include the malleus, incus, and stapes, and the vertebral column comprises different body areas (2).

The cervical area includes the neck and has seven vertebrae. The thoracic or chest region has 12 vertebrae. The lumbar or lower back has five vertebrae. The sacrum has one vertebra and the coccyx, or tailbone, has one vertebra (2).

 The ribs and sternum provide the breast bone of the skeleton or the thoracic region. Twelve pairs of ribs connect in the thoracic region and link to the sternum, while the pectoral girdle or upper limb houses the bones and arm bones in the axial skeleton (2).

The humerus is part of the upper arm and has a socket for the shoulder and the elbow joint in the lower arm. Thus, there are multiple bones in hand. These regions of the hand are called the metacarpals (2).

 In the lower limb, the femur is the most prominent bone in the body and is known as the thigh region. The femur forms the ball and socket of the hip joint (2).

The tibia and fibula are bones of the lower leg and attach to help with balance. The tarsals are small bones at the end of the foot and the heel (2).

 About one-half of the bone matrix is water, and the other half is collagen, which helps to grow different cells, repair injury, and release stored minerals (2).

 Bones are categorized as long, short, flat, irregular, and sesamoid. Long bones are the body’s prominent bones, while short bones are broad and cubed in shape (2).

Flat bones can be irregular in size and are seen mainly in the cranium and the hip bones. Irregular bones have their location in the vertebrae and the sacrum. Lastly, the sesamoid bones form after birth inside the tendons of the muscles (2).

 Concerning the parts of the bone, the epiphysis is the end of the bone. The middle of the bone is the diaphysis, and the articular cartilage is an absorber when bones glide across each other to facilitate movement. Finally, deep in the periosteum is the compact bone that makes up mineralized portions of the bone (2).

 In the compact bone, there are cells called osteocytes that maintain the integrity of the bone. Reticulocytes are cells in the red bone marrow that is known as a region of spongy bone. These keep bones light and robust and are usually in the medullary cavity in the middle of the diaphysis (2). 

Articulations or joints are between bone cartilage, and synovial joints are small gaps between bones that provide a free range of emotion. Fibrous joints exist very tightly and offer no movement. Fibrous joints also hold heat in bony sockets (2).

This summary concludes the overview of the skeletal system (2).

The Reproductive System                         

Reproduction is vital to the survival of a species. Gonads are known as reproductive organs. Some of these organs include the testes, ovaries, and several other organs. Gametogenesis is the process that is known as the reproduction of cells (1). 

Sperm forms in males and goes through spermatozoa, and females have an ovum contributing to their specific sex organ. Gonads secrete several different hormones, such as androgens, testosterone, dihydrotestosterone or DHT, estrogens, and progesterone (1).

 Gametogenesis

  As gametes develop, they are called germ cells. DNA in each cell has 23 pairs of chromosomes, and each pair is homologous to the other. Mitosis is a process that has 46 chromosomes of dividing cells that replicate from each other (1).

 Meiosis is the second stage of gametogenesis. Meiosis includes a process called recombination, a crossing over and breakage of different genes, cells, and chromosomes that contribute to two homologous chromosomes linking up adjacent to each other (1).

Recombination is a process that creates genetic diversity within the body. After the first meiotic division, secondary spermatocytes in the male and secondary oocytes in the female can transfer into polar bodies or sperm cells (1).

 Summary

 For men, it starts with a primary spermatocyte, which goes into the first meiotic division. Then, genes go through a process of crossing over as they swap genetic material. Finally, the 23 chromosome pairs split into secondary spermatocytes, and a secondary mitotic division presents spermatids that form into sperm cells (1).

 For women, it starts with the primary oocytes, which goes into the first meiotic division; crossing over will take place and allow the 23 chromosome pairs to divide into secondary oocytes. Finally, a secondary mitotic division leads to polar bodies and a zygote containing 46 chromosomes (1).

 Sex determination

A genotype is the complete genetic composition of a person and includes sex determination based on the different sex chromosomes. For example, males possess one X and one Y chromosome, and females have two X chromosomes (1).

The ovum only contributes one X chromosome, and half of the sperm can either produce an X chromosome or a Y chromosome. When the sperm meets the egg, they can either form a male or a female, where one of them contributes either to the X chromosome or the Y chromosome or two X chromosomes (1).

When the male contributes an inactive X chromosome, one of the X chromosomes will form into a nuclear mass called Barr bodies (1).

Several chromosome combinations containing an X or a Y chromosome may include three chromosomes, such as an XXX or an XXY (1).

AnPhys3e-Fig-6-01-0R.png

There may even be chromosome pairs that include X and O. The O is absent of a chromosome. These variations are called karyotypes and cause several different disorders, such as Down syndrome or other variations of sex determination (1).

The sex usually begins to develop around the 7th week. In determining if a male becomes a male, the Y chromosome on the SRY gene is expressed and triggers the Y chromosome development (1).  

The SRY gene is what codes the protein that sets in motion for the formation of testes. As for the female, there is a specific reproductive tract that forms called the Mullerian ducts. These ducts are outside of the urinary system and develop the genitalia of the female (1).

AnPhys3e-Fig-6-01-0R.png

If the fetus secretes testosterone, it will also secrete a protein hormone called anti-mullerian hormone. The anti-mullerian hormone is an SRY protein that induces and causes degeneration of the Mullerian duct system (1).

Testosterone will also cause the Wolffian ducts. These ducts differentiate into the epididymis, vas deferens, ejaculatory duct, and seminal vesicles (1).

DHT is a hormone that produces more testosterone to form the penis and the male’s scrotum. Cryptorchidism is a disorder in which the testes fail to develop (1).

Summary 

For male testes and epididymis to continue developing, the Mullerian hormone degenerates and allows the Wolffian ducts to continue developing around eight weeks. Around this time, the vas deferens, epididymis, and the testes develop (1).

 The ovaries allow the Mullerian duct to be formed and prevent the Wolffian duct’s generation for females. Instead, the uterus, ovary, vagina, and urethra develop around 8 to 9 weeks of the female (1).

Disorder of Sexual Differentiation

 A disorder in which there is androgen insensitivity is a syndrome where the genotype XY in the male is present, but the phenotype or the external genitalia is female (1).

This mutation forms when the androgen receptor cannot bind to testosterone and the influence of the SRY protein lacks adequate secretion. In this disorder, the Wolffian ducts cannot respond to testosterone, so the duct system does not develop properly (1).

 Congenital adrenal hyperplasia is when there is too much androgen in the fetus. This condition leads to increased secretion of ACTH and, ultimately, complications in the XX fetus. In addition, when untreated, the XX can have ambiguous genitalia, and it is difficult to determine whether the baby is a boy or a girl. Thus, the XX fetus essentially is masculinized (1).

AnPhys3e-Fig-6-01-0R.png

Summary

The presence of SRY- gonads initiates Sertoli and Leydig cells. As a result, the cells’ anti-Mullerian hormone, testosterone, and dihydrotestosterone develop as Mullerian ducts regress and Wolffian ducts advance (1).

Wolffian ducts transform into the epididymis, vas deferens, seminal vesicles, and ejaculatory ducts. Later forming the development of the penis, scrotum, and prostate (1).

XX Chromosomes have no SRY gene and have a primordial gonad differentiation into fetal ovaries. They have an absence of AMH and an absence of testosterone (1).

AnPhys3e-Fig-6-01-0R.png

Mullerian ducts form in the uterus, fallopian tubes, and inner vagina. Wolffian duct regression initiates the development of the outer vagina, female external genitalia, and other female anatomical structures (1).

Fetal Programming

 Epigenetics is a process that explains the expression of genes. In addition, epigenetics describes how phenotypes are developed and changed in initiating a particular gene and histone modification (1).

Adult phenotypes are expressed in children’s puberty stages and are affected by translating messenger RNA genes into proteins. There is also sexual differentiation in the brain, where certain hormones are released stronger than other hormones for either males or females during puberty (1).

Androgens

Testosterone is part of the steroid hormones and produced by the adrenal cortex. Testosterone converts to dihydrotestosterone by the action of enzyme 5-a-reductase (1).

Estrogens and Progesterone

Ovaries and placenta secrete estrogens. There are three types of estrogens (1).

The ovaries and placenta produce estrone, estriol in pregnant women, and estrogen. These estrogens stem from androgens by the enzyme aromatase (1).

Estrogens are also common in males, from the conversion of androgens to estrogen by the aromatase enzyme (1).

Summary 

• Cholesterol— pregnenolone—17-hydroxypregnenolone–dehydroepiandrosterone–secreted by the adrenal cortex. 
• Pregnenolone–progesterone–17-hydroxyprogesterone–androstenedione–estrone.
• Androstenedione can use aromatase to produce estrogen and testosterone. Then androstenedione is secreted by the testes by 5-a-reductase to form dihydrotestosterone. 

Hypothalamus secretes GnRH in the hypothalamic-pituitary portal vessel and secretes FSH and LH from the anterior pituitary. Finally, it releases the FSH and LH to the gonads, which secrete sex hormones as a form of gametogenesis (1).

AnPhys3e-Fig-6-01-0R.png

Kisspeptin is a neuron peptide that is involved in the activation of the GnRH (1).

FSH- follicle-stimulating hormone

LH- Luteinizing hormone

FSH and LH are hormones that help the maturation of sperm or ova and stimulate sex hormone secretion. 

Inhibin is a hormone that exerts negative feedback on the anterior pituitary gland. 

Summary

1. Fetal life to infancy: GnRH in males and females secrete at high levels.
2. Childhood to puberty: GnRH is low and reproductive function is quiet. 
3. Puberty to adulthood: GnRH increases and shows variations in women during the menstrual cycle and fertility. 
4. Aging: Reproductive diminishes, and there is less response to the gonadotropins. 

AnPhys3e-Fig-6-01-0R.png

Male Reproductive Anatomy and Physiology

Anatomy

Testes of the male are in the scrotum. Spermatogenesis, sperm formation, has seminiferous tubules that converge to ducts and empty into the epididymis. The epididymis connects the testes into the vas deferens, a smooth muscle supplying the testes and bound to the spermatic cord (1).

There are ducts for males called the seminal vesicles behind the bladder and join to form ejaculatory ducts. Ejaculatory ducts enter the prostate gland and join the urethra (1).

The urethra emerges from the prostate gland and enters the penis. Bulbourethral glands are below the prostate and drain into the urethra after leaving the prostate (1).

 Sperm cells are a small percentage of the total volume ejaculated, and semen surrounds the cells. Therefore, semen contributes to nutrients and buffers to protect the sperm and residual acidic urine in the male urethra by increasing sperm motility and prostaglandins (1).

Prostaglandins in semen aid in sperm function and movement in the female reproductive tract. Bulbourethral glands contribute a small volume of lubricating mucoid secretions (1). 

Spermatogenesis

Germ cells are called spermatogonia and divide at puberty. Daughter cells of the first division produce from stem cells in spermatogonia (1). 

Cells continue to divide in mitotic division and differentiation. These cells are primary spermatocytes and undergo the first meiotic division of spermatogenesis (1).

AnPhys3e-Fig-6-01-0R.png

Summary

Spermatogonia divide through mitosis and start with 46 chromosomes and divide into primary spermatocytes. Then these spermatocytes divide into the first meiotic division and form into secondary spermatocytes. Finally, these spermatocytes have a second meiotic division and form into spermatids and spermatozoa. Once in the secondary spermatocytes, there are 23 instead of 46 chromosomes (1).

The final stage of spermatogenesis contains genetic information. Sperm has a head that consists of a nucleus and contains DNA genetic information (1). 

The nucleus’s tip has coverage by the acrosome, a protein-filled vesicle containing several vital enzymes in fertilization. The sperm’s tail is called a flagellum and is a group of filaments that use whip-like movement to propel the sperm at a 1 to 4 nm velocity. Mitochondria from the midpiece of the sperm provide the energy for movement (1).

Sertoli Cells

The seminiferous tubule is the center of each tubule and contains mature sperm cells, called spermatozoa. In addition, the tubular wall has developing germ cells and is called Sertoli cells (1).

 The Sertoli cell extends from the basement membrane and forms an unbroken ring around the seminiferous tubule. The ring of Sertoli cells forms the Sertoli cell barrier (1). 

This barrier prevents chemicals from going into the lumen. Sertoli cells permit stages of spermatogenesis to take place in different compartments (1).

Leydig Cells

Leydig cells are small connective tissue spaces between the tubules. The Leydig cells carry sperm-producing and testosterone-producing functions of the testes (1).

Production of Mature Sperm

Spermatogenesis stimulates local testosterone secretion from Leydig cells and increases the activity of Sertoli cells. Mitotic division differentiates into primary spermatocytes (1).

Primary spermatocytes move through the junctions of Sertoli cells. Sperm formation allows the cytoplasm of the Sertoli cell to contract sperm (1). 

Sertoli cells reach developing germ cells, and the fluid released contains androgen-binding protein. The protein maintains a high concentration of total testosterone in the lumen of the tubule (1).

Sertoli cells influence the germ cell’s environment in response to FSH from the anterior pituitary gland and the local testosterone produced in the Leydig cell, Sertoli cell, and various chemical messengers (1). 

Sertoli Cell Summary

• Provide nourishment for sperm.
• Secrete luminal fluid.
• Respond to testosterone and FSH.
• Secrete protein hormones inhibin that inhibits FSH secretion from the pituitary gland. 
• Secretion of paracrine agents. 
• Phagocytes defective sperm. 
• Secretes AMH, anti-mullerian hormones, and causes female ducts to appear during embryonic life. 

Transport of Sperm

The vas deferens and epididymis store sperm until ejaculation, which is the discharge of semen from the penis (1).

Vasectomy is a surgical procedure and removes a segment of vas deferens as a male contraception tool (1).

Erection

Erection is when blood builds in high pressure in the penis from neural input initiating small arteries of the penis. Complete erection is associated with endothelial cells that release nitric oxide, which relaxes arterial smooth muscle. In addition, different fibers synapse in the lower spinal cord on interneurons and become stimulated by thoughts, emotions, sights, and odors that can induce erection (1).

Erectile dysfunction is the inability to achieve an erection. This occurrence is common in males from ages 40 to 70. The cause can be from multiple sources, including psychological, drugs, old age, and pre-existing conditions like diabetes mellitus and other diseases (1).

cGMP phosphodiesterase type 5 inhibitors are drugs that act as nitric oxides targeting neurons in the penis. Nitric oxide stimulates guanylate cyclase, catalyzes cGMP, and is a second messenger to the transduction pathways leading to the relaxation of arterial smooth muscle (1).

The events terminated by an enzyme-dependent breakdown of cGMP PDE5 block this enzyme’s action to allow more cGMP to exist, which leads to dilation on the penis and maintenance of the erection. In addition, several drugs like Viagra, Levitra, and Cialis help activate these enzymes and release nitric oxide (1).

Ejaculation

Within the penis, the epididymis, vas deferens, ejaculatory ducts, prostate, and seminal vesicles contract. The contraction releases from sympathetic nerve stimulation into the urethra (1). 

During ejaculation, the sphincter at the base of the urinary bladder is closed. As a result, the sperm cannot enter the bladder, nor can urine be expelled. Inhibition of sympathetic nerves allows ejaculation to stimulate the sympathetic nerves in the duct system’s smooth muscles. When the muscular contractions are in rhythm, a psychological change occurs called an orgasm (1).

An increase in heart rate and blood pressure are characteristics of orgasms and include intense pleasure leading to the release of semen. Once ejaculation occurs, there is a latent period where erection limits for a few minutes to a couple of hours (1). 

Control of the Testes Summary

The hypothalamus secretes GnRH, which releases in the hypothalamic-pituitary portal vessel. In the anterior pituitary, FSH and LH are released and target the testes (1). 

There is a stimulation of spermatogenesis for the Sertoli cells and in the Leydig cells. These cells stimulate testosterone, and FSH releases Sertoli cells, and LH releases Leydig cells (1).

The Sertoli cells release inhibin, inhibiting FSH in the anterior pituitary system, and work as a negative feedback loop. Testosterone also inhibits LH secretion in the anterior pituitary system and works as a feedback loop (1).

Testosterone is the hormone that targets other reproductive tracts and organs. Interesting about testosterone is that it can also inhibit GnRH in the hypothalamus (1).

Testosterone Summary

Testosterone initiates the process of spermatogenesis.

Testosterone converts to dihydrotestosterone or DHT. The DHT catalyzes by the enzyme 5-a-reductase which expresses in several androgen target tissues (1).

 Testosterone converts to dihydrotestosterone or estradiol, which has critical pathophysiological implications. Males sometimes lack 5-a-reductase or aromatase in some tissues (1). 

An XY fetus with the 5-a-reductase deficiency will have normal differentiation of male reproductive duct structures but will not usually develop external male genitalia, which requires DHT (1).

DHT stimulates prostate cancer cells. Therefore, these cancer cells receive treatment involving inhibitors of 5-a-reductase to reduce DHT stimulation and release (1).

Male pattern baldness is treated with 5-a-reductase inhibitors because hair follicles express 5-a-reductase, and the locally produced DHT tends to promote hair loss from the scalp (1).

Sex drive or libido increases when there is more testosterone since many male duct glands depend on testosterone for growth and function. Castration or removal of the gonads suppresses testosterone (1). 

AnPhys3e-Fig-6-01-0R.png

Puberty

Puberty is the period during which reproductive organs mature. These usually occur between the ages of 12 to 16 years. The amplitude and pulse frequency of GnRH secretion increases at puberty. Testosterone, as well as other hormones, increases secondary sex characteristics and sex drives (1).

Secondary Sex Characteristics and Growth

Androgens stimulate bone growth, hormone secretions and affect protein synthesis in muscle (1).

Behavior

Androgens are essential for the development of sex drives at puberty. 

Anabolic steroid use is when individuals desire positive effects of muscle mass, and to increase this mass, they take substances in which there is no negative feedback of GnRH, LH, and FSH (1).

This drug results in increased testosterone and spermatogenesis in Sertoli cells. However, the increase will decrease the testicular size and cause low sperm count due to overstimulation (1).

Hypogonadism is a decrease in testosterone release from the testes. Several different physical failures can cause this disease, such as the testes’ failure to supply adequate hormone androgen production (1).

Klinefelter’s syndrome is a disorder in which sexual development alters with an extra X chromosome, so the male birth is XXY. This nondisjunction can have males appear normal before puberty, but both the testes are undeveloped. As a result, Leydig and Sertoli cell function have inadequate distribution, and breast size regarding gynecomastia can increase. In addition, decreased LH and FSH secretion is related to secondary hypogonadism (1).

 Hyperprolactinemia increases prolactin in the blood and can also cause loss of function in LH and FSH. Pituitary gland tumors can also affect prolactin-secreting cells and inhibit LH and FSH in the anterior pituitary (1).

Hypopituitarism is a significant decrease in anterior pituitary gland function. Several causes could be head trauma, infection, or inflammation. The treatment of this condition can be with ACTH or thyroid hormone (1).

Andropause

The testicular function of males decreases around age 40. As a result, libido will also decrease, and sperm becomes less motile. However, many older men continue to be fertile (1). 

Female Reproductive Physiology

Ovulation releases from the ovary during female reproductive physiology. The egg is known as the germ cell (1). 

Menstruation is a cycle in which the uterus is responsible for a reproduction cycle in which the woman will release eggs as differing hormones secrete (1).

AnPhys3e-Fig-6-01-0R.png

Anatomy

The female body’s anatomy includes two fallopian tubes, the uterus, cervix, and female internal genitalia (1).

Fimbriae are fingerlike projections in the ciliated epithelium (1).

The uterus is the hollow, muscular organ between the urinary bladder and rectum (1).

The cervix is a small opening in the cervix leading to the vagina (1).

The female external genitalia includes the mons pubis, labia majora, labia minora, clitoris, and vestibule of the vagina with vestibular glands. The vulva is another name for these structures (1).

The hymen is the vaginal opening of the female. The clitoris resembles the female version of the penis and locates at the top of the vulva (1).

Oogenesis is the process of fetal development of oogonia. Oogonia resemble spermatogonia, and the process starts with fetal life and mitosis differentiation (1).

Oogonia have four oocytes, which will develop into a primary oocyte at birth. The first meiotic division from inception through child and puberty develops into polar bodies after puberty and secondary oocytes. This secondary oocyte will develop in the second polar body, and finally, the second meiotic division will form the ovum (1).

Follicle Growth

Follicles are the structures in which the egg exits. 

Primordial follicles are of one primary oocyte. 

Granulosa cells are cells that surround the primary oocyte. 

Zona pellucida is a material that is thick and surrounds the follicular cells. This area contains glycoproteins, which help with the binding of sperm cells to an egg after ovulation. This layer is also responsible for preventing sperm from binding with the egg after fertilization (1).

Theca cells function together with granulosa cells and estrogen (1). 

Dominant follicles are antral follicles. 

Atresia is the degenerative process by which non-dominant follicles undergo degeneration. Atresia continues through prepubertal life (1).

Cumulus oophorus are cells surrounding the egg and project into the antrum. 

The cumulus secretes the follicle wall, and the oocyte floats free in the antral fluid (1). 

Graafian follicles become large balloon-type cells on the surface of the ovary (1).

Secondary oocytes surround the zona pellucida and granulosa cells on the ovarian surface by antral fluid (1).

Summary 

The oocyte has granulosa cells around the oocyte—the oocyte forms in the primordial follicle and shapes into the primary follicle. The primary follicle contains the nucleus of the oocyte. The primary follicle forms into the preantral follicle. Granulosa cells and zona pellucida are layers that surround the primary follicle (1). 

Early theca cells create the preantral follicle. The preantral follicle forms into the early antral follicle. This follicle contains the antrum, which has the granulosa, and zona pellucida cells (1).

Finally, this forms into a mature follicle. The mature follicle contains a larger antrum, granular cells, and zona pellucida, which protects the oocyte and, finally, the cumulus oophorus and theca cells surrounding the mature follicle (1).

AnPhys3e-Fig-6-01-0R.png

Dizygotic twins are when two or more follicles reach maturity, and more than one egg ovulates. This process leads to twins that are not identical because the eggs carry different sets of genes and receive fertilization by other sperm (1).

Formation of the Corpus Luteum

The corpus luteum secretes estrogen, progesterone, and inhibin. 

There are two phases; the follicular phase and the luteal phase. The follicular phase is the follicle’s maturation, while the luteal phase lasts until the death of the corpus luteum (1). 

AnPhys3e-Fig-6-01-0R.png

Summary of the Corpus Luteum

1. FSH and LH secrete and increase because plasma estrogen is low. 

2. Multiple antral follicles enlarge and secrete estrogen. 

3. Plasma estrogen increases. 

4. One follicle becomes dominant and secretes vast amounts of estrogen. 

5. Plasma estrogen contraction increases markedly. 

6. FSH secretion and plasma FSH concentration decrease.

7. Increasing plasma estrogen exerts positive feedback.

AnPhys3e-Fig-6-01-0R.png

Summary of LH

1. An LH surge triggers.
2. The egg completes its first meiotic division and cytoplasmic maturation while the follicle secretes less estrogen accompanied by some progesterone. 
3. Ovulation occurs.
4. The corpus luteum forms and secretes large amounts of estrogen and progesterone.
5. Plasma estrogen and progesterone increase FSH and LH secretion while plasma concentration decreases.
6.  The corpus luteum begins to degenerate and decrease its hormone secretion.
7. Plasma estrogen and progesterone concentrations decrease.
8. FSH and LH secretions start to increase, and a new cycle begins.

Summary of Major Feedback of Estrogen, Progesterone, and Inhibin

Estrogen causes the anterior pituitary gland to secrete less FSH and LH in response to GnRH and inhibits the hypothalamic neurons. There is also negative feedback of FSH and LH (1).

Inhibin acts on the pituitary gland to inhibit the secretion of FSH. In addition, there is negative feedback inhibition of FSH secretion (1).

Estrogen increases dramatically, causing anterior pituitary gland cells to secrete more LH and FSH in response to GnRH. Then, there is the positive feedback of the LH surge (1).

High plasma concentrations of progesterone in the presence of estrogen secrete GnRH. This presence causes negative feedback of FSH and LH (1).

The sequence of Effects of the LH Surge on Ovarian Function

1. Primary oocytes complete the first meiotic division and undergo cytoplasmic changes that prepare the ovum for implantation. LH effects on the oocyte mediate by messengers related to the granulosa cells. 

2. Antrum seizes blood flow to the follicle. 

3. Granulosa cells release progesterone and decrease the release of estrogen. 

4. Enzymes and prostaglandins increase. 

Functions of Granulosa Cells

1. Nourish oocyte.
2. Secrete chemical messengers that influence the oocyte and theca cells.
3. Secrete antral fluid.
4. The site increases action for estrogen and FSH.
5. Express aromatase, which converts androgen.
6. Secrete inhibin, which inhibits FSH secretion.
7. The location of action for LH induction changes in the oocyte during the formation of the corpus luteum.

The corpus luteum is very important in terms of pregnancy. If pregnancy does not occur, the corpus luteum will degrade. Then, FSH and LH’s secretion will increase, and the cycle will begin again (1).

AnPhys3e-Fig-6-01-0R.png

There are many uterine changes in the menstrual cycle, which usually last one through five days. The proliferative phase is 5 through 10 days and signifies the follicular phase. This phase is also when estrogen is increasing (1).

AnPhys3e-Fig-6-01-0R.png

The luteal phase includes the secretory phase, which goes from 15 days to 28 days. This phase is when there is an increase in both estrogen and progesterone. At this point, the corpus luteum is developing. The last few days, day 28 to the next cycle, are the menstrual phase, also known as the follicular phase. This phase is when there will be a decrease in both estrogen and progesterone (1).

AnPhys3e-Fig-6-01-0R.png

Dysmenorrhea is signified as menstrual cramps and is an overproduction of prostaglandins (1).

Estrogen priming is the presence of estrogen in response to progesterone and is responsible for preparing the endometrium. Premenstrual tension or premenstrual syndrome, PMS, are typical symptoms in which women may have cramping, headaches, anxiety, and irritability during menstruation (1).

Androgens are present in the blood and responsible for stimulating pubic hair, axillary hair, and maintenance of sex drive (1).

 Puberty

 Puberty in females is similar to males; menarche is the first menstruation event, usually occurring around age 12 (1).

Before this, there is an increase in GnRH, which stimulates other hormones. Amenorrhea is the failure to have a menstrual flow. Common causes of secondary amenorrhea can be excessive exercise or even anorexia nervosa (1).

Some of the reasons for anorexia, perhaps causing amenorrhea, is when the brain senses a loss of body fat and decreases GnRH (1).

Precocious puberty defines as a very sudden appearance of secondary sex characteristics, usually with females around the age of six or seven (1).

Several different things can cause these signs, such as possible tumors or infections that inflict the central nervous system to release GnRH. Treatment can often be drugs that decrease LH and FSH release (1).

 Female response to sexual excitement can be a vascular enlargement of the breast, erection of the nipples, contraction of smooth muscle fibers, the clitoris increasing in diameter and length, and during intercourse, the blood flow to the vagina increases (1).

 Orgasm is the same for males, where skeletal muscle activity increases heart rate and blood pressure, and there is a rhythmic contraction of the vagina and uterus. Scientists show that sexual desire is probably more dependent upon androgens instead of estrogen (1).

 Menopause is a cycle around a woman’s age of 48 to 55, where they begin perimenopause. This phase ends when they start to start menopause, which is basically when the menstrual cycle ceases completely (1).

Menopause usually lasts for about 12 months and is essentially leading to ovarian failure. There is a decrease in estrogen and inhibin, which has less negative feedback on the gonadotropin secretions. During this phase, there is a thinning of the vaginal epithelium, and this can cause sexual intercourse to be painful (1).

 There can also be a decrease in bone mass, which can occur from osteoporosis. Hot flashes are also typical of menopause and promote sudden warmth, dilation of the skin arterioles, and sweating. Many of the symptoms associated with menopause have treatment with the administration of estrogen. However, estrogen administration is a debated topic because estrogen administration increases uterine endometrial cancer or breast cancer (1).

 Fertilization and Early Development

 For fertilization to occur, the sperm must enter the female uterus and fertilize an egg. During a woman’s cycle, egg transport involves egg ovulation and extruding on the ovary’s surface. Moreover, the fimbriae’s smooth muscle causes the ovary to allow the egg to move into the fallopian tubes onto the ovarian surface. Thus, if fertilization occurs, it usually does so in the fallopian tube (1).

 We will now go through the process of intercourse, sperm transport, and capacitation (1).

 The act of intercourse is responsible for the fertilization of the egg. Ejaculation occurs when the sperm goes into the vagina to the cervix as the fluid pressure pushes the sperm further inward (1).

Sperm can usually survive up to 122 days within the cervical mucus when released to enter the uterus. Countless sperm discharges because the vagina is in an acidic environment and kills off most sperm (1). 

There are several hundred million sperm that deposit in the vagina in ejaculation, and only about 100 to 200 reach the fallopian tube (1).

The primary reason why there is so much sperm released is that it is a long journey for the sperm to find the egg. Capacitation is when sperm cannot fertilize the egg and has resided in the female track several hours after the secretion (1). 

Sperm moves by the wave-like projections of the tail, whipping it towards the uterus in powerful urges, and the sperm’s plasma membrane alters when it reaches the uterus so that it is capable of fusing with the surface of the egg (1).

 Fertilization

 Fertilization begins with the fusion of a sperm and an egg in the fallopian tube. The zona pellucida has glycoproteins on the surface of the sperm in the sperm head. Many proteins become simultaneously triggered when there is a specific reaction (1).

The sperm binds to the zona pellucida and triggers an acrosome reaction. When the first sperm penetrates the zona pellucida and reaches the egg, it fuses with the egg, and the head of the sperm slowly passes into the egg’s cytosol (1). 

The fertilized egg is now called a zygote. Block to polyspermy is a mechanism that triggers a reaction preventing other sperm from binding. Simultaneously, the egg released has a cortical reaction response where a sequence of events causes the inactivation of sperm and hardening of the entire zona pellucida (1).

Two types of ectopic pregnancies are when the fertilized egg moves backward out of the fallopian tube into the abdominal cavity, where implantation can occur. Also, sometimes miscarriages happen when the egg is in the fallopian tube (1).

 Early Development

 The zygote or fertilized egg remains in the fallopian tube for 3 to 4 days. This process is because estrogen maintains smooth muscle contraction near the fallopian tube and enters the uterus wall (1).

Plasma progesterone will continue to increase in the smooth muscle. Conceptus is a term that derives from the original zygote. The conceptus will undergo several mitotic cell divisions known as cleavage. This division divides the cell from 16 to 32 cells and reaches the uterus in the same size as the original fertilized egg (1).

 Summary

 The sperm moves into the zona pellucida, and one sperm binds the egg plasma membrane. If sperm draws into the egg, it completes the second meiotic division where the nuclei of sperm and egg unite, and the zygote begins (1).

During this process, egg enzymes activate in the egg, release secretory vesicles’ contents, enter the zona pellucida, and block to polyspermy prevents other sperm from coming into the zona pellucida (1).

 When the egg is fertilized and has divided into cleavage, the cells are totipotent or called stem cells. Identical twins can result in this process, and twins will look alike in having the same type of totipotent cells (1).

When the zygote reaches the uterus, the cells are continuing to divide into 100 cells. Then, finally, the conceptus reaches the stage known as a blastocyst (1).

At this point, the cells have lost their totipotent ability and have begun to differentiate. The blastocyst consists of an outer layer of cells called the trophoblast, known as a fluid-filled cavity’s inner cell mass (1).

The inner cell mass will eventually divide into an embryo during the first two months in a fetus. The trophoblast will surround the embryo and fetus throughout the development and be involved in its nutrition (1).

 Implantation

Implantation is when the zygote develops into the blastocyst, anywhere from 14 to 21 days of the typical menstrual cycle. On the 21st day, implantation is the embedding of the blastocyst into the endometrium. Once the blastocyst in the endometrium has contacted, the trophoblast enzymes secrete and allow the blastocyst to bury itself in the endometrial layer (1).

 Placentation

 After the embryo grows for the first few weeks, the placenta will take over the structure. The placenta combines fetal and maternal tissues and serves as the exchange between mother and fetus (1).

 The chorion is an embryonic portion of the placenta and is on the trophoblast cells’ outermost layers. Chorionic villi are single-cell projections of the trophoblast cells. The villi help induce the embryo’s circulatory system and surround enzymes, allowing molecules to secrete from the maternal blood cells (1).

 The umbilical cord is a rope-like structure that connects the fetus to the placenta. Many vessels in the umbilical cord, including the umbilical arteries and the umbilical vein, are responsible for transporting nutrients from the mother to the fetus (1).

 After five weeks, the placenta is well-established and has a heart as well as a pump, and a way of excreting waste products is present. Oxygen and carbon dioxide diffuse into the placenta from the mother (1).

 Amniotic cavity

  There is fluid in the amniotic cavity called the amniotic fluid. The fetus floats in the amniotic cavity and attaches by the umbilical cord to the placenta. Amniocentesis is a process by which a needle inserts into the amniotic cavity, which can determine the sex of the fetus (1).

 Chorionic villus sampling is another technique to determine the sex of the fetus and is usually around 9 to 12 weeks of pregnancy. This process carries a risk of miscarriage, and caution usually associates with this process (1).

Another process to determine the sex of the fetus involves using x-rays or obtaining maternal blood and analyzing it for several days. When there are changes in hormones during the pregnancy, such as human chorionic gonadotropin and estriol, this can cause changes in the placenta. 

 Maternal-Fetal unit

 Maternal nutrition is crucial for the fetus, and if there is malnutrition in pregnancy, this can cause congenital abnormalities. These will exist at birth (1).

B vitamin folate is a vitamin that helps the mother’s nutritional system become sufficient. The mother can take other drugs to help the fetus transport nutrients across the placenta and provide growth and development. Teratogen is an agent that can cause a congenital disability in the fetus, such as medications or alcohol (1).

 Hormonal changes during pregnancy

 There are many changes in hormones during pregnancy. Progesterone is essential because it promotes delivery, and after the first two months of pregnancy, the corpus luteum supplies estrogen and progesterone (1).

The corpus luteum during pregnancy releases a hormone called the human chorionic gonadotropin or HCG. The trophoblast cells secrete this hormone. The detection of this hormone is what is found in the urine to test for pregnancy. This hormone is important because it is responsible for the maternal ovaries to continue to create gonadal steroids and reaches its peak around 60 to 80 days after pregnancy (1).

AnPhys3e-Fig-6-01-0R.png

During pregnancy, the maternal secretion of GnRH, LH, and FSH has limited secretions by high progesterone concentrations in the presence of estrogen. As a result, human placental lactogen mobilizes fats from the adipose tissue and stimulates glucose production in the mother. This hormone will also stimulate breast development and another hormone called relaxin (1).

Relaxin affects the mother’s cardiovascular system and can cause increased arterial compliance as increased blood flow increases in the uterus (1).

 Pregnancy Sickness

 About 10% of pregnant women retain too much fluid and causes edema in the body. In addition, preeclampsia results in low blood perfusion to the placenta (1).

Many women suffer from pregnancy sickness or morning sickness, nausea, and vomiting during the first three months of pregnancy. The cause of this may be an increased sensitivity to aged foods that contain toxic alkaloid compounds, parasites, or infections that could harm the fetus (1).

 Parturition and Lactation

 Parturition is simply the baby’s event and delivery—collagen fibers at the cervix seal smooth muscle cells of the myometrium and disconnect in the uterus. During the last few weeks of pregnancy, the smooth muscle cells form gap junctions between the cells allowing the myometrium to undergo contractions (1). 

Synthesis of enzymes in various messengers and estrogen will induce myometrial receptors from the posterior pituitary and oxytocin, a potent stimulator of uterine smooth muscle contractions (1).

AnPhys3e-Fig-6-01-0R.png

At the onset of labor and delivery, the amniotic sac ruptures and flows through the vagina. Then, downward contractions begin in the upper portion of the uterus(1). 

As contractions continue, the cervix in the vagina starts to breach due to increased abdominal pressure. In addition, the placenta’s umbilical vessels continue to constrict as blood flow to the placenta increases (1).

90% of births are associated with babies’ heads appearing first out of labor to dilate the cervical canal. A breech pregnancy is when the baby orients in multiple directions besides down at the beginning of birth (1).

 A breech presentation may rupture specific organs or require additional surgical delivery of the fetus and placenta and perhaps require a uterine incision to help the baby for safe delivery (1).

 Mechanisms Controlling Parturition

 Smooth muscle cells in the myometrium are responsible for the autonomic contractions that help the muscle stretch from the growing fetus (1).

 Prostaglandins are also responsible for uterine smooth muscle contraction (1).

 Oxytocin is a hormone that directly stimulates uterine smooth muscle and increases prostaglandins around the cervix (1).

 Throughout pregnancy, progesterone is increased and inhibits the sensitivity of the myometrium to estrogen (1).

Once these mechanisms begin, the uterine contractions exert positive feedback upon themselves, increasing hormonal signals in the fetus like ACTH and CRH (1).

 Lactation

 Lactation is the production and secretion of milk by mammary glands located in the breast. After the baby’s birth, milk is produced and secreted by ducts that start in sac-like structures called alveoli. The breast alveoli are milk secretion sites induced by myoepithelial cells. These cells work as contractile cells surrounding the alveoli and secrete milk (1).

During pregnancy, there is a high release of estrogen-progesterone prolactin and human placental lactogen. In addition, the hypothalamus secretes dopamine which inhibits prolactin. Dopamine and prolactin-releasing factor are hormones that reach the anterior pituitary gland and initiate a negative hypophysiotropic control of prolactin’s secretion (1).

Prolactin is a significant hormone that stimulates milk production, and therefore, the body needs to regulate this hormone and prevent milk from being secreted too much. There is also a decrease in estrogen after parturition for many months (1).

There are mediating surges of prolactin that promote lactation, allowing the mother to nurture the baby. Milk initiates and secretes into the lumen of the alveoli. The infant sucks the milk out of the breast after moving into the ducts. Once moved, the milk ejects. This mechanism is called the milk ejection reflex (1). 

Colostrum is a protein-rich fluid that secretes after delivery from the breast. Colostrum and milk antibodies, leukocytes, and other immune system messengers are critical for the baby and help the immune system (1).

 Summary

 Suckling initiates several different events, including nipple mechanoreceptor stimulation. This narrow input to the hypothalamus will increase oxytocin secretion, decrease dopamine secretion, and increase prolactin-releasing factors simultaneously (1).

Oxytocin will continue to increase, and in the breast, there will be a contraction of all cells that will release and eject milk from the breast. During this process, the anterior pituitary increases prolactin and allows milk synthesis to grow in the breast (1).

 It is essential for mothers not to drink alcohol during pregnancy and be careful with certain drugs because breast-milk transmission can transfer either different medicines or viruses or alcohol to the baby and the plasma (1).

 Breastfeeding is typical for most babies from 6 to 12 months. It reduces the severity of gastrointestinal infections and promotes several different health benefits (1). 

 Contraception and Infertility

 Contraception

Sexually transmitted diseases are STDs, such as AIDS, syphilis, gonorrhea, chlamydia, and herpes can transmit during sexual intercourse (1).

Condoms are simple ways that protect from transmitting these sexual diseases (1).

 Several different contraceptives help with birth control or can induce abortions (1).

Some of which are vasectomy, caps, sponges, and spermicides that prevent sperm from reaching the egg (1).

There are also oral contraceptives that decrease the number of different hormones or increase the number of various hormones to trick the body into thinking that it is already pregnant. Thus, the egg has protection from the effects of sperm as the body releases these hormones in high concentrations (1).

For example, progesterone administration protects the cervical mucus and reduces the ability of the sperm to deposit in the cervix. They also inhibit the estrogen effects of the endometrium; therefore, several different oral contraceptives will increase progesterone in the body (1). 

Intrauterine devices or IUDs also work as a form of implantation to prevent the thinning of the endometrial lining and blockage from the sperm reaching the uterus (1).

 Infertility

 6-15% of men and women of reproductive age in the United States are infertile. Failure to conceive can be due to males, 25%, or females, 58% (1).

 There are also unexplained factors, and a number of these issues can happen for females in the fallopian tubes or the uterus. Males that have a condition of hypogonadism can have testicular failure and decrease testosterone (1).

These conditions lead to many couples going to technologies such as in vitro fertilization. This method is when a woman endures injection with drugs that stimulate multiple egg production (1).

The eggs are placed in a dish to receive fertilization from a source of semen. Once the egg fertilizes, the egg can then be transferred inside the woman and initiate a pregnancy (1). 

Summary of Hormones

 During fetal life, there is an anti-Mullerian hormone or an absence of anti-Mullerian hormone based on the sex of the fetus (1).

For males, there is an increase in testosterone. For females, there is an absence of testosterone. Infancy testosterone is still prevalent for males around three-six months, whereas female testosterone is not prevalent (1).

AnPhys3e-Fig-6-01-0R.png

Around puberty, testosterone continues to increase in males, but estrogen and progesterone increase in females. Thus, the effects for males are spermatogenesis, and for females, the menstrual cycle induces (1).

 In adults, testosterone continues to increase, and there is a maintenance of secondary sex characteristics and spermatogenesis in the male. As for female adults, there is still an increase in estrogen or progesterone, which maintains secondary sex characteristics, menstrual cycles, and fertility (1).

 Older men and women have a decrease in the number of different hormones. For example, there is a reduction in testosterone for men around age 55 to 65. In addition, during the age of 48 to 55, women typically go through menopause (1). 

 Menopause means decreased estrogen and progesterone, meaning cessation of menstrual cycles and fertility(1). 

This section concludes the reproductive cycle of men and women and an overview of the skeletal system. 

AnPhys3e-Fig-6-01-0R.png

Sources

Hill, Richard W., et al. Animal Physiology. Oxford University Press, 2018.

Skeletal System – Labeled Diagrams of the Human Skeleton. (n.d.). Retrieved from https://www.innerbody.com/image/skelfov.html#:~:text=The skeletal system includes all the rest of the body.

Vander, Arthur J., et al. Vander’s Human Physiology: The Mechanisms of Body Function. McGraw-Hill Education, 2019.