Penis: Anatomy

Gross Anatomy

Overview

The penis is attached to the pubic bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Bones: Structure and Types by the crura penis. This attachment is the root of the penis, and it is located just below the prostate Prostate The prostate is a gland in the male reproductive system. The gland surrounds the bladder neck and a portion of the urethra. The prostate is an exocrine gland that produces a weakly acidic secretion, which accounts for roughly 20% of the seminal fluid. gland. The urethra Urethra A tube that transports urine from the urinary bladder to the outside of the body in both the sexes. It also has a reproductive function in the male by providing a passage for sperm. Urinary Tract: Anatomy enters the penis at the root and travels within the corpus spongiosum on the dorsal aspect of the penis.

Structure

The penis consists of a root, body, and glans.

Root:

• Internal structure, attaches in the perineum Perineum The body region lying between the genital area and the anus on the surface of the trunk, and to the shallow compartment lying deep to this area that is inferior to the pelvic diaphragm. The surface area is between the vulva and the anus in the female, and between the scrotum and the anus in the male. Vagina, Vulva, and Pelvic Floor: Anatomy
• Begins below the bulbourethral gland
• 3 masses of erectile tissue originate from the urogenital triangle Urogenital triangle Vagina, Vulva, and Pelvic Floor: Anatomy:
  • 2 crura (proximal ends of the corpora cavernosa)
  • 1 bulb (proximal end of the corpus spongiosum)

Body:

• Extending from the 2 crura and 1 bulb are 3 erectile columns that run throughout the shaft:
  • Paired corpora cavernosa
  • Single corpus spongiosum
• Erectile tissue is contained within the tunica albuginea (a dense fibroelastic sheath of connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology).
• Bundles of smooth muscle form cavernous sinuses, lined by endothelium Endothelium A layer of epithelium that lines the heart, blood vessels (vascular endothelium), lymph vessels (lymphatic endothelium), and the serous cavities of the body. Arteries: Histology.
  • Sinuses engorge with blood during sexual arousal → erection
  • The oblique angle of the venous outflow channels Channels The Cell: Cell Membrane become constricted due to the blood volume in the cavernous sinuses during arousal, maintaining an erection.
• Urethra Urethra A tube that transports urine from the urinary bladder to the outside of the body in both the sexes. It also has a reproductive function in the male by providing a passage for sperm. Urinary Tract: Anatomy:
  • Contained within the corpus spongiosum
  • Maintains patency for outflow in the form of micturition and ejaculation

Glans:

• Composed of the corpus spongiosum as it enlarges and assumes a bulbous shape at the head of the penis
• Corona separates the glans from the shaft.
• The glans penis is covered by the prepuce (foreskin).

Skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions

• The dartos fascia Fascia Layers of connective tissue of variable thickness. The superficial fascia is found immediately below the skin; the deep fascia invests muscles, nerves, and other organs. Cellulitis and underlying Buck’s fascia Fascia Layers of connective tissue of variable thickness. The superficial fascia is found immediately below the skin; the deep fascia invests muscles, nerves, and other organs. Cellulitis are loosely attached → highly mobile skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions
• General absence of glandular elements and hair follicles
• Retractable prepuce covers the glans penis and is attached at the frenulum.

Microscopic Anatomy

• Corpora cavernosa:
  • Network of cylindrical sinuses
  • Lined by endothelium Endothelium A layer of epithelium that lines the heart, blood vessels (vascular endothelium), lymph vessels (lymphatic endothelium), and the serous cavities of the body. Arteries: Histology
  • Sinuses are separated by smooth muscle.
• Corpus spongiosum:
  • Network of cylindrical sinuses
  • Similar in appearance to corpora cavernosa
• Tunica albuginea has 2 layers of elastic Elastic Connective Tissue: Histology fiber surrounding the corpora:
  • Inner circular layer
  • Outer longitudinal layer

Neurovasculature and Lymphatic Drainage

Arterial supply

• Internal iliac artery → internal pudendal artery → perineal artery and common penile artery
• Common penile artery has 3 branches:
  • Bulbourethral artery: supplies the bulb, urethra Urethra A tube that transports urine from the urinary bladder to the outside of the body in both the sexes. It also has a reproductive function in the male by providing a passage for sperm. Urinary Tract: Anatomy, corpus spongiosum, and glans
  • Dorsal penile artery: supplies glans penis, corpus spongiosum, and urethra Urethra A tube that transports urine from the urinary bladder to the outside of the body in both the sexes. It also has a reproductive function in the male by providing a passage for sperm. Urinary Tract: Anatomy
  • Cavernous artery: travels near the center of each corporal body, giving off projections directly to the cavernous sinuses

Venous supply

Blood leaving the penis is drained by 1 of 3 venous systems:

• Superficial:
  • Within dartos fascia Fascia Layers of connective tissue of variable thickness. The superficial fascia is found immediately below the skin; the deep fascia invests muscles, nerves, and other organs. Cellulitis
  • Drains from skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions and prepuce
• Intermediate:
  • Under Buck’s fascia Fascia Layers of connective tissue of variable thickness. The superficial fascia is found immediately below the skin; the deep fascia invests muscles, nerves, and other organs. Cellulitis
  • Drains from distal penis including glans
• Deep
  • Drains proximal penis

Lymphatic drainage

• Skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions → superficial inguinal nodes
• Glans → deep inguinal and external iliac nodes
• Body → internal iliac nodes

Innervation

• The glans penis is highly innervated with sensory Sensory Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology nerves from the dorsal nerve of the penis, which is a branch of the pudendal nerve Pudendal nerve A nerve which originates in the sacral spinal cord (s2 to s4) and innervates the perineum, the external genitalia, the external anal sphincter and the external urethral sphincter. It has three major branches: the perineal nerve, inferior anal nerves, and the dorsal nerve of penis or clitoris. Gluteal Region: Anatomy.
• The corpora cavernosa receivse sympathetic and parasympathetic innervation from the cavernous nerve, which is a branch of the pelvic plexus.

Erection and Ejaculation

Erection

• In the resting state → helicine arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology remain constricted, limiting blood flow Blood flow Blood flow refers to the movement of a certain volume of blood through the vasculature over a given unit of time (e.g., mL per minute). Vascular Resistance, Flow, and Mean Arterial Pressure to erectile tissues
• Sexual stimulation (visual, tactile, or auditory) → activation of the parasympathetic reflex
• Release of neurotransmitters (notably nitric oxide Nitric Oxide A free radical gas produced endogenously by a variety of mammalian cells, synthesized from arginine by nitric oxide synthase. Nitric oxide is one of the endothelium-dependent relaxing factors released by the vascular endothelium and mediates vasodilation. It also inhibits platelet aggregation, induces disaggregation of aggregated platelets, and inhibits platelet adhesion to the vascular endothelium. Nitric oxide activates cytosolic guanylate cyclase and thus elevates intracellular levels of cyclic gmp. Pulmonary Hypertension Drugs), which act on cGMP cGMP Guanosine cyclic 3. Phosphodiesterase Inhibitors → smooth muscle relaxation 
• Relaxation of helicine arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology allows influx of blood → filling the sinuses of the corpora cavernosa → penile erection
• Filling and dilatation of the corpora cavernosa → compression Compression Blunt Chest Trauma of venous outflow → maintenance of erection
• Parasympathetic reflex also stimulates the bulbourethral glands Bulbourethral Glands Glands situated on each side of the prostate that secrete a fluid component of the seminal fluid into the urethra. → lubrication of the glans

Ejaculation

• Erection-causing stimuli reach threshold Threshold Minimum voltage necessary to generate an action potential (an all-or-none response) Skeletal Muscle Contraction → induce spinal reflex (at the L1 and L2 level) 
• Causes sympathetic stimulation of genital organs:
  • Constriction of the bladder Bladder A musculomembranous sac along the urinary tract. Urine flows from the kidneys into the bladder via the ureters, and is held there until urination. Pyelonephritis and Perinephric Abscess sphincter 
  • Constriction of reproductive ducts and accessory glands → forces contents into the urethra Urethra A tube that transports urine from the urinary bladder to the outside of the body in both the sexes. It also has a reproductive function in the male by providing a passage for sperm. Urinary Tract: Anatomy (also known as emission)
  • Bulbospongiosus muscles contract rapidly and intermittently → pushes semen along the urethra Urethra A tube that transports urine from the urinary bladder to the outside of the body in both the sexes. It also has a reproductive function in the male by providing a passage for sperm. Urinary Tract: Anatomy 
  • Accompanied by the sensation of pleasure, generalized muscle contraction, rapid heartbeat, and elevated blood pressure

Resolution

• Sympathetic stimulus:
  • Internal pudendal artery constriction → decreased blood flow Blood flow Blood flow refers to the movement of a certain volume of blood through the vasculature over a given unit of time (e.g., mL per minute). Vascular Resistance, Flow, and Mean Arterial Pressure into the corpora cavernosa
  • Contraction of muscles around the corpora cavernosa speeds up drainage
• Followed by a refractory period:
  • Erection/ejaculation cannot occur again.
  • Length is determined by age.

Clinical Relevance

• Disorders of sexual development: Sex chromosomes Sex chromosomes The homologous chromosomes that are dissimilar in the heterogametic sex. There are the X chromosome, the Y chromosome, and the w, z chromosomes (in animals in which the female is the heterogametic sex (the silkworm moth bombyx mori, for example)). In such cases the w chromosome is the female-determining and the male is zz. Basic Terms of Genetics have a pivotal role in sex Sex The totality of characteristics of reproductive structure, functions, phenotype, and genotype, differentiating the male from the female organism. Gender Dysphoria determination. Aberrations of sex chromosomes Sex chromosomes The homologous chromosomes that are dissimilar in the heterogametic sex. There are the X chromosome, the Y chromosome, and the w, z chromosomes (in animals in which the female is the heterogametic sex (the silkworm moth bombyx mori, for example)). In such cases the w chromosome is the female-determining and the male is zz. Basic Terms of Genetics in diverse ways at different stages of development of an individual result in disorders of sexual development (DSD). Penile disorders may include hypospadias Hypospadias A birth defect due to malformation of the urethra in which the urethral opening is below its normal location. In the male, the malformed urethra generally opens on the ventral surface of the penis or on the perineum. In the female, the malformed urethral opening is in the vagina. Penile Anomalies and Conditions, epispadias Epispadias A birth defect due to malformation of the urethra in which the urethral opening is above its normal location. In the male, the malformed urethra generally opens on the top or the side of the penis, but the urethra can also be open the entire length of the penis. In the female, the malformed urethral opening is often between the clitoris and the labia, or in the abdomen. Penile Anomalies and Conditions, phimosis Phimosis A condition in which the foreskin cannot be retracted to reveal the glans penis. It is due to tightness or narrowing of the foreskin opening. Penile Anomalies and Conditions, paraphimosis Paraphimosis A condition in which the foreskin, once retracted, cannot return to its original position. If this condition persists, it can lead to painful constriction of glans penis, swelling, and impaired blood flow to the penis. Penile Anomalies and Conditions, micropenis, and a variety of others.
• Erectile dysfunction Erectile Dysfunction Erectile dysfunction (ED) is defined as the inability to achieve or maintain a penile erection, resulting in difficulty to perform penetrative sexual intercourse. Local penile factors and systemic diseases, including diabetes, cardiac disease, and neurological disorders, can cause ED. Erectile Dysfunction: defined as the inability to achieve or maintain a penile erection that would result in the inability to perform sexual intercourse. Systemic, neurologic, and local diseases can cause erectile dysfunction Erectile Dysfunction Erectile dysfunction (ED) is defined as the inability to achieve or maintain a penile erection, resulting in difficulty to perform penetrative sexual intercourse. Local penile factors and systemic diseases, including diabetes, cardiac disease, and neurological disorders, can cause ED. Erectile Dysfunction. Treatment includes lifestyle modifications, vacuum-assisted erection devices, and phosphodiesterase inhibitors Phosphodiesterase inhibitors Phosphodiesterase (PDE) inhibitors are a group of drugs that act by inhibiting PDE enzymes. Phosphodiesterase inhibitors have various mechanisms of action depending on the subtype of PDE targeted, but their main action is increasing the amount of intracellular cAMP or cGMP, which in turn results in physiologic effects such as reducing inflammation, promoting smooth muscle relaxation, and vasodilation. Phosphodiesterase Inhibitors.
• Kallmann syndrome Kallmann syndrome Kallmann syndrome (KS), also called olfacto-genital syndrome, is a genetic condition that causes hypogonadotropic hypogonadism due to decreased secretion of gonadotropin-releasing hormone (GnRH) by the hypothalamus. The lack of sex hormones results in impaired pubertal development. Kallmann Syndrome: a form of reproductive failures due to severe hypogonadism Hypogonadism Hypogonadism is a condition characterized by reduced or no sex hormone production by the testes or ovaries. Hypogonadism can result from primary (hypergonadotropic) or secondary (hypogonadotropic) failure. Symptoms include infertility, increased risk of osteoporosis, erectile dysfunction, decreased libido, and regression (or absence) of secondary sexual characteristics. Hypogonadism that in males results in small testes Testes Gonadal Hormones, micropenis, undescended testes Testes Gonadal Hormones, and a lack of secondary sexual characteristics Secondary Sexual Characteristics Precocious Puberty. Kallmann syndrome Kallmann syndrome Kallmann syndrome (KS), also called olfacto-genital syndrome, is a genetic condition that causes hypogonadotropic hypogonadism due to decreased secretion of gonadotropin-releasing hormone (GnRH) by the hypothalamus. The lack of sex hormones results in impaired pubertal development. Kallmann Syndrome is also associated with complete loss of the sense of smell Smell The sense of smell, or olfaction, begins in a small area on the roof of the nasal cavity, which is covered in specialized mucosa. From there, the olfactory nerve transmits the sensory perception of smell via the olfactory pathway. This pathway is composed of the olfactory cells and bulb, the tractus and striae olfactoriae, and the primary olfactory cortex and amygdala. Olfaction: Anatomy. Hormone replacement therapy Hormone Replacement Therapy Hormone replacement therapy (HRT) is used to treat symptoms associated with female menopause and in combination to suppress ovulation. Risks and side effects include uterine bleeding, predisposition to cancer, breast tenderness, hyperpigmentation, migraine headaches, hypertension, bloating, and mood changes. Noncontraceptive Estrogen and Progestins is helpful for most individuals.
• Penile cancer Penile cancer Malignant lesions of the penis arise from the squamous epithelium of the glans, prepuce, or penile shaft. Penile cancer is rare in the United States, but there is a higher prevalence in lower socioeconomic regions. The most common histologic subtype is squamous cell carcinoma. Penile Cancer: The most common penile cancer Penile cancer Malignant lesions of the penis arise from the squamous epithelium of the glans, prepuce, or penile shaft. Penile cancer is rare in the United States, but there is a higher prevalence in lower socioeconomic regions. The most common histologic subtype is squamous cell carcinoma. Penile Cancer is squamous cell carcinoma Squamous cell carcinoma Cutaneous squamous cell carcinoma (cSCC) is caused by malignant proliferation of atypical keratinocytes. This condition is the 2nd most common skin malignancy and usually affects sun-exposed areas of fair-skinned patients. The cancer presents as a firm, erythematous, keratotic plaque or papule. Squamous Cell Carcinoma (SCC). The cancer type accounts for about 95% of all penile cancer Penile cancer Malignant lesions of the penis arise from the squamous epithelium of the glans, prepuce, or penile shaft. Penile cancer is rare in the United States, but there is a higher prevalence in lower socioeconomic regions. The most common histologic subtype is squamous cell carcinoma. Penile Cancer. The other malignancies that affect the penis include basal cell Basal Cell Erythema Multiforme carcinoma, mesenchymal tumors, malignant melanoma Melanoma Melanoma is a malignant tumor arising from melanocytes, the melanin-producing cells of the epidermis. These tumors are most common in fair-skinned individuals with a history of excessive sun exposure and sunburns. Melanoma, and metastasis Metastasis The transfer of a neoplasm from one organ or part of the body to another remote from the primary site. Grading, Staging, and Metastasis. Age, promiscuous sexual behavior ( HPV HPV Human papillomavirus (HPV) is a nonenveloped, circular, double-stranded DNA virus belonging to the Papillomaviridae family. Humans are the only reservoir, and transmission occurs through close skin-to-skin or sexual contact. Human papillomaviruses infect basal epithelial cells and can affect cell-regulatory proteins to result in cell proliferation. Papillomavirus (HPV) positivity), tobacco use, and poor hygiene are all risk factors for developing penile cancer Penile cancer Malignant lesions of the penis arise from the squamous epithelium of the glans, prepuce, or penile shaft. Penile cancer is rare in the United States, but there is a higher prevalence in lower socioeconomic regions. The most common histologic subtype is squamous cell carcinoma. Penile Cancer.