37. The anatomy, histology and development of the penis.

37. The anatomy, histology and development of the penis.

Anatomy of Penis

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Penis
covered by sup/deep fascia of penis

• b/w the 2 fascia = dorsal cutaneous v
• below = dorsal v in midline, L & R dorsal a lat to it, and most lat is dorsal n

General Info:

• held in back to perineal body (w/ bulbospongiosus m)
• goes through perineal membrane
• lat = fibromuscular tissue = triangular shape
• Function: sexual intercourse, urination

Fascia and Ligaments of Penis:

• Fundiform lig – from linea alba & membranous layer of sup fascia of abdomen –> splits into L& R parts –> encircles body of penis –> blends w/ superficial penile fascia –> scrotum septum
• Suspensory lig of penis – pubis symphysis and arcuate pubic lig –> deep fascia of penis or body of clitoris
  • lies deep to fundiform lig
• Deep fascia of penis (Buck’s fascia) – continuation of deep perineal fascia, cont w/ fascia covering ext oblique m & rectus sheath
• Tunica albuginea – dense fibrous layer that envelopes both corpora cavernosa & corpus spongiosum
  • very dense around corpus cavernosa –> impede venous return & result in extreme rigidity of structures when erectile tissue become engorged w. blood
  • more elastic around spongiosum, therefore not turgid during erection, permist passage of ejaculate
• Tunica vaginalis – double serous membrane, peritoneal sac @ end of process vaginalis
  • covers front and sides of testis and epididymis
  • closed sac derived from ab peritoneum, forming innermost layer of scrotum
  • parietal layer = adjacent to int spermatic fascia
  • visceral layer = adherent to testis & epididymis

Parts:
1. Root
inf ramus of pubis (crus) –> midline of UG diaphragm (bulb) –> penile urethra
located in superficial perineal pouch, b/w perineal membrane sup, and deep perineal fascia inf
Crus of penis = covered by ischiocavernosus m
Bulb of penis = covered by bulbospongiosus m

* More info on these musc/structures will be covered in male perineum

2. Body (shaft)
3 cavernosus bodies:
2 corpus cavernosa
1 corpus spongiosum

has very little muscular fibers in this part
has thin skin, CT, blood & lymph vessles, fascia and the corpora
fill w/ blood during sexual excitement –> erection

Each cavernous body has strong fibrous CT capsule = tunic albuginea

Corpus cavernosum:

• long rod like structures
• from bulk of penis body
• fibromuscular tissue
• Crus of penis lead to corpus cavernosum
• fuse @ midline
• contains the deep a of penis
• tunica albuginea of corpus cavernosa fuse @ midling = form septum penis

Corpus spongiosum:

• b/w & below carvernosa
• tunica albuginea thinner & weaker & blends w. tunic of cavernosum
• carries the urethra
• bulb leads to corpus spongiosum
• NEVER hardens! (otherwise, would depress urethra, no ejaculation)

3. Glans:

• is the head of the penis
• continous w/ foreskin @ coronal sulcus, and via frenulum
• sep from body via corona glandis (sulcus) and location of glands that release the pre-ejaculate
• exit of urethra is on ant tip of glans = vert slit
• extention of corpus spongiosum, and is therefore also soft in erection
• covered by foreskin

Blood supply:

• br of int pudendal a
  • dorsal a – run in space b/w corpora cavernosa, lat to deep dorsal v
  • deep a – peirce crura, run w/in corpora cavernosa
    • supply cavernosus spaces in erectile tissue of corpora cavernosa
    • gives branches called the helicine a
  • a of bulb of penis – supply post corpus spongiosum, bulbourethral gland
• ext pudendal a – supply penile skin
• vein drainage
  • dorsal v of penis in deep fascia –> prostatic venous plexus
  • superficially, –> superficial dorsal v –> superficial ext pudendal v  or lat pudendal v

Lymph Drainage: superficial lymph nodes

Erection:

1. Deep a of penis –> br into helicine a, that run radially & open into cavernae
2. Veins (which drain cavernae) are located in periphery of corpus cavernosum, beside tunica albuginea
3. Helicine a have special smooth m valves = Ebner’s cushions, usually closed & allows minute amount blood in,  drained easily by veins
4. During sexual excitement, Ebner’s cushions open & blood suddenly flow in and fill up cavernae
5. Blood influx compresses veins, so no blood is drained = ERECTION
6. @ end of erection, Ebner’s cushions close, blood flow dec & vein compression release –> cavernae empty

Begins w/ nervous stimulation –> SNS fiber excitation
also involves contraction of bladder sphincter –> so  no urine –> urethra and no semen goes into bladder
Bulbospongiosus m –> propelling force of ejaculation

Histology of Penis

Embryology of Penis

• The genital eminence, an external mound arising between the umbilicus and the tail, is made up of the genital tubercle and the genital swellings.
• The urogenital sinus opens at the base of the genital tubercle, between the genital swellings.
• These structures form identically in male and female embryos up to 7 weeks gestational age.
• At 9 weeks of gestational age, and under the influence of testosterone, the genital tubercle starts to lengthen.
• In addition, the genital swellings (also called the labio-scrotal folds) enlarge and rotate posteriorly.
• As they meet, they begin to fuse from posterior to anterior.
• As the genital tubercle becomes longer, two sets of tissue folds develop on its ventral surface on either side of a developing trough, the urethral groove.
• The more medial endodermal folds will fuse in the ventral midline to form the male urethra.
• The more lateral ectodermal folds will fuse over the developing urethra to form the penile shaft skin and the prepuce.
• As these two layers fuse from posterior to anterior, they leave behind a skin line: the median raphe.

By 13 weeks, the urethra is almost complete. A ring of ectoderm forms just proximal to the developing glans penis. This skin advances over the corona glandis and eventually covers the glans entirely as the prepuce or foreskin.

Development of the male external genitalia is dependent upon dihydrotestosterone which is produced by the testes. As the genital tubercle is elongating and growing to form the penis, the urogenital folds which lie on either side of the urogenital membrane begin to move towards each other forming a groove, this is known as the urethral groove. The urogenital folds fuse together on the ventral side of the developing penis, enclosing what will now become the spongy urethra. If the urogenital folds fail to close, hypospadias results.

The tip of the penis, which is now called the glans, then begins to form a cord of ectoderm which grows toward the spongy urethra. This cord is known as the urethral plate and when it canalizes, the end of the urethra (external urethral orifice) is at the tip of the penis.

The foreskin is formed in the twelfth week of development. A septum of ectoderm moves inward around the edges of the penis and then breaks down, leaving a thin layer of skin surrounding the penis. During this time, the penis is also developing its corpus cavernosa and spongiosa from proliferating mesenchyme within the genital tubercle.

The labioscrotal folds also grow towards each other and fuse during development to form the scrotum.