Testicle

Testicle
Diagram of inner structures of a human testicle (the labelling "seminal vesicle lobules" is incorrect and should be "testicular lobules" instead)
Diagram of the external features and surrounding structures of the testicles of an adult male
Details
Artery	Testicular artery
Vein	Testicular vein, pampiniform plexus
Nerve	Spermatic plexus
Lymph	Lumbar lymph nodes
Identifiers
Latin	testis
MeSH	D013737
TA98	A09.3.01.001
TA2	3576
FMA	7210
Anatomical terminology

Animation of the migration of spermatozoa from their origin as germ cells to their exit from the vas deferens. A) Blood vessels; B) Head of epididymis; C) Efferent ductules; D) Seminiferous tubules; E) Parietal lamina of tunica vaginalis; F) Visceral lamina of tunica vaginalis; G) Cavity of tunica vaginalis; H) Tunica albuginea; I) Lobule of testis; J) Tail of epididymis; K) Body of epididymis; L) Mediastinum testis; M) Vas deferens.

A testicle, also called testis (pl. testes) is the male gonad in all gonochoric animals, including humans, and is homologous to the ovary, which is the female gonad. Its primary functions are the production of sperm and the secretion of androgens, primarily testosterone.

The release of testosterone is regulated by luteinizing hormone (LH) from the anterior pituitary gland. Sperm production is controlled by follicle-stimulating hormone (FSH) from the anterior pituitary gland and by testosterone produced within the gonads.

Structure

Appearance

Male gonad (testes, left) and female gonad (ovaries, right)

Male humans have two testicles of similar size contained within the scrotum, which is an extension of the abdominal wall. Scrotal asymmetry, in which one testicle extends farther down into the scrotum than the other, is common. This is because of the differences in the vasculature's anatomy. For 85% of men, the right testis hangs lower than the left one.

Measurement and volume

The volume of the testicle can be estimated by palpating it and comparing it to ellipsoids (an orchidometer) of known sizes. Another method is to use calipers, a ruler, or an ultrasound image to obtain the three measurements of the x, y, and z axes (length, depth and width). These measurements can then be used to calculate the volume, using the formula for the volume of an ellipsoid:

$Volume={\frac {4}{3}}\cdot \pi \cdot {\frac {length}{2}}\cdot {\frac {width}{2}}\cdot {\frac {depth}{2}}$

$\approx length\cdot width\cdot depth\cdot 0.52$

However, the most accurate calculation of actual testicular volume is gained from the formula:

$\approx length\cdot width\cdot depth\cdot 0.71$

An average adult testicle measures up to 5 cm × 2 cm × 3 cm (2 in × 3⁄4 in × 1+1⁄4 in). The Tanner scale, which is used to assess the maturity of the male genitalia, assigns a maturity stage to the calculated volume ranging from stage I, a volume of less than 1.5 cm³; to stage V, a volume greater than 20 cm³. Normal volume is 15 to 25 cm³; the average is 18 cm³ per testis (range 12–30 cm³).

The number of spermatozoa an adult human male produces is directly proportional to testicular volume, as larger testicles contain more seminiferous tubules and Sertoli cells as a result. As such, men with larger testicles produce on average more sperm cells in each ejaculate, as testicular volume is positively correlated with semen profiles.

Internal structure

Transverse section through the left side of the scrotum and the left testis

Duct system

The testes are covered by a tough fibrous shell called the tunica albuginea. Under the tunica albuginea, the testes contain very fine-coiled tubes called seminiferous tubules. The tubules are lined with a layer of cells (germ cells) that develop from puberty through old age into sperm cells (also known as spermatozoa or male gametes). The developing sperm travel through the seminiferous tubules to the rete testis located in the mediastinum testis, to the efferent ducts, and then to the epididymis where newly created sperm cells mature (spermatogenesis). The sperm move into the vas deferens, and are eventually expelled through the urethra and out of the urethral orifice through muscular contractions.

Primary cell types

Within the seminiferous tubules, the germ cells develop into spermatogonia, spermatocytes, spermatids and spermatozoa through the process of spermatogenesis. The gametes contain DNA for fertilization of an ovum. Sertoli cells are the true epithelium of the seminiferous epithelium, critical for the support of germ cell development into spermatozoa. Sertoli cells secrete inhibin, a hormone that plays a crucial role in regulating spermatogenesis by inhibiting the secretion of FSH from the pituitary gland, thus helping to regulate sperm production. Peritubular myoid cells surround the seminiferous tubules.

Between the seminiferous tubules are interstitial cells called Leydig cells that produce and secrete testosterone and other androgens important for puberty (including secondary sexual characteristics like facial hair),