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- ACTH Adrenocorticotropic hormone
- cAMP Cyclic adenosine monophosphate
- DHEA Dehydroepiandrosterone
- DHT Dihydrotestosterone
- FSH Follicle-stimulating hormone
- GnRH Gonadotropin-releasing hormone
- hCG Human chorionic gonadotropin
- ICSI Intracytoplasmic sperm injection
- IVF In vitro fertilization
- LH Luteinizing hormone
- mRNA Messenger ribonucleic acid
- PDE5 Type 5 phosphodiesterase
- PRL Prolactin
- SHBG Sex hormone–binding globulin
- TGFβ Transforming growth factor β
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The testes contain two major components that are structurally separate and serve different functions. The Leydig cells, or interstitial cells, comprise the major endocrine component. The primary secretory product of these cells, testosterone, is responsible either directly or indirectly for embryonic differentiation along male lines of the external and internal genitalia, male secondary sexual development at puberty, and maintenance of libido and potency in the adult male. The seminiferous tubules comprise the bulk of the testes and are responsible for the production of approximately 30 million spermatozoa per day during male reproductive life (puberty to death).
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Both of these testicular components are interrelated, and both require an intact hypothalamic-pituitary axis for initiation and maintenance of their function. In addition, several accessory genital structures are required for the functional maturation and transport of spermatozoa. Thus, disorders of the testes, hypothalamus, pituitary, or accessory structures may result in abnormalities of androgen or gamete production, infertility, or a combination of these problems.
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The adult testis is a prolate spheroid with a mean volume of 18.6 ± 4.8 mL. The average length is 4.6 cm (range, 3.6-5.5 cm), and the average width is 2.6 cm (range, 2.1-3.2 cm). The testes are located within the scrotum, which not only serves as a protective envelope but also helps to maintain the testicular temperature approximately 2°C (35.6°F) below abdominal temperature. Three layers of membranes—visceral tunica vaginalis, tunica albuginea, and tunica vasculosa—comprise the testicular capsule. Extensions of the tunica albuginea into the testicle as fibrous septa result in the formation of approximately 250 to 300 pyramidal lobules, each of which contains coiled seminiferous tubules. Within each testis there are over 400 meters of seminiferous tubules, and these structures account for about half of the testicular mass. The approximately 200 million androgen-producing Leydig cells, as well as the blood and lymphatic vessels, nerves, and fibroblasts, are interspersed between the seminiferous tubules.
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