Rectum

The rectum is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. The adult human rectum is about 12 centimetres (4.7 in) long,[2] and begins at the rectosigmoid junction, the end of the sigmoid colon, at the level of the third sacral vertebra or the sacral promontory depending upon what definition is used.[3] Its caliber is similar to that of the sigmoid colon at its commencement, but it is dilated near its termination, forming the rectal ampulla. It terminates at the level of the anorectal ring (the level of the puborectalis sling) or the dentate line, again depending upon which definition is used.[3] In humans, the rectum is followed by the anal canal which is about 4 centimetres (1.6 in) long, before the gastrointestinal tract terminates at the anal verge. The word rectum comes from the Latin rectum intestinum, meaning straight intestine.

Rectum
Retto(anatomia)
The human colon seen from front. The rectum (red) is near the end of the colon.
Rectum anatomy en
Anatomy of the anus and rectum
Details
PrecursorHindgut
Part ofLarge intestine
SystemGastrointestinal system
ArterySuperior rectal artery (first two-thirds of rectum), middle rectal artery (last third of rectum)
VeinSuperior rectal veins, middle rectal veins
NerveInferior anal nerves, inferior mesenteric ganglia[1]
LymphInferior mesenteric lymph nodes, pararectal lymph nodes, internal iliac lymph nodes, Deep inguinal lymph nodes
Identifiers
Latinrectum intestinum
MeSHD012007
TAA05.7.04.001
FMA14544
Anatomical terminology

Structure

The rectum lies in front of the sacrum. It lies behind the bladder in males (left), and the vagina and uterus in females (right).

Gray403
WeiblichesBeckenMedian

The rectum is a part of the lower gastrointestinal tract. The rectum is a continuation of the sigmoid colon, and connects to the anus. The rectum follows the shape of the sacrum and ends in an expanded section called the rectal ampulla, where feces are stored before their release via the anal canal. An ampulla is a cavity, or the dilated end of a duct, shaped like a Roman ampulla.

Unlike other portions of the colon, the rectum does not have distinct taeniae coli.[4]: 397 The taeniae blend with one another in the sigmoid colon five centimeters above the rectum, giving rise to a layer of longitudinal muscle that surrounds the rectum on all sides for its entire length.[5]

The rectum connects with the sigmoid colon at the level of S3, and connects with the anal canal as it passes through the pelvic floor muscles.[4]: 397

Supports of the rectum include:

  • Pelvic floor formed by levator ani muscles.
  • Waldeyer's fascia
  • Lateral ligaments of rectum which are formed by the condensation of pelvic fascia
  • Rectovesical fascia of Denonvillers, which extends from rectum behind to the seminal vesicles and prostate in front.
  • Pelvic peritoneum
  • Perineal body
Gray539

Arteries of the pelvis

Gray1083

Blood vessels of the rectum and anus

Diameters of the large intestine

Inner diameters of different sections of the large intestine, with rectum near rectal/sigmoid junction measuring on average 5.7 cm (range 4.5-7.5 cm).[6]

Microanatomy

Rectum

Cross-section microscopic shot of the rectal wall

Gray1082

Section of mucous membrane of human rectum (60×)

Dogrectum40x3

Dog Rectum cross-section (40×)

Dogrectum400x3

Dog Rectum cross-section (400×)

Function

The rectum acts as a temporary storage site for feces. As the rectal walls expand due to the materials filling it from within, stretch receptors from the nervous system located in the rectal walls stimulate the desire to defecate. If the urge is not acted upon, the material in the rectum is often returned to the colon where more water is absorbed from the feces. If defecation is delayed for a prolonged period, constipation and hardened feces results.

When the rectum becomes full (if the internal and external sphincters are relaxed) the increase in intrarectal pressure forces the walls of the anal canal apart, allowing the fecal matter to enter the canal. The rectum shortens as material is forced into the anal canal. Although peristalsis in the colon delivers material to the rectum, laxatives such as bisacodyl or senna that induce peristalsis in the large bowel do not appear to initiate peristalsis in the rectum. They induce a sensation of rectal fullness and contraction that frequently leads to defecation, but without the distinct waves of activity characteristic of peristalsis.[7] The anal longitudinal muscle also participates in defecation by everting the anus.[8]

Clinical significance

Rectum-2016-12
The inside of a normal human rectum in a 70-year-old, seen during colonoscopy
Rectum-2016-12-hemo
Retroflexed view of the human rectum seen at colonoscopy showing anal verge

Examination

For the diagnosis of certain ailments, a rectal exam may be done. These include faecal impaction, prostatic cancer and benign prostatic hypertrophy in men, faecal incontinence, and internal haemorrhoids.[9]: 179–180

A colonoscopy or sigmoidoscopy are forms of endoscopy that use a guided camera to view the rectum. These may have the ability to take biopsies if needed, and may be used to diagnose diseases such as cancer.

Body temperature can also be taken in the rectum. Rectal temperature can be taken by inserting a medical thermometer not more than 25 mm (1 inch) into the rectum via the anus. A mercury thermometer should be inserted for 3 to 5 minutes; a digital thermometer should remain inserted until it beeps. Normal rectal temperature generally ranges from 36 to 38 °C (96.8 to 100.4 °F) and is about 0.5 °C (1 °F) above oral (mouth) temperature and about 1 °C (2 °F) above axilla (armpit) temperature. In recent years, the introduction of non-invasive temperature taking methods including tympanic (ear) and forehead thermometers, and changing attitudes on privacy and modesty have led some parents and doctors to discontinue taking rectal temperatures.

Route of administration

By their definitions, suppositories are inserted, and enemas are injected, via the rectum. Both of these may be used for the delivery of drugs or to relieve constipation; enemas are also used for a variety of other purposes, medical and otherwise.

Constipation

One cause of constipation is faecal impaction in the rectum, in which a dry, hard stool forms. Manual evacuation is the use of a gloved finger to evacuate faeces from the rectum, and, after the application of stool softeners, is utilised in acute constipation.[10]: 914 It is also in the long-term management of neurogenic bowel, seen most frequently in people with a spinal cord injury or multiple sclerosis. Digital rectal stimulation, the insertion of one finger into the rectum, may be used to induce peristalsis in patients whose own peristaltic reflex is inadequate to fully empty the rectum.

Diseases

Other diseases

Other diseases of the rectum include:

Society and culture

Sexual stimulation

Due to the proximity of the anterior wall of the rectum to the vagina in females or to the prostate in males, and the shared nerves thereof, rectal stimulation or penetration can result in sexual arousal.

History

Etymology

English rectum is derived from the full Latin expression intestinum rectum.[11] The English name straight gut[12] truly expresses the literal meaning of this expression, as Latin rectum means straight,[13] and intestinum means gut.[13] This Latin expression is a translation[14][15] of Ancient Greek ἀπευθυσμένον ἔντερον, derived from ἀπευθύνειν, to make straight,[16] and ἔντερον, gut,[16] attested in the writings of Greek physician Galen.[14][15] During his anatomic investigations on animal corpses, Galen observed the rectum to be straight instead of curved as in humans.[14][15] The expressions ἀπευθυσμένον ἔντερον and intestinum rectum are therefore not appropriate descriptions of the rectum in humans. Apeuthysmenon[17] can be considered as Latinization of ἀπευθυσμένον ἔντερον and euthyenteron[18] has a similar meaning (εὐθύς = straight[16]).

See also

References

  1. ^ Essentials of Human Physiology by Thomas M. Nosek. Section 6/6ch2/s6ch2_30.
  2. ^ "12. Colon and Rectum" (PDF), AJCC Cancer Staging Atlas, American Joint Committee on Cancer, 2006, p. 109
  3. ^ a b al.], senior editors, Bruce G. Wolff ... [et (2007). The ASCRS textbook of colon and rectal surgery. New York: Springer. ISBN 978-0-387-24846-2.
  4. ^ a b Drake, Richard L.; Vogl, Wayne; Tibbitts, Adam W.M. Mitchell; illustrations by Richard; Richardson, Paul (2005). Gray's anatomy for students. Philadelphia: Elsevier/Churchill Livingstone. ISBN 978-0-8089-2306-0.
  5. ^ Sneh Agarwal (January–March 2012). "Anatomy of the Pelvic Floor and Anal Sphincters" (PDF). JIMSA. 25 (1).
  6. ^ Nguyen H, Loustaunau C, Facista A, Ramsey L, Hassounah N, Taylor H, Krouse R, Payne CM, Tsikitis VL, Goldschmid S, Banerjee B, Perini RF, Bernstein C (2010). "Deficient Pms2, ERCC1, Ku86, CcOI in field defects during progression to colon cancer". J Vis Exp (41). doi:10.3791/1931. PMC 3149991. PMID 20689513.
  7. ^ J. D. Hardcastle and C. V. Mann (1968). "Study of large bowel peristalsis" (PDF). Gut. 9: 512–520.
  8. ^ P. J. Lunniss, R. K. S. Phillips (1992). "Anatomy and function of the anal longitudinal muscle". BJS. 79 (9): 882–884. doi:10.1002/bjs.1800790908.
  9. ^ O'Connor, Nicholas J. Talley, Simon (2009). Clinical examination : a systematic guide to physical diagnosis (6th ed.). Chatswood, N.S.W.: Elsevier Australia. ISBN 978-0-7295-3905-0.
  10. ^ Nicki R. Colledge, Brian R. Walker, Stuart H. Ralston, editors (2010). Davidson's principles and practice of medicine. illustrated by Robert Britton (21st ed.). Edinburgh: Churchill Livingstone/Elsevier. ISBN 978-0-7020-3084-0.CS1 maint: Multiple names: authors list (link) CS1 maint: Extra text: authors list (link)
  11. ^ Federative Committee on Anatomical Terminology (FCAT) (1998). Terminologia Anatomica. Stuttgart: Thieme
  12. ^ Schreger, C.H.Th.(1805). Synonymia anatomica. Synonymik der anatomischen Nomenclatur. Fürth: im Bureau für Literatur.
  13. ^ a b Lewis, C. T. & Short, C. (1879). A Latin dictionary founded on Andrews' edition of Freund's Latin dictionary. Oxford: Clarendon Press.
  14. ^ a b c Hyrtl, J. (1880). Onomatologia Anatomica. Geschichte und Kritik der anatomischen Sprache der Gegenwart. Wien: Wilhelm Braumüller. K.K. Hof- und Universitätsbuchhändler.
  15. ^ a b c Triepel, H. (1910). Die anatomischen Namen. Ihre Ableitung und Aussprache. Mit einem Anhang: Biographische Notizen.(Dritte Auflage). Wiesbaden: Verlag J.F. Bergmann.
  16. ^ a b c Liddell, H.G. & Scott, R. (1940). A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with the assistance of. Roderick McKenzie. Oxford: Clarendon Press.
  17. ^ Kossmann, R. (1895). Die gynäcologische Anatomie und ihre zu Basel festgestellte Nomenclatur. Monatsschrift für Geburtshülfe und Gynaekologie, 2 (6), 447-472.
  18. ^ Gabler, E. & Winkler, T.C. (1881). Latijnsch-Hollandsch woordenboek over de geneeskunde en natuurkundige wetenschappen. (2nd edition). Leiden: A.W. Sijthoff.
  • Henry Gray: Anatomy of the human body (Bartleby.com; Great Books Online).
  • Eldra P. Solomon, Richard R. Schmidt, and Peter J. Adragna: Human anatomy & physiology, 2nd ed. 1990 (Sunders College Publishing, Philadelphia). ISBN 0-03-011914-6.

External links

Abdominoperineal resection

An abdominoperineal resection, formally known as abdominoperineal resection of the rectum and abdominoperineal excision of the rectum is a surgery for rectal cancer or anal cancer. It is frequently abbreviated as AP resection, APR and APER.

Anal beads

Anal beads are a sex toy consisting of multiple spheres or balls attached together in series which are continuously inserted through the anus into the rectum and then removed with varying speeds depending on the desired effect (commonly at orgasm to enhance climax). Anal bead users enjoy the pleasurable feeling of the ball passing through the narrow sphincter of the anus.

Anal canal

The anal canal is the terminal part of the large intestine. It is situated between the rectum and anus, below the level of the pelvic diaphragm. In humans it is approximately 2.5 to 4 cm (0.98-1.58 in) long. It lies in the anal triangle of perineum in between the right and left ischioanal fossa.

The anal canal is the short terminal portion of the rectum through which wastes from the large intestine are excreted from the body. The ring at the terminal portion of the anal canal is called the anus.

The anal canal is between 2.5 cm and 5 cm in length and is guarded by two muscles that control the release of waste from the rectum.

The external anal sphincter muscle is the voluntary muscle that surrounds and adheres to the anus at the lower margin of the anal canal. This muscle is in a state of tonic contraction, but during defecation, it relaxes to allow the release of feces.

Movement of the feces is also controlled by the involuntarily controlled internal anal sphincter which an extension of the circular muscle surrounding the anal canal. It relaxes to expel feces from the rectum and anal canal.

Anal canal is divided into three parts. The zona columnaris is the upper half of the canal and is lined by simple columnar epithelium. The lower half of the anal canal, below the pectinate line, is divided into two zones separated by Hilton's white line. The two parts are the zona hemorrhagica and zona cutanea, lined by stratified squamous non-keratinized and stratified squamous keratinized epithelium, respectively.

In humans it is approximately 2.5 to 4 cm long, extending from the anorectal junction to the anus. It is directed downwards and backwards. It is surrounded by inner involuntary and outer voluntary sphincters which keep the lumen closed in the form of an anteroposterior slit.

Behind this lies the anal gland which secretes lymphal discharge and built up fecal matter from the colon lining. In animals,

gland expungement can be done routinely every 24 – 36 months to prevent infection and fistula formation.

It is differentiated from the rectum by the transition of the internal surface from endodermal to skinlike ectodermal tissue.

Anal fissure

An anal fissure is a break or tear in the skin of the anal canal. Anal fissures may be noticed by bright red anal bleeding on toilet paper and undergarments, or sometimes in the toilet. If acute they are painful after defecation, but with chronic fissures, pain intensity is often less. Anal fissures usually extend from the anal opening and are usually located posteriorly in the midline, probably because of the relatively unsupported nature and poor perfusion of the anal wall in that location. Fissure depth may be superficial or sometimes down to the underlying sphincter muscle. Untreated fissures develop a hood like skin tag (sentinel piles) which cover the fissure and cause discomfort and pain.

Anal masturbation

Anal masturbation is an erotic stimulation focusing on the anus and rectum. For humans, common methods of anal masturbation include manual stimulation of the anal opening, and the insertion of an object or objects such as fingers, tongue, phallic-shaped items, water play, or sex toys such as anal beads, butt plugs, dildos, vibrators, or specially designed prostate massagers.

Butt plug

A butt plug is a sex toy that is designed to be inserted into the rectum for sexual pleasure. In some ways, they are similar to a dildo, but they tend to be shorter, and have a flanged end to prevent the device from being lost inside the rectum.

Colorectal cancer

Colorectal cancer (CRC), also known as bowel cancer and colon cancer, is the development of cancer from the colon or rectum (parts of the large intestine). A cancer is the abnormal growth of cells that have the ability to invade or spread to other parts of the body. Signs and symptoms may include blood in the stool, a change in bowel movements, weight loss, and feeling tired all the time.Most colorectal cancers are due to old age and lifestyle factors, with only a small number of cases due to underlying genetic disorders. Other risk factors include diet, obesity, smoking, and lack of physical activity. Dietary factors that increase the risk include red meat, processed meat, and alcohol. Another risk factor is inflammatory bowel disease, which includes Crohn's disease and ulcerative colitis. Some of the inherited genetic disorders that can cause colorectal cancer include familial adenomatous polyposis and hereditary non-polyposis colon cancer; however, these represent less than 5% of cases. It typically starts as a benign tumor, often in the form of a polyp, which over time becomes cancerous.Bowel cancer may be diagnosed by obtaining a sample of the colon during a sigmoidoscopy or colonoscopy. This is then followed by medical imaging to determine if the disease has spread. Screening is effective for preventing and decreasing deaths from colorectal cancer. Screening, by one of a number of methods, is recommended starting from the age of 50 to 75. During colonoscopy, small polyps may be removed if found. If a large polyp or tumor is found, a biopsy may be performed to check if it is cancerous. Aspirin and other non-steroidal anti-inflammatory drugs decrease the risk. Their general use is not recommended for this purpose, however, due to side effects.Treatments used for colorectal cancer may include some combination of surgery, radiation therapy, chemotherapy and targeted therapy. Cancers that are confined within the wall of the colon may be curable with surgery, while cancer that has spread widely are usually not curable, with management being directed towards improving quality of life and symptoms. The five-year survival rate in the United States is around 65%. The individual likelihood of survival depends on how advanced the cancer is, whether or not all the cancer can be removed with surgery and the person's overall health. Globally, colorectal cancer is the third most common type of cancer, making up about 10% of all cases. In 2012, there were 1.4 million new cases and 694,000 deaths from the disease. It is more common in developed countries, where more than 65% of cases are found. It is less common in women than men.

Colorectal surgery

Colorectal surgery is a field in medicine dealing with disorders of the rectum, anus, and colon. The field is also known as proctology, but this term is now used infrequently within medicine and is most often employed to identify practices relating to the anus and rectum in particular. The word proctology is derived from the Greek words πρωκτός proktos, meaning "anus" or "hindparts", and -λογία -logia, meaning "science" or "study".

Physicians specializing in this field of medicine are called colorectal surgeons or proctologists. In the United States, to become colorectal surgeons, surgical doctors have to complete a general surgery residency as well as a colorectal surgery fellowship, upon which they are eligible to be certified in their field of expertise by the American Board of Colon and Rectal Surgery or the American Osteopathic Board of Proctology. In other countries, certification to practice proctology is given to surgeons at the end of a 2–3 year subspecialty residency by the country's board of surgery.

Conic section

In mathematics, a conic section (or simply conic) is a curve obtained as the intersection of the surface of a cone with a plane. The three types of conic section are the hyperbola, the parabola, and the ellipse. The circle is a special case of the ellipse, and is of sufficient interest in its own right that it was sometimes called a fourth type of conic section. The conic sections have been studied by the ancient Greek mathematicians with this work culminating around 200 BC, when Apollonius of Perga undertook a systematic study of their properties.

The conic sections of the Euclidean plane have various distinguishing properties. Many of these have been used as the basis for a definition of the conic sections. One such property defines a non-circular conic to be the set of those points whose distances to some particular point, called a focus, and some particular line, called a directrix, are in a fixed ratio, called the eccentricity. The type of conic is determined by the value of the eccentricity. In analytic geometry, a conic may be defined as a plane algebraic curve of degree 2; that is, as the set of points whose coordinates satisfy a quadratic equation in two variables. This equation may be written in matrix form, and some geometric properties can be studied as algebraic conditions.

In the Euclidean plane, the conic sections appear to be quite different from one another, but share many properties. By extending the geometry to a projective plane (adding a line at infinity) this apparent difference vanishes, and the commonality becomes evident. Further extension, by expanding the real coordinates to admit complex coordinates, provides the means to see this unification algebraically.

Defecation

Defecation is the final act of digestion, by which organisms eliminate solid, semisolid, or liquid waste material from the digestive tract via the anus.

Humans expel feces with a frequency varying from a few times daily to a few times weekly. Waves of muscular contraction (known as peristalsis) in the walls of the colon move fecal matter through the digestive tract towards the rectum. Undigested food may also be expelled this way, in a process called egestion.

Open defecation, the practice of defecating outside without using a toilet of any kind, is still widespread in some developing countries, for example in India.

Fisting

Fisting, handballing, fist-fucking, brachiovaginal, or brachioproctic insertion is a sexual activity that involves inserting a hand into the vagina or rectum. Once insertion is complete, the fingers are either clenched into a fist or kept straight. Fisting may be performed without a partner, but it is most often a partnered activity.

Hematochezia

Hematochezia is the passage of fresh blood through the anus, usually in or with stools (contrast with melena). The term is from Greek αἷμα ("blood") and χέζειν ("to defaecate"). Hematochezia is commonly associated with lower gastrointestinal bleeding, but may also occur from a brisk upper gastrointestinal bleed. The difference between hematochezia and rectorrhagia is that, in the latter, rectal bleeding is not associated with defecation; instead, it is associated with expulsion of fresh bright red blood without stools. The phrase bright red blood per rectum (BRBPR) is associated with hematochezia and rectorrhagia.

Large intestine

The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in vertebrates. Water is absorbed here and the remaining waste material is stored as feces before being removed by defecation.The colon is the largest portion of the large intestine, so many mentions of the large intestine and colon overlap in meaning whenever anatomic precision is not the focus. Most sources define the large intestine as the combination of the cecum, colon, rectum, and anal canal. Some other sources exclude the anal canal.In humans, the large intestine begins in the right iliac region of the pelvis, just at or below the waist, where it is joined to the end of the small intestine at the cecum, via the ileocecal valve. It then continues as the colon ascending the abdomen, across the width of the abdominal cavity as the transverse colon, and then descending to the rectum and its endpoint at the anal canal. Overall, in humans, the large intestine is about 1.5 metres (5 ft) long, which is about one-fifth of the whole length of the gastrointestinal tract.

Lower anterior resection

A lower anterior resection, formally known as anterior resection of the rectum and colon and anterior excision of the rectum or simply anterior resection (less precise), is a common surgery for rectal cancer and occasionally is performed to remove a diseased or ruptured portion of the intestine in cases of diverticulitis. It is commonly abbreviated as LAR.

LARs are for cancer in the proximal (upper) two-thirds of the rectum which lends itself well to resection while leaving the rectal sphincter intact.

Proctitis

Proctitis is an inflammation of the anus and the lining of the rectum, affecting only the last 6 inches of the rectum.

Rectal prolapse

Rectal prolapse is when the rectal walls have prolapsed to a degree where they protrude out the anus and are visible outside the body. However, most researchers agree that there are 3 to 5 different types of rectal prolapse, depending on if the prolapsed section is visible externally, and if the full or only partial thickness of the rectal wall is involved.Rectal prolapse may occur without any symptoms, but depending upon the nature of the prolapse there may be mucous discharge (mucus coming from the anus), rectal bleeding, degrees of fecal incontinence and obstructed defecation symptoms.Rectal prolapse is generally more common in elderly women, although it may occur at any age and in either sex. It is very rarely life-threatening, but the symptoms can be debilitating if left untreated. Most external prolapse cases can be treated successfully, often with a surgical procedure. Internal prolapses are traditionally harder to treat and surgery may not be suitable for many patients.

Rectovaginal fistula

A rectovaginal fistula is a medical condition where there is a fistula or abnormal connection between the rectum and the vagina.Rectovaginal fistula may be extremely debilitating. If the opening between the rectum and vagina is wide it will allow both flatulence and feces to escape through the vagina, leading to fecal incontinence. There is an association with recurrent urinary and vaginal infections. The fistula may also connect the rectum and urethra, which is called recto-urethral fistula. Either conditions can lead to labial fusion. This type of fistula can cause pediatricians to misdiagnose imperforate anus. The severity of the symptoms will depend on the size of fistula. Most often, it appears after about one week or so after delivery.

Suppository

A suppository is a solid dosage form that is inserted into the rectum (rectal suppository), vagina (vaginal suppository), or urethra (urethral suppository), where it dissolves or melts and exerts local or systemic effects. Suppositories are used to deliver both systemically and locally acting medications.

Transverse folds of rectum

The transverse folds of rectum (or Houston's valves) are semi-lunar transverse folds of the rectal wall that protrude into the rectum, not the anal canal as that lies below the rectum. Their use seems to be to support the weight of fecal matter, and prevent its urging toward the anus, which would produce a strong urge to defecate. Although the term rectum means straight, these transverse folds overlap each other during the empty state of the intestine to such an extent that, as Houston remarked, they require considerable maneuvering to conduct an instrument along the canal, as often occurs in sigmoidoscopy and colonoscopy.

These folds are about 12 mm. in width and are composed of the circular muscle coat of the rectum. They are usually three in number; sometimes a fourth is found, and occasionally only two are present.

One is situated near the commencement of the rectum, on the right side.

A second extends inward from the left side of the tube, opposite the middle of the sacrum.

A third, the largest and most constant, projects backward from the forepart of the rectum, opposite the fundus of the urinary bladder.

When a fourth is present, it is situated nearly 2.5 cm above the anus on the left and posterior wall of the tube.Transverse folds were first described by Irish–British anatomist John Houston, curator of the Royal College of Surgeons in Ireland Museum, in 1830. They appear to be peculiar to human physiology: Baur (1863) looked for Houston's valves in a number of mammals, including wolf, bear, rhinoceros, and several Old World primates, but found no evidence. They are formed very early during human development, and may be visible in embryos of as little as 55 mm in length (10 weeks of gestational age.)

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