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Pancreas The exocrine part (C)ispurely serous, and its secretory units, or acini (C15), contain Gross and Microscopic Anatomy polarized epithelial cells. Draining the secre- tory units are long intercalated ducts (C16) The pancreas (A1) is a wedge-shaped organ, that begin within the acini and form the 13–15cm long, that lies on the posterior first part of the excretory duct system. In at the level of L1–L2. It ex- cross-section the invaginated intercalated tends almost horizontally from the C- ducts appear as centroacinar cells (CD17). shaped to the splenic hilum and The intercalated ducts drain into larger ex- may be divided by its macroscopic features cretory ducts which ultimately unite to form into three parts: the . The fibrous capsule sur- Head of pancreas (B2). The head of the pan- rounding the pancreas sends delicate creas, which lies in the duodenal loop, is the fibrous septa into the interior of the organ, thickest part of the organ. The hook-shaped dividing it into lobules. uncinate process (B3) projects posteriorly and Alimentary System inferiorly from the head of the pancreas sur- Neurovascular Supply and Lymphatic rounding the mesenteric vessels (B4). Be- Drainage tween the head of the pancreas and the un- Arteries. Arterial supply to the head of the cinate process is a groove called the pan- pancreas, like that of the duodenum (see p. creatic notch (B5). 200), is provided by branches of the ǟ Body of pancreas (B6). Most of the body of gastroduodenal artery ( common hepatic the pancreas lies in front of the vertebral artery): the posterior superior pancreati- column. The body has an eminence, near coduodenal artery and the anterior superior the neck, called the omental tuberosity (B7) pancreaticoduodenal artery. Both vessels which extends into the omental bursa (see anastomose with the inferior pancreati- p. 222). coduodenal artery from the superior mesen- teric artery. The body and tail of the pan- Tail of pancreas (B8). The tail of the pan- creas receive their blood supply from the creas extends to the splenorenal of pancreatic branches which are branches of the spleen. the splenic artery. The retroperitoneal pancreas is covered on all Veins. Venous drainage is via short veins sides by connective tissue. The transverse named after the corresponding arteries. mesocolon (B9) passes horizontally along They empty via the splenic vein and superior the anterior surface of its head and body. mesenteric vein into the hepatic portal vein. The anterior surface is divided by the root of the mesocolon into an anterosuperior surface Nerves. Sympathetic fibers to the pancreas (B10), which faces upward, and an antero- arise from the celiac plexus; parasympathetic inferior surface (B11), facing downward. fibers arise from the vagus nerve. The 2mm thick pancreatic duct (B12) runs Regional lymph nodes. Lymph from the along the long axis of the gland near its pos- head of the pancreas drains into the pan- terior surface. It usually opens with the bile creaticoduodenal nodes and from there usu- duct onto the major duodenal papilla (B13). ally to the hepatic nodes. Lymph from the In rare instances, the ducts may open inde- body and tail of the pancreas drains to the pendently into the duodenum. A patent ac- pancreatic nodes lying along the superior and cessory pancreatic duct (B14) is not uncom- inferior borders of the pancreas. The pan- mon. It drains above the main excretory creatic nodes drain into the celiac nodes. duct into the minor duodenal papilla. Function. The exocrine pancreas produces a Microanatomy. The pancreas is a predomi- secretion containing lipase which breaks down fat, amylase which breaks down carbohydrates, nantly exocrine gland. The endocrine part and precursors of protease which breaks down consists of the (see p. 324). protein.

aus: Fritsch, Kuehnel, Internal Organs (ISBN 9783135334059), ᮊ 2008 Georg Thieme Verlag KG Pancreas: Gross Anatomy and Microanatomy 221

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aus: Fritsch, Kuehnel, Internal Organs (ISBN 9783135334059), ᮊ 2008 Georg Thieme Verlag KG 222 Alimentary System: Pancreas

Topography of the Omental Bursa patic portal vein the inferior vena cava can and Pancreas be palpated. The pulse of the left gastric artery (B10) can be palpated in the gastro- Omental Bursa pancreatic fold (A4). The omental bursa is a nearly completely closed containing a capillary Pancreas film that lies behind the stomach (A1) and The pancreas lies on the posterior wall of the lesser omentum and in front of the parietal omental bursa.Itsanterior surface is covered -covered pancreas (A2). The by parietal peritoneum, and its head is sur- (arrow) is the only natural rounded by the duodenum. The pancreas entrance to the omental bursa. The peri- lies in close proximity to the large trunks in toneal relations in and around the omental the upper . Running along its supe- bursa have already been discussed in rior border (B11) is the splenic artery (B12) greater detail (see p. 188). which is accompanied by the splenic vein (B13) passing deep to it. Behind the body of

Alimentary System The omental bursa is visible in its entirety only after it has been freed by one of various the pancreas, the splenic vein receives the surgical routes (dividing the lesser omen- inferior mesenteric vein which unites behind tum, , or transverse me- the head of the pancreas with the superior socolon). mesenteric vein (B14) to form the hepatic portal vein (B9). The superior mesenteric Vestibule of omental bursa. The omental artery (B15), which originates from the foramen leads to the vestibule of the omen- aorta, passes behind the pancreas and de- tal bursa which is bounded anteriorly by the scends along the duodenojejunal flexure lesser omentum and posteriorly by the pa- (B16) before proceeding through the pan- rietal peritoneum. Projecting into the vesti- creatic notch to the uncinate process, over bule is the papillary process of the caudate the superior border of the horizontal part of lobe of the (AB3). To the left of the the duodenum and into the root of the papillary process is the prominent gastro- . pancreatic fold (A4) that divides the vestibule from the main part of the cavity. Additional structures lying posterior to the pancreas are, from right to left: the , Main cavity. The greater part of the omental inferior vena cava, aorta, left adrenal gland, bursa consists of the superior recess of omen- left kidney, and vessels of the left kidney. The tal bursa, extending upward between the tail of the pancreas projects into the splenic esophagus and inferior vena cava; the splenic hilum and thus also has a topographical re- recess of omental bursa (A5), extending to the lationships to the left colic flexure and de- left between the splenic and stom- scending colon (B17). ach; and the inferior recess of omental bursa (A6), extending downward between the Clinical note. Disorders of the pancreas (inflam- stomach and transverse colon. mation, cancer of the pancreatic head) can spread to the adjacent duodenum or cause ob- Omental foramen. The anterior boundary struction of the hepatic, bile, and pancreatic of the omental foramen is formed by the ducts with resultant obstructive jaundice. Pan- , a part of the lesser creatic disease can also cause a backup in the omentum. Lying in the hepatoduodenal hepatic portal vein or inferior vena cava. ligament are the (B7), the bile duct (B8), and the hepatic portal vein Diagnosis of pancreatic disease has been greatly im- (B9). On inserting a finger into the omental proved by the use of modern imaging techniques foramen, the hepatic portal vein, lying fur- such as CT and ultrasonography. thest posteriorly in the hepatoduodenal AB18 Right lobe of liver, AB19 , A20 ligament, can be felt at the anterior bound- , AB21 Left lobe of liver, ary of the omental foramen; behind the he- AB22 Spleen

aus: Fritsch, Kuehnel, Internal Organs (ISBN 9783135334059), ᮊ 2008 Georg Thieme Verlag KG Topography of Omental Bursa and Pancreas 223

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aus: Fritsch, Kuehnel, Internal Organs (ISBN 9783135334059), ᮊ 2008 Georg Thieme Verlag KG 298 Pregnancy: Early Development

Early Development tocyst (BC2). At the same time, the en- dometrium (C78) is prepared for blastocyst Ovulation is the release of the egg cell with implantation by progesterone secreted by its surrounding zona pellucida and corona the corpus luteum. The lining of the radiata (= follicular/granulosa cells) and re- thickens and becomes more vascularized ception by the infundibulum of uterine tube and receptive to implantation, allowing the via the abdominal ostium of uterine tube. blastocyst to burrow into it and receive Fertilization must occur within 6–12hours, nourishment. Implantation (C) (nidation) of after which the egg cell is no longer viable. the blastocyst in the endometrium occurs at Fertilization normally occurs in the ampulla a favorable site (from which it will not be of uterine tube. The zygote is transported to easily moved), usually in the posterior (D9) the uterus within 4 or 5days, propelled by or anterior wall (D10) of the uterine cavity. ciliary action of the tubal epithelial cells, the C7 Functional layer of endometrium, C8 Uterine production (flow) of tubal fluid, and con- epithelium tractions of the muscular wall of the uterine tube. All these actions are regulated by hor- Implantation. Implantation (nidation, day mones. 6–7 after conception) involves a series of phases. In the first phase, apposition, the Zygote development is also regulated by hor- blastocyst comes into contact at its embry- mones. The zygote is nourished by substances onic pole (BC4) (implantation pole) with the found in tubal fluid, including pyruvate, lactate, epithelium of the endometrium. The second and amino acids. phase is adhesion, requiring adhesion mol- Cleavage. As it moves through the uterine ecules which are only available for 24hours tube, the zygote undergoes a series of mi- (the so-called window of implantation). totic divisions termed cleavage. With each Only then can invasion occur: the tropho- cleavage the dividing cells, blastomeres,be- blast of the embryonic pole proliferates and come smaller since they remain encased in forms villi, erodes the uterine epithelium, the inelastic zona pellucida (ABC1) (see p. and invades the endometrium (C6). Tropho- 312). blast cells that come into contact with en- Morula. By around the third day after con- dometrial cells form the syncytiotrophoblast Pregnancy and Human Development ception the zygote reaches the 16-cell stage containing multiple nuclei without identi- at which point it resembles a mulberry and fiable cell boundaries. Nonfused tropho- hence is termed a morula (A). The morula blast cells produce the inner layer known can be divided into a central, inner cell mass as the cytotrophoblast. The cytotrophoblast called the embryoblast (BC4) (embryonic consists of a single layer of cuboidal epithe- disc) and a covering layer called the tropho- lial cells. The previously single-layered tro- blast (BC2) which later gives rise to the fetal phoblast now consists of two layers (see p. portion of the placenta. In the blastomere 312). stage the cells resemble each other. In terms of cytology, they are omnipotent cells and Clinical note. Implantation outside of the are indeterminate; thus as late as the 8-cell uterine cavity resulting in extrauterine preg- nancy (ectopic pregnancy) can occur in the stage, complete separation can produce (D11)orovary(D12), dem- multiple offspring. onstrating that the sperm can travel into the Blastocysts. In subsequent stages of deve- abdominal cavity and fertilize an egg cell there lopment, a fluid-filled cavity arises from the (abdominal pregnancy). Most ectopic preg- nancies are tubal pregnancies (D13) (in the confluence of widened intercellular spaces uterine tube). Implantation of the blastocyst in containing fluid secreted by the blastom- the uterine tube can erode the mother’s ves- eres. The zygote is now referred to as a blas- sels and cause life-threatening hemorrhage. tocyst (B), and the fluid-filled cavity is the Implantation in the isthmus (D14) of the blastocyst cavity (BC3). The cells of the inner uterus results in placenta previa in which the cell mass (embryoblast) now lie on one side, placenta obstructs the birth canal. and the cells of the outer layer (trophoblast) flatten to form the epithelial wall of the blas-

aus: Fritsch, Kuehnel, Internal Organs (ISBN 9783135334059), ᮊ 2008 Georg Thieme Verlag KG Cleavage, Morula, Blastocyst, Implantation 299

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aus: Fritsch, Kuehnel, Internal Organs (ISBN 9783135334059), ᮊ 2008 Georg Thieme Verlag KG 314 Human Development: Prenatal Period

Embryonic Period, cont. space in the abdominal cavity causes the in- testinal loop of the midgut to herniate into Stage 10–12 (Week 4). Somite formation the . continues in stages 10–12: there are 4–12 somites in stage 10, 13–20 somites (AB1)in Stage 21–23 (Week 8). The stages in the last stage 11 (AB), and 21–29 somites in stage 12. week of the embryonic period are charac- In stage 10 the neural folds (AB2) begin to terized by differentiation of the typical close to form the neural tube. The brain human features. The head flexure reduces, develops at the anterior end, and the spinal and the neck is established (DE10). The ex- cord forms at the posterior end. The cranial ternal ear (D11) develops and the eyelids and caudal ends of the neural tube remain (D12) appear. The limbs become longer and open, as the superior neuropore (AB3) and in- the fingers (D13) divide into separate digits. ferior neuropore (AB4). In stage 11 the em- The toes establish and chondral ossification bryo is curved and has a cephalic (B5) and a begins. Sex-specific differences begin to be- caudal folding (B6). The first two pairs of come apparent on the external genitalia. branchial arches (B7) appear, and the optic vesicles are visible. The superior neuropore Fetal Period (Overview) closes. In stage 12 there are three pairs of branchial arches. The inferior neuropore The fetal period is characterized by differen- closes and the otic pit is visible. The tiation and maturation of organ systems as primordial heart is composed of a loop in well as a rapid growth of the fetus. The size which contractile activity begins. The limb of the fetus is measured in centimeters or buds of the upper limbs appear. millimeters as crown-rump length (CRL) (sitting height) or crown-heel length (CHL) Stage 13–15 (Week 5). The embryo be- (standing height). In ultrasound examina- comes markedly curved and has 30 or more tions the biparietal diameter (BPD) of the somites (the exact number is difficult to cranium and the femur length can also be ascertain). In stage 13 four pairs of branchial determined to help more precisely assess arches can be seen; the lens placode has size and age. The fetus weighs about 10g at been established, and the limb buds of the the beginning of week 9 and about 3400g

Pregnancy and Human Development lower limbs appear. In stage 14 the lenses by birth. and nasal pit are visible; the optic cup has formed; limb differentiation continues. In Major changes taking place during the fetal stage 15 the cerebral vesicles are present period are measured in months. A main fea- and the hand plates have developed. ture is the apparent disproportionate growth of the head in relation to the trunk Stage 16–18 (Week 6). Stages 16–18 are and limbs. At the beginning of the fetal pe- characterized by continued differentiation riod the head makes up nearly one-half of of the limbs and development of the foot the length of the body; at the end of the fetal plate (C8) and finger rays (C9). In stage 18 period is makes up only one-fourth. the elbow is visible and the toe rays appear. Ossification of the mesenchymal condensa- tions begins. Facial development includes formation of the auricular hillocks, the na- solacrimal groove, the apex of the nose, the eyelids, and retinal pigmentation. Stage 19–20 (Week 7). The flexure of the embryo decreases, since its trunk is length- ening and straightening and its head is be- coming larger relative to its trunk. The limbs are also becoming longer, growing anteri- orly beyond the primordial heart. Restricted

aus: Fritsch, Kuehnel, Internal Organs (ISBN 9783135334059), ᮊ 2008 Georg Thieme Verlag KG Embryonic and Fetal Periods 315

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aus: Fritsch, Kuehnel, Internal Organs (ISBN 9783135334059), ᮊ 2008 Georg Thieme Verlag KG