17. The abdominal wall. The inguinal region. The histology of the lymphatic node. Gastrulation, early differentiation of the intraembryonic mesoderm.

17. The abdominal wall. The inguinal region. The histology of the lymphatic node.

Gastrulation, early differentiation of the intraembryonic mesoderm.

Anatomy = Abdominal Wall

The abdomenal cavity starts from the diaphragm, which projects to the top of the 5^th rib on the right side, and to the bottom of the 5^th rib on the left side. It continues down, and end at the pelvic brim, or linea terminalis.

The entire abdominal cavity is formed by:

Sup border = Diaphragm, changes position during inspiration and expiration

Inf border = Pelvic Brim – therefore, note that the area between the two iliac wings is considered part of the abdomen. There is no connective tissue seperation between the abdomen and pelvis.

The wall itself is formed ant/lat by the ribs and costal margin(sup), and by muscles and fascia (inf).

The lat/inf part of the ab wall is formed by the iliac bones.

Posteriorly, the wall goes from T10 – L5, and is formed by vertebral bodies and the back muscles, like Psoas m. and Quadratus lumborum.

The abdomen is divided into 9 different regions, by 4 main lines:

The 4 lines are:

Horizontal:

• Subcostal line – below the 12^th rib (Sometimes, Transpyloric line is used instead, which is a about 1 vertebrae higher)

• Transtubercular line – a line between the most superior points of both iliac crests

Vertical:

• 2 Midclavicular lines – a vertical line running through the mid point of the clavicle on each side

This divides the abdomen into 9 different regions:

• R & L Hypochodrium (Below costal cartilages)

• Epigastrium (Upper middle region)

• R & L Lumbar regions (Where lumbar plexus branches are found)

• Umbilical region (Middle region, also called mesogastrium)

• R & L Inguinal region (Location of inguinal ligament, and canal)

• Hypogastrium (also called supravesicular region)

Structure of Abdominal wall:

Anteriorly:

Boundaries:

Sup = Costal Margin, and xyphoid process of the sternum

Inf = Inguinal ligament, pelvic bones

Muscles and fascia :

Fascia

1. Superficial fascia – same as hypodermis, contains superficial a/v, lymph vessels, cutaneous n.

• superficial layer of superficial fascia (of Camper) – under the epidermis, mostly fat, crosses into thorax as superficial fascia of thorax and goes down and over inguinal ligament, to make up the superficial perineal fascia
• deep layer of superficial fascia (of Scarpa) –  fibrous layer, continues onto thorax as deep layer of superficial fascia of thorax, continues down and over inguinal ligament to merge with fascia lata of the thigh –> becomes Tunica Dartos over the scrotum

2. Deep fascia – (like investing cervical fascia in head and neck)  fascia of abdominal musc, continues inf as deep perineal fascia, and then external spermatic fascia, then becomes the deep penile fascia

3. Transversalis fascia – inner layer of CT around entire ab cavity, and supports peritoneum.

Muscles

1. Ext Oblique m.  – outer layer of ab wall,

• medially, at  midclavicular line, musc fibers turn into aponeurosis, that fuse at midline, @ linea alba
• runs down to fuse with iliac crest, ASIS, pubic tubercle and crest

• Inf Lumbar Triangle – bordered by post border of ext. oblique, iliac crest and lat border of latissumus dorsi. * Hernia can occur here.

2. Internal oblique m – middle layer of ab wall musc,

• fibers run perpendicular to ext oblique superiorly , but parallel to them inf ( in hypogastric and inguinal regions)
• run inf to attach to inf borders of ribs 10,11,12, becomes aponeuorisis medially that fuses at  linea alba (medially)
• Inf attachment = lumbar vert, iliac crest, inguinal ligament, pubic symphysis
• becomes the cremaster m. withn the scrotal sac

3.  Transverse abdominis m. – innermost layer of abdominal wall,  fibers run horizontally

• at midclavicular line ( same location of semilunar line), become an aponeurosis, that fusues midline at the linea alba
• inserts into costal car 5-11, ribs 11,12, fascia of the lumbar and thoraic regions, inf. into pubic crest, iliac crest
• Inguinal falx (aka Conjoined tendon) –  lower edge of transv. ab m.  that fuses with aponeurosis of int oblique m.

4.Rectus abdomins – midline muscle, from cosstal cart 5-7 & xyphoid  proce ss –> pubic crest

• Semilunar line – lat border
• linea alba – medial border
• Tendinous inscriptions – 3 or 4 tendon lines between muscle fibers – this is what gives the muscle the classic “6 pack” appearence, lie between rectus sheath and the top layer of muscle
• 

  Rectus Sheath

  Rectus sheath – is a covering envelope over the rectus abdominis m, created by the aponeurosis of the other three musc.

  • Above arcuate line –rectus m has aponeurosis of ext oblique, and ant half of int oblique aponeurosis ant, and then behind it, is the post half of int oblique aponeurosis, tansverse ab. aponeurosis  and transversalis fascia.
  • Below  arcuate line – all musc aponeurosis run in front of rectus ab m., leaving only transversalis fascia behind it.
  • The idea is that the lower part of the abdomen leaves into the pelvis, which is tilted in a funny way. Because of this tilt, it looks like the pelvic organs might spill out at any second. To hold them back and keep the tension of the ant wall of abdomen, we need more muscles in front to hold it all back.
  • Where the hell is the Arcuate line? About 1/3 of the way between the umbilicus and the pubic crest.  It is also where the inferior epigastric vessels perforates the rectus abdominus, so if you fine these vessels in the lateral umbilical fold, and follow them, you can point out the arcuate line.

Blood Supply of Ab wall

• Sup Epigastric a – from int thoracic a, in rectus sheath – anastomose with inf epigastric
• Inf epigastric a – from ext iliac a, w/in lat umbilical fold, pierces rectus sheath at level of arcuate line, anatomosis with sup epigastric
• This is an IMP Cavo-caval anastomosis –> CLINICAL NOTE:  Coarcation of  aorta – when there is narrowing in the arch of aorta between where the upper limb arterial branches come out and lower limb arterial branches emerge. There is inc BP in the arch, and less blood sent to lower limb from aorta. This anastomosis b/w sup and inf epigastric a, can allow blood from upper body (via subclavian and int thoracic) to go to lower body. Blood will go from sup epigastric –> inf epigastric  –> ext iliac –> other arteries.

• Other arteries
  • Musculophrenic a (also from int thoracic a) – ant ab wall, diaphragm
  • IC a 9-12 (thoracic aorta) – lat ab wall, anatomose with epigastric a
  • Deep circumflex iliac a (from ext iliac a) – inguinal region
  • Superficial epigastric a (from femoral a) – superficial inguinal region, pubic region

• Venous Drainage
  • • Superficially – towards 4 veins: Sup/inf epigastric v, axillary v, femoral v, portal v (via paraumbilical v)
      • Clincal NOTE – If pressure inc in portal v., due to some blockage, blood can flow into these para umbilical v, enlarging them. =  Caput Medusae (looks like Medusa’s head) *One of 4 PORTO-CAVAL ANATOMOSIS
    • Deep v follow arteries listed above

Lymph Drainage

• Above Umbilicus – Superficial –> axillary, parasternal nodes. Deep –> parasternal nodes. Both eventually go to to  thoracic duct, R lymphatic duct
• Below Umbilicus – Superficial –> superficial inguinal nodes. Deep –> ext iliac nodes –>  para-aortic nodes
  

Innervation

• Intercostal n – (from ventral rami of T7-T12) – supply dermatomes, myotomes (See Embryo section below) – skin, musc, parietal peritoneum of ab wall
  • Lat cut br (each as ant/post br), Ant cut br (each has lat/med br)
• Lumbar n – (L1-L2) –> form lumbar plexus, discussed later.

Inguinal Region

*Clincally Important because common location of abdominal hernias – more on that later.

Inguinal Triangle (Hasselbach’s) → Very common site of direct inguinal hernias

Borders:

Med = lat edge of rectus adbominus (semilunar line)

Lat = inf epigastric a/v

Inf = Inguinal ligament

Inguinal Canal

4 cm long tube like space within the lower lateral part of abdominal wall. It is here that the testis, which develop in the abdominal cavity, descend down into the Scrotum. Because the testis go through this region, this stretches and pulls this area of the ab wall, and makes it weaker. Especially at the areas of the entrance and exit to the canal, the inguinal rings. Hernias are when any tissue or organ, most commonly, loops of the intestines, protude through the adbominal wall – a condition that is very painful. Because females have no testis that descend in this region, their ab wall is not as weak as the males, though they do have structures that pass through this region. Therefore, males are more likely to have abdominal hernias than females.

Borders:

Sup = inf border of internal oblique & transverse abdominis m.

Inf = Inguinal ligament (from ASIS → Pubic tubercle, is an extension of the ext oblique aponeurosis)

Ant = Aponeurosis of Ext & Int abdominal oblique m.

Post = Transverse fascia and Conjoint tendon (the two tendons of Int oblique & Transverse abdmoninis m. combined , also called Inguinal falx (listed as such in Netter))

Entrance – Deep Inguinal Ring

• Slit in the transverse fascia

• Located 1cm above and lat to midpoint of inguinal ligament

• Lat to Inf epigastric a/v

Note that the transverse fascia also surrounds and enters into the inguinal canal with the contents, forming the internal fasica of the canal.

Exit – Superficial Inguinal Ring

• Triangular shaped slit above and just lat to pubic tubercle

• is an opening in the ext oblique aponeurosis

• Med and lat borders of opening formed by splits in the aponeurosis called crura

• Med Crus – attaches to pubic crest

• Lat Crus – formed by fibers attached @ pubic tubercle

• Between the 2 crura are intercrural fibers

Contents:

• Male : Spermatic cord, Ilioinguinal n, Lymph vessels

• Female: Round ligament of uterus, Ilioinguinal n, genital br of genitofemoral n, lymph vessels

Clinical Note: Hernia – Direct v. Indirect

• Direct Inguinal Hernia – Protrudes throught the superficial inguinal ring, directly through the abdominal wall

• Indirect Inguinal Hernia – protrudes through the deep inguinal ring, going through the inguinal canal (and pushing on structures within) and emerging through the superficial inguinal ring. More common than direct

• Increased intra-abdominal pressure caused by the tensing of anterior abdominal muscles can help lower the instance of hernias by pushing the post wall up against the ant wall (ext oblique m.), or push the roof down (int oblique and transverse m.) and therfore closing off any extra space through which the intestinal loops can protrude through.

Histology = Lymph Node Slide #23 *H&E

Lymph node is covered by pericapsular adipose tissue. The paracortex is the area between the cortex and medulla, or is considered the part of the cortex that does not contain any lymphatic nodules. It is the primary site for T & B lymphocytes to undergo antigen dependent proliferation and differentiation. They also produce, store, and recirculate lymphocytes. Most prominent in the inguinal and axillary regions.

Structures to Identify:

• Cortex

• CT capsule

• Subcapsular/cortical sinuses

• Germinal center of secondary follicle

• Hilum

• Lymph nodule

• Medulla

• Reticular cells

• CT trabeculae

• Medullary cords & sinuses

• High Endothelial venules

Flow of Lymph around a Lymph Node

Lymph nodes are filters to trap, capture, and kill antigens. Lymph enters via the vas afferent (around the capsule of the lymph node) → Subcapsular sinuses, lined by endothelium and crossed by stromal endothelial cells → Cortical sinuses, lighter stained area lat to each CT trabeculae → Paracortical sinuses → Medullary Sinuses → vas efferent @ Hilum of lymph node → secondary and tertiary lymph nodes → thoracic duct (for ex, or cisterna chyli or R lymphatic duct)

The lymph node has a outer dark stained cortex, and lighter stained internal medulla.

Cortex (Nodular cortex)

• Darker stained, has many of the lymph follicles, seperated by CT trabeculae that invaginate the parenchyme from the capusle.

• Seperated from the capsule by the subcapsular (marginal) sinus

• Between the CT trabeculae and the nodules are cortical sinuses

• B lymphocytes made here within the follicles. – NOTE that all follicles here are secondary

• Secondary lymph follicles have a lighter germinal center.

Paracortex (Deep Cortex)

• also called the Thymus dependent zone, as it primarily has T lymphocytes

• This is the first sighting of medullary cords and sinuses

• location of HEVs & APCs

• HEVs – High endothelial venules, are the site of entry/exit for circulating lymphocytes in/out of the lymph node. Lined by cuboidal or columnar epithelium w/ receptors for antigen primed lymphocytes, that signal lymphocytes to come to the node – a process called “homing”. Lymphocytes leave HEVs via a process called diapedesis

• APCs – antigen presenting cells,

Medulla

• Medullary cords – cords of lymph tissue, have lymphocytes, macrophages, plasma cells

• Medullary sinuses – drain the lymph from the node to the efferent lymph vessels, lined by reticular endothelial cells, discontinuous. Endothelium, less lymphocytes

• Lymph sinuses not open like blood sinuses – macrophage processes and reticular fibers surround the sinus to form a criss-crossing network which stops free flow of lymph and enhances filtration

• Reticular tissue seen best here: have two major components:

• Reticular cells – make and secrete collagen III, ground substance

• Follicular dendritic cells – w/in germinal centers too, digitate b/w B lymphocytes, in Antigen/antibody complexes & attach via Fc rec’rs. NOTE not APCs.

*Embryo of Lymph Node:

Lymph system begins to develop in 5^th week, may arise as sac-like extension from endoth of veins like jugular, iliac, retroperitoneal.

Embryo: Gastrulation, early differentiation of the intraembryonic mesoderm.

Gastrulation – occurs in the 3^rd week of gestation, and establishes the 3 definitive germ layers – ectoderm, mesoderm, and endoderm, forming a trilaminar (3 layer) embryonic disk by day 21. It begins with the formation of the primitive streak, caused by proliferation of epiblast cells.

• Primitive streak consist of the primitive groove, primitive node, and primitive pit.

• Located caudal to primitive streak is the clocal membrane, where epiblast and hypoblast cells are fused → the cloacal membrane seperates the end of the hindgut from the future anus, the cloaca.

• Ectoderm, intraembryonic mesoderm, and endoderm of trilaminar embryonic disk are all derived from epiblast

• Intraembryonic mesoderm– refers the the germ layer that forms during this process, unlike extraembryonic mesoderm which forms during week 2. Many organs and tissues arise from this, and placenta arises from extraembryonic mesoderm. Intraembryonic mesoderm develops into 5 structures:

1. Paraxial mesoderm – thick plate of mesoderm on each side of midline, organized into segments called somitomeres, that later condense into somites.

• Somites 1-7 = pharyngeal arch, rest form about 44 pairs of somites, that condense down into about 35 pairs in the end.

• Each somite divides into 1 sclerotome (bones and cartilages), 1 dermatome (skin segment), and 1 myotome (muscle)

1. Intermediate mesoderm – connects paraxial mesoderm with lateral mesoderm, forms ridge called urogenital ridge → formation of kidneys and gonadal organs

2. Lateral mesoderm – thin plate of mesoderm that the intraembryonic coelom divides into 2 layers: somatic and visceral mesoderm.

3. Notochord – tube of mesoderm in midline of trilaminar disk from primitive node/pit → prechordal plate, causes overlying ectoderm, to differentiate into nueroectoderm & neural plate.

• Cells of notochord plate detach from endoderm to make definitive notochord

• Formation of vertebral bodies, and nucleus palposus of intervertebral disk

5. Cardiogenic region – horseshoe shaped region of mesoderm in cranial end of trilaminar germ disk, forms