BIOL 320 - Lab 8: Lymphatic, Digestive, Respiratory, Urinary, & Reproductive System Histology

Lymphatic system function

• Plays a role in the body’s defense mechanisms and resistance against disease

• Return fluids to blood via lymphatic vessels, lymph, and lymph nodes.

Lymphoid organs and tissues function

House phagocytic cells and lymphocytes that play a role in the body’s defense mechanisms and resistance against disease.

Two major groups of lymphatic structures occur in connective tissues:

1. Encapsulated lymph organs — lymph nodes, thymus gland, and the spleen; each encapsulated organ is separated from the surrounding connective tissue by a fibrous capsule

2. Diffuse lymph organs — do NOT have a defined boundary that separates them from the connective tissue and this type of tissue is found in virtually every body organ

What happens as lymph passes through a lymph node?

Phagocytes remove debris, microbes, and other antigens.

Lymph nodes

• Each lymph node is encapsulated in dense connective tissue.

• They are scattered throughout the lymphatic system with a high concentration in the upper limbs and in the axillary and cervical regions.

• Connected by a vast network of lymphatic vessels.

The spleen

• The largest lymphatic organ in the body and located lateral to the stomach

• A capsule surrounds the spleen and protects the underlying tissue of red and white pulp

Red vs. White pulp

The color of red pulp is due to the blood that filters through; as blood flows through the red pulp, free and fixed phagocytes in the pulp remove abnormal red blood cells and other antigens from the blood. Blood drains from the sinuses of the red pulp to eventually empty into the splenic vein.

White pulp appears blue due to the lymphocyte nuclei stains; upon exposure to the antigens, the lymphocytes of the white pulp become sensitized to them and produce antiboitics to counteract them.

Lymph follicle

Each lymph follicle within each lymph node is an active center for B cells mitosis.

Dendritic cells, phagocytic cells that eventually make their way to the stratum spinosum of the epidermis, are also closely associated with these follicles.

Within the follicles are germinal centers, which house proliferating B cells.

Dendritic cells

Capture antigens and bring them back to the lymph nodes.

Hassall’s Corpuscles

A diagnostic feature of the thymus and are sites that accumulate dead T cells, but are also involved in the development of regulatory T cells that help prevent autoimmune diseases

Hodgkin’s Granuloma

A cancer of the lymphatic system.

• Untreated cancerous overgrowth can lead to the breakdown of both the splenic red and white pulp

• Reed-Sternberg cells are characteristic of Hodgkin’s disease and are derived from B lymphocytes (considered crippled germinal center B cells), meaning they have not undergone hypermutation to express their antibody

• Seen against a sea of B cells, they give the tissue a moth-eaten appearance

Esophagus

• Scalloped lumen

• Stratified squamous epithelium (provides barrier function)

• Muscularis externa (Two layers)

alternating contractions provide peristaltic movement to swallow food

Gastric pits

• Located in mucosa layer

• Entrance to gastric glands

Gastric glands

• Located beneath the gastric pits, in the mucosa

• Produce and secrete gastric juice

  • Mucus

  • HCL (Gastric acid)

  • Intrinsic factor 

  • Pepsin

DIFFERENTIATING FEATURES OF SMALL INTESTINE AND COLON

The duodenum:

• has Brunner’s glands in its submucosa

The jejunum:

• has prominent villi

• no Brunner’s glands

The ileum:

• peyer’s patch in submucosa

• absorption of B12

The colon:

• lacks villi

Islets of Langerhans

Islets of Langerhans have alpha and beta cells.

Beta cells secrete insulin while alpha cells stimulate the liver to secrete glucose.

Urinary system

• Filters toxins, metabolic wastes, and excess ions from the bloodstream

• Controls extracellular fluid volume in the body

• Kidneys, urinary bladder, paired ureters, and urethra

Urine production

1. Filtration in the glomerulus 

   • Forces fluid and small molecules out of the blood

2. Tubular reabsorption

   • Water and useful molecules filter back into the blood 

3. Tubular secretion

   • Additional solutes from the blood are transported back into the filtrate

Glomerular filtration

• Location: Occurs in the renal corpuscle (glomerulus + Bowman’s capsule).

• Process: Blood enters the glomerulus through the afferent arteriole.
• Due to high blood pressure, water and small solutes (e.g.,glucose, salts, urea) are pushed out of the blood and into the Bowman’s capsule.

• Large molecules (proteins, blood cells) stay in the bloodstream.
• Filtrate: The resulting fluid is called filtrate, similar to plasma but without proteins.

Tubular reabsorption

• Location: Mostly in the proximal convoluted tubule, but also in the loop of Henle, distal tubule, and collecting duct.

• Process: Essential substances like glucose, amino acids, sodium (Na⁺), chloride (Cl⁻), and water are reabsorbed from the filtrate back into the bloodstream via the peritubular capillaries.

• This process is selective and mostly active transport (requires energy).

Tubular secretion

• Location: Primarily in the distal convoluted tubule and collecting duct.

• Process: Additional waste products (e.g., hydrogen ions [H⁺], potassium [K⁺], ammonia, drugs) are actively secreted from the blood into the tubules.

• Helps regulate pH, electrolyte balance, and remove toxins.

Final products: urine

• After reabsorption and secretion, the fluid left in the tubules is urine.

• It flows through the collecting ducts → renal pelvis → ureter → urinary bladder.

• Eventually, it is excreted through the urethra.

Oogenesis

Oogenesis occurs in the ovaries and results in the production of a single mature ovum from a primary oocyte.

Steps of oogenesis

1. Oogonia (2n)

• Diploid stem cells are formed during fetal development.

• Divide by mitosis to increase in number, then begin meiosis I, but get arrested.

2. Primary Oocyte (2n)

• At birth, females have around 1–2 million primary oocytes arrested in prophase I of meiosis I.

• Remain dormant until puberty.

3. Secondary Oocyte (n) + First Polar Body

• At ovulation, meiosis I resumes in one oocyte per cycle.

• Produces a secondary oocyte (haploid) and a polar body (a small, non-functional cell).

• The secondary oocyte begins meiosis II but arrests in metaphase II until fertilization.

4. Ovum (n) + Second Polar Body

• If fertilization occurs, meiosis II completes.

• Produces one large ovum and another polar body.

Outcome:

• One functional ovum is formed per cycle (per month).

• Three polar bodies disintegrate.

Follicular development

Follicular Development (in the female reproductive system) refers to the maturation process of ovarian follicles, which contain the oocyte (immature egg cell) and supporting cells. This process prepares an egg for ovulation and potential fertilization.

1. Primordial Follicle

• Present at birth.

• Contains a primary oocyte arrested in prophase I of meiosis.

• Surrounded by a single layer of flat granulosa cells.

2. Primary Follicle

• Begins to grow at puberty under the influence of follicle-stimulating hormone (FSH).

• Granulosa cells become cuboidal.

• The zona pellucida (a protective glycoprotein layer) forms around the oocyte.

3. Secondary Follicle

• More layers of granulosa cells develop.

• Theca cells appear around the follicle, forming the theca interna and theca externa.

•Theca interna cells produce androgens, which granulosa cells convert to estrogens.

4. Tertiary Follicle (Antral Follicle)

• Formation of a fluid-filled space called the antrum.

• The oocyte becomes surrounded by a layer of granulosa cells called the cumulus oophorus.

•Estrogen production increases significantly.

5. Graafian Follicle (Mature Follicle)

• The follicle reaches full maturity.

• Antrum expands, and the oocyte is now ready for ovulation.

•Surge in luteinizing hormone (LH) triggers ovulation.

6. Ovulation

• The mature oocyte is released from the Graafian follicle.

•It is now called a secondary oocyte and has completed meiosis I, entering metaphase II of meiosis II.

7. Corpus Luteum Formation

• After ovulation, the remaining follicular cells form the corpus luteum.

•The corpus luteum secretes progesterone and estrogen to maintain the uterine lining.

8. Corpus Albicans

• If fertilization doesn’t occur, the corpus luteum degenerates into the corpus albicans (scar tissue).

•Hormone levels drop, leading to menstruation.

Key Hormones Involved in Follicular Development

• FSH: Stimulates follicle growth.

• LH: Triggers ovulation and corpus luteum formation.

• Estrogen: Promotes growth of the endometrial lining and regulates FSH and LH levels.

•Progesterone: Maintains the endometrial lining after ovulation.

Spermatogenesis

Spermatogenesis occurs in the seminiferous tubules of the testes and results in the production of haploid sperm cells from diploid stem cells.

1. Spermatogonia (2n)

• These are diploid stem cells located at the outer edge of the seminiferous tubules.

• They divide by mitosis, producing more spermatogonia and some that become primary spermatocytes.

2. Primary Spermatocytes (2n)

• These enter meiosis I, a special type of cell division to reduce the chromosome number by half.

3. Secondary Spermatocytes (n)

• Produced after meiosis I; each has half the number of chromosomes (haploid).

• These quickly enter meiosis II.

4. Spermatids (n)

• Result from meiosis II; four spermatids are formed from each primary spermatocyte.

• They are immature and non-motile.

5. Spermatozoa (n)

• Spermatids undergo spermiogenesis, a process of maturation and differentiation:

• Formation of flagella (tail)

• Condensation of the nucleus

• Shedding of excess cytoplasm

• Mature spermatozoa are released into the lumen of the seminiferous tubules.