---

Spermatogenesis, Functions of Sertoli Cells, and Structure of Sperm

---

INTRODUCTION

---

PART 1: SPERMATOGENESIS

Site

• Seminiferous tubules of the testes (constitute ~80% of testicular volume)
• Stimulated by FSH (from anterior pituitary) and testosterone (from Leydig cells)
• Begins at puberty (~13 years) and continues lifelong, decreasing in old age

Duration

• Full cycle takes approximately 64-74 days in humans
• ~128 million sperm are produced daily (2 million spermatogonia begin the process daily; each yields 64 spermatozoa)

---

STAGES OF SPERMATOGENESIS

---

Phase 1: Mitotic Phase (Proliferation of Spermatogonia)

• Spermatogonia are the stem cells lying on the basal lamina of the seminiferous tubules
• They undergo repeated mitotic divisions to maintain the stem cell pool and produce daughter cells committed to differentiation
• Type A spermatogonia - dark (reserve stem cells) and pale (actively dividing)
• Type B spermatogonia - give rise to primary spermatocytes
• These cells remain connected by cytoplasmic bridges throughout differentiation, ensuring synchronized development

---

Phase 2: Meiotic Phase (Reduction Division)

• Type B spermatogonia enlarge and undergo DNA replication to become primary spermatocytes (the largest germ cells in the seminiferous tubule; 46 chromosomes, 4N DNA)
• They cross the blood-testis barrier into the adluminal compartment

• Each primary spermatocyte undergoes Meiosis I (reductive division)
• This produces two secondary spermatocytes (haploid - 23 chromosomes each, 2N DNA)
• Secondary spermatocytes are approximately half the size of primary spermatocytes

• Each secondary spermatocyte undergoes Meiosis II (equatorial division)
• This produces two spermatids per secondary spermatocyte
• Total = 4 haploid spermatids from each primary spermatocyte (23 chromosomes, 1N DNA)

---

Phase 3: Spermiogenesis (Spermatid to Spermatozoon Transformation)

Spermatozoa leaving the testis are not yet motile. They acquire full motility and fertilizing capacity during passage through the epididymis - a process called capacitation (completed in the female reproductive tract).

---

HORMONAL REGULATION OF SPERMATOGENESIS

---

PART 2: FUNCTIONS OF SERTOLI CELLS

---

1. Support and Nutrition of Developing Germ Cells

• Spermatids are isolated from the bloodstream and cannot absorb nutrients directly
• Sertoli cells provide nutrients, metabolic substrates (lactate, pyruvate), and growth factors to developing germ cells throughout their maturation
• Spermatids mature embedded in deep folds of Sertoli cell cytoplasm

---

2. Blood-Testis Barrier (BTB)

• Adjacent Sertoli cells form tight junctions (zonula occludens) with each other
• This creates the blood-testis barrier, dividing the seminiferous epithelium into:
  • Basal compartment - contains spermatogonia and early primary spermatocytes (exposed to blood)
  • Adluminal (luminal) compartment - contains meiotic and post-meiotic cells (protected)
• Functions of BTB:
  • Prevents autoimmune attack against haploid germ cell antigens
  • Maintains the unique ionic environment needed for spermatogenesis
  • Protects developing sperm from bloodborne toxins and drugs

---

3. Secretion of Androgen-Binding Protein (ABP)

• Sertoli cells secrete ABP (stimulated by FSH and testosterone)
• ABP binds and concentrates testosterone within the lumen of the seminiferous tubule
• Maintains high local testosterone concentrations essential for spermatid maturation

---

4. Secretion of Inhibin

• Sertoli cells secrete inhibin (a glycoprotein hormone)
• Inhibin exerts negative feedback on FSH secretion from the anterior pituitary
• This is the key endocrine feedback loop controlling spermatogenesis rate

---

5. Secretion of Tubular Fluid

• Sertoli cells secrete an aqueous fluid into the seminiferous tubule lumen
• This fluid transports sperm from the tubule toward the epididymis
• The fluid has a specific composition: rich in androgens, estrogens, K⁺, inositol, glutamic acid

---

6. Secretion of Mullerian Inhibiting Substance (MIS / AMH)

• Also called Anti-Mullerian Hormone (AMH)
• Secreted during fetal development to cause regression of the Mullerian (paramesonephric) ducts
• This ensures male differentiation of the reproductive tract

---

7. Phagocytosis of Residual Bodies

• During spermiogenesis, excess cytoplasm is shed as residual bodies
• Sertoli cells phagocytose and digest these residual bodies

---

8. Aromatase Activity (Estrogen Production)

• Sertoli cells contain aromatase (CYP19)
• They convert androgens to estrogens
• The rete testis fluid is rich in estrogen, which promotes fluid reabsorption and sperm concentration

---

9. Additional Secretions

• Transferrin - transports iron to developing sperm
• Ceruloplasmin - copper-transporting protein
• Plasminogen activator - converts plasminogen to plasmin (involved in remodeling)
• GDNF (Glial cell line-derived neurotrophic factor) - regulates spermatogonial self-renewal
• SCF (Stem cell factor) - supports germ cell survival

---

PART 3: DIAGRAM OF SPERM (SPERMATOZOON)

---

Labeled Diagram Description (for drawing in exam):

                    ACROSOME (cap)
                  ┌──────────────┐
                  │  NUCLEUS     │  ← HEAD (~4.5 μm)
                  │  (condensed  │
                  │  chromatin)  │
                  └──────┬───────┘
                         │ NECK (connecting piece + proximal centriole)
                  ═══════╪══════════ ← MIDDLE PIECE (~5-7 μm)
                  ║ MITOCHONDRIAL SHEATH ║
                  ║ (helical, around axoneme) ║
                  ═══════╪══════════ ← ANNULUS
                         │
                  ───────┼─────────── PRINCIPAL PIECE (~50 μm)
                  (fibrous sheath + axoneme + outer dense fibers)
                         │
                  ───────┼─────────── END PIECE (~5 μm)
                         ▼           (axoneme only, no sheath)

---

Parts of Sperm and Their Functions:

---

SUMMARY TABLE

---

---

Mechanism of Secretion of Insulin, Functions of Insulin, and Diabetes Mellitus

---

INTRODUCTION

---

PART 1: MECHANISM OF SECRETION OF INSULIN

A. Synthesis of Insulin

1. Preproinsulin - synthesized on ribosomes of the rough endoplasmic reticulum (RER); contains a signal peptide
2. Proinsulin - signal peptide is cleaved in the RER; proinsulin is transported to the Golgi apparatus
3. Insulin + C-peptide - proinsulin is packaged into secretory granules in the Golgi, where enzymatic cleavage removes the connecting (C) peptide, yielding the active two-chain insulin molecule (A-chain + B-chain linked by 2 disulfide bonds) and equimolar quantities of C-peptide
4. The secretory granules are stored in the β cell cytoplasm until release is triggered

Clinical Note: C-peptide has a longer plasma half-life than insulin and is not metabolized by the liver. It is therefore a reliable biomarker for measuring endogenous insulin secretion.

---

B. Stimuli for Insulin Secretion

---

C. Step-by-Step Mechanism of Glucose-Stimulated Insulin Secretion

• Rising blood glucose after a meal is taken up by β cells via GLUT-2 transporter (high Km, insulin-independent, acts as a glucose sensor)

• Glucose is phosphorylated to glucose-6-phosphate by glucokinase (hexokinase IV), which acts as the "glucose sensor" of the β cell
• Glucokinase is not inhibited by its product (no product inhibition), so phosphorylation is proportional to glucose concentration

• Glucose-6-phosphate enters glycolysis and the Krebs cycle
• This leads to a rise in intracellular ATP and an increase in the ATP/ADP ratio

• The elevated ATP closes the ATP-sensitive potassium channels (K⁺-ATP channels / K-ATP channels)
• The K⁺-ATP channel is composed of SUR1 (sulfonylurea receptor 1) and Kir6.2 (inward rectifier K⁺ channel subunit)
• In the fasting state, these channels are open, allowing K⁺ efflux and keeping the membrane hyperpolarized

• Closure of K⁺-ATP channels prevents K⁺ efflux
• This leads to membrane depolarization of the β cell

• Depolarization opens voltage-gated L-type calcium channels
• Ca²⁺ flows into the β cell from the extracellular space

• The increased intracellular Ca²⁺ binds to calmodulin, which activates myosin light-chain kinase
• This causes mobilization of insulin secretory granules toward the cell membrane
• Granules fuse with the plasma membrane → exocytosis of insulin (and equimolar C-peptide + small amount of proinsulin) into the portal circulation

Glucose → GLUT-2 → Glucokinase → Glucose-6-P
→ ↑ATP/ADP ratio → K-ATP channel closes
→ Membrane depolarization → Ca²⁺ channel opens
→ ↑Intracellular Ca²⁺ → Exocytosis of insulin granules

D. Biphasic Pattern of Insulin Secretion

• First phase (rapid, 0-10 min): Release of pre-formed insulin granules already docked at the membrane
• Second phase (sustained, 10-60 min): Newly synthesized insulin granules are mobilized and released; requires ongoing glucose metabolism

In Type 2 DM, the first-phase insulin response is characteristically lost, which is an early defect in β cell function.

---

PART 2: FUNCTIONS OF INSULIN

---

A. Effects on Carbohydrate Metabolism (ANTI-HYPERGLYCEMIC)

Brain, RBCs, renal tubules, intestinal mucosa do NOT require insulin for glucose uptake (use GLUT-1, GLUT-3).

---

B. Effects on Fat (Lipid) Metabolism (ANTI-LIPOLYTIC)

---

C. Effects on Protein Metabolism (ANABOLIC)

---

D. Effects on Potassium Balance

• Insulin activates Na⁺-K⁺-ATPase, promoting K⁺ uptake into cells
• After a meal, insulin ensures that absorbed dietary K⁺ is taken into cells → prevents hyperkalemia
• Insulin deficiency → decreased cellular K⁺ uptake → hyperkalemia

---

E. Growth and Mitogenic Effects

• Promotes DNA synthesis and cell proliferation
• Stimulates growth and differentiation of many cell types
• Works synergistically with IGF-1 (Insulin-like Growth Factor-1)

---

F. Summary Table of Insulin Actions

---

PART 3: NOTE ON DIABETES MELLITUS

Definition

---

Classification of Diabetes Mellitus

---

Type 1 Diabetes Mellitus

• Accounts for 5-10% of all diabetes cases
• Most common in children and young adults (peak onset 10-14 years)
• Pathogenesis: Autoimmune "insulitis" - T-cell mediated destruction of β cells → autoantibodies (anti-GAD, anti-islet cell antibodies, anti-insulin antibodies) can be detected
• Genetic link to HLA class II genes (HLA-DR3, HLA-DR4)
• Absolute insulin deficiency
• Without insulin treatment: Diabetic ketoacidosis (DKA) develops

---

Type 2 Diabetes Mellitus

• Accounts for 90-95% of all diabetes cases
• Usually adults; increasingly seen in obese children/adolescents
• Pathogenesis - two key defects:
  1. Insulin resistance: Peripheral tissues (especially skeletal muscle, adipose) show decreased responsiveness to insulin → GLUT-4 recruitment is impaired
  2. Relative β-cell dysfunction: β cells cannot compensate adequately for the increased insulin demand → progressive decline in insulin secretion
• Associated with obesity (80% of cases), physical inactivity, family history
• First-phase insulin response is lost early
• No autoimmune markers; late pathology shows amyloid deposition in islets
• Rarely develops DKA; may develop hyperosmolar hyperglycemic state (HHS)

---

Diagnostic Criteria for Diabetes Mellitus

---

Clinical Features of Diabetes

• Polyuria - osmotic diuresis due to glycosuria (glucose exceeds renal threshold ~180 mg/dL)
• Polydipsia - compensatory thirst due to water loss
• Polyphagia - cellular starvation despite hyperglycemia; hyperphagia to compensate

---

Complications of Diabetes Mellitus

Acute Complications

Chronic Complications

• Diabetic nephropathy - Kimmelstiel-Wilson nodules in glomeruli; leading cause of chronic kidney disease
• Diabetic retinopathy - non-proliferative → proliferative; leading cause of acquired blindness in adults
• Diabetic neuropathy - peripheral (glove-and-stocking sensory loss), autonomic (gastroparesis, impotence, postural hypotension)

• Coronary artery disease - leading cause of death in diabetics
• Peripheral vascular disease - diabetic foot, gangrene
• Cerebrovascular disease - stroke

---

Mechanism of Vascular Complications

1. Advanced Glycation End-products (AGEs) - glucose non-enzymatically glycates proteins → AGEs cross-link collagen → basement membrane thickening
2. Polyol pathway activation - glucose converted to sorbitol by aldose reductase → osmotic damage to cells (nerves, lens → cataract)
3. Oxidative stress - excess reactive oxygen species damage endothelium
4. PKC activation - protein kinase C promotes vascular permeability and proliferation
5. Hexosamine pathway - impairs insulin signaling

---

Management Overview

---

SUMMARY DIAGRAM (for exam drawing)

MECHANISM OF INSULIN SECRETION (Glucose-Stimulated):

Blood Glucose ↑
      ↓
GLUT-2 transporter (β cell uptake)
      ↓
Glucokinase (glucose sensor)
      ↓
Glucose-6-phosphate → Glycolysis → ↑ ATP/ADP ratio
      ↓
K-ATP channel CLOSES (SUR1 + Kir6.2)
      ↓
Membrane DEPOLARIZATION
      ↓
Voltage-gated Ca²⁺ channels OPEN
      ↓
↑ Intracellular Ca²⁺
      ↓
Insulin granule EXOCYTOSIS
      ↓
Insulin + C-peptide released into portal vein

---