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AN70.1-AN71.2 | Glands & Lymphoid tissue — Self-Directed Learning

Exocrine Glands: Classification and Histology (AN70.1)

Exocrine glands secrete their products via a duct onto a surface (skin or mucous membrane), as opposed to endocrine glands which secrete hormones directly into the blood.

Figure: Exocrine Glands: Classification and Histology (AN70.1)

Classification by secretory mechanism:
1. Merocrine (eccrine) — secretion by exocytosis; cell membrane intact (most common — salivary glands, pancreas, sweat glands)
2. Apocrine — apical cytoplasm buds off with the secretion (axillary sweat glands, mammary glands)
3. Holocrine — entire cell disintegrates to form the secretion (sebaceous glands of skin)

Classification by duct system:
- Simple glands: single, unbranched duct
- Compound glands: branching duct system (larger glands — parotid, pancreas, submandibular)

Classification by secretory unit shape:
- Acinar (alveolar): spherical secretory units (grape-like cluster)
- Tubular: tube-shaped secretory units
- Tubuloacinar: combination (most compound glands)

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Serous vs Mucous vs Mixed Acini — The Key Distinction (AN70.1):

Feature	Serous acinus	Mucous acinus	Mixed acinus
Cell shape	Pyramid-shaped cells with round nucleus centrally placed	Tall columnar cells, flattened basal nucleus	Both cell types present
Cytoplasm	Basophilic (blue) granular cytoplasm (zymogen granules)	Pale, foamy, vacuolated (mucinogen granules push out staining)	Mixed appearance
Lumen	Narrow	Wide	Variable
Secretion	Watery, enzyme-rich (e.g., amylase, lipase)	Viscous, mucin-rich (glycoprotein gel)	Both watery and mucus
Serous demilune	Not applicable	Serous cells form a crescent (demilune) around mucous acinus in mixed glands	Present
Example	Parotid gland, pancreatic acini, lacrimal gland	Mucous glands of palate	Submandibular gland (predominantly serous + mucous demilunes); Sublingual (predominantly mucous)

High-yield fact: The parotid is purely serous. The sublingual is predominantly mucous. The submandibular is a mixed gland with serous demilunes (Giannuzzi's demilunes) capping the mucous tubules.

Figure: Serous vs Mucous vs Mixed Acini — The Key Distinction (AN70.1):

Mnemonic: "Pure Sour Milk"
- Parotid = Serous (pure)
- Sublingual = Mucous (mostly)
- Submandibular = Mixed (serous demilunes on mucous acini)

Lymphoid Tissue: Microanatomy (AN70.2)

Lymphoid (lymphatic) tissues are distributed throughout the body and house the cells of adaptive immunity. They range from diffuse aggregates to highly organised organs.

Figure: Lymphoid Tissue: Microanatomy (AN70.2)

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Lymph Node:
- Ovoid organ enclosed in a fibrous capsule with trabeculae extending inward
- Afferent lymphatics (multiple) → penetrate the capsule → subcapsular sinus
- Efferent lymphatic (one) → exits at the hilum
- Blood supply via hilar vessels

Structural zones:
- Cortex (outer): dense primary follicles (resting B cells) and secondary follicles (B cells + germinal centre activated by antigen — pale central zone = germinal centre surrounded by mantle zone of resting B cells)
- Paracortex (middle): predominantly T lymphocytes and high endothelial venules (HEVs) — site of lymphocyte recirculation from blood
- Medulla (inner): medullary cords (plasma cells, macrophages) and medullary sinuses (lymph flows here → exits via efferent lymphatics)

Function: Filter lymph; trap antigens; initiate B and T cell responses; produce antibody-secreting plasma cells.

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Spleen:
- Largest lymphoid organ; filters blood (not lymph)
- Enclosed in a fibrous capsule with trabeculae; no afferent lymphatics; no subcapsular sinus
- Two compartments visible on gross/histology:
- White pulp: lymphoid tissue organised around the central arteriole (periarteriolar lymphoid sheath = PALS, rich in T cells); B cell follicles (primary and secondary) alongside the PALS
- Red pulp: splenic cords (Billroth's cords — reticular cells + macrophages) + venous sinusoids; site of blood filtration, old RBC destruction, and iron recycling; also acts as a reservoir of platelets and RBCs
- Marginal zone: interface between white and red pulp; antigen trapping

Function: Filter blood; remove ageing/abnormal RBCs; immune surveillance; extramedullary haematopoiesis (foetal + pathological).

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Thymus:
- Located in the superior mediastinum; maximal size at puberty; involutes with age (fatty replacement)
- Each lobe divided into lobules by fibrous septa; each lobule has:
- Cortex: densely packed small lymphocytes (thymocytes undergoing selection); epithelial reticular cells providing the microenvironment
- Medulla: fewer, mature thymocytes; epithelial reticular cells; Hassall's corpuscles (concentric whorls of degenerating epithelial cells — pathognomonic of the thymus)
- Blood-thymus barrier (cortex only) — prevents immature thymocytes from encountering peripheral antigens before selection is complete

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Tonsils (Palatine):
- Located at the junction of oral cavity and oropharynx (Waldeyer's ring with pharyngeal and lingual tonsils)
- Covered by stratified squamous epithelium that invaginates to form crypts
- Crypts lined by epithelium + dense lymphoid tissue; secondary follicles abundant in the mucosa
- No capsule on the free (luminal) surface; fibrous capsule on the deep surface
- Function: immune surveillance of ingested/inhaled antigens (MALT — mucosa-associated lymphoid tissue)

Bone Histology: Types and Structure-Function Correlation (AN71.1)

Bone is a specialised connective tissue with a mineralised extracellular matrix. It provides structural support, houses the bone marrow, and is the body's main calcium/phosphate reservoir.

Figure: Bone Histology: Types and Structure-Function Correlation (AN71.1)

Cell types:
- Osteoblasts: Synthesise and secrete bone matrix (osteoid); become trapped as osteocytes
- Osteocytes: Mature bone cells trapped in lacunae within the matrix; communicate via canaliculi (tiny channels); sense mechanical stress
- Osteoclasts: Large, multinucleated; derived from monocyte-macrophage lineage; resorb bone matrix (acid + proteases)
- Bone-lining cells: Quiescent osteoblasts covering inactive bone surfaces

Types of bone:

1. Woven (immature/primary) bone:
- Random arrangement of collagen fibres
- First bone formed: embryonic development, fracture repair, Paget's disease
- Mechanically weaker; temporary scaffold

2. Lamellar (mature/secondary) bone:
- Collagen fibres in parallel sheets (lamellae)
- Stronger; replaces woven bone
- Two forms:
- Compact (cortical) bone: dense; Haversian system (osteon) — concentric lamellae around a central Haversian canal (neurovascular supply); Volkmann's canals connect adjacent Haversian canals; interstitial lamellae between osteons; circumferential lamellae at periosteal and endosteal surfaces
- Cancellous (trabecular/spongy) bone: lattice of trabeculae; marrow spaces contain bone marrow; no Haversian systems (osteocytes nourished by diffusion from marrow)

Periosteum: Fibrous outer covering (outer fibrous layer + inner cellular/cambial layer with osteoprogenitor cells)
Endosteum: Thin layer lining internal bone surfaces (trabeculae, Haversian canals)

Clinical correlation — Osteoporosis:
Loss of trabecular bone (cancellous bone is more metabolically active). Histology: thin, disconnected trabeculae with widened marrow spaces. Risk: femoral neck fractures in post-menopausal Indian women.

Cartilage Histology: Types and Structure-Function Correlation (AN71.2)

Cartilage is an avascular connective tissue. Its matrix is gel-like and resilient — ideal for load-bearing and flexible support.

Figure: Cartilage Histology: Types and Structure-Function Correlation (AN71.2)

Cartilage cells: Chondrocytes occupy spaces called lacunae within the matrix. In some regions, chondrocytes divide to form isogenous groups (pairs or small clusters from a single parent cell).

Perichondrium: Dense fibrous covering on most cartilage surfaces (except articular cartilage and fibrocartilage); contains progenitor cells (chondroblasts) for appositional growth.

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Three types of cartilage:

Feature	Hyaline cartilage	Elastic cartilage	Fibrocartilage
Matrix	Glassy, homogeneous; type II collagen (fibres not visible in H&E)	Type II collagen + elastic fibres (visible with elastic stain)	Dense type I collagen bundles (visible in H&E)
Chondrocytes	In lacunae; isogenous groups	In lacunae; isogenous groups	In rows between collagen bundles
Perichondrium	Present	Present	Absent
Appearance	Blue-grey, translucent	Yellow (gross); elastic fibres visible on special stain	White, fibrous, opaque (gross)
Location	Articular surfaces, costal cartilages, tracheal rings, nose, embryonic skeleton	Epiglottis, auricle (pinna), Eustachian tube	Intervertebral discs (annulus fibrosus), pubic symphysis, menisci, attachments of tendons/ligaments
Function	Smooth, low-friction joint surface; flexible support of airways; growth plate	Flexibility + resilience (bends and returns to shape)	Tensile strength + shock absorption

Clinical correlation:
- Osteoarthritis: Degradation of articular hyaline cartilage (type II collagen + aggrecan breakdown) → pain, loss of joint space on X-ray. Seen in knees and hips of older Indians with high BMI.
- Chondrosarcoma: Malignant tumour of hyaline cartilage; produces lobulated blue-grey tumour mass; abundant myxoid/chondroid matrix on histology.
- Achondroplasia: Most common cause of short-limb dwarfism; FGFR3 mutation → impaired endochondral ossification at the growth plate (zone of proliferating hyaline cartilage). Histology: disorganised growth plate.
- Intervertebral disc prolapse: Nucleus pulposus (vestigial notochord — a form of fibrocartilage with high proteoglycan content) herniates through the annulus fibrosus → compresses nerve roots.