Glands: Structure and Classification

The transcript discusses gland structure, how glands secrete, and how glands are classified.
It uses several terms that are sometimes misspelled or debated in current anatomy literature (e.g., "miracrine" for merocrine, "mammocrine" for merocrine). Wherever the transcript is unclear or inconsistent with standard terms, I note the likely intended meaning.

Endocrine glands secrete products (mainly hormones) directly into the bloodstream or surrounding interstitial fluid.
- Examples mentioned: adrenal, thyroid, and pituitary glands.
Exocrine glands secrete their products into ducts or onto surfaces (e.g., skin surface) and not directly into the bloodstream.
Exocrine glands can be unicellular or multicellular.
- Unicellular exocrine glands lack ducts (example given: goblet cells).
- Multicellular exocrine glands have a duct system.
Unicellular exocrine gland example: goblet cell, located in epithelial tissue, secreting mucin which forms mucus in areas like the airways (trachea, respiratory system).

Unicellular exocrine glands:
- Do not contain ducts.
- Example: Goblet cells secrete mucin to form mucus coating surfaces.
Multicellular exocrine glands:
- Contain ducts and secretory portions.
- Classified by form (duct architecture) and by secretion method.

Simple glands: a single duct.
Compound glands: branching ducts (multiple ducts).
Important note in the transcript: branched glands are sometimes used interchangeably with compound glands; branched refers to arrangement of secretory units, whereas compound refers to duct multiplicity.
Secretory portion shapes:
- Tubular: duct and secretory portion have similar diameters.
- Alveolar (also called acinar in some texts): secretory portion forms a sac-like shape.
Forms of simple glands (single duct):
- Simple tubular
- Simple coiled tubular (often associated with glands like sweat glands)
- Simple branched tubular (still a simple gland because there is only one duct)
- Simple tubular with alveolar center (simple branched to center?)
Forms of compound glands (more than one duct):
- Compound tubular
- Compound acinar (secretory units are sac-like)
- Compound tubuloacinar (mixed tubular and acinar regions)
Key takeaway: simple vs compound relates to the number of ducts; tubular vs alveolar/acinar relates to the shape of the secretory portion.

Merocrine (also called eccrine in some contexts, though the transcript uses "merocrine" and "miracrine"):
- Secretions released via exocytosis as secretory vesicles fuse with the plasma membrane.
- The cell remains intact and functional after secretion.
- Example given in the transcript: salivary glands are merocrine.
- Transcript note: The term used in some parts is "merocrine"; the transcript also uses "mammocrine" in reference to this type, which is likely a misnomer.
Apocrine:
- Secretory products are released with a portion of the cell’s cytoplasm at the apex; the cell repairs itself and continues secretion.
- Examples: mammary glands and some sweat glands.
- The transcript notes a recent debate about whether true apocrine sweat glands exist in humans; some glands formerly labeled apocrine may be better classified as merocrine.
Holocrine:
- Secretory products accumulate within the cell; the entire cell ruptures to release the product.
- New epithelial cells replace the ruptured ones via division.
- Examples: sebaceous glands release sebum; glands in skin and eyelids mentioned.

Merocrine glands (as per transcript):
- Salivary glands (classic example given for merocrine secretion).
- Eccrine sweat glands (watery sweat for thermoregulation) are mentioned as merocrine in the transcript, with caveats about broader classifications.
- Pancreas gland activity is also referenced in the context of merocrine secretion (note: transcript uses a term akin to "mammocrine" here).
Apocrine glands:
- Mammary glands (and some sweat glands) cited as apocrine.
- Dissenting view: human sweat glands may not be true apocrine glands; may be more appropriately classified as merocrine.
Holocrine glands:
- Sebaceous glands produce sebum via holocrine secretion; entire cells disintegrate to release product.
Unicellular exocrine gland example:
- Goblet cells secrete mucin that forms mucus.

Mucin vs Serous secretions:
- Two secretion types mentioned: mucoid (mucin-containing, thick/viscous mucus) and serous (a watery, enzyme-rich fluid).
Goblet cells:
- Located in epithelial tissue; secrete mucin; contribute to mucus layers on surfaces such as respiratory tract.
Sebaceous glands:
- Holocrine glands; secrete sebum; secretion associated with hair and skin surface.
- Erection of hair and sebaceous duct opening described visually in the transcript.
Secretions in digestive tract glands (gastric pits):
- Gland types in the stomach are described as simple tubular glands with various cell types (parietal, chief, mucous cells) within gastric pits.
Simple tubular glands (examples):
- Sweat glands cited as simple tubular.
Simple branched tubular glands:
- Mentioned in the context of glands in the stomach and mouth.
Compound tubular glands:
- Example given: male and female urethral glands secrete a clear fluid that helps cleanse the urethra prior to intercourse.

A note on how gland shapes relate to function, including the arrangement of secretory units and ducts.
Anecdotal references used to illustrate mucus viscosity (high school athletics, sprinting), and a discussion of lactation curves in dairy cattle as a conceptual reminder that production can outpace cellular repair over time.
The transcript emphasizes that future videos will cover structure and function of specific glands in detail.
There is an emphasis on differentiating between true apocrine glands and merocrine glands in humans, signaling ongoing research and potential reclassification.

Endocrine gland: secretes hormones into bloodstream/interstitial fluid; no ducts.
Exocrine gland: secretes into ducts or surfaces; can be unicellular or multicellular.
Unicellular exocrine gland: goblet cell (no duct).
Multicellular exocrine gland: ducts present; secretory portions may be tubular or acinar.
Duct form terminology:
- Simple: a single duct
- Compound: multiple ducts (branching ducts)
- Branched: arrangement of secretory units; sometimes used interchangeably with compound.
Secretory portion shapes:
- Tubular: duct and secretory portion similar in diameter
- Alveolar/Acinar: sac-like secretory units
Secretion mechanisms:
- Merocrine: exocytosis; cell remains intact
- Apocrine: apical portion of cell released with secretory product; cell repairs
- Holocrine: entire cell ruptures to release product
Primary examples to memorize:
- Merocrine: salivary glands, eccrine sweat glands (thermoregulation)
- Apocrine: mammary glands; some sweat glands (debated in humans)
- Holocrine: sebaceous glands (sebum)

Gland classification integrates duct architecture (simple vs compound) with the secretory unit shape (tubular vs alveolar) and the secretion method (merocrine, apocrine, holocrine).
Understanding these distinctions helps predict how glands release products, their potential for damage or cell turnover, and how secretions contribute to body functions (digestion, humidity of airway surfaces, thermoregulation, skin health, etc.).
Real-world relevance includes diagnosing gland-related disorders, understanding sweat/odor physiology, and recognizing that human gland classification can be nuanced and subject to ongoing research.