histo- b1 (SMS) Flashcards
what is the process of bone formation called?
ossification (also osteogenesis)
which germ layer do bones form from?
mesoderm
mesenchyme cells are derived from this germ layer that can then form chondroblasts and osteoblasts (form cartilage & bone cells)
general overview of the cells inside bone
osteoprogenitor cells (mesenchymal stem cells) → give rise to osteoblasts → synthesize bone matrix → calcified
during process, some osteoblasts get trapped in the matrix & differentiate into osteocytes (located in the lacuna)
also have osteoclasts
osteoclasts
multi nucleated giant cells that reabsorb bone
- digest bone matrix
osteoblasts vs osteocyte vs osteoclasts
osteoblasts → form bone
osteocyte → maintain bone
osteoclasts → destroy bone
periosteum & endosteum
periosteum: connective tissue that covers the surface of compact bone
- has an inner cellular layer that contains those osteoprogenitor cells, osteoblasts, and osteoclasts
endosteum: lines narrow cavities (medullary cavity & the trabeculae)
- also has these cells
spongy bone: trabeculae
spongy bone contains rods & plates that forms spaces called trabeculae
spaces b/w the trabeculae are filled w/ bone marrow
diaphysis, epiphysis, & metaphysis of bone
diaphysis: central portion of the shaft of a bone
- has medullary cavity thats filled with marrow
epiphysis: ends of the bone
metaphysis: expanded ends of the diaphysis (b/w the diaphysis & epiphysis)
2 ways that ossification can happen
Intramembranous: bone directly forms from the mesenchyme w/ osteoblasts forming osteoid
- how most bones of the skull & facial vault form, also parts of the clavicle
cartilaginous (endochondral): mesenchyme forms cartilage first which serves as template & that cartilage gets replaced by bone (doesnt turn INTO bone but gets replaced by bone)
- *how most bones in the body form: long bones (humerus, femur), vertebrae, ribs, etc.
entire steps of intramembranous ossification
no cartilaginous template
- mesenchyme condenses where the bone is going to form
- condensed mesenchyme has mesenchymal stem cells which differentiate into osteoblasts (bone matrix forming cells)
- osteoblasts get to work & start synthesizing osteoid (matrix before mineralization occurs- type I collagen & ground substance)
- bone matrix gets calcified
- as osteoblasts form matrix, some get trapped to form the osteocytes in those lacunae
- have cytoplasmic processes that sit in canaliculi (small channels that provide pathway for communication, just like dendrite looking things) - calcified bone matrix organizes into rods & plates to form trabeculae of spongy bone around blood vessels
- b/w trabeculae is hematopoetic tissue (here, mesenchymal cells become blood forming cells - aka MARROW) - mesenchyme around that doesn’t form bone forms the periosteum and the endosteum
- underneath periosteum, compact bone forms
Haversian canal vs canaculi in bones
haversian canals: larger, central canals that run through bone and contain blood vessels and nerves
canaliculi: smaller channels that connect lacunae to Haversian canals & to each other
- allow for exchange of nutrients & waste b/w osteocytes and blood vessels
bone that forms first in any ossification process
immature woven bone
- is irregular & not very strong and most of it gets remodeled to form lamellar bone
entire steps of cartilaginous ossification (endochondral)
- mesenchyme first forms chondroblasts which synthesize hyaline cartilage matrix (type II collagen & ground substance)
- Hyaline cartilage template forms in the shape of the bone thats going to replace it
- has chondrocytes in lacunae and is surrounded by perichondrium - Some of the chondrocytes in the center swell (hypertrophy) & develop vacuoles - these chondrocytes compress the matrix & matrix starts calcifying
- part of perichondrium near middle of the shaft becomes vascularized (starts changing perichondrium to periosteum) - this calcification in the middle causes the hypertrophied chondrocytes to die
- osteoblasts come in and start laying down spongy bone & blood vessels come in too (this forms the primary ossification center)
- once spongy bone formed, osteoclasts start to clear parts of it to make the medullary cavity (which will hold the bone marrow)
- osteoblasts lay down bone around the medullary cavity → remodeling that extends towards epiphysis = replaces cartilage with bone in diaphysis
- after birth, bone formation starts in the epiphyses to create secondary ossification centers (osteoblasts create osteoid first that is then calcified to form immature woven bone thats then remodeled to make mature lamellar bone)
where are the only 2 sites that cartilage remains in the bones?
in the epiphyseal growth plates (in b/w the diaphysis & epiphysis) and articular cartilage (surfaces of epiphysis on both ends)
osteon (aka Haversian system)
basic structural unit of compact bone
look like a tree trunk but with a canal in the center
contains osteocytes within the lacunae
3 types of specialized connective tissue
- bone
- cartilage
- blood
endomysium, perimysium, epimysium of muscles
endomysium: innermost layer, directly envelops each individual muscle fiber
perimysium: covers bundles of muscle fibers (called fascicles)
epimysium: entire muscle is wrapped by this outermost layer
what are the histological differences between fibrocartilage, hyaline cartilage, and elastic cartilage.
in common: all have chondrocytes (cartilage cells), lacunae, and matrix
hyaline cartilage: glassy, smooth-looking matrix with no visible fibers under light microscope
- has few type II collagen fibers but they’re not obvious
- have chondrocytes in lacunae found in small clusters (called isogenous groups)
- have a perichondrium covering cartilage!!!!
fibrocartilage: more flattened & organized cell rows
- densely layered collagen fibers
- Type I collagen fibers ^
- scatters of chondrocytes are arranged in rows
- no perichondrium
elastic cartilage: visible elastic fibers in matrix (the elastic fibers looks like black curls)
- perichondrium is made of outer fibrous later and inner cellular fibrous layer
- chondrocytes in lacunae are more numerous and can be close together
5 layers of the epidermis
- **stratum corneam (“horny layer”): outermost layer, made of dead, flat, keratinized cells
- straight lines, lighter shade of pink -
stratum lucidum (“clear layer”): thin clear layer of dead keratinocytes, ONLY IN THICK SKIN (palm & soles)
- straight line darker shade of pink -
stratum granulosum (“grainy layer”): 3-5 layers of cells that are getting flatter
- helps waterproof skin - stratum spinosum (“spiny layer”): several layers of normal rounded cells that are joined by desmosomes
-
stratum basale (“base layer”): single layer of stem cells that divide to form new skin cells, contain melanocytes, Markel cells (touch), and keratinocytes
- looks like a hill of just single cuboidal cells
mnemonic: before signaling, get legal counsel (from bottom up)
how does thick skin differ from thin skin
thick skin:
- palms of hands, soles of feet
- no hair, no sebaceous glands
- has dermal papillae (fingerprints)
- has 5 layers in epidermis
thin skin:
- everywhere else (arms, legs, face, etc)
- 4 layers in epidermis
- has hair
- has sebaceous glands and sweat glands
- less pronounced dermal papillae
