Bone Histogenesis & Histology BSM Flashcards
Ossification / Osteogenesis
. The process by which bones form
How many germ layers are there in an embryo? Name them?
3 germ layers in an embryo
. The ectoderm, mesoderm and the endoderm
Which of the embryonic layers is used to form bones?
. The mesoderm - the mesoderm forms the mesenchyme and the mesenchymal stem cells are capable of differentitaing into different types of cells like chondroblasts which can form chondrocytes (both are cartilage cells)
& osteoblasts can form osteocytes (both are mature cells)
How many types of cartilage are there? Name them?
. 3 different types of cartilage
- Hyaline cartilage, elastic cartilage and fibrocartilage
Which type of cartilage do bones have ?
Outline the structure + type of collagen in this cartilage
- Hyaline cartilage
- It has cells in ECM - the cells are chondrocytes, which are located in spaces called lacunae (either singly or in groups).
Matrix = consists of collagen, mainly Type 2 collagen & a well hydrated ground substance - both are synthesised by the chondroblasts
Where are chondroblasts located in hyaline cartilage?
. In the perichondrium
. Perichondrium = connective tissue layer which covers the catilage
What are osteoprogenitor cells?
. Osteoblasts develop from mesenchymal stem cells called osteoprogenitor cells
What do osteoblast cells do?
. Synthesise the bone matrix which is then calcified
Osteoblast to osteocyte
Some of the osteoblasts get trapped in the matrix and differentiate into osteocytes which are located in spaces called lacuna - one osteocyte in each lacuna
Appearnce & Function of osteoclasts
. Multinucleated giant cells
. Resorb bone - digest the bone matrix
3 different types of cells found in bone & state their function in 2 words.
1 - Osteoblasts = forms bones (bone formation)
2 - Osteocytes = maintain bone
3 - Osteoclasts = destroy bone (bone resorbtion)
Compact bone versus cancellous bone
Compact bone = dense
Cancellous bone = spongy - has trabeculae - spaces between trabeculae are filled with bone marrow
What is the middle portion of a long bone called? What does it contain?
Cross-section of a shaft (diaphysis) of a long bone
Central portion of diaphysis
- medullary cavity filled with marrow
What are the ends of a long bone called? What are the expanded ends of the diaphysis called?
- Epipysis
- Metaphyses
What is the periosteum
- Surface of compact bone is covered with connective tissue called periosteum and it has an inner cellular layer which has osteoprogenitor cells, osteoblasts and osteoclasts
What are the marrow cavities lined with?
. lined by endosteum which also has the 3 bone cells
How many different types of ossification are there? Name them & state their fundamental difference?
2 different types of ossification
1- Endochondral ossification - involves cartilage as indicated by chondro in the name. The mesenchyme forms cartilage first, which serves as a template and that cartilage gets REPLACED by bone. How most bones form such as long bones like humerus and femur are formed.
2- Intramembranous ossification - bone is going to directly form from the mesenchyme with osteoblasts forming osteoid - examplee most bones of the skull vault & the facial bones. In short, most CRANIOFACIAL bones
Intramembranous Ossificaton Info - Just read
- No cartilage template
- Bone directly forms from mesenchyme
- The mesenchyme in the area where bone is going to be condenses - the condensed mesenchyme has stem cells which differentiate into osteoblasts (the bone matrix forming cells)
. Osteoblasts start synthesising osteoid - which is the matrix before mineralisation happens ( so just the collagen and the ground substance)
. The matrix then gets calcified
. As the osteoblasts form this matrix some get trapped forming osteocytes in the lacunae - the osteocytes have cytoplasmic processes that sit in canaliculi - this is an ossification centre and multiple ossification centres form and fuse
. The bone matrix organises into rods and plates forming the trabeculae of the cancellous bone
. Between the trabeculae = hematopoietic tissue - here mesenchymal cells become bloof-forming cells (marrow)
What is immature woven bone
. Bone that forms first in any process of ossification = immature woven bone
- it is irregular & not very strong
- so the immature woven bone gets remodelled to form lamellar bone
Geography of a bone
Periosteum - underneath the periosteum = compact bone forms - and then on the inside = cancellous bone
Endochondral Ossification Stages
- Mesenchyme first forms the chondroblasts, which synthesize the cartilage matrix (type 2 collagen & the ground substance)
- Cartilage forms in the shape of the bone which is going to replace it - this cartilage is hyaline cartilage and it has chondrocytes in lacunae
- The cartilage is surrounded by perichondrium which is the scaffold for the formation of bone
Learn
Osteoblasts -> Osteoid -> Gets calcified to form IMMATURE WOVEN BONE -> gets remodelled to form MATRUE LAMELLAR BONE
What does the surface of epiphysis have?
. Articular cartilage - becuase they articulate with other bones forming joints
. Articular cartilage = stays for life
What is the epiphyseal growth plate responsible for?
. Longitudnal growth of bones
